2 inductors forward converter

[junchan1122]

Hi all,,

I have a forward mode ps having 2 inductors at the secondary side. one is normal storage inductor, another is in series with the forward diode. Someone said this inductor provides resonant effect to reduce the swtiching loss....

Does any experts know how is this resonant happens? When I probe across the inductor and diode, it is a rounded/sinusoidal-like waveform. But when I probe across the freewheel diode, it is a pretty nice rectanglar pulse that feeding to the storage inductor.

Another thing is about the duty cycle and amplitude of this pulse. While the primary is cyclying at fixed frequency with about 0.4 duty cycle, but the amplitude and duty cycle on secondary seems changing as load dependent. If there is no load, the pluse is about 8V and 0.6 duty cyclye. But if the ps is loaded with 1A, the pulse is 30V and 0.18 duty cycle. The pulse keeks at this fashion even I further increase the load current. I have no idea how and why the ampitude and especially the duty cycle will change with this second inductor.

Thanks in advance,.

Jun.

 

[goldsmith]

Hello Jun
Can you attach your schematic here , please? Thus i can help you .
Best Wishes
Goldsmith

 

[junchan1122]

OIC,, I don't have its schematic. I try to sketch it out and post it.

THaks..

 

[atripathi]

Do you use Simulation? I would recommend you try Simulation tool. There are several free version available, with PSpice being one. It's difficult to comment without seeing the schematic, but just inductor in series with forward diode, does not sound good, unless it resonates with transformer winding capacitance. It would definitely reduce the switch current at turn on. But second inductor current would need a path when switch is getting turned OFF?

 

[junchan1122]

Hi atripathi,

I didn't use any simulations as I have a real workable PS on hand to measure its voltage or waveform. The schematic sketch and some waveforms were attached. I think the amplitude and duty cycle change are the nature of a bluk converter running at discontinuous mode. But I don't understand the duty cycle between the primary and secondary is not the same even it is running on continuous mode as Vo = D * Vi.

It seems there is no that path for the second inductor..

Brgds.
Jun.

 

[goldsmith]

My friends
Hi
About Forward converters ( single ended type ) : we have a transformer with 3 winding , one of them is primary , and the other is secondary . and the other is magnetize discharge . when the switch is on , at the secondary we have voltage in phase with primary , and the diode is on and the out put load will supply . and when the switch is off , the auxiliary winding , will try to discharge the core and thus the mosfet protected .
See below , please:


Good luck
Goldsmith