Speed Controller problems with bootstrap

Discussion in 'The Projects Forum' started by oddball, Jun 5, 2011.

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  1. oddball

    Thread Starter New Member

    May 29, 2011
    I'm trying to design a speed controller for a permanent magnet brushed motor that draws up to 12 amps, running with a 12V supply. I've managed to fry about 10 expensive FETs at this point, figure it's time to ask for help. Need to support a minimum of about 25% duty and a maximum as close as possible to 100%. Only need one direction, so only need half of the h-bridge. The half-bridge drivers I easily found were all surface mount, and now I have a handful of the 4082's, so I'm really hoping they can stay. All my fets are dead, so new ideas there are welcome!

    Note that I'm not very good at electronics, so please use small words. I can solder OK, but needs to be through-hole. I'm currently working on breadboards (other than the high current side, which is soldered), although I think the timing circuits are in good enough shape to build a board. That, of course, means perf board for me. I have three scopes at my disposal, but the youngest is about 25 years old. Each has its own interesting 'quirks', so I switch between them often. That's all to say that I can scope the board, but I'm never certain if noisy/unstable signals are the board or the scope or my cheap probes.

    Current design uses a 555 and comparator for the PWM then feeds a HIP 4082. I'm using IRF1324 for the power MOSFETs (Rds of 1.2mΩ, Id 195A). The bootstrap diode is a STPS2L60RL, a 60A schottky, fast recovery.


    Drat - just noticed an error in the schematic. The bottom of the resistor divider circuit that feeds the 393 (and drives the PWM duty cycle) should tie to ground - not to the non-invert input! Sorry.
    (full size attached below)

    Current issue is the low side fet heats up quite a lot on anything other than 100% and quickly dies closed. I'm pretty sure the high side isn't switching as it's only getting 12V at the gate. A prior revision - using a TL494 to generate the PWM, but otherwise the same - had the high side hitting 24V at the gate for only a small part of its on cycle, then quickly falling to the average motor voltage (ie, 5V @ ~60%). There is a 'swooop' on AHO, where it bounces below ground when turning off and hits just above 12V when turning on. I had also tried a run-of-the-mill diode for the bootstrap, but same effect. I tried upping to a 100uf 50V capacitor, but that one actually exploded. A 47uf 50v - which doesn't seem otherwise different - has survived just fine.
    Obviously if the high fet isn't switching, then the internal diode on the low fet is taking the brunt and causing the heat.
    I also ran this design but using a 6A diode instead of the high fet, but same results (low side overheat, death, fire, destruction).

    Maybe the gate capacitance of the fet is too high? Clearly lots to pay for the high current rating. I was aiming for lowest possible Rds(on) at the time. Maybe the IRL3715 makes more sense? Also loads cheaper!

    So, at this point I'm at a loss. Please let me know where my errors are and what I can do to correct! I'm hoping the primary problem is component selection for the bootstrap, but I just don't know enough to select something else. Many, many thanks!
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
    The pullup resistor on the comparator may be too low. LM393 only guarantees 6mA ssink current. You are trying to sink 12mA.
  3. bertus


    Apr 5, 2008
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