UCC25600: about bias power supply at startup

Hello Kaji-san

The absolute easiest way to do this is to use a resistor from Vin to charge the VCC capacitor on the UCC25600. Place a zener diode across the capacitor so that VCC cannot exceed the 20V abs max rating of the IC. When you connect Vin the VCC capacitor will charge to the UVLO_on threshold at which time the controller will start switching. VCC will then start to drop because of the increased current taken by the controller but the aux winding can then take over and supply the VCC rail before it drops below the UVLO_off threshold. You will need a larger than usual capacitor at VCC - may be about 100uF. The resistor to Vin will always draw current and this will mean that light load and no-load power will be high. The time needed to charge the capacitor at VCC will be quite long because you want to minimise the current in the resistor - 1 or 2 seconds is a reasonable target to aim for.

This approach is inefficient, no-load power is significant and start-up times are long, if these are not concerns for you then this approach can and does work. Here is a sketch of what I mean -

Please let me know if you need any further information

Regards

Colin

Hi Kaji-san

As a follow up - my colleague Yamashita-san suggests that the bias circuit in PMP5141 would work nicely for you and I'd agree with him.

you can find it at www.ti.com/.../slur403.pdf

Regards
Colin