# value of cap in middle of half bridge.

Discussion in 'The Projects Forum' started by iamhere, Jul 12, 2009.

1. ### iamhere Thread Starter Active Member

May 1, 2009
33
0
Hello all,

We can see in many half bridge circuits , there is a capacitor connected to the middle of half bridge(between 'Source' of High mosfet,and 'Drain' of Low mosfet) to power the driver IC , now the question is :

if I want to get 20ma current, what will the value of capacitor connected to the middle of half bridge when voltage =400v , freq=30khz ? I know the way to calculate capacitance and current in AC circuit, but here in the middle of half bridge, voltage is DC near square wave.

Thanks a lot.

attached: example schematic
V=400v
Frequency: 30khz
Question: what is the value of CSNUB if I want to get 20ma current?

Last edited: Jul 12, 2009

Apr 5, 2008
15,802
2,388
Hello,

Can you post a schematic and a link to the datasheet of the driver?

Greetings,
Bertus

3. ### iamhere Thread Starter Active Member

May 1, 2009
33
0

• ###### IR2520D .jpg
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Apr 5, 2008
15,802
2,388
Hello,

I think the capacitor CSNUB is used at startup of the cfl tube.
At startup the tube uses a much higher voltage than it needs to burn.
The capacitor CSNUB makes a booster with the diodes DCP1 and DCP2.

In the datasheet they do not mention the value.
I think it will be someting between 1 and 10 nF.

Others may correct me if I am wrong.

Greetings,
Bertus

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5. ### iamhere Thread Starter Active Member

May 1, 2009
33
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Thanks Bertus,
undoutly , CSNUB provide power source to IC when half bridge works .
now I need to know the capacitance needed for 10ma , 20ma etc.
DCP1 is a zener.

Apr 5, 2008
15,802
2,388
Hello,

Here is an application note with some practical values.

Greetings,
Bertus

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7. ### iamhere Thread Starter Active Member

May 1, 2009
33
0
Thanks Bertus again, unfortunately I will not use IR2320( and I can not find the the current data of Ir2320 from sheet),

BTW: I like your cute cat very much.

Apr 5, 2008
15,802
2,388
Hello,

Wich chip will you be using?
Here is the datasheet of the IR2520.

Greetings,
Bertus

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9. ### iamhere Thread Starter Active Member

May 1, 2009
33
0
I will not use IR2520. I need about 20ma current to power a mcu.

Apr 5, 2008
15,802
2,388
Hello,

So you need 5 Volts 20 mA for a microcontroller.
Do you want to power it right from the mains (240 Volts)?

Greetings,
Bertus

11. ### millwood Guest

I would suggest that you look for a topology with isolation. This circuit, as posted, will expose your entire mcu and you in the process of debugging it to mains voltage.

a \$10 cell phone charger would be better and safer than this.

12. ### iamhere Thread Starter Active Member

May 1, 2009
33
0
I do not want to consider other way to power the IC, capacitor from middle of half bridge is the most economical way.

Vbus= 400 v

Apr 5, 2008
15,802
2,388
Hello,

So you want to calculate the capacitor so it will drop (400 - 18) = 382 Volts at 20 mA.
The impedance will be 382 / 0.02 = 19100 Ohms.

Use the formula:

C=1/(2 X π X 30000 X 19100) = 277 pF.

Greetings,
Bertus

14. ### iamhere Thread Starter Active Member

May 1, 2009
33
0
Thanks Bertus again, this formula is for AC case, but voltage here is DC square wave ( about 200v in average) I do not know if this formula is still proper.

15. ### iamhere Thread Starter Active Member

May 1, 2009
33
0
in datasheet of IR2159 , the Icc about 10ma only, but the CSNUB is 1.5nF . if the formula is proper, current will be about 100ma when 50khz.

16. ### iamhere Thread Starter Active Member

May 1, 2009
33
0
in my application, 1.5n can run the IC well , but in testing , current only about 23ma.