I downloaded it and opened it with 7-zip and it is fine. Stupid windows zip out of date I guess.
Do these files become new symbols? If so, where do I place them?
Thanks,
Mike.
It's still not worth making a buck switcher for 12v to 12v system.
If your 12v panel makes an open circuit voltage of 21v it's probably a crystalline type that produces peak power at about 17v.
On a 12v to 12v system you need to protect the panel with a reverse protect diode, even a decent schottky will drop about 0.4v and you will lose about 0.3v in wiring and terminations etc.
So with your panel at peak power output your SMPS will receive 16.3v input. Now assume your battery is at a nominal 13.8v operating voltage. You have a SMPS with 16.3v input and 13.8v output. Now if you can make a SMPS as good as the commercial one I designed you might get 90% efficiency at 16.3v->13.8v.
So that's (16.3 * 0.90) / 13.8 = current gain of 6.3%.
So you think "that's ok, it's still worth making the MPPT to get 6.3% more current into the battery".
But it's rarely worth it, because if the battery is close to full charge (over 14v) it won't produce much gain at all. Also, the max panel power is rated at say 20W (17v 1.17A) BUT if you connect to a lower load voltage the panel power is on a curve and it will make more current into a 13.8v load than it will into a 17v load.
See this curve, it's pretty typical;
So if you directly connect the panel to the 12v battery you will get about 6% more current into the battery because it is driving a lower load impedance, which is the same power output as you got from the MPPT.
I've already said it above, it's not worth doing on a 12v->12v system which is why there are practically no 12v MPPT devices on the market. And you completely ignored my post, you are asking for people who have experience to help and then ignoring them...
Last edited by Mr RB; 29th September 2009 at 02:17 PM.
Nope, I think you must have missed my reply to you and the part in my post where I stated that I have 2 batteries I can use to make it 24V. I also posted a circuit that is an attempt to make a 12-24V system. I did take on board what you said, did reply and did change my goal because of it.Originally Posted by Mr RB
It's still not worth making a buck switcher for 12v to 12v system.
I've already said it above, it's not worth doing on a 12v->12v system which is why there are practically no 12v MPPT devices on the market. And you completely ignored my post, you are asking for people who have experience to help and then ignoring them...
Mike.
Fine, I'll put in my two cents:
You seem to be neglecting the possability of it being a solar lighting app. Some of those see deep discharge on the batteries overnight after a cloudy day and a bright day following then would be bucking 17V into 10V (dead lead acid) for a gain of 70% with a 95% conversion efficiency possible.
And yes, it is possible with a properly designed switcher and a synchronous reverse polarity diode.
Dan
Last edited by Ubergeek63; 29th September 2009 at 05:18 PM.
Whoops sorry Pommie! I was a bit hasty reading through the thread.
OK, if you want to make a 12->24v converter (which must require a SMPS) you may as well make it a MPPT to get the extra bit of power into the batteries.
The BUZ71 is way obsolete in terms of making high performance SMPS. There are many much better NFETs available. One of the good things about going 12v->24v boost is that you won't gain much from synchronous rectification so you can just use a schottky diode for the rect, which makes it a lot simpler.
To make it a MPPT boost circuit you need to regulate the switching to maintain the INPUT voltage, and keep that at about 17v. That will operate the solar panel at or near max power output. Then if you need the circuit to act as a battery charge controller you use a second system to shut it down when the battery is full, or keep it running and dump load when the battery is full.
You won't find many schematics for boost SMPS that regulate input voltage however.
There's a couple of ways if you are using a PIC as controller;
1. make a fixed period ON pulse whenever the input rises above 17v, this will work fairly well in boost as the input voltage is constant and known, as is the input capacitance and the input current is constant
2. generate a PWM signal and monitor the input voltage and keep adjusting the PWM so that the input voltage remains at 17v (which has its own issues)
Ubergeek63- I agree that with a low load voltage a 12v->12v MPPT can be worth it. Driving 12v pump motors is another good example because the MPPT will make a lot more start current for the motor (when load volts are REAL low) than a panel alone will make, in that case you can use a smaller panel and/or remove the battery altogether.
I've ordered some IRL540s to play with and I had assumed that I could monitor the battery current and alter the PWM to maximise it. Wouldn't this find the max power point? It would actually be easier to regulate the input voltage but I assume that this would limit you to a certain type of panel or is this the same for all panels?
Mike.
Hello. I'm an EE experienced in switcher design but new to the solar power arena. Having built a couple MPPT have come to the same conclusion regarding 12v/12v so I hope to benefit from advice of those here more experienced in the area than myself.
I understand benefits of 12v solar on 24v batteries using boost. However for small (100-200w) systems consumer 12v battery and inverter seems most economical.
Currently using four 22v peak 30w amorphous panels in parallel with a buck converter. Would there be advantages to reconfiguring for 44v into the 12v battery?
Will one or the other make more effective use of morning and late afternoon sun? Or will going back to PWM make more sense. Any opinions?
Last edited by john warfin; 30th September 2009 at 10:03 AM. Reason: typo
It really has to do with the panel output voltage during these low light times. If it is close to the battery voltage already, then MPPT isn't going to do a whole lot.
In general, running your panels at higher voltages would be the most efficient. You'll have less I2R losses getting to the controller, and more voltage headroom for the controller to play with during the lower light scenarios. This all assumes you're using MPPT though. Standard PWM controllers should be run as close to the battery voltage as possible to gain the most power.
Technically a MPPT should adjust to get max power into the battery. But it's not necessary, since the panel peak power output is usually around the same voltage even for different insolation levels, so even at low light levels the panels will still make max power near 17v. So you can regulate for input voltage which is much easier and gets you very close. To do it the other system is more complex, and you need to text the max power output by "detuning" the system away from maximum, to find where the max is, then tune it back to maximum. That adds some losses to the complexity.
As for different panels, you said you already had the panel. So tune it for that. If you are making a device for general use or to sell then stay away from boost, just use 24v panels to charge 24v batteries.
[QUOTE=Mr RB;795809]So you can regulate for input voltage which is much easier and gets you very close./QUOTE]
If done properly doesn't that require sensing light level and complex algorithms? I've been maximizing battery current which involves neither. Just a small extra wire from micros ADC to battery.
I haven't done much testing with the cheaper amorphous panels but they are rarely used for MPPT use anyway as their max power is delivered closer to 15v than 17v.If done properly doesn't that require sensing light level and complex algorithms?
But with the crystalline type that produce their max at 17 or so volts this Vin is fairly constant. The shape of the V/I curve (with max power produced at about 17v) is tied to the physical properties of the panel which remain pretty constant. If you reduce the light level the current output drops but the V/I curve stays pretty much the same. So it still makes around 22v open circuit in full sun and 22v open circuit in cloud. Likewise the max power out in full sun is about 17v and the max power out when cloudy is still about 17v.
When I was working on the commercial MPPT design I had two 12v 80W panels setup for a few days, and I run them into a high power adjustable load and test meters. The max power out was always around the same panel voltage in cloud or full sun etc, and I looked into the option of making a MPPT that just regulated input voltage which is a valid system provided you have an adjuster so set the input voltage when it is first set up and tested. (The peak power point may be different in VERY low light levels but the panel only produces an insignificant % of its total power under those light conditions anyway.)
Testing current into the battery is an excellent system because it auto adjusts for things like the solar panel cable losses and SMPS factors etc. But to "track" the max you need to detune away from max, in both directions, regularly to keep checking that it was ON the max. There is very little difference in total long term power output between a properly adjusted Vin regulated SMPS and a tracking SMPS, so for any small simple system (like Pommie's) I would just go with regulated Vin.
Unloaded voltage makes perfect sense to me however the loaded statement seems somewhat counter-intuitive. Why are these guys spending so much time and money on light sensing sensing systems to stay on the cliff edge? It should be easy to verify with a couple simple experiments.
Apparently I've got a lot to learn.
Hi there, I have boost converter software that helps during the compensation network design phase that was developed as a result of finding errors upon errors on articles found on the web about compensating a boost converter. If you are interested i could send you a copy. It takes into consideration just about everything like inductor esr, capacitor esr, switch resistance, input source resistance, etc., and unlike other types of software this works in the time domain so you can see the results of a component selection in the same way you would if you built a converter and used that component. It also runs very fast (under 1 second per run) because it is made from a set of pre solved equations for a boost converter. The only requirement is that it runs under Windows.
Last edited by MrAl; 2nd October 2009 at 03:41 PM.
I get "invalid or corrupted file" box when I try to download.
Creationists have a world of evidence
Light sensing is completely irrelevant for MPPT! Light sensing is used for tracking the angular position of the sun, to mechanically move the panels (solar tracking) to point them in the direction where they get the most light.
Max power point tracking cares nothing about light it simply adjusts the Vin of the SMPS to provide the maximum power output from the SMPS. It's an adjustable gearbox, that self-adjusts the gear ratio to give the most power output.
As for the simple experiments, yes I did those, like I said I run 2 80W panels for some days into adjustable load and tested power point under varying conditions.
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