[SOLVED] Gate driver for mosfets

[curious#01]

Hi everyone,

I am tryng to design a buck converter. The gate of the n-channel MOSFET of the buck converter is connected to the pwm pin of a microcontroller. I have a few doubts regarding this:


1. What is the difference between normal mosfets and power mosfets. How do I decide which type of mosfet to use in my circuit?

2.When can I directly connect the gate of the mosfet to the pwm pin of the microcontroller?(I was planning to use stm32f series microcontroller which has a pin voltage of around 3.3V)

3.If direct connection is not possible, what kind of gate driver circuit should be used for the mosfet to work properly?

Would be highly thankful if anyone could help me out.

 

[Audioguru]

Almost all Mosfets are power Mosfets. You forgot to tell us the maximum current that you want in your Mosfet.
Most Mosfets need a gate-source voltage of 10V to fully turn on (but 'logic-level" Mosfets need only 5V) so you need a gate driver circuit to boost the 3.3V from your microcontroller. The gate pulses to the Mosfet must be much higher than the buck converter output voltage.

 

[Pulsetronics]

you can't drive any mosfets directly from microcontroller. You surely needs drivers like TLP250 or IR2110 for mosfet to fully turn on.
See Tahmid's Blog for more lnformations

tahmidmc.blogspot.com/2013/02/n-channel-mosfet-high-side-drive-when.html


tahmidmc.blogspot.com/2012/12/low-side-mosfet-drive-circuits-and_23.html


tahmidmc.blogspot.com/2013/01/using-high-low-side-driver-ir2110-with.html

tahmidmc.blogspot.com/2013/05/using-tlp250-for-isolated-mosfet-gate.html

 

[Tunelabguy]

you can't drive any mosfets directly from microcontroller. You surely needs drivers like TLP250 or IR2110 for mosfet to fully turn on.

I was going to say that this is not exactly true. There are logic level MOSFETs that are fully turned on by a 5v microcontroller output. But then I looked again at the OP's micro, and I see that it is only a 3.3v micro. It will be really hard to find a MOSFET that can be fully turned on by 3.3v. Another concern, especially for medium to large MOSFETs, is the gate capacitance. Although a 5v logic signal can fully turn on a logic-level MOSFET, the current available from a typical logic output cannot charge the gate capacitance of the MOSFET very fast. Therefore the gate voltage will rise and fall slowly with the PWM, spending a lot of time in the "partially on" state in which power dissipation in the MOSFET is greatly increased. A properly-selected gate driver will solve both problems. The drive voltage will be 5 or 10 volts, and the drive current will be a good deal more than what a logic output can supply, thus ensuring fast switching.

 

[dick_freebird]

You have to work backward from the load and its primary
power, then to topology, your efficiency goals and so on.
This will lead you a component selection (FET, magnetics,
catch diode or sync diode, etc.).

Then you can figure out what it takes to drive those
devices. A 10mA uC pin is not up to the task, unless
you like a few microseconds of risetime and roasting
the power FET (switching losses will be heinous).

When you know your FET size, the available local gate
drive supply (which may or may not be the primary
input supply) and the logic voltage, you can pick from
the many options one that bridges the gap.

 

[Audioguru]

All the buck converter circuits I have seen use the Mosfet as a source follower so if you want its output to be 5V then its gate pulses must be 10V or 15V. So you need a Mosfet driver that boosts the 3.3V from the microcontroller to 10V or 15V.