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Have I gone mad?

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king.oslo:
Hello there,

Until now I was certain this was true:

P-channel MOSFET conducts when the voltage at the gate is more negative than the threshold voltage relative to the source.
N-channel MOSFET conducts when the voltage at the gate is more positive than the threshold voltage relative to the source.

Is this true?

I am sorry to ask such a fundamental question, but LTspice seems to be suggesting I am wrong (see the attachment). If the above was true, I would expected the green voltage to switch between 172V and 0V @ 25% duty cycle.

Thanks.M


EDIT: What I actually want is the 172V at 75% duty cycle!

IanB:
I have only a sketchy knowledge of FETs, but I wonder, shouldn't we consider source in one case and drain in the other?

amspire:
You have two source followers which are non-inverting and a bit less then unity gain,  it is working as it should.

My thinking is a relay may be the easiest solution particularly if you can arrange for it to switch off when the regulator falls below 14V, or the voltage before the regulator drops to less then 17V. The reason I like a relay is the relay can short out the voltage across the divider, so there is zero volts - a second contact can switch off the 172V as well.

An alternative suggestion is to use a 3 position 2 pole make-before-break switch for an off-standby-on switch.  In the off mode,the input power is off and the divider is shorted. In the standby mode, the input mains is on, bit the divider is still shorted.  In the on mode, then mains is on, and the divider is unshorted. Becuae you have to switch through the standby position, it ensures the power up-power down sequence is correct.

Richard.

Edit: what I think you intended was the P-channel mosfet with a 10K resistor gate to source. Then a N-channel mosfet or NPN transistor with a 180K resistor between the drain/collector and the gate of the p-channel mosfet. Source/emitter to 0v. You then add a simple zener circuit to turn the n-channel/npn on when the voltage of the regulator is over 12V.

Kremmen:

--- Quote from: king.oslo on April 29, 2012, 01:10:56 am ---Hello there,

Until now I was certain this was true:

P-channel MOSFET conducts when the voltage at the gate is more negative than the threshold voltage relative to the source.
N-channel MOSFET conducts when the voltage at the gate is more positive than the threshold voltage relative to the source.

Is this true?

I am sorry to ask such a fundamental question, but LTspice seems to be suggesting I am wrong (see the attachment). If the above was true, I would expected the green voltage to switch between 172V and 0V @ 25% duty cycle.

Thanks.M

--- End quote ---
For starters, there are N channel and P channel devices and both come in 2 kinds: the enhancement mode and the depletion mode transfer characteristic. So you have 4 fundamentally different kinds of MOSFET. I took the liberty of attaching a picture from the 3rd ed. of Physics of Semiconductor Devices by Sze and Kwok, hopefully fair use covers this. If not i will of course remove it. Perhaps that picture clarifies the differences.

Then, your circuit simply has the problem that halfway through you switch the common reference potential from source - to source +. This can be done but it has pitfalls and your circuit falls into one of them. M1 contols the gate of M2 in phase with the clock pulse, switching it between - and circa 11 volts or so. Actually you don't really need the fet because the clock source already does the same thing. Now for M2 the reference (node where the source pin is connected) is the + pole of the voltage source. So nominally the source voltage is 0 and the drain voltage is -172 V. This would be the way to orient a P channel fet voltages. Now the idea of this circuit is that M1 produces voltages alternating between 0V and some -15 V to the gate of M2, in M2 reference orientation above. Unfortunately this is not what happens in practice. M1 only swings some 11V above its own reference which is the -172 V rail in M2 reference frame. In M2 reference it is between -161 and -172 volts. So the "output" (i.e. M2 source) swings just enough to maintain the fet in a conducting state, the voltage keeping a constant difference to that applied on the gate.
You can confirm this by checking the M2 gate voltage in LTspice. You will see that it keeps a constant voltage difference against M2 source. Every time the pulse source and M1 switch, there will be a current spike in M2 gate circuit as the charge rebalances due to the change in source voltage.
This circuit can easily be modified to do what you want. Just take M2 gate voltage from M1 drain and switch the resistor from M1 source to its drain. The output phase of course changes around but that sould not be a major issue.

Psi:
Hm.. This is something i wasn't aware of, and probably should have been.

Learn something everyday i guess.

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