Dear All About Circuits Team,

I hope everyone is doing well,

I have a question about the MOSFET's behaviour, How can I figure out how much heat the Mosfet produces per amp and how much current it can consume when acting as a load?

Below, the Mosfet application shown in the figure, it's an electronic load configuration.

Any suggestions ?

 

You answered by Your own to this question. Take the current through the mosfet, multiply the voltage on it and thats it!. This is the true reason why in high power circuits all are prone to use mosfets in full off/ full on regimes exclusively, aka SMPS.

 

The circuit generates a constant-current through the MOSFET.
Note that the load, whatever it is, may affect the voltage at the MOSFET, so its drain voltage may not be the load supply voltage.
For a series resistive load, the maximum power dissipated in the MOSFET will occur when the voltage at the drain is 1/2 the supply voltage.

 

Be careful ........
Most FETs are designed for "Switching-Duty" ( On or Off ),
and are not well suited to being used as a "Variable-Resistor".

Check the Spec-Sheet for the "Safe-Operating-Area", ( SOA ), Graph of the chosen FETs,
look only at the "DC" Graph-Lines, not the "Pulsed" Graph-Lines,
if there are no DC-SOA-Graph-Lines, then choose a different FET.

Use multiple Op-Amp/FET combinations to divide the Heat over a larger area,
You can have as many parallel Op-Amp/FET combinations as You like.
More is better.
.
.
.

 

For a series resistive load, the maximum power dissipated in the MOSFET will occur when the voltage at the drain is 1/2 the supply voltage.

That's only the case when Rs = 0.

But what holds is that max power dissipation occurs when Vds (not Vd) is half the total supply voltage.

 

For larger electronic loads you should consider using MOSFET devices designed for the purpose such as IXYS' Linear2 series. These have much wider guaranteed safe operating areas (SOA) and a physical construction that eliminates hot spot failure modes more common in devices designed for fast switching when operated in this way.

Heatsinking is critical in this application as the load is continuous.

 

That's only the case when Rs = 0.

Don't follow that.
For Rs equal to zero, Vd would be equal to the supply voltage.

But what holds is that max power dissipation occurs when Vds (not Vd) is half the total supply voltage.

I ignored the Vs voltage since normally there's only a small voltage drop across R1, but your are correct that it should be Vds.

 

Don't follow that.
For Rs equal to zero, Vd would be equal to the supply voltage.

Why would setting the source resistor equal to zero result in the drain voltage being equal to the supply voltage?

 

First of all, I really appreciate all the answers provided by your side.

To be more specific, I want to put this kit that I bought from AliExpress in an enclosure (that I haven't chosen yet), so my question is how big the heat sink that I need to choose and whether I need to add a fan or not. You can see the kit and the scheme of it, as well as the SOA graph of the Mosfet, in the figures.
Any suggestions on how to choose the right heat sinks for further application of this kind of power electronics?

 

1)
What, EXACTLY, do You intend to connect to this Circuit ?
2)
How many Seconds, or Minutes, will the Load be applied ?
3)
Why do You want this Circuit ?, What will be its purpose ?
4)
Do You intend to add a Fuse on the Load-Input ?

There is nothing that will limit the amount of Current, or Heat, generated by this Circuit,
and nothing that will monitor the Heat-Sink-Temperature.
.
.
.

 

To be more specific, I want to put this kit that I bought from AliExpress in an enclosure (that I haven't chosen yet), so my question is how big the heat sink that I need to choose and whether I need to add a fan or not. You can see the kit and the scheme of it, as well as the SOA graph of the Mosfet, in the figures.
Any suggestions on how to choose the right heat sinks for further application of this kind of power electronics?

So, breaking down this circuit, the TL431 gives a 2.5v reference, which is fed to the 4 opamps as a 0 - 1.7v value due to R22/R23. The act as comparators with a little bit of hysteresis due to the 220k feedback resistor, comparing the reference with the voltage across the 0.22 source resistors R1 - R4, giving a constant current sink of 1.7/0.22 = 7.7A per MOSFET or 30A total.

Working from the datasheet , the max junction temperature is 175degC but we'll derate that to 150degC for safety and the thermal characteristics for the TO-220/TO-263 packages is 0.63degC/W thermal resistance junction-case. We generally like case temperature not to exceed 75degC so we calculate max wattage as (150-75)/0.62 = 120W. However, the limiting factor is usually the heatsink. Assuming ambient at 30degC the heatsink would need a thermal resistance of (75 - 30)/120 = 0.375degC/W or about 0.3degC/W allowing for the thermal paste. And that is per MOSFET. Now, 0.3degC/W is doable, but its not immediately obvious how. A quick squint at Mouser or Digikey throws up a huge range of heatsinks but choosing 0.3degC/W gives some very expensive options! The trick here is understanding how heatsink performance is documented and every manufacturer does it slightly differently. As a result the tables given by Mouser and Digikey are not very helpful.

So we'll turn the process round and pick a manufacturer, lets say ATS (catalog attached) and pick a heatsink... lets use ATS-EXL116-300-R0, the key details being:



On the face of it this is nowhere near the 0.3degC/W we need. But look closer... those figures are per 1"(2.54cm) of extrusion, so a 12" length, naturally convected, is actually 5.7/12 = 0.46degC/W and even gently fan assisted at 200lfm (linear feet per minute) its 1.9/12 = 0.16degC/W ! And thats at 10W per inch, or 120W.

Now, here's how I would play this, I'd cut 2 off these in half and fit one MOSFET per section; that gives 0.32degC/W though the amount of metal in the heatsink may not support 120W for long-duration runs. I'd then arrange the 4 sections like this:



Why? Because thats now approx 120mm x 120mm and so we can slap a 120mm fan underneath blowing up. That fan needs to achieve 200lfm per section, or 0.9m3/min or 15litres/sec. A cheap Sunon or RSPro 120mm x 32mm fan will do 2.5m3/min or 600lfm which should get close to 0.2degC/W

 

So, breaking down this circuit, the TL431 gives a 2.5v reference, which is fed to the 4 opamps as a 0 - 1.7v value due to R22/R23. The act as comparators with a little bit of hysteresis due to the 220k feedback resistor, comparing the reference with the voltage across the 0.22 source resistors R1 - R4, giving a constant current sink of 1.7/0.22 = 7.7A per MOSFET or 30A total.

Working from the datasheet , the max junction temperature is 175degC but we'll derate that to 150degC for safety and the thermal characteristics for the TO-220/TO-263 packages is 0.63degC/W thermal resistance junction-case. We generally like case temperature not to exceed 75degC so we calculate max wattage as (150-75)/0.62 = 120W. However, the limiting factor is usually the heatsink. Assuming ambient at 30degC the heatsink would need a thermal resistance of (75 - 30)/120 = 0.375degC/W or about 0.3degC/W allowing for the thermal paste. And that is per MOSFET. Now, 0.3degC/W is doable, but its not immediately obvious how. A quick squint at Mouser or Digikey throws up a huge range of heatsinks but choosing 0.3degC/W gives some very expensive options! The trick here is understanding how heatsink performance is documented and every manufacturer does it slightly differently. As a result the tables given by Mouser and Digikey are not very helpful.

So we'll turn the process round and pick a manufacturer, lets say ATS (catalog attached) and pick a heatsink... lets use ATS-EXL116-300-R0, the key details being:

View attachment 284018

On the face of it this is nowhere near the 0.3degC/W we need. But look closer... those figures are per 1"(2.54cm) of extrusion, so a 12" length, naturally convected, is actually 5.7/12 = 0.46degC/W and even gently fan assisted at 200lfm (linear feet per minute) its 1.9/12 = 0.16degC/W ! And thats at 10W per inch, or 120W.

Now, here's how I would play this, I'd cut 2 off these in half and fit one MOSFET per section; that gives 0.32degC/W though the amount of metal in the heatsink may not support 120W for long-duration runs. I'd then arrange the 4 sections like this:

View attachment 284020

Why? Because thats now approx 120mm x 120mm and so we can slap a 120mm fan underneath blowing up. That fan needs to achieve 200lfm per section, or 0.9m3/min or 15litres/sec. A cheap Sunon or RSPro 120mm x 32mm fan will do 2.5m3/min or 600lfm which should get close to 0.2degC/W

Good Afternoon Mr. Irving,

This was a really good tutorial to my question, this was very useful, thank you so much for your time, I really appreciate it.


Good Afternoon Mr. LowQCab,

My question was about what Mr. Irving explained, Thank you very much for proceeding with me in my question.

Best Regards,
Ahmed.