# Bootstrap Capacitor Calculation Issue

Discussion in 'General Electronics Chat' started by DumbDummy77, Feb 1, 2010.

1. ### DumbDummy77 Thread Starter Member

Jan 21, 2010
43
0
Hi

I am currently learning how to calculate the bootstrap capacitor for IR2110 Application. So far I have identified every parameters apart from one:

Qgate 27E-06
Frequency 21600
Icbs(leak) 0
Qls 5.00E-09
Vcc 14
Vf 0.78
Vls 4.5
Iqbs(max) 230E-06
Vmin ??

I use the forumla from Application Note AN-978 on page 6. At the moment I am not certain on how to obtain Vmin, I use a half bridge configuration and the voltage is about 100V and the IGBT is IRG4BC20UD.

Please explain me how to obtain Vmin?

2. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
Vmin = the minimum voltage between Vb and Vs.
Vb = the high side of Cboot.
Vs = the junction of the high side MOSFET source and the low side MOSFET drain; the load connection point.

3. ### DumbDummy77 Thread Starter Member

Jan 21, 2010
43
0
Can you please give me an example in number?

Regards

MM

4. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
It depends on the MOSFET you're using.

If it's a standard level MOSFET, you'd want Vmin to be 10v.
If it's a logic level MOSFET, you'd want Vmin to be 5v.
If it's a very low threshold MOSFET, you could use a lower number for Vmin.

Cboot is what supplies the current to charge the MOSFET gate to turn it on. In order for a standard level MOSFET to be considered fully turned on, you need to get the Vgs (gate relative to the source voltage) up to 10v.

5. ### DumbDummy77 Thread Starter Member

Jan 21, 2010
43
0

Right I get the picture, for my application, I use IGBT such as RG4BC20UD at the moment. According to the datasheet, the Vge is +/-20V so that mean I apply 20V in Vmin. Is it correct?

I am struggling to understand it, to be honest with you. Can't you give me a design example? Hopefully i grasp the idea quickly

Regards

MM

Last edited: Feb 1, 2010
6. ### S_lannan Active Member

Jun 20, 2007
247
2
Nothing too complicated. Just make that voltage a suitable voltage to turn on the IGBT.
Try 15v it's what most IGBT gate drive circuits i've seen use.
It's somewhere inbetween VGE(th), which is the voltage where the device beings to conduct, and VGE(max), the maximum voltage you can put on the gate.

Jan 21, 2010
43
0

8. ### DumbDummy77 Thread Starter Member

Jan 21, 2010
43
0
Vmin is the difference between Vb and Vs. Hence since Vb is connected to Vcc which make 12V (Vcc = 12V). I am not sure about Vs to be honest. If I am mistaken about this, please correct me asap

9. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
You need to look at the Vce(on) specification in the datasheet.
There you will find Vge=voltage - for that particular IGBT, it's 15v.

If the design is simply handed to you, how much would you learn in the process?

You say you already have all the other numbers - plug them into the formula, and you'll have your result.

10. ### DumbDummy77 Thread Starter Member

Jan 21, 2010
43
0
Thank you SgtWookie and I have another question to ask you do I need to connect correct resistor from the IR2110 to the gate of IGBT. If yes, what forumla should I use?

11. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
Go back to that same Application Note, and start reading on page 9.

Note the reference to DT97-3 on page 11.

12. ### legaspito New Member

Nov 21, 2011
1
0
i believe you should use a resistance value that does not make your voltage drops under the specified Vg you know what i'm saying

13. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
This topic has been asleep for 21 months, and the original poster has not returned since the end of May. It is not likely that they still require help on this.