Bootstrap Calculation for IR2110

Discussion in 'General Electronics Chat' started by adailton, Jul 31, 2011.

  1. adailton

    Thread Starter Member

    Apr 1, 2011
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    Hi,

    http://forum.allaboutcircuits.com/showthread.php?t=33685

    Firstly, I'm sorry I'm going on same topic after years, but I'm suffering from same thing. I read this topic many times and I searched, but I didn't found anything satisfying.

    In AN-978, there is a formula for bootstrap calculation but I don't understand for a week. Vf, Vls and Vmin aren't clear. I didn't see alternative way from simulation results for these voltages.

    In this topic, in first message, DumbDummy77 used some values for Vf and Vls. I didn't understand that where I take these values(Are they in simulation results?).

    If somebody help me, I will be grateful.
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Vf - forward voltage drop of the bootstrap diode, depends on the diode chosen, in general between 0.4 to 0.7V
    Vls - Voltage drop across the low-side fet (conducting)
    Vmin - is the minimum acceptable Voltage you want for the high side driver Vb-Vs, where Vb is positive and Vs is Gnd (for the high side!)

    Have a look at the http://www.irf.com/technical-info/designtp/dt98-2.pdf
    as well.
     
  3. adailton

    Thread Starter Member

    Apr 1, 2011
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    I use uf4007 diode( I didn't find UF4001) and its Vf=1.7 V(its constant)
    For IR2110, min Vb=Vs+10 =>> Vmin=10V ??

    For Vls calculation, I use IRFZ44n and its Rds=17.5 mOhm, I used 4A current for conducting current, Rds*Icond= 0.07 V ??

    With these values, I found bootstrap cap. at least 1,15 uF , its suitable.Isn't it?
     
  4. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    I'd use a diode with a lower forward drop, as long as the DC bus voltage doesn't exceed the diode's reverse voltage, of course.

    So you have at most 10V to drive your MOSFET? Is this enough to drive it safely, keep it's heat dissipation low and does it not negatively affect the turn-on turn-off times? Have a look at it's dataheet. edit: if it's the IRFZ44n as well, it should be ok.

    Voltage drop on the FET should be ok.

    I didn't make the calculation, if it gives you 1,15uF, then you should be good with this value, take the next bigger one, 2,2uF.
    When choosing the cap consider that this capacitor is the one where you take the gate charge current from.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    These definitions are given in the application note; my notes in parenthesis afterwards:
    Vf = Forward voltage drop across the bootstrap diode. (You obtain this information from the datasheet of the diode that you are using. The diode should be a fast recovery or Schottky type.
    Let's say you were using a UF4004 type. A datasheet is here:
    http://www.fairchildsemi.com/ds/UF/UF4004.pdf
    Looking at the datasheet, you can find that the Vf for the UF4001 through UF4004 is 1.0V @ 1.0A. UF4005 through UF4007 diodes have a Vf of 1.7v @ 1.0A)

    VLS = Voltage drop across the low-side FET or load. (You can calculate the voltage drop across the low-side FET by multiplying the FET's Rds(on) by the expected drain current. If you are using a current sense resistor on the source terminal, you need to add that voltage drop in as well.)

    VMin = Minimum voltage between VB and VS. (Vb = the high side of Cboot, the boot capacitor, and Vs = the junction of the high side MOSFET source terminal and the low side MOSFET drain; the load connection point.)

    You really should take the values from manufacturer's datasheets downloaded from their websites instead of simulation results. Don't rely on datasheets obtained from generic datasheet websites, as they are frequently outdated; also the manufacturer may have discontinued that part, and may be recommending a newer/better part as a replacement.

    [eta] Gee, I should have done a screen refresh after being on the phone and several interruptions...
     
  6. adailton

    Thread Starter Member

    Apr 1, 2011
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    Thanks for all helps. I'll take care that you said.

    Still I don't be sure about Vls, but its contrubition is very small. I use 0.07 V.

    For diode, I chose 1N4148, not only faster, but also lower Vf(0.7 V). And I will use this diode in resistor/diode block for dead-time generating. No need schottky, allright?
     
  7. SgtWookie

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    Jul 17, 2007
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    1N4148/1N914 diodes are extremely fast, but have limited current carrying ability, and their Vf rises rapidly when near their maximum.

    I tested a random 1N4148 several years ago and created this plot to show Vf over a range of If:

    [​IMG]

    I recommend that you use a fast/ultrafast diode with a higher current rating.
     
  8. adailton

    Thread Starter Member

    Apr 1, 2011
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    Is Vf(max) still low? What is problem? And how I decide that diode has higher current rating?I'm sorry, I want to learn.
     
    Last edited: Jul 31, 2011
  9. SgtWookie

    Expert

    Jul 17, 2007
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    You really need to look at datasheets for the diodes that you are considering using.
    The problem with using diodes like the 1N4148/1N914 is that they are only rated for 75mA to 100mA current, and your peak currents can be very high. If your MOSFETs have a very low gate charge, you might get away with it, but you have not provided any information about them.

    A Schottky diode is constructed differently from a silicon diode; it is N-doped silicon bonded to metal; chromium, platinum and a few other metals can be used; I can't remember offhand. When a standard PN junction goes from forward-biased to reverse-biased, it takes time for the junction to shut down due to the charge carrier depletion region. But since the Shottky diode doesn't have that region, there is virtually no reverse recovery time. This is important for your boost cap supply diode and your gate diode.

    Also, Schottky diodes generally have a lower Vf than a silicon diode counterpart. The low Vf is important from an efficiency standpoint.
     
  10. adailton

    Thread Starter Member

    Apr 1, 2011
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    I really thank you. I understood of all. I'll go for schottky diodes.(for example: 1N5819( ST brand BYV-10) schottky))

    For information, I'm building full bridge class D amplifier. My mosfets IRFZ44n and have low gate charge.

    This necessity for only bootstrap diodes I learned. For dead-time generation block(diode/resistor on mosfets gate) , I use 1N4148.
     
    Last edited: Jul 31, 2011
  11. SgtWookie

    Expert

    Jul 17, 2007
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    The 1N5819 will be OK for the boost cap charge if your supply voltage is under 35v. If it is higher, you will need a higher voltage diode. A 1N5817 would be a better diode to use on the gate, as it has a much lower Vf than the 1N5819. It's Vr is only 20v, so not useful for a boost cap charge diode over about 15v.

    The IRFZ44 doesn't exactly have a "small" gate charge; Qg is 63nC, which is fairly significant. But, we don't know what you are planning on for your whole project, as you have not posted a schematic, so it's impossible to suggest what might work better for you. The IRFZ44 is fairly old.
     
  12. adailton

    Thread Starter Member

    Apr 1, 2011
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    My supply voltage is 20V in full bridge.But all components isn't found in Turkey. On gate, is 1N4148 useless?

    Irfz44n has a bit high Qc but Rds(on) very low (17 mOhm). best mosfet I found is it :)
    my projects one half is here:

    [​IMG]
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    You can try it and see how it works for you. However, if the 1N4148 fails on you, you may not realize it in time to prevent your MOSFETs from burning up due to insufficient dead time.
     
  14. adailton

    Thread Starter Member

    Apr 1, 2011
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    In simulation, big gate resistor= big dead time. In real world, I don't know yet. So, I'll use 33 Ohm tomorrow, and I'll reduce. You recommend me that sch. diode for in gate, I'll try it. Are there any suggestion for me, for my circuit?
    Thank you for all.
     
  15. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Should be the same in real world. in general: the bigger the gate resistor the longer it takes for the gate to be charged.

    Adailton, I can't resist, your name lets me think that you are from Brazil. Is that so?
     
  16. adailton

    Thread Starter Member

    Apr 1, 2011
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    Haha, don't resist :) yes adailton from Brazil, but I'm from Turkey. Adailton is a footballer, was a wonderkid in Football Manager PC game. I saw his future was brilliant and I liked his name and I use. But he didn't explosion :) I'm keeping alive his name :)
     
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