Help me understand this buck converter

[Mike90250]

point to jflorey2 5-10%
but i still can't recommend super caps

[mberg2007]

All you need is a very simple Voltage Regulator. It can be either a Linear Series VR or a switching VR that can operate from any input voltage from Voc down to about 1.5 above output voltage. So if you use say a 12 volt battery panel and the load is 5 volts, 6.5 volts to 22 volts. Piece of cake, it can be with a simple 3-terminal 5-volt VR like a 7805.

No. A simple voltage regulator will not do the trick. The simplest design will run the 40V/0.5A (20 watts) panel at 5V, which will yield me at most 2.5W (5V*0.5A=2.5W).

A buck converter tries to do it smarter but runs into problems when the panel cannot meet the power needs of the load. The DC/DC will keep decreasing its impedance to try to pull more power from its source. If the source was a voltage source that would work. But it's not - once you start moving to the right side of the VI plot for a panel it becomes more like a current source. So the power goes DOWN as it decreases the impedance. That makes the voltage drop further until you "crash" - hit the lowest voltage the DC/DC will work at. It will then stay there, and not return until the power from the panel at the new lower voltage exceeds the power the load needs. In practice this means the system works fine until a cloud passes in front of the sun, at which point one would see a rapid drop to close to zero power, with no recovery until you were in bright sun. And even then I've seen the panel being unable to recover.

For example if you load is say 5 watts at 5 volts, just use a large panel like 100, 200, or a 1000 watts with a LM7805. That way just after sun rise the regulator will come to life and will last to almost sun set when the panel voltage dips below the 6.5 volt threshold. If you used say a 10 watt panel, you would only get an hour or two of operation around solar noon, and if any clouds or shade occur during that time, it will crash. But if you used a much larger panel, even moderate shade or clouds can still generate 5 watts.

I don't really want to brute-force the problem by throwing a huge panel at it. I figure I have 40 watts of power, I should be able to run 10 watts of load. The panel only delivers 40 watts at its Vmp so it should be driven at that power. Not 5V. You see the problem? Check out the eBay item I linked to. It's a buck converter that has an MPPT dial. It's not MPPT in the sense that it actually tracks that point (at least I don't think it is) but it might be useful to me. My initial post was to hear from people with experience with these units and how they worked and how they might help me get more power out of my panel without resorting to batteries.

-Michael

[mberg2007]

Correct. And it will continue to drop until the input goes back into regulation.

That's what most DC/DC converters do; it's a function called UVLO. (undervoltage lockout.)

Since many loads (lights, fans etc) will reduce their current draw as voltage drops, reducing the output voltage, rather than cutting it completely, can work to reduce total power.

Most DC/DC converters don't do this. Most of the cheap ones on eBay don't. They keep trying to switch on and function even when the load draws much more power than what is available, resulting in some bizarre on-off-brownout type of condition that the system just never recovers from.

Did you see the item I linked to on eBay? The buck with the display and the "MPPT dial"? You know anything about precisely what this does and how?

-Michael

[mberg2007]

Most MPPTs use a buck converter, except the feedback is from the input side instead of
the output side. It finds the panel best operating point, and tries to dump all available
energy into a large sink at the output. Trouble is your loads are fixed, not capable of
absorbing whatever energy is available. A battery does that.

It could discard the excess power as heat. In my case I potentially have 30 watts of waste power but that's fine. I only need 10.

A regulated output buck draws just what it needs to keep up the output load voltage. As
load increases or sun decreases, it will just keep pulling more of the panel available current
until the panel V slides below MPPT voltage. At that point the buck converter will not get
enough power, and will collapse the panel voltage trying to.

Yes this is precisely the problem I am describing. The buck I linked to on eBay seems to have some setting that might be an UVLO voltage. Do you have any experience with this item or similar items? Or an idea about what it does, exactly?

-Michael

[mberg2007]

[B][SIZE=14px]Super Caps are NOT batteries.

Have you noticed that you are the only person here discussing super caps? You don't have to, if you don't want to.

I only mentioned supercaps as a way to smooth out power fluctuations or build up a bit of a surplus current on the input side of the buck. Given the voltage and current we're talking about here, I figured that super caps were the only ones sized for the job. That's all.

-Michael

[mberg2007]

They will, for a short, limited time, behave like batteries, but only for the upper 5% or 10% of their power power curve that intersects battery voltage.
Once below standard battery voltage LVD (11v, 22v, 45v) the gear powered by them will shut down or go bonkers because of low voltage, Still gobs of
un-useable power in the cap, but it cannot work as well as a battery,

BTW you can combine a supercap with a buck converer. This allows you to use up almost all of the power in the supercap while providing a stable output at almost any voltage. Right down to the lowest input voltage the buck will operate at, which is often as low as 4-5V.

-michael

[bcroe]

It could discard the excess power as heat. In my case I potentially have 30 watts of waste power but that's fine. I only need 10.



Yes this is precisely the problem I am describing. The buck I linked to on eBay seems to have some setting that might be an
UVLO voltage. Do you have any experience with this item or similar items? Or an idea about what it does, exactly?

-Michael

The simplest ideal system might use a 7V mpp panel and a linear reg as Sunking described.
The loads would work in good sun and brown out under inadequate. I don't see load
requirement numbers, so its not possible to do calculations.

The nature of a switching supply is to pass energy through, NOT dissipate significant energy
beyond efficiency losses. Switchers have been around since power transistors; I'm not
familiar with the variety of stuff now on the market except as to how they might handle energy.

You have the general understanding, looking for a fix. I see the only reason to use a super cap is to get some
extended running past the time of good sun. That, if the benefit justifies the cost. A secondary benefit could
be some intermittent operation under somewhat inadequate sun, in a manner like the Linear Current Booster.

Keep in mind, your 40W panel will practically never attain 40W. So avoiding more panels requires decent efficiency.
One approach I see is to use a proper MPPT to charge a big cap with the load turned off. The cap must be capable
of operation at the panel Vmax (a problem with super caps) or you must have a way to limit the voltage (clamp or
open the charging path). I'd turn on the load only when the cap is perhaps 75% charged. Now use a simple buck
converter to convert the cap voltage to the required load voltage. Turn off the load when cap voltage is too low
to maintain load regulation. With good sun the system will start properly and keep running till sun is inadequate;
then it will oscillate while gathering what energy is available. good luck, Bruce Roe

[jflorey2]

Did you see the item I linked to on eBay? The buck with the display and the "MPPT dial"? You know anything about precisely what this does and how?

Already answered that. It's a linear (i.e. not on-off) LVD.

[Sunking]

No all you need is a simple Linear or Switched voltage regulator. Only way it will work. If you use a 40 watt panel on a 10 watt load means you do not run or operate more than a couple of hours around noon. But please do not take my word for it as I prefer you to waste a lot of time and money learning that yourself. Keep doing what you are doing. Sooner or later you will figure it out and try a $1 LM-7805 on a hundred watt panel and will work just great most of the day.

[mberg2007]

No all you need is a simple Linear or Switched voltage regulator. Only way it will work. If you use a 40 watt panel on a 10 watt load means you do not run or operate more than a couple of hours around noon.

I *am* using a voltage regulator. I just call it a Buck converter but we mean the same thing. Bucks are more efficient than your LM-7805 but they achieve the same objective. The problem is that the darn thing will collapse the panel and not recover from that situation.

I posted here to find people who knew more about the buck converter I posted a link to. Most bucks have simple regulation of output voltage, and for some CC types you can regulate the output current as well, but this unit also claims to have an "MPPT" setting that can be dialed in. I simply want to know what it does, precisely, and how it works. Forget all about my installation, forget what I'm trying to do, just help me understand what that specific buck converter does.

try a $1 LM-7805 on a hundred watt panel and will work just great most of the day.

As I said I prefer not to put up a huge panel as it takes up a lot of space and because frankly it is ridiculously inefficient considering the 10 watt load. I prefer a smarter, even if costlier, solution.

-Michael

[Sunking]

I *am* using a voltage regulator. I just call it a Buck converter but we mean the same thing. Bucks are more efficient than your LM-7805 but they achieve the same objective. The problem is that the darn thing will collapse the panel and not recover from that situation.

I understand perfectly, but a switching regulator does not buy you anything in this application. It just cost more, more complicated with more things to go wrong. You are essentially using a PC for a watch.

As I said I prefer not to put up a huge panel as it takes up a lot of space and because frankly it is ridiculously inefficient considering the 10 watt load. I prefer a smarter, even if costlier, solution.

Quit being stupid. It is the only way it can be done without batteries.

Let's say you want this 10 watt device to run 24 hours a day. Once you take efficiency of charging batteries into account it would take a 100 watt panel to generate 240 watt hours of usable power for the device.

A solar panel does not generate its specified power from sunrise to sunset. It only gets to about 85% of rated power, but only for a few minutes around noon. A 100 watt panel does not generate 10 watts of power until a few hours after sun rise, and falls below 10 watts in the afternoon. If you were to use say a 20 watt panel, it is only going to work for an hour or so in a 24 hour day. If you want any significant run time, you must use a brute force panel.

It does not take a MPPT converter to do what you want. You cannot take advantage of MPPT because your POWER HAS NO WHERE TO GO after 10 watts. The most power you can pull is 10 watts from a 10,000 watt panel. Using anything more than a LM-7805 is just a waste of money and your time. You need something like a 100 to 200 watt panel so by early morning it can generate the 10 watts you need to run the device and run until late afternoon when the panel power drops below 10 watts and your toy goes dark.

Bottom line is this. You can use a linear regulator like a 3-pin $1 LM-7805, or your way using an expensive complicated Buck Converter. Both will perform equally. Use a 20 wat panel and you get an hour of run time around noon. Use a 100 watt panel or more and you wil have several hours of run time.

But please do it your way. I like seeing people waste their time and money after they have been told. Sooner or later you will figure it out and have that Homer Simpson moment.