I try to simulate boost circuit by multisim but it does not work can someone help me

 

"Does not work" is not very descriptive. That's like saying "my car doesn't run."
How does it not work?
There appears to be a missing connection between R4 and the gate of Q1.

 

"Does not work" is not very descriptive. That's like saying "my car doesn't run."
How does it not work?
There appears to be a missing connection between R4 and the gate of Q1.

there is connection already , I mean that I expect 400v output ,and output is about 220 and is not stable

 

On what basis do you expect 400V? Calculation? Having a real circuit?

What is the relationship between the two different ground symbols in Multisim? Are they Ohmically connected?

The opamp cannot work with the power supply connections shown.

 

On what basis do you expect 400V? Calculation? Having a real circuit?

What is the relationship between the two different ground symbols in Multisim? Are they Ohmically connected?

The opamp cannot work with the power supply connections shown.

what is wrong on opamp it is working as error amplifier , and also if you connect the 2 ground ,the problem still the same

 

and also this my first time to do simulation by Multsim, so forgive me if something is wrong

 

As Mike stated, the op amp needs plus and minus supplies to operate in your circuit.
It cannot operate with a single supply with the + input grounded.
If you look at the data sheet for that op amp (gasp) you will see that the input common-mode range can go no lower than about 3V above the negative voltage rail.

 

what is wrong on opamp it is working as error amplifier , ...

The OpAmp cannot work with pin 11 tied to Gnd.

 

and also this my first time to do simulation by Multsim, so forgive me if something is wrong

Maybe the op amp is just drawn upside down and Vcc is 15 volts and ground is ground, but in either case the - input will always be higher than the + input (ground) so the output will always be ground.
What is U1 and the amplitude and frequency of the sawtooth feeding it?

 

Maybe the op amp is just drawn upside down and Vcc is 15 volts and ground is ground, but in either case the - input will always be higher than the + input (ground) so the output will always be ground.
......................

You missed V3, -5V(apparent reference voltage) being summed into the (-) input.

 

As Mike stated, the op amp needs plus and minus supplies to operate in your circuit.
It cannot operate with a single supply with the + input grounded.
If you look at the data sheet for that op amp (gasp) you will see that the input common-mode range can go no lower than about 3V above the negative voltage rail.

I did change all these mistake but I still can not get 400v out so what else wrong

 

Maybe the op amp is just drawn upside down and Vcc is 15 volts and ground is ground, but in either case the - input will always be higher than the + input (ground) so the output will always be ground.
What is U1 and the amplitude and frequency of the sawtooth feeding it?

amplitude is 15 v and frequency is 120khz

 

You don't show the input polarity of the comparator, U1.
That would be a problem if it's incorrect.

Measure the voltage waveform at:
the output of the op amp,
the output of the comparator,
the sawtooth waveform,
the MOSFET drain.
Post those with the vertical and horizontal scale indication.

The voltage rating of D2 is too low. It needs to be over 400V.

What's the purpose of D4?

 

You don't show the input polarity of the comparator, U1.
That would be a problem if it's incorrect.

Measure the voltage waveform at:
the output of the op amp,
the output of the comparator,
the sawtooth waveform,
the MOSFET drain.
Post those with the vertical and horizontal scale indication.

The voltage rating of D2 is too low. It needs to be over 400V.

What's the purpose of D4?

D4 to turn off MOSFET fast
and for the sawtooth , I can not measure it because it is inside a package

 

D4 to turn off MOSFET fast
and for the sawtooth , I can not measure it because it is inside a package

With a 10 ohm resistor in series with the gate, the diode likely will have little effect on the turn-on time.

The source voltage of Q1 should go to ground when it is ON and it's not, so that's a problem.
You need to determine why it's not going to ground.
Check the current through L1.

 

thanks so much, the circuit in the lab it works , but I do not know why the simulation is not , the source connected to the common when the gate is on in the lab but in simulation is not

 

thanks so much, the circuit in the lab it works , but I do not know why the simulation is not , the source connected to the common when the gate is on in the lab but in simulation is not

If you can show the current through L1, that could give a clue.