High inrush current for LED drivers

[Ivan Seeking]

One issue I ran across recently involved 24-volt IR LED drivers for IR cameras, in a factory. The LED units were each rated as 0.5 amp loads. The camera was on the same circuit and only pulled about 250 ma. The engineer had specified 2 amp fast-acting fuses to protect the camera circuit and they were popping all over the factory. I investigated and found that we were pulling over 6 amps on each LED driver circuit, on startup. And that was just what I could catch on a meter. The current then dropped to the nominal value.

This particular manufacturer didn't list the inrush current in the specifications which is why our engineer had missed it. I did a little checking and found that LED driver circuits, and particularly IR LED drivers, have high inrush currents that can be as much as 100 times greater than the nominal load.

Challenge of High Inrush Current
As with any power supply, the LED Driver contains internal capacitors. For the typical LED Driver, the capacitors charge in less than one millisecond after power is turned on. This rapid charging creates https://adlt.com.au/%EF%BB%BFled-driver-inrush-currents/. Compared to older lighting modalities, this is 6 times the inrush current of an incandescent lamp and 4 times the inrush current of the magnetic ballast used in fluorescent lighting.

https://www.ametherm.com/blog/inrush-current/inrush-current-protection-led-lighting-retrofits/

Technologies exist to limit the inrush in LED circuits. In our case, we were able to exchange the drivers for some that had current limiting.

 

[Baluncore]

The switching converters used in LED current drivers generate significant EMI unless plenty of reservoir capacitance is employed at the power input. That capacitance results in the higher inrush currents observed.

I expect to find thermistors used in those situations where the product is mass produced. But I have trouble finding one-off thermistors to fix occasional problems. I guess LED drivers without thermistors are at the cheaper end of the product line, and if you are lucky, you get what you pay for.

Semiconductors are notorious for blowing faster than fast fuses. It takes electronic protection to protect electronics. Fuses prevent short circuits from melting insulation, making smoke and starting fires.

I always have the feeling that thermistors must run hot while the equipment is in use, and that waste of thermal energy is inefficient. I guess a thermally well insulated thermistor can be more efficient, but then the fuse can be blown when there is a momentary power outage, shorter than the thermistor thermal recovery time.

Where I have encountered inrush problems in low voltage equipment I have found that replacing the fast-blow fuse with a slow-blow fuse, or with a thermal circuit breaker (manual reset), overcame the problem.

 

[Ivan Seeking]

Where I have encountered inrush problems in low voltage equipment I have found that replacing the fast-blow fuse with a slow-blow fuse, or with a thermal circuit breaker (manual reset), overcame the problem.

We tried 4 amp slow-blows but they were still popping. And we didn't want to go any bigger on the camera, which had some very small wires. So the manufacturer agreed to send another model. But I did see that they actually make soft starts for LED drivers.