I'm designing a step-up constant current source to drive LED strips (nominal load voltage is about 54 VDC). Requirements:
- Vin: 18..32 VDC
- Iout = 0.2 A
- Vout = 54 VDC (nominal) - 57 VDC (maximum)
Since the circuit should have an on-off input, I decided to use LM2586SX-ADJ.
A hand-made fast prototype worked fine at the R&D stage, so we manufactured a hundred of the circuit. The circuit works fine after energizing. However, after some time (I really can't say an exact duration, but it varies between 15 minutes and 1 hour) the inductor starts to buzz, overheats and then finally fails permanently (burns) in a few seconds. I've to say that both the IC and the inductor keep quite cool during normal operation.
What I've tried
- At first, I thought that the problem comes from the DC resistance of the inductor. So I replaced the inductor with 7447709681 from Würth. It didn't help.
- Increased the switching frequency to nearly 200 kHz. It didn't help.
- Placed a 0.1 µ capacitor across the input of the LM2586. It didn't help.
- Placed a snubber (47 Ω and 10 nF) across the SW pin. It didn't help.
- The bottom layer is completely GND with neither cuts nor holes.
- There's a pi filter (100 µF elco - 68 µH - 100 µF elco) before the input, VX. But it's in another sheet, so I couldn't show it here.
- BL input comes from the microcontroller (5 V or GND).
So I'm stuck at this problem. Any help will be greatly appreciated.