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Electronic DC Load - Design and Build to test PSU Project

peteroakes

Back tot he Main Modular Bench power System Project The Modular Bench Power Supply ++, The Essential DIY Build for Every EE Student and Old Timer alike...

 

I keep promising myself I will build an Electronic DC Load and have kept defering it, well no longer, I finally ot it designed and tested on a breadboard and will be following up with a completed unit in a project case

 

It can go way above 30V (60 to be precice and upto 5Amps), the MOSFET I used the IRFP064 is able to go to 70Amps but thats more than my wiring would stand and way more than i need so I designed the load to go upto 5Amps for now. All I need to do though is change the current sense resistor and it can easily become a 10Amp unit

 

For the heatsink I tested with a standard one but subsiquently found an old CPU heatsing with a Fan attached which looks like it will do a grand job of cooling the MOSFET etc so it will be the one I use in the build.

 

Testin showed I can load as little as 250mV and still control upto an Amp which is great and once im over 1V source voltage I can go all the way to 5Amps with ease.

 

Now you all know the math "Power = Volts * Amps) so therefor will know that at 30V an 5Amps theres 150W being dissipated in the MOSFET and thats a lot of heat. I am pretty sure the CPU heatsing even with the fan will still get pretty hot and I look forward to characturizing it once the bukd is done. till then I have no idea how well it will work but I am sure it will be better than the one I tested the circuit with

 

for now, here is the schamitic I came up with

image

 

It will use between 12 and 24V for the DC in, has a REF02 Voltage reference to provide the manual setting via a 10 Turn pot and has a trimmer to accuratly set the top output to precisly 5A if so desired.

I also included a BNC input as an alternate control allowing a signal generator to control the load enabling many more test scenarious to be executed like transient response testing, noise response, Waveform based profile, pulse response etc.

 

The design does have a frequency limit before the output falls off of only a few hundred Hertz but that should be more than enough for this. It may be way better once it is off the bread board and the wiring is significanly shorter too but I wont be able to test that till the buld is complete. I will add a video to this blog once that is done and tested

 

anyway, enough banter, here is the video (Yes Im back to my 1+ hour videos... sorry I tried)

 

Hope you enjoy

 

Thanks to Vishay Precision Group ( VPG) Bulk Metal Film division for their kind donation of the current sense resistors and precision metal film resistors used in the resistor divider part of the design. It takes the use of this project to the next level by bringing stability to the key parts of the design in the order of a few parts per million... oh yaaaa.

Parents

  • Dear Peter

     

    How do one go about selecting the MOSFET.
    I would have thought 5A 30V would be ample. The 70 Amps mosfet (IRFP064) has a very steep Id Vgs curve at low amps. Is this relevent?

    I would have thought a requirement would be a 'Linear MOSFET'. What does it mean?

     

    Would any of these be usable

    1. IRL540N

    2. IRF730B

    3. CEP83A3

    4. HRF3205

    5. S20N60C3S

     

    How would I parallel more that one to spread the heat better? Would you need some sort of balancing resister?

     

    Why did you use a 50K for the current adjustment pod and not a 10K?

     

    As you can see. I'm quite the novice!

     

    Kind regards

    Stephan

  • in reply to stephanc

    I first take my design parameters and look for matching mosfet.

    1. Max Volts I need to handle + some wiggle room

    2. Max current I want to handle + wiggle room

    3. Total continuous power.

     

    This is where the fun starts. A large number of MOSFETS are characterized for switching (On or Off) and therefor their Power Rating reflects this and can even show things like 1KW, but when you dig deeper you find this is only good for a 10uS pulse so for a DC  load, almost totally useless.

    Just because a MOSFET is shown as a switching MOSFET does not mean it won't work for a linear application. the ones I listed can handle very high power in pulse mode but this drops rapidly in DC conditions. the latest two I mentioned

    the IXFN140N30P is a pulse mode MOSFET but also shows a good DC characteristics in the data sheet (300V, 110A, 0.024Ohms, Vgs(th) 3V)

    image

    the other one is actually DC characterized (Linear by design) but also shows pulse ratings that allow it to go to the 700W published power. (500V, 46A, 0.16Ohms, Vgs(th)3V)

    image

    I have not looked at the ones you listed yet but you can see, the DC figures are way under the pulse ones, but with some cleaver software one can allow more current at lower voltages and less at hi voltages to control the total power, also if the DC load is programed to support pulse loads then you can go way way higher (Assuming your wiring can handle it).

     

    So once I narrow down the available selections, I look at how to drive them, both the above support a Vgs(th) or 3V typical and max of only 5V so relatively easy to drive (Ignoring the capacitance of the gate image.

     

    I am no MOSFET expert but this is my initial sort criteria, also are the parts available for free from one of my sponsors comes into play of course.

    Another thing to consider is if your planning on paralleling up the mosfets to increase reliability and power capability, often you will find the Switching kind do not parallel well below a certain Vgs(th), their resistance goes down as they warm up, this can cause run away effects and the like, others have a much lower Vgs and will operate with the opposite effect. pretty much all MOSFETS of this type will have a transition between negative and positive temperature coefficient and its picking the one to suit your needs or designing arround the issue,

     

    This is a great resource I use, pretty sure this guy is an expert and he explains a lot of stuff way better than I can currently

    Kerry D. Wong » Blog Archive » Linear MOSFET and Its Use in Electronic Load

     

    Regards

    peter

Comment
  • in reply to stephanc

    I first take my design parameters and look for matching mosfet.

    1. Max Volts I need to handle + some wiggle room

    2. Max current I want to handle + wiggle room

    3. Total continuous power.

     

    This is where the fun starts. A large number of MOSFETS are characterized for switching (On or Off) and therefor their Power Rating reflects this and can even show things like 1KW, but when you dig deeper you find this is only good for a 10uS pulse so for a DC  load, almost totally useless.

    Just because a MOSFET is shown as a switching MOSFET does not mean it won't work for a linear application. the ones I listed can handle very high power in pulse mode but this drops rapidly in DC conditions. the latest two I mentioned

    the IXFN140N30P is a pulse mode MOSFET but also shows a good DC characteristics in the data sheet (300V, 110A, 0.024Ohms, Vgs(th) 3V)

    image

    the other one is actually DC characterized (Linear by design) but also shows pulse ratings that allow it to go to the 700W published power. (500V, 46A, 0.16Ohms, Vgs(th)3V)

    image

    I have not looked at the ones you listed yet but you can see, the DC figures are way under the pulse ones, but with some cleaver software one can allow more current at lower voltages and less at hi voltages to control the total power, also if the DC load is programed to support pulse loads then you can go way way higher (Assuming your wiring can handle it).

     

    So once I narrow down the available selections, I look at how to drive them, both the above support a Vgs(th) or 3V typical and max of only 5V so relatively easy to drive (Ignoring the capacitance of the gate image.

     

    I am no MOSFET expert but this is my initial sort criteria, also are the parts available for free from one of my sponsors comes into play of course.

    Another thing to consider is if your planning on paralleling up the mosfets to increase reliability and power capability, often you will find the Switching kind do not parallel well below a certain Vgs(th), their resistance goes down as they warm up, this can cause run away effects and the like, others have a much lower Vgs and will operate with the opposite effect. pretty much all MOSFETS of this type will have a transition between negative and positive temperature coefficient and its picking the one to suit your needs or designing arround the issue,

     

    This is a great resource I use, pretty sure this guy is an expert and he explains a lot of stuff way better than I can currently

    Kerry D. Wong » Blog Archive » Linear MOSFET and Its Use in Electronic Load

     

    Regards

    peter

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  • in reply to peteroakes

    Dear Peter

     

     

     

    I greatly appreciate your reply, help and advice.

     

     

     

    I’m testing and experimenting with your DC load circuit.

     

     

     

    Kind regards

     

     

     

     

     

    Stephan Cilliers

     

    Professional Engineer

     

     

     

     

     

     

     

     

     

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