Hello everybody !
I have this Basic Variable Power Load w IRFZ44N cct. This is not my cct, I find it online and I am uncertain of its functionality.
I know there are these 2 types of driving a mosfet: linear mode and switching mode.
The usual way I know how to use my IRFZ44N is in full switching mode, with minimal RDSon as possible, from 10VGS up to maximum 20VGS as in its datasheet.
Here is what I understand from this cct:
This cct is made, by the look of it, in linear mode. Especially when the VDD is less than 10V.
That DZ10V and its 10k current protection, are there to limit higher voltages to a maximum of 10V on the transistor Gate. To protect it from over voltage and not burn its gate. The POT47k can be any value, I used 47k because is a rare value and I have a bunch left. Its role is to linearly change the voltage on the Gate, playing the role of a voltage divider. The current is very low, thanks to the 10k in front of the POT.
So,from my understanding, when the Gate receives less than <10V, RdsON will increase, lowering the performance of the transistor, increasing the power dissipated as heat over the transistor in the same time, and conducting way less current compared when is fully open with >=10V.
So, it is better to drive any mosfet as fully open as possible, with a minimal RdsOn as possible, to allow as much current to pass through it.
So this is the theory and practice I know so far. I kept away from driving mosfets in their linear region <10VGS.

Now... to correct this cct, Im thinking on a boost DC converter. I already have a cct for it, that I can build. Its a bit complicated, including a 555 oscilator module, that drives a small mosfet included in a current pump with a 500uH coil module and in the end, a current rectification module, to liniarize the oscilations, making it as DC and smooth as possible pretty much. But is a big-ish cct to implement. What it does, it is taking a 5V input, and outputs 60V or so. I already made the circuit for another application, but Im thinking to make it again, this time to specifically drive my mosfet in this Power Load cct presented in the picture. But 60V or so is way over nominal VGS. That DZ10V is there to limit and assure a clean 10VGS. So Im safe at this point. Why to use this DC converter? For the simple reason, of driving everything from 5V ! My mosfet will be driven in switching mode, with a precise 10V on its gate, and everything else in the cct will run at 5V.
The problem is that POT, that is transforming everything I said until here into crap. Because whatever Im doing until the POT, doesnt matter. As long that POT is there, it will add a less than 10V on Gate and make it run more hot than I want it.
Whats the solution here? Where Im doing wrong? Is my logic so far ok? Do I miss anything? Any comment is welcomed.
One approach I believe you will come up with is to use PWM on this mosfet to pass less or more current through it, all in the switching mode, no linear mode. I thought of it as a possile solution, but I want to see what you say too.
Thank you.

 

So the question stands: What useful thing does this circuit provide??
WHAT is the whole purpose??

 

If you want the FET to act as a variable load then (1) Rds will be greater than the minimum and (2) it will therefore run hot when providing a heavy load.

 

You have two options:

A)Waist the power:
1.Mosfet in linear mode- the energy will be dissipated on mosfet(s).
2.Mosfet in switched mode- the energy will be dissipated in some other element like power resistor, heating element, bulb.

B) Use the power for something useful like charging the battery, supply some device, transfer the energy back to source - but the energy has to be used immediately.

 

Your described “fix” is called PWM, which is not at all the same thing as a variable load.

Here s how that circuit is supposed to work (it does not, because the input voltage is too low, as you pointed out.)

At Vgsth the MOSFET starts conducting. Near that voltage, it acts as a constant current (not power) source. As the voltage rises, it begins to look more like a resistor. In either case, the MOSFET drops the entire input voltage across D and S, and dissipates power proportional to the voltage and current.

Converting it to PWM with a 10V gate voltage, is basically shorting the power source with a very low (Rdson) resistor.

P.s. A dummy load must dissipate the power.

 

Here s how that circuit is supposed to work (it does not, because the input voltage is too low, as you pointed out.)

well...it is working but the mosfet gets very hot. So I am doing all I can to run it as cool as possible in a comfortable size.
Here is what I made so far.

Another issue is the knob of the POT rotates like 10% and the mosfet is surging like 200-400mA at about 11Vdc.
You can actually see a litle white marker on the knob relative to it's maximum.
Im watching my varPSU amper-meter to know when Im turning the POT too far.
And as expected, at that power 11V*0.4A = 4.4W !!! with this dinky radiator,the mosfet gets hot in about 4 seconds.
So with a small delay, which is good, otherwise it was burned already.
What I dont like is the VERY small travel of the POT knob, to reach very high power disipation.
The cct is good and simple - I like it very much for its simplicity, but.... its too sensitive, too abrupt !!!
My money is because I'm running the mosfet in linear mode !!! - thats why I open this discussion.

 

So the question stands: What useful thing does this circuit provide??
WHAT is the whole purpose??

Like this comercial Electronic Load Tester you can find on ebay/aliexpress. You can find them explained on youtube as well.

 

What I dont like is the VERY small travel of the POT knob, to reach very high power disipation.

That's because the gain of the MOSFET is very high once you reach its Vgs(th) voltage.
To get a linear variation in current with pot rotation you could make a constant current circuit with an op amp and a small series resistor (below):
Vset is the voltage from the pot.
The op amp can be any single-supply or rail-rail type amp such as the LM324.
The op supply can be Vcc as long as it doesn't exceed the op amp voltage rating.

For that circuit the load would be between Vcc and the MOSFET drain terminal.

Like this comercial Electronic Load Tester you can find on ebay/aliexpress.

Notice the beefy heat-sink with the attached cooling fan.

 

I don't understand what you're trying to do. The circuit you show uses the MOSFET as a variable resistor, but you're not doing anything other than heating the MOSFET. There's no 'DC Converter' functionality here. This arrangement is commonly used for a constant current load eg for battery testing to determine capacity, typically using an opamp to control the gate voltage while monitoring the current through the MOSFET. ([edit] as shown in the post above)

btw MOSFETs intended for switching perform really badly in linear mode except at low currents as they tend to fail due to hot spots developing in the die. If the Safe Operating Area (SOA) chart for your MOSFET doesn't show a DC line its probably not suited for this mode. There are MOSFETs designed specifically for Linear operation, but they are expensive (I have a 2400W dummy load using IXYS Linear2 MOSFETs rated at 200W each on a massive water cooled heatsink).

If you're looking to go from 10v to 5v (at what current?) a standard 3-pin voltage regulator eg LM7805 is the simplest way to go,but the dissipation will always be (10-5) x I Watts. If you're looking to go from 4 - 60v to 5v that's more complex because you need a boost-buck topology, but in any case dropping from anything above 10 or so volts to 5v needs a switching solution else you're just throwing energy away as heat.

 

Like this comercial Electronic Load Tester you can find on ebay/aliexpress.

Yes, but don't believe the specs on those. They claim 150W but anything above about 30W the fan is screaming, and 2 failed very quickly!

 

That's because the gain of the MOSFET is very high once you reach its Vgs(th) voltage.

So probably, that abrupt raise in current & power I am experiencing, from the POT 10% rotation, is because the mosfet was practically closed, and when is seeing Vgs(th) as you say, about 3V, (2V-4V) range, is starting to conduct. Very interesting detail - I didnt see it myself.

you could make a constant current circuit with an op amp and a small series resistor

Yes, that was planB actually. I am aware of this cct with the opamp and I did build a couple, like 2-3 in my past experiments.
But the idea with this basic cct, this guy who made it and use it, he said he used it for about 20 years.
He might had a finer pot than mine, although he is saying his POT was scrapped from an old radio. So, I dont think it was so different than mine - in function. It was an old POT model, much bigger in size. Hmmm. I'm finding it fascinating !

 

I don't understand what you're trying to do.

Well ... I want a lot of power dialing ! Either rotational like from my POT, or incremental, like some jumpers.
From a few miliWatts to a couple of Watts or even 10's of Watts. Like 50W for example. And why not, even 100W !
I know mosfets are very good at conducting current through them. But I dont think they are that good when used in linear mode. Is my impression.
Thats why I wanted to use it as a switch in the first place, using that boost DC converter.
I think, if I want all that power, I will have to PWM its gate and run it at 10V minimum on its gate to completly and correctly drive it.

Yes, but don't believe the specs on those. They claim 150W but anything above about 30W the fan is screaming, and 2 failed very quickly!

I know and Im aware and I agree with you !
At it's naked basic circuit diagram, I believe those comercial power load are the same as my circuit here.
The only diference is they have some MCU both for handling the actual power on the mosfet, rotating the knobs, incrementally, and then displaying it.
I do not have a comercial one ! thats why I want to build one myself in DIY mode.

 

you could make a constant current circuit with an op amp and a small series resistor (below):

Here is a picture from an old experiment I did on 2024-03-01 (march). Im using the opamp cct as a driver for the power transistor. In this specific experiment I linear drive a NPN, I think it was BD681. Its on that aluminium CPU heatsink clamped to it. With those thick blue wires. I think I try it with a mosfet at some point and I hit the same issues of very high power dissipation as heat over the transistor in test. You can see the cct diagram on the table !
Im using 1R5W for the Load resistor and you can barely see it in the left bottom corner, a white brick. Its also specified in the diagram as R6.

 

when is seeing Vgs(th) as you say, about 3V, (2V-4V) range, is starting to conduct.

And the transconductance gain shown right below the Vgs(th) value is 19S (A/V) or 19mA/mV.

 

One thing to note is that there is variation in Vgs(th) of about 3mV/°C, which equates to about 50mA/°C increase in current as your heatsink warms up. Don't take your eyes off it otherwise you will have thermal runaway.

 

An added note:
If you want to use the op amp circuit, it can be converted into a high-side current regulator (with grounded load) if that's preferable, using a RR op amp and a P-MOSFET.

 

Didn’t we have a thread that went on for weeks before on this same ill-conceived idea? And I really mean ill-conceived literally. TS could not explain what he was trying to do then and he still has not done so.

He has shown us a (not very good) circuit for a variable dummy load. But then he complains that it gets hot! Where does he think the energy is going to go? He wants to put up to 100 W into a MOSFET channel and have it stay cool. Good luck with that!

 

One thing to note is that there is variation in Vgs(th) of about 3mV/°C, which equates to about 50mA/°C increase in current as your heatsink warms up. Don't take your eyes off it otherwise you will have thermal runaway.

@q12x Which is another reason for using the opamp with -ve feedback. Thermal runaway is more problematic with certain MOSFET constructions. The Safe Operating Area (SOA) chart on the left below is for an Infineon IRFZ44N TrenchFET, while the one on the right is for an NXP IRFZ44N HEXFET. You wouldn't want to ever consider the LH one for linear use, but the RH, suitably de-rated by 70%, might work OK. Why de-rated? Because those curves are for a junction temperature of 175C and a case temperature of 25C which is near impossible to achieve. More realistic would be a case temperature of 80C and an ambient temperature of 30C which, assuming a total thermal resistance case to ambient of 2.5C/W, gives a dissipation of (80 - 30)/2.5 = 20W on a 2C/W heatsink (allowing 0.5C/W for thermal grease between heatsink and case), and a junction temperature of 80 + 1.5 * 20 = 110C (based on 1.5C/W junction to case) which is OK. Forced air cooling on a suitable heatsink might increase the dissipation to 30W or a little more (but nowhere near the theoretical spec of 110W!). A 2C/W heatsink is a substantial heatsink.

 

@q12x Which is another reason for using the opamp with -ve feedback. Thermal runaway is more problematic with certain MOSFET constructions. The Safe Operating Area (SOA) chart on the left below is for an Infineon IRFZ44N TrenchFET, while the one on the right is for an NXP IRFZ44N HEXFET. You wouldn't want to ever consider the LH one for linear use,

Have you noticed that the "secondary breakdown" slope on a MOSFET SOA graph only appeared in the last decade? Before then, there were plenty of MOSFETs with the Zero tempco point at a very high current, but with the same SOA graph as the 2SK134!
Then suddenly, honesty took over!

For an active load, Rds(on) has no relevence, so the Exicon Lateral MOSFETs would give a stable solution, and they are hard to break.

 

I have an idea. I collected this image as part of the specifications for this product from aliexpress page.
I never had this thing, but if you have it,(or similar) it will help me.
If this thing is running from USB at 5V, then 35W/5V=7A. Hmmm.
Maybe to use 2 mosfets in parallel ?to split the power dissipation ? I am not that good at this chapter, but is an idea.
Probably this is true at its absolute maximum settings.
My idea is to reimagine it's cct, to make a cct close to that 35W at 5V.
Or, if you have one, or you know a website with a skematic for it, that will be more helpful than reimagining it.
I can see it has 2 rotary encoders (green+silver knob). Very interesting detail about fine tuning and coarse current.
But until that stage I want the "how to" and the recipe for the basic cct.
I do see its (not so large) aluminium radiator, and the computer fan. I also think it is having a thermal sensor. Which I had a lot of problems making one work in practice. I never being able to make a basic thermometer because all my thermistors are weird. I also got transistors instead of LM35 thermistors. I had a lot of bad luck with getting simple working thermistors. So that might be a big problem.But until that point... lets see what you can come with. Thanks !