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Hi Squiggles! I've had quite a few discussions with people on the EV Tech list. Here's what I've learned:
Replace the freewheel diodes with mosfets. The diodes' ANODE (input) is replaced with the mosfets' SOURCE leg. The diodes' CATHODE (output) is replaced with the mosfets' DRAIN leg. Let's call the freewheel mosfets gates "GATE2", and the regular mosfets gates "GATE1".
You need a microcontroller that has 2 separate PWM outputs with programmable deadtime. The AT90PWM3B was made just for this purpose. Let's say PWM1 is connected to GATE1, and PWM2 is connected to GATE2.
Let's also assume we are starting with GATE1 High, and GATE2 low. So, the regular mosfets are in conducting state, and the freewheel mosfets are in the BLOCKING state.
Now, when you want to switch to the state where the regular mosfets are off and the freewheel mosfets are conducting, do this:
Bring PWM1 output low. The chip waits for a specified dead time (like 1/2000000 sec), and then PWM2 output goes HIGH.
When you want to switch to the state where the regular mosfets are ON and the freewheel mosfets are BLOCKING, do this:
Bring PWM2 output low. The chip waits for a specified dead time longer than the trr of the intrinsic diode (like 1/2000000 sec), and then PWM1 output goes HIGH.
Repeat this process each cycle. So, I don't think there is much external circuitry that needs to be added. Just an extra driver circuit and gate resistors. The power section would have to be modified so that there would be mosfets on both sides of the heat spreader.
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