CHAPTER 5: REGULATED POWER SUPPLIES

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1 HAPTE 5: EGUATE POWE UPPE A regulated power supply can be built usg a simple zener diode as a voltage regulator. everse breakdown voltage of the zener diode is used to hold the put voltage constant at a certa load condition. To improve voltage regulation, a negative feedback configuration is used the regulator. Higher efficiency power supplies use switchg regulators which employ fast switchg devices. The quality of a power supply depends on its load regulation, le regulation, and put resistance.. UPPY HAATET. oad regulation The load regulation dicates how much the load voltage changes when the load current changes. The load regulation is defed as: oad N F regulation 00% F where N load voltage with no load current ( 0) F load voltage with full load current max ) The smaller the load regulation, the better the power supply. A well-regulated power supply can have a load regulation of less than % (i.e., the load voltage varies less than % over the full range of load current).. e regulation Any change the le voltage of the nomal value (i.e., 0 ac) will affect the performance of the power supply. The le regulation is defed as: N F N where e regulation H 00% H load voltage with high le load voltage with low le The smaller the le regulation, the better the power supply. A well-regulated power supply can have a le regulation of less than 0.%. 3. Output resistance The put resistance of a power supply determes the load regulation. f a power supply has a low put resistance, its load regulation will also be low by the relationship: EE33- egulated Power upply 07

2 N F TH F oad regulation TH 00% (m) where TH put resistance of the power supply F full load current (occurs when the load resistance is mimum) (m) mimum load resistance. HUNT EGUATO The le regulation and load regulation of an unregulated power supply are too high for most applications. The regulations can be improved by usg a voltage regulator. A lear voltage regulator uses a device operatg the lear region to hold the load voltage constant. The shunt voltage regulators have the regulatg device parallel with the load. ome of the shunt regulators are shown below. ener oltage egulator mproved ener oltage egulator BE EE33- egulated Power upply 08

3 3 ( BE ) Higher Output oltage egulator 3 mproved egulation One advantage of shunt regulators is that they have built- short circuit protection. f there is a short circuit across the load termals, none of the components of the regulator will be damaged, all that happens is that the put current creases to. The regulator has an efficiency of: P Efficiency 00% P The shunt voltage regulators have low efficiency due to the power wasted by the regulator components, P reg, and most of this power is dissipated across series resistor. This type of regulator is used applications where efficiency is not important. n addition, this regulator is very simple. P reg P - P Example: 4-, 4-, 4-3 (page 94) EE33- egulated Power upply 09

4 . EE EGUATO ompared to the shunt voltage regulators, the series regulators have higher efficiency (50%- 70%), fairly simple to design and good enough for small load power applications (<0W).. imple series regulators P ( ( BE ) ) The ener follower 3 4 Q Q P ( Efficiency ( ) 00% BE ) Two-transistor regulator The headroom voltage is defed as the different between the put and put voltage: Headroom voltage The lower the headroom, the higher the efficiency (i.e., the best efficiency is obtaed when the put voltage as large as put voltage). Transistors Q and Q is usually replaced by a arlgton connection (a low power transistor, Q, drives the power transistor, Q). This connection allows larger values of and 4 to be used order to improve the regulator efficiency further. Example: 4-7, 4-8 EE33- egulated Power upply 0

5 . mproved regulation An op-amp can be used to improve regulation even further (usg negative feedback). 3 P ( ) The series regulator has no short-circuit protection as the shunt regulator even it has higher efficiency, therefore some form of current limitg has to be cluded to protect the regulator. The resistor 4 the figure below is a current sensg resistor the regulator protection circuit. f the current is small such that the voltage across 4 is smaller than BE, the regulator works normally. When the load current creases such that BE is slightly greater than 0.7, Q turns on, the collector current flows through 5. This decreases the base voltage to Q, which reduces the load voltage and the load current. When the load is shorted, Q conducts heavily and brgs the base voltage of Q down to approximate BE, eventually reduces the put voltage delivered to the load ( BE ). The value of 5 has to be chosen such that it will provide the ga for E amplifier Q and also provide enough current to drive Q under normal condition. A A 3 5 Q Q 4 BE 4 Figure below depicts the current limitg circuit response. reg BE 0.6 BE 0.7 Example: 4- (page 95) EE33- egulated Power upply

6 3. Foldback current limitg Q Q reg max max 6 K BE 4 7 K 7 ( K) K 4. MONOTH NEA EGUATO Positive voltage regulator: M78XX Negative voltage regulator: M79XX M78XX 3-3 M79XX - Example 4-, 4-3, 4-4 (page 933) EE33- egulated Power upply

7 . -to- ONETE ometimes, it is necessary to convert a voltage of one value to a voltage of another value. This is accomplished by usg a -to- converter. -to- converters are very efficient because they switch the transistors on and off to convert voltage. ce the transistor is switchg on and off, the power dissipation is greatly reduced and the converter has efficiency between 65%- 85% WTHNG EGUATO A switchg regulator falls to the general class of -to- converters because it converts a put voltage to a put voltage. The switchg regulator cludes a voltage regulation, typically pulse-width modulation controllg the on-off time of the transistor. By changg the duty cycle, a switchg regulator can hold the put voltage constant under varyg le or load conditions. The pass transistor is switched between cut off and saturation. When the transistor is off, the power dissipation is zero. When the transistor is saturated, the power dissipation is still very low because E(sat) is much less than the headroom voltage of the series regulator. As the results, switchg regulator can have the efficiencies from 75% to more than 95%. Many topologies of switchg regulators have been developed. ependg on applications, the designer will pick the right topology for the voltage regulator, of course, the cost of the power supply is always a ma factor design. EE33- egulated Power upply 3

8 . Buck regulator Figure below shows a buck regulator, the most basic topology for switchg regulator. This regulator always step downs the voltage and the switchg device, Q, can be either FET or BJT. Q Pulse width Modulation EF FB - - kick losed switch Open switch hoke-put filter A comparator controls the duty cycle of the pulses and this duty cycle is controlled by the feedback voltage from the voltage divider &. The rectangular signal of the pulse modulator closes or opens the switch. When the pulse is high, the switch is closed. This reverse-bias the diode, so that the current flows through the ductor. This current generates a magnetic field around the ductor and the amount of stored energy the magnetic field is given by: Energy 0.5i This current also charges the capacitor and supplies current to the load. While the switch is closed, the ductor voltage has the polarity as shown. As the current through the ductor creases, more energy is stored the magnetic field. When the pulse goes low, the switch is open. At this stant, the magnetic field around the ductor starts collapsg and duces a reverse voltage across the ductor. This reverse voltage is called the ductive kick. The diode is forward-biased and the current through the ductor contues to flow the same direction. At this time, the ductor returns its stored energy to the circuit, it acts as a source and contues supplyg current to the load. urrent flows through the ductor until the ductor returns all of its energy to the circuit (discontuous mode) or until the switch closes aga to start a new cycle (contuous mode) whichever comes first. The capacitor will also source the load current durg part of the time that the switch is open. This way, the ripple across the load is mimized. EE33- egulated Power upply 4

9 The average value of the put voltage of the choke-put filter is related to the duty cycle and is given by: The larger the duty cycle, the larger the put voltage. When the power is first turned on, there is no put voltage and no feedback to the comparator, the duty cycle approaches 00% of the modulator. As the put voltage builds up, the feedback voltage, FB, reduces the comparator put voltage, which turn reduces the duty cycle. At some pot, the put voltage reaches an equilibrium value at which the feedback voltage produces a duty cycle that gives the same put voltage. n this case, the feedback voltage FB EF and the put voltage can be derived as: After equilibrium sets, any attempted change the put voltage (by the le or by the load), will be almost entirely offset by the negative feedback. f the put tries to crease, higher feedback will reduce the modulator duty cycle to reduce the put voltage. f the put voltage decreases, lower feedback will crease duty cycle to offset the loss.. Boost regulator EF The boost regulator always steps up the voltage as shown the figure below. When the pulse is high, the switch is closed and energy is stored the magnetic field. When the pulse goes low, the magnetic field collapses and duces a reverse voltage across the ductor. The put voltage now adds to the ductive kick. This means the peak voltage on the right end of the ductor is p kick And of course, kick is proportional to the duty cycle. The rectangular voltage appears at the put to the capacitor-put filter. Therefore the regulated put voltage,, approximately equals the peak voltage given above which is always greater than. Q Pulse width Modulation EF FB EE33- egulated Power upply 5

10 P - peak P Open switch apacitor filter 3. Buck-Boost regulator The buck-boost regulator produces a negative put voltage when driven by a positive put voltage as shown figure below. Q - Pulse width Modulation EF - kick - P losed switch Open switch apacitor-put filter When the pulse goes high, the switch is closed and energy is stored the magnetic field. At this time, the voltage across the ductor equals to the polarity shown. When the pulse goes low, the switch opens. Aga, the magnetic field collapses and duces a kick voltage across the ductor. This kick voltage is proportional to the energy stored the magnetic field which is dependg on the duty cycle. f the duty cycle is low, the kick voltage approaches zero, if the duty cycle is high, the kick voltage can be greater than, dependg on how much energy stored the field. EE33- egulated Power upply 6

11 The diode and capacitor-put filter produces an put voltage equal to p. ce the magnitude of this put voltage can be greater or less than the put voltage, this regulator is call buck-boost regulator. An vertg amplifier is used the regulator to convert the feedback voltage before it reaches the vertg put of the comparator. The voltage regulation then works as previously described. Attempted creases put voltage reduce the duty cycle, which reduces the peak voltage. Attempted decreases put voltage will creases the duty cycle. Either way, the negative feedback holds the put voltage almost constant. 4. Monolithic switchg regulator witchg regulators have been manufactured monolithic form (i.e., tegrated circuit). The monolithic switchg regulators usually require external components which could not be tegrated such as ductors or capacitors. ome regulators provide the external feedback connection order to change the put voltage suitable for specific applications. All of the switchg regulators topologies have been tegrated to. For example the T074 is a buck regulator, MAX63 is a boost regulator and the T074 can be connected as a buck-boost regulator. Example 4-5 (page 946) EE33- egulated Power upply 7