Calling all FAPpers - FAP800 - precautions and driving advice

[Alaskan]

Thanks for the schematic Mike. I'm collecting information on this project and saving here:

Fiber Lasers and Drivers

I found this information regarding how overcurrent loads are handled with the Texas Instruments regulators:

Overcurrent Protection:

For protection against load faults, all modules incorporate output overcurrent protection. Applying a load that exceeds the regulator's overcurrent threshold causes the regulated output to shutdown. Following shutdown, a module periodically attempts to recover by initiating a soft-start power-up. This is described as a hiccup mode of operation, whereby the module continues in a cycle of successive shutdown and powerup until the load fault is removed. During this period, the average current flowing into the fault is significantly reduced. Once the fault is removed, the module automatically recovers and returns to normal operation.

The way I read this, when an over current situation occurs the output is shut down but keeps trying, or "hiccuping" an output until the FAP cools enough to have a higher impedance, or lower current draw, but to reach overcurrent with the 50A module it is far too high at 85 amps to safely run the FAP that way. Choosing a lower current rated TI module which has close to a 50 amp overcurrent threshold (depending upon which FAP you have) such as the PTH08T210W rated for 30 amps continuous appears to be the way to handle this, but perhaps 50 amps is too high for your particular FAP. There are several versions of the TI high current voltage regulators to choose from, so you probably can find a suitable one with a lower over-current threshold. If running the FAP in intermittent duty cycle, this ought to work out for us, but I wouldn't try it for 100% duty cycle as the module would probably heat up too much.

 

[Hemlock_Mike]

I picked the 50 Amp module so it would be running at 80% at full 40 Amp module rating. Sometime, I may want to try
the FAP at rated 40 Amps. It is unlikely a massive module failure could draw 85 Amps and if it did, the FAP was
trashed anyway. The other failure would be an external short which would isolate the FAP.
NOT TO WORRY !!!!! -- Mike

 

[Alaskan]

True, the diode would be destroyed before it reached 85 amps, so it would never reach that, but that is why I don't want to use the PTH08T250W 50A regulator with the over-current threshold so high, I want the voltage regulator to reach cutoff before the FAP draws too much current to damage it. However, my 40 watt FAP is rated to produce full output power at less than 54 amps, so that can be close to 4 amps over cut-off using the module rated at 30 amps continuous which means I couldn't run the FAP at 40 watts out, likely, unable to run it at all because with a freshly charged battery, every time I would turn it on, the FAP would go into over-current shutdown with that regulator. However, there is a way around this, adjust the voltage output lower so the FAP cannot normally draw that much current, it would be less than 40 watts out, but 30 watts is plenty for me anyway, and the battery would last longer, so that's really a better output level for me.

All of that said, as well as your observation, neither of our FAP's have gone into current runaway, so all of this may be unnecessary at the duty cycles we would use them at in a portable unit. If so, why bother using a regulator anyway if using a direct connection to a 2.0 VDC 25 AH Cyclon lead acid battery which would run down before the diode could get too hot? Of course, this only applies if a huge heatsink is used and if using a small heatsink, the laser very well could get so hot it could become damaged before it reached the over-current threshold.

Even though neither of us have experienced thermal runway with our FAP's to where they draw too much current, perhaps that can happen, we just didn't get them hot enough to see it. If the reason we didn't see it is they don't easily over-current when hot, that's a problem when depending upon a cut-off to kick in during over-current to prevent the unit becoming so hot it could be damaged. In that case, I should also incorporate a thermal cut-off device.

 

[Hemlock_Mike]

That's why I clamped my TI PS to a voltage which under "normal" conditions
would provide 30 Amps for test purposes. If the FAP decided to suddenly draw
85 Amps at a voltage which can provide only 30 Amps, Something went WAY wrong.
Mike

 

[Alaskan]

Agreed, and I see what you were doing now. 85 amps would probably be a short, I expect a diode failure would not cause that much current. Still, it was such a good deal I ordered five of the TI units (edit: at $10 each) rated for 30 amps continuous with built in cut-off at 50 amps. Should work without overheating at the limited duty cycle I will use it at. If I’m wrong and it starts shutting off thermally at the reduced duty cycle, I will need to use the 50 amp regulator which I bought two years ago, untouched.

 

[Junkers]

I finally got around to getting everything together, got a pair of decent safety glasses for a reasonable price, and voila...

...I can't post links or photos. Damn, what's with the new rules?

Anyway... I notice these FAP units have a very low threshold with respect to operating temperature. The current datasheet states 30C max and I've seen some older datasheets state 40C. That doesn't exactly leave a lot of headroom. Is that the point of failure of just where premature wear begins?

I have a couple of peltier modules here which I would like to use. Is that advisable? What's the conventional approach?

 

[Cyparagon]

Like all other manufacturer recommendations, the maximum operating temp is just the maximum suggested. Beyond that point, the manufacturer doesn't guarantee operation. It may work fine with minimal degradation in lifespan, or it may pop at 42 degrees. It's a risk you're welcome to take if you can afford it. Typically, a reduction in lifespan (proportional to the overtemp of course) is to be expected with most electronics.

TEC or chilled water are both common solutions.

 

[Alaskan]

Same thing said a different way:

If the laser diode temperature continues to rise exceeding the maximum operating temperature, the diode can be catastrophically damaged or the long term performance may degrade significantly. If the laser diode’s operating temperature is reduced by about 10 degrees C., the lifetime will statistically double.

Let it run 10 or more C. hot and if it doesn't have a catastrophic failure, or some of the diodes in the bar go dark in short order, I expect it will just have reduced life. Since they are rated for many thousands of hours, perhaps for your use, not a big deal when you can buy them surplus cheap? But then again, they are probably cheap due to nearing what is considered end of life where the output drops by 20% or more.

 

[Junkers]

Well I setup the peltier modules this evening and they worked well. They managed to keep the body of the laser at around 18C at around 5V/1.8A while the laser ran at 24A. I haven't pushed it any further as of yet, haven't got the cojonies. That day will come though. While messing around I did manage to melt some powdered aluminum which was impressive given the beam isn't focused and the absorptance is supposedly only around 10%. I'm going to order a focusing lens very soon. Will be interesting to see what I can achieve with that.

 

[Alaskan]

Glad to see the TEC’s worked so well for you