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elegant solution for discharging the main capacitor when power is cut off

evs
Genius 5910 points
Other Parts Discussed in Thread: TL431, TLV431, UCC29950, UCC28630

Hi,

I'm looking for a solution to discharge the main capacitor when the power is cut off. Something like a switched on bleed resistor when the power is off.

 Any suggestions?

Thanks,

  • 0
    TI__Mastermind 40580 points
    Hello EVS,

    A general question like this without supporting background information can only be answered with a generalized response based on a number of assumptions. However, perhaps the solution examples proffered can spark ideas for adaptation into a number of applications.

    Assuming an AC-to-DC converter where the “main capacitor” refers to the input bulk capacitor and power cut-off refers to input AC power, you’ll need a switch circuit that biases the switch on when AC voltage is not detected. Essentially, a high-voltage NPN transistor with low-value resistors on the collector to the bulk voltage can self-bias with high-value resistors from its base to the bulk. Without another circuit to short the base to emitter to turn it off, this switch and bleeder network will always be on whenever voltage exists on the bulk rail. Since there is no mention of how fast or slow the discharge should occur, it should be noted that a fast discharge can result in considerable power loss during start-up until the turn-off circuit becomes operational.

    The bleeder turn-off circuit needs to detect whether AC input is present or not, and needs its own power to do that, so I expect it to be attached to the primary-side auxiliary supply (assuming that there is one). As an alternative, the turn-off circuit can be powered directly by the AC line, however this will raise standby power considerably, since it will be running from high voltage all the time, even during no-load conditions. A detection and switch-off circuit running from the low-voltage auxiliary supply may be more complex, but can contribute much less to the standby power. A less complex detection method may be to infer loss of AC by detecting loss of the auxiliary power which occurs when the bulk capacitor runs down below its minimum design point, if the delay between loss of AC and detection of aux dropout is acceptable.

    Adding details beyond these examples will require making more assumptions about a specific application, but I hope this is enough to get the design ideas rolling.

    Regards,
    Ulrich
  • 0 in reply to Ulrich Goerke
    Genius 5910 points
    Ulrich, Thanks for the reply.

    If you take the forum name and main capacitor bleed resistor in account thinks that explains a lot what I'm looking for. The only missing information if there is an aux supply, and there is.

    I was wondering if TI has standard solutions for that?? something like a digital NC relay for high voltage/low current. I did assume is it is a very common problem.
    It is not very difficult to make a circuit myself. Only you need a reference(zener) to detect if the aux power is present or not. That reference(zener) need to be powered from the main capacitor and needs at least 1mA to work properly. And that is in the same scale as the bleed current. For that to work properly, I'm looking for a very low current reference from the main supply.
  • 0 in reply to evs
    TI__Mastermind 40580 points
    Hello EVS,

    I am not aware of any ICs specifically dedicated to bulk-cap discharge. That may be a concern is certain applications where the bulk rail may be exposed to touch after power down, but it doesn't appear to be a very common problem in my experience, and is usually solved on an ad-hoc basis. Aside from a zener-type reference, one can consider a TL431 or TLV431 (for lower bias current) or a similar shunt regulator to use as an adjustable reference and switch within the bleeder turn-off circuit. There are also several logic-level output voltage monitor ICs that might be adapted to this application.

    A more common problem to my knowledge (specifically for un-pluggable power supplies) is rapid discharge of the X-capacitors in the EMI filters, to meet safety requirements for residual voltage on the exposed AC-plug prongs. There are at least two primary-side controllers by TI, to my knowledge, that directly incorporate this feature: UCC28630 off-line flyback controller and UCC29950 PFC+LLC combo controller. There may be other digitally-based controllers that can provide the signals, but not the direct interface to accomplish the discharging.

    I hope this helps you.
    Regards,
    Ulrich