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