Hi, Not much good with analogue, would be grateful for any comments on this please :) (or simple picture) That's half an H-Bridge, we'll have 2 in full H-Bridge configuration driving a 30V LED garden light string from a Raspberry Pi (details below). R2/R3 are dummy loads for simulation, please ignore. Problems: ========= A) It's possible I might be exceeding gate-source voltages on TR2 or TR4 (TR4 max g-d V is 20V). Is there a standard solution to this? Something with a zener maybe? B) Transistor selection is not final - and TR1 may not work with 3.3V input (I designed to 5V initially) so we may need a different device or a buffer gate. I can solve that one way or another - just noting I am aware of this :) Many thanks, Tim What and why: ============= Application is Raspberry Pi GPIO (3.3V) to 30V LED lighting string. There'll be 2 of these in full H-Bridge configuration as the LED string is a mixture of forward and reverse connected substrings. Son wants to make the patterns more interesting and likes Pi's so it's a fun project. What I've done so far: 1) Not driving an inductive load, so lose the flywheel diodes common in motor drives; 2) I've tried it in CircuitLab with and without the TR2/3 driver stage. The switching speed and power dissipation is a lot better with that stage at higher switching rates. Whilst we probably won't drive this much over 1kHz, I prefer to not have software errors able to overheat things and we need small heatsinks as this is going in a waterproof box on the lawn; 3) Ignore R2/R3 - this is a dummy load for CircuitLab simulation; 4) R4 is a dummy internal resistance for the PSU (otherwise we get impossibly high switching transients). In reality the 30V supply is a current limited switch mode SELV** PSU. ** Separated Extra Low Voltage - UK designation for a "safe to touch" PSU for wet applications -- Email does not work

You need to redesign it, no good with oveestressing Vgs Use level shift with capacitive method, if you have guaranteed minimum duty cycle Otherwise, add gate to upper part for reshaping of the drive signal Cheers Klaus

H-Bridge with level shift - MOSFET gate voltage issues?

Jan 02, 2019 64 Replies

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krw 7 years ago

But in this case the RFI isn't harmonics of 60Hz. It's harmonics of the parasitic elements of the switching circuit.

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Joe Chisolm 7 years ago

I gave you a TI part, DRV8871, .565ohm rds, 45 v, 3.5a and you can get a breakout board from adafruit for $7.50USD

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Chisolm Republic of Texas

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bill.sloman 7 years ago

"Parasitics elements " don't have harmonics, frequencies do.

In this case the PWM waveform is nominally a square wave which has odd harmonics whose amplitudes decline in proportion to the harmonic number. If the square starts off faster than 60Hz, the harmonics are going to bigger.

Bill Sloman, Sydney

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Piotr Wyderski 7 years ago

Thanks, indeed a very interesting part. But I don't see your other posting where you gave it to me. On my end this is the first message from you.

Best regards, Piotr

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Joe Chisolm 7 years ago

easy to get lost in all the post. pardon my tying mistakes. sliced my finger before the holidays so i'm still one handed typing for a while.

If you are still thinking about rolling your own, the FDC5614P is a logic level P. 60V, 3A, Vgs -1.6 NTF3055L108 - logic level N channel

60V 3 A Drive them from gpio pins on the Pi. dont remember if the PI gpio has pull up or down. watch for that on the gate drive.

Chisolm Republic of Texas

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