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UCC28632: Webench AC to DC design, no power after AC power cycle

Aric Gerspacher
Prodigy 120 points
Part Number: UCC28632

Here is the Webench link:

webench.ti.com/.../SDP.cgi

(edit) links to Webench not working

System runs fine, but will not output power after an input AC power cycle.

Prototype built from this properly supplies power on secondary side on initial power up, system runs fine.

120VAC supplied through a power strip to this prototype.

After flipping the switch to the power strip off for any duration of time,  then turn back on, no power.

Lots of volts on Cin and Cin2 when power is off.

Discharging Cin and Cin2 while power strip is off, then turning on the power DOES work.

Is there a better way to power cycle with this than discharging the capacitors?

They seem to discharge very slowly.

  • 0
    TI__Mastermind 24145 points

    Hi Aric,

    Thank you for reaching out.

    Do you have any bleeder resistor connected across the capacitor? This should discharge them to safe electrical operating levels before you can power cycle again.

    Regards,

    Harish

  • 0 in reply to Harish Ramakrishnan5
    Prodigy 120 points

    A bleeder resistor and a switch have been recently soldered in place to discharge them. However such a resistor is not placed in the Webench design nor the example schematic in the  UCC2863x datasheed (Figure 44, Typical Application Circuit for 19.5V / 65-W Adapter).

    It initially seems like simply placing a permanent bleed resistor across the capacitors is a bad idea. It will be a constant power drain, and the 350V across the rectifier bridge will require a large value resistance to keep power low but conversely that will lead to very long discharge times. 

    Is it common to have a bleed resistor in AC adapter circuits?

    Is a better answer to have some independent circuit that monitors the AC side and safely brings down power in events like this?

  • 0 in reply to Aric Gerspacher
    TI__Mastermind 24145 points

    Hi Aric,

    Thank you for reaching out.

    I have seen some chargers which hold bulk voltage for a long time even after the power supply is turned off.

    Generally the Xcap discharge feature takes care of this or I have seen resistors placed across the capacitor. But it can affect low standby requirement.

    Regards,

    Harish