Comparison of reverse recovery in flyback vs forward?

[grizedale]

Comparing a flyback and a 2-transistor forward converter (both CCM)......both suffer reverse recovery of output diodes............but considering the resultant current spike in both topologies, which reverse recovery transient causes the worst EMC problem?....

--is it the flyback's

or the 2-transistor forward converter's

(generally speaking, i.e. if they are of similar power)

 

[goldsmith]

Dear grizedale
Hi
Fly back converter , has bad out put spikes and regulation ratio . and you can not get the out put power , more than 350 watt with best designing ! . but the forward converters can give you powers up to 1KW with good ripple rejection ratio (load regulation) . and without bad output spikes(approx) .for more information and designing methodology , you can read books in this field , such as switching power supply design of abraham i press man , or switching power supply cookbook of marty brown or .....
Best Wishes
Goldsmith

 

[grizedale]

Thanks,

May i ask what you mean by "Bad output spikes"?

CCM Flyback output diode current peaks are high, but not massivley higher than a Two transistor forward converter.

-in flybacks, ...how high the peaks are depends on how deeply continuous-mode you are......the more deeply continuous-mode, the lower the secondary peak current.

 

[Mehrshad74]

Usally flyback transformers have larger air gap and consequently more leakage inductance that will limit the dI/dt.
on the other hand the current of output diode in flyback is very larger usually because the load current=d.Im but in forward IL=Im.
I guess fly back will generate more EMC because the diode current is 2-5 times of forward converter.

 

[goldsmith]

Dear grizedale
Hi
When the stored energy in inductance of transformer , want to supply the load , it will have instantaneous value , ( because of that the on / off time) .
but you can improve it (approximately) with some snubber networks or adding a good filter in your out put .
Best Wishes
Goldsmith

 

[grizedale]

Mehrshad74:-

Here are 1 Transistor Flyback (CCM) vs Two Transistor Forward currents for supplying the same load of +/-40V, 4A from Vin = 385VDC:-

1. FET Current RMS = 1A95 for Flyback; 2A for Forward
2. FET current peak = 5A12 for Flyback; 5A9 for Forward.
3. Diode Current Peak = 7A6 for Flyback; 6A4 for Forward
4. Output cap ripple = 2A63 for Flyback; 1A3 for Forward


.....each switched at 66.7 KHz.

These numbers are pretty much the same.......there seems to be no reason why a Flyback cannot be used for the 320W power if a Two Transistor Forward can.

So why is the Flyback so hated for these high powers?
-i see no reason other than perhaps a drop in efficiency of maybe 5% due to the switching loss's and loss from the leakage.
-but our application is not bothered about efficiency that much.......its a guitar amplifier power supply.