Author Topic: MPPT function – hardware implementation  (Read 12896 times)

0 Members and 1 Guest are viewing this topic.

jack11

  • Jr. Member
  • **
  • Posts: 95
  • Country: us
MPPT function – hardware implementation
« on: March 29, 2013, 03:52:12 PM »
It's not difficult for me to understand the mppt concept of sliding up and down the PV array's I-V curve in a conventional mppt controller, computing the array's output power at each point, and selecting that operating point where the V*I product is maximum for given conditions.
But, what's less clear to me is how this is implemented in the actual hardware at the front-end of a mppt controller, this operation of a PV array at different operating V-I points.

I am thinking some sort of a “programmable???” resistor (from very low ohm to some pretty large ohm value) that makes the array operate with variable resistive loads, at different V-I points (then the microcontroller can multiply these Vs*Is to find the max power point).
Another way of saying this may be to operate the array at the point where its output resistance is always tracked and matched by the input resistance of the controller, for max power transfer. Generally, from the array's I-V curve, I believe I could compute and plot the array's output resistance curve, just like I can compute and plot its power curve, at different operating V-I points.

Or, this could be done by the use of some sort of programmable resistance-matching DC-to-DC converter???

Or, some other ways???

Anyone experienced in designing mppt controllers, that can comment on this?

Also, I have seen some mppt implementations that do not measure the array's V and I parameters or maximize its output power, but instead measure some load parameters (load V and/or load I). Then they drive the array in a way to maximize the power transferred to the load as inferred from these load-based measurements (no V*I multiplier needed). But they still would need some similar hardware implementation in the controller's front-end to change the operating point of the array???

joestue

  • Hero Member
  • *****
  • Posts: 1759
  • Country: 00
Re: MPPT function – hardware implementation
« Reply #1 on: March 29, 2013, 05:48:05 PM »
basically MPPT for solar amounts to making a current to current converter.

because the battery voltage stays the same, you change the power flow by changing the current, the dc-dc converter has to be operated in current mode to do this.

in practice, you change the pwm duty cycle running into a voltage or current mode dc-dc converter, and measure the power flow out of the solar panels, or to simplify things, into the battery.
then you have some kind of an algorithm ride the curve.

take a look at this
http://www.timnolan.com/index.php?page=arduino-ppt-solar-charger
My wife says I'm not just a different colored rubik's cube, i am a rubik's knot in a cage.

altosack

  • Jr. Member
  • **
  • Posts: 92
Re: MPPT function – hardware implementation
« Reply #2 on: March 29, 2013, 06:18:37 PM »
Commercial MPPT controllers use what is called a buck converter, which is a DC-DC converter that, given a input voltage and current, converts it to a lower voltage and higher current.  It does this by varying the duty cycle of the primary power transistor (usually a MOSFET, but it could be an IGBT for high voltage applications); the ratio of the output voltage to the input voltage is equal to the percentage of time the transistor is on, called the duty cycle.  This switch is turned on and off very rapidly, usually in excess of 100 kHz, to reduce the size of the inductor and capacitors needed for the circuit (they don't need to store as much charge if it operates very rapidly).

This rapid cycling is called PWM (phase width modulation), and is what is controlled by the software in the MPPT controller.  You don't actually need to compute the V*I product; you can just measure the output current and maximize that, since the output voltage (the battery) will vary quite slowly compared to the update rate of the controller.  However, since the update rate is usually slower than 10/s, >100 ms is an eternity for even a slow 8-bit microcontroller doing software double precision floating point, so with all that time on your hands you might as well compute the product to have more information to display.

OperaHouse

  • Hero Member
  • *****
  • Posts: 1308
  • Country: us
Re: MPPT function – hardware implementation
« Reply #3 on: March 29, 2013, 08:30:30 PM »
Depending on your power requirments there may be suitable switching converters on ebay that can be modified to work at the power point.  These you can set to the maximum voltage you want the battery to see.  The modification then varies the duty cycle so panel voltage never drops below a set panel voltage.  As mentioned before power point varies mostly with temperature.  A seasonal adjustment gets you 90% of the advantage of actual tracking.  $15 on ebay will get you a converter that will beat the pants of anything you can buy for four times that amount.

A buck converter is not the only way to go.  For those with the random 12V panel and a 24V system a boost converter is ideal.  Much higher power at a lower cost and generally more efficient.  The same power point mods will work. 

rossw

  • Hero Member
  • *****
  • Posts: 834
  • Country: au
Re: MPPT function – hardware implementation
« Reply #4 on: March 30, 2013, 02:46:46 AM »
A seasonal adjustment gets you 90% of the advantage of actual tracking.

I've got actual, real-world measurements that say otherwise.
I have 6 identical arrays. 2 are fixed to due north and adjust seasonally ONLY. The others track.

Before we get to the graph, explanation:  only read the right hand side (for reasons that will become evident).
The blue line is the solar power available as actually measured by a precision pyranometer.
The yellow line is the power measured from one seasonal-only adjustable array.
The red line is the power measured from one tracking array.
Remember, this is the ACTUAL POWER MEASURED. Both arrays feeding MPPT chargers (Outback FlexMax 80)
The big drop at 11am was when the batteries went into float.

The entire area under the red curve, and above the yellow, is power I wouldn't have got without tracking. (And it's symmetrical, the same thing happens at the end of the day if I can use it)

Madscientist267

  • Hero Member
  • *****
  • Posts: 1181
  • Country: us
  • Uh oh. Now what have I done?
Re: MPPT function – hardware implementation
« Reply #5 on: March 30, 2013, 04:11:47 AM »
Quote
A buck converter is not the only way to go.  For those with the random 12V panel and a 24V system a boost converter is ideal.  Much higher power at a lower cost and generally more efficient.  The same power point mods will work.

The only thing worse than no information is bad information.

Using a boost converter in that way is just irresponsible. LOL

There's a reason MPPT starts with higher voltage PV (or wind) and ends in a lower voltage battery. So while yes, it will WORK, Its highly INEFFICIENT to do it the other way.

Steve
The size of the project matters not.
How much magic smoke it contains does !

OperaHouse

  • Hero Member
  • *****
  • Posts: 1308
  • Country: us
Re: MPPT function – hardware implementation
« Reply #6 on: March 30, 2013, 04:34:20 AM »
" Both arrays feeding MPPT chargers "

Are we talking apples and oranges here?  I'm not talking about tracking the sun but rather tracking the voltage of the panel vs a fixed power point.

Mad......I think you have been up to long.  Get some sleep/

rossw

  • Hero Member
  • *****
  • Posts: 834
  • Country: au
Re: MPPT function – hardware implementation
« Reply #7 on: March 30, 2013, 04:39:51 AM »
Are we talking apples and oranges here?  I'm not talking about tracking the sun but rather tracking the voltage of the panel vs a fixed power point.

When you say "Season adjustment" and "tracking" without clarifying what "tracking" you meant, particularly given you'd used the term "MPPT", I assumed you meant "mechanical tracking of the arrays".

Even so, the numbers I collected before and after going MPPT indicate FAR more gain day on day, most of the day, almost every day, from MPPT. And the gain is cumulative. Better aligned panels get more power, and MPPT gives you around 30% more watts over just feeding the panels directly to the batteries. I discussed this quite some time back (>2 years?) and provided hard numbers.

MPPT might seem expensive, but if you have any non-trivial array, it's well worth the cost IMO.

OperaHouse

  • Hero Member
  • *****
  • Posts: 1308
  • Country: us
Re: MPPT function – hardware implementation
« Reply #8 on: March 30, 2013, 04:59:40 AM »
I wish I could track the path of sun.  Technically, my panels aren't even on my property so I have to keep them mobile and low profile.  Data is nice.  Maybe this summer I will get around to having two side by side panels, One with a fixed power point voltage and the other that tracks the voltage.  True it was an economy of words with a discussion of controllers with  "some sort of a “programmable???” resisto."r

jack11

  • Jr. Member
  • **
  • Posts: 95
  • Country: us
Re: MPPT function – hardware implementation
« Reply #9 on: March 30, 2013, 03:01:30 PM »
So, on a high level, according to timnolan.com (thanks joestue for providing this link), the answer seems to be a dc-dc converter in which the voltage (or maybe current) conversion ratio can be controlled by a microcontroller, by modulating the pulse width input to the power transistor. Then, for a fixed system (battery) voltage, this varying ratio means that the PV array may be set to operate at any voltage one wants (0 to Voc of the array, or really Vbatt to Voc).

Also, based on some other posts here, I have a feeling that there is more to the actual circuit to be built. However, I'd like to eventually build such a controller to try different mppt algorithms, or for some other experimental reasons. Perhaps I can control the dc-dc converter via an I/O port of a standard PC, if the bandwitdh of the control signal is not too high.

I could't find on timnolan.com any references to a specific dc-dc converter that may be used for this mppt purpose, or maybe some evaluation board the manufacturer sells. Can anyone recommend a manufacturer/model of a dc-dc converter that may be used pretty much off-the-shelf to experiment with this?

DamonHD

  • Administrator
  • Super Hero Member Plus
  • *****
  • Posts: 4125
  • Country: gb
    • Earth Notes
Re: MPPT function – hardware implementation
« Reply #10 on: March 30, 2013, 03:25:55 PM »
Just FYI I happened to run across this today in my Twitter feed:

http://dangerousprototypes.com/2013/03/30/app-note-get-the-maximum-power-out-of-a-solar-panel/

Rgds

Damon
Podcast: https://www.earth.org.uk/SECTION_podcast.html

@DamonHD@mastodon.social

Treehouse

  • Jr. Member
  • **
  • Posts: 69
Re: MPPT function – hardware implementation
« Reply #11 on: March 30, 2013, 07:00:21 PM »
I seen this a while ago, nice simple one chip design too bad a dip version is not  avalible.. :(

 http://www.linear.com/product/LT3652

Madscientist267

  • Hero Member
  • *****
  • Posts: 1181
  • Country: us
  • Uh oh. Now what have I done?
Re: MPPT function – hardware implementation
« Reply #12 on: March 31, 2013, 01:18:24 AM »
Quote from: OperaHouse
Mad......I think you have been up to long.  Get some sleep/

No, Just been feeling a little too "phase width modulated", whatever that is. Perhaps you could explain this mysterious new technology to us?

Steve
The size of the project matters not.
How much magic smoke it contains does !

joestue

  • Hero Member
  • *****
  • Posts: 1759
  • Country: 00
Re: MPPT function – hardware implementation
« Reply #13 on: March 31, 2013, 02:00:27 AM »
Quote
A buck converter is not the only way to go.  For those with the random 12V panel and a 24V system a boost converter is ideal.  Much higher power at a lower cost and generally more efficient.  The same power point mods will work.

The only thing worse than no information is bad information.

Using a boost converter in that way is just irresponsible. LOL

There's a reason MPPT starts with higher voltage PV (or wind) and ends in a lower voltage battery. So while yes, it will WORK, Its highly INEFFICIENT to do it the other way.

Steve

this is utter nonsense, a boost converter is a buck converter working backwards.
both have identical power inductor, and power switch requirements.
however, most people just assume that you can throw any mosfet at an inductor and a diode and think it will work.
both converters have identical capacitor requirements as well.

My wife says I'm not just a different colored rubik's cube, i am a rubik's knot in a cage.

Madscientist267

  • Hero Member
  • *****
  • Posts: 1181
  • Country: us
  • Uh oh. Now what have I done?
Re: MPPT function – hardware implementation
« Reply #14 on: March 31, 2013, 03:12:36 AM »
Have any of you ever even USED a boost converter?

I love the "armchair engineering" going on here...

Boost converters have their uses, but should ONLY ever be used when "the right way" is either impossible or highly impractical. Getting 19V for a laptop in a car is an acceptable case, as going from 12V DC to 120V AC just to go back to 19V DC is just a waste of power (and more importantly in a car, extra equipment and wiring to get in the way).

The components may be the same as in a buck converter, but they are subject to higher currents. Switching losses increase as a result of switching higher currents. Anyone who has ever used a boost converter knows that they run hotter than a buck converter driving identical wattage for this very reason.

They should be considered a last resort for most circumstances. Charging a battery that is a higher voltage than the PV panel by going through a boost converter is like pushing your car away from the gas pump after you fill it up: Stupid.

Unless of course something else is wrong with the car...

Steve
« Last Edit: March 31, 2013, 03:20:47 AM by Madscientist267 »
The size of the project matters not.
How much magic smoke it contains does !

DamonHD

  • Administrator
  • Super Hero Member Plus
  • *****
  • Posts: 4125
  • Country: gb
    • Earth Notes
Re: MPPT function – hardware implementation
« Reply #15 on: March 31, 2013, 05:14:05 AM »
I think that this thread is getting a little personal.

Lots of converters of both flavours, not just for PV, do exist out there commercially.

Rgds

Damon
Podcast: https://www.earth.org.uk/SECTION_podcast.html

@DamonHD@mastodon.social

OperaHouse

  • Hero Member
  • *****
  • Posts: 1308
  • Country: us
Re: MPPT function – hardware implementation
« Reply #16 on: March 31, 2013, 10:39:25 AM »
I had an unrelaated post ready.   I think I'm just going to say it is tax time and that will keep me busy for two weeks.  After that I will be posting some fun inverter projects.  One using my favorite ebay boost converter.

oztules

  • Hero Member
  • *****
  • Posts: 1477
  • Country: aq
  • Village idiot
Re: MPPT function – hardware implementation
« Reply #17 on: March 31, 2013, 05:56:44 PM »
"Have any of you ever even USED a boost converter?

I love the "armchair engineering" going on here..."

Yes I have designed and built my first boost converter here:  http://www.fieldlines.com/index.php/topic,128250.0.html

I didn't know what has happened to all the files in the old system, but most of them are there.

It shows that even someone who does not know what they are doing, can design and build a booster that can run up to 60A without problem. Without half the FETs on board, even 200-300 watts runs without heat sinks for short periods, a modest heatsink will do for long term..... and do it all in half a day.

I don't see where your information comes from, but as I see it, a step up transformer is no worse than a step down transformer, and the same applies with boost and buck .
Higher currents just means thicker wires and more or lower Rds on fets to keep the loses the same.

If the village idiot can design and build one in a day, on a remote outpost of civilization, it can't be that hard, or difficult to achieve good results to charge a 12v battery from another 12v battery at 30-50 amps, or step up to whatever you need to do.

Thats not to say I'm not stupid..... coz I am.

There were some more stories I did on that converter stuff, but I don't seem to be able to find them or their files.

And this is not that smart: "Charging a battery that is a higher voltage than the PV panel by going through a boost converter is like pushing your car away from the gas pump after you fill it up: Stupid."

I recall Flux used boost converters to good effect doing exactly the same as you propose, but with wind not solar,... I don't for a micro second consider him stupid.

Maybe you have the comfy arm chair.



.....................oztules
Edit: spelling is not my strong point some times.... I think the keys are too close together.... my excuse and I'm sticking to it.


« Last Edit: March 31, 2013, 06:01:32 PM by oztules »
Flinders Island Australia

Madscientist267

  • Hero Member
  • *****
  • Posts: 1181
  • Country: us
  • Uh oh. Now what have I done?
Re: MPPT function – hardware implementation
« Reply #18 on: March 31, 2013, 05:57:27 PM »
<facepalm>

To each his own... I wouldn't say it is getting personal, but there are certainly more practical and less practical ways of accomplishing a task. I digress either way.

Steve

EDIT - Oz - Its not the transformer, its the silicon. Higher current means more power dissipation in them. I never said it can't be done, just that it should be a last resort. As for running a turbine at a lower voltage just to boost it up before it hits the battery? Maybe I need to look in a dictionary, or its possible that copper prices have dropped to the point that its cost effective to run really heavy wires down a tower... ? You tell me.
« Last Edit: March 31, 2013, 06:07:16 PM by Madscientist267 »
The size of the project matters not.
How much magic smoke it contains does !

oztules

  • Hero Member
  • *****
  • Posts: 1477
  • Country: aq
  • Village idiot
Re: MPPT function – hardware implementation
« Reply #19 on: March 31, 2013, 06:28:35 PM »
"Oz - Its not the transformer, its the silicon. Higher current means more power dissipation in them."

Think carefully about this ..... if we use HV fets, then the Rds on is high, if we use LV fets, the Rdson is lower.... do you see a pattern here?

Eg, for boosting low voltages, elcheapo irf3205 will give you 110A@55V and an RDSon of .008 ohms..... this is pretty low, and a few in parallel will get you down to .004 ohms, and for 2 bucks worth, you can go down to .002 ohms easily....

For bucking, you need much higher voltage ceiling, and would probably keep to 200v plus for your fets , and you will need to go with higher Rdson chips. eg even if you go for only 100v 27A chips (IRF540 etc) then the Rdson is up to .055ohms..... so if you doubled the current for the 3205, or halved the current for the 540, the losses in the 540 would far exceed the losses in the higher current 3205 for twice the current. ( the 3205 could run 6 times the current for the same channel losses)

A lot of very low Rds on chips have been developed for push pull inverters (12vdc-240vac hobby inverters), and are as cheap as nuts, and very high current ratings, and very low Rdson.

I think your argument needs supporting evidence before I can take it as a serious excuse to not see boosters as an effective solution if the environment requires it.


............oztules

Flinders Island Australia

Madscientist267

  • Hero Member
  • *****
  • Posts: 1181
  • Country: us
  • Uh oh. Now what have I done?
Re: MPPT function – hardware implementation
« Reply #20 on: March 31, 2013, 07:52:22 PM »
Ok, let's say I give you the silicon. What about the ridiculous cost of the copper to do the lead ins?

I'm sorry, but I fail to see any legitimate use of a boost converter to charge a battery from a turbine or PV. Especially PV. Its too easy to just reconfigure the panels for a higher voltage and then go DOWN to the voltage you need for the batteries. At least with a turbine, its set up to deliver a certain nominal voltage during the build. A later upgrade to the storage would render the turbine useless without a boost converter if the voltage of the bank were increased. But to design a turbine from the git go with a lower voltage output than the bank its going to charge with the idea in mind to use a boost converter to make it work, is, you guessed it, stupid.

Steve
The size of the project matters not.
How much magic smoke it contains does !

rossw

  • Hero Member
  • *****
  • Posts: 834
  • Country: au
Re: MPPT function – hardware implementation
« Reply #21 on: March 31, 2013, 08:14:34 PM »
What about the ridiculous cost of the copper to do the lead ins?

Don't plan to get involved in this, however from the sidelines - this isn't a problem at low power levels.
If you had say, a 12V/80W panel and wanted to top-up your 24V batteries with it, it's a fairly modest cable.

My own mid-sized arrays (600W per array) are arranged for a nominal 100Vmp to keep current down and I would consider boost converters in this application quite impractical given the 60m cable runs.

oztules

  • Hero Member
  • *****
  • Posts: 1477
  • Country: aq
  • Village idiot
Re: MPPT function – hardware implementation
« Reply #22 on: March 31, 2013, 08:32:20 PM »
Gee Steve,
The world is a much more complicated place than you seem to envisage it.

I am one of the stupids... I designed a 48v system with a turbine that cut in at a nominal 24v... not 48v. It needed to achieve almost twice the cut in rpm, to get to charging voltage of the 48v bank.

Now to you this is obviously silly, but if you take the time to learn from the real Guru's of the world (Flux), then you would come face to face with what really matters.... and that is  matching the load. Without too much dribble it is like this. Wind is linear, power from the wind is cube function, power from genny in rpm terms is squared.

We need to match three curves. Only at zero will they converge, from that point on, they diverge... but a Flux points out, once you get past the lower parts of the curves, the squared and cubed curves are not quite so disparate, and he shows curves to illustrate this point.... but at the lower parts of the graph, the divergence is quite marked.

If we build the turbine for 1/2 the envisioned voltage, we can use heavier wire in the same space for the same configuration. a big I^2 R win for starters.
Now between "cut in" and twice cut in, the power may be modest, and a smallish (600 watts for me) booster can take the 24v cut in low power to 48v low power, with light loss. We can also tailor the gain of the boost section, to better mimic the near square function differential we would expect to see between the bank and the turbine.... so we have a sort of lower level mppt going on before the turbine reaches 55v.... by then, we can let the turbine drive the bank directly, current limit the booster ( pwm duty cycle has gone from 50% to zip), and now we have the turbine with 1/4 the original 48v design resistance, and the exponential graph arms are heading north at not similar rates, but not as severe as at lower power.... ie the curves are a bit less dramatic, and the stator has  a better chance of following the rpm/voltage from the mill higher without the same heat degradation ( because of 1/4 the resistance), as it would have done for a 48v stator.....and the turbines characteristics now better match the wind as real power cut in is twice the design of the tsr originally considered ( less stall)

A full dissertation can be found in Flux's article of matching the load..... one smart hombre!
So it is done for simple, and very useful reasons. We can drive the turbine much harder at high winds, and control stalling in light winds.... whats not to like?

Similarly, with panels.... where could we use boost to advantage.

If we have a 24v bank, we will be using the solar to drive it up to 28v most of the time, and floating ... other times we want to terrorize the bank with w good equalize charge.

The best panels for the money, will be 30wv panels, as for the same power as a 37wv panel, the current will be much higher... so we can drive the batteries directly without losing any where near the losses that a 37wv panel would suffer (60 cell versus 72cell panels). I think Chris O mentiones that he has the lower volt panels, and they rock into a lower charged bank.

So now, we need only consider the panels a bit feeble above 28v with low light charging, and/ or charging a near full or full bank, or equalize charging.

Funny thing is, a  booster would be perfect for this, and so you get the best of all worlds, without the main current going through any converter at all..... my preference, less power going through fets... the better, and then boosting for the rest of the charge cycle.... which can be low current anyway.

It's all about matching the load. Some folks want a classic or similar to control their destiny... I'm not one of them, I'm too remote to bother with their stuff at all. In a remote setting in particular, simpler is better, and in both the turbine and panel situation outlined above, a blown booster will not impact the system mortally, and you will still have most of the power whilst you find parts to ameliorate the difficulties.

Those in comfy armchairs with manufacturers just up the road can try buck toys, as they have backup from the right people, but most folks around the world won't have midnight up the road, and need systems which are near bullet proof. Boosters used as above give you this , bucks would  terminate the use of the turbine if failed, or seriously compromise the output of a HV solar system into the bank whilst the buck gets fixed, thrown out and replaced etc etc.
My turbines peaked above 5kw, and I have now gone way past the 9MWH with the solar panels I have built. The Buck for the turbine/s would have been very substantial and way expensive ( and blown up I suspect in the real peaks.. I am in the roaring forties after all)... the boosters only needed to play in the sub 600w range.

Everything is not as simple as it seems on the surface. The current in a booster is of little relevance, and can be catered for in the design phase. The resultant outcomes is what is important..... sometimes a boost is just what you need for reasons that don't jump out at you first off.

edit:
Rossw fair comment.... but also 600w is pretty tame, and you could use a booster to push the V up to 350 volts or so for the cable run... rather than boosting at the bank end...... and then using a inverter to come down again ( drive the H bridge directly with the 350dc)...... yes pedantic, but actually doable...

 In my case, I can use a 350volt array, and then go into a grid tie unit which is driving into the output of the 6kw house (PSW) inverter. Any excess the house does not use, can charge the battery bank through the inverters internal inverter charger ( up to 70A@55v or so.... actually reverse driving the fets back from the transformer), and when the clouds come over, the batteries just feed the deficit between grid tie output, and load requirements... neat system.  ( needs dump load on batteries) The grid tie drives the house direct most of the time in daylight, and the batteries just float until the clouds or night turn up.




.....................oztules
« Last Edit: March 31, 2013, 09:02:53 PM by oztules »
Flinders Island Australia

joestue

  • Hero Member
  • *****
  • Posts: 1759
  • Country: 00
Re: MPPT function – hardware implementation
« Reply #23 on: March 31, 2013, 11:50:30 PM »
yes, i built a buck converter, and got 98% efficiency at 60 KHZ using irfz44 mosfets hard switched at 5 amps per pair.
50 volts going in, 24 volts going out. so i lost 10 watts. --it wouldn't have mattered if it was a boost converter working backwards.

one whole watt of power was lost in those two 680uF 200 volt capacitors. the 1uF film caps were required to prevent the mosfets from going into avalanch during the switching transition, and the two toroidal inductors killed the ripple going into the power supply (this was required, as there was half a volt of ripple IIRC on the input to this thing.), but the inductor on the output was not required, because the current ripple was only 2 amps or so.
http://johansense.com/bulk/50_buck1.jpg

I would estimate that half the mosfet losses was due to driving them from a signal generator with a 50 ohm output impedance, and then adding 20 more ohms to turn the fets on, as there should have only been 2.2 watts lost per fet at 5 amps (they were in parallel to get 10 amps out of this machine) instead, i was at double that, at 60KHz. this used to not be that bad.

now consider that for a few extra dollars mosfets today are probably one third the resistance, twice the voltage and half the gate charge.
so any common man should be able to throw a proper half bridge driver at two mosfets and get 98% efficiency as either a buck or a boost converter.

heck, consider the stats of the induction heater i'm working on right now:
4x 500 or 600 volt mosfets, i can't recall which, at 70 milliohms each.
20 amps at 400 vdc going into the HBridge, out of the H bridge comes a square wave into a resonant load at 60-140KHZ.
24$ worth of mosfets, switching 8 Kilowatts, (20 amps average turns into 28 amps RMS, 28^2* 70 milliohms is 55 watts) 55 watts per switch lost, but each pair is only on half the time, so it becomes 110 watts lost at 8 KW.
since the body diode is canceled out, and at resonance there is no flyback diode loss, i can get 98.6% efficiency.
but just double the cost and that is halved, not to mention the inductance of the packages probably limit me to 4KW, not to mention the fact that if i bought new, i would need to spend 5$ in film capacitors to hold the DC bus together.

so lets say i run 24$ worth of mosfets at 14 amps rms each, this turns into 13 watts at 50% duty cycle which is 26 watts lost turning 400 volts dc into 100khz ac at 14 amps rms or 10 amps average.
If i spend 20$ on silicon carbide diodes, i could turn this into a hard switched two phase boost or buck converter and get about the same efficiency, as high as 99%. so i'm at about 25$ per kilowatt plus an inductor and capacitors.

heck, the last induction heater i didn't even need to put heatsinks on the mosfets, which may have been why it generated noise sufficient to block out FM stations.. lol. but the point is, the waveform should look like this:
http://johansense.com/induction_heater/6/P1020775.JPG
now, in the case of this waveform, there is 500 nanosecond of dead time between the switches, and its at resonance, which is why the voltage climbs so slowly. (2 volts per nanosecond.

When you get to hard switching, as in the case with buck or boost converters, things get a bit more sketchy.
http://johansense.com/induction_heater/prototype3.1_W.JPG
you'll see that there's an extra 30 volts that shows up across like, one inch of pcb trace between the mosfet and the catch diode, and that's only at 100 volts on the dc bus, what happens when there's 400 and twice the current draw?
all that energy can potentially be absorbed in avalanche with low voltage power circuits, HIGH currents (say running 30 amps at 20% duty cycle through a TO-222) and people wonder why they get warmer than the back of napkin math said it should.

so in the case of boost or buck converters, since we have to hard switch big inductors, we throw more phases at it, and then we add: (  :)  )
a resonant snubber.
these things are really not that hard to design, but they typically add a lot of $$ to the problem. 
My wife says I'm not just a different colored rubik's cube, i am a rubik's knot in a cage.

joestue

  • Hero Member
  • *****
  • Posts: 1759
  • Country: 00
Re: MPPT function – hardware implementation
« Reply #24 on: March 31, 2013, 11:59:17 PM »
Honestly i don't see much point in trying to switch more than 30 amps per package if you want to do 60+ KHZ, unless you want to use a three layer or four layer pcb board. unless you use a resonant snubber, you simply can't get more than 99% efficiency because you're going to be burning up a lot of energy in the snubber to keep the fets from going into avalance simply due to the 10nH of inductance per inch of wire.
or you do as some do, and just let them go into avalanch, charge people 3 times more than they should for the dc-dc converter, and hand them a 10 year warranty and say hey we will replace it when the capacitors dry out and the fets blow up.

10 nH at 30 amps and 60 KHZ is .27 watts.
so figure 100nH of inductance between the drain, boost diode, and the boost capacitor and you're doing really good... and you've got 3 extra watts of power that Must go somewhere, and it goes somewhere at 30 amps if the fet avalanches.
My wife says I'm not just a different colored rubik's cube, i am a rubik's knot in a cage.

rossw

  • Hero Member
  • *****
  • Posts: 834
  • Country: au
Re: MPPT function – hardware implementation
« Reply #25 on: April 01, 2013, 03:22:39 AM »
edit:
Rossw fair comment.... but also 600w is pretty tame, and you could use a booster to push the V up to 350 volts or so for the cable run... rather than boosting at the bank end...... and then using a inverter to come down again ( drive the H bridge directly with the 350dc)...... yes pedantic, but actually doable...

Yes, like I said, 600W per array is what I call "modest" (6 arrays total = 3600W, still a small install, but does me most of the time).

Going up to 350V DC pushed me into a different (and more expensive) insulation class for cables etc. Also, I'm not mad keen on putting my electronics out in the field where they're going to be exposed to extremes of temperature, potential fire, definate insect problems, lightning etc.

In my case, I figured 150Voc (100V nominal at max power) was "about right". Means I can feed straight into the FlexMax, and my isolation schottky diodes were easy to come by, and the switchgear wasn't anything particularly special, and well, just a good compromise. (I rarely see more than about 45A in from the arrays even with 70+A into the batteries.

jack11

  • Jr. Member
  • **
  • Posts: 95
  • Country: us
Re: MPPT function – hardware implementation
« Reply #26 on: April 01, 2013, 08:08:16 PM »
that Linear LT3652 Treehouse provided seems to be a solution one can use to build a mppt controller.
I quickly looked at the data sheet, and it looks like it has built in mppt, battery charger, temp compensation, etc.
It has limitations at input voltage and output current, so a workaround would be needed to make it usable, but in 12V maybe 24V systems only.

One question is: can it be stacked at the output (bat pin)?
Say 30 of them on a circuit board would be needed to make a 60A charger, etc.
I did not see any references to stacking in the data sheet.

But, it seems that if one connected the outputs in parallel, then each of the LT3652 would behave the same if multiple LT3652s were connected to a large lead-acid battery (see app circuit in data sheet).
It's as though each of the LT3652 would not know that there are more LT3652s charging the same large battery, and each one would be responsible for its "2A slice" while it thinks it charges a small battery which it was designed to handle.

Can someone look at this data sheet to see if this is possible?
http://cds.linear.com/docs/en/datasheet/3652fd.pdf

joestue

  • Hero Member
  • *****
  • Posts: 1759
  • Country: 00
Re: MPPT function – hardware implementation
« Reply #27 on: April 01, 2013, 09:01:56 PM »
unfortunately the lt3652 isn't really a real MPPT.

Jack what are your electronics skills? if you can make sense of this, i could easily build and ship you a few kilowatts of buck converters to go with it. i'm not very interested in demultiplexing that circuit.
there is a higher resolution image of it available, perhaps someone here knows were the pdf of the original article went.
http://electronicdesign.com/site-files/electronicdesign.com/files/archive/electronicdesign.com/files/29/6262/figure_01.gif

http://www.idl.ku.edu/ARA/AARPS/Documents/PV/LTC1149-MPPTracking.pdf
« Last Edit: April 01, 2013, 09:06:24 PM by joestue »
My wife says I'm not just a different colored rubik's cube, i am a rubik's knot in a cage.

GoVertical

  • Hero Member
  • *****
  • Posts: 691
  • Country: us
Re: MPPT function – hardware implementation
« Reply #28 on: April 01, 2013, 10:29:11 PM »
http://www.idl.ku.edu/ARA/AARPS/Documents/PV/LTC1149-MPPTracking.pdf

Hi, is fig 1,2,3 available for the above PDF???
Learn from the past, live in the present, plan for the future
kilroyOdin is not here ;)
SEMPER FEROX

joestue

  • Hero Member
  • *****
  • Posts: 1759
  • Country: 00
Re: MPPT function – hardware implementation
« Reply #29 on: April 01, 2013, 11:00:37 PM »
yes, i linked to figure 1.
you don't need figure 2 or three

i've already modified the circuit to use 2 mosfets instead of a 4053 3x SPDT switch.
however, you'll need to use some other circuit to produce a 50% duty cycle square wave with a fine adjust on the duty cycle.
notice that i've drawn the switches as relays.
link: http://johansense.com/bulk/analog_mppt.JPG

the only part i can't make sense of in my head is the additional 2 M ohm resistor (R 5 in the .gif file ) to the P gate drive in the original schematic, since that pin would be drug low when the high side switch is on, and during normal operation the inverting input of amplifer A2 is around 5 volts, which is half the supply voltage, that resistor seems to act as some kind of feed back loop stabilizer to the mppt function.
i'm not sure its really necessary, because the integrating capacitor around A2 should slow everything down well enough by itself.

seems to me it would make more sense to referance A1 to the mid point of the analog loop (5volts) and reset the capacitor c1 to that voltage as well. as it is, it would appear the author needed the offset null (which appears to adjust the duty cycle), not to modify the duty cycle, but to adjust for the offset voltage of A1.
with some additional modifications, A1 can be configured to get zero offset, but it would be ac only, as it is, leakage and whatever else will add a dc offset to whatever 100uv the LTC6062 has.
« Last Edit: April 01, 2013, 11:30:19 PM by joestue »
My wife says I'm not just a different colored rubik's cube, i am a rubik's knot in a cage.

GoVertical

  • Hero Member
  • *****
  • Posts: 691
  • Country: us
Learn from the past, live in the present, plan for the future
kilroyOdin is not here ;)
SEMPER FEROX