PSFB output ripple current issue

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pxidr

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Hi everyone,

A bit new here, seeking help for my issue after many attempts to solve the problem myself...

I'm currently developping a 100kHz, 24kW PSFB for an EV charger, using a full-bridge SiC MOSFET at the input (700VDC nominal), a custom-made planar transformer, and full-bridge SiC diode rectification at the output (4x2 STPSC30H12CWL arranged in parallel for current handling), the goal of the design is outputting a variable 50-500VDC at 64ADC max.

My output inductor is 33uH and my output capacitance is 200µF, using 4x50µF low ESR film caps (TDK B32776Z5506K000).

I'm currently testing my PSFB on a 300VDC lab power supply and a 2kW resistive load, voltage and current regulation at the output are fine.

At 110VDC output, 10ADC, my voltage ripple is acceptable (about 1,26V pk-pk), but my current ripple is way too high (776mA pk-pk).

I'm unable to resorb this current ripple. The weird thing is that current ripple doesn't increase if I increase the load, it is pretty constant at 2A, 5A, 10A etc.

Any suggestions for filtering out that current ripple ?

 
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I use 8x STPSC30H12CWL 30A TO-247 schottky diodes (2 diodes per package), however with all that diodes in parallel, I increase the total diode parasitic capacitance, also all these TO-247 long leads also adds stray inductance.

So I will soonly switch to a SOT-227 diode module do reduce all of these undesired parameters. Also, easier to mount and heatsink.

Synchronous rectification is more suitable for lower voltages, but for high voltages, I've read it is less.
SiC diodes also exhibits better switching performance (virtually no trr) than their Si counterparts.

Also, all the converter (3ph AFE PFC + PSFB + magnetics) is mounted on a quite large heatsink (500*300*40) with forced ventilation on it, with 97% targeted efficiency the total losses should not exceed 800W at full load, so I don't believe thermals are an huge issue here. It is not as small nor confined as an OBC, where you effectly need watercooling.
 
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Yes, i think for your case, the heatsink needs to be like a "tunnel" of metal, with fan air blowing through it, and the semico's mounted along the outer faces of that "tunnel".
 

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cupoftea said:
Yes, i think for your case, the heatsink needs to be like a "tunnel" of metal, with fan air blowing through it, and the semico's mounted along the outer faces of that "tunnel".
Click to expand...
Exactly this, with two 14W 120mm fans blowing thru it...
 

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