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I designed a simple buck converter to meet the following specs (I have attached the schematic):

  • Vin = 17.4V
  • Vout = 12V
  • D = 0.69
  • Switching Frequency = 50kHz
  • Inductor Ripple Current = 20% of Output Current
  • Output Voltage Ripple = 2% of Vout

Background Info: All the parts and component values listed on the schematic are parts I'm implementing in the circuit. I know the inductor and capacitor values are not matched perfectly to the specs listed above, but they are the closest that I could find. The PWM signal is being generated by the Arduino Uno and I am using the TC4427 IC to drive the signal at the gate of my MOSFET.

Problem:My MOSFET is not turning on. I know for it to turn on Vgs > Vth. I am using the TC4427, because the max output voltage of the Arduino is only 5V. What I'm confused about is why my gate driver isn't supplying the necessary turn on voltage. I thought I understood how it works, but not apparently. The max output of the TC4427 is only as high as the supply voltage I feed into it (from measurements I've taken), which doesn't make sense to me. I've looked on the TC4427 datasheet, but it doesn't give me any information on the max voltage output coming from the gate driver. Does this mean I need to supply an input volts higher than V1 to the supply just to turn on my MOSFET? Or how exactly is a gate driver supposed to work?

schematic

simulate this circuit – Schematic created using CircuitLab

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The TC4427 by itself can not generate a gate drive voltage greater than its own supply voltage. That follows from the functional block diagram on p.2 of the datasheet.

It would make the question clearer, if you add the gate driver to the schematic on the O.P. and show its supply rails.

For the time being, I'll assume that the TC4427 is powered from the same +17.4V that goes to the input of the buck. In that case, the source of the N-channel MOSFET will not be any higher than 17.4 V - Vth.

Typically, buck converters have a bootstrap gate driver1 and an N-channel MOSFET, or a P-channel MOSFET.

1 Bootstrap gate driver (see also here) is not the only option for N-challel MOSFETS. Other options: gate driver transformer, additional supply rail for gate driving with elevated voltage.

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Your N ch fet is wired as a source follower and this is bad for this type of power supply (switching) but OK for a linear regulator! You should either: -

  1. Consider using a P channel MOSFET
  2. Use a bootstrapped driver

If you use a bootstrapped driver it will provide a gate volate that is at least 10V higher than the source voltage and turn the damn thing on properly.

What you have at the moment is (at best) 17.4V delivered to the gate and maybe 14 or 15 volts out of the source - now that is hardly turning the MOSFET on is it?

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