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USB Charger and Buck converter

TokyoDave

Feb 17, 2023
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I make small LED lamps for craft projects, powered by USB chargers. I recently started making my own aluminium PCBs with 3.3V SMD LEDs. (Total current in the range of 0.5-1.0A)
Problem: Too much heat generated from resistors to drop the voltage from 5V to 3.3V. So I ditched the resistors, and tried using a buck converter to drop the 5V from the USB charger, but the LEDs starting flickering extremely badly. I then tried with a 'low ripple' buck converter, which didn't work either. The combination which did work was 5V from bench power (or random 5V wall adapter) and the 'low ripple' converter.

I'd like to understand the relevant technical difference between the USB charger and the 5V wall adapter, if there is any way I can still use my stock of USB chargers, or if not, how I can make sure that any 5V wallwart I buy is not going to have the same problem.

Thanks!
 

Martaine2005

May 12, 2015
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What is the current rating of your USB chargers?
If they’re old stock, they might only be 500mA which means they’re overloaded.
The fact that a wall wart and bench PSU work also confirms this.
Edit: why not put another LED in circuit so the voltage required is higher?. Then a smaller value current limiting resistor can be used.

Martin
 

TokyoDave

Feb 17, 2023
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What is the current rating of your USB chargers?
If they’re old stock, they might only be 500mA which means they’re overloaded.
The fact that a wall wart and bench PSU work also confirms this.
Edit: why not put another LED in circuit so the voltage required is higher?. Then a smaller value current limiting resistor can be used.

Martin
I tried 4 chargers. One is an old 1A, two are recent vintage 2A USB chargers, and one is a multi-voltage smart charger. The bench PSU says the LED PCBs only require < 1A at 3.4 volts. The wall wart I tried says its rated for 0.6A, and it worked fine with the better buck converter, but flickered badly with the cheapo one. So my intuition is there is some kind of ripple introduced by the buck converters and by the USB chargers, and they don't work together well. Unfortunately, I don't have a scope to confirm it with.

Leds in series might work, but again a pair of 3.3 LEDs might not produce much light at 5.1V from the charger. Its a pain to test SMD chips without a PCB.
 
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Martaine2005

May 12, 2015
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I wouldn’t believe the 2A rating unless they are a good name brand.
Have you got anything that you can use to load test them?.
Try using more LEDs to load them and see what current is drawn before it shuts down.

Martin
 

Harald Kapp

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Nov 17, 2011
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The wall wart I tried says its rated for 0.6, and it worked fine with the better buck converter, but flickered badly with the cheapo one.
The problem seems to be with the "el cheapo" converter. Possibly the input is not well filtered and the cheap converter draws high peak currents during the switching operations whcih may lead to overload of the wall wart and excessive voltage drop as a consequence.
You can try an additional filter or smoothing capacitor between wall wart and step down converter to buffer the current peaks. As a rule of thumb (and only rule of thumb!) 1000 µF per 1 A of current are good, so in this case a 16 V / 1000 µF electrolytic capacitor should be more than suficient (16 V or higher rating for safe and long lasting operation on 5 V).

Note that you will need to limit the current even when using the step-down converter. LEDs are current controlled, not voltage controlled.
 

TokyoDave

Feb 17, 2023
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The problem seems to be with the "el cheapo" converter. Possibly the input is not well filtered and the cheap converter draws high peak currents during the switching operations whcih may lead to overload of the wall wart and excessive voltage drop as a consequence.
You can try an additional filter or smoothing capacitor between wall wart and step down converter to buffer the current peaks. As a rule of thumb (and only rule of thumb!) 1000 µF per 1 A of current are good, so in this case a 16 V / 1000 µF electrolytic capacitor should be more than suficient (16 V or higher rating for safe and long lasting operation on 5 V).

Note that you will need to limit the current even when using the step-down converter. LEDs are current controlled, not voltage controlled.
The better buck also flickers with the USB charger, so I'd presume using the better one is necessary but not sufficient. I wondered about use a cap to buffer at the input or output of the buck, but the biggest I had at hand was 220uF. It might have helped very slightly. I'll give the 1000uF a try.
 
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TokyoDave

Feb 17, 2023
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I wouldn’t believe the 2A rating unless they are a good name brand.
Have you got anything that you can use to load test them?.
Try using more LEDs to load them and see what current is drawn before it shuts down.

Martin
PSU says around 600mA. I also put one of those USB V/A meters in circuit, and it fluctuates all over the place, but highest it reads is < 0.5. So maybe all of the USB chargers (incuding the Apple one) are bogus, and also the (cheap) bench PSU, but then what's a poor boy to do :)Just to note: the 'low ripple' stepdown I'm using is the DSN-MINI-360.
 
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dave9

Mar 5, 2017
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You are working with a quite low, 1.7V max vDrop from your regulation circuit. I would try to find a PWM buck, current regulating driver instead of what looks like you are just setting a voltage regulated driver to "about" what the LED would need for a target current.

I used to get such things from Fastech, but they went out of business recently. The smaller and less expensive ones are often in a form factor meant for flashlight integration, but you can simply solder wires to them and remove the spring if one is present. If the driver has a series diode as a reverse voltage protection, you can jumper around that to avoid the extra vDrop across that diode.

You can get a smaller vDrop from a linear type regulator such as those based on a 7135 chip, but being linear it sheds all extra voltage as heat which is something you seem opposed to, hence this topic.

Is using a resistor really too much heat, or you simply need a higher wattage rated resistor and/or better thermal isolation from the rest of your project? Since it is mains powered rather than battery, I think I could accept a bit more waste heat and the simplicity and low cost of using a resistor, and yet, it puts more load on a random USB AC/DC charger adapter as well, which since those are typically sealed units, running a light for hours at a time multiplied by many days per year, could result in less than ideal lifespan.

Perhaps your better option is simply use a one part solution, a mains AC to (3W?) constant current driver board. Why buy a 5V PSU when what you really want is a current regulated/limited, 3.3V drive?
 
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