EEVblog #102 - DIY dummy load - why does the current cap out at 1.3A?

[waspinator]

I'm trying to understand what limits Dave's design to 1.3A. The MTP3055 MOSFET has a drain current of 12A. Is that not the specification that's important for how much current can go through it? Is there another transistor or opamp that can be used to increase the maximum current draw, or is this more complicated than that?

Thanks

edit: which graph in the datasheet corresponds to what Dave shows?

[David Hess]

The transistor and the sense resistor have power dissipation limits as well.  At high voltages, the transistor is likely to have either secondary breakdown limits or something similar.

[mij59]

Take a look at page 6 figure 12 of the data sheet.

Assuming static operation, the current and voltage must be below the line  DC.

[waspinator]

Take a look at page 6 figure 12 of the data sheet.

Assuming static operation, the current and voltage must be below the line  DC.

so it looks like if you connect a 10V power supply you should be able to pull 4A. or am I reading the graph wrong?

If the opamp could provide a gate-to-source voltage of 6.5V would I be able to pull 12A?

Thanks again

[mij59]

Take a look at page 6 figure 12 of the data sheet.

Assuming static operation, the current and voltage must be below the line  DC.

so it looks like if you connect a 10V power supply you should be able to pull 4A. or am I reading the graph wrong?

yes, but only with a case temperature of 25 C.

If the opamp could provide a gate-to-source voltage of 6.5V would I be able to pull 12A?

yes, but for static operation you'r outside the safe operating area 

[waspinator]

oh okay, thanks. so do you know a transistor that could handle more current off the top of your head? (that could be used in the original circuit)

the FQP30N06L seems like a good candidate, but I'm not sure.

[mariush]

I'm using fqp50n06l in my own load, but i've used the slightly modified circuit shown here : http://www.sleepyrobot.com/?p=136

And it's not a transistor, it's an n-channel mosfet

[waspinator]

I'm using fqp50n06l in my own load, but i've used the slightly modified circuit shown here : http://www.sleepyrobot.com/?p=136

And it's not a transistor, it's an n-channel mosfet

so how was your performance? could you get 3 or 4 amps at 12V?

um, a mosfeT isn't a transistor? what's the T stand for?

[mariush]

Well, yes.

But it's like saying "a vehicle" instead of "a bus"... when most people think of regular cars when they hear the word "vehicle".

I think most people would think of npn or pnp transistors when you say "transistor".

[Leestons]

Pretty much. When I think transistor the last thing I think of is a MOSFET.

[mij59]

oh okay, thanks. so do you know a transistor that could handle more current off the top of your head? (that could be used in the original circuit)

the FQP30N06L seems like a good candidate, but I'm not sure.

Dissipation of 80W is not possible, the temperature of the junction gets too high, the thermal resistance is too high.
A TO247 package transistor would be better, e.g. IRFP460.

[waspinator]

oh okay, thanks. so do you know a transistor that could handle more current off the top of your head? (that could be used in the original circuit)

the FQP30N06L seems like a good candidate, but I'm not sure.

Dissipation of 80W is not possible, the temperature of the junction gets too high, the thermal resistance is too high.
A TO247 package transistor would be better, e.g. IRFP460.

thanks, I'll try that.