External Charge Pump For High Side MOSFET Driver

Discussion in 'The Projects Forum' started by Unjust, Oct 20, 2009.

  1. Unjust

    Thread Starter New Member

    Oct 14, 2009
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    Hi all,

    I am in the process of building a PWM MOSFET slot car controller. My 'control' stage works fine and now I am on to the 'power' stage. The 'control' stage is to send a 10kHz PWM signal to either the 'drive' or 'brake' FETs via an L6387E High & Low-Side FET Driver. The L6387E contains an internal Bootstrap Diode which charges an external Bootstrap Capacitor for the High-Side Driver (Throttle). As most of slot car racing is performed at full throttle (ie 100% duty), the Bootstrap Capacitor would quickly lose its charge. Would my circuit in the attached file be the correct way to connect an external Charge Pump (555 Voltage Doubler) to the L6387E, to keep the Bootstrap Capacitor filled during the 100% duty stages? Do I even need the Bootstrap Capacitor if I use an external charge pump?

    Any feedback would be greatly appreciated.


    Ken

    L6387E: http://pdf1.alldatasheet.com/datasheet-pdf/view/242080/STMICROELECTRONICS/L6387E.html
     
    Last edited: Oct 20, 2009
  2. Thav

    Member

    Oct 13, 2009
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    The capacitor wouldn't necessarily keep draining once you've turned it on the first time. It turns on once, transfers charge to the FET gate at the transition, and then is only discharged by leakage current in the FET gate and self discharge of the capacitor. Both of these are small and could eventually cause your FETs to turn off.

    I'd say look up those characteristics from the FETs and capacitors you're using and calculate out the effect of the gate leakage and self discharge. You should see an image like this on the FET data sheet.
    [​IMG]
    In that case you would maybe drive to 12V gate to source, and your gate can drop until the flat on that curve at ~2.5V before the FET starts to turn off.

    By the way, also look up your FET's input capacitance (given as either Cis and or Cgs, they are not the same but close enough for this purpose) and make your bootstrap value about ten times that. 0.1uF is probably enough, but I'd recommend getting a film or ceramic cap instead of an electrolytic, because they will charge and discharge faster.
     
  3. Unjust

    Thread Starter New Member

    Oct 14, 2009
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    If the longest straight is say 2s long, then at a PWM freq of 10kHz, the FET Gate is being recharged ≈20,000 times. With a 2980pF Ciss, the Bootstrap Capacitor will be drained: 2980pF x 20,000 ≈ 59.6μF, before leakage and self discharge? That would be a big, expensive non-electrolytic capacitor!

    So if I change my code so full throttle = Pin High instead of a full duty pulse, the gate is only being charged once per straight? Then the 0.1μF Bootstrap Cap would survive: 0.1μ / 2980pF ≈ 33 full throttle applications without being replenished?

    Thanks
    Ken

    MOSFET: http://pdf1.alldatasheet.com/datasheet-pdf/view/68150/IRF/IRF3711.html
     
  4. Unjust

    Thread Starter New Member

    Oct 14, 2009
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    If I modify my code so full throttle = pwm pin high (instead of a 100% duty pulse), then the gate capacitance will only have to be charged once for a continuous full throttle application, and will last 0.1uF / 2980pF = 33 applications (according to the Ciss of an IRF3711). Will the Bootstrap Capacitor recharge during a part-throttle application (ie. High-side and low-side FETs off) or will it only charge when the low-side FET is on (ie.When brake is applied)? As I understand from the documentation, the Bootstrap Cap only charges when the low-side FET is on, therefore a slot car does not brake long enough per lap to equal the recharge of the Bootstrap Cap to supply full and part-speed applications for the following lap.

    I guess what I am wanting to know is if I apply my original circuit, even just as a safeguard, is it the correct way to recharge the Bootstrap Cap (will it overcharge the cap and damage the IC?). Just can't get my head around these voltage-driven applications that still depend on a current supply.

    Thanks,
    Ken

    IRF3711 MOSFET: http://pdf1.alldatasheet.com/datasheet-pdf/view/68150/IRF/IRF3711.html
     
    Last edited: Oct 21, 2009
  5. Thav

    Member

    Oct 13, 2009
    82
    0
    Yes, the bottom FET would have to turn on to charge the bootstrap cap. You won't really be able to recharge the bootstrap cap while your top FET is gated, not without another converter anyway.

    I see where you're going with the 33 applications, but if you run the 100% duty cycle you're not having to reapply the gate voltage. If you just gating and ungating the top gate you probably will run out of steam. You'll have to turn the bottom FET on.

    You could go for a P channel FET for the top devices and not have to use the bootstrap driver. Then you can just gate with your single supply. The drawback will be that PFETs have higher channel resistance, so you might have to get a slightly more expensive device to get the larger chip area in the TO-220 or bump up to TO-247 (a big jump).
     
  6. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Ken, I don't know if this will help you but I found it researching my own problem.
    http://www.irf.com/technical-info/designtp/dt92-4.pdf


    It's a charge pump circuit that allows continuous on time from a gate driver. Hope it helps you.

    Cary
     
  7. Unjust

    Thread Starter New Member

    Oct 14, 2009
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    Hi Cary,

    Did you use the circuit in the end? I simulated it and it worked, so I'll give it a go. I found the attached circuit in a datasheet for a different FET Driver. It also simulates alright, but only for 1%-99% duty as it doesn't have its own oscillator. Less components though. But as slot car racing is ≈80% full throttle, I'll implement your suggestion.

    Many thanks,
    Ken
     
  8. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Hi Ken, haven't used it, just found it in my research for my project. Glad it helped out!

    Please post the out come when you try it out.

    Cary
     
  9. Unjust

    Thread Starter New Member

    Oct 14, 2009
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    0
    Hi Cray,

    I tried the charge pump from DT92-4. I simulated it...pass. I breadboarded it with its own final storage capacitor...pass. Removed the final capacitor and ran leads to my half-bridge...epic fail (eg.pop fizzle :eek:), half-bridge doesn't go anymore. Lucky they come in packs of 5! Probably just a wrong connection, but I replaced the circuit with my original voltage doubler but modified it to resemble DT92-4 (see attachment). Perfect! Can run full duty all day. Might look at zenering the +V of the 555 so I can up the voltage of the circuit, but first I'll sort out noise and back-emf protection.

    Thanks for the ideas and tips guys. Maybe everyone will now run high-side N-FETS!

    Ken
     
    Last edited: Oct 28, 2009
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