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Vgs (Max) 2.5 V - Will I kill the MOSFET with a 3.5 V signal


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I am looking at MOSFETS as drivers and ST has one that looks good (low Rds at .02 ohm) for the price.

 

http://www.mouser.com/ProductDetail/STM ... A%252bI%3d

 

But the Vgs (max) is only 2.5V according to the data sheet. I assume that it will be fully on below this point and after this point will break down. Is that correct? I wouldn't want to drive it with 3.5V and have it self destruct. But I like the rest of the characteristics for the price. Curious on that.

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Interesting and you're right. What is the Vgs threshold (Max) parameter that they give on page 4 of the data sheet?

 

And by the qualifications of chart on figure 7 at just 3.5V trigger most MOSFETs won't have the low Rds values that we'd like to operate at to keep the device efficient. So are MOSFETs not the device of choice for logic power?

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Vgs threshold (Max) is the highest gate voltage that the MOSFET will need to begin to turn on in a worst case situation. In an analog application it would be the highest bias voltage needed.

 

The Vgs threshold voltage(s) are often misinterpreted to mean how much voltage is needed to turn the MOSFET fully on. What is really means is the voltage needed to *begin* to turn the MOSFET on. A typical TO-220 package MOSFET (such as the IRFxxx series) requires 8 to 12 volts to be "fully on." There are logic level MOSFETs that require much less, but most of them are only available in SMD packages.

 

The Vgs threshold is important for analog applications because the MOSFET will typically be biased near that voltage. For switching applications you have to look at charts like figures 6 & 7 to determine the gate voltage needed to ensure minimum Rds and maximum Ids.

 

You could use this MOSFET with the MSP430 @ 3.6V, but the current it can switch will be limited. It won't be able to get anywhere near the 27 Amp maximum. A few amps will probably be fine. Boosting the gate voltage to 6 volts or more would allow for more current to be switched.

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How about STP55NF06L?

 

Here's another idea (untested,) what if we use voltage doubler to drive MOSFETs? MCU drives BJT, BJT drives MOSFET with 7.2V.

 

On multichannel boards this would be actually cheaper than getting some fancy MOSFETs.

 

That's what I was also thinking. Any BJT should be able to supply the gate current of a MOSFET. But first I'll look at SMD logic MOSFETs. Seems like a simpler solution if they can handle the current and maybe cheaper with less parts.

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Keep in mind the gate of a MOSFET is essentially a capacitor. This is Ciss in the spec sheet.

 

STP27N3LH5: 470 pF (0.47 nF)

STP55NF06L: 1700 pF (1.7 nF)

 

As a general rule, higher current MOSFETs will have higher gate capacitance.

 

Whatever is driving the gate should be able to quickly charge and discharge this capacitance. The typical output impedance of a MOSFET gate drive circuit is 20 to 100 ohms (including the gate resistor).

 

If you drive a MOSFET directly with a microcontroller or with a relatively high impedance source like a pullup resistor, then the switching time may be rather long and result in unwanted heat dissipation. In some extreme cases the output port voltage of the microcontroller many droop at MOSFET turn on/off and cause unexpected behavior of other circuitry connected to the port.

 

The MSP430 specs don't characterize the output port in enough detail to know how it will respond with a 1.7 nF load. So this is something that should be characterized empirically. Connect a 2.2 nF cap to an output pin and look at the rise/fall times on a 'scope. Also look at the droop on another port pin.

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...If you drive a MOSFET directly with a microcontroller or with a relatively high impedance source like a pullup resistor, then the switching time may be rather long and result in unwanted heat dissipation...

If I am not mistaken, 777's intended use for this is switching solenoids/relays (no PWM) so this should not be a problem.

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That's true Rob G. These are simple ON/OFF type circuits. However I do have a variable voltage needed in the next project. So that will change things. I guess that's where I'll have to get the scope out.

 

This MOSFET was recommended by someone and it looks like it will work for a few applications where ULTRA high currents aren't needed.

 

http://www.mouser.com/ProductDetail/Int ... DAhg%3d%3d

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