100nF capacitor charged up to 30V will damage LED array?

[Flyback]

Hello,
We are designing a led driver, spec = vin=30vdc, vout=19vdc, iled = 2.5a
If somebody disconnects the led load, then the output capacitor (100n) gets charged up to 30V….Then when someone re-connects the led load, the 30V , 100n capacitor suddenly discharges through the 19v led array….will the led array be damaged?….the energy is 0.5.C.V1^2 – 0.5.C.V2^2
Where C = 100n
V1 = 30V
V2 = 15V
So total energy = 33.8uJ…….though what does this do to the led array?….will it weaken it , incidiously, making it fail in the field prematurely?-the mlx10803 led driver we are using doesn't facilitate open led protection, (at least not without many extra components)......but we stay using the mlx10803 because it is the only one that jitters the switching frequency......we need this because there are 24 of these cards inside the enclosure, so we need the jittering , or else we will get ticking on the supply ...which is two PSP-500-24 PSUs.....each supplies 12 mlx10803 driver cards.

 

[tvtech]

Hi Flyback

The questions never seem to stop

You seem to have the theory waxed with your nice calculations and so on.
Truth is, 100 NanoFarads is really not able do any damage anywhere ever...

Unless of course it is charged up to Gazillions of Volts (big one) and you touch an IC's leg with it..and the huge (still 100NF decides to discharge).

Other than that, no real capacity there.

Hey, maybe I am wrong.

Only a humble tvtech here.

Regards,
tvtech

 

[ronsimpson]

Look at the LED data sheet. Probably the diode can take 10X the current for a short time. Probably 100X current for a very short time. The diode array is very strong.
Think about this question;
You have a 10 watt 18 volt Zener diode. Would you feel good about pulsing it with a capacitor? (one time)

 

[Flyback]

well, putting it like that, i think it seems as if it will be ok...however , the bonding wires inside leds are super fine and delicate, so im not so sure...

Unlike non-optical components, to reduce visibility, the thinnest shortest smooth radius goldwire bond is attached which gives much less margin (undefined due to customer solder stress) than a power diode which can be pulsed 100x its average rated current.

LEDs are far more delicate than zener diodes.....I reckon the 100nF could blow up the leds.

 

[JimB]

If I understang correctly, the LED array operates at 19v 2.5A, so it is dissipating 47.5 Watts.
47.5Watts is 47.5 Joules/Second.

You are worried about 33.8uJoules.

If those joules are dissipated in 1uS, the power will be 33.8Watts (in 1uS).

We do not know how fast the capacitor will discharge into the LED array, the resistance of the circuit will have a big influence on the peak current etc.

If you are really worried by this, why not set up at test jig.
Have a 100nF capacitor charged to 30 volts, and discharge it through a LED array.
Do this once, then 10 times, then 100 times, and then let it run for a week if you must.
Then check the LEDs for degradation in their performance.

A practical test trumps all the theoretical soul seaching and hand wringing.

JimB

Of course if the LEDs were inductively coupled, that would probably slug the capacitive discharge transient.
(I am sorry but the devil just made me say that. )

 

[tvtech]

(I am sorry but the devil just made me say that. )

LOL..this is funny

Love you Guys

Regards,
tvtech

 

[Flyback]

thanks, btw, I haven't even thought about those inductive coupler people for some time (I wont mention their name, but we know who inductively couples energy to led lamps)

 

[Nigel Goodwin]

well, putting it like that, i think it seems as if it will be ok...however , the bonding wires inside leds are super fine and delicate, so im not so sure...

Unlike non-optical components, to reduce visibility, the thinnest shortest smooth radius goldwire bond is attached which gives much less margin (undefined due to customer solder stress) than a power diode which can be pulsed 100x its average rated current.

LEDs are far more delicate than zener diodes.....I reckon the 100nF could blow up the leds.

Sorry, but you don't appear to have the faintest idea about it

It's perfectly common and normal to pulse LED's at far higher currents than they are rated - the 'normal' failure mode of LED's (as with most semiconductors) is due to overheating - short term pulses cause no such problems, and the thought of a tiny 0.1uF charged to only 30V causing problems is laughable.

Check all the IR remotes in your house, they pulse the LED's at 1A or higher - the LED's in them rarely fail (and most reasons are mechanical damage).

 

[Dick Cappels]

Your concerns are well placed. Multiple narrow pulses of high current can cause eventual failure of components, usually in the form of metal migration, which occurs when the current density (amps/square cm) exceeds some level.

Check the LED datasheet and don't exceed the maximum peak current and things will be ok. You might want to add a resistor between the capacitor and the connector for the LED array to limit the peak current to something below the specified maximum.

To be sure, use a digital scope in the single shot mode and some sort of current sensing to view the actual current pulse. Make sure the current sensing means does not significantly limit the current to the circuit.

 

[ronv]

I agree. It may not fail right away. But do it enough and it will.

 

[Diver300]

I haven't done the calculations, but it just doesn't seem like there is enough energy to do any damage. The temperature rise will be tiny.

Where semiconductors heat up rapidly by more than 60 °C or so, then you can get fatigue damage. Power semiconductors that are shorted and are protected by their over temperature protection may be damaged, and only survive hundreds or thousands of such events. However, that would typically be a semiconductor that survives 12 V supply, 100 A short circuit current for 1 ms or so. That is over 1 J of heating. The capacitor looses 34 μJ, spread across 6 LEDs, and it is of a completely different scale to anything that would do damage.

 

[Nigel Goodwin]

I haven't done the calculations, but it just doesn't seem like there is enough energy to do any damage. The temperature rise will be tiny.

I agree, it's a crazy suggestion that such a tiny capacitor could cause any damage.

 

[Flyback]

At one place where i worked......I do know that a 15uF ceramic charged up to 80V, and then discharged through a 5W, 3.5V LED lamp destroyed it.....this happened a couple of times, and then they took all the led lamps away as they were expensive. It was a sepic led driver that had OVP clamp the vout at 80V.

 

[ronv]

All rather non-committal.

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CB4QFjAA&url=https://www.cree.com/xlamp_app_notes/electrical_overstress&ei=aXxVVNSMNZDAiQK3v4G4DQ&usg=AFQjCNESbAEFgKz--RX643SulmIgYSv-UA

 

[Diver300]

At one place where i worked......I do know that a 15uF ceramic charged up to 80V, and then discharged through a 5W, 3.5V LED lamp destroyed it.

That is 10,000 times as much energy compared to what the OP asked about.