11/18/2023 0 Comments Transistor gate charge![]() Give or take real numbers for a given device, and give or take load current (e.g. Most of the drain voltage swing occurs during the Miller plateau, which consumes about half the gate charge, so there you go. Load (drain voltage) switching time will be some fraction of gate switching time, usually around half. And likewise, Ipk isn't going to be any more than Vgs(on) / R_G, no point in using a 12A gate driver on a 5 ohm gate for example. Note that R includes internal gate resistance (R_G, sometimes given in the datasheet), driver resistance, and anything you add externally. Then you need a gate driver either with resistance low enough, or peak current high enough, to achieve the desired switching time (assume gate rise/fall time is about t = 2.2 R Ceq, or t = Qg(tot) / Ipk). For this purpose, you can use the equivalent capacitance, i.e., assuming the gate were a capacitor that stores the same amount of charge over the same voltage range. For switching purposes, you aren't so much concerned with when, and how much, current flows, just so long as it's done in the desired time frame. Meanwhile, the APT20M18 has a gate capacitance of 9880 pF, but a total gate charge of 330 nC at 10 VGS! Yet, it's rated at 200V, 100A! How does this work? And on a different, yet related, note, why do people still use the standard IRFP4xx - IRFP2xx FET's so much, if there's much better FET's seemingly available nowadays? Is it because they're easy to drive? Or just more or less "tradition" in some sense?Ĭharge is the integral of current or conversely, current is the derivative (rate of change) of current. ![]() Example, these two mosfets: The IRF200P222 has a gate capacitance of 9820 pF, and a total gate charge of 135 nC at 10 VGS. whereas gate capacitance does vary, but not by quite so much a margin. But, what is gate charge? What is it "measuring"? How does this affect gate driving? I know gate capacitance seems to be the biggest thing most people look for in a mosfet, but yet, gate charge can seemingly vary WIDELY between FET's even of similar current and voltage ratings. I understand that as you increase gate drive voltage, gate charge increases. There's gate capacitance, and gate charge. I've been spending a lot of time researching the difference between various power MOSFETs, and I'm a tad bit confused.
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