bootstrap capacitor calculation

[khushi_123]

i m making a dc motor driving ckt using MOSFET and IR2110..f=4khd, vcc=12v, vdd=5v duty cycle=50% mosfet=IRFZ44N
can any one tell me the value of bootstrap capacitor..i m not getting how to calculate it from the formula that is given in the application note of IR2110...HELP..

 

[Tahmid]

I don't think it's too critical. While there is a formula in page 6 of AN-978, I found that the value isn't too critical. You can use something like 10uF or 22uF and adjust if necessary (I don't think it will be necessary).

Hope this helps.
Tahmid.

 

[khushi_123]

kindly chek this circuit..and tell me if there is any mistake cux one of the ic is getting hot..help..

 

[Tahmid]

Place a 1k resistor between the gate and source of each of the MOSFETs. I think that should solve the problem. You should also use a 0.1uF ceramic or tantalum type capacitor across the bootstrap and filter caps.

Hope this helps.
Tahmid.

 

[ch wazir]

kindly chek this circuit..and tell me if there is any mistake cux one of the ic is getting hot..help..

Dear Khushi,
I checked yours cicuit,it is pretty good one and it should work.What kind of problem are you facing?Don't connect any resistor like 1k .
plz mention the problem so that I may guide you further.

 

[Tahmid]

I don't see any specific problem in the schematic. I think connecting the 1k resistor will solve the problem.
One more thing: there is no need to connect the 1N5817 diodes across the MOSFET. At 4 kHz frequency and 50% duty cycle, the body diode of the MOSFET is good enough.

Hope this helps.
Tahmid.

 

[khushi_123]

i checked the ic after swapping..the same ic is getting hot..is it faulty ic??..wats the fault??..shud i chnge th ic??..i am giving the attached pwm at input..bt still its nt working..will it work at 50% duty cycle??..help..

 

[Tahmid]

Have you placed the 1k resistor like I said before?

 

[khushi_123]

yes i have placed the 1k resistor..and then i have checked the circuit..i m getting correct output at LO but i m not getting anything at HO..help me..motor is not moving..help..:sad:

 

[Tahmid]

If you check HO with a scope, you won't see anything meaningful as it's not ground referenced. Do the ICs burn anymore?
Is the motor you connected a 12V DC motor? Then it's not really going to move. Since you say you have 50% duty cycle, the motor receives 12V in one direction and then 12V in the other. So, basically it moves in one direction for 1 pulse and then the other direction for the next. So it doesn't move. Try using a 12V AC motor instead. Or connect some other load such as lamp.

Hope this helps.
Tahmid.

---------- Post added at 15:19 ---------- Previous post was at 15:17 ----------

Also check to see if you get any output at the bridge, using a voltmeter or the oscilloscope.

 

[khushi_123]

there was a connection problem on breadboard..i made it on vero board..and now its working and the motor is moving..thnk u so mch for ur great help..thx alot..u r really a good helper..:smile:

 

[khushi_123]

kindly tell me what is the purpose of 1k resistor between gate and source of mosfet..??as u told me to do so..

 

[khushi_123]

also plz tell me the purpose of capacitors used between vcc and common and also the capacitors used between vdd and vss...???

 

[Tahmid]

kindly tell me what is the purpose of 1k resistor between gate and source of mosfet..??as u told me to do so..

To prevent accidental turn on of the MOSFETs. Usually it doesn't cause a problem at low voltage, however, it's better safe than sorry.

also plz tell me the purpose of capacitors used between vcc and common and also the capacitors used between vdd and vss...???

For filtering and also to provide currents for short time when required.