Mosfet based Dc motor controlled circuit

[Ashkar]

Goldsmith I got your point.The mosfet is been charged but the capacitor built inside the gate tends to open the gate continuously even if gate is disconnected.So would a 1k resistor solves the problem or a high resistivity value is to be used.

 

[alexan_e]

I don't see a way that anything like this can happen while the mosfet is connected in the circuit of post #1, the gate charge is constantly controlled by the driver, it it not floating at any time so I don't see the purpose of a pull down resistor.

Alex

 

[goldsmith]

you're right alex
but you can test it . different between resistor and without gate resistor in actual ( not with simuling with simulator).In class D amplifiers if you dont use this resistor , your mosfets will going explosion. you can test it(if your driver has not internal of this resistor )
for last post about totem pole you're right.
Thank you.

 

[alexan_e]

Are you saying that in the circuit if post #1 if you don't connect a resistor between the gate and source the mosfet will burn... no way.
I have no idea of the conditions in a class D that could lead to a failed mosfet but in this case I don't see it.

Alex

 

[goldsmith]

Dear Alex
Hi
In design with mosfets we have assure coefficients . for example: discharging gate capacitor(with resistor) . and use snubbers and anti parallel diode (at switching ) and etc. you can see these things in professional designs. you can test it (capacity of gs effect in actual. ) (what is the highest frequency that you work until now?) at high frequencies we have very big problems with mosfets. in middle or low frequencies problems are a few . but gs resistor has most important effects in circuits(some of mosfets have this resistor (internal))(like anti parallel diode).
Wish you the best
Goldsmith

 

[alexan_e]

If this (external) resistor is so important why haven't I seen it in any of the tutorials or application data of mosfet drivers or in any of the designs I've seen?

**broken link removed**
http://www.irf.com/technical-info/appnotes/an-978.pdf

Alex

 

[Tahmid]

If this (external) resistor is so important

I've found it to be very important and, in many circuits, the single difference between a working circuit and a burnt one.

In all circuits I've made involving IR2110 or L6385 or for the matter, high voltages, without that resistor the MOSFET instantly burns. This has been discussed before here:
https://www.edaboard.com/threads/197011/

And has been found out that placing the resistor there solves the problem. It is due to the Miller charge between the drain and the gate.

Hope this helps.
Tahmid.

 

[alexan_e]

Yes I remember that thread but my question still stands, why doesn't any of these manufacturers mention that a resistor like that should exist, especially if the circuit is impossible to work without it (because the mosfet will burn).

Also you mentioned in that thread

This happens due to a capacitance in the MOSFET that is present between the drain and gate and causes accidental turn-on when high voltage is applied.

there is no high voltage in the circuit of this thread, the power supply is just 12v

Alex

 

[Tahmid]

Yes, sometimes the problem can also been seen with low voltages, but much more rarely than higher voltages. Only once or two have I faced this problem at 12V. But I did - once or twice. That's why I always place it there.
I don't know why the manufacturers don't mention it but in commercial inverters (at least the ones that I've seen), there is always a resistor placed between gate and source. I've found the value to range from 1k to 4.7k(ones involving high frequency) and 1k to much higher, even 47k (in ones at low frequency eg 50Hz). Nevertheless, it is placed there.

Hope this helps.
Tahmid.

 

[FvM]

I agree with Alex, that the pulldown resistor suggestion can't be found in the data sheets or application notes of standard gate drivers. Referring particularly to IRF devices like IR2110 that implement an undervoltage-lockout circuit, the outputs are pulled actively low, if the supply voltage falls below the threshold of e.g. 9.5 V. So a pulldown resistor would only serve a purpose, if you manage to tristate the driver outputs somehow. According to the MOSFET characteristic, the pulldown action may possibly stop below a supply voltage of 3V or so. But the output voltage will be still clamped to the driver supply by the internal supply diodes.

I must confess, that I never checked this condition, because systems, where I have used IR2110 drivers won't enable the bus voltage with missing driver supply. Perhaps someone should measure the IR2110 pulldown capability with a ramped supply voltage.

If the power semiconductor (MOSFET or IGBT) is however separated from the driver and connected by a cable, I would place a pulldown resistor and a zener diode to prevent from floating gate or overvoltages.

If you aren't sure about requiring the pulldown resistor, I don't see a problem to place it as a precaution.

 

[Tahmid]

I agree with Alex, that the pulldown resistor suggestion can't be found in the data sheets or application notes of standard gate drivers. Referring particularly to IRF devices like IR2110 that implement an undervoltage-lockout circuit, the outputs are pulled actively low, if the supply voltage falls below the threshold of e.g. 9.5 V. So a pulldown resistor would only serve a purpose, if you manage to tristate the driver outputs somehow. According to the MOSFET characteristic, the pulldown action may possibly stop below a supply voltage of 3V or so. But the output voltage will be still clamped to the driver supply by the internal supply diodes.

I must confess, that I never checked this condition, because systems, where I have used IR2110 drivers won't enable the bus voltage with missing driver supply. Perhaps someone should measure the IR2110 pulldown capability with a ramped supply voltage.

If the power semiconductor (MOSFET or IGBT) is however separated from the driver and connected by a cable, I would place a pulldown resistor and a zener diode to prevent from floating gate or overvoltages.

If you aren't sure about requiring the pulldown resistor, I don't see a problem to place it as a precaution.

All of this makes sense. However, I found the problem even when a driver is driving the MOSFET. Using the 1k resistor helped. Other users have also mentioned this. Like you said, there is no problem with placing the resistor as a precaution. Better safe than sorry!

 

[alexan_e]

I can understand using the resistor as a precaution in case it is needed but apart from protection from a floating gate I can't understand any other purpose that this resistor can serve.
It is like a big secret shared between the circuit manufacturers and used in commercial devices as you said but not mentioned in any kind of paper I've seen and doesn't seem to have an explanation of what exactly it does in a circuit where the gate is properly connected.

Alex