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| DL24 Atorch type electronic loads: considerations on how to choose the best mosf |
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| Filippo52:
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 |
| elecdonia:
--- Quote from: Filippo52 on January 31, 2023, 09:12:49 pm ---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 --- End quote --- 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). 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. |
| Filippo52:
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 |
| Silicium81:
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! |
| elecdonia:
--- Quote from: Filippo52 on February 02, 2023, 07:03:42 pm ---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 --- End quote --- Thank you for this helpful discussion. 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. |
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