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Li’l PowerHouse A new 40V/1A switchmode power supply with LCD readout Is your old power supply so old it has germanium tran­sistors? Maybe it has Dymo® labels on the front panel and a fab­ric-covered power cord? Er, does it use a copper oxide rectifier? If you have an ancient power supply, now is the time to give it the heave-ho and get this up to the minute design. By PETER SMITH & LEO SIMPSON 56  Silicon Chip T HIS NEW POWER SUPPLY has a big power output for its size. It can give DC voltages up to 40V and the output current can be as much as 1.2A, depending on the voltage setting. And it can be varied right down to less than 1.5V while still giving out 1.2A. This is great for testing battery circuits that operate at 1.5V, 3V or whatever. In times past when we have looked at doing a compact power supply, the natural approach would be to produce an analog design with an analog meter on the front panel. An example of this was our dual tracking 18.5V supply published in the January 1988 issue. The analog approach has the virtue of simplicity and it gives good results. But that’s in the past. And it is boring. Nowadays we can do a lot better with a switchmode design. It is more efficient so big heatsinks are unnecessary and you can get a higher maximum DC voltage output for a given secondary voltage from the power transformer. And you can get a lower minimum DC voltage at full current with having problems with the high power dissipation of a conventional series regulator. In fact, this circuit will have losses of less than 10W (including transformer losses) under worst case conditions, meaning that it does not need any heatsink apart from that pro­vided by the back panel of the case. By contrast, if we had gone to a conventional regulator using the same power transformer, it would have losses (ie, heat) of around 30W when delivering 1.5V at 1.2A and it would need a fairly substantial finned heatsink on the rear of the case. Another good feature of the design is the low level of ripple and hash in the output, and this is not always the case with switchmode designs. We have achieved this with critical attention to the circuit layout and two stages of LC filtering. Digital panel meter The new supply has a 3.5-digit LCD panel to monitor the voltage or current, as selected by a toggle switch. As well, you can set the current limit by pressing a button on the front panel and then rotating the knob closest to the LCD panel. A 10-turn potentiometer lets you precisely set the output voltage which can be done before you connect the output load by means of the load switch. In addition, there is a LED on the front panel, just next to the “set current” knob, to indicate when an overload occurs. The supply is protected against short circuits by the way. The voltage vs current characteristic is shown in the graph of Fig.1. As it shows, the supply will deliver 1.2A over the range from less than 1.5V to 30V. At higher voltages, the avail­able current drops off because the transformer is only rated at 30VA which means that it could only deliver 1A at 30V if the circuit had no losses at all. In fact, we are over-rating the transformer to get 1.2A at 30V but it Fig.1: the voltage vs current characteristic of the supply. It is capable of delivering 1.2A over the range from 1.23V to 30V. Beyond that, the current falls off due to the transformer regulation. quite good too. In fact, apart from the output noise and ripple, the perfor­ mance is actually a little better than our previous 40V 3A power supply published in the January & February 1994 issues. Based on switcher Fig.2: how a switching regulator operates. When S1 is closed and S2 is open, current flows to the load via L1 which stores energy. When S1 opens and S2 closes, the energy stored in L1 maintains the current through the load until the switches toggle again. does not appear to be a problem – the transformer appears to be conservatively rated at 30VA. At 40V, the output current from the prototype supply was around 160mA which is pretty respectable for a small supply. As shown in the specifications panel, the load regulation of the circuit is excellent and the line regulation is The circuit is based on the National Semiconductor LM2575HVT high voltage adjustable switchmode voltage regulator. This is almost identical to the switcher chip used in the 40V/3A power supply men­tioned above. However, this new 40V/1A circuit is not simply a cut-down version of the 1994 design; apart from the use of the switcher chip, it is different in a number of aspects. Let’s have a brief look at how the switcher works. Fig.2 shows how a switching regulator operates. In operation, S1 and S2 operate at high speed and are alternately closed and opened. These two switches control the current flowing in inductor L1. When S1 is closed and S2 is open, the Main Features • • • • • • • • • • • Output voltage continuously variable from 1.23V to 40V Output current of 1.2A from 1.23V to 30V LCD panel meter for voltage & current 10-turn pot for precise voltage adjustment (optional) Adjustable current limit LED current overload indication Output fully floating with respect to mains earth Load switch Low output ripple Short circuit and thermal overload protection Minimal heatsinking JUNE 2000  57 Fig.3: a basic regulator using the LM2575 switcher IC. In this circuit, a switching transistor takes the place of S1 in Fig.1 and diode D1 takes the place of S2. The output voltage is set by the ratio of R2 & R1 which feed a sample of the output voltage back to an internal comparator. current flows to the load via inductor L1 which stores up energy. When S1 subsequently opens and S2 closes, the energy stored in the inductor maintains the load current until S1 closes again. The output voltage is set by adjusting the switch duty cycle – the longer S1 is closed and the current flows through it, the higher will be the output voltage. Fig.3 shows a complete voltage regulator based on the LM2575 IC. It is a 5-pin device which requires just five extra components to produce a basic working circuit. Its mode of opera­tion is the same as that described in Fig.2 except that here an internal switching transistor is used for S1, while an external diode (D1) is used for S2. What happens in this case is that when the transistor is on, the current flows to the load via inductor L1 as before and diode D1 is reverse-biased. When the transistor subsequently turns off, the input to the inductor swings negative (ie, below ground). D1 is now forward-biased and so the current now flows via L1, through the load and back through D1. The output voltage is set by the ratio of R2 and R1 which form a voltage divider across the output (Vout). The sampled voltage from the divider is fed to pin 4 of the switcher IC and then to an internal comparator where it is compared with a 1.23V reference. This sets Vout so that the voltage produced by the divider is the same as the reference voltage (ie, 1.23V). Apart from the comparator and the switching transistor, the regulator IC Specifications Minimum no load output voltage........................................................ 1.23V Maximum no load output voltage.......................................................... 40V Output current............................................................................... see Fig.1 Current limit range.................................................................. 10mA to 1.2A Current limit resolution........................................................................10mA Line regulation..............................0.1% for a 10% change in mains voltage Voltmeter resolution..........................................................................100mV Current meter resolution.......................................................................1mA Meter accuracy......................................................................2% plus 1 digit Load regulation no load to 1A <at> 24V.......................................................................1.2% no load to 1A <at> 12V.......................................................................1.5% no load to 1A <at> 6V.........................................................................1.8% no load to 1A <at> 3V.........................................................................3.3% Output noise and ripple 3V to 24V <at> 1A............................................................ 25mV p-p (max) 58  Silicon Chip also contains an oscillator, a reset circuit, an on/off circuit and a driver stage with thermal shutdown and current limiting circuitry. The incoming supply rail is applied to pin 1 of the IC and connects to the collector of the internal switching transistor. It also supplies an internal regulator stage for the rest of the regulator circuit. In essence, the LM2575 uses pulse width modulation (PWM) to set the output voltage. If the output voltage rises above the preset level, the duty cycle from the driver stage decreases and throttles back the switching transistor to bring the output voltage back to the correct level. Conversely, if the output voltage falls, the duty cycle is increased and the switching transistor conducts for longer periods. The internal oscillator operates at 52kHz ±10% and this sets the switching frequency. In theory, this frequency is well beyond the limit of audibility but in practice, a faint ticking noise may be audible due to magneto­strictive effects in the cores of the external inductors. One very useful feature of the LM2575 is the On/Off control input at pin 5. This allows the regulator to be switched on or off using an external voltage signal and we have used this to provide the adjustable current limiting feature, as we shall see later on. Circuit details Fig.4 shows the full circuit of the new power supply. Transformer T1 is supplied with mains power via fuse F1 and power switch S1. Its 30VAC secondary is JUNE 2000  59 2000 10F 16VW +5.1V C- 1 5 -5.1V 10F 16VW LED A K -5.1V -5.1V 4 x 0.1F GND 3 LM2575 4 IC5 ICL7660 8 V+ 2 5 C+ OUT 0V 30V D1-D4 1N4002 100 100 2 3 4 VR3 10k 6 1 4 100k 100k 1 2 1 OUT 1k 100k 2 2 1 6 5 2 3 5 METER ZERO VR5 100k 1 4 IC4 TL071 7 +5.1V -5.1V 6 S4: 1 - MEASURE CURRENT 2 - SET CURRENT 1M 100k 100k 1M 330pF 5 8 7 2 300 RFH 9 10 ROH 11 DP3 .8.8.8 6 1M 27k VOLTS CAL VR4 5k  0.1F 63VW 13 2 1 V- V+ S3b 2 1 + EARTH _ OUTPUT 1.23-40V 1A +5.1V CASE 0.1F 250VAC 0.33F 63VW LOAD S2 DP1 Q0570 DIGITAL PANEL METER 1 INLO COM RFL INHI 4 IC3b LM393 470 8 2 x 47F 63VW + LM336-2.5 REF1 _ D6 1N4148 3 *SEE TEXT *R1 0.005 L2 47H OVERLOAD LED1 3 x 470F 63VW 1k S3: 1 - MONITOR CURRENT 2 - MONITOR VOLTAGE 7 4.7k 680 5W L1 470H D5 MBR360 IC3a LM393 +2.5V +5.1V 5 100k S4a 2 S3a 3 FB ON/ GND OFF IN IC1 LM2575HVT-ADJ CURRENT LIMIT VR2 1k S4b OP77GP 0.1F 1.5k CURRENT CAL -5.1V IC2 7 330F 63VW 15k 2 x 2200F 50VW VOLTAGE ADJUST VR1 50k 40V/1A ADJUSTABLE POWER SUPPLY _ + ADJ LM336Z 10F 16VW +5.1V 100F 16VW ZD1 5.1V 1W 1k 5W CASE POWER S1 250VAC T1 M6672L Fig.4: the circuit is based on IC1, the LM2575 switcher controller. It runs at around 50kHz and the resultant DC output is filtered with inductors L1, L2 and the associated capacitors. IC2 & IC3 provide the current limit fea­ture while IC4 drives the LCD panel meter. SC  E N 240VAC A F1 500mA Parts List 1 PC board, code 04106001, 171 x 127mm 1 M6672L 30V 30VA mains transformer 1 DPDT 250VAC 6A plastic rocker switch with neon indicator (S1) 2 S1345 DPDT miniature toggle switches (S2,S3) 1 DPDT momentary pushbutton switch (S4) 1 LCD panel meter (Altronics Q-0570) 1 M205 panel-mount safety fuse-holder (F1) 1 500mA M205 fuse 1 470µH toroid inductor (L1) 1 47µH toroid inductor (L2) 1 TO-220 insulating bush and washer 2 15mm knobs 3 captive binding post terminals (1 red, 1 green, 1 black) 1 cordgrip grommet for mains cable 1 13-way 2.54mm SIL header plug (for connection to panel meter) 4 3.2mm solder lugs 22 PC stakes Hardware for pre-punched metal case 1 pre-punched metal case 3 M4 x 10mm screws 4 M4 nuts 2 M4 internal star washers 8 M3 x 6mm screws 1 M3 nut 1 M3 flat washers 4 10mm tapped spacers Hardware for plastic instrument case 1 plastic case, 200 x 155 x 65 mm (W x D x H) with metal front and rear panels (Altronics Cat. H-0481F & H0484F) 2 M3 x 10mm screws 1 M3 x 15mm countersunk screw 5 M3 nuts 1 M3 flat washer 4 M3 internal star washers 2 M4 x 10mm screws 2 M4 nuts 2 M4 flat washers 4 self-tapping screws (to mount PC board) Semiconductors 4 1N4002 1A 100V diodes (D1-D4) 60  Silicon Chip 1 MBR360 3A Schottky diode (D5) (SR306 or 31DQ06 also suitable) 1 1N4148 small signal diode (D6) 1 LM2575HVT-ADJ high voltage switchmode controller (IC1) 1 OP77GP op amp (IC2) 1 LM393 dual comparator (IC3) 1 TL071 op amp (IC4) 1 ICL7660 switched capacitor voltage inverter (IC5) 1 LM336Z-2.5 voltage reference (REF1) 1 1N4733 5.1V 1W zener diode (ZD1) 1 3mm red LED with bezel (LED1) Resistors (0.25W, 1%) 3 1MΩ 2 1kΩ 6 100kΩ 1 1kΩ 5W 1 27kΩ 1 680Ω 5W 1 15kΩ 1 470Ω 1 4.7kΩ 1 300Ω 1 1.5kΩ 2 100Ω Potentiometers 1 50kΩ 16mm linear pot (VR1) OR 1 50kΩ multi-turn linear pot 1 1kΩ 16mm linear pot (VR2) 1 10kΩ horizontal trimpot (VR3) 1 5kΩ horizontal trimpot (VR4) 1 100kΩ horizontal trimpot (VR5) Capacitors 2 2200µF 50VW PC electrolytic 3 470µF 63VW PC electrolytic 1 330µF 63VW PC electrolytic 1 100µF 16VW PC electrolytic 2 47µF 63VW PC electrolytic 3 10µF 16VW PC electrolytic 1 0.33µF 63VW MKT polyester 6 0.1µF 63VW MKT polyester 1 0.1µF 250VAC MKT polyester 1 330pF MKT polyester Wire and cable 1 2-metre 250VAC mains lead with 3-pin plug 1 600mm length of green/yellow mains wire 1 200mm length of 13 way ribbon cable 1 60mm length of 0.4mm enamelled copper wire Miscellaneous Cable ties, heatshrink tubing, heatsink compound, solder, hook-up wire. full-wave rectified using diodes D1D4 and filtered using two paralleled 2200µF 50VW elec­ trolytic capacitors. The resulting 42V DC supply is applied to the switching regulator (IC1). The additional 330µF capacitor connected between pins 1 & 3 of IC1 is included to prevent cir­cuit instability and is mounted as close to the IC as possible. Diode D5, inductor L1, the three 470µF capacitors and potentiometer VR1 form the basic switchmode power supply block (see Fig.4). D5 is a 3A Schottky diode which has been specified instead of a conventional fast recovery diode because of its low forward voltage drop. As a result, there is very little heat dissipation within the diode and this leads to increased effi­ciency. The output from IC1 feeds directly into L1, a 470µH induc­ tor. The 10-turn potentiometer VR1 and its associated 1.5kΩ resistor provide voltage feedback to pin 4 of IC1, to set the output level. When VR1’s resistance is at 0Ω, the output from the regulator (pin 2) is equal to 1.23V. This output voltage increas­es as the resistance of VR1 is increased. The 680Ω 5W resis­tor connected across the regulator output discharges the three 470µF capacitors to the required level when a lower output vol­tage is selected. 2nd filter circuit Inductor L2 and its associated 47µF and 0.1µF capacitors provide a second stage of filtering to further attenuate the switching frequency ripple. The resulting filtered voltage is then applied to the output terminals via load switch S2. Addi­tional filtering is applied at this point using a 0.33µF capaci­tor across the terminals and a 0.1µF capacitor between the nega­tive terminal and the case. Current limiting The current sense resistor (R1) is wired into the negative supply rail adjacent to inductor L2 and consists of a short length of 0.4mm enamelled copper wire. The voltage developed across it is multiplied by 200 using op amp IC2, so that IC2’s output delivers 1V per amp of load current. IC2 is specified as an OP77GP which has the required low input offset voltage (typically 50µV) and a very low input bias current (typically Despite the relative circuit complexity, the power supply is easy to build. This view shows the prototype PC board for the supply, with all parts in place. The full assembly details will be published in next month’s issue. 1.2nA – that’s nanoamps!) This is necessary to ensure that IC2’s output is at 0V when no current is flowing through R1. Because its inputs operate at close to ground potential (ie, 0V), IC2 must be powered from balanced positive and negative supply rails. The +5.1V rail for IC2 (and for the remaining ICs) is derived from the output of the bridge rectifier via a 1kΩ resistor and 5.1V zener diode ZD1. For the negative rail we use IC6, an ICL7660 switched capacitor voltage converter which oper­ates at 10kHz to provide a -5.1V supply. Comparator stage IC3a monitors the output voltage from IC2 and compares this with the voltage on its inverting input, as set by the current limit control VR2. This 1kΩ potentiometer and its associated 1kΩ resistor form a voltage divider network which is connected across the 2.5V reference, REF1. In operation, VR2 sets the voltage on pin 6 of IC2 at bet­ween 0V and 1.25V, corresponding to current limit settings of 0-1.25A. Because IC3a is an open collector device, its output at pin 7 is connected to the +5.1V rail via a 4.7kΩ pull-up resis­tor. If the voltage at the output of IC2 is greater than that set by VR2, pin 2 of IC3a is pulled high by this resistor. This also pulls pin 5 of IC1 high and switches off the regulator to provide current limiting. At the same time, pin 2 of IC3b is pulled high via diode D6 and so pin 1 switches low and LED1 lights to indicate current limiting or an overload condition. When the current subsequently falls below the preset limit, pin 7 of IC3a switches low again and the regulator turns back on. Thus, IC3a switches the regulator on and off at a rapid rate to provide current limiting. The 1MΩ resistor and 330pF capacitor at pin 2 of IC3b provide a small time delay so that LED1 is powered continuously during current limiting. Digital panel meter The LCD panel meter we’re using for this circuit has sim­pler interfacing requirements than those we have used in the past. It requires a +5V supply which comes from ZD1 and the resistors across its pins 5,6,7 & 8 configure it to read 2V full scale (or 1.999V to be precise). Op amp IC4 is connected as a unity gain amplifier with level shifting to provide for an offset at the input of the LCD panel meter. The non-inverting input (pin 3) of IC4 takes its DC input from switch S3 and S4 (current limit set) to monitor the current or voltage output. The current monitoring is simple because the output of op amp IC2 is 1V per amp, as already discussed. For the voltage output, we use a voltage divider consisting of a 27kΩ and 300Ω resistors in series with 5kΩ trimpot VR4 which is used for calibration. The second pole of switch S3 selects the decimal points on the LCD panel meter so that it can read up to 1.999A in current mode and 199.9V in voltage mode. In practice, the maximum reading will be around 1.2A in current mode and 40.0V in voltage mode. This means that we have more meter resolution in current mode than in voltage mode. More resolution could be obtained by range switching for the panel meter but we wanted to keep the circuit as simple as possible. Next month we will present the full constructional details of the power SC supply. JUNE 2000  61