-
DL24 Atorch type electronic loads: considerations on how to choose the best mosf
Posted by Filippo52 on 29 Jan, 2023 09:52 -
Mine is meant to be only a contribution from an enthusiast.
If we look in the forums and on Youtube there are many cases of Mosfets that sometimes explode in a pyrotechnic and dangerous way.
We neglect the case of FAKE components and limit the discussion to the original ones.
In products such as the DL24, in its various versions; powerful but cheap Mosfets are used: currently comes with IRPF264.
Enthusiasts who go to replace the mosfet often rely on the most visible parameters such as Amps and Watts supported. The voltage is related to the type of source to be tested.
If we want to give an example, the IRFP90N20D 200v 94A and 580W type il Mosfet is certainly considered better than the IRFP264 currently mounted in the DL24pcb and which has the following data 250v 38A and 280W.
In reality both of these Mosfets are intended for common use for MOSFETs, i.e. as switches and therefore in dynamic regime and not to act as a static resistor in an electronic load and this, wanting to simplify a lot, means that currents and dissipations are possible only for very short instants in which the MOSFET works in switching therefore in conditions where in most of the time the Mosfet is either closed or open and therefore dissipates very little.
In an electronic load this thing is not respected and the MOSFET is asked to behave like a resistor, returning to a state of partial conduction and therefore of dissipation for even very long times.
To evaluate these things in the Mosfet Datasheets, the manufacturer puts a graph called briefly "SOA" or in full "Forward bias Safe Operating Area" or even "Maximum Safe Operating Area" where there are curves remaining "under" which the manufacturer guarantees "survival" of the Mosfet under those indicated conditions.
Below is an image where there are both MOsfets we just talked about
The first IMPORTANT, indeed very important, thing to note is that the curve with the longest time is 10 msec, ie the manufacturer does not supply any data for continuous operation in "DC"; this makes it clear that we will not be able to have guarantees, quite the opposite.
I have traced some curves in red imagining a deterioration which generally exists with the increase of time, but it is only my hypothesis: no guarantee from the manufacturer.
We can make comparisons with the curve at 10msec but perhaps it is better to use my curve to get values which, even if not guaranteed, are more compatible with continuous use.
let's take a trivial voltage of 20 v the IRFP90n20D at the crossing on the red line will give about 8 A while the "less powerful" IRFP264 will give 20A; the same difference can be found using the manufacturer's official curves at 10 msec.
This means that the IRFP264 is more suitable for use, always improper let us remember, what we want to do with it even if it has lower general data.
But it's not exactly always true; for example at 4 volts (just the voltage of a lithium element) we see that the IRFP90n20D will carry 70A while the IRFP264 only 50 and if we use the official curves at 10 msec at 7v the IRFP90n20D will carry 200A while the IRFP264 will it will stop just above 100 A.
In short, the comparison is not as successful as looking at the main current and wattage specifications where the IRFP90n20D clearly doubles the IRFP264.
In practice, who can say with which voltage and current pairs it will be used and since the "DC" curve is invented, the result will be ...any. But we have found that the general data "alone" is not enough to find the best MOsfet and it is necessary to look at the SOA graph.
Let's take a step forward, armed with these considerations, let's look for Mosfets that have large currents, large wattages but also a nice "DC" curve in the SOA graph and we exaggerate to be clearer.
Here is a good example IXFB100N50P, a real monster with 500v 100A and 1890W of dissipation, with a large plus264 container and with an impressive weight of 10.28gr (IRFP90n20D or IRFP264 weigh just under 6g and incidentally their fake 4,4gr)
This "monster" as seen from the SOA graph also has the DC curve
and at 20 v we read 100A, in short we arrive almost at 2000W and we will be limited to 1890W by the dissipation.
let's take into account that the initial mosfets are still available and cost from 5 to 7 €, I'm always talking about the original ones; this monster is instead more difficult to find in this period (so beware of fakes) and costs between €25 and €30.
So is everything okay? do we have the solution? just pay?
Yes and no
There's still one thing that can escape and instead it's VERY IMPORTANT.
If you look at the SOA graph of the IXFB100N50P, if it is true that there is a DC curve BUT it is also true that the whole SOA graph refers to these two temperature conditions
Tj of 150°C and Tc of 25°C.
If you think about it for a moment in an electronic load and in a situation of continuous dissipation how do you keep the case at 25 °C?
It is practically impossible except by liquid cooling, and not like water but strong things like nitrogen and the like.
This happens because even the IXFB100N50P monster has not been designed to work in a linear way and therefore the manufacturer is not able to give guarantees if you put it in an electronic load with the case at 50°C for example, a rather normal situation instead.
Here is the last point.
There is a category of MOSFET called linear, born to do the work like of the electronic load.
Let's take an example here too
the IXTK110N20L2 less powerful than the previous one with "only" 200V 110A and 960W and weighing only 6.8 gr but which costs at least another 10€ more and is even less easy to find in this period.
here is its curve
Before you tell me the monster was better, look at the test conditions: Tj equals 150°C but Tc is 75°C. Here's the BIG difference is this; the "survival" of this Mosfet is guaranteed by the manufacturer in the real conditions in which an electronic load works and therefore this Mosfet if cooled well but with feasible things will be able to give a real 600W electronic load coupled with a heatsink of those for very high-end PCs powerful. We must remeber that il DL24 Atorch mosfet is not the only limit for 600W, many other things must be enforced like diode schottky , shunt resistors and also some parts of PCB
I don't know how clear and useful it was but I'll stop here, any insights on this question.
I apologize to those who already knew these things.
-
Thank you for explaining this.
1) It is critically important to select MOSFETs which are intended for “linear operation” where both current and voltage are >0 simultaneously. Vendors like Digikey have a “linear” parameter which can be selected when searching their inventory.
2) The “safe operating area” (SOA) graph is an essential guide for selecting the type and quantity of MOSFET devices required to handle the maximum expected load current/voltage.
I’m beginning to think of alternative electronic load circuit designs which dissipate most of the power in a bank of resistors. This could take the form of a “high power” DAC with binary weighted load resistors. Each load resistor will have a MOSFET in series to select it. But these MOSFETs will always be entirely on or entirely off. Therefore standard low-cost switching MOSFETs can be used here.
There could also be one linear MOSFET used as a “vernier” for the LSB. This MOSFET won’t have a series resistor.
If the binary-weighted load resistor bank is properly scaled then this LSB linear MOSFET will dissipate only 10% (or less) of the total power. -
I welcome you to share my thoughts on SOA and linear MOSFETs.
Your idea of a resistor bank is very suggestive; but I fear that in the end it would be much more expensive than even the most expensive linear mosfet that solves all the problems with only one component -
Thank you for this informative article.
My DL24 electronic load has just seen one of its mosfets drop today in full use!
I was discharging a 48V/20Ah lithium-ion battery with a modest power of 300W (that's a 600W load made up of four mosfets/heatsinks). The mosfet of the main board is shorted after 2 hours of operation, the voltage was then 46V... The measured temperature of the heatsink was 58°C.
The mofets present were IRFP260M.
I'm thinking of replacing the mosfets with IRFP264s
The boxing:
-
Dear Silicon81
Congratulations for the particular assembly of the 4 DL24 modules.
From what I see they are last year's models, now each module has 4 Mosfets and in theory they are more resistant, I mean in theory because they put 4 tiny heatsinks for each Mosfet.
The differences between the IRFP260M that you have on your modules and the IRFP264 that you would like to use are not relevant, there is an improvement but it is not very big and if we don't try to understand what happened you may not solve it.
Let's try together if you like.
I understood that you were discharging a 48V battery with a load of 300W (half of what the 4 DL24 modules should bear), if I'm not mistaken the current should have been 300W/48V= 6.25A.
These divided among the 4 modules are 1.56 A for each module and therefore for each mosfet.
The mosfets should have held.
I ask you
1 - you checked the temperature but I think your model has a sensor only on the main module; so the 58°C is only on the main module? Can you confirm?
2 - you know that the sensor is only close to the Mosfet and does not detect the true temperature of the component.
3 - have you had the opportunity to check if the current is divided equally between the four modules?
4 - have you checked the contact between the mosfet and the dissipator, the chinese guys sometimes make mistakes
Let's start discussing these things, let's try to understand what could have happened and then if you want to solve it by changing the Mosfet, do you have where to buy guaranteed original ones?
Since you're intervening, perhaps it's time to think of something a little better like a Mosfet so you don't think about it anymore.
But first it's important to understand today if your mosfets were all carrying the same load and if they were all cooled properly.
a greeting -
Another thing Dear Silicium 81
unfortunately the Chinese often put Mosfets in their devices that are not original but fake.
This would explain why the mosfet burned out
I can help you verify also for the other module; although you will have to unsolder the mosfets to test it
In the meantime, do one thing if you don't have a burnt Mosfet yet.
Weigh it with a precision scale and come here and tell us how much it weighs
hi -
Thank you for your interest in my breakdown!
To answer, it's the main mosfet that burned out (the one whose temperature is displayed), I had put thermal grease on this mosfet and the intensity for the four mosfets was around 7A at the time of the failure . I don't have the desoldered mosfet with me, I will weigh it as soon as possible (I have a precision scale). I'm curious if it was an original model or a copy... -
Don't worry, I enjoy discussing these things and I'm happy to collaborate with you.
Know that if it weighs less than 5g it is most likely a fake.
There is another even more important test but it must be done on the remaining mosfets, because it is probable that they are either all original or all fake.
The other test is the RDS-ON measurement; generally the FAKEs have much higher values than those specified, obviously you have to unsolder them, but if you want to replace them all you will have to do it.
So in the meantime you prepare yourself to know how to test the mosfets you will buy.
to test the RDS-ON you need a power supply of at least 12 volts and at least 2 A, better if you have two power supplies of which the second must be much less powerful, just 10 v 100ma.
I'll send you the schematic if you're interested. So you will become a MOSFET expert like I have for the past few months
I bought IRFP260N and IRFP90N20D from Aliexpress and they were cheap but they were all FAKE: they weighed 4.4 gr and the RDS-ON was 3-4 times the specified value.
Then I got them from Reichelt.com and from Ebay from a German seller and they were Original: 5,6 Gr of weight and RDS-ON within the specifications.
See you soon bye -
I weighed the slammed mosfet: 6.03g. I desoldered another one which weighs 5.45g and does not have exactly the same case...
The shorted one is on the left, it stinks of fake !
I ordered 4 IRFP264PBF from mouser to replace them all.
-
Are you interested in trying RDS-on on the mosfet you unsoldered and which I believe is working?
It is a more probative test to understand if it is a fake -
Here is the result of the RDS-on measurement on mosfet ok: 85.4 milli-ohm!
The expected value being 40 milli-ohm, we are indeed in the presence of a fake IRLP260M...
The test circuit used: (I made the simulation display the actual measured values)
-
I welcome you to share my thoughts on SOA and linear MOSFETs. Your idea of a resistor bank is very suggestive; but I fear that in the end it would be much more expensive than even the most expensive linear mosfet that solves all the problems with only one component
I agree with you. A less costly solution is to provide a modest selection of fixed high-power resistors to place in series with the MOSFET load tester.
Example:
Testing 48V power supply with 10A load current.
480W total dissipation.
Place 4 ohm high-power resistor bank in series with MOSFET load tester.
Connect + voltage sensing input of MOSFET load tester to high end of 4 ohm series resistor
This permits load tester to measure total voltage drop for calculating Watts
40V across resistor = 400W
8V across MOSFET load tester = 80W
8V across MOSFET should be well within SOA
-
okay
but in this way you will have to prepare a type of resistor for each measurement situation, you will no longer have a flexible instrument that is valid in any load situation -
OK Silicium
well done
you are already a Mosfet expert.
Now your hypothesis of replacing all the Mosfets with the same number of originals makes sense.
Just try to find a reliable supplier.
Hello and good job -
Hopefully they will be ok:
-
Mouser is very good seller; i'm sure that you will get original mosfets with good quality.
The result will be good and I believe the 300w load will be supported by the 4 modules.
However, remember that you are using mosfets not intended for this use and therefore without any guarantee from the manufacturer.
I am doing like you.
In the future, if linear mosfets return to having affordable prices, I will be able to think of replacing the current IRFP264 with a linear type.
it was a pleasure talking to you -
okay
For many years I’ve been repairing and sometimes modifying large audio power amplifiers. The same issues with output device power dissipation and “safe operating area” apply to both audio amplifiers and electronic loads. The output stages of high-power audio amplifiers often require a large number of parallel devices, for example a high-quality 200W audio amplifier may have 4 to 6 paralleled output devices rated at 16-20A each, but derated to only carrying 2A each in order to accommodate that these devices might have 75V from emitter-collector (or source-drain if MOSFETs).
but in this way you will have to prepare a type of resistor for each measurement situation, you will no longer have a flexible instrument that is valid in any load situation
My thinking is that the 4 ohm series resistor will greatly reduce the problem of excess voltage across the MOSFETs in the electronic load. The goal is to keep the MOSFET operating condition well within the SOA limits. Adding one resistor to the circuit will not make it excessively complicated. To continue the comparison with audio amplifiers, a 4 ohm load resistor is the most commonly used load for testing audio amplifiers. -
Dear Electonia;
she forgives but maybe we didn't understand each other well.
It is clear to me how a 4 ohm resistor can simplify "life" for the mosfet that connected it in series.
But, as I wrote to you, you will no longer have a universal and versatile tool.
Let's just take the example of the 4 ohm resistor. If you wanted to discharge a 4 volt lithium battery with a 1A current you would already be in crisis and would have to replace the resistor.
That is, as I told you, each pack of resistors you emit will be usable in a certain range of voltages and currents and you will need to be able to insert many, different from each other, to adapt your instrument to many situations.
I hope I was able to explain myself better -
I received the new mosfets from mouser. They are now in place and everything works normally. Without surprise, the finish of the cases has nothing to do with the desoldered ones... This confirms the more than dubious origin of the ones that were in the original place!
-
Dear Electonia;
Thank you for this helpful discussion.
she forgives but maybe we didn't understand each other well. It is clear to me how a 4 ohm resistor can simplify "life" for the mosfet that connected it in series. But, as I wrote to you, you will no longer have a universal and versatile tool. Let's just take the example of the 4 ohm resistor. If you wanted to discharge a 4 volt lithium battery with a 1A current you would already be in crisis and would have to replace the resistor. That is, as I told you, each pack of resistors you emit will be usable in a certain range of voltages and currents and you will need to be able to insert many, different from each other, to adapt your instrument to many situations. I hope I was able to explain myself better
I should have mentioned that the 4 ohm series resistor is an option which only needs to be used when the voltage of the power source being tested is higher than what the MOSFETs can easily handle.
I agree that we don’t need the 4 ohm resistor when the source voltage is <20-30V. My recommendation to use the series resistor applies only to situations where the voltage is high enough to overstress the MOSFETs, for example when I described load-testing a 48V power source.
I frequently use an older load tester with only a single MOSFET to measure the Ah of power tool batteries (12V-18V). In this case I do not add any series resistance. But on a few occasions where I was testing a power source with 48v-60V, then I added the 4 ohm series resistor.
So, rather than making the 4 ohm series resistor a permanent component of the load tester, my idea is it can simply be wired into the test circuit temporarily when needed.
-
I should have mentioned that the 4 ohm series resistor is an option which only needs to be used when the voltage of the power source being tested is higher than what the MOSFETs can easily handle.
I agree that we don’t need the 4 ohm resistor when the source voltage is <20-30V. My recommendation to use the series resistor applies only to situations where the voltage is high enough to overstress the MOSFETs, for example when I described load-testing a 48V power source.
I frequently use an older load tester with only a single MOSFET to measure the Ah of power tool batteries (12V-18V). In this case I do not add any series resistance. But on a few occasions where I was testing a power source with 48v-60V, then I added the 4 ohm series resistor.
So, rather than making the 4 ohm series resistor a permanent component of the load tester, my idea is it can simply be wired into the test circuit temporarily when needed.
It's a good idea to dissipate some of the power in an external resistor. It can be convenient to use car headlight bulbs. These are generally 50W bulbs under 12V, i.e. a resistance of approximately 3 ohms crossed by an intensity of 4 Amps.
By using a headlight bulb, we know that the voltage drop will be 12V under 4A, easily allowing resistive arrangements to be made
It is therefore sufficient to place in series with the battery to be discharged a bulb for 4A, two bulbs for 8A.
The diagram is this (for 8A):
-
It's a good idea to dissipate some of the power in an external resistor. It can be convenient to use car headlight bulbs. These are generally 50W bulbs under 12V, i.e. a resistance of approximately 3 ohms crossed by an intensity of 4 Amps.
+100 for suggesting 12V incandescent light bulbs of appropriate wattage. Multiple 12V light bulbs can be connected in series or in parallel if needed. All incandescent light bulbs have a built-in current limiting feature: Their resistance when the filament is hot is much larger than the resistance when it is cold (dark). This causes them to work like a current limiting device.
For many years I've used a ""dim-bulb tester" to safely apply AC mains power to items I am repairing. This works just as well with low-voltage bulbs as it does with mains voltage bulbs. Please note that the light bulbs must always be traditional "incandescent" bulbs. Not CFL or LED. -
Elettronia
Ok now everything is clear;
I agree perfectly with you that this is for the higher voltages that put the mosfet in more difficulty.
I had said that you need various resistances if you make different uses that you have to insert or remove from the connection each time.
It is an effective solution if you have a few different situations to deal with
Otherwise it becomes a bit uncomfortable and if you have to have many high wattage resistors even expensive.
That's all, however yours is a simple idea to implement even now that linear power mosfets are not on sale
a greeting -
12 volt incandescent bulb for automotive it is exactly the method I used before I had an electronic load.
It is very cheap.
Electronic load is the "automatic solution" for all for the entire operating range of the electronic load.
Unfortunately DL24 is a Chinese product. It's cheap but has many flaws, including the seller's unacceptable arrogance.
let's say that in the end we have a good "semi-automatic" product that tries to combine the advantages of the two solutions: cheap and with a wide range of action
-
Hopefully they will be ok:
The operating point you draw means 450W which is much more than the max. allowed dissipation power Pd of 280W! At 100V, I assume you get max. 2A (rather 1A) from a IRFP264.