bootstrap capacitor value for IR2110 HiLo MOSFET driver

[Dale Gregg]

Hi,

I'm trying to set a value for the bootstrap capacitor to go in between VB and VS. VCC=15V, IR2110 Iout=20A, diode volt drop is 1V, Giss=120nC

I'm new to this and have been playing around with numbers since this morning. I'm getting results but I'm don't know if there right.

http://www.irf.com/technical-info/appnotes/an-1123.pdf

I've tried using the calculations in this but its more for analysis after you know what your values are (ie, where would I get this value from "Ileak takes into account the DC current that has to be supplied to the gate driver high side circuitry", the example uses 200uA)

The way I've done it so far,

VB=VCC-Vf
VB=15-1=14 (current limiting resistor for mosfet)
Q=C*dV/dt
C=Q*dt/dV
C=120E-9*(163.2E-6/14)=2.1428E-12 (163.2E-6 been the max on time)
2.14pF? Seems small, is this just the minimum it can be to charge the gate?

A point in the right direction would be great, Thanks, Dale

 

[FvM]

I don't see how AN-1123 could lead you to this curious bootstrap capacitance value.

What should be the meaning of Q=C*dV/dt? It can't be right if you look at the units of involved quantities.

 

[Dale Gregg]

Looking at it again with a fresh head this morning, I haven't a clue where that came from. I think I've confused C=I(dV/dt) with Q=CV :-?.

AN-1123

This says that the maximum voltage that the bootstrap capacitor (VBS) can reach is dependent on the volt drop across RBOOT, VF of
the bootstrap diode, the drop across the low-side switch (Vds(on)drop).

Average I through Rboot=
Ibsr=(Qg*f+Ileak)/D -Qg-gate capacitance(120E-9),f frequency = 3922, Ileak - DC current to be supplied to the gate driver high side circuitry (how would I find this? is it the IQBS Quiescent VBS supply current @ 230E-6A?), D - lowest duty cycle 10%

Given those values Ibsr=7mA

The VD across Rboot is then,

Vrboot=Ibsr*Rboot But Haven't decided on a value for Rboot yet. Would I do this by first finding the required current for the cap (1), or use the above Ibsr (2)?

(1)
I=C(dV/dt), dV = 14V (?Vcc-Vf?), lowest DC 10%/Period = 25.5E-6S, estimating C at 1E-6F
I=549E-3A

Rboot=V/I V=14V (?Vcc-Vf?), I=549E-3
25.5Ohm
OR
(2)
Rboot=Ibsr*(Vcc-Vf)
98E-3 Ohm (seems too low?)

Vrboot=Ibsr*Rboot Ibsr=7E-3A Rboot=25.5Ohm
178E-3V

Total Q from Cboot to HI-side circuit
Qtot=Qg+Ileak*(1-D)*Period Qg=120E-9, Ileak=230E-6, D=0.1, P=255E-6
172.78E-9C

Vbs ripple amplitude,
Vbsrip=Qtot/Cboot
173E-3V

So then Vdrop will be,
Vdrop=Vrboot+Vf+Vbsrip/2^3 Vrboot=2.68, Vbsrip=173E-3 (I've also added Vf here)
1.2V

So then the Vb-Vs or gate voltage would be Vcc-Vdrop? 13.8V? Would I need to consider the Vdrop of the low side switch Rdss(on)?

The next section is about varying duty cycle which I can play around with in excel and frequency response.

I think I'm understanding a little better but do these calculations look right? I think I've gone astray with the Rboot value, is it just a matter of playing around with the C + R values in excel?

Regards,

Dale