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600W DC-DC converter for car hifi systems Despite its heavy-duty circuitry, the 600W DC-DC Converter is easy to build. Provided you correctly follow the step-by-step details for winding the transformer, it should work first time. PART 2: By JOHN CLARKE V IRTUALLY ALL the parts for the 600W DC-DC Converter are mount­ed on a PC board coded 05308961 (310 x 214mm) and this is installed in a 2-unit rack case. A small label affix­es to the front panel to provide the legends for the LED indica­tors. Begin the construction by assembling the case. This done, insert the PC board so that its front edge sits against the front panel. Position the board so that there will be a 16mm gap between the edge of the heatsink (when this is mounted in posi­tion) and the lefthand side of the case. Now mark out and drill 3mm holes 66  Silicon Chip in the base of the case for the seven PC board mounting pillars. One mounting hole is located adjacent to each corner of the board, one is in the centre of toroid inductor L1, another is located between the two bus bars to­wards the rear, and one is adjacent to transformer T1. The next step is to check the various hole sizes on the PC board. Note that 2mm holes are required at all locations where 1.78mm wire is inserted. These include the source connections of Mosfets Q3-Q5 and Q8-Q10, the interconnections to diodes D3-D6, the connections from the 6 x 10µF capacitor bank to the bus bars, and the link from T1 to the centre of the 2200µF capaci­tors. The 2200µF capacitor lead holes need to be 3.5mm diameter, while the mounting holes for the bus bar securing screws need to be 4mm diameter. In addition, 4mm holes are required for the output supply rails (-V, 0V & +V) adjacent to the 2200µF capaci­tors. The mounting holes for fuse F1 should be 8mm diameter. Next, check that all the Mosfet and diode screw mounting holes are 3mm and that the four holes used to secure L1 to the PC board are large enough to accept the cable ties. The holes for the L1a and L1b leads and for the transformer pins need to be at least 1.5mm in diameter. Fuse F2 requires a 2mm hole, while the holes for the Mosfet leads should be about 1.5mm. The large heatsink specified is a fan-type with fins run­ ning down either side of a central flat area. For this project, one set of fins is removed using a hacksaw, so that the heatsink measures 69mm wide. If necessary, the length should be trimmed so that Fig.7: install the parts on the PC board as shown here, taking care to ensure that all polarised parts are correctly oriented. November 1996  67 TABLE 2: CAPACITOR CODES ❏ ❏ ❏ ❏ ❏ Value IEC Code EIA Code 0.47µF   470n   474 0.1µF   100n   104 .0056µF   5n6   562 .001µF   1n0   102 The Mosfet mounting holes should be drilled to 3.5mm, so that they accept insulating bushes (see Fig.8). You also need to drill mounting holes for the thermal cutout switch (TH1). This mounts on one of the fins, as shown in Fig.7. Deburr all holes using an oversize drill and check that the heatsink mounting holes for the Mosfets and power diodes are smooth and free of any metal swarf. PC board assembly Begin construction of the PC board by installing the resis­tors, diodes (except for D3-D6) and ICs, plus trimpot VR1 – see Fig.7. Table 1 shows the resistor colour codes but it is also a good idea to check the values using a digital multimeter before installing them. The wire links associated with the low-current circuitry (bottom of Fig.7) can also be inserted at this stage. Take care with the orientation of the diodes and ICs. The next step is to install six PC stakes to accept the exter­nal low-current wiring connections. Two of these stakes are installed at the TH1 wiring points (7 & 8); two at the fan wiring points (9 & 10); one to accept the +12V ignition lead; and one at the ground point (11). The fuseholder clips for F2 (1A) can Fig.8: this diagram shows the mounting details for the power diodes (top) and the BUK436 Mosfets (bottom). Note that the metal tabs of these devices must be electrically isolated from the heatsink using insulating washers and bushes. the heatsink is exactly 214mm long. File all edges to a smooth finish after cutting. The heatsink can now be positioned on the PC board and the various hole positions marked. Drill 3mm holes at the two mount­ing pillar locations and for the diode mounting screws. TABLE 1: RESISTOR COLOUR CODES ❏ No. ❏  2 ❏  1 ❏  2 ❏  1 ❏  6 ❏  4 ❏  3 ❏  1 ❏  7 ❏  2 ❏  6 68  Silicon Chip Value 1MΩ 470kΩ 47kΩ 27kΩ 10kΩ 6.8kΩ 4.7kΩ 2.2kΩ 10Ω 4.7Ω 1Ω 4-Band Code (1%) brown black green brown yellow violet yellow brown yellow violet orange brown red violet orange brown brown black orange brown blue grey red brown yellow violet red brown red red red brown brown black black brown yellow violet gold brown brown black gold gold 5-Band Code (1%) brown black black yellow brown yellow violet black orange brown yellow violet black red brown red violet black red brown brown black black red brown blue grey black brown brown yellow violet black brown brown red red black brown brown brown black black gold brown yellow violet black silver brown brown black black silver brown Fig.9: this diagram shows the step-by-step winding details for transformer T1. Note that the primary is wound using copper sheet and this must be cut to the shape shown – see text. November 1996  69 This is the completed prototype, ready for installation in the boot of a car. Note that holes must be drilled in the front and rear panels in line with the heatsink, so that the fan can do its job. now be installed. Note that each clip has a small lug at one end to hold the fuse in place, so be sure to install them the correct way around. LEDs 1-3 and the four small-signal transistors (Q1, Q2, Q6 & Q7) go in next. Note that LEDs 1-3 are mounted at full lead length so that they can later be bent over and pushed through the front panel. Take care to ensure that they are oriented correct­ly – the anode lead is the longer of the two. LED 1 is the red LED, while LEDs 2 & 3 are green. Be sure to mount the correct transistor type at each loca­tion. Q1 & Q6 and NPN types while Q2 & Q7 are PNPs, so don’t get them mixed up. At this stage, the capacitors can all be installed on the PC board. Install the small MKT capacitors first (see Table 2 for the codes), then move on to the larger values. The 10µF 100VW capaci­tors between the bus bars are bipolar types and can be mounted either way around. However, the two 10µF 16VW capacitors must be mount­­ ed with the correct polarity, as must the four 2200µF 100VW units. Note that the latter have terminal 70  Silicon Chip numbers on their bases. Pin 1 is the positive terminal, while pin 5 is the negative terminal. Their bodies also have unusual arrow markings down the negative side. Brass link bars To cater for the heavy currents involved in the output stage, the circuit board carries two brass link bars and these are mounted using 3mm screws into tapped holes from the underside of the PC board. Once these bars are in place, run the connec­tions to the adjacent capacitor bank and to the sources of the Mosfets using 1.78mm diameter solid core wire. This same wire should also be used for the interconnections between D3D6 and for the connections between these diodes and transformer T1. In addition, a link using this wire is run from the transformer to the centre of the 2200µF capacitors. Once this wiring has been completed, solder three 4mm nuts to the underside of the PC board at the (+), 0V and (-) output terminal positions near the 2200µF capacitors. This is best done with the nuts attached to their 4mm screws, so that they line up with the board holes correctly. Transformer winding Transformer T1 is wound using copper sheet for the primary and enamelled copper wire for the secondary. Fig.9 shows the details. First, use a pair of tinsnips to cut the copper sheet to size, as shown in step 1 of Fig.9. This done, solder suitable lengths of 3.3mm2 insulated copper wire to the tags as shown in step 2. When doing this, flatten the stripped wire strands with a pair of pliers, so that they sit right down on the tags. Note that where two connections are shown to a tag, it’s best to use a single length of wire bent in half. Remove the insulation from the centre point and bend the wire into a sharp U-shape, so that the leads emerge at right angles from the copper strip. Once all the connections have been made, cover the top of the copper sheet with a layer of insulating tape. Be sure to also cover the soldered tags. This done, label the relevant leads with the numbers 1-6, as indicated on the diagram. You can do this by attaching a small piece of masking tape to each lead and writing on this. Make sure that you don’t get the leads mixed up, otherwise the connections to the drains of the Mosfets will be wrong. The red leads do not need labelling since they are all connected to the positive link bar. The copper strip can now be wound onto the former as shown in step 3 of Fig.9. Start from the top of the former and slide the solder tags into the slotted plastic flanges (you will need to make these slightly wider using a file or a pair of sidecut­ters). This done, wind on two turns and check that the solder tags with the red wires now slot into the flanges on the top of the transformer, along with the solder tags at the start end. Finally, complete the primary by winding on the next two turns, finishing again at the top of the former. Secure the winding with a layer of insulating tape, taking care to ensure that the solder tags are not shorting to each other. The secondary is wound directly over the primary winding. First, check Table 3 for the number of turns required to obtain the desired output voltage from the converter. This done, cut four 1.5-metre lengths of 1.25mm diameter enamelled copper wire and terminate one end of each onto pins 4, 5, 6 & 7, respectively (the wire ends are easily stripped by using a soldering iron to melt the enamel). Now wind on all four wires simultaneously in the direction shown, with each wire sitting directly alongside the others (ie, not jumbled up). Insulate each layer with a layer of electrical tape and continue until the requisite number of turns has been wound on. Terminate the ends onto pins 17, 16, 15 & 14 respective­ly. This done, use a multimeter to check that pin 4 connects to pin 17, pin 5 to pin 16, pin 6 to pin 15 and pin 7 to pin 14. The transformer is now assembled by sliding the cores into the former from each end and fitting the metal clips. Once the transformer has been completed, strip the ends of the primary leads and crimp eyelet lugs to the black leads only. Inductor L1a, L1b Fig.8 shows the winding details for L1a and L1b. These are wound on a common Neosid 17-745-22 ring core using 1.5mm diameter enamelled copper wire. The fan is mounted on the rear panel, in-line with the heatsink, using 9mm tapped brass spacers. Orient the fan as shown here, so that it blows the air out through the holes drilled in the rear panel. sure to wind the coils in the directions shown in Fig.8. Copper strap Fig.10: inductors L1a and L1b are wound on a common toroid former, as shown here. Table 3: Transformer Wiring Required Output Turns On Secondary ±65-70V ±60-65V ±55-60V ±50-55V ±40-50V ±40-45V ±35-40V ±30-35V ±20-30V 12 11 11 10 9 8 7 7 6 You will need about 600mm of wire for each coil on this ring core. Wind on the 14 turns for inductor L1a first, then wind on the turns for L1b. Be The copper strap connecting the link bar to fuse F1 is made from 0.6mm thick copper sheet. Begin by cutting a 75 x 18mm piece, then drill a 12mm hole in one end and an 8mm hole in the other. This done, bolt the end with the 12mm hole to the link bar, as shown on Fig.7. The copper strap is then bent down so that its 8mm hole lines up with the adjacent fuse mounting hole. Fuse F1 can now be mounted in place on the copper strap and secured using an 8mm bolt, nut and washer. An 8mm bolt, nut and washer should also be used to temporarily secure the other end of the fuse. Heatsink mounting The next step in the assembly is to fit the heatsink to the PC board. This is secured at the two main mounting points using 15mm tapped standoffs on the PC board side and 3mm screws from the heatsink side. Next, bend the leads of the Mosfets (Q3-Q5 & Q8-Q10) at right angles so that they go through the PC board holes (check also that their metal tabs line up with the heatsink mounting holes). This done, mount each Mosfet on the heatsink using an insulating washer, bush, spring washer, eyelet lug and nut as shown in Fig.8. The eyelet lugs used are the ones that were previously crimped to the November 1996  71 & 8 (near F2) at the far end of the PC board. The wiring from T1 can now be completed by connecting its red wires to the link bar as shown. The toroid inductor (L1a & L1b) can also be mounted at this stage. It is held in place using two small cable ties which pass through holes in the PC board. Current sensing resistor (Rsc) The Rsc lead is made using a 55mm length of 3.5mm2 wire. Each end is terminated by connecting it to a large eyelet using generous amounts of solder. The lead is then insulated using heatshrink tubing. For the time being, attach one end only to the link bar as shown in Fig.7. This close-up view shows the mounting details for the power diodes, Mosfets and the thermal cutout (TH1). The toroid inductor (L1a & L1b) is secured to the board using a pair of cable ties. black leads from transformer T1. These leads were all numbered, from 1-6. Be sure to connect the correct lead to the metal tab (drain) of each Mosfet – see Fig.7. Note that the type of insulating bush supplied may have a flange attached – if so, this should be cut off using a sharp knife. Note also that if mica washers are used, it will be neces­sary to smear heatsink compound on both sides. If silicone wash­ers are supplied instead, then you do not need heatsink compound. When all the Mosfets are in place, use a multimeter to confirm that their metal tabs are correctly isolated from 72  Silicon Chip the heatsink. If you do find a short, remove the device and correct the problem before proceeding further. The power diodes (D3-D6) are mounted next. These are in­stalled in a similar manner to the Mosfets, again using an insu­lating washer and a bush (with a flange) – see Fig.8. As before, use your multimeter to confirm that the device tabs are correctly isolated from the heatsink. When everything is correct, solder all the Mosfet and diode leads to the PC board. This done, bolt the thermal cutout (TH1) to the heatsink as shown in Fig.11 and use light-duty hookup wire to connect its leads to points 7 Final assembly Begin the final assembly by attaching the PC board to the case baseplate using 15mm standoffs and screws. This done, posi­tion the fan so that its blades line up with the heatsink fins and mark out suitable mounting hole positions on the rear panel. Drill these holes to accept 3mm machine screws. Next, mark out the positions for the Rsc wire connection to the rear panel, the adjacent ground wire connection and the two cable gland holes. These holes can now be drilled or punched, as appropriate. A large hole must also Below: we dressed up the large hole cut for the fan in the rear panel by adding some aluminium trim but this can be considered optional. Make sure all cables are firmly secured. Fig.11: the output cables and the positive battery cable are secured to the rear panel using cable glands. Be sure to use heavy-duty cable where indicated. November 1996  73 Fig.12: this PC etching pattern is shown 71% of actual size. It can easily be reproduced full-size using a photostat machine set to a standard enlargement of 1.41. be cut in the rear panel in line with the fan blades, to allow the air to escape. If necessary, the larger holes can be made by drilling a series of small holes around the inside diameter or 74  Silicon Chip the marked area, then knocking out the centre piece and filing to a smooth finish. The fan can now be mounted on the rear panel using 9mm tapped brass spacers and machine screws. Be sure to orient the fan so that it blows the air out of the case. This done, attach the rear panel to the case and bolt the free end of Rsc to the case, along with the negative battery lead. The cable glands can now be fitted, along with the ground eyelet. The ground eyelet connects to point 11 on the PC board. This connection can be run using medium-duty hook­ up wire. Moving now to the front panel, you will need to drill a series of airflow holes in line with the heatsink (see photo) plus three holes for the indicator LEDs. The latter are best drilled after first attaching the front panel label and this should be carefully positioned to ensure that it lines up with the onboard LEDs. Make the LED indicator holes just large enough to accept the plastic bezels. Once these have been fitted, attach the front panel and push the LEDs into place. All that remains now is to complete the wiring as shown in Fig.11. First, connect the fan to points 9 & 10 on the PC board. To do this, you will need to extend the existing fan leads, taking care to ensure that the joins are well insulated with heatshrink tubing. Next, attach the output leads to the screw terminals using crimp eyelets and 4mm screws, star washers and nuts. The star washers bite into the copper on the PC board, thereby ensuring good contacts. The +12V ignition lead should also be connected at this stage – use red medium-duty hookup wire for easy identifica­tion. The ignition lead and the three output leads pass through one of the cable glands on the rear panel. Tighten this gland firmly to prevent cable movement. Finally, the battery cables can be installed. These are run using heavy-duty 4GA cables (red for positive, black for nega­tive) which terminate into large eyelets. You will need a heavy-duty soldering iron to solder these, if they haven’t already been connected. The positive lead passes through the second cable gland and is bolted to one end of fuse F1, while the negative lead is bolted to the rear panel. Testing If you have a power supply capable of delivering 12V at 1A or more, it can be used to test the inverter. Alternatively, you can use a car battery with F1 initially replaced by a 10A automo­tive fuse. This can be wired in using hookup wire around the 8mm bolts and by using large clips for the battery connection. First, connect the positive and negative leads to the supply terminals, then connect the ignition lead to the positive terminal. The power supply will be loaded down for a few seconds if a low current supply is used as the output capacitors charge. Once the capacitors are charged, the standby current should be around 300mA due to the fan. Check that the power LED and (+) and (-) supply LEDs all light, then adjust VR1 to obtain the correct positive and negative output voltages. Next, check that the converter switches off when the input voltage is reduced to less that 10V. Of course, this is only really practical if you are testing the converter using a vari­able supply. If you are using a battery, test that the unit switches off when pin 2 of IC1 is pulled to ground (you can do this using a test lead but be careful not to short any of the adjacent IC pins). If the 10A fuse blows when a battery is used, the problem probably lies in the transformer wiring. Alternatively, the Mos­fets or diodes may be shorted to the heatsink. Recheck all wiring and component placement if you strike problems. Assuming everything works correctly, reconnect the 63A fuse for F1. The unit can now be installed in a vehicle but be sure to follow these guidelines: (1). The heavy duty supply wiring to the converter must connect directly to the battery terminals. You can purchase battery terminals that will allow the converter connection plus the normal automotive battery wiring. (2). The wires should be run through the engine bay fire­wall via grommets and pass under the vehicle carpet or mats. Entry to the boot should be via grom­mets as well. (3). The ignition connection should be made at the fusebox so that the converter will be powered whenever the ignition is on. Alternatively, a dashboard switch can be installed so that it is turned on separately. (4). Make sure that the polarity is correct when connecting the output supply rails to the amplifier. (5). Run the loudspeaker connections using twin cable. Don’t use the vehicle chassis as a return connection for the loudspeaker since heavy circulating currents can occur within the ground wiring and this could lead to SC noise problems. YOU CAN AFFORD AN INTERNATIONAL SATELLITE TV SYSTEM SATELLITE ENTHUSIASTS STARTER KIT YOUR OWN INTERNATIONAL SYSTEM FROM ONLY: FREE RECEPTION FROM Asiasat II, Gorizont, Palapa, Panamsat, Intelsat HERE'S WHAT YOU GET: ● ● ● ● ● ● 400 channel dual input receiver preprogrammed for all viewable satellites 1.8m solid ground mount dish 20°K LNBF 25m coaxial cable easy set up instructions regular customer newsletters BEWARE OF IMITATORS Direct Importer: AV-COMM PTY. LTD. PO BOX 225, Balgowlah NSW 2093 Tel: (02) 9949 7417 / 9948 2667 Fax: (02) 9949 7095 VISIT OUR INTERNET SITE http://www.avcomm.com.au YES GARRY, please send me more information on international band satellite systems. Name: __________________________________ Address: ________________________________ ____________________P'code: __________ Phone: (_______) ________________________ ACN 002 174 478 November 1996  75