MOSFET Question

[insane4evr]

Power mosfets typically have low on resistance, RDson (fraction of an ohm) and have high drain current rating, Id. Once turned on, mosfet heat dissipation can be calculated and usually is low, i.e., it runs cool. Example, RDSon = .02 ohm, on current = 4 ampere, dissipation = .02 x 4 = .08 watt.

However, when you are continuously and rapidly switching it on and off via its gate and the switching waveform is slowly rising and falling like a triangular wave, it places the mosfet in its linear resistance state which is much higher than RDson for a certain percent of its fully on/off time, the device can generate heat and has to be bolted to a heatsink.

Simplified example (just to illustrate):
Let us say you have a triangular wave frequency with its positive peak at 10 volts. Assume mosfet gate voltage to be fully on = 5 volts. This means that 50% of the time the mosfet is in its linear resistance state. For simplicity only, assume linear resistance is a constant 2 ohms and ID is 5 amps. So 50% of the time it is dissipating 2 x 5 x 50% = 5 watts (again for simplicity only as calculation is much involved and sometimes requires empirical data). Assuming the mosfet comes in TO-220 package. If not mounted on an adequate heatsink, the mosfet will be damaged by excessive heat. Note that it is percentage dependent. So, if your triangular wave is short in duration and with low repetition rate, you will not have heat problem.

So, depending upon your application, you most probably will need to add mosfet driver at the output of your triangular wave generator to create fast rise and fall times to drive the gate of the mosfet.

I know this still does not answer the thread owner's question. So, use it or thrash it.

[Farlander]

So am I to understand that when looking at the front of a mosfet, from left to right, is GDS?

Gate = on/off switching voltage
Drain = Load
Source = Ground

What type is advisable to use in HHO production?  NPN or PNP?

Thanks

[Spewing]

sounds good to me

[insane4evr]

So am I to understand that when looking at the front of a mosfet, from left to right, is GDS?

Gate = on/off switching voltage
Drain = Load
Source = Ground

What type is advisable to use in HHO production?  NPN or PNP?

Thanks

You have to refer to the manufacturer's datasheet for the terminal identification of the part number you are going to use.
NPN and PNP apply to bipolar junction transistors (BJT) and not to mosfets. The mosfet equivalent for NPN is N-channel and for PNP is P-channel. The type depends upon your circuit.

[Farlander]

Roger wilco, yeehaw@