Buck Converter for LED Batten 310v to 100v

Hello Members

I have a LED Batten tube (~ 4ft) which stopped working recently. I opened it up and found it is using a buck converter with a switching controller IC (NON ISOLATED). Nothing was burnt or non functional so the switching controller must be the culprit. Also I can't read its part number, nor I can order a new one. The LED strip has a total of 87 surface mount LEDs with 29 in series and 3 such in parallel (3P-29S configuration). Rated power of the batten is 20W with Power Factor of 0.9. So I am assuming each LEDs are going to consume around 200mW (20W x 0.9 = 18W / 87 = ~200mW). I don't know the LED's part number, but I have to assume their breakdown voltage will lie in between 3.3v to 3.6v. Given 29 in series I have to supply around 100v.



I was planning to build my own LED driver. I designed a circuit using a flyback transformer (coiled by myself; 150:50 just to experiment with) but hit with the common problem a flyback trafo has: transient voltage spikes. I tested with just 12v and my multimeter gave reading of over 1000v :bsdetector: on both primary and secondary sides. I am worried what would happen if I try with a rectified 220v. I don't have enough room to implement more components and suppress the spikes. So I am looking for a solution with lower part count.

So I want to stick with the same method they were using in their product: a buck converter. Also I don't have enough room for isolation. So a simple non isolated circuit would be my best choice.

So before I proceed, I wanted to know if it is suitable to step down from 310v to 100v using a non isolated buck circuit, or are their any caveats. Also, what are the things which I need to consider and keep in mind?



P.S.: I know high voltage DC can kill me. I can take care of that! :fight:

Here is a buck converter controlled by 2 transistors (sziklai pair). Suitable for high voltage power supply.

The inductor is placed near 0v ground (rather than the conventional position). It is the current-sensing component, providing a signal which turns the leftmost transistor on and off. The sziklai pair multiply gain, creating snap action which switches current to the inductor.



Adjust the potentiometer to create the desired current to the load.

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(The above was modified from a schematic found on the internet.)

Thanks a lot!!! Its simpler than I was expecting. As this circuit has a current sensing side, I don't have to implement any complex feedback controller circuit.

I see a PNP and an NPN transistors are being used. Also the switching frequency will be quite high depending on the PNP transistor. Can you tell me what kind of transistors and diodes are needed.

Thanks.

Yes, it is safer for led's to make it current controlled, rather than voltage controlled.

The transistors should be chosen to tolerate 310V.

Common advice says to use a Schottky diode when you're running a fast switching frequency.

If you increase the inductor value a slower switching frequency results.

I have MJE13003 and 13005 transistors with me. They are NPN type. They can easily tolerate over 400v. Can I use them in place of the PNP??

Thanks..

To make the circuit oscillate is easier with the NPN PNP sziklai pair. It creates the NOT logic function. Then combined with high gain it produces the desired snap action.

However making it work is not as easy with NPN only.
A single transistor does not provide snap action.

With some effort it may be possible to arrange two or more NPN's to control a buck converter reliably.

The idea is to make it safe being exposed to 300v, and to locate an NPN at the high or low side so it turns on and off properly.
Here is a schematic using a theoretical op amp which is safe to be exposed to 300v:

HV9910B can do a non isolated buck for you...and can run off 450v

https://ww1.microchip.com/downloads/en/DeviceDoc/20005344A.pdf

Also, the AL9910
https://www.diodes.com/assets/Datasheets/AL9910_A.pdf

..The schems are in the datasheets

Hi. Thanks for your suggestions.. I was designing a circuit using 555 ic and a mosfet. Hardly getting enough time for this project. But I will post my result along with a complete circuit diagram to help others as soon I can test my circuit. I am also thinking about using an ATTINY85 (C language is not my bright side), but I will get back to this thread as soon I have something reliable.

we took apart one of these (similar one)
and found it had a little non isolated buck converter inside it.
https://www.elesi.com/8w-e27-gls-le...MI3Oq1-s3M4QIVzpztCh3WoQq-EAYYCCABEgIX_PD_BwE

i will get back with the name of the controller ic we found in it

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This used a BP9912C in buck converter configuration.
It had a 3u3 electrolytic (400v)
The CIRCUIT in the bulb was as in the BP9912C datasheet but there is a CLC input filter…the UPstream C is a 47Nf FILM CAP. (DOWNstream is the 3u3 electrolytic)
There is no fuse, no fusible resistor, and no mov, no NTC, no inrush resistor, and no components upstream of the bridge rectifier.
There is no second winding on the buck inductor (eg for zcd detection)……its just a straight inductor.