Gene Heskett wrote: > Ultra fast, Si, or Schotkey? Si seems worthless but I have seen the amp > or so rated Schotkey's make a difference. Their Vf is only about 2/3rds of > a germanium diode, and a very low impedance once conduction starts. > > Ultrafast Silicon. It is a 200 V diode, so Schottky is out. There's so much capacitance already, I don't worry a whole lot about turn-off, the problem was to get the diode to turn ON fast enough. These are supposedly rated for 4 ns, the ES3D. (I also use the ES1D in the high-side driver bootstrap circuit). So, you have the situation the high-side transistor is on, sourcing 20 A into the load through the 47 uH filter inductor. Then, the high-side transistor shuts off, and there is a couple hundred ns of dead-time before the low-side transistor turns on. But, there's still 20 A flowing out through the filter inductor, and you have to keep the node between the two transistors from going below ground. I don't have to worry about small negative voltages, the IR chips guarantee tolerance to -5 V, I have seen them handle up to about -7 V without abnormal functioning. The ES3D diode installed with suitably short traces solved the problem, and held the worst-case negative spike to less than -4 V.
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