Electronic Transformers

Transcription

1 Electronic Transformers There are many types of transformers. What distinguishes an electronic transformer from other types of transformers? Electronic transformers are simply transformers used in electronic applications. This is a very broad definition; consequently there are many types of electronic transformers. Examples of types of electronic transformers include ( but not limited to ) power, pulse, instrument, current, switching ( or switch mode ), inverting, signal, step-up, step-down, impedance matching, high voltage and saturable. Some of the preceding types can be divided into more sub-types. Types of switching transformers include ( but not limited to ) flyback, feed forward converter ( also called buck ), and boost. Gate drive transformers and trigger transformers are types of pulse transformers ( depending on who you talk to). The feed forward type includes a push-pull center-tap and a half bridge configuration. It becomes apparent from the preceding type designations that the type designation of an electronic transformer is determined by its intended application. To learn more about a particular type, click on one of the available links for electronic transformer types. Electronic transformers may be further described by their basic structure and/or construction style. Many current transformers are wound on toroidal cores; hence the transformer is referred to as a toroidal current transformer. Many transformer coils are wound on bobbins ( spools ) or tubes. The transformer core is inserted into and around the coil. These transformers may be referred to as bobbin wound or tube wound structures. There are many core shapes available; E, E-I, U, U-I, Pot, RM, PQ, EP, EFD, and others. Electronic transformers may be further described by the methods of mounting and electrical terminations. Transformers mounted on printed circuit boards may be pinthru or surface mount. Transformer windings are terminated to bobbin pins or surface mount pads. The pins or pads are then soldered to the printed circuit board. Some transformers have lead wires. These wires are often referred to as flying leads. Electronic transformers may be used to supply power, transmit signals, establish voltage isolation between circuits, sense voltage and current levels, modify voltage and current levels, provide impedance matching, and filtering. Lightly loaded transformers may perform some inductor-like functions, such as storing energy and limiting current flow. Do electronic transformers have any characteristics common to all electronic transformers? Not really. Most electronic transformers can easily be held in your hand, even in a child s hand, but there are some too large to hold. Due to ever-higher operating frequencies, more electronic transformers are being made from ferrite core materials, but some specialized applications use other core materials.

2 Despite the many types of electronic transformers, their theory of operation does not differ. Electrical functions are usually similar but design characteristics can differ in certain ways. Some examples are; unipolar versus bipolar core utilization, saturating or not saturating, degree of energy storage, regulation, and transformer impedance. Butler Winding can make ( and has made ) electronic transformers in a wide variety of shapes and sizes. This includes; various standard types of core with bobbin structures ( E, EP, EFD, PQ, POT, U and others ), toroids, and some custom designs. Our upper limits are 40 pounds of weight and 2 kilowatts of power. We have experience with foil windings, litz wire windings, and perfect layering. For toroids, we can ( and have done ) sector winding, progressive winding, bank winding, and progressive bank winding. Butler winding has a variety of winding machines, bobbin/tube and toroid. That includes two programmable automated machines and a taping machine for toroids. Butler winding has vacuum chamber(s) for vacuum impregnation and can also encapsulate. To ensure quality, Butler Winding purchased two programmable automated testing machines. Most of our production is 100% tested on these machines. For more information on Butler Winding s capabilities, click on our capabilities link. Toroidal Transformer Toroidal transformers are the high performers among transformers. They offer the smallest size (by volume and weight), less leakage inductance, and lower electromagnetic interference (EMI). Their windings cool better because of the proportionally larger surface area. A 360 degree wound toroidal transformer has a high degree of symmetry. Its geometry leads to near complete magnetic field cancellation outside of its coil, hence the toroidal transformer has less leakage inductance and less EMI when compared against other transformers of equal power rating. Toroidal transformers with a round core cross section are better performers than toroidal transformers with a rectangular cross section. The cancellation is more complete for the round cross section. The round cross section also gives a shorter turn length per unit of cross sectional area, hence lower winding resistances. The toroidal transformer also has better winding to winding magnetic coupling because of its toroidal shape. The coupling is dependent on the winding being wound a full 360 degrees around the core and wound directly over the prior winding, hence sector wound windings do not couple as well and have higher leakage inductance. As winding turns are positioned further away from the core less complete coupling will occur; hence toroidal transformers with multi-layered windings will exhibit more leakage inductance. Toroidal transformers can be used in any electronic transformer application that can accommodate its shape. Although usable, toroidal transformers are not always practical for some applications. Gapped toroidal transformers usually require that the gap be filled with some type of insulating material to facilitate the winding process. This is an extra

3 expense. Split core current transformers can be assembled directly on a conductor while toroids must be passed over a disconnected end of the conductor. A toroid can be split in two, but a suitable clamping mechanism (difficult and costly) is required. Some printed circuit boards are space critical. Mounting a toroidal transformer flat on the board may take up too much precious board area. Some applications also have restricted height so the toroid cannot be mounted vertically. Generally speaking toroidal transformers are more expensive than bobbin or tube wound transformers. Sufficient winding wire must first be wound (loaded) onto the winding shuttle, then wound onto the toroidal transformer s core. After that, the best situation, from a cost perspective, is no insulation required over the winding and the next winding uses the same wire size. If the wire is different, then the leftover wire must be removed and the wire for the next winding must be loaded. However, if the winding must be insulated, then if must either be insulated (taped) by hand or the toroidal transformer must be removed and taken to a separate taping machine, then placed back on the toroid winding machine after taping. The shuttle must then be loaded with the wire size and type for the toroidal transformer s next winding. A toroidal transformer with a single winding (auto-transformer, current transformer) wound on a coated core will probably be cost competitive with an equivalent bobbin or tube wound transformer since the toroidal transformer will not require a bobbin or tube. The cost differential will then depend on the method and cost of mounting the transformers. Toroidal transformer cores are available in many materials: silicon steel, nickel iron, moly-permalloy powder, iron powdered, amorphous, ferrites, and others. Silicon steel and nickel iron are available as tape wound cores or laminated pieces. Non-magnetic toroids are also available to make air core toroidal transformers. Butler Winding manufactures toroidal transformers in a wide variety of materials and sizes. To ensure quality, Butler Winding purchased two programmable automated testing machines. Most of our production is 100% tested on these machines. For more information on Butler Winding s capabilities, click on our capabilities link. Toroidal Transformer - Ferrite Core Transformers As today's electronic designers pack more components into less space, there is increasing demand for high performance components. Toroidal transformers with a ferrite core are in the high performance category. A 360 degree wound ferrite core toroid (and toroids in general) has a high degree of symmetry. Its geometry leads to near complete magnetic field cancellation outside of its coil, hence the toroidal transformer has less leakage inductance and less EMI when compared against other coils of equal power rating. Today's electronic devices are being operated at ever increasing high frequency. Ferrite core toroidal transformer manufacturers have developed core materials that can operate

4 above 1 megahertz at low gauss levels. Use of a ferrite core toroidal transformer combines the performance features of the toroidal shape and the low loss feature of the ferrite core material.. In high frequency applications a ferrite core toroidal transformer can offer smaller size (by volume & weight) and lower losses. Core materials have been developed for power applications and filtering applications. Some have a temperature coefficient designed to offset capacitor temperature drift for tuned filter applications. Core material initial relative permeability can range from 750 to Materials for the highest frequencies usually have lower permeability. Toroidal transformers with ferrite cores are commercially available in a variety of sizes ranging from one tenth to five and one half inches outside diameter. The weight ranges from a fraction of an ounce to pounds. Larger ones exist but they are specialty items. Some larger specialty They can be purchased with or without an insulating coating. Several voltage ratings are available. Butler Winding makes toroidal transformers and ferrite core toroid coils in a wide variety of materials and sizes. Butler Winding also does "bobbin wound" and "tube wound". Butler winding has a variety of winding machines, bobbin/tube and toroid. That includes two programmable automated machines and a taping machine for toroids. Butler Winding has vacuum chamber(s) for vacuum impregnation and can also encapsulate. To ensure quality, Butler Winding purchased two programmable automated testing machines. Most of our production is 100% tested on these machines. For more information on Butler Winding's capabilities, click on our "capabilities" link. Toroidal Transformer - Tape Wound Transformers Tape wound core toroidal transformers are made by wrapping thin long strips of magnetic material around a winding mandrel. Originally, tape wound core toroids were developed to replace vacuum tubes. Vacuum tubes were fragile and required frequent replacement. The tape wound core toroidal transformers were more reliable. Magnetic coupling permitted the mixing of signals while maintaining electrical isolation between circuits. Tape wound core toroidal transformers also developed along another path. Early toroidal transformers used thin ring shaped laminations stamped from electrical steel. The steel from the center was waste material. A core was made by stacking these rings to the desired height. The laminated stack reduced core eddy currents. Lower eddy currents result in lower core losses. The thinner the laminations, the lower the losses were, but the more time it took to process and stack the laminations. Designers adapted the tape wound core process to general-purpose transformers as well. Winding tape wound core toroidal trsnaformers were much faster than stacking toroidal cores; hence use of thinner material became more practical. The process was then adapted to rectangular cores known as C cores.

5 Today, tape wound core toroidal transformers can be made with strip as thin as They are available in alloys of silicon steel, nickel-iron, cobalt-iron, and amorphous metals. Some materials are processed to enhance square loop properties. With appropriate gauss de-rating, the thin strip extends the useful frequency range up to 10 to 20 kilohertz depending on the type of material. Ferrite cores have lower core losses and cost less per unit weight, but their saturation levels are much lower. Low weight and minimal space are desired features for aviation and aerospace applications. Consequently tape wound core toroids are usually preferred over ferrites for these applications provided the operating frequency is not too high. Tape wound core toroids wound with nickel-iron alloys are particularly sensitive to shock and vibration. These cores need to be place in a protective box with a damping medium such as silicon oil. Silicon steel alloys are the least sensitive. Silicon steel is frequently used without a protective box. It depends on the particular application. Butler Winding produces tape wound core toroidal transformers in a wide variety of materials and sizes. For more information on Butler Winding s capabilities, click on our capabilities link. Flyback Transformers - Kickback Transformers A simple and low cost power supply is bound to be quite popular. The single ended flyback circuit topology fits this description. The flyback transformer utilizes the "flyback" action ( also known as "kickback" ) of an inductor or flyback transformer to convert the input voltage and current to the desired output voltage and current. Figures 1A and 1B show simple flyback transformer schematics for an inductor and a flyback transformer. These schematics do not show any parasitic effects ( such as leakage inductance and winding capacitance ). Modern flyback transformer and circuit design now permit use in excess of 300 watts of power, but most applications are less than 50 watts. By definition a transformer directly couples energy from one winding to another winding. A flyback transformer does not act as a true transformer. A flyback transformer first stores energy received from the input power supply (charging portion of a cycle) and then transfers energy (discharge portion of a cycle) to the output, usually a storage capacitor with a load connected across its terminals. An application in which a complete discharge is followed by a short period of inactivity (known as idle time) is defined to be operating in a discontinuous mode. An application in which a partial discharge is followed by charging is defined to be operating in the continuous mode. See figures 2A and 2B for illustration. Gapped core structures increase the magnetizing force needed to reach saturation and lower the inductance of the flyback transformer (or inductor). Consequently, a gapped

6 flyback transformer (or inductor) can handle higher peak current values, and thereby storing more energy, most of which is stored in the magnetic field of the gap. For these reasons almost all flyback transformers (or inductors) are gapped. The gap may be a discrete physical gap, several smaller discrete physical gaps or a distributed gap. Distributed gaps are inherently present in low permeability powdered cores. The bulk of the stored energy is stored in the magnetic field of the gap(s). Most modern flyback transformers are operated at high frequency hence gapped ferrite core materials are typically used. Butler winding can make (and has made) flyback transformers in a wide variety of shapes and sizes. This includes; various standard types of core with bobbin structures (E, EP, EFD, EC, ETD, PQ, POT, U and others), toroids, and some custom designs. We have experience with foil windings, litz wire windings, and perfect layering. For toroids, we can (and have done) sector winding, progressive winding, bank winding, and progressive bank winding. Butler winding has a variety of winding machines, bobbin/tube and toroid. That includes two programmable automated machines and a taping machine for toroids. To ensure quality, Butler Winding purchased two programmable automated testing machines. Most of our production is 100% tested on these machines. For more information on our capabilities, click on our "capabilities" link. How does a flyback transformer ( or inductor ) work? Flyback circuits repeat a cycle of two or three stages; a charging stage, a discharging stage, and in some applications idle time following a complete discharge. Charging creates a magnetic field. Discharging action results from the collapse of the magnetic field. The typical flyback transformer application is a unipolar application. The magnetic field flux density varies up in down in value ( 0 or larger ) but keeps the same ( hence unipolar ) direction. Charging Stage: The flyback transformer ( or inductor ) draws current from the power source. The current increases over time. The current flow creates a magnetic field flux that also increases over time. Energy is stored within the magnetic field. The associated positive flux change over time induces a voltage in the flyback transformer ( or inductor ) which opposes the source voltage. Typically, a diode and a capacitor are series connected across a flyback transformer winding ( or inductor ). A load resistor is then connected across the capacitor. The diode is oriented to block current flow from the flyback transformer ( or source ) to the capacitor and the load resistor during the charging stage. Controlling the charging time duration (known as duty cycle) in a cycle can control the amount of energy stored during each cycle. Stored energy value, E = ( I x I x L ) / 2, where E is in joules, I = current in amps, L = inductance in Henries. Current is defined by the differential equation V(t) = L x di/dt. Applying this equation to applications with constant source voltage and constant inductance value one obtains the following equation; I = Io + V x t / L, where I = currents in amps, Io = starting current in amps, V = voltage in volts across the flyback transformer winding ( or inductor ), L = inductance in Henries, and t = elapsed time in seconds. Note that increasing L will decrease the

7 current. Stored energy will consequently decrease because effects of the current squared decrease will more than offset the effects of the inductance increase. Also be aware that the flyback transformer ( or inductor ) input voltage is less than the source voltage due to switching and resistive voltage drops in the circuit. Discharge Stage: The current ( which creates the magnetic field ) from the source is then interrupted by opening a switch, thereby causing the magnetic field to collapse or decrease, hence a reversal in the direction of the magnetic field flux change ( negative flux change over time ). The negative flux change induces a voltage in the opposite direction from that induced during the charging stage. The terms flyback or kickback originate from the induced voltage reversal that occurs when the supply current is interrupted. The reversed induced voltage(s) tries to create ( induce ) a current flow. The open switch prevents current from flowing through the power supply. With the voltage reversed, the diode now permits current flow through it, hence current flows into the capacitor and the load across the capacitor. If current can flow, then the resulting flow of current is in the direction, which tries to maintain the existing magnetic field. The induced current cannot maintain this field but does slow down the decline of the magnetic field. A slower decline translates to a lower induced flyback voltage. If current cannot flow, the magnetic field will decline very rapidly and consequently create a much higher induced voltage. In effect, the flyback action will create the necessary voltage needed to discharge the energy stored in the flyback transformer or inductor. This principle, along with controlling the duration of the charging stage, allows a flyback inductor to increase or decrease the voltage without the use of a step-up or step-down turns ratio. In the typical flyback circuit, the output capacitor clamps the flyback voltage to the capacitor voltage plus the diode and resistive voltage drops. For a sufficiently large & fully charged capacitor, the clamping capacitor voltage can be treated as a constant value. The equations V(t) = L x di/dt, and I = Io + V x t / L can also be applied to the discharge stage. Use the inductance value of the discharging winding and the time duration of the discharging stage. The time will either be the cycle time minus the charging time ( no idle time ), or the time it takes to fully discharge the magnetic field thereby reaching zero current. The cycle time equals the period which equals 1 / frequency. Idle Stage: This stage occurs whenever the flyback transformer ( or inductor ) has completely discharged its stored energy. Input and output current ( of the transformer or inductor ) is at zero value. Other Principles of Operation Equal Ampere-Turns Condition: A magnetic field is created by the current flow through the winding(s). The current creates a magnetizing force, H, and a magnetic field flux density B. A core dependent correlation will exist between B and H. B is not usually linear with H. By definition H is proportional to the product of the winding turns and the current flowing through the winding, hence ampere-turns. In classical physics, the magnetic field flux cannot instantaneously change value if the source of the field ( the current flow ) is removed. When the source current is removed from the flyback transformer ( or inductor ) the charging stage ends and the discharge stage begins. The

8 value of the magnetic field will be the same for both stages at that point in time ( cannot instantaneously change to another value ). The same magnetic core is used for both stages, hence if the magnetic field is the same, then the magnetizing force, H, must be the same. Consequently the ampere-turns at the end of the charging stage must equal the ampere-turns at the start of the discharge stage. If there are multiple outputs then the total amperes turns of all outputs at the start of the discharge stage must equal the ampereturns at the end of the charging stage. The same condition applies at the start of the charging stage. The total ampere-turns of all outputs at the start of the charging stage must equal the ampere-turns at the end of the discharge stage. Note that there are zero ampere-turns at both the start and end of an idle stage when an idle stage exists. Zero Average Voltage: During steady state operation, the average voltage across the charging winding must equal the average voltage across the discharge winding, or equivalently, the volt-seconds of the charging stage must equal the volt-seconds of the discharge stage. If not, flux density increases over time and the core saturates. Assuming a 1:1 turns ratio, then from V1 x t1 = V2 x t2 one can obtain t1 / t2 = V2 / V1 for both continuous and discontinuous modes of operation. For continuous mode operation, t1 + t2 = 1 / operating frequency. Conservation of Energy: Power out cannot exceed power in. Sum up output power ( V x I ) of each output at maximum steady state load plus allowances for parasitic output power losses ( diode and resistive losses ). Divide power in watts by operating frequency. The result is the energy in Joules that must be discharged each cycle into the output storage capacitor during steady state operation. It is also the amount of energy that must be added to the flyback transformer ( or inductor ) during the charging stage. The energy being transferred equals ( Ipeak x Ipeak Imin. x Imin. ) x L /2. If operating in the continuous mode, the stored energy will exceed the energy being transferred because the starting level of stored energy is above zero ( Imin. > 0 ). The flyback transformer ( or inductor ) must be designed to handle the peak stored energy, Ipeak x Ipeak x L / 2. The power source will have to supply the transferred energy plus the parasitic switching and resistive losses of the charging circuit, plus some power allowance for transient conditions. Take this value and divide by the power supply voltage. The result will be the average input current. Need additional information about Flyback Transformers? Contact Butler Winding. Ask for engineering assistance.

9 Power Transformers - Switch Mode What differentiates a power transformer and a switch mode power transformer from other transformers? Power transformers (and inductors) are essentially A.C. (alternating current) devices. They cannot sustain transformer operation from a fixed D.C. (direct current) voltage source. However they can sustain transformer operation in a transient condition(s) that allows resetting or reversal of the transformer s magnetic flux levels. An A.C. voltage source keeps reversing the polarity of the voltage being applied across the transformer. Consequently the magnetic fields keeps reversing. Voltage reversal can also be accomplished with a D.C. source such as a battery. The connections between the D.C. source and the transformers are repeatedly switched, thereby reversing the voltage polarity across the transformer, hence reversing the magnetic field. The transformer can also be switched off from the D.C. source. In this case the magnetic field simply collapses until it reaches its residual value (ideally equal to zero). This collapse resets the transformer s magnetic field. Switch mode power transformers (and supplies) get their name from the switching action needed to sustain transformer operation. By controlling the amount of on time and off time of the switches, one can also control the amount of power delivered to the transformer s load (or load circuit). The voltage can be fed to the switch mode power transformer in voltage pulses. The pulse duration is a portion of an overall cycle time. The cycle time is equal to the inverse of the operating frequency. The terms duty cycle and pulse width modulation arise from the control of the switching on time and off time.

10 Switch mode power transformers are used extensively in electronic applications, usually within a switch mode power supply. A switch mode power supply is usually powered from a D.C. source, such as a battery. The switching mode power supply converts the input D.C. source to one or more output D.C. sources. The power supplies are often referred to as DC to DC converters. In similar fashion, the switch mode power transformers are often referred to as DC to DC transformers (or DC-DC transformers). A switch mode power transformer can have several secondary windings. Consequently, the switch mode transformers permits multiple outputs which can be electrically isolated from one another. Transformer action permits one to step up or step down the voltage as needed via an appropriate turns ratio. Pulse width modulation is used to provide voltage regulation. Many electronic applications require some sort of power supply which converts power from the conventional low frequency sinusoidal A.C. wall socket (for example, 115V 60 Hz) to the necessary voltage, current, and/or waveform required by the circuit. Typically the circuits need a well-regulated D.C. voltage. Designers often choose either a rectifier type circuit (to convert A.C. voltage to D.C. voltage), a switch mode power supply, or both. For the both case, the A.C. voltage is first rectified to provide a D.C. voltage. The D.C. voltage varies as the A.C. voltage varies, hence good voltage regulation cannot be assured. One or more switching mode power supplies follow the rectifying circuitry. The switching mode power supplies provide a more tightly regulated output voltage. A.C. rectification is not a necessity. Although tricky, it is possible, through switching actions, to divide ( chop ) the A.C. waveform into a series of pulses, which are directly fed into the switching mode power transformer. Pulse width modulation is used to control the regulation. Butler Winding can make (and has made) switching mode power transformers (and /or inductors) for Buck, Flyback, and Boost applications (discussed below) in a wide variety of shapes and sizes. This includes; various standard types of core with bobbin structures (E, EP, EFD, PQ, POT, U and others), toroids, and some custom designs. Our upper limits are 40 pounds of weight and 2 kilowatts of power. We have experience with foil windings, litz wire windings, and perfect layering. For toroids, we can (and have done) sector winding, progressive winding, bank winding, and progressive bank winding. Butler winding has a variety of winding machines, bobbin/tube and toroid. That includes two programmable automated machines and a taping machine for toroids. Butler Winding has vacuum chamber(s) for vacuum impregnation and can also encapsulate. To ensure quality, Butler Winding purchased two programmable automated testing machines. Most of our production is 100% tested on these machines. For more information on Butler Winding s capabilities, click on our capabilities link. Switching Mode Power Transformers, Basic Application Circuits The design of a switch mode power transformer will differ depending upon the type of circuit used. There are many variations of switching mode power supplies, but they can be narrowed down to three basic circuit configurations (each also has a mirrored configuration); Buck, Boost, and Flyback. Be aware that the name for the Buck

11 circuit varies from industry to industry and from person to person. It may also be referred to as an inverter, D.C. converter, forward converter, feed forward, and others. There are also unipolar and bipolar (push-pull) versions. The basic Buck circuit is illustrated in Figure 1A with an inductor and in Figure 1B with both a switch mode power transformer and an inductor. A push-pull version is shown in Figure 4. The basic Flyback circuit is illustrated in Figure 2A with an inductor and in Figure 2B with a switch mdoe power transformer. The basic boost circuit is illustrated in Figure 3A with an inductor, Figure 3B and 3C with a transformer and in Figure 5 with a push-pull forward converter type of switch mode power transformer. The circuits shown in Figures 1A, 2A, and 3A, which have no switch mode power transformers, are the simplest circuits. They are useful for explaining the operating theory. The Forward Converter (Buck) Circuit The inductors in all of the buck circuits act as filtering elements to smooth out the ripple and reduce peak currents. Since they must store energy for part of a cycle they usually have a discrete air gap(s) or a distributed air gap in the magnetic core path. The switch mode power transformer in the Buck Circuit of Figure 1B couples energy from the input side (primary) to the output side (secondary). An ideal transformer does not store any energy and consequently does not provide any ripple filtering. The inductor does the ripple filtering. Ideally, a Buck circuit transformer couples energy without storing it (hence it meets the true definition of a transformer). The transformer does not need to do any ripple filtering. The transformer should have minimal air gap. The on time on the transistor (switch) controls how much energy is delivered to the capacitor hence it regulates the output voltage. Note that for the inductor circuit of Figure 1, the average capacitor voltage can never be more than the source voltage even for ideal circuit components. Real life voltage drops (diode, transistor, winding resistance) ensure that the average output voltage will be less than the source voltage. The transformer in Figures 1B remove this voltage limit and can also provide electrical isolation between input and output. The circuits of Figures 1A and 1B are unipolar applications of forward converters. Pushpull versions, such as that shown in Figure 4, are bipolar applications. Unipolar and bipolar applications are explained further below. Click on the available link for more information about push-pull switching mode power transformers.

12 Inductive Flyback (Kickback) in Switch Mode Power Transformers Unlike the Buck transformer; the flyback inductor, flyback transformer, boost inductor, and boost transformer intentionally store energy during the on time (charging portion) of a cycle and then discharge energy during the off time portion. (Technically, since they intentionally store energy, the switch mode flyback and boost power transformers are not true transformers.) They usually have a discrete air gap(s) or a distributed gap in their core s magnetic path. The transistor is turned on and current flows into the inductor or transformer (which has inductance). When the transistor is turned off, the input current that formed and maintained the core s magnetic field become zero. The magnetic field collapses causing a voltage reversal to occur in the inductor or transformer. The collapsing magnetic field induces sufficiently high voltage (known as inductive kickback voltage) to discharge energy into the capacitor connected to the inductor or to the switch mode power transformer secondary. Inductive discharge into the capacitor continues until the magnetic field completely dissipates or power is restored to the input. Restoring the power starts the inductive charging cycle again. The use of inductive kickback permit the output voltages of the inductor circuits of Figures 2A and 3A to be either lower, equal, or greater than the input source voltage. A transformer step up is not needed to achieve voltages higher than the source voltage. Flyback transformers are usually preferred over flyback inductors. The appropriate turns ratio can optimize current levels. The transformer can provide voltage isolation between input and output, and removes a polarity restriction that comes with a flyback inductor design. Boost Inductor Circuits You might ask what distinguishes the boost inductor application from the flyback inductor application. One characteristic is the polarity reversal of the output capacitor due to the placement of the circuit components. Compare the circuits of Figures 2A and 3A. The diode in the flyback circuit, Figure 2A, completely blocks direct flow of current from the input source to the capacitor regardless of the capacitor s voltage value. The capacitor can only be charged by the inductive kickback. The diode in the boost circuit, Figure 3A, permits current flow from the input source to the capacitor without the use of inductive kickback if the capacitor voltage is sufficiently low. Consequently it both stores energy and passes through energy during the charging portion of a cycle. Pass through current flow stops whenever the capacitor voltage approaches the value of the source

13 voltage minus the diode voltage drop. (Further increase requires the inductive kickback voltage.) This may be a desirable feature for rapid power supply startup Few designers are aware of the boost transformer circuit shown in Figure 3B because the circuit is not very practical. With only half-wave rectification it is either a forward (Buck) converter transformer application or a flyback transformer application depending on choice of polarity. Full wave rectification, as shown, permits it to duplicate the boost inductor actions discussed in the prior paragraph; both storing energy and passing through energy (by transformer coupling like a Buck transformer) during the charging portion of a cycle if the secondary capacitor voltage is sufficiently low. It acts likes a flyback transformer during the discharging portion of the cycle. It is rarely used with the full wave rectification as shown. It has seen some limited use as modified in the circuit shown in Figure 3C. The transformer has two secondary windings. One is used as a Forward (Buck) converter. The other is used as a flyback. It effectively divides the fullwave rectification into two half-wave applications. A more common boost inductor application is shown in Figure 5. A boost inductor is used with a push-pull (Buck) transformer. High power power supplies might use this type of circuit. In this application both switches are not open at the same time. Both switches are closed to charge the inductor, otherwise the switches are alternated on and off with one closed and one open.

14 Unipolar versus Bipolar What is the difference? When a current flows through an inductor or a transformer a magnetic field is created in its core. The value of the magnetic field will be greater than zero and it will have a direction associated with it. This direction is also referred to as the polarity of the field. If the value of the current varies, then the value of the magnetic field will vary accordingly, but the field polarity (direction) will remain the same as long as the current direction does not reverse. When an inductor or transformer continually operates with the same magnetic polarity it is a unipolar application. The circuits shown in Figures 1 through 3, including A thru C versions, are all unipolar applications. Applications were the magnetic field polarity is continually reversing are bipolar applications. A.C. applications are bipolar applications. Push-Pull types of forward converters (Buck) are bipolar applications. Push-pull transformers are often used in inverter circuits to create A.C. voltage from a D.C. source. A push-pull center-tap application is shown in Figure 4. There are several types of push-pull applications. More information about push-pull transformer applications is available on this website. Click on the available link. Electronic Transformer - Inverter Transformer The term "inverter" is associated with several different electronic applications. In logic circuits "inverter" may be a logic inverter, the equivalent of a "Not" gate. In analogue signal processing an inverter can be a circuit which inverts the phase of the signal being transmitted. In power conversion applications an inverter is an electronic transformer which converts power from a Direct Current (D.C.) source into Alternating Current (A.C.) power. Power conversion inverters can be divided into two sub-categories,

15 voltage-fed inverters and current-fed inverters. Voltage-fed inverters are more common than the current-fed inverters. The electronic transformers used in inverter circuits are often called inverter transformers. Inverters produce A.C. power by switching the polarity of the D.C. power source across the D.C. power source s load. The early inverters used mechanical switches to do the switching. Vacuum tubes replaced mechanical switches in low power applications. Eventually semiconductor based switches (diodes, transistors, F.E.T.s, S.C.R.s, etc.) replaced both mechanical and vacuum tube switches. The schematic in Figure 1A illustrates a very simple inverter circuit. The circuit does not have an inverter electronic transformer. The switches are alternated on and off ( cycled ), but are not on at the same time. The load will see alternating square wave pulses of voltage equal to the source voltage minus the circuit s resistive voltage drops. The pulse voltage cannot be adjusted, but the average load voltage can be made less than the source voltage by holding both switches open ( off ) at the same time. The portion (ratio < 1) of time during a cycle that a switch is on is called the duty cycle. The inverter schematic in Figure 1B utilizes a capacitor and another switch to provide a lower load voltage. One switch controls the amount of charge delivered to the capacitor hence it also controls the capacitor voltage. The set of two switches alternately switches the polarity for the connection between the capacitor and the load. The load voltage cannot exceed the input source voltage. The inverter schematic of Figure 1C adds an electronic transformer inverter with two secondary windings. The switching action sends alternating current through the inverter transformer s primary winding. This is referred to as push-pull action. The core has bipolar utilization. Bipolar utilization is discussed further below. The inverter transformer s turns ratio can permit either higher or lower load voltage. The inverter transformer s output is an A.C. square wave. Output filter networks can be used to obtain sine wave output. The inverter transformer can also provide electrical isolation between the inverter transformer s input and output sides. Full wave rectification can be applied to the inverter transformer s outputs to obtain a D.C. voltage of different value than that of the input source. This is shown in the schematic of Figure 1D. Compare the schematic of Figure 2A to the one in Figure 1D. Note in figure 2A the center-tap connections on the electronic transformer windings, a set of two switches instead of a set of four switches on the input side, the two diodes on the secondary instead of four, and the output filter inductor between the capacitor and load. The inverter transformer center-taps allow use of fewer switches and diodes. The inductors smooth out the current surges from the rectification thereby maintaining tighter output voltage regulation (less ripple voltage). The circuit in Figures 2A depicts a typical Push-Pull Forward Converter circuit. Be aware that the name for a Forward Converter circuit (and transformer) varies from industry to industry and from person to person. It may also be referred to as Buck, inverter, D.C. converter, feed forward, and others. There are also unipolar versions and there are bipolar versions that utilize saturable transformers to trigger transistor switching.

16 Butler Winding makes electronic transformers and inverter transformers in a wide variety of shapes and sizes. This includes; various standard types of core with bobbin structures (E, EP, EFD, PQ, POT, U and others), toroids, and some custom designs. Our upper limits are 40 pounds of weight and 2 kilowatts of power. We have experience with foil windings, litz wire windings, and perfect layering. For toroids, we can (and have done) sector winding, progressive winding, bank winding, and progressive bank winding. Most of our production is 100% tested on these machines. For more information on Butler Winding s capabilities, click on our capabilities link. The Difference between Bipolar and Unipolar Applications

17 Since the connections of the electronic transformer "inverter" are alternated, the current direction through the electronic transformer will also alternate. Consequently the magnetic field polarity of the inverter transformer s core will alternate between positive and negative flux directions. This is known as bipolar utilization of the inverter transformer s core. This is graphically illustrated in Figure 2B. The B-H curve shown is also known as a hysteresis loop. The area inside the loop is related to the core loss. A thinner loop means less core loss. Also note the residual flux density point. In a Unipolar application the flux density, B, would never return to zero value. It would stop at Br when the current (hence also the magnetizing force, H) returns to zero. The applied voltage reversal (by switching action) ensures that the flux density returns to zero. Bipolar utilization permits use of a smaller core than unipolar utilization because it permits a larger change in the core s flux density. Fewer turns are needed to handle the same amount of power. Compare Figure 2B to Figures 3C, 4C, and 5C. Unipolar utilization occurs if the magnetic flux remains in one direction. The value may vary up and down but does not cross zero value. A unipolar application is illustrated in Figures 3A, 3B, and 3C. Some designers may refer to the transformer in Figure 3A as an inverter transformer, but it is not. It is serving as a pulse transformer with a resistive load. If we assume it to be an ideal transformer, then there is no core loss, no leakage inductance, does not store any energy, and the residual flux density is zero. Figure 3B shows the expected output if a rectangular voltage pulse is placed across the transformer (turn switch on, then off). The output will also be a rectangular pulse without any distortion. There will be a change in amplitude because of the transformer s turns ratio. The ideal transformer s lack of stored energy eliminates the possibility of an inductive kickback voltage spike. This circuit does not produce an A.C. output, hence no true inverter action. A non-ideal electronic transformer has finite inductance hence it stores some inductive energy in its magnetic field. A lower inductance results in more stored energy. Consider the non-ideal gapped transformer in the circuit shown in Figure 4A. The gap lowers the inductance of the transformer; consequently more current can flow when the switch is closed (compared to no gap). When the switch is closed the transformer directly couples power to the load plus it stores energy in its magnetic field. The field is created by the magnetizing current. The current flow due to the load does not contribute to the stored energy. When the switch is opened the magnetic field collapses. The collapse creates an inductive kickback voltage of reversed polarity. The induced secondary voltage causes

18 current to flow through the load resistor in the reversed direction. (This is how a flyback transformer functions.) The load sees alternating current although it usually has an asymmetrical waveform. One could claim that the circuits and transformer have inverter action. The energy stored in the electronic transformer s magnetic field is dissipated as heat produced by current flowing through the load resistor. Current of declining value will continue to flow until either all of the stored energy is dissipated or the switch is closed again. If completely dissipated, then the output shown in Figure 4B and the generalized hysteresis loop of Figure 4C apply. The transformer is said to be operating in discontinuous mode. The load voltage and load current reach zero value, and the core s flux density reaches its residual value. Note that the flux density averaged over time is greater than zero. This holds for all unipolar applications. If the switch is closed again before all the energy is dissipated, then the output shown in Figure 5B and the generalized hysteresis curve of Figure 5C applies. The transformer is said to be operating in the continuous mode. The load voltage and load current remain above zero value, and the flux density does not reach its residual value. The output waveform in Figure 5B is more rectangular than that of Figure 4B. The circuits in Figures 4A and 5A are not very practical inverter transformer circuits. To be useful the transformer must store as much energy as it directly couples to its load. Consequently, the transformer will tend to be lightly loaded and designed to have appreciable magnetizing current. Output filters would be required to produce a more symmetrical output waveform. These circuits find little use as shown here. There are

19 D.C. biased unipolar applications, which function as inverters. They are not discussed here. Saturable Transformers as Inverter Transformers Figure 6A shows a Royer Inverter Circuit schematic that uses saturable transformers. The saturable transformer also functions as the inverter transformer. Figure 6B shows a Jensen Circuit which uses a saturable transformer and a power transformer. The power transformer functions as the inverter transformer. Both of these circuits make use of push-pull switching to achieve the inverter action. The feature of these two circuits is the transistor switching action that is activated by a voltage spike created when the saturable transformer enters saturation. An oscillation develops which maintains the necessary switching action. The theory of operation is not discussed here. It may be available on this website at some future date from the issue date of this website page. Check the available links. Buck Boost Transformer - Push Pull Transformer When it comes to power conversion, the buck boost or "push pull" transformer application is well known. The buck boost transformer configuration is widely used in converting direct current (D.C.) voltage into another value of D.C. voltage, and in inverters. Inverters convert direct current into alternating current (A.C.). The push pull transformer is usually the preferred choice in high power switching transformer applications exceeding one kilowatt. It is usually used in a circuit known as a "forward converter" circuit. Be aware that the name for the "forward converter" circuit varies from industry to industry and from person to person. It may also be referred to as an "inverter", "D.C. converter", "buck", "feed forward", and others. A basic "forward converter" transformer circuit is illustrated in Figure 1A. It is not a push pull transformer application. The output inductor reduces ripple voltage. Pulse width modulation is used to control the value of the output voltage

20 A center-tapped buck boost transformer application circuit is illustrated in Figure 2A. Figure 2A only shows one output. Multiple voltage outputs are possible by using either a tapped secondary winding or using multiple secondaries. Some other buck boost transformer versions are discussed further below. They are illustrated in Figures 3, 4, 5, and 6. (These include some push pull transformers without the center-taps.) The core of the transformer in Figure 1A is operated in a unipolar fashion. Unipolar operation is depicted graphically in Figure 1B. The core's magnetic "B-H" loop remains in one quadrant of the "B-H" grid. A loop occurs once every cycle. The flux density "B" and the magnetizing force "H" never cross zero hence always retain the same (or one) polarity. "H" does not have to return to zero value. The core in a push pull transformer has bipolar operation. Both "B" and "H" cross zero value and reverse polarity. Bipolar operation is depicted graphically in Figure 2B. Note that the "db" value (change in B) in Figure 2B for the bipolar push pull transformer can be more than twice the "db" value shown in Figure 1B for the unipolar forward converter (assuming the same core material). Push pull transformer (bipolar) operation permits one to handle the same amount of power in a smaller package than for that of a unipolar operation. There are tradeoffs. The buck boost transformer operation requires more switching elements and its control circuitry is more complicated. Consequently a push pull transformer application is more expensive. The voltage pulses must be adequately controlled to avoid phenomena known as saturation walk. Center tapped push pull transformers have winding capacitance issues at higher frequencies. Winding imbalances can contribute to saturation walk. Power ratings for push pull or buck boost transformer can vary from a fraction of a Watt to Kilowatts. Megawatts is possible, but definitely beyond Butler Winding's capabilities. Size correlates with power hence size (and weight) can vary from a fraction of a cubic centimeter (several grams) to multiple cubic meters (thousands of kilograms). Buck boost transformers can be wound on toroids, bobbins, and tubes. Core materials vary depending on the application. Laminated or tape wound grain oriented silicon steel is common for low frequency inverter buck boost transformers. Ferrite core materials are common for high frequency switching push pull transformers. If minimal size is a requirement, nickeliron alloys may be chosen for the 1 to 20 kilohertz range. Minimal energy storage is desired so cores have minimal air gaps in their structure. Butler Winding manufactures buck boost transformers in a wide variety of shapes and sizes. This includes; various standard types of core with bobbin structures (E, EP, EFD, PQ, POT, U and others), toroids, and some custom designs. Our upper limits are 40 pounds of weight and 2 kilowatts of power. We have experience with foil windings, litz wire windings, and perfect layering. For toroids, we can (and have done) sector winding, progressive winding, bank winding, and progressive bank winding. Butler winding has a variety of winding machines, bobbin/tube and toroid. That includes two programmable automated machines and a taping machine for toroids. Butler Winding has vacuum chamber(s) for vacuum impregnation and can also encapsulate. To ensure quality, Butler Winding purchased two programmable automated testing machines. Most of our