High current LM317....with FET?

High current LM317....with N channel FET?

Is it possible to implement a high current LM317 circuit, similar to the transistor based one in the datasheet, but using a N channel FET.
I have a bunch of really high current capacity FETs.

Possibly but are your FETs really rated for high current in linear mode? Most are rated for high current when used in switching applications where Id is high but Vds is very low so the overall power dissipation is also low. The rules of W=V*I apply to FETs as well as BJT when used in linear operation.

Brian.

betwixt said:

Possibly but are your FETs really rated for high current in linear mode? Most are rated for high current when used in switching applications where Id is high but Vds is very low so the overall power dissipation is also low. The rules of W=V*I apply to FETs as well as BJT when used in linear operation.

Brian.

Click to expand...

Salvaged from flat screen tvs, e.g. IXTH75N15 150V-75A

But also IRLZ44N 5V LOGIC 55V-47A

Re: High current LM317....with N channel FET?

I see no advantage in using them as a current amplifier. The main reason devices like those are used is they have a very low Rds, in other words they work well as on/off switches but when used in linear mode the Rds will effectively be the same as the collector-emitter resistance of a BJT and therefore dissipate exactly the same amount of heat.

Their big disadvantage is poorly controlled gate threshold voltage which means you may need far more than the 0.6V of a BJT to start them conducting. In designed operation, they would be run with zero gate-source voltage to turn them off and a high gate-source voltage to make them fully conductive. Holding it somewhere between would be harder than with a BJT and have no performance advantage.

Also look closely at the de-rating graphs you will see, for example, that the IXTH75N15 is rated at 75A in pulsed operation but only rated for ~10A maximum continuous current

Brian.

Re: High current LM317....with N channel FET?

betwixt said:

I see no advantage in using them as a current amplifier. The main reason devices like those are used is they have a very low Rds, in other words they work well as on/off switches but when used in linear mode the Rds will effectively be the same as the collector-emitter resistance of a BJT and therefore dissipate exactly the same amount of heat.

Their big disadvantage is poorly controlled gate threshold voltage which means you may need far more than the 0.6V of a BJT to start them conducting. In designed operation, they would be run with zero gate-source voltage to turn them off and a high gate-source voltage to make them fully conductive. Holding it somewhere between would be harder than with a BJT and have no performance advantage.

Also look closely at the de-rating graphs you will see, for example, that the IXTH75N15 is rated at 75A in pulsed operation but only rated for ~10A maximum continuous current

Brian.

Click to expand...

Bummer!


What is you opinion of this high current linear voltage regulator, except with an LM317lz or LM317 in place of the fixed output regulator? I have both these transistors on hand.
https://www.electronicspoint.com/th...-current-output-regulator-not-working.287591/

Also would you be able to give me some feedback on this circuit: https://www.electronicspoint.com/threads/20-30v-input-12v-high-current-output-regulator-not-working.287591/
This post is related to it.






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Brian I just found these on ebay: **broken link removed**

Vastly more convenient than trying to design a high current LM317.

One or two units would supply ample current for my purposes.

Up until now I have only ever found fixed voltage versions of these.

All should work but both transistors should be PNP so it's basically a darlington pair.

The concept of the 'wrap around' transistor is that they drop the bias voltage for the pass transistor across the resistor in series with the regulator input pin. At first, it seems counter-intuitive to monitor the input of the regulator rather than the fixed output voltage but it does work, and it works well. The values have to be calculated so the input resistor still allows some current to flow through the regulator, I usually design for about 75% of its rated current to flow through the regulator with Vbe (say 0.7V) across the resistor. That means that up to that 75% figure, the resistor drops less than 0.7V and the transistor doesn't conduct. Now, if the load current increases, the transistor starts to conduct and everything above that 75% figure passes through the transistor instead. The voltage is still fully regulated because the input current to the regulator depends upon it's output voltage.

The big advantage, and reason for running the regulator at relatively high current, is that if it goes into thermal shutdown, it stops drawing current and the pass transistor shuts off too. A good thermal bond between them also helps.

The downside of that configuration is how it reacts to a sudden short circuit, before the regulator has time to shut down, there is a potential for a high current to flow from the reservoir capacitor through the small transistor and straight through the main transistor base junction.

The schematics would work better if the small transistor was removed completely and a darlington PNP power transistor used instead. If you try that, remember the voltage dropped across the input resistor has to bias TWO PN junctions so calculate for 1.4V drop at 75% regulator load.

Brian.