Linear current regulator is OK to supply 3.2A , pulsating current?

[grizedale]

Hello,

We are in Spain and are having big trouble (and angry customers) with a 6.8W flashing LED light which is a warning light…………

The linear regulator (LM350) keeps blowing up (its case is blown wide open).

Here is the schematic.
http://i48.tinypic.com/2cfxwqq.jpg

(the schematic uses LT1085 regulator because LTspice doesn’t have a model for LM350)
-also, as you can see, we’ve already put a 3R9, 25W resistor in series with the LM350 to allow the LM350 to run cooler, but LM350 is still blowing up.
WH25 resistor series:
http://www.welwyn-tt.com/pdf/datasheet/wh.pdf


Here is the current in the 0R39 resistor (also same as input current)
http://i49.tinypic.com/35bt3c5.jpg

Here is the voltage on the 100uF output cap (the voltage that drives the LEDs)
http://i46.tinypic.com/5yg4ly.jpg

Here is the voltage at the “IN” terminal of the linear regulator
http://i46.tinypic.com/orj69s.jpg

Also:-
Vin = 28V
Power dissipated in LM350 is 6.2W.
Power dissipated in the 25W input resistor is 11W

The LEDs are Osram platinum dragon (LY W5SN)
LED datasheet:
http://www.thorlabs.com/Thorcat/21700/21732-M01.pdf


The LM350 is attached to a heatsink with an “LR20” insulating (also thermally conductive) spacer made by Denka:

Datasheet of thermally conducting material:
http://www.reipower.it/pdf/REI-Denka.pdf

LM350 Datasheet
http://www.ti.com/lit/ds/symlink/lm350-n.pdf

---------------------------
The LED flash pattern is

ON for 44ms
OFF for 10ms
ON for 44ms
OFF for 10ms
ON for 44ms
OFF for 298ms

…..then repeat endlessly
----------------------------

We originally get a contract company to design this for us but now they’ve disbanded.

The only thermal test data which we can find on this product is a thermal camera image of the PCB, which indicates that the surface of the LM350 was 123 degC.
-Unfortunately it doesn’t show what the ambient temperature was when this image was taken, or whether or not the PCB was enclosed in its enclosure (I suspect it wasn’t in its enclosure, as they wouldnt have been able to take the thermal image if it was)

This product is placed outside in the sunshine, and we wonder if the product’s amber diffusor is acting as a heat-trap, making the internal ambient far hotter than the external ambient temperature?


First of all, the the thing that concerns us is the flashing nature of this product.
…..So the LM350 linear regulator is conducting 3.2A pulses of current with the above flash pattern.

…..this means the “IN” terminal voltage of the LM350 is wildly going up and down as shown above.

….i am wondering if this is a harbinger of instability problems in this linear regulator and maybe linear regulators are just not meant for such pulsating current flow?

There are input and output caps connected up to the LM350 but they are not that physically near to it……..they are each about one-and-a-half inches of PCB track away from the LM350.

The heatsink on which the LM350 is mounted is not that big, and has no fins……its about 1mm thin metal………..the crazy thing is that the base of the product, on which the LM350 PCB + heatsink sits, is a huge aluminium bar and it’s unfortunate that the LM350s weren’t thermally coupled up to that in some way.




(Another point is that the LEDs are in parallel with no series equalising resistors, but we find that’s OK)


Anyway, I wonder if you believe that this ON/OFF operation of a linear regulator is a bad idea? (i.e. with pulsating 3.2A current)

 

[mister_rf]

In my opinion it’s better to change for a step-down converter. See for example the LM2678. You have also an input to provides an electrical ON/OFF control of the power.
https://www.ti.com/lit/ds/symlink/lm2678.pdf

 

[grizedale]

sorry but switching regulators take too long to get through approvals, and we dont have that time.

We are looking at a quick fix here, perhaps simply increasing the series resistor value to 4R7.

bUT THE HIGHLY PULSATING current through the LM350, and the highly pulsating input voltage of LM350 (DUE TO THE DROPPING RESISTOR) concerns us, as it may cause instability of the LM350?

 

[mister_rf]

Place some 100nF-220nF ceramic decoupling capacitors very close to the IC input and output pins to the GND.

 

[Gofs]

Try to use 2 or 3 linear regulator in parallel. But connect them through low voltage diode and small resistor. You will have less voltage but the current could be bigger. I never didn't such a experiment but this is coming to my mind in your case when you can't use step down converter. Here you have the schematic. You must to choose R value during tests.

 

[mtwieg]

There shouldn't be anything wrong with pulsing currents through the regulator. It sounds to me like this is just a matter of too much average power dissipation and insufficient cooling. It is conceivable that stability issues are coming into play, but I strongly feel that it's minor compared to the overall thermal issues. You can go ahead and damp its response by putting compensation networks in the feedback, but I doubt it will make a difference. Putting additional output bypass caps directly on the output doesn't really make sense since it's a current regulator, not a voltage regulator.

Sounds like the people who designed the thing were incompetent and didn't due their thermal calculations correctly (if at all). How much latitude are you given with solving this issue? Can you change the enclosure/heatsink? Can you change the arrangement of the LED load (putting mode LEDs in series would lessen the dropout voltage on the regulator)? Can you change the input voltage?

Increasing the input resistor might be a quickest fix, but it's not a very robust one. Is that resistor thermally coupled to the regulator at all? Do they share a container? If so, then even if you try and shift more dissipation to the resistor, it probably won't make such a huge difference if they all occupy one packed enclosure. Can you post a picture of the product, inside and out?

edit: also, decreasing the input bypass cap might actually help as well. In reality you want the regulator input to swing wildly. That's the point of that resistor, it decreases the dropout on the regulator. But if there's a huge cap there then it might not be doing its job so well.

edit: the LM350 claims to have temperature overload protection... but apparently it's not working correctly. I think you should consider looking for a better regulator. Preferably one that actually specifies its package thermal properties...

 

[grizedale]

thanks all, Mister_RF:

We cannot put those capacitors there as they will resonate with the stray inductance between the regulator IN/OUT pins and the input/output electrolytic capacitors.

-you can see this by adding them in the simulation.