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UCC25600: UCC256404 Resonant Capacitor Cr - Capacitive Divider?

Part Number: UCC25600
Other Parts Discussed in Thread: UCC256404, UCC25640EVM-020

Some of the UCC256404 literature shows the resonant network's Cr as a capacitive divider with the upper capacitor from +400V to the junction and a lower capacitor from the junction to GND. I haven't see this method used on most other QR Converters I have looked at.

I doubt it would be a capacitive divider anyway.

The Excel Spreadsheet UCC25640x Design Calculator Rev1.0 show's the more typical single capacitor method on its schematic page (Same as on the "schematic-less" UCC256404 data sheet).

The UCC25640EVM-020 Evaluation Module contains full schematics which do show the upper and lower capacitors.

For proper UCC256404 operation, based upon the Excel Spreadsheet UCC25640x Design Calculator outputs, what should I use?


Peter Baxter

  • Hi Peter,

    The "capacitor divider" is known as a split resonant capacitor structure. The only difference between the split resonant capacitor structure and the traditional single cap connected to ground is that for the split structure, half of the resonant capacitance is connected to ground and half is connected to Vin. Split resonant capacitor will have the possibility to "pre-charge" the capacitor mid point to 1/2*Vin more quickly during startup. The benefit of this is the transformer will be become symmetrically driven faster and have less risk for hard switching during startup. A single resonant capacitor to ground will start with 0V dc bias. The single resonant capacitor will need to be charged up before the transformer can be driven symmetrically, so there is more risk for hard switching during startup. The benefit of a single resonant capacitor is the layout is a little easier as you do not need to rout the resonant capacitor back to the input voltage. Both approaches are perfectly acceptable to use with UCC256404. For the UCC25640 EVM, 30nF was used for the resonant capacitance. The two 15nF capacitors could either be both connected to ground or have 1 connect to ground and the other connect to Vin. 

    Best Regards,

    Ben Lough

  • Hi Ben,

    Excellent response.