how to choose appropriate bootstrap diode for MGD

Discussion in 'The Projects Forum' started by etiquoe, Jul 12, 2010.

  1. etiquoe

    Thread Starter New Member

    Jul 11, 2010
    hi everyone

    i wanna built 24V 3 phase bldc motor driver.
    i'm using IRF540N MOSFET for the bridge, with 24V highside voltage rail,
    and i'm gonna use ir2110 for driving high and low side.

    i'm trying to understand about choosing bootstrap components based on AN978 and DT-98-2, but still confused about choosing bootstrap diode. i mean about its VRRM for selection concideration.
    anyone can help please?

    Last edited: Jul 12, 2010
  2. SgtWookie


    Jul 17, 2007
    Well, you want a pretty fast diode, preferably with a low Vf. A 1N5819, 1A, 40V could be a likely choice. If you just want to make certain that the Vr won't be exceeded, you could go with a MUR120 or the like - but it's not a Schottky; just a fast recovery diode.

    People seem to have a lot of trouble with the IR2110, because they don't really understand how the bootstrap works, and the datasheet and application notes don't make it really clear how it works either.

    In order to charge the bootstrap cap initially, you need to have the high side OFF, and the low side ON - both sides - for a period of time.

    I suggest for the bootstrap cap to use a 0.1uF and 47uF cap in parallel.

    However, in order to make certain there is adequate storage available locally, you also need a couple of caps that are at least that large (preferably several times larger) on the other side of the diode, so that when it returns to ground reference, it can be immediately charged back up, particularly if the load is not inductive.

    If your load is inductive, make certain that you have "flywheel" diodes on either side of the bridge.

    Remember that you cannot keep the high side MOSFET on indefinitely. If you attempt to do so, the charge will bleed off the boost cap, and you will wind up with fried high-side MOSFETs. You need to turn it off for a moment (with the low-side MOSFET on) in order to recharge the cap.

    If you use the SD (shutdown) input, you will discharge the boost cap; so you must re-charge the boost cap before starting the bridge back up. Do not use the SD input for a bridge inhibit; only use it if you wish to shut the bridge down for an extended period of time.
  3. etiquoe

    Thread Starter New Member

    Jul 11, 2010

    you in DT-98-2 app note?
    using two capacitors with different type (electrolytic and ceramic)?