Hi,

i need help with my circuit as it is not working as should.

using the application notes(AN486) from silicon labs i have calculated the required bootstrap capacitor which is anything above 151nF .

the problem is that when simulating this circuit in multi sims 14 ...the high output of the IR2011PBF stops pulsing the mosfet after 400uS...then the output voltage of the buck decreases til it gets to 6v then starts pulsing again. i have check varies points on the circuit and made sure undervoltage lock out is not activated on any of the pins according to the data sheet. and i have no control loop on this circuit so dont know why it is doing that

i am totally lost as what is wrong ....

input voltage 40V
Freqency is 200Khz
Duty Cycle is 0.225
output voltage should be 9V

Click this image to show the full-size version.

Last edited: Today at 6:20 PM

 

What's the voltage across C7?
Try increase the value of C7 to 1μF or more.

 

its 14V with around 1.3V ripple ..i have tried that and get same problem

 

why rise - i gess it accumulates the carge for VS sensing it has to stay off (i never done anything with high-low side drivers - i don't want to go gessing here futher on)

 

Hi,

i need help with my circuit as it is not working as should.

using the application notes(AN486) from silicon labs i have calculated the required bootstrap capacitor which is anything above 151nF .

the problem is that when simulating this circuit in multi sims 14 ...the high output of the IR2011PBF stops pulsing the mosfet after 400uS...then the output voltage of the buck decreases til it gets to 6v then starts pulsing again. i have check varies points on the circuit and made sure undervoltage lock out is not activated on any of the pins according to the data sheet. and i have no control loop on this circuit so dont know why it is doing that

i am totally lost as what is wrong ....

input voltage 40V
Freqency is 200Khz
Duty Cycle is 0.225
output voltage should be 9V

Click this image to show the full-size version.

Last edited: Today at 6:20 PM

A little hard to explain, but I'll try.
The output voltage overshoots because there is no closed loop. When that happens the inductor current goes to zero so there is no inductive kick to make the left side of L1 go to ground so the boost cap doesn't charge.
It actually works for me in Ltspice, but I don't have the same components.
You could add another FET to make it synchronous or if you close the loop it will probably be ok.