half bridge Smps wave forms ..... 1

[ani12]

hi ,
i have built a half bridge smps with pfc front end for 2 X 200W class - D . it works fine but the wave forms that i see is troubling me .... Can any one have a look and tell me if i am doing it wrong ..... the smps has a open voltage loop . wat is troubling me is primary waveform at 90% load the wave form should be flat during the dead time which it is not ....

 

[ani12]

schematic of DC -DC half bridge converter...


regards

 

[ani12]

no load transformer primary waveform
TRANSFORMER PRIMARY AT NO LOAD.JPG

 

[ani12]

the PFC is operating at 100khz and Dc- DC half bridge is operating at 90khz . Maximum loading is 500W


Regards

 

[Comcokid]

First, the disclaimer - I haven't worked with half-bridge converters, just push-pull and full bridge.

Now, what I see is that when both switches go off, the energy in the transformer creates a spike. Amplitude of this spike will depend upon the switching speed of the back-diode in your switching devices (or the switching speed of an optional Schottky diode you put here for that purpose). Then there is the ramp. Checking some of the literature out there, I see in resonant half-bridge converters there is a small ramp caused by CDS of the MOSFET along with the capacitor. In your half-bridge, I would expect your two divider capacitors to be >> CDS, but if you have a snubber across your MOSFETs, then it might be the snubber capacitor parallel with CDS in series with your divider capacitors.

One other possible thought is that if the type of capacitors you're using for your divider capacitors exhibit a lot of dielectric absorption, then these capacitors may be changing their voltage during the off state from this effect. This will also cause you to see a ramp in the off time.