mosfet microcontroller protection

[yassin.kraouch]

Hi,

i like to drive this MOSFET https://www.fairchildsemi.com/ds/FD/FDC6401N.pdf with a microcontroller https://focus.ti.com/lit/ds/symlink/msp430f2101.pdf, but i have a doubt on the protection of the PIN of the MCU, i suggest ti insert a resistor between the gate and the PIN of the MCU, on order to limit current, have you an idea on how to calculate the value of the resistor ?

 

[alexxx]

A push pull amplifier is a very common drive method for power mosfets, especially for higher frequency input signals (PWM for instance). Small gate resistors result in faster switching speed and larger resistors underdump the oscilations but extend switching time, getting the power MOSFET warmer.

Hope that helped

 

[mister_rf]

I prefer to drive a FET with a series resistor (~100 ohms) between gate and pin and perhaps clamp diodes(Schottky) to the logic rails for ESD protection.
Of course you will pay for that with a reduced rise time on the load and maybe it will heat up the FET a little more than otherwise.
Always include a pull down resistor on the gate (~10k), as leakage current can turn on the FET and you have no control of what happens during the first milliseconds before your MCU is configured.

Read also the discussions in this topic:
https://www.edaboard.com/threads/130012/

 

[Tahmid]

The safest way would be to use a totem-pole driver and use a gate resistor of 10-22 ohms. This makes the MOSFET switch on and off quickly and also ensures that if the MOSFET requires higher gate current, it can be provided. This prevents the microcontroller pin from "frying". If you are just going to switch the MOSFET on and off slowly (used as an on/off switch) at low frequency, then just a gate resistor will suffice. If you're going for high frequency, I recommend you use the totem-pole. alexxx has already provided a link to the circuit for the totem-pole. There's an app note by Fairchild Semiconductors about choosing the gate drive resistor.

I think this should clear your doubts:
www.fairchildsemi.com/an/AN/AN-9068.pdf - Gate Resistor Design Guidelines for SupreMOS

Hope this helps.
Tahmid.

 

[alexan_e]

A very important question is what will be the switching rate of the mosfet and the Vds voltage/current.
If you are going to control a load that turns on/off once in a while then you can easily use a resistor (like 1K) assuming that you will drive the gate above the gate threshold voltage.

If we are talking about fast rate switching (PWM or something) then you need a proper driver.

Alex

 

[yassin.kraouch]

The safest way would be to use a totem-pole driver and use a gate resistor of 10-22 ohms. This makes the MOSFET switch on and off quickly and also ensures that if the MOSFET requires higher gate current, it can be provided. This prevents the microcontroller pin from "frying". If you are just going to switch the MOSFET on and off slowly (used as an on/off switch) at low frequency, then just a gate resistor will suffice. If you're going for high frequency, I recommend you use the totem-pole. alexxx has already provided a link to the circuit for the totem-pole. There's an app note by Fairchild Semiconductors about choosing the gate drive resistor.

I think this should clear your doubts:
www.fairchildsemi.com/an/AN/AN-9068.pdf - Gate Resistor Design Guidelines for SupreMOS

Hope this helps.
Tahmid.

thank you about your helpful information, but for what frequency we are talking about high frequency? for example do you consider 1khz to 15 khz higher frequency ?

---------- Post added at 17:33 ---------- Previous post was at 17:31 ----------

A very important question is what will be the switching rate of the mosfet and the Vds voltage/current.
If you are going to control a load that turns on/off once in a while then you can easily use a resistor (like 1K) assuming that you will drive the gate above the gate threshold voltage.

If we are talking about fast rate switching (PWM or something) then you need a proper driver.

Alex

Yes i wil drive with PWM with a frequency range from 1khz to 20khz,

---------- Post added at 17:47 ---------- Previous post was at 17:33 ----------

have you a link of a driver ? in order to drive this MOSFET ?

---------- Post added at 18:05 ---------- Previous post was at 17:47 ----------

what do yo mean by totem-pole driver???????

 

[alexan_e]

You are using a N mosfet, do you intend to use it as a low side switch or as high side switch?
In case of high side switch things get more complicated because you will also need to use bootstraping to generate the proper gate voltage.

The totem pole looks like


Consider it like a push-pull emitter follower


**broken link removed**

Alex

 

[yassin.kraouch]

You are using a N mosfet, do you intend to use it as a low side switch or as high side switch?
In case of high side switch things get more complicated because you will also need to use bootstraping to generate the proper gate voltage.

The totem pole looks like
View attachment 57502

Consider it like a push-pull emitter follower


**broken link removed**

Alex

what do you mean by low side switch ? and bootstraping?? simply i will connect each gate of the MOSFET to an output of the MCU that generate two PWM signal with dead time,

 

[alexan_e]

A low side switch is a mosfet or transistor connected at the low (voltage) side of the load and it provides the ground, it it like a mechanical switch that connects/disconnects the gnd to the load.
In this case the source of the Nmosfet will be connected to the ground and the drain will be the output providing the gnd to the load.

A high side switch is a mosfet or transistor connected at the high (voltage) side of the load and it provides the positive supply.
In this case the drain of the Nmosfet will be connected to the positive supply and the source will be the output providing the positive supply to the load.
This is where you need a bootstrap circuit to be able to generate a voltage that is higher than the one connected to the drain, it is basically a diode-capacitor circuit, the capacitor gets charged and works like a battery connected between the source and the gate, the voltage of the capacitor adds on the voltage of the source (like two voltage sources in series) and this ensures that the gate will have always a voltage higher than the source to achieve proper Vgs bias.

Alex

 

[yassin.kraouch]

look at this shematic, i will switch the capacitor between the two two cells in order to charge and discharge it to equalize the two cell, i will drive the two MOSFET with the mcu, what do you think should i insert a bootstrap and how ?

 

[Tahmid]

Think of it like this:
In high side configuration, the load is between the source of the MOSFET and ground, the drain is connected to the positive rail. This means that the MOSFET sources current - the MOSFET provides the positive voltage.
In low side configuration, the load is between the positive rail and the drain of the MOSFET, the source is connected to the negative rail. This means that the MOSFET sinks current - pulls the negative end of the load to ground.

In high side configuration, the MOSFET has a floating source - its source is not connected to ground and to switch the MOSFET on, you need an additional 10v (above the level of the positive rail) by charge pump or boost or bootstrap or other methods, or you could have an isolated power supply and switch the MOSFET with respect to the isolated power supply.

If you are driving something like a lamp, motor or relay and it is possible to use the MOSFET as a low side switch, do so as this simplifies the circuit design. What will you do with the PWM signal? That will answer whether you require a high side or low side configuration.

Hope this helps.
Tahmid.

---------- Post added 22-06-11 at 00:01 ---------- Previous post was 21-06-11 at 23:59 ----------

Q1 and Q3 are in high side configuration (I explained this just now), so require high side gate drive (bootstrap would be easy). You could build a discrete driver, but you will get better results (better rise/fall times) by using a driver IC such as IR2110.

---------- Post added at 00:04 ---------- Previous post was at 00:01 ----------

I'm not exactly sure what you're trying to do. Is it possible to use a different configuration, one employing low side drive?

 

[alexan_e]

Is that 3.3v power supply the real value you will use in your circuit?
If it is then with a simple 6v-12v supply you will be able to drive the high side mosfets with a proper Vgs.

Alex

 

[yassin.kraouch]

i have no idea about bootstrap ? how can i do this ?
as i axplained above this capacitor will equalize the two cells so that they will have the same voltage,
so i need to switch this capacitor between the two cell in order to charge it and discharge it

---------- Post added at 19:12 ---------- Previous post was at 19:10 ----------

Is that 3.3v power supply the real value you will use in your circuit?
If it is then with a simple 6v-12v supply you will be able to drive the high side mosfets with a proper Vgs.

Alex

Yes Alex i will is the power from the cell in order to supply the circuit

---------- Post added at 19:17 ---------- Previous post was at 19:12 ----------

the true MOSFET that i will be use in my project is https://www.fairchildsemi.com/ds/FD/FDC6401N.pdf

 

[alexan_e]

will you have any higher voltage available (>6v) or the only power supply are the cells?

If you are going to use the bootstrap solution then maybe you should use a proper mosfer driver but your supplies are very low and you may have problems there...

This is an example of a discrete mosfer driver with bootstrap (it was for a 12v supply), note that your voltages are very low, from the 3.3v you will loose about 0.7v on the totem pole and about 0.5 on the bootstrap diode, you will barely get 2.5v Vgs


Alex

 

[yassin.kraouch]

will you have any higher voltage available (>6v) or the only power supply are the cells?

If you are going to use the bootstrap solution then maybe you should use a proper mosfer driver but your supplies are very low and you may have problems there...

This is an example of a discrete mosfer driver with bootstrap (it was for a 12v supply), note that your voltages are very low, from the 3.3v you will loose about 0.7v on the totem pole and about 0.5 on the bootstrap diode, you will barely get 2.5v Vgs
View attachment 57507

Alex

i will use oonly the power supply from the cell, but i have not 12 V in my design, yes but i can use this in my shematic, no?? and replace the 12 V with the 3.3V
the Vgsth of the mosfet is 1.5V and i think this can work, what do you think ?

 

[Tahmid]

This is an example of a discrete mosfer driver with bootstrap (it was for a 12v supply), note that your voltages are very low, from the 3.3v you will loose about 0.7v on the totem pole and about 0.5 on the bootstrap diode, you will barely get 2.5v Vgs
Alex

And the problem is? If you look at Figure 5 in the datasheet, 2v at gate will allow you the MOSFET to be used for maximum current. According to Figure 4, RDSon is also acceptable at 2v. Remember, this MOSFET has a gate threshold voltage of only 0.9

Hope this helps.
Tahmid.

 

[yassin.kraouch]

And the problem is? If you look at Figure 5 in the datasheet, 2v at gate will allow you the MOSFET to be used for maximum current. According to Figure 4, RDSon is also acceptable at 2v. Remember, this MOSFET has a gate threshold voltage of only 0.9

Hope this helps.
Tahmid.

the problem is can i use the same component as mentionned in the picture that alex give to us ? and only change the voltage 3.3V with the 12 V ?

 

[alexan_e]

This circuit has a high consumption on R1, so a more efficient circuit would look like

https://obrazki.elektroda.pl/89_1307374762.gif

the bootstrap diode and capacitor have to be added in this circuit

Alex

 

[Tahmid]

the problem is can i use the same component as mentionned in the picture that alex give to us ? and only change the voltage 3.3V with the 12 V ?

Yes you can. In the 2nd circuit, the addition of the transistor means that you drop another 0.6v. You could use a TLP250 with a diode and capacitor to provide bootstrap as well.

Hope this helps.
Tahmid.

---------- Post added at 00:58 ---------- Previous post was at 00:58 ----------

But then again, that depends on if you can find a TLP250 or if it is readily available in your area.

 

[yassin.kraouch]

Yes you can. In the 2nd circuit, the addition of the transistor means that you drop another 0.6v. You could use a TLP250 with a diode and capacitor to provide bootstrap as well.

Hope this helps.
Tahmid.

---------- Post added at 00:58 ---------- Previous post was at 00:58 ----------

But then again, that depends on if you can find a TLP250 or if it is readily available in your area.

TLP250 ?? phototransistor ?? this will produce a response time, and again we face the same problem, no ?

---------- Post added at 20:21 ---------- Previous post was at 20:09 ----------

This circuit has a high consumption on R1, so a more efficient circuit would look like

https://obrazki.elektroda.pl/89_1307374762.gif

the bootstrap diode and capacitor have to be added in this circuit

Alex

in the attached picture you put two circuit what is the difference ?