UCC28951: Inquire SCH review

Hello Edward

I'd make the following comments

You have a shim inductor in place, L3, but no clamp diodes. These diodes are needed to control the peak voltage on the output rectifiers - they are shown as DB and DC in Fig 48 in the UCC28950 data sheet. You should check the rectifier voltages and add the diodes if necessary.

You should also add 10k resistors from the SR gates to their sources - as you have done on the primary side MOSFETs.

Please make sure that the layout of the bootstrap diode circuit on U1 and U2 charges the bootstrap capacitor fully during the first switching cycle of a burst - this is especially important for U2 driving the 'C' and 'D' MOSFETs - see the note at Figure 1 in the UCC28950 Data Sheet.

There are two reasons you may not be achieving ZVS - insufficient energy storage and incorrect delay times. Check the value of the shim inductor according to the information in the Data Sheet. Given that there is sufficient energy to achieve ZVS then you need to adjust the timing resistors (DELAB, DELCD, DELEF, ADEL and ADELEF) to achieve the best balance of ZVS across the load range.

Regards

Colin

Hello Edward

TI e2e is a public forum and there is no 'private' option.

The only option is to move this conversation to email - if you wish to do this then please contact me directly at colingillmor@ti.com     

Regards

Colin