UCC25630-1EVM-291: Burst start from 30% of max.load

Hi Hiroshi-san,

Thanks for your interest in UCC25630-1EVM. The burst mode of UCC25630x is configurable through the resistor divider connected from RVCC to the LL/SS pin. The burst mode % of max load is driven by system requirements such as standby power. There is no right or wrong value for the burst mode threshold setting.

In practice, you will want to make sure the LLC is bursting frequently enough so that VCC is replenished from the bias winding. If VCC drops below 10.5V during a long burst-off period, then the HV startup circuit will charge the VCC capacitance back up (this can lead to higher standby power if the HV startup circuit is periodically getting enabled at light load).

Best Regards,
Ben Lough

Ben-san,

Thank you for your advice.
This thread is for UCC25630-11EVM as is condition.
If the output load focusing from 1.2w to 2W, can you select the value of R14 and R15 for the best average efficiency ?

Hi Hiroshi-san,

To decrease the power consumption of the LLC converter, you will want to increase the resistance of R15. you just need to guarantee the burst-off period is not too long in order to prevent the HV startup function from enabling to charge VCC.

Best Regards,

Ben Lough

Ben-san,

Which is better R14=R15= 510k or R14=R15= 750k ?

Hi Hiroshi-san,

R14=R15=510k should give you a higher burst mode threshold.

Best Regards,
Ben Lough