skylogger wrote: What do you think of using a 555 to generate gate test signals? I've adjusted the resistor values to give a close to 10khz frequency square wave output. The transistor inverts signal so high side and low side get 180 deg switching.
Still a few problems.
First, 555s, while strong, may not have sufficient rise and fall slew rates.
Gate series resistors are usually around 22-47 ohms, not 1K.
A 1K pullup is not good; it's far too slow. Outputs need to be push-pull. Really you need a proper gate driver chip.
You also need to isolate the drive for the upper transistor. As soon as it turns on, its emitter goes wildly positive, so to keep it on you will need a gate drive that goes more positive than the emitter. This requires boost diodes. If you don't do this, the gate will go hundreds of volts negative with respect to the emitter, which will surely destroy it.
But finally, and perhaps most importantly, you need guaranteed dead time. IGBTs and MOSFETs take time to turn off; tens of nanoseconds or more. They also take time to fully turn on, but you can't rely on one being slower than the other. So merely inverting the low signal for the high switch isn't good enough.
The dead time is to prevent "shoot through", where the upper and lower transistors are conducting at once, shorting out the power supply. Even if you have a current limited power supply, those 3x680 μF capacitors can pack a lot of energy and blow up the MOSFETs. That's why I prefer to test with a ~50 V current limited supply; the energy in the capacitors (=½CV²) is 64x less with an 8x lower supply voltage. 52 V at the mains terminals turns out to be enough to turn on the power supply for the Elcon chargers. Maybe we can find a similar test voltage for the iMiEV chargers, or find another way to power the control circuit and test with about 12 VDC current limited.
What might work is four 555 timers (say two dual 555 devices). Connect them to trigger off the previous timer's falling edge, with the first one triggered by the last one in a sort of ring oscillator. Initial conditions will be tricky. Arrange the times as say 30 μs, 1 μs, 68 μs, and 1 μs. The two 1 μs timers are for the dead time; they don't connect to gates. The 30 μs and 68 μs outputs connect to say upper and lower gates respectively (only drive one pair for initial testing). So you should get 30 μs high and 68 μs low, representing a 30% pulse width modulation (total period is 30+1+68+1 = 100 μs, or 10 kHz). You'd still need an isolated driver for the upper transistor, so you might as well use a proper gate driver chip. You likely don't have one of those at home, and Jaycar etc likely won't have one you can buy off the shelf.
So the next thing might be to trace the driver circuitry, and find out where they are driven from, and connect their inputs to the four 555 circuit. It's starting to sound pretty complex. You will also need to ensure that the driver chips are powered up.
That might be as simple as supplying 12 VDC or perhaps 14 VDC from a power supply. The power supply for the drivers seems to come from a circuit near the yellow-taped transformer that's on its own. But I don't see how it gets power; it could be 12 V coming through the very fine ribbon cable connecting the top board to the charger board, or more likely from the PFC output (400 V bus). It's likely a high frequency power supply as you'd find in a small plug-pack (wall wart) power supply these days, using the boosted mains voltage rather than merely rectified mains. If so, it depends on the design how low in voltage it will work at. It might be easier to find the outputs of the power supply and supply from a bench power supply, rather than use the power supply in the charger.