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Regulated SMPS strange mosfet waveforms :-(
Regulated SMPS strange mosfet waveforms :-(
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Old 14th January 2010, 06:00 PM   #21
Eva is offline Eva  Spain
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Unregulated SMPS works perfectly ok without output inductor, but it's required for regulation without high component stress.

The output inductor will try to short the secondaries of the transformer while it's discharging resulting in square waves. Resonance only shows up when there is no output inductor, or when the output inductor discharges before next cycle begins (and this just happens at low loads).

The following waveforms show the effect and were captured from an off-line SMPS that I did some time ago (half bridge). Blue trace is primary current, red trace is primary voltage. Pictures are no load, medium load and full load.
Attached Images
File Type: gif 100A_3F.gif (22.6 KB, 203 views)
File Type: gif 100A_3G.gif (24.2 KB, 184 views)
File Type: gif 100A_3H.gif (26.5 KB, 179 views)
I use to feel like the small child in The Emperor's New Clothes tale

Last edited by Eva; 14th January 2010 at 06:05 PM.
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Old 14th January 2010, 06:16 PM   #22
pergo is offline pergo  Italy
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eheheh, stability problem, transient response, phase margin, gain margin....
...very complex..but not too complex

for now there aren't these problems. my smps work fine for me. obviously they can work better with proper skills, but i'm young..and lazy

now i understand "why" of that strange waveform, and i wanna say a "BIG thank you" for the replies, always accurate but extremely simply to understand.

thank you to everyone!!

EDIT: Eva, i see now the image you posted. Very examplary. Thank you!!

Last edited by pergo; 14th January 2010 at 06:18 PM.
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