Fullscreen HTML5Med Res (??MB)HTML4

A 2kW 24V/240VAC sinewave inverter; Pt.4 This month, we will begin construction of the 2kW inverter by presenting the· assembly details for the four main PC boards. Each PC board can be tested so that any faults can be fixed before they are wired into the chassis. By JOHN CLARKE Make no mistake, this is a major construction project and should not be attempted unless you are an experienced constructor. You will need access to a dual trace oscilloscope, a digital multimeter, an adjustable power supply capable of 24V at 1A, a crimping tool , plus light and heavy duty soldering irons. Also a small pro58 S1LICON CHIP pane torch will be necessary for soldering lugs to 150A cable unless an hydraulic crimping machine is available. We will begin by discussing the assembly of the four PC boards. These are the DC-DC Inverter Board, the HPack Board, the Sinewave Generator Board and the Rectifier Board. Check each PC board for shorted tracks, open circuit tracks and other faults which may be evident on a visual inspection. Repairs can be made to shorted tracks by cutting away copper with a sharp utility knife, while open circuit tracks can be repaired by soldering a length of tinned copper wire across the break. Look at the overlay diagrams and check that the holes in the PC boards are the correct size for mounting the components or hardware. Most of the component holes should be 1mm diameter while the PC board mounting and regulator tab mounting holes should be 3.5mm diameter. Holes for the 4mm screws (for the cable lug connections) should be 4mm in dia- *SOLDER 4mm NUT TO COPPER SIDE Fig.12: parts layout on the DC-DC Inverter Board. Install all the parts as shown except for the Mosfet transistors and the lMQ resistor marked "see text". This resistor is installed later only ifit is necessary to increase the "dead-time". meter. Drill or ream out any holes which need to be enlarged before you install any of the components. Most of the links and all low wattage resistors for these PC boards are spaced on 12.7mm (0.5 inch) centres and it may be expedient to make up a small jig so that the resistor leads and the link leads can be bent to match these centres. Do not use IC sockets We do not recommend using IC sockets for any of the ICs in this project. Apart from the fact that quality types can cost more than the ICs them selves, they can reduce the reliability of the circuit. Readers may notice from the photographs that we used a socket for IC23; ie, the OTP-ROM (One Time Programmable ROM. However, this was only so that we could easily remove and replace the OTPROM during the circuit development. DC-DC inverter board Let's now start with the DC-DC In- verter Board which is coded 11309921 and measures 204 x 15 7mm. The component overlay is shown in Fig.12. Note the 12 large pads, six on each side of the board. These are drilled to accept a 4mm machine screw. You will need to solder a brass nut to each of these large copper pads. This is best done by fitting all the screws and nuts in position on the board. Each nut is then soldered to the copper pad using a heavy duty iron. It is also a good idea at this stage to tin the Gate, Source and Drain pads for the Mosfets (Q5-Q16). Now insert the nine PC stakes and solder them in place. The 3-terminal regulator (REG1) is mounted flat on the PC board and secured with a screw and nut. Bend the leads first so that they fit precisely into the IN, GND and OUT pads when the regulator is secured in place. The links and low wattage resistors can now be soldered in place but omit the 1MQ resistor marked "see text" at this stage. This done, install the three ICs, taking care with their orientation. When soldering, check that adjacent pins are not bridged except where dictated by the copper pattern. This can easily happen with a medium size soldering iron tip. The capacitors can now be all installed, followed by trim pot VR1, transistors Q1-Q4, the diodes and zener diodes. Make sure that these components are all installed the right way around. The 82Q 5W wirewound resistor should be mounted so that its body is about 1mm above the PC board to allow the air to circulate beneath it for cooling. Finally, install the inductor (Ll). Do not insert the Mosfets at this stage - that step comes later. H-pack board Fig.13· shows the parts layout on the H-Pack Board. It measures 204 x 157mm and is coded 11309922. Assembly of the H-Pack Board begins in the same way as the previous board; ie, by soldering the brass nuts to the large pads on the copper side. There are 16 insulated links on the board and these are made using 15A mains-rated cable. These wires carry JANUARY 1993 59 CURVED LINKS 15A MAINS WIRE *SOLDER 4mm BRASS NUTS TO COPPER SIDE Fig.13: parts layout for the H-Pack Drive Board. Note that the 16 curved links must be run using 15A insulated mains wire. The four IGBTs & the four diodes are moun~ed on the underside of the PC board (see Fig.15). the heavy current from the IGBTs and diodes to the H-drive terminals (+365V, 0V, X and Y). Once these leads are in, the 14 PC stakes can be installed, followed by the links, resistors, ICs, diodes and zener diodes. The capacitors and varistors (the S14K275 devices) can then be installed, followed by the BOBBIN 12T CLOSEWOUND 10T CLOSEWOUND T2· H WINDINGS WRAP WITH INSULATING TAPE Fig.14: here are the winding details for transformers T2-T4. Use 0.125mm enamelled copper wire & make sure that there is at least 5mm clearance between the two windings'. 60 SILICON CHIP MOVs. The latter can be oriented either way but the electrolytic capacitors must be oriented as shown in Fig.13. Winding the transformers Transformers T2, T3 and T4 are wound using 0.125mm enamelled copper wire - see Fig.14. Each transformer bobbin is wound in the same way. The windings are 10 turns between pins 1 and 4 for the primary and 12 turns between pins 5 and 8 for the secondary, on a Philips EFD 15/8/ 5 bobbin. Wind the turns as close together as possible and solder the leads to the outside pins. The actual direction that each winding is wound is not critical but the completed bobbin should have at least a 5mm clearance between the primary and secondary windings to provide high voltage isolation. Complete the job by winding a couple of layers of insulation tape around the bobbin to keep the turns in place. Once the bobbins are wound, insert the two ferrite core halves and secure them in place with the spring clips. The three finished transformers can then be installed on the PC board, taking care to ensure correct pin orientation. The four IGBTs (Q17-Q20) and the four diodes (D10-D13) are mounted on the underside of the PC board as shown in the mounting detail diagram of Fig.15 (this diagram also applies to Mosfets Q5-Q16 on the DCDC Inverter Board). To install these devices, first bend their leads at right angles 3mm from their bodies as shown in Fig.15. This done, attach four 9mm standoffs to the PC board, then install and solder each device so that its face sits flat against the resting surface (ie, the workbench). Siiiewave generator board The Sinewave Generator Board measures 181 x 131mm and is coded 11309923. Its component overlay diagram is shown in Fig.16. You can begin this board by installing the 17 PC stakes, followed by the wire links. The remaining parts can then be installed. ~----3mm Note that the 7805 regulator (REG2) is fitted with a finned heatsink and is secured to the PC board with a screw and nut. Smear the metal tab of the regulator with heatsink compound before bolting the assembly together. The 150Q 5W resistor is mounted about 1mm above the PC board to allow the air to circulate beneath it for cooling. Rectifier PC board This board mainly functions as an assembly platform for the two high voltage capacitors (680µF 400VW & 25µF 370VAC), the three large toroids (12 , 13 & L4), and a terminating panel. Apart from that, it incorporates the fast rectifiers (D3-D6) and the optocoupled feedback circuitry for the DCDC inverter, involving IC4 and IC5. The board measures 214 x 162mm and is coded 11309924. The first step is to solder the seven 4mm brass nuts to the relevant pads on the copper side of the PC board. The heavy current links must be run using 15A mains-rated wire. Keep Fig.15: the IGBTs & diodes are installed on the underside of Hpack board as shown in this diagram. Make sure that the mounting face of each device is exactly 9mm below the bottom of the PC board. these links as short as practicable. Now install the fuse clips, the tinned copper wire links, the resistors and zener diode and, finally, the two ICs. The DC filter choke toroid, 12, is wound with six turns of 2mm enamelled copper wire spread evenly around the core. Strip the enamel insulation from the wire ends, tin them with solder and then solder them to the PC board. The finished toroid is anchored to the PC board using two small cable ties. Solder in the four fast recovery diodes (D3 -D6) but do not trim their leads under the board yet since their tabs have to be aligned with matching holes in chassis when the board is finally installed. Inductors 13 and 14 and the two high voltage capacitors are not installed at this stage. Testing DC-DC Inverter Board: to test the in- verter board, you will require a power supply capable of delivering 24V DC at 0.5A. You will also need a dual trace oscilloscope and a multimeter. Bridge the pins for the thermal cutouts and connect your DC power supply to the +24V and GND IN pins. Connect your scope probes to the 4. 7Q resistors at the emitters of Ql and Q2 and the emitters of Q3 and Q4 respectively. Now apply power; the oscilloscope should show two 20kHz square waves with an amplitude of 15V peak. The waveforms are as shown in Fig.9 which is on page 30 of the November 1992 issue. If no waveforms are present, you will have to backtrack through the circuit to find the malfunction. First check the outputs of inverter IC2 and then the outputs of ICl (pins 9 & 10). If nothing still, check that the 15V supply is present at the relevant pins of ICl and IC2. Now switch off the power and switch on again to observe the soft start feature. Each waveform should start off with a low duty cycle and increase to the maximum duty cycle of slightly less than 50% in about one second. Check the "dead time" between each waveform. This is measured as the time between the falling edge of one waveform and the rising edge of the other. It should be about 5µs. If the dead time is less than lµs, then the lMQ resistor between pins 4 & 14 of Fig.16: parts layout on the Sinewave Generator Board. Make sure that all polarised parts are correctly oriented & use PC stakes for the 17 external wiring connections. Don't use sockets for any of the I Cs. JANUARY 1993 61 CURVED LINKS ARE 15A MAINS WIRE *SOLDER 3mm BRASS NUTS TO COPPER SIDE Fig.17: parts layout for the Rectifier PC Board, minus L3, L4 & the two high voltage capacitors. As on the H-Pack Board, the curved links must be run using 15A mains-rated cable. ICl should be installed. This will increase the dead time. Test the under voltage cutout by winding down the power supply. At a voltage of around 20V from the power supply, the circuit should stop operating. The feedback control is tested by connecting a lOOkQ resistor between the "A" pin and the +15V pin. Adjust This close-up view shows the H-Pack Board. The tµF 500VAC filter capacitor that straddles the centre of the board will be added next month when the chassis wiring is described. 62 SILICON CHIP VRl slowly to check that the duty cycle can be varied from maximum to minimum. Note that maintaining a steady duty cycle with VRl is very difficult but this test will confirm that the control circuitry is operating. Finally, the Mosfets (Q5-Q16) can be installed. These are mounted at right angles on the underside of the PC board as shown in Fig.15. Sinewave Generator Board: connect a DC supply set to 15V between the GND and +15V pins on the PC board. Connect your oscilloscope probes to the A and B pins and check that these two waveforms are 180° out of phase and operating at about 1MHz. Similarly check that the E & F outputs-and the C & D outputs are also at 1MHz and 180° out of phase. If there is a fault here, check for correct supply voltages on IC14, IC15 and IC16. By the way, the supply rail for these three ICs was incorrectly shown as +5V on the circuit of page 70 in the December 1992 issue. It should be +15V. Disconnect the supply from the Fig.18: these 5V p-p waveforms should be present on the collectors ofQ21 (top) & Q23 (bottom). +15V pin and then bridge the G and H pins, the Kand L pins and the Mand N pins respectively. Now connect a 24V supply to the +24V pin and GND pins. Check that +5V is present on the relevant pins of ICl 7-IC23. To check the signals present on the collectors of QZl and Q23, you will need to trigger your oscilloscope externally using the 50Hz signal present at pin 1 of IC17. You should obtain signals similar to those shown in Fig.18. The top trace shows the signal at the collector ofQZl while the lower trace shows the signal at the collector ofQ23. A similar waveform should be seen at the collectors ofQ22 and Q24 when the external trigger is set to the opposite trigger polarity (see Fig.18) Note that these waveforms show only the active lOms half of the ZOms period. For the other lOms period, the collector ofQ21 is low and the collector of Q23 is high. The same remarks apply to QZZ and Q24. H-Pack Board: this board can only be tested when wired to the Sinewave Generator Board. You will need to cut 14 x 250mm lengths of hook-up wire and connect them between the A-N pins on the Sinewave Generator Board and the A-N pins on the H-Pack Board. An additional DC supply or the DCDC Inverter Board will be needed to provide +15V to the Sinewave Generator Board. Connect 24V to the DC-DC Inverter Board and to the Sinewave Generator Board and connect the +15 V rail from the Inverter Board to the + 15V pin of the Sinewave Generator Board. The GND pins should also be connected to Fig.19: check for these 15V p-p waveforms at the collectors of Q18 (top) & Q20 (bottom) the power supply (ignore the GND OUT on the DC-DC Inverter Board at this stage). Connect your scope's probes to the gates of Q18 and QZO, with the earth clips on the Y terminal and on the OV rail. Switch on the power. The waveforms should be as shown in Fig.19; ie, with an amplitude of 15V. Similar waveforms should be obtained on the gates of Ql 7 and Ql 9 with the earth clips on the OV rail and on the X terminal. If these waveforms are incorrect, check that there is a +15V supply across ZD7 and the supply pins (1 & 8) ofIC7, across ZD9 and the supply pins ofIC9, and across ZDl 1 and ICl 1 and the supply pins ofIC13. Unfortunately, the Rectifier PC Board is not easily tested until completely wired into the inverter chassis. After that, we can do some further tests to check that the circuits are functioning correctly before we finally switch on full power and drive 240VAC loads. Next month, we will describe the construction of the DC-DC transformer and the terminal board, and show how the PC boards and other components are wired into the chassis. SC J. The Rectifier PC Board serves mainly as an assembly platform for the two high voltage capacitors (680µF 400VW & 25µF 370VAC), the three large toroids (L2, L3 & L4), & a terminating panel. It also incorporates the fast rectifiers (D3-D6) and the optocoupled feedback circuitry (IC4 & IC5). JANUARY 1993 63