XL6009 Boost Buck input voltage issues

[TWilson]

I have purchased a generic XL6009 boost buck converter and intend to use it for running a daylight LED module. The whole thing will run off two 12v 10w solar panels in parallel. The LED module is running at around 21v at half power.
The issue I have discovered is that for some reason the input voltage is being dragged down to between 3.1 and 3.6 volts. The current rumps up but I can't seem to get the intended voltage out of the panels.
I tested it on a bench power supply and as I slowly brought up the voltage, the amps started to climb. I limited the current and found the voltage capped as it did on the panels. I slowly brought up the current and at around 2 amps the current suddenly dropped to 0.4 amps and the voltage jumpped up to the intended 18 volts.
Can anyone explain this phenomenon?

[Zero999]

The solar panels can't provide the current at the required voltage.

Another thing: is the LED designed to run off a constant voltage? Normally they need a constant current, unless it has a built-in driver.

[TWilson]

I thought it was just the solar panels but I duplicated it on the bench power supply. If I raise the put put voltage via the chip's pot, then there is no issue.
As for the LEDS,  I'm wanting variable current to give it a more skylight feel to the light and not run them at full current. This is why I'm sticking to the fixed voltage.

[bktemp]

A switch mode regulator draws a constant input power while regulating the output:
If your input voltage is low, it needs a high current to deliver the necessary power. If your power source has a high impedance or a current limit, it results in this behaviour you see, because the switching regulator draws so much current when starting at low input voltages.
One possible workaround is implementing some undervoltage shutdown that disables the switching regulator when the input voltage is too low.

[Seekonk]

Those little converters work great with solar panels, but you have to add a few extra parts to monitor the input voltage and prevent it from dropping below the panels power point.  These are fast enough that you can switch the enable pin or fake the feedback pin into thinking the output voltage is too high.  A transistor or TL431 are an easy implementation. Done it to lots of them.

[Zero999]

I thought it was just the solar panels but I duplicated it on the bench power supply.

Yes, you made your power supply model the solar cells by setting the current limit too low to power the converter.

If I raise the put put voltage via the chip's pot, then there is no issue.
As for the LEDS,  I'm wanting variable current to give it a more skylight feel to the light and not run them at full current. This is why I'm sticking to the fixed voltage.

Running LEDs off a constant voltage source is not a good idea. One problem is Vf decreases with temperature, causing the current to shoot up. You might be able to get away with it by experimentally increasing the voltage, until the current drawn is a fraction of the maximum rating but it's still a bad idea.

[Seekonk]

What exactly is the power of this LED and is there a battery involved.  Lighting a LED with only a solar power is an odd application. Out of two 10W solar panels you would probably have trouble getting 10W. You  could put the panels in series with no converter, that would at least prevent them from dropping too much.

[TWilson]

What exactly is the power of this LED and is there a battery involved.  Lighting a LED with only a solar power is an odd application. Out of two 10W solar panels you would probably have trouble getting 10W. You  could put the panels in series with no converter, that would at least prevent them from dropping too much.

The LEDS are designed to run at 24 volts at 0.56 amps but in reality draw 1 amp. I'm intending to run them at 21 volts at a measured 0.35 amps or so. And no, there is no battery involved. It is designed to mimic a sky light.
I understand I won't get the full 10W. That was the intent in getting the two in parallel.
I was originally going to have them in series but I was concerned that the output voltage from the panels would go higher than what the LEDS would tolerate as the panels can get up to 18V each in full sun. And I couldn't figure a suitable circuit to cap the voltage if it did go over.

[james_s]

LEDs need current limiting or they will try to draw an infinite amount of current, limited only by the impedance of the supply. You could try a simple current limiting circuit, like using a LM317, it's not very efficient but the power from the solar panels is essentially "free" anyway and any excess will be wasted no matter what you do unless you have something to store it.

[Seekonk]

I bought some 120V 18W ceiling lights for my camp.  The LED was 41V and I ran them at 35.  Something like that would be ideal to be powered by two solar panels in series.  The solar panel will self limit to the maximum current specified.  It will not over voltage as the current will drag down the voltage.  The LED themselves would act like a zener and not conduct at all below the minimum voltage.  So putting the panels in series would be the quick and dirty way.  Your boost converter would work very well too if it was set to the maximum voltage desired and the input voltage was monitored.   This could be as simple as a voltage divider to a transistors base.  Collector connected to a pull up resistor  and a diode to the sense pin of the chip.  This would always send the maximum power to the LED for given sun.

[Zero999]

What exactly is the power of this LED and is there a battery involved.  Lighting a LED with only a solar power is an odd application. Out of two 10W solar panels you would probably have trouble getting 10W. You  could put the panels in series with no converter, that would at least prevent them from dropping too much.

The LEDS are designed to run at 24 volts at 0.56 amps but in reality draw 1 amp. I'm intending to run them at 21 volts at a measured 0.35 amps or so. And no, there is no battery involved. It is designed to mimic a sky light.
I understand I won't get the full 10W. That was the intent in getting the two in parallel.
I was originally going to have them in series but I was concerned that the output voltage from the panels would go higher than what the LEDS would tolerate as the panels can get up to 18V each in full sun. And I couldn't figure a suitable circuit to cap the voltage if it did go over.

All right it seems like the LEDs in question are designed to run off a 24V source.

You'll probably find that running them in series won't do any harm, as the voltage will drop to a safe level once the LEDs are connected. If you're really worried you can connect a 25V shunt regulator in parallel with the solar panels to limit the voltage.

Another thing you could consider is a light pipe, which uses no electronics at all.

[TWilson]

Update:

After some experimenting I decided to go with just hooking the LED up to the solar panels directly. This caused the voltage to peak at 24.2 volts when the panels were cold and a max current draw of 0.78 amps. This is the maximum I was aiming for so I am extremely happy.

Thank you for all the great ideas.

[Kjelt]

Sorry to bring this zombie topic back alive but only encountered this phenomenon myself yesterday and was quite puzzled.

So indeed there needs to be at least 5V but also at least 4A at startup to get the thing going otherwise it will start with low voltage max current.
If you put the PSU on 5V (CV) and 2.5A (CC) and the output load for 12V 0.2A the thing will draw the max 2.5 A at 3.x V for like a minute before the endvoltage of 12V is reached and only then switch back to 5V 0.7A.
This means that this thing is somehow burning up 7W during the unstable phase (ic is getting pretty hot).
Bizar.... IMO unusable for many cases.

[LinuxHata]

These ICs have a bad habit - on startup, they consume a lot of current, so if input voltage sags, the built-in transistor remains open and short circuits the power supply. It is same for other ICs of that company.

[liaifat85]

There is a discussion about a similar problem here.
https://electronics.stackexchange.com/questions/538791/voltage-drop-when-connecting-dc-dc-booster-xl6009-to-my-solar-panel-generating-1
You can check this out.