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The CLASSiC-D ±35V DC-DC Converter Delivers up to ±35V & 125W from a 12V battery with high efficiency By JOHN CLARKE This compact DC-DC converter was designed to mate with our CLASSiC-D Amplifier (published in November & December 2012). It presents an efficient way to run the CLASSiC-D amplifier module from a battery to make it a compact powerhouse. Of course, it can also be teamed up with other SILICON CHIP amplifier modules too, if you already have them on hand, and its output voltage can be adjusted over a small range. T HIS DC-DC CONVERTER is designed to deliver ±35V DC supply rails from a 12V DC input. At that setting, it will enable the CLASSiCD Amplifier to deliver some 100W into 4Ω and 60W into 8Ω. This is certainly less than the CLASSiC-D’s 30  Silicon Chip maximum output of 250W when powered from ±55V supply rails but we have chosen this setting as a good compromise between power output and battery life. And while the DC-DC Converter can be used with other power ampli- fier modules which have a similar supply rail requirement, they will not be as efficient as the CLASSiC-D module and therefore will not give you as much audio output for a given battery current. The DC-DC Converter is housed in a siliconchip.com.au FUSE F1 V+ (+35V)  Q1 G E2 S –IN1 IC1 TL494 IC2b Q5* 0V Q4* Q6* D Q2 G E1 Vss Q3* T1 D IC2a Vcc SECO NDARY TH1 THERMAL CUTOUT PRIM ARY +12V S +IN2 V– (–35V) * DIODES SHOWN FOR CLARITY (MOSFETS USED IN FINAL CIRCUIT) VOLTAGE FEEDBACK PWM CONTROLLER DRIVER LOW LOSS FULL WAVE RECTIFIER TRANSFORMER MOSFETS Fig.1: a simplified diagram of the DC-DC Converter. It uses a TL494 switchmode PWM controller (IC1) to drive Mosfets Q1 & Q2 in anti-phase and these drive transformer T1 at about 25kHz. The transformer secondary then drives a rectifier stage to derive ±35V rails. rugged diecast box measuring just 119 x 94 x 57mm. Just add the CLASSiC-D Amplifier module and a 12V SLA battery and you have the basis for a powerful portable PA amplifier or a really punchy busking amplifier, with good battery life. DC-DC converter basics The DC-DC converter works by alternately switching 12V to each half of a centre-tapped transformer primary winding. The resulting AC waveform is then stepped up in the transformer’s centre-tapped secondary, rectified and filtered to provide the plus and minus supply rails. Fig.1 shows the basic schematic of the DC-DC Converter. It operates at a switching frequency of about 25kHz and uses a high-frequency ferrite transformer. Mosfet Q1 drives the top half of the step-up transformer, while Q2 drives the bottom half. The secondary winding’s centre-tapped output is fed to a bridge rectifier and filter capacitor stages to develop the plus and minus DC output rails. The Mosfets are driven via separate drivers, IC2a & IC2b, by a TL494 switchmode chip (IC1) which has feedback to keep the positive DC voltage to a set value (ie, 35V). This feedback controls the width of the pulses applied to the gates of the Mosfets. If the voltage rises above the set value, the width of the gate pulses is reduced and vice versa. The two Mosfets are switched in anti-phase, so that when one half of the winding is conducting, the other is off. Fig.1 shows the rectifiers as diodes siliconchip.com.au Main Features & Specifications Features • • • Compact housing Efficient rectifier circuitry Thermal shutdown • • Fuse protection Power indication Specifications Power supply: 11.5-14.4V using a 12V battery (or 24V with modifications) Power rating: 50W continuous, 125W peak (enables the CLASSiC-D amplifier to deliver up to 100W into 4Ω on normal program material) Standby current: 130mA at 12.6V Standby Current with CLASSiC-D Amplifier connected: 220mA in protect mode; 490mA in run mode with no signal DC supply ripple at 60W load: less than 2V but in reality they are Mosfets, hence the Q numbers (eg, Q3, Q4 etc). The reason for using Mosfets instead of fast recovery diodes is that they are far more efficient, since they have less forward voltage drop than diodes. The circuit also incorporates a low voltage cut-out and over-temperature protection. If the battery voltage drops below 11.5V, the converter switches itself off. This is essential if you are powering the converter from a 12V SLA battery. If these batteries are allowed to discharge much below 11.5V, they will be rendered useless. That can be expensive and frustrating! Over-temperature protection is provided by a thermal cut-out attached to the inside the diecast case. If the case temperature exceeds 60°C, the thermal cut-out opens and the converter shuts down. When it cools sufficiently, normal operation resumes, with no harm done. Circuit details Fig.2 shows the full circuit of the CLASSiC-D DC-DC Converter while Fig.3 shows the internal circuitry of the TL494. It is a fixed frequency pulse width modulation (PWM) controller containing a sawtooth oscillator, two error amplifiers and a PWM comparator. It also includes a dead-time control comparator, a 5V reference and output control options for push-pull or single ended operation. The PWM comparator generates the variable width output pulses by comparing the sawtooth oscillator waveform against the outputs of the two error amplifiers. The error amplifier with the highest output voltage sets the pulse width. May 2013  31 o TH1 (60 C) S1 (OPTIONAL)  13k* CON2 D3 1N4148 A K 2 C1 11 C2 12 1 F Vcc 100 F MMC –IN1 12V INPUT 3 x 4700 F – TP GND 16V LOW ESR* ZD1 A 16V 1W 100nF 10k TP AC1 1 F 1 E2 3 100nF 4.7k 15 TP REF Vss 3 10k F1 S2 S2 PRIMARY SECONDARY (ET029) D 5 4 F1 F2 IC2: TC4427 REF TP AC2 Q2 STP60NF06 10 G F2 S CTRL 10 F 4 100nF X2 9 SECONDARY PRIMARY S 7 S1 S1 Q1 STP60NF06 10 IC2a IC2b E1 13 7 D G –IN2 4.7k 14 6 2 10k 1M 47k 10 FB IC1 TL494 T1 MMC TPVcc +IN1 1M 1M + 10 K 8 CON1 F1 10A* +IN2 DT 47k RT 6 10k CT 5 10k 16 OUTPUT VOLTAGE VR1 100k 100nF 10k 1nF * = VALUES FOR 12V VERSION SC 2013 CLASSIC-D DC-DC CONVERTER Fig.2: the full circuit of the CLASSiC-D DC-DC Converter. It uses Mosfets Q3-Q5 to rectify the AC from transformer T1’s secondary and these are controlled by four IR11672 secondary side driver (SSD) ICs (IC3-IC6). Each SSD monitors the voltage across its Mosfet to determine when to switch the Mosfet on or off via the VGATE output. Pin 13 selects single-ended output or push-pull operation. In our design, push-pull operation is selected and the outputs appear at the transistor emitters, with the collectors tied to the positive supply. Dead-time comparator The dead-time comparator ensures that there is a brief delay between one output going high and the other going low. This means that the outputs at pins 9 & 10 are both low for a short time at the transition points. This dead-time period is essential, since without it, the Mosfet driving one half of the transformer would still be switching off while the other Mosfet would be switching on. This would destroy both Mosfets as they would effectively create a short circuit across the 12V supply. 32  Silicon Chip One of the error amplifiers in IC1 is used to provide the under-voltage protection. Pin 2 monitors the +12V rail via a voltage divider consisting of 10kΩ and 13kΩ resistors. Noninverting input pin 1 connects to IC1’s internal 5V reference at pin 14 via a 4.7kΩ resistor. When the voltage at pin 2 drops below 5V (ie, when the battery voltage drops below 11.5V), the output of the error amplifier goes high and the PWM outputs at pins 9 & 10 go low, thus shutting the circuit down. The 1MΩ resistor between pins 1 & 3 provides a small amount of hysteresis so that the output of the converter does not rapidly switch on and off if the battery is close to the 11.5V threshold. The over-temperature protection operates with a 60°C thermal cut-out (TH1) connected in series between the voltage divider on pin 2 and the positive supply rail. If the case temperature reaches 60°C, TH1 opens and so the circuit shuts down by turning the PWM off. The second error amplifier in IC1 is used to control the output voltage of the DC-DC Converter. This amplifier has its inputs at pins 15 & 16. The feedback voltage is derived from the positive side of the bridge rectifier and is attenuated using a voltage divider consisting of VR1, a series 10kΩ resistor plus a 10kΩ resistor to ground. The resulting voltage is then fed to pin 16 of IC1 and compared to the internal 5V reference which is applied to pin 15 via a 4.7kΩ resistor. Normally, the attenuated feedback voltage should be close to 5V. Should this voltage rise (due to an increase in the output voltage), the output of the error amplifier also rises and siliconchip.com.au K 2 4 1 F MMC 3 1 Vcc OVT VD D IC3 IR11672 VGATE EN MOT 75k VS GND 7 D1 UF4003 TP3 5 10 8 A G S A K D2 UF4003 Q3 IRFB23N15 DPBF 4.7k TP5 2 1 F 4 MMC 6 3 1 Vcc OVT A VD IC5 IR11672 VGATE EN MOT 75k VS GND 7 5 D 8 10 G S  LED1 Q5 IRFB23N15 DPBF K 6 1000 F 35V 100nF LOW ESR CON3 V+ +35V AC1 0V AC2 2 4 1 F MMC 3 1 Vcc OVT VD MOT VGATE 75k VS GND 7 1.5k D IC4 IR11672 EN TP4 5 10 8 G S Q4 IRFB23N15 DPBF K ZD2 15V 1W 6 2 1 F 4 MMC 3 A 75k 1 Vcc OVT VD D IC6 IR11672 VGATE EN MOT GND 7 100nF 5 VS 8 10 G S V– –35V 1000 F 35V LOW ESR Q6 IRFB23N15 DPBF 6 VOLTAGE FEEDBACK STP60NF06, IRFB23N15 D LED 1N4148 ZD1, ZD2 UF4003 A A A K K G K K D A S OUTPUT CONTROL Vcc 13 6 Rt INSIDE THE TL494 OSCILLATOR 5 8 D DEADTIME COMPARATOR Ct Q Q1 FLIP FLOP 0.12V CK 0.7V 9 11 Q Q2 10 DEADTIME 4 CONTROL PWM COMPARATOR 0.7mA ERROR AMP 1 Vcc 12 UV LOCKOUT ERROR AMP 2 4.9V 5V REFERENCE REGULATOR 3.5V 1 2 3 FEEDBACK PWM COMPARATOR INPUT 15 16 14 REF OUTPUT 7 GND Fig.3: the internal circuit of the TL494 Switchmode Pulse Width Modulation (PWM) Controller. It is a fixed-frequency PWM controller containing a sawtooth oscillator, two error amplifiers and a PWM comparator. It also includes a deadtime control comparator, a 5V reference and output control options for push-pull or single-ended operation. siliconchip.com.au May 2013  33 + 100nF 10 F1 10A 4700 F D3 16V T1 100 F 47k 4700 F Q4 TP4 4003 D2 10 1 F MMC Q6 10 ZD2 100nF 75k IC5 1 F MMC 10 DC-DC CONVERTER LOW ESR 11104131 13140111 C RET2013 REV N O C CD- CD + 1000 F 35V IC3 75k TP3 1.5k TP5 Q5 Q3 TP AC1 LOW ESR 0V V– D1 10 100nF 75k 1000 F 35V + CON3 V+ 16V 10k 4003 TP GND VR1 100k 75k 10k 10k 15V 10k + 1nF TP AC2 1M 10k IC1 10k 1 F MMC 1M 1M 47k 4.7k 13k 16V 4148 ZD1 16V 16V + 4.7k IC2 10 F TL494D LED1 4700 F 4.7k 3x 100nF A TC4427 10 10 – TP REF TP Vcc Q2 STP16NF06 1 F MMC CON2 S1 Q1 + TH1 12V CON1 IC4 IC6 IC3, IC4, IC5 & IC6: IR11672 Fig.4: install the parts on the PCB as shown on this layout diagram, starting with the SMD ICs (IC1-IC6). Be sure to orientate the ICs, Mosfets, diodes zener diodes and electrolytic capacitors correctly this reduces the output pulse width. Conversely, if the output falls, the error amplifier output also falls and the pulse width increases. The gain of the error amplifier at low frequencies is set by the 1MΩ feedback resistor between pins 3 & 15 and by the 4.7kΩ resistor to pin 14 (VREF). These set the gain to about 213. At higher frequencies, the gain is set to about 9.5 by virtue of the 47kΩ resistor and 100nF capacitor in series across the 1MΩ resistor. This reduction in gain at higher frequencies prevents the amplifier from responding to hash on the supply rails and ensures stability. The 10kΩ resistor and 1nF capacitor at pins 6 & 5 respectively set the internal oscillator to about 50kHz. An internal flipflop divides this by two to give the complementary 25kHz output signals at pins 9 & 10. Note that while most of the inverter circuitry could run at much higher speed, “skin effect” in the windings of the ferrite-cored inverter transformer set the practical limit for switching the Mosfets to around 25kHz. Pin 4 of IC1 is the dead-time control input. When this input is at the same level as VREF, the output transistors are off. As pin 4 drops to 0V, the dead-time decreases to a minimum. At switch on, the 10µF capacitor between VREF (pin 14) and pin 4 is discharged. This 34  Silicon Chip prevents the output transistors in IC1 from switching on. The 10µF capacitor then charges via the 47kΩ resistor and so the duty cycle of the output transistors slowly increases until full control is gained by the error amplifier. This effectively provides a soft start for the converter. The complementary PWM outputs at pins 10 & 9 of IC1 are fed to Mosfet drivers IC2a and IC2b which drive the gates of Q1 and Q2. Note also the 100nF capacitor and the three 4700µF low-ESR capacitors between the centre tap of the transformer primary and the ground. These are included to cancel out the inductance of the leads which carry current to the transformer. They effectively provide the peak current required from the transformer as it switches. Mosfet rectification As previously mentioned, the AC from the transformer secondary is rectified by Mosfets instead of a conventional diode bridge. This increases the overall efficiency of the DC-DC Converter. The rectification process employs both the intrinsic diodes of the Mosfets and their normal channel conduction. The intrinsic diode in a Mosfet is a reverse-connected diode that is part of the substrate layer. Originally, these intrinsic diodes were notoriously slow acting but are now quite fast. Now if the Mosfets were prevented from conducting, their intrinsic diodes are connected to operate in the same way as a conventional bridge rectifier. The Mosfets themselves are then controlled to act as “helpers” for each diode, switching on when the intrinsic diodes begin to conduct and switching off just before reverse conduction. Each Mosfet is controlled using an IR11672 secondary side driver (SSD). Each SSD monitors the voltage across its Mosfet to determine when to switch the Mosfet on or off via its VGATE output. When the voltage between drain and source is greater than -50mV, the Mosfet is switched on to bypass the intrinsic diode. When the voltage drops below -6mV, the Mosfet is switched off. Using the Mosfets saves valuable power compared to conventional diode rectifiers. For example, at a current of 3.5A, a Vishay V10150C Schottky diode would have a forward voltage close to 0.9V, resulting in a power loss of 3.15W for each diode. By using the specified IRFB23N15 Mosfets, the voltage drop at 3.5A is less at 0.25V, giving a power loss of 875mW. Overall, the Schottky diode rectification would have a 6.3W loss compared to 1.75W for the Mosfet rectifiers; remember that only two diodes are conducting at any one time. The low power dissipation means that these Mosfets do not require heatsinking and the higher efficiency means less battery current for a given power output. Of course, there is some power loss associated with the Mosfet drivers. This amounts to about 267mW for the four devices in the bridge. The IR11672 includes a minimum on-period to prevent the Mosfet switching off immediately it switches on, which could otherwise happen due to the decreased voltage between drain and source. The minimum on time is set by the resistance at the MOT (Minimum On Time) terminal. Using the 75kΩ resistor, this is around 3μs. Note that the IR11672 is designed for high-frequency switchmode supply rectification up to 500kHz. Power for each IR11672 is derived from the -35V supply rail via a 1.5kΩ resistor that feeds 15V zener diode ZD2. The initial -35V supply is obtained by the rectification provided siliconchip.com.au by the intrinsic diodes in the Mosfets. Then, as each IR11672 receives a supply, rectification using the switched Mosfets begins. Both IC4 and IC6 share the same common 15V supply via ZD2. This is possible because these ICs also share the common -35V supply as their negative rail. The supply for IC3 & IC5 is derived via diodes D1 & D2 respectively. When Mosfet Q4 is switched on, Q3’s source is pulled to the -35V supply rail and so power from ZD2 can flow through D1 to charge the 1µF supply capacitor for IC3. Similarly, when Q6 is switched on, Q5’s source is pulled to the -35V supply and IC5’s supply capacitor is charged from ZD2 via D2. Indicator LED (LED1) provides power indication. It also serves as a minimum load for the +35V supply. This minimum load is required to match the load on the -35V supply that delivers power to zener diode ZD2. Since it is the +35V supply that is monitored with IC1 for voltage regulation, the minimum load ensures that the PWM drive to maintain voltage regulation is sufficient to maintain the -35V supply. For correct operation, it is important that this minimum load is not disconnected. So if LED indication is not required, the LED connections on the PCB should be bridged to ensure that the LED resistor is still connected between the +35V supply and ground. Construction All the parts for the CLASSiC DC-DC Converter are mounted on a double-sided PCB coded 11104131 and measuring 110 x 85mm. This fits neatly inside a metal diecast case measuring 119 x 94 x 57mm. The diecast case not only makes for a rugged assembly but also provides shielding plus heatsinking for Q1 & Q2. This view shows the completed PCB assembly. It’s earthed to the metal case via an earth lead soldered to TP GND. solder pin 1 first. That done, check that the device is correctly aligned. If not, remelt the solder and adjust it as necessary. The remaining pins are then soldered, starting with the diagonally opposite pin (pin 16 or pin 8), after which you should resolder pin 1. Don’t worry if you get solder bridges between adjacent pins during this process. These bridges can be quickly cleared using solder wick – just press the solder wick against the bridge using a hot soldering iron. A dab of noclean flux paste will aid this process. Once all the ICs are soldered in, the next step is to install the remaining low-profile parts. Note that component values shown on Fig.4 are for a 12V supply. If you wish to use a 24V supply, then it will be necessary to change a few component values, as detailed in the accompanying panel. Start with the resistors, diodes and zener diodes. Table 1 shows the resis- CAUTION It’s a good idea to switch off and let the 1000μF output filter capacitors discharge (ie, blue LED out) before connecting (or disconnecting) this DC-DC Converter to an amplifier. It’s also a good idea to avoid touching the ±35V (70V total) supply rails during operation to avoid the possibility of a shock. Fig.4 shows the parts layout on the PCB. Begin the assembly by installing IC1-IC6. These are all SMDs in SOIC packages and are quite easy to solder in place due to their (relatively) wide 0.05-inch pin spacing. Each IC is mounted on the top of the PCB and must be orientated as shown on the overlay diagram of Fig.4. To solder an IC in place, align its leads over the PCB pads and tack Table 1: Resistor Colour Codes o o o o o o o o o siliconchip.com.au No.   3   4   2   1   7   3   1   7 Value 1MΩ 75kΩ 47kΩ 13kΩ 10kΩ 4.7kΩ 1.5kΩ 10Ω 4-Band Code (1%) brown black green brown violet green orange brown yellow violet orange brown brown orange orange brown brown black orange brown yellow violet red brown brown green red brown brown black black brown 5-Band Code (1%) brown black black yellow brown violet green black red brown yellow violet black red brown brown orange black red brown brown black black red brown yellow violet black brown brown brown green black brown brown brown black black gold brown May 2013  35 DIGITAL MULTIMETER DIGITAL MULTIMETER 0.05 20A 2A COM 0.03  20A F2 S2 F1 S1 2A COM  S2 F1 F2 1 WIND 21 BIFILAR TURNS OF SECONDARY (1.0mm ECW) IN THREE LAYERS, THEN COVER WITH PVC TAPE S1 2 WIND 7 BIFILAR TURNS OF PRIMARY (1.25mm ECW) IN ONE LAYER, THEN COVER WITH PVC TAPE CPH-ETD29-1S-13P BOBBIN (VIEWED FROM UNDERNEATH) Fig.5: the winding details for transformer T1. The secondary is wound first using 21 bifilar turns of 1mm-diameter enamelled copper wire and is covered with a single layer of insulation tape. The primary is then wound on using seven bifilar turns of 1.25mm enamelled copper wire – see text. Running The DC-DC Converter From 24V Although we have not tested this DC-DC Converter at 24V, it can be done with some circuit changes. However, 24V operation is not ideal because the winding wire needs to be a smaller diameter so that the extra turns required can fit on the transformer bobbin. For 24V operation, the secondary is wound with 21 turns of 0.8mm enamelled copper wire. The primary is then wound with 14 turns of 1mm enamelled copper wire. Note that this has to be run in two layers and so once completed, the wires will need to be run back across to the other side of the bobbin (ie, at right angles to the windings on the underside) to return the wire to the finish terminals. In addition, the fuse must be changed to 5A, the capacitors changed from 4700µF 16V to 1000µF 35V, the 10Ω resistor for ZD1 changed to 1kΩ and the 13kΩ resistor at pin 2 of IC1 changed to 36kΩ. The parts list below shows the new parts. Parts List Changes For 24V Operation 1 M205 5A fast blow fuse (F1) (instead of 10A) 5 1000µF 35V (instead of 3 x 4700µF 16V PC low-ESR electrolytic and 2 x 1000µF 35V PC low-ESR electrolytic) 1 1kΩ 0.25W resistor for ZD1 (instead of 10Ω) tor colour codes but you should also check the values with a multimeter, as some colours can be difficult to distinguish. Be sure to orientate the diodes and zener diodes as shown on Fig.4. The 36  Silicon Chip 1 36kΩ 0.25W resistor (instead of 13kΩ at pin 2, IC1) 1 2.6m length of 0.8mm-diameter enamelled copper wire for T1’s secondary 1 1.8m length of 1mm-diameter enamelled copper wire for T1’s primary zener diode type numbers are shown in the parts list. The PC stake at TP GND is next on the list, followed by LED1. The latter is mounted with its leads bent down by 90°, so that its lens can later pushed through a matching hole in the side of the case. To install it, bend its leads down about 3mm from its body, then solder it in position so that the centre line of its body sits about 9mm above the PCB. Be sure to install the LED with the correct orientation. Its anode lead is the longer of the two. Mosfets Q1-Q6 can now go in. These should be installed so that the tops of their metal tabs are 20-25mm above the PCB. Follow with the capacitors. The electrolytic types must all be orientated with the correct polarity (ie, with the negative side towards the left edge of the PCB). Once they’re in, install trimpot VR1, then fit screw terminal blocks CON1, CON2 & CON3. Now fit the fuse clips. These each have an end stop at one end, so that the fuse will not slip out when installed. Make sure these end stops go to the outside, otherwise you will not be able to later install the fuse. Transformer winding The PCB assembly can now be completed by winding and fitting the transformer. Fig.5 shows the winding details for the 12V version (refer to the accompanying panel for the winding details for the 24V version). The secondary windings are wound on the bobbin first. Begin by cutting a 2.6m length of 1mm-diameter enamelled copper wire into two 1.3m lengths. That done, strip 5mm of the enamel insulation from one end of each wire using a hobby knife, then solder these wires to terminals S1 & S2 (start) as shown in Fig.5 (these go on the side with the seven terminals). Now carefully wind on seven bifilar turns (ie, both wires laid side by side) to the opposite side of the bobbin, then another seven turns back towards the start terminals and finally another seven turns back to the opposite side (ie, 21 bifilar turns in all). Once all the turns are on, secure them in place using a single layer of insulation tape, cut to fit the width of the bobbin. Now set your multimeter to read ohms and use it to determine which wire is connected to S1. That done, trim this wire to length, strip 5mm of enamel insulation from the end and solder it to terminal F1. The other wire is then connected to F2. Finally, use your multimeter to confirm that there is close to zero ohms siliconchip.com.au between S1 and F1 and close to zero ohms between S2 and F2. Check also that there is a high impedance (>1MΩ) between the windings, eg, between S1 and S2. The primary winding is also bifilar wound but consists of just seven turns of 1.25mm enamelled copper wire. Note that the orientation of the bobbin is also important when installing this winding. First, check that the bobbin is orientated so that the side with the six terminals is to the left, as shown in Fig.5 (ie, with the terminals facing towards you). That done, cut a 900mm length of 1.25mm enamelled copper wire in half, strip one end of each wire and solder them to the primary S1 & S2 terminals. Now wind on seven bifilar turns in the direction shown, taking care to ensure that the wires are close together (otherwise they won’t fit into the bobbin). Cover this winding with another layer of insulation tape, then identify which wire connects to S1 and connect it to F1. The other wire is then connected to terminal F2. Note that the primary F1 and S1 terminals are diagonally opposite each other, as are S2 and F2. By contrast, S1 and F1 are directly opposite each other for the secondary winding (as are S2 and F2). Once again, use a multimeter to confirm that S1 and F1 are connected, that S2 and F2 are connected, and that there is a very high impedance between the two windings. Check also that there is no connection between any of the primary and secondary windings. Once the windings are in place, the transformer assembly is completed by sliding the two ferrite cores into the bobbin and securing them in place using the supplied clips. The transformer can then be installed on the PCB. Par t s Lis t 1 double-sided PCB, code 11104131, 110 x 85mm 1 diecast box, 119 x 94 x 57mm (Jaycar HB-5064 or equivalent) 1 ETD29 transformer (T1) (1 x 13-pin former [element14 Cat. 1422746], 2 x N87 cores [element14 Cat. 1781873], 2 x clips [element14 Cat. 178507] 1 thermostat switch (60°C, normally closed) (Jaycar ST3821, Altronics S5600) (TH1) 2 IP68 cable glands, 4-8mm cable diameter 1 2-way screw terminals (5.08mm pitch) (CON1) 2 3-way screw terminals (5.08mm pitch) (CON2,CON3) 2 M205 PCB-mount fuse clips 1 M205 10A fast-blow fuse (F1) 1 SPST or SPDT toggle switch (S1) (optional – see text) 4 M3 x 9mm tapped spacers 2 TO-220 silicone insulation washers 2 insulating bushes 2 M3 x 10mm screws 6 M3 x 6mm screws 4 M3 x 6mm countersunk screws 4 M3 nuts 1 solder lug 1 2.6m length of 1mm enamelled copper wire (for T1 secondary) 1 900mm length of 1.25mm enamelled copper wire (for T1 primary) 1 length of 24/0.2mm (0.75mm2 cross section) figure-8 cable 3 lengths of 19/0.18mm (0.48mm2 cross section) or 14/0.2mm (0.44mm) wire 1 200mm length of medium-duty hookup wire 1 PC stake (TP GND) Semiconductors 1 TL494CDR SOIC-16 Switchmode Pulse Width Modulation Controller (IC1) 1 TC4427ACOA SOIC-8 Dual Mosfet Driver (IC2) (element14 Cat. 1467705) 4 IR11672ASPBF SOIC-8 Smart Rectifier Controller (IC3-IC6) (element14 Cat. 1827123) 2 STP60NF06 N-channel Mosfets (Q1,Q2) 4 IRFB23N15DPBF 150V, 23A N-channel Mosfets (Q3-Q6) (element14 Cat. 8648735) 2 UF4003 fast rectifier diodes (D1,D2) 1 1N4148 switching diode (D3) 1 16V 1W zener diode (1N4745) (ZD1) 1 15V 1W zener diode (1N4744) (ZD2) 1 3mm blue LED (LED1) Capacitors 3 4700µF 16V low-ESR electrolytic 2 1000µF 35V low-ESR electrolytic 1 100µF 16V electrolytic 1 10µF 16V electrolytic 6 1µF 50V monolithic multilayer ceramic (MMC) 1 100nF X2 class 275VAC MKP metallised polypropylene 5 100nF 63/100V MKT 1 1nF 63/100V MKT Resistors (0.25W, 1%) 3 1MΩ 6 10kΩ 4 75kΩ 3 4.7kΩ 2 47kΩ 1 1.5kΩ 1 13kΩ 7 10Ω 1 100kΩ mini horizontal trimpot (VR1) Preparing the case You now have to drill holes in the diecast box to mount the PCB and to mount Q1 & Q2 and the thermal switch. Another hole is required for the LED, while two large holes are required to accept cable glands. First, sit the PCB assembly inside the box and mark out the four mounting holes. Drill these out to 3mm in diameter and countersink them from the outside to suit the specified countersunk screws. That done, attach four M3 x 9mm siliconchip.com.au Nylon spacers to the PCB assembly using M3 x 6mm screws, then sit the PCB inside the diecast box. Once it’s in position, mark out the mounting holes for the tabs of Mosfets Q1 & Q2 plus a hole at one end to accept the indicator LED. Drill these out to 3mm in diameter, then slightly countersink the holes for Q1 & Q2 to remove any sharp edges. This is necessary to prevent damage to the silicone insulating washers that fit between the Mosfet tabs and the case (a sharp edge could puncture a washer and short a metal tab to the case). The cable glands are placed 15mm down from the top of the case and 20mm in from the sides (see photo). The thermal cut-out is mounted midway between the two cable glands, with its top mounting hole 7mm down from the top edge of the case. It’s a good idea to solder an M3 nut to one lug of the thermal cut-out. This can then be used in the lower mounting position, making the unit easier to May 2013  37 SILICONE WASHER INSULATING BUSH 10mm LONG M3 SCREW M3 NUT Q1, Q2 PCB REAR OF CASE Fig.6: the mounting details for Mosfets Q1 & Q2. The metal tab of each device must be isolated from the case using an insulating bush and a silicone washer. Using The Converter To Power The SC480 Amplifier If you want to run a pair of SC480 amplifier modules using this DCDC Converter, you can do so but they will give slightly less than their specified power output since they were originally designed to run from ±40V rails. However, they will run quite happily from ±35V. attach when the PCB is in place. Once all the holes have been drilled, install the PCB assembly in the case and secure it using four countersunk screws. Attaching Q1 & Q2 Q1 & Q2 are each attached to the side of the case using an M3 x 10mm screw and nut, along with a silicone insulating washer and an insulating bush. Fig.6 shows the details. Do the screws up firmly, then use a multimeter to check that both tabs are correctly isolated from the case. You can do this by measuring the resistance between the case and the Mosfet tabs. You should get a high ohms reading in each case but the meter may initially show a low ohms reading as various on-board capacitors charge up when the probes are connected. A permanent zero ohms reading means that there is a short which has to be fixed. The case itself is earthed to the GND 38  Silicon Chip Modifying The CLASSiC-D Amplifier For ±35V Rails As presented in the November and December 2012 issues of SILICON CHIP, the CLASSiC-D Amplifier is designed for ±50V (or ±55V) supply rails. However, if you intend using this DC-DC Converter to power the amplifier, you need to make a few changes to the amplifier to suit the converter’s lower ±35V supply rails. This involves changing several resistors and zener diodes, as shown in Table 1 on page 68 of the December 2012 issue (ie, in the article describing the construction of the CLASSiC-D amplifier module). The new zener diode type numbers are also shown in this table. Once the necessary parts have been changed in the amplifier, the supply wires from the DC-DC Converter can be connected to it using three lengths of 19/0.18mm (0.48mm2 cross section) or 14/0.2mm (0.44mm2) wire. Make sure the connections are made with the correct polarity. PC stake on the PCB via a short length of hook-up wire. That’s done by first attaching a solder lug to one end of the wire, then attaching this to the case using the same mounting screw that’s used to attach the top lug of the thermal cut-out. The other end of the wire is then soldered to the GND stake. Once it’s in place, fasten the bottom mounting lug of the thermal cut-out to the case, then solder two 80mm-long leads to its terminals and insulate these with heatshrink. The other ends of these leads can then be stripped and connected to the TH1 terminals on CON2. The S1 switch terminals on CON2 can either be connected to an external switch or simply bridged with a short piece of tinned copper wire. The switch (or bridging wire) does not carry significant current (less than 50mA), since it doesn’t carry the full DC-DC Converter current. Basically, S1 is will probably only be needed if there’s no power switch for the external power supply. Completing the assembly The assembly can now to completed by installing fuse F1 and connecting the power supply leads. The supply leads can be made using a suitable length of 24/0.2mm (0.75mm2) figure-8 wire. Connect the striped lead to the negative terminal of CON1 and the other lead to the positive terminal. You can use a pair of needle-nose pliers to push the wires into their terminals on CON1. Testing Before connecting the external supply, go over the assembly carefully and check that the parts are all correctly positioned. In particular, check that the electrolytic capacitors are the right way around as these things have a nasty habit of exploding if they are installed with reverse polarity. That done, wind trimpot VR1 fully anticlockwise, then fit the lid on the case (just in case an electrolytic is in the wrong way around). If possible, use a current-regulated power supply to initially test the DC-DC Converter. If you don’t have one, then a non-regulated supply or a 12V battery can be used. Be sure to get the supply polarity correct; if you connect it the wrong way around, the fuse will blow. Once it’s hooked up, apply power and let the unit run for several minutes. If it powers up safely (ie, no explosions from capacitors), you can then remove the lid and check the voltages between the 0V and the +35V and -35V terminals on CON3. With VR1 wound fully anticlockwise, you should get around +10V and -10V on these terminals. Assuming all is well, carefully rotate VR1 clockwise until you get +35V and -35V readings. Do not set the outputs any higher than ±35V, as the output capacitors are not rated for higher voltages (ie, they only have a 35V rating). Finally, the three output leads can be made up using 24/0.2mm wire and connected to CON3. The other ends of these leads can then be fitted with coloured heatshrink sleeves to identify them: red for +35V, green for 0V (GND) and blue for -35V. You new DC-DC Converter is now ready for use with the CLASSiC-D Amplifier. However, before connecting it up, the amplifier needs a few minor modifications in order to operate from ±35V rails – see the above panel. SC siliconchip.com.au