# PWM Voltage calculation

• posted

Is there a direct formula for calculating the voltage from a PWM circuit?

say I had a PWM chip controlling a FET with X volts feeding the FET, is there a direct calculation?

I work better with real numbers and felt using real numbers would result in people thinking I'm asking a homework problem, however, using whole numbers would be easier.

Say 10 volts at the drain and 100kHz at the gate, what would I get out (assuming there is a direct calculation).

• posted

What are you given? What are you calculating?

Bill

-- Fermez le Bush--about two years to go.

• posted

Salmon Egg wrote in news:C1D8F174.5B785% snipped-for-privacy@sbcglobal.net:

I was using a made up circuit. Many times I'll see circuits where a FET is driven by a SG3525 and I'm not sure how to calculate the output voltage of the FET.

• posted

PWM stands for Pulse Width Modulation. So what 'pulse width'? with 100 kHz, it could be 'on' 50% and 'off' 50% (that would give you a nice square waveform). But it also could be 'on' 10% and 'off' 90%. It would still have a frequency of 100 kHz, but obviously a lot less power would get through.

Calculating the 'voltage' is a bit confusing. Do you mean to calculate the 'average' or 'rms' level? Average is easy, it is just the 'on' fraction times the 'on' voltage. For example, if it is 'on' 10% of the time, and

10V, then the average would be just 1V (neglecting drop through the FET when full on).

RMS is equal average for a square wave, so there you are.

daestrom

• posted

Well I guess we can agree to calculate in RMS and ignore the FET's voltage drop. If it's just the ON-time multiplied by the 'on voltage' then what role does the frequency play?

So if I have a FET with 10 volts on the drain, and 100kHz with 10% on-time on the gate, then I can expect 1 volt on the source?

What about if I change my frequency to 500kHz and keep the same 10% on- time?

• posted

Think about it.. If it is on for 50% of the time in one cycle then it will be on for 50% of all the time.

Who cares if each cycle is 1ms long or 1 micro sec long.

Any percentage of one cycle will be the same percentage of the total time.

-- John G

• posted

"John G" wrote in news:45b5c231\$0\$7998 \$ snipped-for-privacy@per-qv1-newsreader-01.iinet.net.au:

That doesn't seem to make sense. Why the different frequencies then?

You're saying if I have a 1Hz square wave or a 100MHz square wave both having a 50% duty cycle, my FET's output voltage will be the same in both cases?

• posted

The *average* or RMS output will be the same (theoretically). Of course, extreme frequencies start to bring in parasitic capacitances that literally change the overall circuit.

The frequency choice is a bit of trade-off. Too high and you have parasitic capacitances. Too low and any inductances or transformers will have to be much larger in size for the same power. If you're trying to reproduce some low frequency supply with the PWM output (say, to drive an AC motor), then you probably want the pulse frequency to be high enough that you can get 'good' waveform reproduction.

daestrom

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.