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?

 

Hello,

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

Greetings,
Bertus

 

 

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

 

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

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.

 

Hello,

Here is an application note with some practical values.

Greetings,
Bertus

 

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.

 

Hello,

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

Greetings,
Bertus

 

Hello,

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

Greetings,
Bertus

I will not use IR2520. I need about 20ma current to power a mcu.

 

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

 

I will not use IR2520. I need about 20ma current to power a mcu.

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.

 

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

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

 

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

 

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.

 

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.

 

in my application, 1.5n can run the IC well , but in testing , current only about 23ma.