# Using a Mosfet

• posted

I'm still working on the Mosfet power switching design I was fussing about a fortnight ago. I'm a bit hamstrung by the fact that I've used Mosfets before in other peoples designs but have never had to actually think about them. I can make a stab at bipolar transistor design but FETs are a closed book to me.

I've selected my Mosfet, a BUZ72A which has the current and voltage rating I want. The question is, how to turn it on. Flowers, chocolate, liquor and soft music don't work.

The data sheet Max Ratings says the max Gate/source voltage is +/- 20V.

The electrical characteristics say that the gate threshold voltage is Min 2V Typical 2.9V and Max 4V The same section gives the static Drain/Source resistance (low, as I want it) for Gate/Source at 10V and Drain current of 5A.

I'm intending to send the gate low with an NPN transistor (BC548) turned on hard when I want the FET turned off.

Question, what is the optimum voltage to apply to the FET gate when I want the FET turned on hard? 10V?

Furthermore. Phil Allison in his reply to my previous post suggested a 63V Zener, drain to source, to protect the FET from inductive spikes. I've since seen a suggestion (on a 'Net hobby page) that a 15V Zener connected gate to source will protect the gate from spikes. It occurs to me that when I have decided on the optimum gate voltage, I should connect a zener of that voltage gate to source. Feed it with a resistor from the 12V supply which is also the load resistor of the NPN transistor. When NPN transistor is not conducting, the zener-controlled voltage is applied to the FET gate. When NPN transistor is turned on hard, the gate/collector/resistor point goes low. Certainly lower than the the 2V Min threshold voltage of the FET. Obviously, I'll choose a load resistor for the NPN transistor which limits the collector current to a safe value. The attraction of this idea is that the zener will alsodetermine the voltage to be applied to the gate as well as protect against spikes.

Question, is it a stupid idea? Has any real designer ever done it this way?

PH

• posted

"Peter Howard"

** Yep.

** Nope.

** Probably lots.

........... Phil

• posted

Thanks for the succinct reply. I went ahead tonight and built up and tested my design as described. The FET passed the desired current of 1A for several hours with no signs of distress. Measured 0.15V across drain/source so I guess it's turned on hard. Trivial for the experienced but a big thrill for me when something actually goes as planned. PH

• posted

tested

several

for

Oddly enough, the thrill never goes away...... :-)

Ken

• posted

lol

if you exceed that, you'll blow a hole thru the very thin gate oxide layer, and bugger it. Some FETs have built-in gate zeners, in which case you'll fry them.

its worth noting that many low-Vth FETs have +/-12V Vg_max.

Vth varies with temeprature, and from device to device. Use more than

4V, and it will always turn on.

there is probably a curve of Rdson vs Vgate. Rdson drops significantly around Vth, then continues to decrease with increasing Vth, but only slowly. If I^2Rdson is an issue, use more Vgate to get lower Rdson.

Rdson *increases* with junction temperature. worst case this can lead to thermal runaway. If you have a lousy heatsink, the device can run wuite a bit hotter than expected, if the load current is constant (eg if Rdson is

as long as Vcesat is seen a suggestion (on a 'Net hobby page) that a 15V Zener connected gate to

Cheers Terry

• posted

I

Buy Dilbert books and use the knowledge against the dark side of management.

Ken

• posted

It does when you get paid fuck as designer all and only ever recognised when things go wrong.

• posted

again, many FETs have these diodes built in. but yes. it doesnt even have to be a zener - a diode to a +15V (or say +9V etc) supply, with a decent cap (say 100nF) is often cheaper, and works just fine.

look at the datasheet for a zener. often the voltage is pretty loose, and moves with temperature. the higher Vz, the greater the tempco.

Anyway, this is actually unnecessary. if you have a 12V supply and

+/-20V Vg_max, then any zener from 13-18V will work fine (note I kept the zener voltage *below* Vg_max and above Vcc, that way you can ignore its tolerance)

in general more Vg is better, as long as the FET doesn't break.

most designers pick the zener to clamp gate spikes rather than "control" Vg. Check zener tolerance and voltage variation with zener current....

the disadvantage with this approach is that the NPN transistor Ic constraint sets Rgate, and the gate time constant is then whatever it is

- Rgate*Cgs. slow turn ON, fast turn OFF. If switching at a low frequency, then who cares - I once built 17,000 OEM UPS' (inside terabyte RAID arrays) that had 47k pullups, no zener but otherwise your circuit.

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