1
\$\begingroup\$

I've tried to search for this but I have [not] seen any combination of which resistors I need and which I don't need, so I'm not sure what to do.

I'm planning to drive an IRFZ44 MOSFET with a TC4420 gate driver. I'm doing that instead of driving it directly because I couldnt find a MOSFET that could provide the current I need at a given VDS, besides that, I'm learning something new.

I use 3.3 V as the input for the driver and 12 V as VDD for the driver. To my understanding the gate resistor reduces ripples and protects whatever is connected to the gate from to much current being drawn and the pulldown resistor lets the gate discharge fast when the mosfet turns off (is that correct so far?). But I'm not sure if these reasons still hold true for the TC4420. Does it need ripple protection? The datasheet says it can provide up to 6 A at 18 V VDD, so protecting it from overcurrent shouldn't be a problem. No idea about the ripples and pulling the gate low though. Could someone clarify that?

Example schmematic: enter image description here

\$\endgroup\$
6
  • 1
    \$\begingroup\$ Show your proposed schematic and where you'd connect the resistor. What ripple supply levels are you worried about and on what supply rail? A lot of info needed for those who are unable to read minds. \$\endgroup\$
    – Andy aka
    Commented Sep 7, 2020 at 16:24
  • \$\begingroup\$ Link to the datasheet. \$\endgroup\$ Commented Sep 7, 2020 at 17:02
  • \$\begingroup\$ @Andy aka It was more of a general question regarding these resistors in combination with a gate driver. Im not worried about ripple, i just read that you the gate resistor takes care of them, and i wondered if gate transistors usually take care of that. I'll add a schematic and a link to the datasheet to my original post in a second \$\endgroup\$
    – rogerg
    Commented Sep 7, 2020 at 17:21
  • \$\begingroup\$ @rogerg What frequency will the MOSFET be switched at? \$\endgroup\$ Commented Sep 7, 2020 at 19:01
  • \$\begingroup\$ @AndrewMorton Around 200Hz. \$\endgroup\$
    – rogerg
    Commented Sep 7, 2020 at 20:03

3 Answers 3

1
\$\begingroup\$

The gate driver actively pulls the gate voltage high or low, so a pull-down resistor is not needed.

The gate driver datasheet says that, due to its low output resistance, it may be a good idea to add some sort of short-circuit protection, e.g. the MOSFET could fail as a short, so I suggest that you calculate the gate resistor value to limit the power dissipation in the driver to whatever your design can cope with.

Further reading:

\$\endgroup\$
1
\$\begingroup\$

Suggestions (I've killed a few gate_drivers, so I share these ideas)

  • Use a Ground Plane under the Gate_driver and the Power MOSFET

  • run the Driver output to the FET gate over the Ground plane, with no slits under that trace

  • bypass the GateDriver VDD (+12v in your case) to the Ground plane

  • install some small value Resistor between Driver and the Power FET Gate; this resistor may not be needed, but various inductances in the FET leads, along with Cgate_drain, may result in an oscillator; that Gate resistor will serve to dampen/prevent the oscillation

  • what value of Gate_Dampening resistor? try sqrt(L / C) where L is 10 nanoHenries and C is 10 nanoFarads (note the Miller Effect Plateau will boost the capacitance 2X or 3X or 5X during Drain transitions); the math suggests ONE OHM.

  • be very cautious about how the MCU (control signal generator) connects to the Gate Driver. With high dI/dT such as 6 amps/6 nanoseconds (if the TC4420 is that fast in turnoff or turnoff) and 5 nanoHenry inductance of the GateDriver GROUND PIN/PCBrouting, you may see MINUS FIVE volts on the GateDriver silicon substrate (on chip, inside the package).

  • Given the unknown ESD structures of GateDrivers (often turning on at -0.6v, or at VDD + 0.6v), there may be troublesome interactions between MCU and GateDrivers

  • Some datasheets specify OK TO -5 Volts on input PIN; some companies promise that for all input pins, some do not promise that for ENABLE pins.

  • I'd ADD 1Kohm between the MCU and the GateDriver, just to ensure negative voltage swings do not inject charge back into the MCU

\$\endgroup\$
1
\$\begingroup\$

To my understanding the gate resistor reduces ripples and protects whatever is connected to gate from to much current being drawn and the pulldown resistor lets the gate discharge fast when the mosfet turns off (Is that correct so far?).

No. The gate resistor can reduce ripples a little bit but it's purpose is to protect both the gate and the output. At low frequencies (less than 1 Khz) 1K is good. At higher frequencies you need less impedance but not less than 140 ohms. The pulldown resistor is necessary when there is a possiblity that the gate be not connected neither to GND nor to Vcc, in other words, be on High Impedance sate. Then it becomes unstable. But the driver as any output from most ic's is supposed to pull the gate up or down, doing the job of the pulldown resistor. This resistor can be omitted if you are sure the gate can't be in High impedance state. It can be if, for instance, there is an 'output enable (or disable)" function which explicitly put the output in high impedance and you use this function.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.