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Effect of change in frequency or duty % on transformer output

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polashd

Member
In case of a smps transformer of 1:2 primary and secondary turn ratio roughly the output voltage will be double & current will be half of input.

My questions are what effect will be on output, if

1) Frequency increase or decrease (when duty % is constant)

2) duty % increase or decrease (when Frequency is constant)

would anyone please advise me
 

audioguru

Well-Known Member
Most Helpful Member
A transformer works poorly at low frequencies and at high frequencies. It works best at the frequency it is designed for.
If you change the frequency or the duty cycle then the output level will be reduced.
 

polashd

Member
If you change the frequency or the duty cycle then the output level will be reduced.
Thanks for your reply.
If both are kept constant then how the output to regulate. As I know depending on the feedback frequency and/or duty % is adjusted to get a desired output in an smps. Am I missing something.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Thanks for your reply.
If both are kept constant then how the output to regulate. As I know depending on the feedback frequency and/or duty % is adjusted to get a desired output in an smps. Am I missing something.
I think AG was a little confused?, and thinking of conventional transformers rather than switch-mode ones, but even then conventional transformers work perfectly over a fairly wide frequency range.

As far as your question goes, you are perhaps slightly confused as well? - to alter the output level you change the duty cycle, the frequency doesn't matter (within reason), and some types of SMPSU change both (with the frequency change irrelevant, and simply a by-product of the way the duty cycle modulation is created). Another way is somewhat confusing called 'frequency changing', but it still works by changing the duty cycle.

a) Duty cycle change - the frequency is constant (although there's no need for it to be so), you alter the duty cycle by changing BOTH the mark and space. So for a 100uS timing, 50% duty would be 50uS mark, and 50uS space - 25% would be 25uS mark, and 75uS space. the overall time (and thus frequency) remains at 100uS. However, it doesn't matter if the frequency does change, as long as the ration between mark and space stays at the value you desire - in this example it's 10KHz.

b) 'Frequency' change - in this the 'mark' (or the space) remains constant, so for the same 50% ratio mark would be 50uS and space would be 50uS (maintaining the same values as previously), BUT for 25% the mark remains at 50uS, and the space is increased to 150uS, keeping the ratio at 25%, but lowering the frequency from 10Khz to 5KHz. So while you're actually changing the frequency, that's only a side effect, you're actually changing the duty cycle, just as before.

So altering frequency alone, as in your question 1), won't alter the output as the duty cycle remains constant - until you get way outside the capabilities of the circuit, where things will stop stop working correctly,

So you can either change mark AND space, or you can change mark OR space, the end result is the same.
 
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