36x LED 5730 in series: diodes burnt when LED driver (based on BP9833A or MT7854BD) powered up

Question

Background:

• I have 18W, 36 LED lamp fixture which I cannot change.
• Originally it has 2x parallel 18off 5730 LEDs and 300mA driver. It works for some time, but diodes starts burning. I thought that the problem is 300mA when one of the LED strings gets faulty - remaining parallel string cannot handle 300mA and it ends up with both branches destroyed. Below table describes 5730 LED which I bought to replace burnt diodes:

Solution which turns into problem:

• I manage to re-wire all 36 LEDs in series (=3.3Vx36=118,8V, 120mA) & renew faulty diodes.
• I bought driver based on BP9833A and assembly everything altogether. Please see below picture of my BP9833A driver:
• After re-installation of fixture two LEDs get burnt during power-on. I repeat previous steps and got another few LEDs burnt (not only those which I renew). Finally I install 3x in series 390 varistor parallel to the LEDs. It allows me to start-up the driver without harm to the diodes. With UT203 multimeter I have: 115-120mA, 117Vdc. Driver idle voltage is ca. 180Vdc. When loaded it is switching off around 120Vdc. Nearly perfect except this idle voltage which I suppose should be not the case as I have the load connected.
• ROVP has 10kOhm, resistor on pin CS seems to be fine as driver stabilises around 120mA.
• I thought my bad experience is a flaw of BP9833A. I bought driver based on MT7854BD with lower maximum voltage. Please see below picture:
• I reduce string to 24 LEDs (=3.3Vx24=79,2V, 120mA) as it cannot handle more than that - it simply stays in safe mode. Current stays around 120mA, loaded voltage 75Vdc. Surprisingly same fault happens without varistors. Few LEDs are damaged again during power-up phase. It seems to be worse when single pole wall switch is in use. They survive few starts when switched by Schuko type plug with round pins on my bench.

Question:

1. Why do I burn 150mA diodes with 120mA driver during power-up phase?
2. What can I do to limit excessive initial voltage/current in both BUCK converter designs during start?

Answer 1

I think it's because you attempt to use these constant power (18W) power supplies at or near their maximum voltage. It should be used closer to the minimum voltage. And/or because you use constant power and not constant current power supply. When you try at the upper voltage limits, the driver doesn't see 18W even thought it outputs 120 mA and rises voltage much higher in order to reach 18W.

You need a constant current power supply for exactly 120 mA (Not 100~240 mA or the likes) with a power range between 15W to 30W. Then theorically it should outputs a voltage within the LED limit. The goal should be to keep 120mA, not 18W or x volts.

Besides, I advise you to divide the series and use two or even three series in parallel, each with their own suppy. It's an extra cost but it's much safer against electro shocks. At 120 VDC it's already quite dangerous.