Need help selecting a MOSFET

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Marcwolf

Senior Member
Hi Folks
I need some help finding a MOSFET with the following characteristics.. and being common as well

Logic level Gate so can be driven straight from a picaxe
Can handle 65V min
Can handle 5A min

Hopefully someone can answer this tonight and I can source the part tomorrow. I have an old MIG welder that I have never been able to get the wirefeed right on it. It uses a feed off the main trasnformer to drive the motor and depending on that voltage is the speed the wire is fed. No real adjustments

I've since made up a PicAxe board that output PWM to a MOSFET to drive the motor. It works very well except the MOSFET is getting too hot. I suspect that I am not driving the gate hard enout. It's an P50N06L

Many thanks..
Dave
 
T

tracecom

Senior Member
I have had excellent results with two different MOSFETS being driven by PICAXE's: IRLZ44N and IRLU8721. The former is older but may be more available than the latter. Both are rated for much more than 5A, but I use them without heat sinks.
 
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Goeytex

Senior Member
FET selection is driven by 1. Availability, 2. Specifications, 3. PACKAGE TYPE

The problem with hobbyist requirements is we want Logic Level, Low RDSON, Low capacitance, through hole (TO220) FETS. This drastically limits our choices with most large suppliers. I did parametric searches at Element14, and RS Australia and there is nothing in stock that I can find that meets these kind of specs.

If you can do surface mount the world of choices opens up. If you can use a FET driver, the world of choices opens up even further.
 
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John West

Senior Member
Keeping a power MOSFET from getting too hot depends on two thing beyond selecting a device that meets the required specifications - proper heat-sinking, and switching the FET on and off fast enough (if doing PWM control work) so that it doesn't spend much time between ON and OFF states generating heat.

There is no MOSFET yet made that will handle large amounts of current and fast PWM activity without some sort of heat-sink and a gate driver device beyond a PICAXE. All high power MOSFET's have significant gate capacitance that must be overcome. At some level of current and switching speed you simply must take these things into account. There are very good FET's available today, but there is no "magic" FET.
 
D

Dippy

Moderator
Exactly. The MOSFETs suggested would not drive well from PICaxe if the PM is the usual fast-ish motor value.

One further thing we need to calculate is the ratio of the transition time to the duty period.
If it takes 1 microsecond to switch on or off and the power-on time period is 2 microseconds then we may be in trouble.
But if the total period is 10mS then it probably not problem. Do the power calcs.

We also need to calculate the peak gate drive current to see if this damages (or potetnially damages) weedy drivers like PICs.
The calculation is so simple I don't need to tell you.

This means there are several parameters we need to calculate to determine good driving.
Don't be tempted to make switching too quick. Obviously you'll know why.
MOSFET and IGBT driving (esp very high currents and high voltages) is a balance.

To determine a minimum driving spec you will need to define a PWM duty/Frequency value.
This maybe well OTT for such a small current but would be good education for you and others.
 
G

Goeytex

Senior Member
@Dave

What PWM frequency are you using ?
Why must the FET be driven directly by the Picaxe ?
Are you using a heat sink ?
What voltage is the motor ?

A good (through-hole) logic level FET like a FQPF33N10L will need to dissipate about
1 watt of heat given a 24 volt supply, a 5 ohm load, and a PWM frequency of 1Khz. This is
very manageable with a relatively small heat sink.

Your FET has a rather large gate capacitance (> 2000 pf) and and relatively slow on time (> 200ns). Given the
low drive current from the Picaxe it is less than ideal for high speed switching while being directly driven by the
Picaxe, (especially if there is no heat sink). The heat problem can be exacerbated by an unnecessarily high PWM
frequency given the low drive current and less than stellar switching time of that FET.

However, adding a FET driver to your existing FET along with a decent heat sink should be adequate for your
application and make the heat issue manageable.
 
B

bluejets

Senior Member
Seen problems like this before .......simple wire feeder needing some sort of speed control and the "fix" becoming so complicated it is just not worth the hastle. So why not just run the motor off full supply with a few series resistors and bridging switches.
 
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Goeytex

Senior Member
You must not have welded much with a good wire feed welder. Good smooth wire feeding is essential to a nice weld.
I don't see where upgrading a wire feed welder to feed the wire smoothly & with a good variable speed control is a
hassle at all. Using a Picaxe to do it actually uncomplicates the upgrade if the components are properly matched.
Seven or 8 parts on a perfboard and few lines of code doesn't seem very complicated to me.
 
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John West

Senior Member
Goeytex, the rest of us were helpful, but your post #9 was a good, complete, straightforward answer to the question. Well done.
 
M

Marcwolf

Senior Member
Hi All
Many thanks for your comments Goeytex. Having done a little welding I could see what the issue was with the MIG. Really bad base design.

Re your questions
What PWM frequency are you using ? - 4khz

Why must the FET be driven directly by the Picaxe ? - less components and less issues re additional voltage to drive the Mosfet. Most mostfets I have seen need 10 to 15v Vgs and I need to generate that as well.

Are you using a heat sink ? - Yes about 4 Sqr inches of alum. Might swap to a old CPU heatsink with fan.

What voltage is the motor ? - 60V max. The original design basically apples 40,50,60v to the motor depending on what secondary was selected so not much in variation.

I had no troubles with the PWM setup with a 08M either, got the code working nicely so it ran from 70% to 100% PWM which corresponded to the motor starting to turn from 20RPM to full speed.

How ever the MOSFET area was where things were comin unstuck. The only one I could find locally were non-logic one which meant some additional components to try and drive the MOSFET fully on.

Anyway - I was able to find a supplier of IRL540's which are 100V 5A logic mosfets. They are used a lot in Pinball machines. Got 5 coming in tomorrow so I'll give it another try again.

Many thanks for all of your replies.
Dave
 
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fritz42_male

Senior Member
Pardon my ignorance but I have never MIG welded although I have a borrowed machine sitting in my garage ready for when I have some spare time. I understand the need for different feed rates but would you control this via a pot on the welder itself or via a variable trigger on the gun?

I would have thought a variable trigger on the gun would be the best bet as you could increase and decrease feed rates as you negotiate the weld line.
 
M

Marcwolf

Senior Member
At the moment it will be a pot on the front of the welder. Through experimentation I should be able to mark in colors the best feed for a particular setting. A pot on the gun would be nice but then I'd have to run some additional wires along the cable and there is also interference.

Ideally (flight of fancy here) I could have a IR transmitter on the gun that would allow me to change the wire speed -10% to +10% from the initial setting.. or a little box to wear on my belt..
Oh the possibilities once I get the intial PWM running and driving.

Many thanks
Dave
 
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Goeytex

Senior Member
If you drive the IRL540 directly from the Picaxe through a 1K resistor
And the motor draws 5 amps peak @ 50 volts.

With a 4khz PWM the FET will have to dissipate about 14 watts. It will get pretty danged hot.

Reduce the resistor to 330 ohms and the heat dissipation will be 8 watts. Still way too hot but better.

If you then reduce the frequency to 1khz the heat dissipation will be reduced
to about 2.1 watts. Better yet & Managable

If you use a FET driver at 1khz with a 20 ohm gate resistor it is heat dissipation
is reduced to about 1 watt. Much better. Very managable

My point being ... these logic level FETs with no driver do not like high speeds and low gate drives.
 
M

Marcwolf

Senior Member
Many thanks for the info Goeytex.
I can reduce the frequency to 1khz easily and I can live with 2.1 watts as the heatsink can be placed infront of the welder cooling fan.

Still learning a lot of things about mosfet so many thanks for your advice.

Many thanks
Dave
 
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