Booster driver for string LED systems

I have some DIY LED circuits which I’m using and building for lighting in my RV. Since this is powered by 12vDC. As such, sometimes battery V drops to a point things don’t work so well, my lighting systems give up. They’re not using a buck booster, which is how the LED’s I purchase as for auto replacements for the incandescent units are standard are constructed, to maintain high intensity when V drops. I have looked at schematics I could construct but haven’t found one I’m satisfied with, so now I’m looking for a very compact circuit board with a bridge rectifier, a driver chip, a coil and other components that can drive up to 6 strings of 4 LED’s (white) to incorperate in my fixtures. I’m using a single push on/push off circuit I construct which I want to put these boosters in so any input would help.

Hello 92monterodo-bat, welcome to the DigiKey TechForum.
Can you add a drawing of your schematic or circuit? Also do you have any information on the LEDs that you are using, as for the voltage and current ratings? Also a bridge rectifier would convert AC to DC voltage. Are you using AC to power the LEDs? Hopefully if you can add some more info, one of the engineers can help with this project.

I’m looking at the data sheet for the IRS254, which would likely require me to reconstruct the LED section of the circuits. I’ve been dealing with several issues on this system, the latest was DC isolation between the batteries and the inverter and all. My battery bank is suffering from age lately, not taking a full charge or holding it. The schematic I’m using for the switching is from, I don’t seem to be able to transfer it from the web page to email attachment. It uses a pair of Transistors, 2n3904 and 2n3906 with 5 diodes, a MOSFET and a momentary switch. So the only reason for the bridge rect would be polarity protection and a means to get as much ripple out as possible, I would place it at the very front of the circuit along with with a linear Vreg section maybe. I think I would want to do this to keep the input voltage at around say 8 to 9 volts and then use a boost circuit as a final driver to supply the rest of the circuit with a steady supply as I am planning on switching from the current 12v battery system to a 24v system and install some system between the 24v to convert DC to DC 12 ( a converter or a MPPT charge controller ) to keep a secondary 12v system.

sorry, I missed a number on that IC, it’s an IRS2541, an obsoleted part.

As @David_1768 points out, before we can consider solutions we really need to know the specs of your LEDs to be able to give any sound advice. Are these individual LEDs or a module with 4 in series?

If individual LEDs, what are their forward voltage and current ratings (or give us a part number we can look up).

If a 4-in-series module, does it have a built-in current limiting resistor(s) or are they just a string of LEDs on a board with no built-in resistor? What voltage and current is the string rated for (or what is the part number so we can look it up)?

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I’m using SMD white LED’s, single pieces I’m installing on boards. The
LED’s have a Vf of around 2.5v with 20 mA. So I assembled them in
strings of 4 with the appropriate dropping R’s to power them with a
nominal 12v. All the circuitry is what I assembled myself, installed
in a poly case I found that I was able to work with. I tested the
completed circuits and they are drawing between 40 to 60mA. I have 2,
one uses a 4 string and the other a 6 string. It looks like I’m going
to have to reconstruct the light boards to run at a lower voltage
input with different R’s unless I can figure a way to make the lights
function better at varying voltage.
So I guess I would say modules I assembled using individual parts. The
light board is 1 board and the switch is another board.

I’m not an engineer, just a guy that can sort of read schematics and
put stuff together and sometimes it works and sometimes it doesn’t.

No problem, just need to understand what the requirements of your LED strings would be.

So, a string of 4 LEDs rated at 2.5V @ 20mA would be about 10V at 20mA. For a 12V system, you would want to have a series resistor of about 100Ω ((12V - 10V) / 0.020A = 100Ω), and 1/4W rating would be plenty. Six strings would add up to about 120mA at 12V, so your driver would need to be able to supply at least 1.5W, and preferably a bit more.

If you go with a 24V system, then there would probably be no need to go with a boost topology, which tends to be less efficient than a buck topology.

For example, if you had a 24V battery supply, you could use something like the R-78W12-0.5, which will take anywhere from 15V to 32V and regulate down to a fixed 12V output for up to 500mA. This could power up to 25 of your 12V, 20mA strings in parallel.

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I’m thinking that this part would not be a possible arrangement as the RV would require to remain supplied by nom. 12v. The only part that would be 24v would be the main battery bank which would be charged by the MPPT/solar system which would need to drive the main inverter and include a 24v to 12 v converter or charger to keep a secondary bank or battery fully charged to run periodic heavy loads such as starting the engine. There will also be an isolation system with some manual bypass solenoids. You’re pretty close to the supply I started with 12v -10v and think I used around 100R maybe a bit more or less. I don’t recall, I did use 1/4w. Whatever I used, I do recall testing the strings at around 10 to 15 mA. The schematic I used for the switching part was using a 12v source. The 1 circuit with 6 strings draws around 60 mA and the other of 4 draws around 40 as far as I can recall. I will check out the R-78w12-0.5

Ok I went to look at the Recom page and I found a driver, the RCD-48 which appears to be an interesting piece I might be able to use for another system of lighting I have with some work. I might work for this also aside from being able to supply a lot more than I would need to run the current circuits I’m working with no I think that isn’t a problem as less draw is easier to deal with than more current. I have some fixtures that need a dimmable control and some not.

I just got an email ad from Digkey and found the Flex PUC0512 series and even though it shows a 70% rating, I think this will suit my projects very adequately. I will have to use a linear 5 v circuit to regulate the input which is fine because if the power supply fluctuates, it will never be below a limit that a simple linear reg will function good, current draw will always be acceptable, it will fit on a standard 2.54 mm spacing which is what I use for most stuff. It is small enough I can fit in the poly cases for the DIY lights. The regulated output is just what I was looking for and external component requirement appears to be minimal, and I should have on hand. I’m going to study this more.

A couple of notes regarding the RCD-48 . . .

The output voltage has to be at least several volts lower than the input voltage. Unfortunately, the datasheet does not clearly define how much lower, for every input voltage, but at its maximum input of 60V, the output can be no higher than 56V. Assuming this 4V as the typical drop, at 12V, you may not get more than about 8V out.

It is an LED constant current driver rather than a constant voltage driver. This has some advantages and some disadvantages, depending on what you’re trying to do. If you use a constant current driver, you don’t want to have the resistor in series because that resistor’s purpose is to control current and drop the excess voltage from the supply down to what the LEDs need. The constant current control takes care of that by raising or lowering the voltage to maintain constant current.

The real key to this part is that it will try as hard as it can to push out whatever its current spec is, and it doesn’t care what might burn up in that process. The RCD-48 comes in several current ratings from 350mA to 1.2A, and your LED strings will need to be capable of drawing that much (whatever the rating of the particular model), or they will be overdriven and burn out.

For the reasons above, based on your description of the LED strings you have, I would not recommend RCD-48. It’s meant for high-power LED modules rather than strings of low current LEDs.

Here are a few constant voltage output buck/boost supplies (input can be above or below output) that may be suitable for your 12V strings:

  • These are more like hobbyist types, but may work just fine for you:

The Flex PUC0512S1B is more like the following, but they won’t require the drop to 5V first.

An additional note about using the PUC0512 along with a linear regulator to drop from 12V (nominally) to 5V and then boost it back up. It will be very inefficient. A little back of the napkin calculation for that configuration, using four strings of four LEDs with a 100Ω current limiting resistor (about 20mA per string), I get about 24% efficiency. The LEDs themselves will take about 0.8W and the battery would have to supply about 3.3W.

The primary culprit in excess power draw is the linear regulator. By dropping from 12V to 5V, it is only 42% efficient. The rest of the losses come from the boost regulator, at around 70% efficient, and the 100Ω current limiting resistors, causing a 17% loss in the LED strings (83% efficiency). If you used a switching regulator (such as one of these linear regulator replacements) rather than a linear regulator to drop from 12V to 5V, you could improve your regulator efficiency from 42% to between 80% and 90%, depending on the specifics of the model and how many LED strings you power. This would drop your power consumption from around 3.3W down to around 1.7W.

However, going with one of the isolated DC-DC converters I linked above would be the most efficient, with a total power consumption of around 1.3W for four strings (efficiency in the 62% range, including resistor and converter losses).

I guess that would be a mistake on my part making the statement of
using a linear as I was aware linears are less efficient than
switching. My bad, in fact now that I think about it, I have some
switching reg boards on hand, I need to look at them again. I may even
have what I should be using already. I purchased some tiny boost
boards for another project I never used. They are configurable by
solder jumpers already for input/output V. Using these may solve all
my issues.

Quite likely so. Good luck!

Ok so I’ve been doing a bit of homework and I’ve found this IC which
is a stable battery switching reg circuit. The pretty neat looking
MC34063 which appears that I would not have to worry about dropping V
then boosting it again, pretty minimal component count on the
externals. The schematic I’m looking at uses a TIP31 and indicates a
load up to 1.5 A which is well above what my lights draw so probably
wouldn’t need much of a heat sink.I would likely just mount the Q to a
small piece of aluminum in the poly case like I did the MOSFET in the
switch circuit. I can order some of these in the THD package, install
them on small boards using SMD for the rest of the pieces. I have a
hand full of 8050 NPN’s too which are rated for 1.5 A. Do you think
those would work instead of the TIP31 since these circuit don’t draw
any more than 60mA? If they will then I wouldn’t need the aluminum
piece for the TIP31. I’ve been using these SMD 8050’s for some other
stuff and solder a piece of 20g wire to the collector pin and plug it
on both sides of that pin in the perfboards to carry as much heat as
this can. Let me know what you think if you have a mind to do
so.Frankly I don’t see the point in using a TO-220 packaged Q if
there’s no way the load is going to be more than what a 2N3904 or any
other transistor in that class can handle. I have the data sheet on
the Vreg and going over it.

Hello 92monterodo-bat,

It warms my heart to see folks working to solve challenges such as this.

As I understand, you are currently using a nominal 12 VDC system. You appear to be accepting significantly reduced battery life by allowing the voltage to drop below 12.0 VDC. This implies a maximum equalization charge voltage up to 15 VDC. In the future you wish to transition to a 24 VDC system.

You have series connected 4 ea 2.5 V_F LEDs operating at 20 mA. With a nominal 12 VDC system you are likely using a 100 \Omega current limiting resistor.

Provided you do not require isolation, the MC34063 would be an excellent solution for boost in a 12 VDC system or buck in a 24 VDC systems. Here we assume the LEDs are reconfigured in series strings of 6 or 7 LEDs each. This reconfiguration would provide overhead avoiding the voltage extremes in either the 12 of 24 VDC system.

The SS8050 is not recommended as the V_{CE} is rather low. This is especially true when we consider an automotive environment with the associated charging spikes. I do believe you are correct that the TO-220 packaged TIP31 is excessive. It may also be hard to drive with its relatively low gain. My suggestion is to select something from this list. It included the classic BB139 which may be just what you are looking for.

Just in case you haven’t already done so, here are some application notes and evaluation board documentation from onSemi. Be sure to search keyword “MC34063”.

Please keep us in the loop. We are curious to know your progress including stumbling points (learning opportunities) and ultimate success.

Happy holidays,


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As per the given reply options @ the bottom of emails, all of those. I am still doing my research on this for now and will look at those recommendations. I hadn’t considered recon-ing the LED strings as in the end the wiring of the system requires remaining @ the nom. 12v @ the end of the system. Only the 24v part will be from the solar panels via the MPPT charge controller for the bulk of the power storage so a 24v to 12 converter is a required item also. For the first light fixture, I
m going to go ahead with the LM1117-5.0 in an SMD piece on a board with peripheral components to power another board I have that is a regulated 12v output to drive the switching circuit (on/off). Since this circuit only draws a max of 60mA or slightly less with 16 LED’s, I’m not really concerned about how much it is wasting for now… I just need the LED’s to be supplied with a steady 12v. They aren’t left on constantly for the most part. As soon as I can re - fund operations after the 3rd, I will be purchasing items needed for circuits, battery storage upgrades, etc.meaning more solar panels, LiFe batteries and converter or another MPPT system. I need to contact my solar items supplier to discuss this with them ASAP.

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