Yet another burned LED driver - help needed

Discussion in 'General Electronics Chat' started by Preben Hansen, Nov 30, 2018.

  1. Preben Hansen

    Thread Starter New Member

    Nov 24, 2018
    2
    0
    Hi folks,
    I need some advice from you experts to repair the CC LED driver in my LED lamps. 3 out of 7 lamps don't work any longer, after 8 years of use. The supplier has no spare parts and no other lamps in the same size (needed because the lamps are built into a concrete wall). Due to these circumstances, an option is now to try to repair the LED drivers myself - hopefully with some help from you.
    As may be visible from attached pictures, at least the fuse and a capacitor are blown - presumably due to sub-optimal low cost circuit design.
    I've added all the information I have in below circuit diagram, drawn from reverse engineering the extracted LED driver. From studying LED drivers in the internet, I had expected to find a couple of resistors (in addition to the VDR), but I don't find and can't measure any.
    All LEDs seem to be OK. All 3 LED drivers appear to be failing due to a blown capacitor and fuse (symptoms). The other components look OK, at least there are no sign that they are burned. The black parts under the components on the board are rests from the black foam in which the components were sealed.

    Components
    F1 Fuse 250V 2A
    VDR 7D 471k Smart Lightning Protection Zinc Oxide Varistor blue
    IC1 DB107 1A 1000V Bridge rectifier
    C1-4 CBB Capacitance 104J, 400V 0.1µF/100nF Feet Distance 10mm
    C5 Monolithic Ceramic Chip Capacitor yellow 104, 50V 0.1µF/100nF
    C6 Ceramic Capacitor blue 103M 1kV 0.01µF
    TR1 Transistor SiHV 9921 0843
    D1 MUR 160, 1.0A Super-Fast Rectifier 600V
    L1 Inductor
    L2 Inductor

    My questions are:
    1. Is the LED drivers failing due to poor design or age?
    2. Will it make sense to exchange all capacitors C1 - C4 (C1 is blown in all 3 LED drivers)?
    3. Is a fast or timed fuse the most appropriate?
    4. What other components are most likely blown as well and should be exchanged?
    5. What is the inductance of L1 and L2?
    6. I don't find any documentation on TR1 in the internet. What would be a suitable substitute?

    I don't expect all questions to be answered. Any advice is very much appreciated!
    Thanks a lot in advance :)
    Artin2 LED-Driver.jpg Artin2 LED-Driver2.jpg Artin 2 LED Driver Circiut Rep.png
    Red marked components are blown.
     
  2. ebp

    Well-Known Member

    Feb 8, 2018
    2,332
    811
    TR1 is a Supertex HV9921. Supertex is now part of Microchip.

    Does anything apply any sort of mechanical force to C1? It is very strange that it should be the blown in all failed units. Capacitors like that (probably metalized polyester film, commonly marked "MKT") generally don't fail unless an overvoltage is applied, but if something puts pressure on the part, it might make it more failure prone.

    L1 will be primarily for radio frequency interference suppression. L2 is the main "buck" inductor. Sometimes inductors like that have the value marked on them, either on the sleeve or on the end, usually as 3 digits in microhenries (e.g. 101 would be 10 x 10^1 = 100 µH).

    My guess is that all of the parts except the failed capacitor and the fuse have survived, however the capacitor may have resulted in failure of the bridge rectifier.

    Capacitors to replace the originals will be very inexpensive and easy to obtain. I see no particular merit in replacing all of them, though perhaps they are of marginal quality. The fuse looks unusually small to me, which might make it more of a problem to obtain.
     
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  3. Preben Hansen

    Thread Starter New Member

    Nov 24, 2018
    2
    0
    Thanks a lot ebp! Excellent information.
    Just knowing what TR1 is helped me to find the datasheet on the Microchip website. There is even a User Guide that has a LED driver circuit basically identically to mine, including a component list - just with only 10 LEDs instead of the 24 that I have. There are even steps and formulas for dimensioning most of the components yourself.

    The formula for calculating the inductance of the buck inductor L2 is V(out) * 10.5 µs / (30% ripple * I(out).
    I've now measured the LED V(f) to 3.3V @ I(out) = 20 mA, hence
    L2 = 24 * 3.3V * 10.5 µs / (0.3 * 20 mA) = 139 mH - which seems big to me...
    I can't see anything written on L2 in the LED driver (except from '&'), I've fully opened. Perhaps I can dig out the L2 number you mention, from one of the other drivers.

    If my measuring is correct, the rectifier is still OK.
    I checked the 3 LED drivers again. In one (below), C4 is burned and not C1 as for the two others. Additionally, a thin copper wire is visible next to L2; seems suspect. I'll remove the black foam to check where it comes from. LED Driver 2 C4+L2.png

    I open one of the still working lamps to check if anything is putting pressure on the capacitors. Except from the neighboring capacitor and potentially a cable seems to touch the burned L1 capacitor. C1 is sitting 5 mm over the PCB to avoid being squeezed between the rectifier and C2.
    LED lamp case.png

    I've found small fuses with dimension 3.6x10mm on amazon. The choice is between fast and timed. Not sure yet what is better.
     
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