Looking for high power buck boost design reference

[Ichan]

Hi,

I need to build a 24V 10A non-inverted buck-boost solar charger controller, i wold like not to start it from zero so a similar design is what i seek for...  .

Anyone?

-Ichan.

[Gallymimu]

LT8705 is a really nice part we just used for a non-inverting buck-boost.

BUT, for a solar charge controller you should use something more sophisticated like an MPPT controller (it scans for the optimal power point of the solar cell output and adjusts loading accordingly)

[zab]

I think best option is sm72442 specially design for solar . it has same 4 switch pattern. for buck boost operation.
I think for simplicity better to used buck interleaved design option for high current. I came across a charger that changes its frequency to wide range .Initially only high side conducts while low side switches on for a small duty cycle.and it start working in synchronous mode .
when interleaved mode is used ripple current come to half and becomes easy to get high power.

[Ichan]

Thank you, i will dig into that two chip soon - first thing is to checking the price, both are expensive especially the LT.

I am not sure about MPPT, we draw some amount of current from the panel at the point where the panel start to decrease its output voltage (maximum power) - it will assume that we have a free unlimited place to dump that amount of current. How if the battery is full and the load is a constant current (LED)? Or, how if the battery is Li-ion where we need to charge it with constant current?

The solar panel had been determined by the other, it has more or less 38V open circuit and 29V maximum power output voltage - that 29V is too marginal to charge 24V lead acid battery (2x12V in series) and i believe it is for full sun irradiation. So i think i can not use buck converter in here, am i right?

-ichan

[zab]

First thing is to start from simple. My brother just add few more panels to get more voltage instead of using just one or two. I have seen 3 to 4 panels for 24v and 6 to 8 for 48v .Higher the volt less current is required for same amount of power so less wastage in wires and thinner wire may carry more power. I have used 4 panel in series to charge 24 v  batteries. so that is not the problem.
Doing buck boost simultaneously require complex circuitry.But nothing impossible.
About MPPT current and voltages are measured to get power rating.You start from low duty cycle and gradually increase it. the out put voltage start increasing as it reaches above battery level.you get current reading. just multiply both to get power.As you go on increasing duty cycle you get more power.at some stage your voltage start decreasing so you get less resultant power.this is the point to avoid go back to previous duty cycle to get higher power. The reading are taken at regular small interval. so you increase and decrease duty cycle to maintain the power at maximum level.

[solutions]

At 26.4V on the lead acid (a dumb, but cheap, choice for green power,especially where you are - people discard dead batteries in a way that poisons the ground water), it sounds like you don't care about MPPT, so I'd look at operating a buck at close to the dropout input voltage and you may be fine with the charge current you get. I think your bigger problem will be partial shading of your panel(s).

[Old_but_Alive]

please dont flame me ( or mute, or chastise, thats for you pickit2)

this is sort of off topic, but relevent.

I am currently in the UK, and like the idea of the IKEA system.
I am planning to move lock stock and barrel from blighty ( ah what's happened to the land I was born in 64 years ago )
to asia.
I want to be totally stand alone with PV system, battery's, inverter etc. so NOT connected to the local electricity company

any suggestions ?

Mike

[pickit2]

please dont flame me ( or mute, or chastise, thats for you pickit2)

this is sort of off topic, but relevent.

I am currently in the UK, and like the idea of the IKEA system.
I am planning to move lock stock and barrel from blighty ( ah what's happened to the land I was born in 64 years ago )
to asia.

Mike

well what can I say?, a warning 2 years ago, kept you on the short and narrow.
as to moving to asia, their beer is not as good as the ones in the the crack, the dizzy or the belly
some of the best pubs in Liverpool.

Translation "Ye Old Crake" were John Lennon drank in the war office, the home to Dougie, more famous than Lennon at the time.
"The Dispensary"where you can get great beer, with an insult or too for free.
"The Belvidere" The best of the three, free Wi-Fi = I can mute from there

[solutions]

...not to mention your (was actually mine until reality sunk in) fantasy of off-grid is, er, "damped" by the fact that there's a monsoon season of about 8 months - at least where I go there is. For that significant oversight, I'd suggest to Pickit2 to put you on a 48 hour "courtesy mute"

Backyard nuke is the answer, IMO, and easy, regulationwise, if you pick N. Korea as your new home: http://www.world-nuclear.org/info/Nuclear-Fuel-Cycle/Power-Reactors/Small-Nuclear-Power-Reactors/

[Old_but_Alive]

@solutions, i guess thats no then.
@pickit2 can i post a really good scouse joke, or should i pm it?
for 2 years, i have been scared to post, 'cos the fastest gun in the west is trigger happy on the 'ol mute button.
sending this from Abu Dahbi airport

[pickit2]

I'm not scouse, I'm manx, the one drunk that left the other 50,000 clinging to that rock in the Irish Sea.
 
damn I was gonna buy you a pint in the "monkey & **** or the much better real ale pub down the road.

[Gallymimu]

Thank you, i will dig into that two chip soon - first thing is to checking the price, both are expensive especially the LT.

I am not sure about MPPT, we draw some amount of current from the panel at the point where the panel start to decrease its output voltage (maximum power) - it will assume that we have a free unlimited place to dump that amount of current. How if the battery is full and the load is a constant current (LED)? Or, how if the battery is Li-ion where we need to charge it with constant current?

The solar panel had been determined by the other, it has more or less 38V open circuit and 29V maximum power output voltage - that 29V is too marginal to charge 24V lead acid battery (2x12V in series) and i believe it is for full sun irradiation. So i think i can not use buck converter in here, am i right?

-ichan

MPPT isn't as simple as just picking an operating point based on a one time measurement or the spec sheet.  Depending on the operating conditions it will shift, which is why the MPPT charge controllers scan periodically to make sure they are keeping the power maximized.

Yeah the LT stuff is VERY expensive but it's also VERY good and just works!  Depends on your volume of manufacturing what makes sense I guess.

[Ichan]

Don't get me wrong about MPPT, i do care about it - seems i do not yet fully understand it. As zab said, it will be easier to use more solar panel and use buck converter - but i simply can not do that  .

I am now staring at sepic & inverse sepic converter, triggered by a new microchip app note that use inverse sepic for this purpose.

Questions:
- what is the advantage / disadvantage between sepic and inverse sepic?
- stated every where that sepic converter is difficult to control - anyone can explain?
- at the app note above the pwm used is fixed on time (variable frequency) - any particular reason?
- if i want high side current sensing for the converter (eg. LTC6101), where should i put it, before or after the switches?

-ichan

[zab]

I have not seen it practical example in real life. so not in position to comment it now . I will study it as I was personally interested to make a high capacity mppt controller. the example is only 100 w only.How to increase it? Waiting to see result of your trial.

[Ichan]

Solved by diplomacy  .

The project owner agree to return back the panels to the supplier asking for replacement with another panels with 37 Vmp, this cost them some but i finally can persuade them that it is a worth thing to do  .

So this will go with Buck converter, but I still interested with that inverse sepic app note, i may built it one - as it is first, then try to change it to use GS family of dspic.

-ichan