High-side switching

Hey guys, I'm working on a pretty crazy project at the moment. Basically, it's going to be a HUGE RGB, individual segment colour controlled digital wall clock, spanning over 4 metres in length.

I have been messing around with the circuit design, and I think I have the highside switching sorted, but kinda wanted to confirm that the resistor/zener combo is a viable way of controlling P-chan MOSFETs from a uC output in this situation.

Please excuse the messy diagram around the LED "display".

The zeners are 7-8V, probably 8.2V.

Along the top are the 7 segment controlling high side switchers.

LED's are missing resistors because I will be using LED strip, which already has inbuilt current limiting resistors.


The problem I see with using zeners to level shift the gate drive is that the zener voltage is quite critical. Too low and the P-channel mosfets will turn on a little when they are supposed to be off; too low and the reverse. Using 8.2V zeners and a 12v supply, the mosfets will get just under 3.8v of gate drive in theory, which is a little on the low side for standard logic level mosfets. If the 12v supply is well-regulated, then it will probably work, although 7.5v zeners would probably be better. You should add a resistor in series with each mosfet gate to limit the charging current when the gate is pulled low.

I would drive each mosfet via an NPN transistor instead. With careful choice of components, you can omit the gate series resistor, so the number of components required (including the base drive resistor) is no greater than you have at present, i.e. 2 resistor and a transistor instead of 2 resistors and a zener diode.

I'll drive MOSFET's using one ULN2003 / 2803, no zeners.

With bipolar drivers the p-channel MOSFETs don’t need to be logic-level either - and your microcontroller pins are safe should a MOSFET fail.