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Anonymous Poster #1

Biasing Enhancement mode N-chanel MOSFETs

04/29/2022 12:31 PM

Hi,

What will happen if a 50 KHz, 15 V suqare wave AC (30 V pk to pk) is applied between the gate and source of an enhancement N-chanel MOSFET?

Thank you.

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#1

Re: Biasing Enhancement mode N-chanel MOSFETs

04/29/2022 1:48 PM

The Drain will develop psychological distress. "What am I doing here", " Why it had to happen to me" etc. S.M.

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#2

Re: Biasing Enhancement mode N-chanel MOSFETs

04/29/2022 4:08 PM

A depletion type MOSFET is normally on. An enhancement type MOSFET is normally off. An n channel MOSFET increases conduction with a positive voltage on the gate. So applying an AC signal to an n-channel enhancement MOSFET would turn the FET on during part of the positive-going signal on the gate.

"How to Turn on a N-Channel Enhancement type MOSFET

To turn on a N-Channel Enhancement-type MOSFET, apply a sufficient positive voltage VDD to the drain of the transistor and a sufficient positive voltage to the gate of the transistor. This will allow a current to flow through the drain-source channel.

So with a sufficient positive voltage, VDD, and sufficient positive voltage applied to the gate, the N-Channel Enhancement-type MOSFET is fully functional and is in the 'ON' operation.

How to Turn Off an N-Channel Enhancement type MOSFET

To turn off an N-channel Enhancement MOSFET, there are 2 steps you can take. You can either cut off the bias positive voltage, VDD, that powers the drain. Or you can turn off the positive voltage going to the gate of the transistor."

http://www.learningaboutelectronics.com/Articles/N-Channel-MOSFETs

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#3
In reply to #2

Re: Biasing Enhancement mode N-chanel MOSFETs

04/29/2022 4:31 PM

Hi Guru Rixter,

I realize that to turn an N-chanel enhancement MOSFET a +ve gate voltage greater than the threshold voltage but less than the maximum allowable gate voltage.

The maximum gate voltage of 11N65M5 specified as +/- 25 does it not indicate that a negative voltage up to 25 volt can be applied to the gate without damaging the device?

as we already know that this MOSFET will conduct only when a positive voltage is present on the gate terminal. logically, when the applied voltage swings to the negative side, won't the MOSFET be turned OFF? I know that tis is the case for bipolar NPN transistors. Won't the same apply to the MOSFET also?

Thanks,

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#5
In reply to #3

Re: Biasing Enhancement mode N-chanel MOSFETs

04/29/2022 5:10 PM

Driving the gate voltage of a MOSFET farther OFF than needed will only tax the circuitry driving the MOSFET. More charge needs to be removed from the gate and parasitic capacitances. This can lead to an asymmetric switching cycle compared to the incoming signal. Usually, this asymmetry is nothing but a trivial effect that may even be hard to measure.

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#16
In reply to #5

Re: Biasing Enhancement mode N-chanel MOSFETs

05/03/2022 1:36 PM

Sometimes negative gate voltages are used to shorten the turn-off switching time of the MOSFET by pulling the charge out of the gate with a higher current.

It does however complicate the gate drive circuitry and perhaps the controller for the switching circuit, especially if you want a fast turn-on as well, because now you have to dissipate the negative potential across the gate before you can begin turning the MOSFET back on again.

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#20
In reply to #16

Re: Biasing Enhancement mode N-chanel MOSFETs

05/03/2022 2:22 PM

Dear Sir Robin,

"Driving the gate voltage of a MOSFET farther OFF than needed will only tax the circuitry driving the MOSFET. More charge needs to be removed from the gate and parasitic capacitances. This can lead to an asymmetric switching cycle compared to the incoming signal. Usually, this asymmetry is nothing but a trivial effect that may even be hard to measure." -- Advice from Guru Redfred.

Paying attention to this advice, a switching diode, 1N4448, was incerted between terminal 3 of the transformer and the gates. The gate gets charged to 15 V within n neconds but does not turn OFF after the ON time. I tried a gate resistor (1K) for pulling down the gate voltage but did not help.

No the problem is to figureout how to discharge the gate capacitot at the end of the positive portion of the wave.

Kind regards;

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#4
In reply to #2

Re: Biasing Enhancement mode N-chanel MOSFETs

04/29/2022 4:48 PM

Yes, you have probably answered what the OP wants but you are assuming a load will be attached to the drain. The OP did not specify what if anything will be connected to the drain. Similarly, the OP did specify what will be the threshold voltage. A threshold voltage greater than 15V is rare but not unheard of in a MOSFET.

Proper engineering is very detail-oriented.

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#6
In reply to #4

Re: Biasing Enhancement mode N-chanel MOSFETs

04/29/2022 9:40 PM

I agree. It was a generic answer to a generic question. I should have added that you need to refer to the datasheet for the particular device to determine its limitations and that the operation of the device is dependent on the entire circuit.

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#7

Re: Biasing Enhancement mode N-chanel MOSFETs

04/30/2022 7:27 AM

Please give us more details:

1.) What's connected to the Drain?

2.) What else is connected to the Source?

3.) Which device are you using? (data sheet?)

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#8
In reply to #7

Re: Biasing Enhancement mode N-chanel MOSFETs

05/02/2022 4:18 PM

Dear Guru,

Please see the circuit below.

Kind regards,

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#9
In reply to #8

Re: Biasing Enhancement mode N-chanel MOSFETs

05/03/2022 3:37 AM

Oh look. A pulse-width-modulated thermostat.

Whatever is the point of that?

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#15
In reply to #9

Re: Biasing Enhancement mode N-chanel MOSFETs

05/03/2022 1:02 PM

Dear Guru,

Not really a thermostat;

It is a power reulator. Instead of using a phase control circuit, I was just experimenting with this circuit. The dead time is kept as 2 uS. The pot is set so that the frequency is set at 50 Kz; (20uS / cycle) If the dead time is kept constatnt ans frequency is varied, the duty cycle can be varied. By varying the frequency from 50KHz to 250 KHz, by changing the potentiometer setting, the power can be varied from 95% to 50 %. I may be wrong.

Kind regards;

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#11
In reply to #8

Re: Biasing Enhancement mode N-chanel MOSFETs

05/03/2022 6:59 AM

You appear to have a short directly across the secondary.

Plus the top MOSFET has a short between Gate and Source, so that it cannot turn on.

I'm sure this is equivalent to a "typo", but, we need to see a corrected version.

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#12
In reply to #11

Re: Biasing Enhancement mode N-chanel MOSFETs

05/03/2022 9:34 AM

Sorry Guru;

Please see below the corrected circuit.

Regards;

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#13
In reply to #12

Re: Biasing Enhancement mode N-chanel MOSFETs

05/03/2022 10:53 AM

Looks kind of OK.

I'm not sure but I think the Q1, Q2, Q3 & Q4 circuitry may be a bit over the top:

The current from pin 4 of the transformer has no where to go (gates of MOSFETs are essentially small capacitors), so you could probably get away with driving a much smaller transformer directly with the resistors which have existing values of 56 Ohms.

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#14
In reply to #13

Re: Biasing Enhancement mode N-chanel MOSFETs

05/03/2022 11:23 AM

The circuitry around the four transistors (Q1-4) worries me. The only thing limiting the collector to emitter current for these transistors is the primary winding reactance of the transformer. (I am assuming the +15V supply does not collapse.) With the transformer's secondary current loop containing two gates to sources capacitances in series the coupled impedance is negligible.

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#18
In reply to #13

Re: Biasing Enhancement mode N-chanel MOSFETs

05/03/2022 1:48 PM

Dear Guru Randall,

Since the MOSFETs are voltage , instead of current, driven devices, there is no question of current flow. Current flows to the gates only during the gate-source capacitor is being charged. I believe that a pulse which is large enough to bring the charge of the capacitor over the threshold voltage should bring the MOSFETs to conduction and keep ON untill some how the gate charge is removed.

I used the larger transformer since I thought that it is necessary to provide the necessart charge current so that the gate charge could reach as quick as possible.

Kind regards;

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#21
In reply to #18

Re: Biasing Enhancement mode N-chanel MOSFETs

05/04/2022 7:45 AM

The total gate capacitance CISS is 644pF, so, if the driving impedance was 56Ω then the RC time constant would be about 36 ns. The falling Drain voltage acting through Crss would act against this, but, not too significantly.

What is not happening as you expect?

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#10

Re: Biasing Enhancement mode N-chanel MOSFETs

05/03/2022 5:13 AM

Assuming it survives the experience and is correctly biased, it will switch at <...50Khz...>.

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#17

Re: Biasing Enhancement mode N-chanel MOSFETs

05/03/2022 1:40 PM

For a power regulator, wouldn't an SCR or Triac do the trick? 50 kHz sounds like a very high bandwidth controller for such an application.

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#19
In reply to #17

Re: Biasing Enhancement mode N-chanel MOSFETs

05/03/2022 2:01 PM

Dear Sir Robin,

A triac or an ACR will certainly do the trick. as a mater of fact an ofthe shelf dimmer can be used. But phase control can only work with AC supply. This circuit can be used for both AC and DC supplies.

Reagrds;

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#22
In reply to #19

Re: Biasing Enhancement mode N-chanel MOSFETs

05/04/2022 10:01 PM

Very true for the D.C. case. I just assumed that since one of the schematics had the supply labeled as phase, I thought we were talking about an AC power source. No worries. Cheers !

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