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Revised 40V 8A power supply is short-circuit proof Do you need a big power supply? One which will deliver lots of current but is short circuit proof? Well this is for you. Its output is adjustable from 0-45V and it can deliver up to 8 amps. 56  Silicon Chip Specifications Output voltage .......................................................................... 150mV-45V Output current .................................................. 8A below 35V, 6.6A at 40V Load regulation ...................................................................................0.5% Ripple and noise ................................................................ 60mV p-p at 8A Current limit adjustment .................................................................... 1A-8A Over temperature cutout .....................................................................80°C circuit is completely different. While it uses the same power transformer and main bridge rectifier, from there on it is different. Features Pt.1: By JOHN CLARKE I T MIGHT NOT LOOK all that big from the photos but trust us, this is a really big power supply, delivering up to 280 watts, depending on the voltage and current settings. In the past, a power supply with that much output capability would be a monster and it would weigh a tonne as well. But this is a switchmode design and so it is highly effi­cient. The result is that it does not need a really big power transformer and big heatsinks. It uses the same operating princi­ples as the switchmode power supplies employed in millions of personal computers. Before we go too much further we should state that this power supply is a revised and updated version of one we published in the January & February 1992 issue of SILICON CHIP. Externally, the revised design looks much the same as the original version and it has much the same features but its The revised power supply is housed in a large plastic in­strument case and has generously sized meters for voltage and current. There are two knobs to adjust the output: one for vol­tage and one for current. Just below the voltage knob is a toggle switch which allows the supply to deliver a fixed 13.8V which is handy if you are working on any automotive device. Below the current adjust knob is a pushbutton switch which allows the maximum current to be set and below that again is the load switch. This is another handy feature because it allows the voltage output to be set precisely before the load is connected. The ammeter shows current from 0-10A and has three modes of operation. Normally this meter shows the current delivered to the load but when the current set switch is pressed and with the load switched off, it shows the current limit setting. This is variable from 1-8A using the current adjust knob. Reserve current When the power supply is delivering current to a load you can press the current set switch to display the reserve current available. This is the difference between the set current limit and the current delivered to the load. It is a handy feature which can allow you to set the current limit to a certain value over the normal quiescent current drawn by the load. Above each of the voltage and current adjust knobs is a LED to indicate “regulator dropout” and “current overload”, respec­tively. As its name suggests, the regulator dropout in- dicator shows when the difference between the load voltage setting and the unregulated DC input voltage is insufficient to allow the regulator to work properly. This will normally only occur when the output voltage and current are both high. When the regulator dropout LED indicator comes on, you can keep using the supply and no harm will occur because it is fully protected but the hum and noise superimposed on the output will be quite a lot higher than normal. Similarly, when the supply goes into current overload or exceeds the current output setting, it will produce an audible squealing which gives you a further warning that its settings are being exceeded. Three binding post terminals are provided for the supply’s output, red for positive, black for negative and green for Earth. Neither side of the supply is tied to Earth so it may be operated as a fully floating supply or Main Features •  Large voltage and current meters •  Adjustable current limit •  Load switch •  Regulator dropout indication •  Current overload indication •  Variable or fixed 13.8V output •  Can be used as a current source •  Over temperature cutout •  Floating output can be earthed on + or - terminal •  Reserve current (headroom) indication April 1998  57 Fig.1: IC1 drives the two Mosfets to vary the output voltage and also control the current delivered. The use of several stages of LC filtering provides low ripple and switching noise in the output and also isolates the Mosfets from heavy surge currents when short circuits occur. either the red or black terminals can be linked to the green Earth terminal if you desire. New design While the original design was basically sound, there were a number of problems with it. First, it used a special optical fibre link between the control and regulator sections and this component was often difficult to obtain. Second, it had current foldback protection which caused problems when the supply was called upon to drive big incandescent lamps or DC motors; as soon as these loads were connected, the initial surge current caused the supply to go into foldback and so no power would be deliv­ered. Third, the main power Mosfet used for voltage regulation turned out to be prone to destruction under short circuit condi­ t ions and with high power delivered to the load there was a tendency for the toroidal inductor to overheat. In addition, some users also wanted the ability to operate the supply as a constant current source and that is not possible in a circuit with foldback protection. Hence, we had a number of reasons to reassess the design and to produce a new version which was considerably more rugged. This new design is now short circuit proof and only runs warm Fig.2: block diagram of the TL494 switchmode controller. It contains an oscillator, pulse width mod­ulation (PWM) comparator, error amplifiers and output drivers at pins 9 & 10. Other refine­ments include a dead-time control and under-voltage (UV) lockout. 58  Silicon Chip when delivering high currents. The supply can easily drive DC motors without causing current overload on startup. Fig.1 shows the simplified circuit for the new 40V 8A ad­justable power supply. It is a switchmode circuit with two Mos­fets (Q1 & Q2) used to drive transformer (T2). By varying the duty cycle of Q1 & Q2 we can control the output voltage. In essence, the circuit operation is as follows. Transform­er T1 delivers 35VAC to the bridge rectifier BR1 and its output is filtered with C1 which comprises five 4700µF capacitors. The result is smoothed DC of about 50V. A regulator reduces this to 12V to feed IC1, the TL494 switchmode controller. IC1 controls a push-pull switchmode converter comprising the two switching Mosfets Q1 & Q2, transformer T2, bridge recti­ fiers D1-D4, inductor L1 and C1, which is two 1000µF capacitors. Mosfets Q1 & Q2 operate pretty much like any other push-pull switchmode converter. When Q1 is switched on, the full +50V is applied across the top half of the primary winding of T1 and so, by transformer action, -50V appears across the other half of the transformer winding and at the drain of Mosfet Q2. When Q2 switches on, the reverse action occurs across the transformer primary. Transformer ac- Fig.3: these waveforms demonstrate the operation of IC1. The top two waveforms are the gate signals for Mosfets Q1 & Q2, at pins 9 & 10. The lowest waveform is the oscillator waveform (CT) with the feedback voltage superimposed on it. tion also causes current to flow in the secondary winding and via the bridge rectifier BR2 to the LC filter consisting of L1 & C2. Following C2 is another LC filter consisting of L2 & C3 and this further filters the output of bridge rectifier BR2. The voltage developed across C3 is determined by the load current and the length of time that Q1 & Q2 are alternately switched on. The duty cycle is always less than 50% for each Mosfet but it can be a lot less than that, when the load current is low and the re­quired output voltage is also low. IC1 monitors the voltage produced across C3 using voltage divider re- Fig.4: these are the gate signals to Q1 (top trace) and Q2 (lower trace) when the supply is delivering low voltage and low current. sistors R2 & R3 and adjusts the duty cycle of the switching signal applied to Q1 & Q2, to obtain the voltage re­ quired. Similarly, the output current from C3, which flows to the load via LC filter L3 & C4, is monitored by resistor R1. If the current limit is exceeded, IC1 reduces the duty cycle of the switching Mosfets and this in turn reduces the voltage and hence the current. Importantly, even though IC1 acts to control the output voltage and current by continuously adjusting the switching signal, the reason why this new circuit can withstand repeated short circuits is that the three LC filters (L1, Fig.5: much wider gate signals are applied to Q1 and Q2 when the supply deliv­ers higher voltage and current to the load. April 1998  59 Fig.6: output ripple and noise from the supply when it is deliv­ering 8A at 35V to a resistive load. the gate capacitance of the Mosfets. IC2 & IC3 have their supply decoupled with 0.1µF capacitors to prevent supply lead inductance affecting the drive signals. The gates of Q1 & Q2 are each driven via a 47Ω resistor and these slightly slow the switching times, to reduce electromagnet­ic interference. A series diode and 150V zener diode is connected between the gate and drain of each Mosfet to protect them against transients. If a voltage spike of more than 150V occurs at the drain of Q1, for example, ZD1 conducts to turn the Mosfet momentarily on to safely clamp the transient. Thus the voltage spike is limited to about 155V, as set by the zener voltage plus the series diode, plus the turn-on voltage of the Mosfet gate. Dropout detection C2, L2, C3, L3 & C4) provide very good isolation between the load and Mosfets Q1 & Q2. No matter what peak currents might be drawn by overload­ing, the LC filters smooth it all out so that the Mosfets do not have to supply high instantaneous currents. Fig.2 shows the internal workings of IC1. It contains an oscillator, pulse width modulation (PWM) comparator, error ampli­fiers and output drivers at pins 9 & 10. Other refinements in­clude a dead-time control and under-voltage (UV) lockout. The basic operation of IC1 is shown in Fig.3. The top two waveforms are the gate signals for Mosfets Q1 & Q2, at pins 9 & 10. The lowest waveform is the oscillator waveform (CT) with the feedback voltage superimposed on it. The voltage and current signals from the power supply are applied to the error amplifiers 1 & 2 and their outputs are combined at pin 3. This feedback voltage at pin 3 is compared against the sawtooth oscillator waveform in the PWM comparator and the resulting rectangular waveforms are produced at pins 9 & 10. If the feedback signal is high on the sawtooth waveform, then the pulses from pins 9 & 10 are narrow, while if the feed­back voltage is low on the sawtooth, then the pulses are wider. The oscilloscope waveforms of Fig.4 show the gate signals to Q1 (top trace) and Q2 (lower trace). These are quite narrow pulses which occur when the supply is delivering low voltage and low current. Fig.5 shows much wider 60  Silicon Chip gate signals, representing a higher voltage and current to the load. Fig.6 shows the output ripple from the supply when it is delivering 8A at 35V to a resistive load. Circuit details Fig.7 shows the full circuit of the revised power supply. While it looks a good deal more complicated than the simple diagram of Fig.1, you should still recognise the main supply chain from T1 through T2, L1, L2 & L3, along the top of the circuit diagram. The main differences are associated with IC1, showing all the external components plus the metering, overload and overcurrent LED indication circuitry. The 3-terminal regulator REG1 provides a 12V supply for IC1 and the associated low voltage circuitry. It runs from the main +50V supply rail via a 470Ω 5W dropping resistor. Pins 9 & 10 of IC1 produce the gate signals for Q1 & Q2. However, they don’t drive the gates directly. Instead, each pin is buffered by four inverters, in IC2 or IC3. Pin 9 is buffered with IC2a and then by the paralleled trio IC2b, IC2c & IC2d, while pin 10 is buffered with IC3a and then with paralleled trio IC3b, IC3c & IC3d. These inverter/buffers perform several functions. First, they increase the gate drive signal to the full 12V swing of the supply rail. Second, they “square up” the gate signals to produce fast pulse rise-times and fall-times and at the same time high current drive to Inverters IC2e & IC2f buffer the pin 2 output of IC2a; ie, the gate drive signal to Q1. This signal approaches 50% duty cycle when the power supply is called upon to deliver full power. A 10kΩ resistor and 0.1µF capacitor filter the pulse signal to produce a DC voltage which represents the “average” value of the waveform. This approaches 6V when the gate drive is close to 50% duty cycle. The inverting input (pin 2) of op amp IC4 monitors this voltage and compares it to the +4.8V at pin 3 set by the 33kΩ and 22kΩ resistors across the 12V supply. Normally, the output of IC4 is high (close to 12V) since its pin 2 input is lower than pin 3. When the gate drive signal approaches 50% duty cycle, pin 2 goes above pin 3 and so pin 6 of IC4 goes low (close to ground) and drives the dropout LED (LED1) via the 2.2kΩ resistor. Soft start IC1 oscillates at close to 44kHz, as set by the components at pins 5 & 6. The actual Mosfet drive frequency is half this at 22kHz. At power up, the Fig.7 (right): IC1 drives the two Mosfets via paralleled inverters to obtain fast switching and low dissipation. The five op amps are there to provide minimum loading (IC5c & IC5d), current limit drive to the meter (IC5a), dropout indication (IC4) and current limit indication (IC5b). April 1998  61 Parts List For 40V 8A Power Supply 1 PC board, 80 x 94mm, code 04304981 1 large instrument case, 355 x 250 x 122mm (Altronics H-0490) 2 aluminium panels for front and rear of case 1 front panel label, 350 x 120mm, to suit case 1 steel baseplate (Altronics H-0492) 1 MU-65 panel meter 1mA FSD (0-10A scale) (M2) 1 MU-65 panel meter 1mA FSD (0-50V scale) (M1) 1 35V 300VA toroidal mains transformer (Altronics M-4092) (T1) 1 ETD44 transformer assembly with two cores (3C85 ferrite), 1 bobbin and two retaining clips (T2) 1 ETD34 transformer assembly with two cores (3C85 ferrite), 1 bobbin and two retaining clips (L1) 2 10 x 5 x 0.5mm material to gap L1’s cores 1 44mm OD Neosid iron powdered core 17-745-22 (L2) 1 33mm OD Neosid iron powdered core 17-742-22 (L3) 1 single sided fan heatsink 105 x 225mm 1 red panel mount binding post 1 black panel mount binding post 1 green panel mount binding post 1 SPST neon illuminated rocker 250VAC switch (S1) 1 10A SPST or SPDT toggle switch (S2) 1 DPDT momentary pushbutton switch (S3) 1 normally closed, 80°C, 10A thermal cut out switch (TH1) 1 3AG panel mount 250VAC safety fuseholder (F1) 1 7.5A 3AG fuse 1 5kΩ linear potentiometer (VR1) 1 50kΩ linear potentiometer (VR2) 2 22mm knobs 2 5mm LED bezels 1 10A mains cord and plug 1 cordgrip grommet for mains cord 1 3-way 10A mains terminal block 7 solder or crimp lugs 2 TO-218 mica or silicone insulating washers 4 TO-220 mica or silicone insulating washers 6 TO-220, TO218 insulating bushes 1 1m length of red medium duty hookup wire 1 1m length of black medium duty hookup wire 1 1m length of green medium duty hookup wire 1 1m length of yellow medium duty hookup wire 1 1.5m length of red heavy duty hookup wire 1 500mm length of black heavy duty hookup wire 1 200mm length of 10A green/ yellow mains wire 1 500mm length of 10A brown mains wire 1 11m length of 0.8mm diameter enamelled copper wire 1 3m length of 1.25mm diameter enamelled copper wire 1 160mm length of 0.8mm diameter tinned copper wire 1 100mm length of 1.25mm diameter tinned copper wire 23 PC stakes 4 6mm standoffs 12 3mm screws x 25mm 2 3mm x 10mm countersunk screws 3 3mm x 10mm screws 17 3mm nuts 5 3mm star washers 8 self-tapping screws to secure baseplate to case 1µF capacitor and 100kΩ resistor at pin 4 set the “dead time” at maximum. Dead time is the time between one Mosfet turning off and the other turning on, so that there is no chance of both being on at the same time, which could have disas­trous results. By setting the dead time at maximum, 62  Silicon Chip Semiconductors 1 TL494 switchmode controller (IC1) 2 4049 CMOS hex inverters (IC2,IC3) 1 TL071, LF351 op amp (IC4) 1 LM324 quad op amp (IC5) 2 BUK436-200A or BUK436-200B 19A 200V Mosfets (Q1,Q2) 2 BC639 NPN transistors (Q3,Q4) 1 7812, LM340T12 12V regulator (REG1) 1 FB3502 35A 200V bridge rectifier (BR1) 4 MUR1560 15A fast recovery diodes (D1-D4) 2 1N4148, 1N914 signal diodes (D5,D6) 2 150V 3W zener diodes (ZD1,ZD2) 2 5mm red LEDs (LED1,LED2) Capacitors 5 4700µF 50VW PC electros (C1) 5 1000µF 50VW PC electrolytics (C2,C3) 1 220µF 35VW PC electrolytic 2 10µF 16VW PC electrolytics 1 1µF 16VW PC electrolytic 1 0.1µF 250VAC MKT polyester (C4) 3 0.1µF MKT polyester 2 .01µF 250VAC MKT polyester 1 .01µF MKT polyester 1 .001µF MKT polyester Trimpots 1 5kΩ horizontal trimpot (VR3) 1 50kΩ horizontal trimpot (VR4) 1 500Ω horizontal trimpot (VR5) Resistors (0.25W, 1%) 1 1MΩ 4 2.2kΩ 1 220kΩ 6 1kΩ 2 100kΩ 2 470Ω 3 47kΩ 3 100Ω 1 33kΩ 2 47Ω 1 27kΩ 2 10Ω 2 22kΩ 2 1kΩ 5W 1 18kΩ 1 470Ω 5W 1 12kΩ 1 39Ω 5W 2 10kΩ 1 10Ω 5W 1 4.7kΩ 2 0.1Ω 5W Miscellaneous Heatshrink tubing, cable ties, solder, etc. no power is supplied to transformer T2 by the Mosfets. As the voltage at pin 4 drops towards 0V, the dead time gradually decreases Most of the parts are mounted on a single large PC board, so the construction is straightforward (full details in Pt.2 next month). until it is at a minimum and so the Mosfets provide a “soft start”, bringing the set voltage up gradually. Error amplifier Pin 14 of IC1 is a +5V reference for the error amplifiers. The output voltage of the power supply is fed to a voltage divid­er consisting of 100kΩ and 12kΩ resistors and monitored at pin 1 (see Fig.2). The inverting input at pin 2 connects to the wiper of switch S4 via a 4.7kΩ resistor. This resistor and the 1MΩ resistor between pins 2 & 3 set the amplifier gain at 213. A 47kΩ resistor and series .01µF capacitor roll off the high frequency response of the amplifier to a maximum gain of about 11 above 16Hz. The wiper of switch S4 connects either to potentiometer VR1 (the voltage control) or to VR3. Both potent­ iometers are connect­ ed to the +5V reference. VR3 is adjusted to set the fixed 13.8V output while VR1 sets the variable output. If VR1 is set to give 5V at its wiper, the switchmode circuit acts to produce the same voltage at pin 1. The power supply therefore produces 46.66V because this is reduced by the 12kΩ and 100kΩ resistive divider to 5V at pin 1. For intermediate settings of VR1, the circuit maintains this same voltage at pin 1. Since VR1’s wiper can vary between +5V and 0V, the output voltage can be varied from 46.66V down to almost 0V. Current limiting The current delivered by the power supply is detected using two paralleled 0.1Ω 5W resistors and the resulting voltage is monitored at pin 15 of IC1 via a 100Ω resistor. VR2 sets the current limit and operates as follows. With no current flowing through the two paralleled 0.1Ω resistors, pin 15 is set to some small positive voltage by VR2. When current is drawn from the supply, the voltage developed across the 0.1Ω resis­tors acts to pull pin 15 lower. If pin 15 is pulled below 0V, which is lower than pin 16, then the output of error amplifier 2 goes high to reduce the pulse drive to the Mosfets. This limits the current. When no current is flowing through the 0.1Ω resistors, VR2 can be adjusted to provide from +0.45V down to 0.01V. The resist­ance of the two paralleled 0.1Ω resistors is 0.05Ω and so 8A will produce a 0.4V drop across them. Thus, if VR1 is adjusted to set pin 15 to 0.4V then current limit will occur at 8A. When VR2 is set to give 0.05V at pin 15, current limit will occur at 1A. A 1mA meter, M2, is used as the ammeter. When switch S3 is in position 1, the meter is connected across the 0.1Ω current sensing resistors but in series with trimpot VR5 and a 100Ω resistor. The meter therefore displays the load current. We’ve already discussed how pin 15 of IC1 is biased by VR2 to set the current limit. The voltage at pin 15 is buffered with unity gain amplifier IC5a and its output drives meter M2 April 1998  63 A large finned heatsink is bolted to the rear panel to prevent the output devices from overheating and self-destructing. when switch S3 is in position 2. The meter thereby indicates the current limit setting in amps, when the load switch S2 is off (ie, no current actually flowing to the load). But if the load switch S2 is on, the load current produces a voltage drop across the 0.1Ω resistors and this is subtracted from the current limit voltage applied to pin 15 of IC1. In this condition, when S3 is in position 2, the ammeter displays the difference between the load current and the current limit. In other words, it shows how much more current can be delivered to the load before limiting occurs. This can be a handy feature when driving some loads where the current swings need to be con­trolled. As discussed previously, current limiting occurs when pin 15 of IC1 approaches 0V. Pin 15 is buffered by op amp IC5a and its output, as well as driving the ammeter, is connected to op amp IC5b which is connected as a comparator. Its non-inverting input at pin 10 sits at about +5mV, as set by the 220kΩ and 100Ω resistors across the 12V supply. When pin 9 goes below 64  Silicon Chip pin 10, which happens as the circuit goes into current limiting, pin 8 of IC5b goes high to drive overcurrent indicator LED2 via a 2.2kΩ resistor. Minimum loading Op amps IC5c & IC5d and transistors Q3 & Q4 provide a mini­mum load for the power supply. This is necessary to ensure that the regulator works reliably at low values of load current. If we don’t provide a minimum load, the switching pulses to Q1 & Q2 become extremely narrow and tend to become irregular as the circuit tries to maintain a fixed voltage. This minimum loading is achieved with three sets of resis­tors. Firstly, two 1kΩ 5W resistors in parallel are permanently connected across the supply (near C2 on the circuit of Fig.7) and these provide sufficient current drain for voltage settings above 10V. For voltage settings below 10V, Q3 is used to switch in a 39Ω 5W resistor while for settings below 5V, Q4 switch­ es in a 10Ω 5W resistor. Op amps IC5c & IC5d are connected as comparators to control the switch- ing of Q3 & Q4. The non-inverting inputs (pins 3 & 5) are tied to a divider string consisting of a 22kΩ resistor and two 470Ω resistors. The inverting inputs (pins 2 & 6) of each op amp monitor the supply output voltage via a voltage divider consisting of 18kΩ and 1kΩ resistors. The resistive divider strings are set so that IC5d’s output is high when the power supply voltage is between 0V and 5V and IC5c’s output is high when the voltage is between 0V and 10V. When IC5d’s output is high, it drives the base of Q4 via a 1kΩ resistor to connect the 10Ω resistor across the supply, while IC5c’s high output drives the base of Q3 via its 1kΩ resistor to connect it to the power supply rails. Note that IC5c & IC5d both have 47kΩ feedback resistors. These provide some hysteresis to prevent the output from oscillating at the verge of switching. Note that the 10Ω, 39Ω and 1kΩ load resistors are connected across the supply before the 0.1Ω current sensing resistors. This prevents them from affecting the ammeter reading or the current limit setting. Next month, we will give the full SC construction details.