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Could this FET be used to switch 12V/30A from a PIC

shumifan50

Jan 16, 2014
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http://www.proto-pic.co.uk/content/datasheets/N-CHannel-MOSFET60V30ADatasheet.pdf

I have read the data sheet and do not understand it, unfortunately.

Some questions:
1. Is it capable of driving 12V/30A? I read on some posts that it requires 2 voltages, a charge pump was mentioned. Will it switch 12V if driven with a 5V TTL signal from a PIC or will it not be fully switched on? Similarly, the posts mentioned switching off being a problem. Will setting the pinto 0 on the pic cause it to fully switch off.
Note. The blurb on protopic says it should work , but on the funspark website there seems to be confusion.
2. If I need to drive more amps, can these be put in paralle(gates together, sources together and drains together)l?

Sorry if these are stupid questions.
Thanks for any help in advance.
 
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(*steve*)

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30A is right on the edge for this transistor. You would have to be careful that you turn it on and off quickly, more-so if you're doing it many times per second.

A 5V signal will turn it on, but depending how you're using it, you might want to consider a gate driver.

Yes, mosfets can be paralleled, and that's what I'd recommend for 30A. You don't connect the gates together though, you drive them separately. This might be done using a single gate driver and low value (let's say 1 to 10 ohms) resistors between it and the gate, or you may decide to have 2 gate drivers. Gate drivers are often available in pairs, so depending on what else you're doing, it may actually be simpler to have 2 gate drivers.
 

KrisBlueNZ

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http://www.proto-pic.co.uk/content/datasheets/N-CHannel-MOSFET60V30ADatasheet.pdf
1. Is it capable of driving 12V/30A? I read on some posts that it requires 2 voltages, a charge pump was mentioned. Will it switch 12V if driven with a 5V TTL signal from a PIC or will it not be fully switched on? Similarly, the posts mentioned switching off being a problem. Will setting the pinto 0 on the pic cause it to fully switch off.
If you use it with the source connected to 0V and the load connected between a +12V supply and the drain, it will switch 30A at 12V. But...
(a) If the load is inductive, you must connect a diode rated for at least 30V across the load (with its cathode to the +12V rail) to suppress the back EMF from the load. Otherwise, when the MOSFET turns OFF, the back EMF (due to the load inductance) will create a high-voltage pulse on the MOSFET's drain which will exceed the 60V rating for the MOSFET and will damage it;
(b) It's never a good idea to run a component at its rated maximum (current or voltage);
(c) Running at 30A that MOSFET will dissipate up to about 42 watts and it will need a significant heatsink.

You should use a MOSFET with a higher current rating and a lower RDS(on) specification. Here's what you need to know.

1. I would use a MOSFET that's rated for a continuous current at least 50% higher than the maximum expected current. In this case that would mean a 45A continuous drain current specification.

2. The amount of power dissipated by the MOSFET when it's conducting continuously can be calculated from the drain current and the RDS(on) value (also called the ON-resistance) using the formula P = I2 R, where I is the drain current in amps, and R is the RDS(on) resistance in ohms. So for a given current, a lower RDS(on) is better because it means less power is lost in the MOSFET and less heat needs to be dissipated.

3. Heatsinks are specified by their thermal resistance to ambient, which is measured in °C/W (degrees Celsius per watt). This figure is the number of degrees Celsius that the heatsink will rise above the ambient temperature for every watt of power dissipated. The required heatsink can be calculated from the maximum allowable temperature rise divided by the heatsink's thermal resistance to ambient. For example, for a maximum heatsink temperature of 75 °C at a maximum ambient temperature of 35 °C, the allowable temperature rise is 40 °C. If the device is dissipating 10W, your heatsink needs a thermal resistance of 40 / 10 = 4 °C/W.

4. RDS(on) is usually specified with typical and maximum values, and often, at several VGS voltages - often 10V and 4.5V, and sometimes lower voltages as well. At higher VGS voltages, the MOSFET conducts more strongly, so RDS(on) is lower. From this point of view, it's good to drive the gate with as much voltage as possible; this can be achieved using a level shifter made with transistors.

5. If you are switching the MOSFET regularly at some frequency, instead of just turning it ON and OFF periodically, you should use a MOSFET gate driver IC. These can be powered from a 12V supply (typically) to provide enough gate voltage to bias the MOSFET into heavy conduction, and a high current that charges and discharges the MOSFET's gate-source capacitance quickly and ensures that it changes quickly and cleanly between fully OFF and fully ON.

6. If you drive the MOSFET gate from a microcontroller output, you should use a MOSFET that's designed for low RDS(on) drive; these are also called "logic level" MOSFETs. These have data sheet specifications for RDS(on) at VGS voltages of 4.5V and lower. MOSFETs that aren't optimised for low VGS drive will conduct at 5V gate-source voltage, but not as heavily as they do at 10V gate-source voltage, and RDS(on) at 4.5V gate-source voltage will be significantly higher than the value specified for 10V gate-source voltage, so the power dissipation will be higher as well.

7. N-channel MOSFETs generally have lower RDS(on) values than P-channel MOSFETs. Most modern MOSFETs are only available in SMT (surface-mount technology) packages. Many of these can be hand-soldered, but no-lead packages (e.g. DFN, PQFN, SON - most package names that include 'N', and BGA packages) require reflow soldering and x-ray inspection. Many modern SMT MOSFETs have low VDS limits, and low VGS limits as well. These can easily be damaged by voltages that are quite safe for older, larger MOSFETs. Zener diodes or other clamping devices can be connected between gate and source, and between drain and source if necessary, to protect them against spikes or unexpected circuit conditions.
2. If I need to drive more amps, can these be put in paralle(gates together, sources together and drains together)?
Yes, but it's simpler and cheaper to use a more modern MOSFET. The specifications of devices you can get nowadays are really impressive. Here are a few for you to check out:

http://www.digikey.com/product-detail/en/BUK951R6-30E,127/568-9859-5-ND/3672466
http://www.digikey.com/product-detail/en/PSMN0R9-25YLC,115/568-6720-1-ND/2674297
http://www.digikey.com/product-detail/en/BUK962R5-60E,118/568-9569-1-ND/3431418

It's only a coincidence that these are all NXP devices. Other manufacturers such as STMicroelectronics, Fairchild, and especially Alpha & Omega, make some very impressive devices as well.
Sorry if these are stupid questions.
No, they're good questions.
 

shumifan50

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Thanks you very much for your responses - I will now embark on doing my first FET based circuit.
@kris thanks for your detailed explanation, it is much appreciated as now I can even calculate the heatsink required. I will look for a higher rated FET, say 60A leaving some headroom.

@Steve: Could I use a BJT to drive the gate(a bit like driving a relay) so it will be driven at 12V and therefore switching on the FET harder. In my specific application (LiPo protector) I will just use it as an on/off switch for the battery.
Sorry, Kris already answered this in his point (4) - it will work for a simple on/off switch.

On funspark they say use a LTC1155, but those are expensive and seem like an overkill for what I want to do. That seems to solve the case Kris mentioned in his (5), but I won't need this.


@kris
Those FETs you gave links to all seem to have Max Power of about 360 watt, so I think at 12V they would be limited to 30A in any event, which would still be close to limits in my case.

Would using a high rated BJT maybe be a better solution?
 
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pilko

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"Those FETs you gave links to all seem to have Max Power of about 360 watt, so I think at 12V they would be limited to 30A in any event, which would still be close to limits in my case."
You have calculated the current incorrectly.---- Use KBNZ's formula in post 3 item 1.
P = I^2 * Rdson therefore I =sq.root (P/Rdson).
 

shumifan50

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@pilko:
Now I am even more confused. In that case what is 'Max Power' in the data sheet. That is specified as around 360 watt. I think we are talking abour different things here.
 

pilko

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You are using the formula P=V * A, which is OK but when the mosfet is on, the V across the source / drain is very low. When the mosfet is off, the voltage across the source / drain is around 12V but now the current is almost zero. This is why it is better to use KBNZ's formula.
 

(*steve*)

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Thanks you very much for your responses - I will now embark on doing my first FET based circuit.
@kris thanks for your detailed explanation, it is much appreciated as now I can even calculate the heatsink required. I will look for a higher rated FET, say 60A leaving some headroom.

You need to be aware that as you increase the current carrying capacity, reduce the Rds(on), and reduce the voltage needed at the gate to switch the device, the gate capacitance rises.

The problem with gate capacitance is that it means you need a higher gate current (possibly several amps) to allow the gate capacitance to be charged and discharged so you can actually turn the device on and off in a reasonable time.

This may not be too important if the current is switched once every second, but it will be critical if it is switched 40,000 times per second.

This is because the mosfet is actually in one of three states.

  • Off - where the power dissipation is the leakage current times the supply voltage (and is essentially zero)
  • On - where the power dissipation is given by I2RDS(on)
  • Switching - where the power dissipation averages 1/2 of V * I (in your case 12 * 30 / 2 = 180W)
The last one is very important. If it takes 1ms (1/1000 sec) to switch the mosfet, then if you do it once per second you add an average of 0.18W to your total dissipation -- which is negligible. However, if you're switching for PWM at a rate of 200Hz, the additional power dissipated will be 72W (180 * 0.001 * 400 = 72W)

If you can make the mosfet switch 100 times faster, then your switching losses fall to less than a watt.

So that's why I asked how often you intend to switch the 30A.

@Steve: Could I use a BJT to drive the gate(a bit like driving a relay) so it will be driven at 12V and therefore switching on the FET harder. In my specific application (LiPo protector) I will just use it as an on/off switch for the battery.

Aaaah. It's for that. A shame you didn't say so earlier.

Sorry, Kris already answered this in his point (4) - it will work for a simple on/off switch.

Just the output of the microcontroller should work for that. You're only switching once in a blue moon. However you really need a mosfet with a lower RDS(on) because you don't want this dissipating 42 watts.

On funspark they say use a LTC1155, but those are expensive and seem like an overkill for what I want to do. That seems to solve the case Kris mentioned in his (5), but I won't need this.

It's the correct solution, but there are circuits you can bake using biolar transistors that should work jut as well. They will also allow probably a larger gate voltage swing, and that will reduce the power dissipation in the mosfet.
 

shumifan50

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Guys thanks so much for this discussion. Even though It will not make me be an electronics engineer, it certainly helps to understand how to use, and calculate values, for mosfets. Thanks again to all contributors..
 
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