I'm creating a dead simple motor driver:


simulate this circuit – Schematic created using CircuitLab

I apologize for any mistakes in the schematic, I tried to portray the motor as it's equivalent components in the editor but I don't really know what the actual values for them should be. It is a 36V ~25A motor.

I'm planning the frequency of the PWM signal to be 22kHz.

I learned that there come a bunch of problems when you start switching FETs fast such as voltage spikes, increased current consumption for energizing the gates, something about capacitance between gate and source which I don't fully understand and possibly more.

To tackle all of those I should use a MOSFET driver IC. I found TC4420 which has a continuous output current of about 1.5A which should be well above what those gates will waste. Is it possible to use a single TC4420 IC to control all 8 FETs?

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    \$\begingroup\$ Look at the data sheet for your MOSFETs and find the "gate charge" for your conditions. Then determine the current you need to supply all that charge in the time available, some small fraction of the 22 kHz period. \$\endgroup\$ – electrogas Jun 25 at 1:49
  • \$\begingroup\$ @electrogas thank you for the pointers. It is apparently more complicated than I anticipated. So I might be wrong but looking at fig. 6 on page 4 I determine that total gate charge will be 35nC. The time in which the FET must switch is 1 / 22000 = ~50 us but 50 micro seconds is the entire length of the period, so I'm guessing the switching needs to be faster than that, say 5us? In which case I'm a little stuck. Looking at this answer I'm understanding that ... \$\endgroup\$ – php_nub_qq Jun 25 at 17:07
  • \$\begingroup\$ ... the switching will happen in about 2.3 * R * C, C I calculate as 35nC / 5V = 7nF. I currently have no resistors on the gates but let's just pick an arbitrary 1k value, switching time becomes 2.3 * 1000 * 0.000000007 = 16 us. However in this calculation I don't see current anywhere and this confuses me further.. \$\endgroup\$ – php_nub_qq Jun 25 at 17:11

So current is charge/time, and you need to transfer 35nC in whatever the switching time is. Now picking 1/10 the the period probably isn't good enough, because you want the interval during which the switching takes place to be short because this is the time when the current flowing through your device and voltage across the device are both large, hence power dissipation. From the looks of your data sheet you might want to shoot for a switching time of about 100 ns, so then your gate current will want to be about 35/100 or about .35 amps. You could probably do it with one of your TC4420 drivers since its peak current rating is up there around 7 amps I believe. I think you should put separate series resistors in each gate line, perhaps 2 ohms, to equalize the currents.

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  • \$\begingroup\$ Thank you so much, it makes sense to me now, until I get confused again :D \$\endgroup\$ – php_nub_qq Jun 26 at 21:11

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