question about switching regulators

xsr_Gx

So, i am using a LM2651 (http://www.national.com/pf/LM/LM2651.html#Overview) switch regulator that takes 4-14V and outputs an adjustable voltage.

To know the value of the inductor, the datasheet uses this formula:

L = (Vin - Vout)*Vout / ( Vin x Iripple x 300khz)

they said that a reasonable value is to set up the ripple current to be 30% of the DC output current.

The input to this regulator comes from a 6V, 850mA panel. How do i determine those values? If you look in the datasheet, there are some values for teh resistors, capacitors and the inductor based on the typical application. Does this mean that using these values, we get the features/specs they mentioned in their datasheet?

Any help will be appreciated. Thanks

lorelec

Re: question about switching regulators

Ripple current of anywhere from 20-50% is typical, and determining what its exact value should be isn't usually all that critical unless there's a reason you need it to be. The lower the ripple current, the greater the value of your inductor will need to be (in high current applications, inductors usually get physically larger, heavier, and more expensive when you increase their inductance). At 300kHz and in your current range, the inductor's going to be pretty small no matter what you do. On the flip side, a smaller inductor will cause the power supply to enter discontinuous mode (DCM) earlier (at a higher output current), and may result is slighlty worse regulation. A larger inductor will increase conduction losses some, a smaller one can increase switching losses a little. It's all a tradeoff.

So I would do as the datasheet recommends: plug your numbers into the formula. Use your highest expected Vin. Take 30% of your highest expected Iout value and use it for Iripple. The result will be in henries.

Marc

Solar Guppy

Re: question about switching regulators

Won't Work ...

Switching regulators do not have regulation on the input side, which is required for PV

As soon as the load on the regulator is more than the PV output, the regulator will over load the array and the voltage will collapse, probably causing the regulator to reset when the voltage in drops below the minimum required to operate

lorelec

Re: question about switching regulators

Just make sure the load is sized so that it doesn't drag the PV beyond its rated capacity (good advice when loading any power source). Then, even at 100%D/C (the LM2651 is limited to 95%) the PV voltage will still be above the undervoltage lockout of the regulator.

Marc

mike95490

Re: question about switching regulators

lorelec wrote: »

Just make sure the load is sized so that it doesn't drag the PV beyond its rated capacity (good advice when loading any power source). Then, even at 100%D/C (the LM2651 is limited to 95%) the PV voltage will still be above the undervoltage lockout of the regulator.

Marc

And tell the weatherman to not send any clouds. Any shadow, bird, plane, cloud will glitch the solar, and thus the regulator, unless you are only at 10% of the max solar

BB.

Re: question about switching regulators

Unless this regulator's input is buffered by a battery--just running it on a solar panel will just about be useless--unless the solar panel is way over-sized... If you can use an over-sized solar panel, no battery, and live with power interruptions (shadows, clouds, nighttime)--it can work--but a simple 3T-regulator (analog) would work too.

The reason to use Maximum Power Point Tracking type power supplies is to maximize energy from the solar panels (for example) to the load--which usually would be a battery, utility grid, or well pump where they can absorb excess power for use later or to run a variable load (such as pumping).

If this is to drive a small electronic sensor, etc., that has no power storage--there is not much reason to use a switching power supply. Yes, in theory, it would be more efficient (85% vs 50% for a 6 volt to 3 volt conversion)--but it may not be worth the complexity.

Another issue with digital electronics is to get clean resets/reboots and to save the data before the power goes away (state or data saving, closing files, etc.).

It can be done--but need to look the whole system to ensure that all the pieces play together nicely.

-Bill

lorelec

Re: question about switching regulators

We really don't know anything about what he's trying to power with this regulator. Given the size of his PV, I'm assuming it's something pretty small -- which might work just fine even with weather issues factored in. If not, then he should increase the size of the PV. I've done it with a variety of switching regulators, and can tell you it works so long as you match the load to the PV and allow a little fudge factor. To say it flat out "won't work" isn't helpful at all. A linear regulator is certainly a possibility too, as Bill said, especially if you need low-noise -- but again you're going to be constrained by the often variable output of the PV.

Marc

BB.

Re: question about switching regulators

I think this is related to his other threads about a remote sensor network of a bunch of small sensors (mic, camera, etc.) for monitoring field sites.

From the other thread, was looking for 4 hours a day "on time" for some sensors. Night-time use for others. All would check in / dump data back to a central server (if I understood the setup/questions correctly). All out in Santa Cruz CA -- which can have some pretty foggy/cloudy weather (depending on seasons and geographic locations).

There are a bunch of different devices and power consumption. Knowing what would is acceptable power... It is possible that the solar array charging a "super cap" with some sort of down converter/regulator would be enough. However, I would still wonder if a solar panel + regulator without any power storage/buffer would be a pain for digital devices with state information that needs to be stored/transmitted.

Certainly, an over sized solar panel can make up for low solar radiation (cloudy weather). But--I think that some sort of local energy storage is really needed. Whether battery or capacitor. The type of regulator is probably a secondary decision point (analog, digital, up/down/up-down converting, etc.).

At this point, I will be the first to say I don't know what I don't know.

-Bill

Solar Guppy

Re: question about switching regulators

lorelec wrote: »

We really don't know anything about what he's trying to power with this regulator. Given the size of his PV, I'm assuming it's something pretty small -- which might work just fine even with weather issues factored in. If not, then he should increase the size of the PV. I've done it with a variety of switching regulators, and can tell you it works so long as you match the load to the PV and allow a little fudge factor. To say it flat out "won't work" isn't helpful at all. A linear regulator is certainly a possibility too, as Bill said, especially if you need low-noise -- but again you're going to be constrained by the often variable output of the PV.

Marc

It won't work due to the VI curve of a solar panel and how feedback on the regulator works. As soon as a load exceeds the VMP point, increasing current draw will immediately cause array collapse. All these regulators know only one thing, voltage out low, increase the duty cycle = collapsed array

Its very likely it won't even power up. For this to work with solar, your control loop has to be off the input, the output is secondary

lorelec

Re: question about switching regulators

Solar Guppy wrote: »

As soon as a load exceeds the VMP point, increasing current draw will immediately cause array collapse.

If you keep the PV operating in its constant voltage region (i.e. to the right of the knee on its power curve) by loading it down at no more than its Imp (making allowances for weather, etc.), then there's no problem.

You have the same issues with linear regulators. Heavier load is going to cause the pass transistor to turn on harder, which will eventually drag the PV into its constant current region. Doesn't mean you can't derive a regulated voltage from a solar panel. Just make sure you match the load to the panel and allow for some wiggle room so that you never approach the Imp.

Not that battery or some other form of backup wouldn't be a good idea (maybe the poster is using backup, too?)...

Marc

Solar Guppy

Re: question about switching regulators

lorelec wrote: »

If you keep the PV operating in its constant voltage region (i.e. to the right of the knee on its power curve) by loading it down at no more than its Imp (making allowances for weather, etc.), then there's no problem.
Marc

There in lies the problem, these regulators are output based feedback, not input, so as they are intended use the won't work ... One has has to deal with power the solar starting from zero and startup won't happen, possibly ever depending on how large the panel to load is ...

BB.

Re: question about switching regulators

lorelec wrote: »

If you keep the PV operating in its constant voltage region (i.e. to the right of the knee on its power curve) by loading it down at no more than its Imp (making allowances for weather, etc.), then there's no problem.

I guess that is where I am confused... P=I*V; If you are in the constant current region and the regulator has to supply a tiny increase in output power, the regulator will pull that little bit more current--which will collapse the solar array's output (I more or less constant, falling Volts, means Pin available collapses with Vin collapse). More or less positive feedback making a basic Flip-Flop circuit.

data sheet for a crystalline silicon panel

To my way of thinking--you have to keep to the RIGHT [fixed--Marc's next post corrected my error] of the Vmp Power knee (array needs to be higher wattage than "optimum" and need to be way below Imp of the array in normal operation)... So that when the regulator needs to supply a little bit more power on the output, it can pull an increase in current without collapsing the voltage (i.e., operating in the voltage source area of the IV curve).

One of the start up issues, that I would see with a "simple" switching power supply is that it will try to draw maximum current until the output is stable (voltage and current). Otherwise, it will stick near Vin minimum (4 volts or so) and never develop enough power (I*V) to get the output stable until the "Vin" can get back above Vmp.

If there was a large capacitor (or battery) on the input, and the power supply was held off until Vin>Vmp--then turned on--it could then work (unless there was not enough sun and the input voltage collapsed again).

Again, not saying that this cannot work--but a lot of thought about the load's power requirements and the overall operational requirements for the system would have to be nailed down.

-Bill

lorelec

Re: question about switching regulators

SG,

Like you said, these regulators know only one thing: output voltage drops, duty cycle increases. So why does it matter that the feedback comes from the output? The output voltage drops for lack of current through a given load. Duty cycle increases to compensate. If your load is light enough, there's no way, even at 100%D/C, that you're ever going to drag the PV into the constant current region. Try putting a 470 ohm resistor (or whatever) across the output of a panel, and tell me what the voltage reads. I'll bet you it's not below the Vmp.

As for startup, you'd be hard pressed to find a regulator that wouldn't load the PV enough for the few uA or Ma that it needs to get going. No buck regulator is going to immediately start its duty cycle out on the high end -- that would be a recipe for destruction of whatever's attached to it. The regulator in question (LM2651) features soft-start, too. From the data sheet:

"The switch duty cycle, which is controlled by the level of the current limit,
starts with narrow pulses and gradually gets wider. At the same time, the output voltage of the converter increases towards the nominal value, which brings down the output voltage of the error amplifier."

I've had no problems using ICs from Maxim and Linear to generate low current, regulated buck supplies from a solar panel. They work without a glitch, from dawn til dusk.

Bill,

Sounds like you have it right, except that the constant current region is to the left of the knee, and that's what we want to avoid if we're using the typical negative feedback regulator. Once we're there, an increase in load will only draw us deeper in, and will drop the voltage in the process.


Marc

BB.

Re: question about switching regulators

Marc,

Oops... You are correct--to the right of the knee is voltage source region.

-Bill

xsr_Gx

Re: question about switching regulators

Thanks a lot guyz. You are all very informative.