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Old 12-31-2009, 10:57 PM   #182 (permalink)
NiHaoMike
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Quote:
Originally Posted by adamj12b View Post
OK, so the isolation uses a minimum of 4 isolated DC-DC converters. One for the 3 low sides and 1 for each high side. They way this work, is because of the internal configuration of the IPM. The low side of the IGTB is connects to the negative battery terminal, this is also connected to the negative supply terminal for the logic of the IPM.

So a little off track for the High side. The way the bootstrap caps work is they charge when the phase is pulled to the negative side of the pack. Since this is common to the negative supply of the logic, the cap charges with 15VDC. when the IGBT goes high, the logic uses the power stored in the caps to power the gate.

Now the reason for not having a common isolated supply for the whole module has to do with the internal wiring distance, delay and inductance. So when you have 3 IGBT's it is not possible to pick a common point to tie the the supply to, So you use 3 so they are all powered at the same time with the same delay, because they are all symmetrical.

So, with the Ebay IGBT modules, you will need 2 small isolated DC-DC converters to power each one. One for High side and one for Low side. There will be two opto-isolator's, One for High and one for Low. Each will get its power from the respective DC-DC converter.

Thats what I have for now. Some time tomorrow, I will post a schematic showing what I mean.
You only need one DC/DC with multiple isolated outputs. Adding additional outputs to an unregulated push pull converter is just a matter of adding more secondary windings and rectifiers.

Switchmode Power Supply For Car Audio
That's a starting point (start with the simplified version). Just use smaller components, change the turn ratios to get +-15v or so, and add more secondary windings. Leakage inductance will cause the output voltage to rise with no load, so use zener diodes to keep it within the ratings of the components. (The normal output voltages should stay below the zener diode voltages.)

As for signal isolation, Analog Devices makes some very impressive isolators. The ADUM7441 is good for 3 signals in one direction and one the other, such as 3 phase drive one direction and fault indication in the other.
ADUM7441 | 1 kV RMS Quad-Channel Digital Isolator | Digital Isolators | Interface | Analog Devices
I have used another Analog Devices chip that isolates USB and I'm very impressed at its performance.

And although the high voltage circuit would be isolated, you'll want some connection to ground to stop charge buildup and EMI but still keep isolation. I suggest a series string of two or more large resistors (1M total resistance or greater) in series from each side to ground, along with two Y2 capacitors from each side to ground. That will "soft ground" the midpoint as well as act as a discharge resistor for the main capacitors.
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