High & Low Side NMOS Driver & Cboot

Discussion in 'The Projects Forum' started by Herman.C, Aug 5, 2010.

  1. Herman.C

    Thread Starter New Member

    Jul 29, 2010
    Hi everyone,

    I am currently trying to get the L6385 NMOS driver to work with my 3 phase inverter bridge. I thought that the use of the driver would simplify things down a bit and allow me to use 6 NMOSs (don't have to match with PMOS).

    I am using the STP16NF06L NMos.

    I am trying to calculate the size of the CBoot. I have used the application notes from the manufacturer tp calculate a value for Cboot, but I got a ridiculously low figure.
    View attachment CBOOT.pdf
    The formula for it is

    Cboot = Q(tot) / Vboot

    Vboot = Vcc - Vf - Vgs_min

    where Vf is the bootstrap diode forward voltage and Vgs_min is the minimum gate source voltage for my high side NMos.
    I have used 12 volts for Vcc, assumed Vf to be 1V and Vgs_min is 1V.
    I am not sure with Vgs_min being 1V, the datasheet says 1 but Id doesn't really get optimal until around 6V.

    Vboot = 12 - 1 - 1 = 10V

    Q(tot) = Qgate + (Iikgs + Iik_cap + Iqbs + Iik + Iik_diode)*Ton + Qis

    Qgate is High side gate charge = 10nC
    Iikgs = High Side gate to source leakage current = 100nA
    Iik_cap = Bootstrap capacitor leakage current = 0 as I don't intend to use an electrolytic capacitor
    Iqbs = Bootstrapped Section Quiescent Current = 200 uA
    Iik = Bootstrapped Section Leakage Current = 10uA
    Iik_diode = External bootstrap diode leakage current = 0 I am not using an external bootstrap diode
    Ton = High side switch on time = 100u - I am not sure where to find this but it was the value the datasheet example used, STP16 give like rise time of like 30 ns, so it didn't seem appropriate?
    Qis = Charge required be internal level shifter = 3nC


    Q(tot) = 10 nC + (100nA + 0 + 200uA + 10 uA + 0) * 100us + 3nC
    = 34nC

    Cboot = 34nC / 10 V = 3.4nF

    This seems very small? As the manufacturer application notes says typical values are 100 nf to 500 nf!?!

    What have I done wrong in my calculations? The final answer seems too small!

    Another question that I would like to ask is that say that I have Cboot worked out, what is an effective way to test it? I need to 'initialise' i.e. charge up the Cboot by driving the low side on first before I can drive the high side 'on'. This seems like a process that can only be done via the use of the MCU?