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Topic: Understanding MOSFET datasheets (Read 15947 times) previous topic - next topic

localbroadcast

Feb 04, 2015, 05:22 am Last Edit: Feb 04, 2015, 05:24 am by localbroadcast
Hello.

I am planning on purchasing some mosfets for a number of projects I have on the go at the moment.  I want to make sure that I am buying the right mosfets for the job, because these things can be rather expensive.

I want to make sure that I am reading and understanding the datasheets, and was hoping some of you people could clarify things for me.

The basics that I need to know are the following:
Maximum continuous voltage the mosfet will allow from Source to Drain
Maximum continuous current the mosfet will allow from Source to Drain
The voltage / current needed to fully turn the mosfet ON
The turn on / turn off time

Do mosfets work strictly fully ON or fully off?  Or do they operate like a transister, where there is a region that it is only slightly on, depending on the gate current / voltage?

Also, will the mosfet turn off when the gate voltage is dropped to 0?  Or does the source voltage have to be 0 as well for the mosfet to turn off?  I remember there is some type of device that only needs a pulse at the gate to be turned on, and won't turn off until the source voltage = 0..  I just don't remember if this is how a mosfet works too.

Here is a mosfet I was thinking of buying.. If anyone could tell me the vital information that I need to know about it, that would be very helpful.

IRFP460 mosfet

Here is a link to the DATA SHEET

Thanks again!

weedpharma

The maximum specifications are just that, maximum that can't be exceeded. They are considerably higher than what it will run with continuously.

MOSFETS are like transistors in that they have part on areas. Read the data to see what the series resistance and possible current flow at a particular gate voltage.

These are curves. EG, at 2 volts on the gate the current is very low. At 5 volts it may be 5A while at 10 volts it may be 50A. It varies according to the device.

With the arduino, you need is a logic level gate mosfet. The gate needs to be fully on or the resistance of the drain/source is not at its least so causes heat.

My favourite small mosfet is NTD5867NL. It works with 5v on the gate and handles a few amps with ease. They are cheap. I bought mine from RS Components.

There are others but make sure they are fully on at 5v on the gate.

When you remove the gate voltage, there is a time taken to discharge the gate capacitor (internal). Therefore it is best to have a discharge resistor from gate to Gnd. This is important especially when you use PWM as it will get excessively hot without the R.

Weedpharma

JimboZA

Do mosfets ... operate like a transister
MOSFETS are like transistors
Not to be too picky, but mosfets are transistors  ;)
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Your answer may already be here: https://forum.arduino.cc/index.php?topic=384198.0

localbroadcast

Yea.. I've been doing some reading, and I think for my purposes I'll be needing a mosfet driver.  There's so many different drivers to choose from.. it's hard to decide!  Any suggestions on a driver that works well with the arduino outputs?

weedpharma

Not to be too picky, but mosfets are transistors  ;)

Good to meet a fellow pedant!!  ;)

Weedpharma

localbroadcast

Not to be too picky, but mosfets are transistors  ;)

yea.. yur right.. but I hope you all understood what I meant by what I said.. I was just comparing mosfets to yur standard sized bipolar transistors..

So what value in the datasheet shows the voltage that needs to be applied to the gate in order to turn the mosfet completely ON?

After doing some reading.. it sems like the gate pin operates like a capacitor.. and the mosfet turn on time is dependent on how quickly the gate is charged.. so a higher current applied to the gate results in a quicker turn on time.  This is the Q value in the datasheet, given as a value measured in nC.  Q = IT, where T is the turn on time in seconds, I is the current applied to the gate, and Q is the value in the datasheet given by
Qg or Qtot.. Some datasheets give more than one value for Q, which is confusing.. Qtot Qgs and Qgd... I'm not sure what the purpose of all 3 is yet.. still need to figure that out..

michinyon

That capacitance stuff matters mostly when you are going to use a very high PWM frequency -  like megahertz.

It's less important at lower frequencies typically used for driving robot motors  and that the arduino can practically generate.

Quote
So what value in the datasheet shows the voltage that needs to be applied to the gate in order to turn the mosfet completely ON?
That mosfet is not a great choice.  Look at Fig 1 in the first post you linked.    If your gate voltage is 4.5 volts,    you will only get 4 amps through the circuit,  even for large Vds.    This implies a fairly high apparent resistance and therefore heating.     To turn that on,   as far as it will go,   you need a gate voltage of around 7 or 8 volts.

If you look at Fig 8,   that fet looks good for controlling voltages around 100-200 volts  but not so good for controlling 12 V or 24 V to a robot because the on-resistance is rather high.




michinyon

The   Rds resistance of your mosfet -  even with 10 volts on the gate voltage,  is 270 milliohms at 12 A load current. 

The Rds for the one weedpharma prefers,  with only 4.5 V on the gate and a 10 A load current,   is only 50 milliohms.

That's 80% less heat you are generating,  right there.       And the performance of your mosfet will be even worse than that,  if you can only drive the gate to 5 volts.

MarkT

That capacitance stuff matters mostly when you are going to use a very high PWM frequency -  like megahertz.

It's less important at lower frequencies typically used for driving robot motors  and that the arduino can practically generate.

Except that it matters from a few kHz upwards...  Switching losses for a high power load
will dominate, and to keep them under control you normally want the switching time to be
< 1% of the PWM cycle time, so for 8kHz PWM that's 1us or better.  That usually means
needing 10's to 1000's of mA drive to charge the gate capacitance (for instance a gate
capacitange of 2nF charges to 10V in 1us with 20mA or more, but a large MOSFET with
20nF needs 0.2A
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MarkT

#9
Feb 04, 2015, 01:24 pm Last Edit: Feb 04, 2015, 01:24 pm by MarkT
The key parameters you need to look at in a datasheet are:

Vds(max)
Rds(on)  and the Vgs at which this is achieved
Qg (total gate charge) for the gate voltage you use.

You may also be interested in the body-diode's performance and the Vgs(max) and
the plateau voltage (usually only hinted at in graphs).  Its best to use a gate drive
voltage about twice the plateau voltage as this gives roughly symmetrical switching
times.

You don't need to look at the max current because if you do your sums right
with Rds(on) you'll never be anywhere near this, its an extreme thermal limit.
And if you have an Internation Rectifier datasheet they lie blatantly about
max current anyway.
[ I DO NOT respond to personal messages, I WILL delete them unread, use the forum please ]

localbroadcast

well.. I'm not using the mosfets to drive 24 volt robot motors.. I'm using them for a load at 100v - 150v at 10amps.  So I need the bigger mosfets.

I am thinking of driving the mosfet gate using a push-pull setup with npn pnp BJT transistors.. This way the arduino can trigger the transistors, which can provide a high current to the mosfet gate, allowing for fast gate charge / discharge of the mosfet.

I am trying to find a 3 or 4 amp transistor that the arduino can turn on.. and from what I'm reading, the arduino can only provide about 5 volts @ 20mA to do this..

I was looking at the MJE243 MJE253 npn pnp transistor set.. but I'm not sure the arduino can provide enough current to completely turn it ON.. Here's the datasheet stats for those transistors:

Collector−Emitter Saturation Voltage = 0.3v , 0.6v
(IC= 500 mAdc, IB= 50 mAdc)
(IC= 1.0 Adc, IB= 100 mAdc)

Base−Emitter Saturation Voltage = 1.8V
(IC= 2.0 Adc, IB= 200 mAdc)

Base−Emitter On Voltage = 1.5V
(IC= 500 mAdc, VCE= 1.0 Vdc)

From this information.. what is the current required to fully turn this transistor on?

rockwallaby

I'm sure we've been here before with this, but I'll suggest these drivers again to you from that other thread we were having discussions.

Quote
And to drive them nice and cleanly with these UCC27424P MOSFET drivers
The drivers are high speed dual drivers with a paralleled MOSFET and bipolar transistor complimentary output for good sourcing and sinking of main power MOSFET gate charge.
This driver can be driven directly by logic, I'll be connecting some of these to a DUE soon.

They also have a nice feature, with an enable pin, which you can connect back into some other safety cut-out system if you need.

I think I paid AUD$1.35 each from element14 and recieved another tube of them today. UCC27424P

These drivers have an output stage that has both bipolar and MOSFET devices in parallel.
Check out the datasheet for the specs.

Surely, it's got to be easier to use such a device as this that is designed and proven to work with power MOSFETS, than to mess about with discrete components.
_____
Paul
Paul - VK7KPA

localbroadcast

rockwallaby.. yes I ordered a handful of those drivers.  So they work fine with the pwm from the arduino?  Also.. I hope they work with the mosfets I ordered. 

IRFP460 20A 500V N Channel MOSFET Transistor

Any advice how to wire this driver up with my arduino and this mosfet?

MarkT

well.. I'm not using the mosfets to drive 24 volt robot motors.. I'm using them for a load at 100v - 150v at 10amps.  So I need the bigger mosfets.
Then you should seriously consider changing over to IGBTs as a possibility.
[ I DO NOT respond to personal messages, I WILL delete them unread, use the forum please ]

MarkT

I am thinking of driving the mosfet gate using a push-pull setup with npn pnp BJT transistors.. This way the arduino can trigger the transistors, which can provide a high current to the mosfet gate, allowing for fast gate charge / discharge of the mosfet.
No, use proper MOSFET driver chips, they work :)

Quote
I am trying to find a 3 or 4 amp transistor that the arduino can turn on.. and from what I'm reading, the arduino can only provide about 5 volts @ 20mA to do this..
Not likely, you need fast switching so darlingtons are out of the question, and
single BJTs don't have the gain.  The only option is a pair of MOSFETs to drive
the gate on the big MOSFET. Funnily enough that's exactly what you get in a MOSFET
driver chip....

Quote
I was looking at the MJE243 MJE253 npn pnp transistor set.. but I'm not sure the arduino can provide enough current to completely turn it ON.. Here's the datasheet stats for those transistors:


Collector−Emitter Saturation Voltage = 0.3v , 0.6v
(IC= 500 mAdc, IB= 50 mAdc)
(IC= 1.0 Adc, IB= 100 mAdc)

Base−Emitter Saturation Voltage = 1.8V
(IC= 2.0 Adc, IB= 200 mAdc)

Base−Emitter On Voltage = 1.5V
(IC= 500 mAdc, VCE= 1.0 Vdc)

From this information.. what is the current required to fully turn this transistor on?
Undefined, you haven't said what collector current or what power loss you can
tolerate.

Just get a MIC4422 and some good ceramic decoupling for it.  At that power level
(> 1kW load) you need really clean fast switching - during switching transients the
peak power dissipation in your MOSFETs will be similar to the load.  You keep
the switching time down to 100ns or so to prevent nasty things happening.  The
MIC4422 can drive many amps if needed. 

Use a fast opto-coupler on the input to the MIC4422 (with that power level if
you blow a MOSFET the MIC4422 will explode too, you don't want to take out
everything else and give yourself a lethal shock either).  Something like ACSL64000
series?

If you are PWM'ing the load you'll need to calculate your losses back-of-the-envelope
style...
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