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[MOVED]what is burst-mode PFM in power converter design ??

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noahsoon

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I have read from one ''power electronics Tech" notes:

"there are several variations on PFM, such as single-pulse PFM, multipulse PFM and burst-mode PFM."

Could anyone tell me the different between these PFM modes? :-(

---------- Post added at 14:38 ---------- Previous post was at 14:30 ----------

I roughly know that, for fixed on-time PFM, if the inductor current returns to zero in every switch cycle, then it is single-pulse PFM. If the load current is higher, and the inductor current ramps up before fully returning to zero in maybe 2 or 3 seccessive cycles, it enters multi-pulse PFM. multi-pulse PFM has larger output voltage ripple compared to single-pulse PFM.

but what is burst-mode PFM?
 

carkiller

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burst mode came out of the energy star and other 'green' regulations requiring less than one watt limit of standby power for many electronic appliances (and the limit is dropping). How does this burst mode do this?
Take a typical 100 watt off line smps, flyback, forward, resonant whatever. It might be 85% efficient at 100 watts full load, but at no load it still may sap 2-4 watts from the line. Most electronic devices never completely shut off; some electronic causes them to go from standby to operate - they bootup quicker that way - since we cant even toast bread in this world without a uP or two.
Anyway, lets say a given appliance needs 100 mW of power in standby; the switcher above still draws 2-4 watts, so its eff in standby is 2-5% (YUK!)... but lets say turn off the switcher completely and we moniter the 5 VDC output, when it drops to 4.75 VDC we briefly turn the switcher full bore for a few cycles until it gets the voltage to 5.1 or 5.2 volts and then turn it off again until the voltage falls back to 4.75 again... GUESS WHAT? we are now operating at about 85 % eff while delivering only 100 mW average to keep things alive in standby.
 

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