Need a simple DC output current limiter

Hi

I looking at having to protect an 24VDC output from overload/short circuit. I have a design that works with a sense resistor which trips a comparator to turn off a MOSFET, but I would like to find a method to avoid too many parts and complexity.

There's polyswitch PPTC, but I don't like the temperature dependancy. Are there any other options that will provide a resonable precise trip point?

Point of Load device with this inside Some special Reg/LDO with internal current limit other?

Thanks

Klaus

Reply to
Klaus Kragelund
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How fast does it have to trip? How accurate does the trip point have to be? How easy does it have to be to reset? Why doesn't the thing have a plain ol' circuit breaker?

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Tim Wescott
Wescott Design Services
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Tim Wescott

If you have a high load current you could use an LDO CONTROLLER with external PFET.Set the current limit useing a sense R.

LP2975

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Most LDO with integrated pass device come with some sort of protection thermal or (ocp) current trip.

Or a zetex current sensor with a High side Pfet and use a TL431 as a comparator to flip the PFET open when the sensor reaches the threshold current.Read the app notes from zetex (now DIODES INC) for specific details.

Reply to
Hammy

Perhaps a current sensing mosfet driver can be hacked for your app.

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D from BC myrealaddress(at)comic(dot)com BC, Canada Posted to usenet sci.electronics.design

Reply to
D from BC

Hello Klaus,

As others said also, I don't know your requirements, characteristics of the source and load (like short circuit behavior), so the text below may not be applicable for you.

When a MOSFET with comparator are too many components, there is not much space left for other solutions.

I was involved in the design of current limiters/shut down circuitry for supply busses (hot-swap application). I like the current source approach (that is normally not acting as a current source). By integrating the voltage across the pass element (minus small offset), you can have a good estimate of the behavior of the short and the dissipated energy in the pass element. When the integrated voltage across the pass element reaches a threshold, the current source is shut down and a cool down time is inserted before reset. This scheme allows for charging of capacitors and other inrush phenomena.

For every semiconductor current limiting/short circuit protection circuit, you have to assess the peak currents and duration that occur during a short circuit. A saturated current source has a response time, so for a short time, the current through the pass element can be far above the design current.

Many (modern) power MOSFETs show similar failure mechanism in linear operation as occur in BJT (second breakdown effect). Look to the Vgs - Id curve. Many MOSFETs show negative temperature in the graph up to a certain Id.

Never rely on you electronic solution alone, so have some additional means of protection (normal fuse rated for the voltage and expected short circuit current).

Best regards,

Wim PA3DJS

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you can use the PM, but forget a, b and c.

Reply to
wimabctel

How much current? How much voltage drop can the application tolerate?

It probably can be built around PROFETs but the downside is that those are expensive:

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Another option might be (sufficiently large) motor or stepper driver chips that have an over-current sense function, operated in static fashion. That's like using a hammer do smack a mosquito but if they are low enough in cost, why not?

The brute-force method would be a LM317 with one resistor so it becomes a current source, and then let that run into thermal shutdown. Could be too slow for the supply though.

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Regards, Joerg

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Reply to
Joerg

[snip]

Except it doesn't go into thermal shutdown, it just absorbs the necessary voltage drop when the current limit is exceeded.

It might ultimately go into thermal shutdown... doesn't matter, the main circuit would have current-limited immediately.

Think CURRENT SOURCE ;-)

The only caveat is whether Klaus' circuit can stand the ~1.5V drop during normal operation.

...Jim Thompson

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| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Jim Thompson

Yep, I was just thinking about an el-cheapo mounting method sans heat sink. Then it reaches 180C or whatever rather quickly depending on current. If heatsinked then this problem goes away. But it might have to dissipate >30W and those need to go somewhere.

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Regards, Joerg

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Joerg

You missed my points.

Current Limit occurs quickly.

_Then_ thermal shutdown occurs.

If Klaus has the headroom, it sounds like a viable solution.

...Jim Thompson

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| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

Yes, that would be the scenario. But there are apps where letting a device shoot up to 180C until it thermally shuts down isn't considered kosher. For example, in medical the inspector would frown ;-)

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Regards, Joerg

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Joerg

125°C to 150°C dependent on version, hardly stressful for Silicon.

...Jim Thompson

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| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

In many markets the silicon doesn't matter. What matters is how hot the pastic body and tab get and when that's above of what the regulatory guy said is acceptable -> back to the bench ;-)

The typical scenario in med is that the unit goes into the "sweat room" and dozens or hundreds of thermocouples are fastened inside. The QC folks have a knack for finding the parts that are most likely to fail.

For the LM317 it's often 180C, see page 2:

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Regards, Joerg

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Reply to
Joerg

"Pastic" ?:-) In MEDICAL ?:-)

So buy the National part.

...Jim Thompson

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| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

Isn't this the sort of thing that you're supposed to be learning from the textbook and instructor?

Thanks, Rich

Reply to
Rich Grise

It's the new stuff :-)

Then some day the design engineer is on vacation, the purchaser can't get National, approaches young buck whether ON Semi is ok, young buck sez it's ok, ECO gets signed ...

Plus I can't remember seeing thermal shutdown in the table of the National part other than some vague statements on page 10:

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Regards, Joerg

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Reply to
Joerg

For relatively low current levels, a simple latching "soft fuse" can be used. One power Darlington, two small-signal transistors, four resistors, one capacitor. It's similar to your comparator/MOSFET system, and allows some degree of control over the shutdown time (fast-acting or slower-acting).

It introduces a couple of volts of drop in the Darlington and in the sense resistor (less at low currents, more just before it trips) and however much power dissipation this drop will cause at the current levels you're looking for.

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is the version I cobbled up to prevent my ham-radio screwdriver antenna motor from damage if I accidentally leave the switch in the up/down position (as I did after bumping it with my knee one day :-(). With the specified components, it trips when the current exceeds around 350 mA, with a trip time of under a second at a 500 mA load... quite a bit faster if I actually short-circuit the output.

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Dave Platt                                    AE6EO
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Dave Platt

...a transistor, E-B across the series current sense resistor, the collector used to limit drive to output pass transistor: 2 parts.

Reply to
Robert Baer

3 parts: Series resistor to base. Else -> short circuit -> pass transistor shutdown a smidgen too slow -> base sees a spike of several amps -> tsssk ... *BANG*
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Regards, Joerg

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Joerg
[...]

Don't get me started! I once made a hero of myself by diagnosing a nasty intermittant fault in a $40,000 Diazo duplicating machine that had stumped every field engineer in the company before me. I measured voltage levels, eyeballed the controller board, & discovered that the bug was caused by some moron in manufacturing substituting (IIRC) a

74HC14 for a 74ACT14 in a tray-detect switch input. I replaced it with the correct part ($1), & the problem never recurred.
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    W
  . | ,. w ,   "Some people are alive only because
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Reply to
Bob Larter

I've got to pay Joerg a visit. The sound effects from his lab are down-right fascinating ;-)

...Jim Thompson

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| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

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