Bootstrap Capacitor For High Mosfet Switching

[Gokkul55]

Hello All

For any one new joining this discussion here are the links to my previous questions.

http://www.eng-tips.com/viewthread.cfm?qid=350101

http://www.eng-tips.com/viewthread.cfm?qid=350473

So that being said i am currently in the part of the design stage where i need to choose the bootstrap capacitor value.

Done a few calculations myself, as always need to verify my math.

Spec:

Mosfet Driver IC - IR2110
Mosfet - IRF3710
Bootstrap Diode - UF4002
Frequency of switching - 200 KHz

(The initial idea was 500 KHz but then most design by TI and ST had the frequency at around 200 KHz. So it made me think may be at that frequency the switching losses may be high. Not sure, so came down to 200 KHz)

As per the attached App-Note the bootstrap capacitor is to be calculated using the formula given in page-6.

http://files.engineering.com/getfil...1-25a7-46c1-a43c-23e2698967c5&file=AN-978.pdf

Voltage across low side mosfet - R_DS(on) X Current
23mOhm x 10A = 0.23V

Voltage across low side shunt - R X I
1.25mOhm x 40 A = 0.05V

V_LS = 0.23 + 0.05 = 0.28 -> lets make it 0.3 to be safe

Q_g = 130 nC
freq = 200 KHz
I_cbs = 0 A (non electrolytic cap)
I_qbs = 230 uA
V_cc = 12 V
V_f = 1 V
V_LS = 0.3 V
V_min = 2 V
Q_ls = 5 nC

Substituting the values i arrive at 61.184 nF

As per App-Note DT98-2 the capacitor value obtained should be multiplied by 15 to reduce the risk of overcharging

http://files.engineering.com/getfil...5-66df-48f1-b696-1c11244ca0a5&file=DT98-2.pdf

Hence the new value of bootstrap Capacitor - 917.759 uF --> nearest value 1uF

Please check and let me know if i have done the math right

Thanks
Regards
Gokkul

 

[zappedagain]

If you haven't done the math correctly than your high side FET will shut off before it is supposed to. Try it and see. Think about what the bootstrap cap does.

Z

 

[Gokkul55]

If you haven't done the math correctly than your high side FET will shut off before it is supposed to.

I know that, i just need someone to verify. Cos implementing the circuit will take some time and if it fails i will have to do the process again.

So i am just trying to check if my calculations are correct.

Thanks
Regards
Gokkul