I am attempting to design a powerful, yet efficient soft switching fullbridge converter. Achieving that requires high side switching with an N Channel MOSFET.

Now while I have succesfully achieved a drain to source voltage drop of but 0.2mV when a stable dc voltage is applied. However, when a switching signal of 100kHz is applied to the gate of the MOSFET, via a Darlington transistor (see diagram below), strange things start to happen. With strange things I mean:

This is my first attempt at high side switching with an N Channel MOSFET and I am completely lost on what I should do to fix this. Why is it not a pretty square wave like the signal? My guess is that the gate is charging too slow.

As additional information, this is the circuitry of the test setup: circuit

I hope someone can help me with this. Thanks in advance!


1 Answer 1


The only means you have of charging the gate source capacitor (possibly somewhere in the realm of 1 nF), is a 4 kohm resistor. Go work how long this takes to charge. You need a push pull driver for high speed switching.

  • \$\begingroup\$ A push-pull driver, ofcourse! I'm going to try it out now! Thank you! I'll come again if it works! \$\endgroup\$
    – Azagro
    Feb 9, 2017 at 21:24
  • \$\begingroup\$ Hold on, the 30 volt supply for the gate is too high. Got for 15 volts or you'll pop the mosfet \$\endgroup\$
    – Andy aka
    Feb 9, 2017 at 21:27
  • \$\begingroup\$ I may have rounded the voltages to their nearby tens, but i'll make sure not to cross the +/- 20V difference! Thank you. \$\endgroup\$
    – Azagro
    Feb 9, 2017 at 21:36
  • 1
    \$\begingroup\$ what is being suggested is what the datasheet permits and there is a very high chance it is 20V max and thus 15V is recomended \$\endgroup\$
    – user16222
    Feb 9, 2017 at 21:42
  • 1
    \$\begingroup\$ ok I follow you. You need a bootstrapped push pull driver. Fairchild and international rectifier make them but there will be others. They steal a bit of output power and create a higher supply rail automatically. \$\endgroup\$
    – Andy aka
    Feb 10, 2017 at 10:14

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