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I was trying to figure out how to calculate the internal gate resistance of the MOSFET. For the purpose of analysis, Let's take the MOSFET as - BUK7Y3R5-40E

From most of the documents I read, best way to get the internal gate resistance is to measure it out.

With my current circumstances, I would like to go with a mathematical calculation but I could not find anything as reference.

Can someone suggest how to do calculate the MOSFET internal gate resistance. If it is difficult to calculate the value accurately, any approximation will be sufficient.

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  • \$\begingroup\$ How do you want to calculate this? the gate resistance depends on so, so many parameters that are hidden from you. \$\endgroup\$
    – Joren Vaes
    Jul 12 '18 at 9:41
  • \$\begingroup\$ Basically this MOSFET is driven by a MOSFET driver circuit. It is a part of forward converter. I have not fixed the driver IC. But few options are under discussion. And can take 3.3 Ω external gate capacitance for discussion \$\endgroup\$
    – vt673
    Jul 12 '18 at 9:45
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You can extract some typical values from the SPICE model for the MOSFET you have in mind. For example, for the one you mention in the question the model has the following lines:

LG GATE 1 7.81292696075128e-10

RLG GATE 1 1.96360271543439

So the gate acts like it has about 2 ohms in series with 0.8nH

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    \$\begingroup\$ Nice, that 2 ohms is close to my answer's "a couple of ohms". And fyi, the inductance LS is mainly from the package (leads, bondwires etc), not the MOSFET die itself. \$\endgroup\$ Jul 12 '18 at 11:28
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A simplified way to model/look at the gate of an NMOS can be this:

schematic

simulate this circuit – Schematic created using CircuitLab

There will be a series resistance Rs

There will be a parallel resistance Rp

There will be the gate capacitance Cgate

Although not so clear from your question I think you are asking about Rs, the series resistance.

Asking about Rp would not make much sense anyway as it is often too high to even measure. Some manufacturers specify a gate input current and then Rp should be a current source in the model or even better (more accurate), a diode with a leakage current. This diode is sometimes present for ESD protection but it often leaks more than the gate itself!

But back to Rs, this Rs depends on the shape and material used to make the gate structure inside the MOSFET. There is no way for you as an end user to know what size, shape and material (and it's resistance) the gate is made off. Semiconductor companies regard this as their "trade secret".

Luckily, for almost all of the normal applications of these MOSFETS, you generally do not need to know the value of Rs as it will almost always be low enough. My guess is that for most power MOSFETs the value of Rs will be a few ohms at most.

The only applications I see where the value of Rs does matter is for extremely fast switching. Fortunately the fastest switching speed or largest delay is often mentioned in the datasheet.

I have seen other cases where the gate resistance was indeed relevant but that was for an on-chip CMOS filter. There the value of Rs was calculated by the MOSFET models so I could optimize my MOSFETs to minimize Rs. But that's on-chip low power design, this has little to do with power MOSFETs for switching applications.

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  • \$\begingroup\$ Great answer. In microwave engineering we also spend a lot of time worrying about Rs - We want to tune out Cg with a inductor or matching network, but we can't get past Rs as this limits the efficiency and Q-factor of our tuning network, and thus also limits the bandwidth. People spend weeks designing the network to connect the gates to the top level metals for on-chip amplifiers to reduce this gate resistance without increasing the capacitance too much (optimize RC product to some extent). \$\endgroup\$
    – Joren Vaes
    Jul 12 '18 at 11:08
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MOSFET gate resistance is a very high value (much higher than megohms) and so should be of next to zero concern when working out how to drive the gate. On the other hand the gate of a MOSFET will have a capacitance. On a power MOSFET this capacitance can be a very respectable amount and the data sheet for the part will provide specifications that characterize this capacitance.

It is the gate capacitance that you need to be concerned with when you are selecting the gate driver. A large gate capacitance that needs to have the gate switch very quickly (to permit efficient switching behavior of the MOSFET) may require a driver that has to dynamically switch multiple Amps of current. The reason you often see a small valued resistor between a gate driver and the MOSFET gate is to limit the maximum dynamic current flow to a level that the driver can handle. Of course slowing down the maximum dynamic current flow has some other advantages as well including reducing voltage rail and GND switching noise.

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  • \$\begingroup\$ thanks for the response, Could you please check the following doc (ti.com/lit/ml/slup170/slup170.pdf) page -43. Kindly note, the IRFP450 MOSFET Gate resistance was calculated as 1.6 Ω. I think I just missed to convey what I meant by internal gate resistance. \$\endgroup\$
    – vt673
    Jul 12 '18 at 10:33
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    \$\begingroup\$ That document states that this "parameter is the internal gate mesh resistance (R<sub>G,I</sub>), which is not defined in the data sheet. This resistance is an equivalent value of a distributed resistor network connecting the gates of the individual MOSFET transistor cells in the device". This is the resistance of the interconnections between paralleled MOSFETs on a power device. The document even says that you need to use a measurement technique to determine this value so unless you have proprietary data from the device manufacturer you are not going to be calculating this value. \$\endgroup\$ Jul 12 '18 at 10:52
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Depends upon the material used in making the gate electrode.

1) metal gate process was used in early 15 volt Motorola logic devices, and is still used in some integrated circuits, because of the very low cost (5 to 7 masks); a square of "gate" would be 0.02 ohms, for example.

2) "more modern" processes use silicon-gate, specifically polysilicon; polysilicon resistance can vary at least 100X in its resistance, say from 10 ohms (low voltage FETs) to 1,000 ohms (high voltage FETs)

All this is from memory. No guarantees.

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