Re: PWM in a switching power supply

BradBrigade wrote: 
Hi,

First of all, I'm trying to figure out how switching power supplies
work (the ones in PCs).  I've found very basic info, but I want more
technical stuff.  If anyone has some good links please let me know.
These are questions I have yet to find an answer for.

Anyway, here's my question.  One thing I read was that the output
voltage of the supply is fed back to the PWM which changes it's duty
cycle accordingly to keep the output voltage constant.  But I thought
that the input-to-output ratio of a transformer is fixed.  If the PWM
is outputting 100V at 20KHz to a 10:1 transformer, you get out 10V at
20KHz, right?  What does it matter what the duty cycle is?  It's still
100V at 20KHz.  What am I missing?

If the transformer is a voltage output (produces some ratio of the primary voltage when the switches are on) and zero the rest of the time), then, yes, the peak output voltage is essentially independent of the duty cycle. but those kind of transformers also require an additional LC filter that outputs a voltage about equal to the average input voltage, not the peak. Holding the peak voltage for a smaller part of the cycle lowers the average voltage.

In supplies, where the transformer acts as an energy storage device (apply input voltage, till the primary current ramps up to some value, then cut the primary current, forcing the stored energy to reverse the winding voltage and go up till an output rectifier connects the transformer secondary to some storage capacitor). the average energy throughput depends on how high the energy each charge-discharge cycle, and how many cycles per second, and if the stored energy is all dumped each cycle, or only some of it (whether or not the primary switch is left off till the transformer dumps all its magnetic energy, or is turned back on while the dump is in progress).

Second, why does a switching power supply break without a load?

Break? As in Kablooie? I don't know about that, but many malfunction, because the control loop gain is dependent on the load current. That is, the gain goes up as the load decreases. Stability requires that the loop gain fall as frequency rises, so that before the frequency is reached where the loop phase shift swings by 180 degrees (compared to low frequencies) the gain has fallen below 1, so that the negative feedback (converted to positive feedback by the extra phase shift) cannot generate a self sustaining echo.

Third, in all my years in electronics, I have never used a choke, now I
see them all over these power supplies.  Can someone clue me in about
what they do, and why they are in these things?

There are just inductors. They store energy proportional to the inductance and proportional to the square of the current passing through them. Once you get up into significant currents, they become as useful and necessary for energy storage as capacitors. Whereas capacitors pass current in order to control the rate of change of their voltage, inductors generate voltage across them to control the rate of change of the current through them. If you want to absorb current pulses and stabilize voltage, you use a capacitor. If you want to absorb voltage pulses and stabilize a current, you use an inductor. And we are back to that averaging filter that is needed to smooth out the current from that pulsing voltage, variable duty cycle, constant peak voltage rectified transformer so that it can be connected to a storage capacitor where the voltage is to be regulated.

I appreciate any info at all.  Thanks a lot.

.

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