PWM Voltage calculation

[Peter]

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).


Thanks in advance.

 

[Peter]

What are you given? What are you calculating?

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

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.

 

[Peter]

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

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?

 

[John G]

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?

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.