Dual-Output Linear Power Supply

Transcription

1 ECE 2B Lab #2 Lab 2 Dual-Output Linear Power Supply Overview In this lab you will construct a regulated DC power supply to provide a low-ripple adjustable dual-output voltage in the range 5-12 VDC at 0.2 Amps (maximum) load current from a 120 VAC power outlet. This will be used to provide power to circuits you will construct in later labs in ECE 2. Figure 2-1 Photo of the finished product (some components not shown). 1 Bob York

2 2 Dual-Output Linear Power Supply Power supplies are essential components of any microelectronic system. This lab is intended to provide a basic understanding of the design and operation of linear supplies, and techniques for construction of hardwired electronic circuit modules. In this lab you will gain experience with: Power transformers Diode bridge rectifiers Capacitive loading for ripple rejection Use of IC voltage regulators Basic soldering techniques The objective of the lab is not simply to create a working circuit; it is to learn about circuits. So, as you progress through the lab, try to understand the role of each component, and how the choice of component value may influence the operation of the circuit. Ask yourself questions such as: Why is this resistor here? Why was this particular integrated circuit chosen? How could the system be improved? It is only when you can answer such questions that you will progress towards designing your own circuits. Table of Contents Background information 3 Basics of Linear Power Supply Design 3 Voltage Regulation 4 Pre-lab Preparation 6 Before Coming to the Lab 6 Parts List 6 Full Schematic and Board Layout for Lab #2 7 In-Lab Procedure AC Power Transformer Full-Wave Bridge Rectifier Filtering Capacitors Adjustable Voltage Regulators 12 LM317 Positive Regulator Circuit 12 LM337 Negative Regulator Circuit Finishing Touches 14 Optional Enclosure 14 Bob York 2

3 Background information 3 Background information Basics of Linear Power Supply Design The simplest type of DC power supply is a so-called linear supply, shown schematically in Figure A transformer is used to step down the AC line voltage to a smaller peak voltage V m, which must be somewhat larger than the ultimately desired DC output. A diode circuit rectifies the AC signal and a capacitive filter bank is then used to smooth or filter the rectified sinusoid, producing a waveform with predominant DC component. Under normal loading conditions there is always some residual periodic variation or ripple in the filtered signal. If the application requires very low ripple and constant DC output over a wide range of loading conditions, then active regulation is required to further reduce or eliminate this residual ripple. Most active regulator circuits will require a certain minimum input-output voltage differential for proper operation. V m sin t I out To AC line Transformer Rectifier Filter Regulator + V out - Load V out Figure 2-2 Components of a typical linear power supply In Lab 1 we discussed diode rectifiers so the transformer and rectifier combination should be easy to understand. Let s therefore start by focusing on the selection of the shunt capacitor. Figure 2-3 shows an unregulated supply with a fullwave bridge rectifier followed by a filtering capacitor C and a load resistance R connected at the output terminals (thus forming an RC time constants). By making the shunt capacitance big enough we can insure that the timeconstant for discharging the capacitor through the load is long Transformer Bridge Rectifier compared to the oscillation period, so the output remains approximately constant. In this way the capacitor effectively filters out the rapidly varying component of the rectified sinusoid to produce a nearly constant DC voltage. 1 This is a simple but somewhat bulky and inefficient power-supply. So-called switch-mode or switching power supplies are more compact and efficient, but a bit more difficult to understand. C + I out V out Figure 2-3 Simple DC supply without active regulation. + - R 3 Bob York

4 4 Dual-Output Linear Power Supply The detailed action of the filter capacitor and load Voltage V resistor combination is shown V p in Figure 2-4. As the rectified sinusoidal waveform begins to increase the T/2T capacitor charges up and the Time voltage across it increases. Charging Discharging On the downward portion of Figure 2-4 Effect of the filtering capacitor on the rectified the rectified waveform the sinusoid under resistive loading conditions. capacitor discharges into the load and the voltage across it decreases. The cycle then repeats, resulting in a periodic ripple of magnitude V in the output waveform. To minimize this voltage droop, we must choose a sufficiently large capacitor so that the RC time constant is much greater than the oscillation period T 1/ f. Clearly the choice of this capacitor is critically dependent on the load resistance, or maximum desired load current. As the load resistance goes down, the required capacitance goes up. Another equivalent way to understand this point is as follows: during the time period when the rectified sinusoid is low, the load current required to maintain a constant output voltage must come entirely from the stored charge on the capacitor. If the load requires a large current, a large amount of charge must be stored, requiring a large capacitor. We can quantify these points in a simple way using the governing equation for a capacitor. During the discharging period the current is related to the capacitance and voltage droop by dv V I T V I C C C out (2.1) dt T /2 2 V 2R f V where we assumed that the voltage droop was relatively small so that the derivative could be approximated. This equation can be used to find the required filtering capacitance for a given load current and desired voltage droop. Clearly we can never make V zero with practical capacitors, and hence there is always some residual voltage droop. That brings us to a discussion of active regulator circuits. Voltage Regulation An active regulator circuit is inserted between the filtering capacitor and the load as shown in Figure 2-5. There are many different ways of achieving active regulation; we describe one simple scheme, outlined in Figure Transformer Bridge Rectifier 2-6. The basic idea is to use feedback to control the amount of current going to the load. In the figure, a voltage-controlled pass resistor is used to control the current flow (this is usually realized with a transistor). A differential amplifier (such as an op-amp) compares the output signal against a reference voltage (provided by a zener diode in this case) and generates an error signal based on this difference. The action of this negative feedback loop forces the output to remain constant at some value that is proportional to V ref, where the proportionality factor is set by the resistor divider at the output. If the output goes too high, C + Active Regulator I out + V out Figure 2-5 Use of active regulators to minimize ripple or droop. - R Bob York 4

5 Background information 5 the series pass resistance is increased; if the output goes too low, the pass resistance is reduced. These kinds of circuits can operate very quickly, stabilizing the output even when the unregulated DC input may fluctuate rapidly. Voltage-controlled resistor I out Power supplies with very low ripple can be Unregulated made using active regulation, but please note DC V out R that the conceptual regulator schematic Figure V ref 2-6 leaves out many important details, e.g. the implementation of the voltage-controlled resistor, or the biasing of the difference amplifier. Such details can increase the Figure 2-6 A simple active regulator (conceptual schematic). complexity of the circuit significantly. Practical regulator circuits may also include additional functionality such as current limiting, where the output current is not allowed to exceed a certain threshold, and thermal shutdown in which case the output is shut off if the temperature of the circuit exceeds a certain threshold. Designers of regulator circuits may also use more sophisticated circuits to produce a stable reference voltage. Nowadays it is rare to design your own regulator circuit from scratch since there are many useful ICs on the market that were designed specifically for this purpose. You will use two of them in this lab, the LM317 and LM337. The kind of regulator we ve just discussed is simple and easy to use, but is very inefficient. To operate correctly the input voltage must be substantially higher than the output voltage. For example, a regulated 5V output might require an unregulated 9V input, so at a load current of 1A there would be 4Watts dissipated in the regulator circuit. In addition to being very inefficient (5W out/9w in = 56% in this example), the regulator circuit will get very hot and require special provisions for heat removal. These kinds of problems are overcome nowadays using switch-mode power supplies, but these circuits are a bit more difficult to understand. We will explore a simple type of switching converter in Lab 5. The final point of discussion concerns making dual DC supplies with both positive and negative output voltages. This is usually done using a center-tapped transformer, shown in Figure 2-7, where a third Center-Tapped Bridge wire is attached to the Transformer Rectifier Positive + V pos middle of the secondary + Regulator winding. If this terminal is C taken as the common Common ground point in the secondary circuit, then the C Negative voltages taken at opposite Regulator V neg ends of the winding will be Figure 2-7 Basic dual-supply system using a center-tapped positive or negative with transformer and two regulators. Note the polarity of the filtering respect to this point. We capacitor in the negative supply circuit in the figure above. can then add separate positive and negative regulator circuits as shown. Many op-amp circuits have historically used dual-output power supplies of this type. It is possible to add additional circuitry to force the outputs to track each other precisely, so that the positive and negative supply voltages are exactly the same in magnitude; this is often used in precision instrumentation or highquality audio applications. We will not construct this type of tracking power supply. Instead, we will make two independently adjustable positive and negative output voltages. 5 Bob York

6 6 Dual-Output Linear Power Supply Pre-lab Preparation Before Coming to the Lab Read through the lab experiment to familiarize yourself with the components and assembly sequence. Before coming to the lab, each lab group should obtain a parts kit from the ECE Shop. If you have not yet done so, remember to purchase a soldering iron (one for each group) stand, and roll of solder, as well as small tools (wire cutter/stripper, needle-nose pliers, screwdriver, etc.). As of 2007 all ECE labs at UCSB use lead-free solder, and this requires a somewhat more expensive soldering iron than you might find at a local hardware store. Optional: simulate the regulated supply using Circuit Maker or MultiSim (circuit files available on the course web site). Parts List This kit has a lot of parts. Be sure you have all the parts below and can identify each component. If you do not have some of the resistors in your kit, please locate the correct values in the resistor cabinet in the lab. Qty Description 1 Power Transformer (output 1 In-line 3AG fuse housing, #18 wire 1 120V 3-prong power cord w/pigtail 2 3AG Fuse, 0.5A 250V 1 SPST Power Switch 4 Silicon Rectifier Diodes, 1N4002 (1A, 100V, DO-41) 1 LM317T 3-terminal Adj. Pos Regulator (TO-220) 1 LM337T 3-terminal Adj. Neg. Regulator (TO-220) 1 Red LED (T1 3/4, 20mA) 1 Green LED (T1 3/4, 20mA) 2 5k trimpot (Bourns 3/8" thumbwheel type) Ohm 1/4 Watt resistor 2 1k-Ohm 1/4 Watt resistor 2 10uF 25V electrolytic capacitor (PC lead) To be assembled by ECE Shop w/heatshrink tubing over 120V connections uF 35V electrolytic capacitors (PC lead, 0.197" (5mm) spacing) Ohm, 0.5 Watt resistor /4" machine screw (for transformer) nuts 2 TO-220 Heatsink 80C) 2 Mounting hardware for TO-220 heatsink 1 Female Header 3-pos 0.1" spacing (tin) 8 Male breakaway headers 0.1" spacing (tin) 1 PC Board (Sunstone BY001) 4 3/8" threaded standoffs (4-40) 8 machine screws 4-40, 3/16" long (for standoffs) Bob York 6

7 Pre-lab Preparation 7 Full Schematic and Board Layout for Lab #2 Figure 2-8 Complete schematic for ECE 2 Dual-Output Adjustable Regulated Power Supply. Figure 2-9 PC Board for Lab 2 7 Bob York

8 8 Dual-Output Linear Power Supply In-Lab Procedure Follow the instructions below CAREFULLY. Failure to do so could result in serious damage to the lab equipment, destruction of parts, and possible injury to you and your lab partner. Each critical step begins with a check box like the one at the left. When you complete a step, check the associated box. Follow the instructions below and carefully document your results for inclusion in your lab report. 2.1 AC Power Transformer You will be given a power transformer that is prewired to a 120V AC power cord with an inline fuse and power switch, similar to Figure All 120V AC connections have been covered with heat-shrink tubing to protect you from accidentally touching the leads. Nevertheless, take care when handling these wires; do not yank or twist the connections. There should already be a fuse in the fuse-holder, and you have extras in your parts kit. AC power On/Off Fuse 1A Earth ground Figure 2-10 (a) Power transformer with AC power cord and in-line fuse attached; (b) Equivalent circuit for the assembly. The transformers for this lab are designed for a 120 VAC input (primary) and an 18VAC center-tapped output (secondary), and rated for ~1 Amp in the secondary. The ECE shop is always looking for the lowest cost parts so you may not receive the same transformer pictured in Figure 2-10, and depending on the actual unit the leads may not have the same color coding. Two possibilities are shown in Figure Blue Jameco Yellow Black Stancor P-8691 Blue 240 VAC 120 VAC Red 9 VAC Black 9 VAC 120 VAC 9 VAC Blue w/ Stripe 9 VAC Black Yellow Black Blue Figure 2-11 Schematics for two transformers that may be used in this lab. If the primary has a center-tap (as for the Jameco part shown in Figure 2-10a) then one of the leads is not needed and will be covered up by the shop (a center-tapped primary allows the transformer to be used in other countries where wall outlets are at 240V instead of the Bob York 8

9 Full-Wave Bridge Rectifier 9 120V U.S. standard). It is important to remember that VAC implies an rms voltage, so 18VAC means something in excess of V peak-peak. Transformers are usually rated conservatively at the maximum current level, and due to the properties of the magnetic cores the output voltage may be significantly higher than the spec at low current levels. First attach the four 3/8 hex standoffs and 3/ machine screws (Figure 2-12) to the four holes on the corners of the PC board. These will keep the board elevated off the lab bench and help avoid the possibility of accidentally shorting some of the solder connections on the backside. Next, use the ¼ 4-40 machine screws and nuts to securely attach the power transformer assembly to the circuit board. Attach the green earth ground to the case of the transformer using one of these screws. Now solder the three wires for the secondary into the appropriate place on the PC board. Do the Figure 2-12 PC Board standoffs. center-tap first. If you aren t sure which wire is the center-tap, please consult with your TA. Insert the wire through the hole and bend slightly to hold in place. Remember: a good solder joint is shiny, with the solder flowing nicely around the wire and solder pad. A bad joint is a easily identifiable as a dull gray lumpy blob, and occurs when the wire and copper pad are not sufficiently hot. Trim the excess wire afterwards. 2.2 Full-Wave Bridge Rectifier A full-wave diode bridge is used to rectify the AC signal. The diodes must be capable of sustaining the maximum expected current and voltage. We will use a 1N400x device rated at 1A and >100V. These devices are usually Figure 2-13 Diode symbol and axial-lead packaged as shown in Figure 2-13, with a white package markings. A 1N4005 is shown here band marking the cathode. Solder the bridge rectifier (Diodes D1-D4) onto the circuit board. Carefully note the polarity of the diodes, and trim the excess leads afterwards. Find the breakaway header pins in your kit as shown in Figure They can be broken off individually with small pliers and soldered into the various test points on the PC board. Do this for the test points marked GND, +Rect Out, and Rect Out (see Figure 2-9). It may help to have a lab partner hold the pin in place with pliers while soldering. Check to see if there is a fuse in the fuse-holder. If not, insert one from your parts kit (you should also have a spare). Double check your connections and soldering with a TA and Figure 2-14 Breakaway header pins. then apply AC power. Using your oscilloscope with the common (black) leads connected to the circuit GND, record the positive and negative rectified waveforms at the +RectOut and RectOut test points in your LAB RECORD, taking care to note the peak amplitude 9 Bob York

10 10 Dual-Output Linear Power Supply in each case. Also note and record the oscillation period (in milliseconds) and mark on your graph. You may observe some distortion in the rectified sinusoid. Note: If at any time your circuit doesn t appear to be working, first check the fuse! Errors in assembly or soldering during the lab may lead to a short-circuit which might blow the fuse. If this happens (and it often does), be sure to identify and fix the error before replacing the fuse. 2.3 Filtering Capacitors The next step is to smooth the rectified waveform to obtain a constant DC voltage. By adding a sufficiently large shunt capacitance at the output of the bridge rectifier we can insure that the time-constant for discharging the capacitor through the load is long compared to the oscillation period. Clearly the loading conditions play a big role in determining the required size of the filtering capacitor. If the load resistance is small, a large capacitor is needed to keep the RC time constant sufficiently large. Another way of thinking about this is that the capacitor must supply all the current to the load during the time that the rectified signal is low. If the load resistance is small it will draw a significant current, thus requiring a large capacitor to store the necessary charge. Large capacitors are usually of the electrolytic type as shown in Figure Recall from ECE 2A that these have a specific polarization and maximum voltage. Markings on the case indicate which lead is positive (+) or negative (-), as well as the capacitance value and maximum voltage. Remember: Mounting the capacitor backwards or exceeding the maximum voltage WILL cause the device to explode, sometimes dramatically and dangerously. In addition, since large-value capacitors can store a lot of charge they are capable of delivering an unpleasant electrostatic zap. In this lab we will eventually provide a resistive discharge path for the capacitors through some LEDs, but as a general precaution: Value and voltage rating printed on package Long-lead is positive - sign on package indicates negative lead Figure 2-15 Electrolytic capacitor. Avoid touching the leads on a large electrolytic after it has been charged With the power off, add one of the electrolytic capacitors (C1 and C3) at the outputs of the bridge rectifier, with a small 0.1Ω resistor (Rs) in series with C1 as shown in Figure Solder the components in place after double-checking the capacitor polarity. Attach the power resistor decade box as the load impedance on the positive supply output as shown in Figure 2-16, and adjust to 555,555. This setting will protect you initially against accidentally shorting out the supply circuit. Now turn on the power and record the output voltage with a multimeter. Examine the output waveform on the oscilloscope. For this large load resistance the output voltage should be almost perfectly constant. The capacitor charges quickly to this peak value, and the small load current does not discharge the capacitor significantly. Bob York 10

11 Filtering Capacitors 11 Our objective is to supply at least 200mA at the maximum +Rect DC voltages. What is the smallest load resistance that we 1A C 1 can use without exceeding 0.2 R L R A? (Hint: use Ohms law). s 0.1Ω Record this value in your GND notebook, and set the decade box to this value and record the C 3 -Rect output voltage waveform. You should see evidence of voltage Figure 2-16 Schematic relevant to most of step 2.3; use droop. It may help to AC couple the bench decade box for the load resistance R L. the scope input so you can expand the vertical scale. Record the droopy waveform in your notebook. Observe and record the voltage waveform across the sense resistor Rs at the test point marked Charging Transient. You may want to add another of the breakaway header pins to make it easier to connect to this node. Using a small series resistance is a common way of measuring or sensing the current flow in a certain path without significantly perturbing the circuit. From your measurement, calculate the peak current flowing into the capacitor during the charging period, and record this in your notebook. Before proceeding, turn off the AC power!! In order for our voltage regulator to work effectively, the input voltage must stay well above the maximum desired output DC voltage, so we must keep the voltage droop small. What is the minimum required capacitance that will maintain <1V ripple under maximum current (minimum load resistance) conditions? Record this in your notebook, and solder extra capacitors (C2 & C4 in Figure 2-8) as needed. Power up and record the output waveforms under maximum current (minimum load resistance) conditions with the additional capacitors in place. Repeat this step for the negative supply output as well. Before proceeding, turn off the AC power!! Now add the LEDs and associated bias resistors (R3 and R6; see the complete schematic in Figure 2-8). Be sure to insert them in the correct polarity! The PC should indicate the orientation of the flat, but if you aren t sure about the polarity, ask the TA or review information in Lab 1. The LEDs here serve a dual purpose: they not only indicate when the circuit is on and functioning, they also help discharge the large capacitors when the power is disconnected. When you are finished, turn on the circuit to verify that the LEDs are functioning, then turn off the power: you will observe that the LEDs continue to stay light for a brief period while the capacitors are discharging. 11 Bob York

12 12 Dual-Output Linear Power Supply 2.4 Adjustable Voltage Regulators LM317 Positive Regulator Circuit We will use two popular voltage regulator ICs the LM317 (positive voltage) and LM337 (negative voltage). Figure 2-17 shows a portion of the data sheet showing a typical connection for an adjustable positive supply using the LM317. The resistor divider provides feedback to the IC proportional to the output voltage, so that it can increase or decrease the current as needed. The chip is designed so that the output voltage is given by R2 Vout R2 (2.2) R1 In our circuit we are using 470Ω for R1; can you see why? Calculate the range of V out that you can expect from (2.2) for the input voltage that you measured in the previous section. The datasheet provides guidance on selecting the two capacitors shown in Figure The capacitor at the V in terminal is only recommended if the device is more than 6 inches from the filter capacitors, and a capacitance at the V out terminal of F is recommended (we chose 10μF). Before soldering the LM317 into place, attach it to the small aluminum heatsink using the mounting hardware included in your kit. If your heatsink has mounting tabs like the one shown in Figure 2-18, either break them off or orient them upwards away from the PC board. Note the insulating gasket is not Figure 2-17 Circuit connection for LM317 adjustable regulator and pin assignments in the TO-220 package. Figure 2-18 Heatsink and mounting hardware, and photo of the assembled regulator circuit in place. critical in this circuit; these are only necessary when it is important to electrically isolate the metal case of the LM317 (here at V out potential) from the heatsink. Note: heatsinking of active devices is always critical for the circuit to function properly and to maintain a long operating lifetime. Since this is an adjustable regulator, the output DC voltage can be significantly different from the input voltage. This means that there may be a large voltage drop across the device ( V out V in ). This voltage drop multiplied by the maximum load current is the maximum power that must be dissipated. Heatsinks provide Bob York 12

13 Adjustable Voltage Regulators 13 additional surface area to help remove the waste heat by convection to the surrounding air. For a given power dissipation, the heatsinks must be designed to keep the device case temperature below some value specified in the datasheet. The small heatsink we are using is specified to keep the temperature rise below 80ºC at power levels less than a few Watts. With AC power disconnected, solder the additional regulator components (C5, R1-R2, and the LM317) into the positive supply circuit as shown in Figure Resistor R2 is an adjustable trimmer potentiometer or trimpot, which can be adjusted from 0-5k. You may also wish to add male header pins to the test point marked Vout. Connect the power resistor decade box to the output terminal to provide a >100k load. Power up the Wiper Figure 2-19 Positive regulator circuit and the thumbwheel-type potentiometer used for R2. circuit. Using a multimeter to monitor the DC output voltage, adjust R2 and record the range of DC output voltages obtainable. Set the output voltage to around +12V and reduce the load resistance to give a current of 0.2A). Now observe the output voltage waveform on the oscilloscope. You should no longer observe any large droop in the waveform, since the IC regulator is working to maintain a constant output voltage. Zoom in on the waveform using your oscilloscope (AC couple the input and expand the scale): you can still see some residual ripple, but it is very small; nevertheless, in high gain, high sensitivity circuits, such minor supply irregularities can cause problems. Bigger filter capacitors and more complex voltage regulation is required to reduce this ripple further. Now adjust the output voltage to +5V and adjust the load resistance for 0.2A at this new output voltage. Leave the circuit on for about 5 min., and then touch the heatsink on the LM317 (if you left off the insulating washer the heatsink will only be at +12V potential which is harmless). Is it warm? Before proceeding, turn off the AC power!! LM337 Negative Regulator Circuit Repeat the above assembly steps for the negative supply regulator circuit using the LM337. You may wish to consult the full schematic in Figure 2-8 and the data sheet for the LM337. Note that the pin assignments for the LM337 is different than the LM317. Turn on the AC power and verify the correct operation of the negative regulator circuit with a multimeter. You do not need to repeat the steps involving waveform observations and loading effects. 13 Bob York

14 14 Dual-Output Linear Power Supply 2.5 Finishing Touches The last step makes the circuit a little easier for us to use later on: Your kit should have a 3-terminal female header block similar to the one shown in Figure Solder this into the designated place on the PC board. Although these headers are designed to mate with the male header pins used earlier, they also work really well with the #22 AWG jumper wires that we use with our solderless breadboards. So this header block allows for very convenient power connections to the solderless breadboards. Finally, demonstrate your working power supply to the Figure 2-20 Female header TA. block for output connections. Optional Enclosure In practical prototyping work it is smart practice to complete a circuit by placing it in an enclosure of some sort. This protects the circuit and also provides a convenient surface for attaching indicator lights, output terminals, fuses, switches, buttons, etc. While it is not necessary for this lab, the shop does sell some suitable enclosures and associated hardware for this lab. Congratulations! You have now completed Lab 2 Notes on the Report Important items for inclusion and discussion include (but are not limited to) the following: Bridge Rectifier: Include plots or sketches of the measured and annotated rectified waveforms. (simple grids are provided in the powerpoint file Lab 2 pics ). Filter capacitors: Describe the unregulated open-circuit output voltage and the load resistance for a 0.2A load current. Include plots or sketches of the voltage waveforms under maximum load current conditions, and discuss the capacitance required to achieve <1V droop under these conditions. Also include the charging current waveform measured with the help of the sense resistor Rs. Regulator circuits: What range of output voltages were you able to realize? What was the residual output ripple under maximum output conditions 0.2A)? Bob York 14