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LM5050-1: Disable diode-mimic operation

Part Number: LM5050-1

I want a simple FET driver to connect between a diode bridge and bulk filter capacitance. The bridge output will be 0 to about 25VDC (bus voltage) and it will switch at around 120 Hz. I'd like to use the LM5050 but I need the FET to deliver current in both directions (in and out of the cap)  - could I connect the LM5050 "In" and "Out" pins together to disable the diode mimic function of the part? The FET would only connect to the "Gate" and "In" pins of the LM5050.

If this is not possible, can you suggest an alternative configuration, or an alternative part. Some of the half bridge drivers have under voltage lock out so not suitable - I need the FET to turn on with little or no bus voltage.

David

  • Hello David,

    Based on your question, you would like to allow reverse current to flow from bulk filter capacitance to bridge?

    Could you share the amount of current that you want to allow and MOSFET you plan to use with LM5050.

    Additionally may be required to block reverse current in all the scenarios?

    LM5050 may not be able to support operation down to 0V (even with external DC supply bias).

    Other option is to look at LM74610 device or LM74670 which can operate with even 0.5V bus voltage.

    Regards,

    Kari.

  • Hi Kari,

    I want current to flow both directions, essentially the FET is a short circuit from the capacitor to the bridge output. 

    Current will be probably 20A max. I haven't chosen a FET yet - I dont see a big problem here, since my risetime and current requirements are easy to meet.

    FET is either on or off as controlled by the system via the driver, no diode-like blocking required in any scenario.

    While the bus may go to 0 the driver would be supplied from a separate DC supply of my choosing, made to match the FET driver. While I stated operation of the driver at 0V was required that was a bit of an exaggeration and 3 or 4 v is probably enough.

    I'll look at your suggested devices tomorrow.

    Thanks

    David

  • Hi Kari,

    Neither of the devices you mentioned have an external control input ie to allow my system to control the FET, they are fully automatic diode emulators.What I really need is the top half of a half bridge, without all of the UVLO functionality.

    David

  • Hi David,

    Yes, if you would like to control MOSFET reverse current blocking (diode functionality) ON/OFF, then adding a PnP transistor and diode would work.

    Connect a small signal diode from IN to OUT (with Anode connected to IN and Cathode connected to OUT) and a resistor 100k from OUT to GND.

    Next connect a PNP transistor between drain of MOSFET and OUT pin, with emitter of PNP connected to drain of of POWER MOSFET and collector of PNP connected to OUT pin. Next Base of PNP transistor is used to control the PNP. Pulling the base of PNP to GND via a resistor/another signal MOSFET is needed.

    Pulling base of PNP to GND, turn on PNP transistor and connects OUT pin to drain of power MOSFET. Leaving base of PNP open, keeps the PNP transistor off. But small signal diode from IN to OUT and resistor OUT to GND, keeps OUT pin always below IN and the external Power MOSFET is always ON.

    Regards,

    Kari.

  • Hi David,

    If you think your question is answered, click "this resolved my issue button" at the end of the thread.

    Regards,
    Kari.