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I understand transformers only work with AC because there is electromagnetic induction only when the current going through is AC. In switch-mode power supplies, the pulse transformers are powered by high frequency signals generated by MOSFETs which means they can be smaller because the high frequency will compensate for the missing inductance.

I also learned that, when the output voltage drops (due to a load in the secondary), the PWM controller increases the duty cycle of the signal generated by the MOSFETs, in order to compensate and keep the voltage constant. However, the frequency of the signal is the same. Given that transformers only work with AC and that the frequency of ON/OFF switching of the MOSFETs (variation) is the same when the duty cycle is increased, how does that result in higher output power?

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My observations plus your comments suggest that in a switching power supply, the step down transformer is powered from a rectified standard AC, fed to the transformer in the form of high frequency pulses. If the frequency is fixed, then the power can be varied by changing the width of each pulse.

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  • $\begingroup$ But the signal fed to the setp down transformer is DC, right? because it has a capacitor after the bridge rectifier. So the only "AC" part comes from the mosfet switching... if the variation is the same, regardless of the duty cycle, how does a higher duty cycle result in higher inductance? $\endgroup$
    – Dinis
    Commented Aug 27, 2021 at 1:15
  • $\begingroup$ The inductance of the transformer limits the rate of increase of the current in each pulse. A longer pulse lets the current go higher before being switched off. $\endgroup$
    – R.W. Bird
    Commented Aug 27, 2021 at 13:47
  • $\begingroup$ Alright, I understood now. Thank you $\endgroup$
    – Dinis
    Commented Aug 31, 2021 at 22:36

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