WaveTheory
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I think, it's the other way around. The regular way to prevent body diode conduction is to keep deadtime low and perform strictly synchronous switching.Alternatively/additionlally you could increase deadtime of the FETs.
I'm building a 2000W sensored brushless DC motor driver. I'm using IR2110 drivers with FDP2532 FETs. The drivers are fed 6 synchronized PWM signals at 72khz. High-side signals have dead-time controlled by the microcontroller.
72khz may seem extreme for a motor. Ill soon be trying for 100khz. This is one of the reasons I opted for building a custom controller. There are many reasons I did this.
Firstly, higher frequency means less ripple in the power rail, and that means my electrolytic capacitors run cooler. They get mad hot below 30khz due to increased ripple amplitude. Increasing the frequency even higher opens the possibility of eliminating electrolytic caps all-together, and replacing them with mylar or ceramic. In theory, this would allow a well-designed controller to have an almost indefinite operating life.
Second, it reduces eddy-current in the motor core.
Third, it reduces the size of inductive and capacitive filtering elements on signal and power lines.
The efficiency gains in the capacitors and motor core may not entirely make up for the higher switching losses, but as switches and drivers become more advanced their losses will become less pronounced. In the present, this design saves cost at the expense of increased switching loss. In the near future, it's a free lunch. I see a future with switching speeds in the hundreds of kHz make electrolytic capacitors as obsolete as selenium rectifiers, and make tantalum capacitors overkill. It will reduce the size of inductors in filters, and power converters, and make 300k hour service lives seem common. When that future comes, Ill be ready for it!
Yes. I investigated my PWM signals with the scope. I previously had my dead-time set to about 4 clock cycles (72mhz clk), and the FDP2532 datasheet has a total on-off delay of about 250us, which translates to about 20 clock cycles. I can now spin the motor up to 35v, under load, without failure. It wont go much faster without crashing the MCU due to noise on the hall-effect sensor lines (but that's a different matter). It's not guaranteed, since I only tested for about 30min, but it's a huge improvement. I'd say the problem likely was the deadtime.
I had thought that too-little deadtime would cause enough shoot-through current to make the FETs hot, as this was my experience in the past. I hadn't imagined that it could kill them so suddenly while they were cold, and with a small enough current as not to register on my power supply. I'm now the wiser for it thanks to everyone's help.
As for the capacitors, film capacitors and ceramic capacitors cannot achieve nearly the same density of farads as electrolytics. Using the minimum number of film caps, especially at lower frequencies, would take up more space then the whole rest of the device. Tantalum capacitors are an option, but they are far too expensive to practically replace electrolytics in power levels this high. They also have a danger of thermal runaway.
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