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Registered Member #4650
Joined: Sat Apr 28 2012, 04:29PM
Location:
Posts: 3
I have been working on an h bridge driver for some time now. My intention was to design something that could work for n channel H bridges designed to handle over 600V. There are a lot of drivers out there for 200 and a few for 600, but I noticed your options become scarce once you surpass 600V. To be honest, I never fully understood how the whole bootstrap capacitor thing worked. I just never understood why one would charge a capacitor to charge a gate rather than just charging the gate directly without a capacitor. I suspect it has to do with producing a floating voltage, but I cannot be sure.
I went through several designs and about a ton of mosfets, but I finally have something that is simple enough that even I can understand it. It consists of only a multivibrator and two isolation transformers. The nice thing about the design is that the voltage through the h bridge is limited only by the isolation voltage of the transformers--which tends to be far higher than anything you would send through an h bridge. The bad news is that there is no way to control dead time. Unless you use very large isolation transformers, this technique will only work for high frequencies >~10kHz.
I have found that using the same power source to drive upper and lower mosfets always leads to your power source acting as a short circuit between the drain and source of the upper and lower fets respectively. The short circuit goes through the gates of both and blows them both out. The only way around this problem I can understand is using isolation transformers (I presume the bootstrap capacitor method does something similar, but I could never figure out exactly how that works.) Furthermore, I found that you can use the same power source to drive the lower mosfets, but not the upper mosfets. The reason is that the sources of the lower mosfets are tied together and their drains connected by the load. So you can send a voltage across the gates and current will flow from gate1 to source1 to source2 to gate2 and effectively turn one mosfet on and the other off at the same time. The upper mosfets have their drains tied together and their sources connected by the load, so the same technique will not work. This is where the isolation transformers come in. I have found that I can use one isolation transformer to drive each high side transistor so the voltage applied across the gate and source are floating. Since each of the high transistors are driven by separate floating voltages, there is no net voltage difference between the two sources or the two drains.
The final circuit is attached below. The multivibrator is composed of two op amps with their inputs cross coupled. I use a TCA0372 for the op amps and a 100KOhm dual potentiometer from radioshack for the resistors. The frequency will be RC*ln(2) when the resistors and capacitors match.
To the best of my knowledge, this topology requires the least number of components to drive an H bridge. Please leave any comments on the design or possible improvements.
Registered Member #2288
Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179
I would suggest using a GDT for all four gates if you're going to use it for any. It helps to prevent shoot-through.
Also, using op-amps to directly drive GDT's isn't the best idea, although you could probably squeeze by with those 1A ones you linked to. You're best off just using some gate drivers like the UCC37322/1 designed specifically for high pulsed current.
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Simple H bridge Driver
