Bs170-mosfet

[dark]

Hello Board,

I just want to confirm if its ok to pullup this NFET to 12V ? .
I couldnt see 12V VGS for this part instead the datasheet ,only says about 10V VGS . Datasheet Absolute maximum rating asks for VGS of +/-20V , is it ok to go with this , I am hesitant .

https://www.electro-tech-online.com/custompdfs/2009/03/BS170.pdf

Thanks

 

[crutschow]

10V Vgs is just the recommended value to turn the FET completely on.

The maximum Vgs rating is 20V, thus it's perfectly ok to apply 12V to the gate.

 

[smanches]

Most MOSFETS start to turn on at around 4.5v, although it's the amount of charge that actually controls the on/off state, not the voltage. At 4.5V the charge builds slowly, so the FET will turn on slowly (propgation delay), but there will be lower current to the gate. If you increase the voltage to 10V, the current to the gate will go up and charge it much faster, lowering the propgation delay, although your now using more power just for activating it.

Basically, the higher the voltage to the gate, the higher the current draw and the faster the field can charge and turn the MOSFET on. The downside is the higher current draw, and the more powerful your MOSFET drivers will have to be.

Most datasheets that I've seen use either 10v or 12v to determine their propagation delays, timing, etc.

 

[crutschow]

Most MOSFETS start to turn on at around 4.5v, although it's the amount of charge that actually controls the on/off state, not the voltage. At 4.5V the charge builds slowly, so the FET will turn on slowly (propgation delay), but there will be lower current to the gate. If you increase the voltage to 10V, the current to the gate will go up and charge it much faster, lowering the propgation delay, although your now using more power just for activating it.

Basically, the higher the voltage to the gate, the higher the current draw and the faster the field can charge and turn the MOSFET on. The downside is the higher current draw, and the more powerful your MOSFET drivers will have to be.

Most datasheets that I've seen use either 10v or 12v to determine their propagation delays, timing, etc.

It is normal to say that MOSFETS respond to gate voltage, but there is a small charge that is needed to charge the gate capacitance when you change the gate voltage (which is the charge under the gate). If you have a low impedance drive to the gate than the response time will be fast, independent of the gate voltage (above the threshold voltage). Once you supplied this capacitance charge then no further gate current is required.

A higher gate voltage will, of course, require more capacitive charge and that can require some significant power when driving the transistor at high frequencies.

A higher gate voltage causes the MOSFET to conduct more current. The MOSFET is a voltage-controlled current source with the current proportional to the voltage squared (above the threshold voltage).

If you use a MOSFET as a switch, then you want a high gate voltage to completely turn it on and minimize it's "on" resistance and dissipation.

 

[dark]

10V Vgs is just the recommended value to turn the FET completely on.

The maximum Vgs rating is 20V, thus it's perfectly ok to apply 12V to the gate.

Thanks Carl , as the datasheet lacks details or maybe not required can the VGS go as high as 20V? or it doesnt matter above 10V to 20V causes a constant on state resistance.

 

[smanches]

Most of the datasheets I've seen, and I've looked at a alot lately, have the max voltage on the gate around 18-20V. It's recommended to put an 18v zener at the gate to prevent overvoltage conditions.

Someone might be able to clarify that better.

Now that I've actually looked at your datasheet, the max gate voltage is the Vgss figure: 20v. Going higher would decrease the turn on time somewhat, as the gate will charge faster, but it shouldn't lower the on resistance more than 10v. It's interesting that I don't even see a gate charge figure there. Could it be because it's such a small device?

 

[audioguru]

Most of the datasheets I've seen, and I've looked at a alot lately, have the max voltage on the gate around 18-20V. It's recommended to put an 18v zener at the gate to prevent overvoltage conditions.

It is required only if the input voltage has spikes higher than 20V. Most don't.

Now that I've actually looked at your datasheet, the max gate voltage is the Vgss figure: 20v. Going higher would decrease the turn on time somewhat, as the gate will charge faster, but it shouldn't lower the on resistance more than 10v. It's interesting that I don't even see a gate charge figure there. Could it be because it's such a small device?

I am surprised at its very low capacitance which is thousands of times less than a power Mosfet.

It is the current that charges its input capacitance, not the voltage.
The rise and fall times are spec'd with a generator with a low output resistance for high charging/discharging current.

Its datasheet shows its on-resistance decreasing less as the gate voltage rises. The difference between 10V and 12V is almost nothing.
Its on-resistance is pretty high at 5 ohms max.