# Buck converter design issue

I want to design buck converter for 385VDC to 48VDC for that I calculated the values of component and simulated it with proteus but I am not getting the desired output. What is wrong? PWM has a 20v amplitude as given in IRF740 data sheet. And 20kHz frequency and 12% duty cycle.

• For an N channel device, the gate must rise above Vout by a large enough voltage to fully enhance the device. A boost circuit is required to get what you want. – Peter Smith Mar 24 '19 at 13:30
• I did not get it. Means I need to boost gate pwm pulses amplitude? I am getting 385vdc already from a boost converter. – Deep Mar 24 '19 at 13:44
• You have your N-channel MOSFET configured as a "source follower". This means that the source terminal cannot rise higher than $V_{gate} - V_{threshold}$. Threshold voltage for the IRF740 is in the range of 2 to 4 V. If you want to turn it on fully, bringing the source terminal to +385 V, you'll need to drive the gate with a signal that rises to +390 V. This can be done with a "charge pump". – Dave Tweed Mar 24 '19 at 13:49
• Can you please suggest changes in circuit. – Deep Mar 24 '19 at 13:53
• The gate needs to be driven to Vout + the fully on Vgs at a minimum, so about 53V in this case. Note that this will have significant heating effects on the FET. – Peter Smith Mar 24 '19 at 14:05