Using MOSFETS for a Level Shifter

Question

I want to use a pair of N and P channel mosfets in a totem pole arragement to shift a VFD display on and off using a 74LS247 decoder/driver. I would put the P channel with the source pin on the top and connected to the 25 volts the VFD requires. The P channel drain pin would be connected N channel drain pin. The junction of both drains would be my output (25 Volts). The source pin of the N channel would be connected to ground.

The gates of both the P and N channel mosfets would be tied together and powered from the "Open Collector" output of the 74LS247 decoder/driver to turn on and off the appropriate mosfet. I plan on using a Fairchild FDS8958A mosfet complementary pair.

Questions:

• Do I need a series resistor in series with the LS247 output and the two gates?

• What value do I need?

• Do I need pullup / pulldown resistors between the source pin and gate pins?

Answer 1

Sorry, but this isn't going to work, or at least not directly. Yes, you need a pullup, but if you use one you'll destroy the 247. Reason? The 247 outputs have a max voltage rating of 15 volts, and you need to drive to 25. If you do manage to drive the P-type gate to 25 (to turn it off) you will violate the gate-source maximum voltage Vgss of 20 volts for the N-type. And if you drive the 247 output low you will exceed the gate-source voltage rating of the P-type.

Answer 2

If you want to use a logic level to switch 25V, then I'd suggest an arrangement like this - a classic high-side switch.

The voltage divider on the gate of the P-Ch is to avoid Vgs exceeding the 20V limit for that part.

^{simulate this circuit – Schematic created using CircuitLab}

The update that I've made to the circuit incorporates an extra P-channel MOSFET, M3, and a pullup resistor to 5V, R4. This can be pretty much any modest P-Ch MOSFET; it should be reasonably fast but it doesn't need to be high voltage or high current. Its purpose is to convert the open-collector output from your 74LS247 into a 5V logic level. Alternatively, you could use a pullup resistor (like R4) and an inverter, e.g. a single gate from a 74LS04.

If you could choose a different decoder IC than the 74LS247, one with a logic level output, then you could go back to the previous circuit.

Please do some testing with a single instance of the circuit to satisfy yourself that this does indeed work before you design and assemble a 42-channel version!