Will ANY MOSFET turn fully on with between 5 to 12v?

[bigal_scorpio]

Hi to all,

After doing some MOSFET research online I realise I don't fully understand how they are turned on. Since all the FETs I have used have always been switching lowish voltages (the most was 30v) I have never come up against what they actually need to saturate with the high voltage ones.

The ones I plan on using are STP9NK60Z N-CHANNEL 600V - 0.85r - 7A and will be switching about 270v dc for a DC universal motor.

I was wondering if I can switch it with the output from a 555 timer for PWM or even from a PIC micro?

Nowhere in the specs does it state the actual voltage or current needed to switch it, and up to now I didn't know it was a concern.

Any help appreciated

Thanks Al

 

[Roff]

See Rds(on) on page 3 of the datasheet. This is the ON resistance when Vgs=10V. It will typically NOT be fully on at Vgs=5V.

 

[bigal_scorpio]

See Rds(on) on page 3 of the datasheet. This is the ON resistance when Vgs=10V. It will typically NOT be fully on at Vgs=5V.

Hi Ron,

Yes I see the RDSon but where is the part that tells you where the level that will saturate is?

Is there a relation between the voltage being switched and the gate voltage required?

Also when I have rectified AC and get more DC voltage than the RMS AC volts, can I treat the neg side as Gnd as I do when I have rectified AC but through a transformer?

Thanks Al

 

[Roff]

Your third question implies that you are working directly with the AC line (mains), without any sort of transformer. This can be deadly, especially to a novice. I won't answer any more questions until you completely explain your application.

 

[Electroenthusiast]

There are two Ttypes of MOSFET: nMOS and pMOS, so it depends on what you are using.

 

[bigal_scorpio]

Hi Guys,

Ron, I am a novice in the use of FETs but not with working with high voltages.

I was an electrician for 30 years, both industrial and domestic, so I have a healthy regard for the invisible stuff!

I am trying to slow down a 20,000rpm 230v 0.22hp universal motor. I have already geared down the output by pulleys but my biggest and smallest pulleys still give about double the desired rpm. Most speed control circuits use AC and then DIAC/TRIAC combinations, but I don't have any DIACs unless I can find an old board with one on. The holidays are not helping buying things.

I do have the 600v N-FETs though so I thought why not rectify the AC and PWM the FET on the low side. This is when my knowledge of FETs let me down. I have used them before for the odd PWM LED circuits but never above 30v or so and cannot understand the correlation between required gate voltage/current to source/drain voltage/current, if there is one.

My basic question is just that. Is there a formula or rough guide I can apply and understand?

 

[Roff]

You need 10V Vgs to get the MOSFET turned on. This is independent of Vds.

I don't understand this question:

Also when I have rectified AC and get more DC voltage than the RMS AC volts, can I treat the neg side as Gnd as I do when I have rectified AC but through a transformer?

A schematic might help.

 

[crutschow]

..................

Yes I see the RDSon but where is the part that tells you where the level that will saturate is?

Is there a relation between the voltage being switched and the gate voltage required?

................

The value of Vgs that they use to measure RDSon is the minimum Vgs required to saturate the transistor and obtain this value of RDSon. Any voltage less than that may not fully turn on the transistor (there is some manufacturing variable in the value). So you never use less than that value of Vgs to saturate the transistor.

There is no significant relation between the voltage being switched and the required gate voltage.

 

[RCinFLA]

The use of the term 'saturation' applies better to bipolar devices then to MOSFET's. MOSFET have an Rs versus gate voltage in the non-constant current region of low Vds voltage. The parts are spec'd for a given Rs at a given gate voltage. You will get lower Rs for higher gate voltage (up to the spec'd limit of part, typically 20 vdc) and a higher Rs if there is less gate voltage applied.

Using a term from old vacuum tube days, the constant resistance slope region of MOSFET's are sometimes referred to as the 'triode' region. So if you want to relate 'saturation' to MOSFET's you may relate it to the triode region where a given gate voltage translates to a nearly constant resistance slope.

If you want to drive a MOSFET from a 5v logic look for a low Vgs spec'd part. General trade offs for MOSFET's are: Higher breakdown voltage the higher the Rs for a given die size. Lower the Rs the bigger the die for a given breakdown voltage spec resulting in higher input capacitance to drive. You won't find a low gate drive MOSFET's with a high breakdown voltage.

There are low gate drive MOSFET's targeted for single cell LiIon battery powered devices like cell phones. They don't need high breakdown voltage.

 

[bountyhunter]

Will ANY MOSFET turn fully on with between 5 to 12v?

The ones called "logic level" turn on very strongly at VGS values as low as about 3V.

 

[Boncuk]

Hi bigal_scorpio,

if you want a circuit well suiting your requirements you could refer to my design. The device has been tested on AC motors of 2.5KW without any triac misfires.

The triac used is a bit overkill since it can handle up to 40A, but I preferred the BTA41 because it's fully isolated.

The circuit corrects for mains frequency changes between 46 and 54Hz automatically thanks to a Schmitt-Trigger circuit with very short pulses for the input of the microcontroller.

The output circuit is standard. You might omit the snubber circuit for the power triac and replace it with a VDR rated 275V/0.6W if your motor has little power (max 250W).

An option is the animated LED circle indicating motor rotation with increasing speed. If maximum rpm is reached (full AC output) the 12 blue LEDs (arranged like a clock face) extinguish and the center red LED illuminates.

For rpm adjustment I used an optical rotary encoder (Grayhill 62P) to make the operation of the device fool-proof. (Thais even destroy pots by overturning them. )

Here are the schematics and PCB layouts. I used two PCBs mounted as a sandwich to fit into a standard US wall outlet box.

The hex file for the ATtiny2313 will be provided if you decide to use that circuit. If you don't posses an ATMEL programmer I might ship a programmed chip to you compensating for the chip and shipment.

Boncuk

 

[tvtech]

@ Boncuk

You go out of your way to help people. These are things that a Forum does not necessarily require. You do it anyway.

God Bless you friend,

You are worth Gold with your sharing attitude

Cheers,
TV Tech

 

[Boncuk]

@ Boncuk

You go out of your way to help people. These are things that a Forum does not necessarily require. You do it anyway.

I just do my best to help out. I designed the circuit for my own use and why not share it?

God Bless you friend,

You are worth Gold with your sharing attitude

Cheers,
TV Tech

Being a refugee from East Prussia during WWII I received a lot of help from good neighbors far from home in Bavaria.

So I'm just giving back what I have received.

Kind regards

Boncuk

 

[bigal_scorpio]

Hi Boncuk,

The schematic looks good and as TV Tech said, thanks for sharing.

I am also impressed with another of your skills. For someone who lives in Thailand and was born in Bavaria you still speak better English than many of my fellow countrymen.

Thanks again Al

 

[RCinFLA]

Hi Guys,

Ron, I am a novice in the use of FETs but not with working with high voltages.

I was an electrician for 30 years, both industrial and domestic, so I have a healthy regard for the invisible stuff!

I am trying to slow down a 20,000rpm 230v 0.22hp universal motor. I have already geared down the output by pulleys but my biggest and smallest pulleys still give about double the desired rpm. Most speed control circuits use AC and then DIAC/TRIAC combinations, but I don't have any DIACs unless I can find an old board with one on. The holidays are not helping buying things.

I do have the 600v N-FETs though so I thought why not rectify the AC and PWM the FET on the low side. This is when my knowledge of FETs let me down. I have used them before for the odd PWM LED circuits but never above 30v or so and cannot understand the correlation between required gate voltage/current to source/drain voltage/current, if there is one.

My basic question is just that. Is there a formula or rough guide I can apply and understand?

Using MOSFET's for AC motor speed controller is very difficult since the MOSFET gate drive must be isolated.

Since it is a 20,000 rpm motor I assume it is brush based motor. Have you tried a simple lamp dimmer or a ceiling fan speed controller.

 

[Sceadwian]

There's no simple math unfortunately, the voltage that is required at the gate of a Mosfet depends on both the source-drain current and the source-drain voltage, as Crutshow mentioned saturation and linear modes of operation are not the same for a Mosfet compared to a Bipolar transistor, the terms are in fact almost exactly reversed, you actually desire to run the FET in it's linear range.
https://en.wikipedia.org/wiki/MOSFET
Wikipedia's entry on FET's is a bit hard to chew for those that haven't delved into semi conductor theory before, but it explains everything pretty well.

 

[Jeff Birkle]

Hi all,
If anyone is still looking at this thread I have a few Q's about MOSFET switching as well.

If I have a high voltage (60volts DC) input.

To a 24 volt battery bank (such as in a buck controller).

Using a P-channel (IRF9530) for example on the positive side of the circuit with a Vgs of +,- 20volts. Turning it on will require grounding the Gate.

But turning it off is a different story, does the Gate have to go to the same voltage potential as the source?

 

[Roff]

Don't ground the gate if the source is more positive than 20V. As you noted, Vgs(max)=±20V.
You do in fact have to make Vgs=0 to turn it off.

 

[crutschow]

..................
But turning it off is a different story, does the Gate have to go to the same voltage potential as the source?

At a minimum you have to reduce the gate-source voltage to less than the minimum threshold voltage. Typically the voltage is just reduced to near zero. This has to be forced by the drive circuit, or by a resistor between gate and source if the drive is open. A floating gate may go to to any voltage, likely not zero.

 

[crutschow]

The use of the term 'saturation' applies better to bipolar devices then to MOSFET's...................

I understand that. I was using the word "saturation" in its somewhat generic sense of meaning fully on. Technically, of course, that's true for BJTs but not MOSFETs. In the future I will use the term "fully turned on" to avoid any confusion.