# Sepic(Buck-Boost) Converter

#### ericgibbs

Joined Jan 29, 2010
19,082
hi P,
Check the d/s 2.31V at 0.454 Amps!
So 7 panels in Series will only give 16.17V when loaded with a 36R,ie; 0.454A ,,, 8Watts, they cannot supply the current required by the Sepic.

Have you studied Ohms law and Power maths.?

E

Note: when producing 0.454A the Onload voltage is only 2.31V

Corrected a value.

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#### ericgibbs

Joined Jan 29, 2010
19,082
Hi P,
The left circuit is a crude representation of your Solar.
Ie: Current limited, 16V at 0.454A

The right circuit is what happens when you connect the Sepic
Which requires say 10+ Amps to run at 16V input.
Look at the sim plot.
E
I realise that the sims are 'crude' but you should get an idea about what is wrong with the Solar.

You are trying to convert a 8Watt source into a 80Watt output.!

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#### Peter523

Joined Aug 15, 2021
97
Hi P,
The left circuit is a crude representation of your Solar.
Ie: Current limited, 16V at 0.454A

The right circuit is what happens when you connect the Sepic
Which requires say 10+ Amps to run at 16V input.
Look at the sim plot.
E
I realise that the sims are 'crude' but you should get an idea about what is wrong with the Solar.

You are trying to convert a 8Watt source into a 80Watt output.!

View attachment 246359
Thank you for the explanation, it is quite understanding. What i understand is that the load that sepic adds on the output od the solar is too large and as a result solar panels cannot transfer the required power to the SEPIC so that SEPIC can convert it to 8.4V. Correct me if i misunderstood something.

#### ericgibbs

Joined Jan 29, 2010
19,082
hi P.
You are correct, the Solar panels, 7 off. will only output 0.454A at 16V, which is about 8Watts, the Sepic load is about 85Watts.

You would need at least an array of say 20 *7 panels [ 20*0.454A=9A] [7*2.31v =16v]
Series and parallel connect panels, about 140 in total.

More problems would occur at lower Solar panel light levels.

May I ask why you chose a 3V minimum input for the Sepic.??

E

#### Peter523

Joined Aug 15, 2021
97
hi P.
You are correct, the Solar panels, 7 off. will only output 0.454A at 16V, which is about 8Watts, the Sepic load is about 85Watts.

You would need at least an array of say 20 *7 panels [ 20*0.454A=9A] [7*2.31v =16v]
Series and parallel connect panels, about 140 in total.

More problems would occur at lower Solar panel light levels.

May I ask why you chose a 3V minimum input for the Sepic.??

E
It represents the condition that only one cell is sun lighted (~2.61V).

#### ericgibbs

Joined Jan 29, 2010
19,082
It represents the condition that only one cell is sun lighted (~2.61V).
Hi P,
I don't understand what you mean.?

E

#### Peter523

Joined Aug 15, 2021
97
Hi P,
I don't understand what you mean.?

E
I mean that i chose 3V as the minimum SEPIC input cause the 3V (~2.61V to be more accurate) is the condition where only 1 solar cell from the 4 of the panel is full illuminated. Hope i was more specific.

#### ericgibbs

Joined Jan 29, 2010
19,082
hi,
Have you finally understood that the project using say only 7 Solar Panels of that type is not feasible.?

E

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#### Peter523

Joined Aug 15, 2021
97
hi,
Have you finally understood that the project using say only 7 Solar Panels of that type is not feasible.?

E
Of cource, after all simulations etc. i understood that it is not feasible, but i tried to explain you the reason i took as minimum the 3 Volts. That requirement was set at the beggining (the range 3V-19V) of the project and now i just explain from where this requirement came from. I am at the last year of my studies.

#### ericgibbs

Joined Jan 29, 2010
19,082
hi P,
I would recommend that when posting a technical query, you should point out the details of the supply source, in this case a Solar panel.

What are you planning to do with this question set by your tutor.?

E

#### Peter523

Joined Aug 15, 2021
97
hi P,
I would recommend that when posting a technical query, you should point out the details of the supply source, in this case a Solar panel.

What are you planning to do with this question set by your tutor.?

E
Thank you for the advice and sorry for the misunderstanding and waste of time.
About the question : do you know any other ways to convert the solar panels voltage range to 8.4V for the three DC-DC converters? Or (better) an IC?
Thanks again.

#### ericgibbs

Joined Jan 29, 2010
19,082
Hi P,
There is no easy fix.
Work out the total Power load for the 3 converters, what is that load Power.?
E

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#### Peter523

Joined Aug 15, 2021
97
Hi P,
There is no easy fix.
Work out the total Power load for the 3 converters, what is that load Power.?
E
You mean the power that consumes every converter?

#### Danko

Joined Nov 22, 2017
1,855
About the question : do you know any other ways to convert the solar panels voltage range to 8.4V for the three DC-DC converters? Or (better) an IC?
Yes. Use IC based SEPIC, which is able to transfer whole power of your 7 solar panels
with V_in from 3V to 19V, V_out=8.4V and efficiency 80% to 90%.
https://www.pololu.com/product/2572

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#### ronsimpson

Joined Oct 7, 2019
3,203
and waste of time.
Not a waste, this is how we learn. You will be in this place in your work years from now. It is common to have a project that can not be done the way you are told. Misunderstandings are common.

#### ericgibbs

Joined Jan 29, 2010
19,082
Use IC based SEPIC, which is able to transfer whole power of your 7 solar panel
Hi Danko,
The Solar panels he is proposing can only output 2.3V at 0.454A = ~1 Watt.
His final load at 100% η is 3V*2A= 6Watt + 5V*3A= 15Watt + 12V*3A =36W , Total 57Watts.

Assuming a conversion eff' of say 90% , thats approx 63 Watts.

He would require an array of panels 9 * 7 =63 and thats on bright Sunlit day.

E

#### ronsimpson

Joined Oct 7, 2019
3,203
While we are talking about solar. Most panels will never output what they say. I live at high altitude, clear days, 10% humidity and can't get the rated output power. At sea level, 95% humidity the output must be way down but I don't know. The rating is based on the Sun being directly over head. If you live North (or South) this never happens. Lets say you can get 60 watts to run the SEPIC from 11:45 to 12:15 on sunny days.

Sorry I am being depressed about life.

#### Peter523

Joined Aug 15, 2021
97
Yes. Use IC based SEPIC, which is able to transfer whole power of your 7 solar panels
with V_in from 3V to 19V, V_out=8.4V and efficiency 80% to 90%.
https://www.pololu.com/product/2572
So this is unable to convert solar voltage to the appropriate level? Maybe any other solutions?

#### Danko

Joined Nov 22, 2017
1,855
So this is unable to convert solar voltage to the appropriate level?
You can convert voltage or convert current to almost any level you need.
But it is impossible to increase power of source in process of conversion.
Power is like money - amount of it decreases with every conversion.
Maybe any other solutions?
Only one - to use powerful (>63W) solar panel.