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I want drive a buck converter with bootstrap technique. but this circuit does not work! enter image description here

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  • \$\begingroup\$ Try putting an earth on the negative end of V4 and see what happens. At the moment it looks floating to me and I don't think it would properly work if you built it without that cross-connection to the driver. \$\endgroup\$ – Andy aka Jan 28 '16 at 23:10
  • \$\begingroup\$ The error is telling you the step size is too small. Fix it. \$\endgroup\$ – Matt Young Jan 29 '16 at 13:27
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    \$\begingroup\$ Did you edit this to be a completely different controller now? Looks nothing like the circuit I initially based my answer on. And the PSPICE error is missing? \$\endgroup\$ – Krunal Desai Jan 30 '16 at 0:06
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The IR2118 doesn't seem to be hooked up correctly. Its floating high-side bootstrap pin (VB) isn't actually connected to anything -- it should be hooked up to at the least a capacitor to the VS pin, and the application diagram calls for an external diode connecting VCC to that node as well:

IR2117/IR2118 annotated.

Right now, it has no ability to generate the boosted drive voltage for your transistor because the bootstrap supply isn't working / isn't connected correctly. Since you are using a N-channel enhancement mode device, it requires a positive VGS to function. The IR2118 driver has a floating pin (VB) which supplies the positive voltage to the MOSFET, referenced against whatever the source of the transistor is connected too:

Block diagram.

In the typical connection diagram, thanks to the diode between VCC and VB, it's creating a simple voltage multiplier to generate the voltage needed to drive the transistor. You could also hook up some arbitrary high-voltage supply to VB.

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