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Old 10-01-2018, 12:03 PM   #1
AdrianM
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Join Date: Dec 2011
My Country: UK
Line Voltage: 240VAC +6%/-10% 50Hz
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Default High side Mosfet swich design

I need to switch a 30Amp resistive DC load from a 5V microcontoller pin. This isn't PWM just occasional on/off.

I'm familiar with the design considerations for high speed switching - rapidly getting charge in and out of the gate to minimise time spent in the linear region but in this application I'm pretty sure I can safely simplify the design a great deal because it's only used as a discrete on/off control.

What I've come up with does seem uncomfortably slow so I thought I'd ask around in case I'm missing something!



Q1 would actually be IPD50P04P413ATMA1 a 12mOhm P-Channel MOSFET, 50 A, 40 V. The total gate charge is less than 50nC so charging/discharging at around 5mA should take no more than 50nS x 1/0.005 = 10uS

This keeps it in the top right corner of the safe operating area:



The DC input could be as high as 30V so to limit the gate voltage low swing, the 470R source resistor makes M1 act like a constant current sink (its threshold being around 2.5V). R1 maintains around 10V across it until the supply drops below 12V

LTspice sim of ~30uS switch-off for part with even higher gate charge (120nC) seems OK:


Green is gate voltage, red ouptut voltage and blue power dissipation in Mosfet. Switch-on should be a bit sharper and it is at <10uS:



Too good to be true? I'm so used to the pitfalls of Power PWM design that this simple application has me jittery
Attached Images
File Type: png sw.png (14.5 KB, 24 views)
File Type: png safe.png (28.3 KB, 23 views)
File Type: png plot.png (29.7 KB, 23 views)
File Type: png onplot.png (31.1 KB, 23 views)
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