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I've been studying some ESC (electronic speed control) circuits for BLDC motor control and something always catch my attention. In many of these schematics there's often a resistor connecting the controller IC to the power MOSFET. As the MOSFET gate is an isolated connection what's the reason for such component? I mean, if I remove the resistor shouldn't the circuit behave the same?

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ @EugeneSh. But the datasheet doesn’t have a specification of maximum gate current. How to design such component without this specification? \$\endgroup\$ – PDuarte May 16 '18 at 14:09
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    \$\begingroup\$ It is use to damp parasitic oscillation. \$\endgroup\$ – Long Pham May 16 '18 at 14:10
  • \$\begingroup\$ My initial comment was incorrect or incomplete. Here is a somewhat duplicate question electronics.stackexchange.com/questions/68748/… \$\endgroup\$ – Eugene Sh. May 16 '18 at 14:13
  • \$\begingroup\$ on some U1s, the pin can float away during reset/boot, which can cause a motor to unexpectedly rev up at an inopportune time. by pulling it down, such MCU blips don't cause a noticeable output effect. That's among other reasons, but apt for you. \$\endgroup\$ – dandavis May 16 '18 at 16:13
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One reason a gate resistor is used is to slow down the turn-on and turn-off of the MOSFET. (This is more relevant to power circuits that switch a fair amount of current.) While it may seem that very fast switching is desirable, because of lower switching losses, it can result in ringing due to parasitic inductances, leading to electrical noise problems.

Other factors, besides switching speed, affect the amount of ringing: the physical layout of the circuit; the properties of the MOSFET, especially the switching characteristics of its body diode; any snubbers or capacitors that might be present; the level of the switched current.

A typical gate resistor value would be a few ohms to a few dozen ohms. Sometimes a second resistor in series with a diode is used in parallel with the gate resistor to separately adjust turn-on and turn-off behavior.

The choice of value is a trade-off between switching losses and ringing; the value may not be settled until the circuit is built and tested. One way to choose a value is use the smallest value that reduces ringing below the level that allows the product to pass any EMC certifications that are needed.

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  • \$\begingroup\$ So a high resistor reduces ringing ? \$\endgroup\$ – Meenie Leis May 16 '18 at 14:26
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    \$\begingroup\$ @MeenieLeis - Generally, yes; I've expanded the answer \$\endgroup\$ – user28910 May 16 '18 at 15:55
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The resistor slows down MOSFET turning on/off. That means that (unless ZVS is implemented ) losses will be higher, which means MOSFET will be hotter. The advantage may be that bandwidth of square wave on the wires is lower, meaning less electrical interference.

I would argue that really hard problem of EMI is quite low on the spectrum, so fighting it this way is not very effective. In fact this is a good way for slowly switching stuff like a switch for solenoid or something. For a motor- use good layout, shielded connectors, bypass capacitors, etc.

And remember that EMI issues raise arguments as hot as between muslims and christians, so stay away from thay.

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  • \$\begingroup\$ Nice comparison. \$\endgroup\$ – Long Pham May 16 '18 at 16:08
  • \$\begingroup\$ Almost said "muslims and jews", but it's not that violent. \$\endgroup\$ – Gregory Kornblum May 16 '18 at 16:13
  • \$\begingroup\$ There is something many people miss about motor cable. Total current in it is zero, so magnetic phenomena exist only very close to the wires. On the other hand voltage is worst possible- high amplitude square wave - meaning it covers wide spectrum with high power junk. So in some cases some fractions of pF may be enough to pass interference and kill communication or analog sampling. \$\endgroup\$ – Gregory Kornblum May 16 '18 at 16:18

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