Comparison of Low-Parts Current Limiters

theboom said:

TL431

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Here's another TL431 current regulator. Unsure how it's different than the circuit above.

theboom said:

Which of these limiters will run cooler, all things being equal?

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Unless it's a switching regulator limit, they all will dissipate the same power (Ilimit * Vin-Vout).

theboom said:

Will they all simply regulate the current, ie continuous limit? Or will they turn off current on overcurrent?

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Unless it's an electronic-fuse type, it will just continuously limit the current.

theboom said:

Which would have fewest parts in a real-world practical circuit?

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I believe you can do a parts count as well as I can.

theboom said:

Are they all temperature-compensated?

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Only those with a compensated reference, such as the LM317, and TL431 circuits.

theboom said:

Which would have the smallest PCB footprint, ie can they be implemented with all-SMD parts?

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You would have to see if the parts are available in SMD packages.

theboom said:

How do costs compare, including external parts?

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You can do that by looking at the supplier cost for the parts.

crutschow said:

I believe you can do a parts count as well as I can.

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But these schematics may be simplified compared to real-world practical circuits. You may be as ignorant of that as i am.

crutschow said:

Unless it's a switching regulator limit, they all will dissipate the same power (Ilimit * Vin-Vout).

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I've been told they won't dissipate the same heat. For example, it may depend on the thermal characteristics of a selected transistor. Do you have hands-on experience with any of these circuits?

Here's a simple current-limit circuit that requires only two transistors and two resistors.
The current limit value does increase about 0.3%/°C due to the change of Q1's base-emitter voltage with temperature.

theboom said:

But these schematics may be simplified compared to real-world practical circuits.

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In general the number of parts seem close to what the actual circuit would have.

theboom said:

I've been told they won't dissipate the same heat. For example, it may depend on the thermal characteristics of a selected transistor.

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Whoever told you that was wrong.
The heat dissipated is solely determined by the formula I showed.
The only thing the thermal characteristics will affect is how that heat will be dissipated to the ambient air.

crutschow said:

Here's a simple current-limit circuit that requires only two transistors and two resistors.

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Great! Why might someone choose this design over any of the designs i posted above?

Will it handle 4A? That FET is marked -13.5A, –20 V. Will your circuit support positive V's?

Will the output V follow the input V?

When it's in limit-mode, is its behavior equivalent to a constant current device?

crutschow said:

The current limit value does increase about 0.3%/°C due to the change of Q1's base-emitter voltage with temperature.

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Is that the same as saying it's not temperature compensated?

How would that affect Vout in cold vs warm ambient temperature? I think "0.3%/°C" means current goes up as temperature goes up. And it looks like, in limit-mode, Vout goes down as current increases. Therefor, as temp goes up, Vout goes down, correct?

Could a TL431 help here?

crutschow said:

The only thing the thermal characteristics will affect is how that heat will be dissipated to the ambient air.

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"How"?

theboom said:

Why might someone choose this design over any of the designs i posted above?

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Minimum number of parts.

theboom said:

Will it handle 4A?

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Yes, if the power can be dissipated by the FET.

theboom said:

That FET is marked -13.5A, –20 V. Will your circuit support positive V's?

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Yes.
The voltage across the P-MOSFET is positive source to drain (which is minus drain to source which is how the MOSFET voltage is specified).

theboom said:

Will the output V follow the input V?

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If it's not in the current-limit mode, yes.
See below:
Note how the output voltage (yellow trace) closely follows the input voltage (red trace) until the current limit is reached.

theboom said:

s that the same as saying it's not temperature compensated?

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Yes.

theboom said:

as temp goes up, Vout goes down, correct?

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No.
As temperature goes up, the current-limit goes up.

theboom said:

Could a TL431 help here?

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The current limit would be more stable, but the sense resistor would be about 4 time larger, giving a higher voltage drop when operating normally with no current limit.

theboom said:

"How"?

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How what?
Do you understand what thermal resistance is and how power is dissipated from a transistor?
Thermal resistance is the temperature rise due to the transistor power dissipation.
The thermal resistance from the transistor die to ambient air must be low enough so the transistor does not overheat (usually about 125°C).
For power above about a watt (depending upon the transistor) an added heat sink is usually required.

Hi,

Some corrections:

crutschow said:

they all will dissipate the same power (Ilimit * Vin-Vout).

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correct formula: Ptot = I_limit * (V_in - V-out)
Mind the position of the brackets.

crutschow said:

If it's not in the current-limit mode, yes.

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even if the MOSFET is in the saturated mode there will be a voltage drop:
V_drop = I * (R_DS_ON + R1)
V_out = V_in - V_drop

crutschow said:

No.
As temperature goes up, the current-limit goes up.

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The bjt determines the thermal behaviour. V_BE has negative temperature coefficient.
Thus V_BE goes down with higher temperature.
This I_limit goes down with higher temperature.

Klaus

Thanks for correcting my comments, Klaus.
My mind was obviously a little fuzzy.

crutschow said:

How what?
Do you understand what thermal resistance is and how power is dissipated from a transistor?
Thermal resistance is the temperature rise due to the transistor power dissipation.
The thermal resistance from the transistor die to ambient air must be low enough so the transistor does not overheat

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Yep, i understand that.

You wrote:

crutschow said:

The only thing the thermal characteristics will affect is how that heat will be dissipated to the ambient air.

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Sorry for being pedantic. It seemed the word "how" might be misleading. "How" implies "manner", as in "the manner in which heat will be dissipated".

Wouldn't it be more accurate to say "how much heat will be dissipated"?

Question: Is it possible that different devices, with identical die-to-ambient thermal resistance, can withstand different degrees of heat?