# Sepic(Buck-Boost) Converter

#### Peter523

Joined Aug 15, 2021
97
Hi P,
With that circuit you drawing almost 60A from the 3V supply.
E
Yes, what could i change?

#### ericgibbs

Joined Jan 29, 2010
19,121
hi Peter,
Using a 3V voltage source is adding to the design problem.

Consider that you have 8.2Vout into a 0.77R load, thats 8.2v/0.77R = 10.65A, so even at 100% conversion efficiency, it will draw 8.2V/3V = 2.7 * 10.65A =28.8 Amps!

E

#### Peter523

Joined Aug 15, 2021
97
hi Peter,
Using a 3V voltage source is adding to the design problem.

Consider that you have 8.2Vout into a 0.77R load, thats 8.2v/0.77R = 10.65A, so even at 100% conversion efficiency, it will draw 8.2V/3V = 2.7 * 10.65A =28.8 Amps!

E
And what should i use instead of the voltage source?

#### ericgibbs

Joined Jan 29, 2010
19,121
hi P,
May I ask, is this simply a simulation project for say a College assignment, where the circuit conditions are specified in the assignment, eg: Vsource =3V.?

or is it a project you intend to build.?

E

#### ronsimpson

Joined Oct 7, 2019
3,221
Eric,
Add this to the spice file.
K1 L1 L2 0.99
Don't use 1.0

#### ericgibbs

Joined Jan 29, 2010
19,121
hi Ron.
Tried that idea on the '87%' version.
MOS Power dissipation down, but MOS current is high.

Will try the other version later.

E

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#### Peter523

Joined Aug 15, 2021
97
hi P,
May I ask, is this simply a simulation project for say a College assignment, where the circuit conditions are specified in the assignment, eg: Vsource =3V.?

or is it a project you intend to build.?

E
Hi eric,
my goal is to build(not only simulate) this sepic converter for a college assignement. The assignment specifies that the converter should take voltages coming from solar panels and ranging from 3V-19V and convert them to 8.4V to "feed" three DC-DC converters.

#### ronsimpson

Joined Oct 7, 2019
3,221
A professor can make an assignment that makes no real world product. ("ours is not to reason why" Lord Alfred Tennyson )
Solar panels act like a constant current source and not a constant voltage source.
Approximant numbers for the PWM. Vin 3V @ 50A, 6V @ 25A, 12V @ 12A.
Start out with good light on the panels and the PWM is working. We get 12V at 13A so this PWM is pulling 12A and all is good. As the light reduces the potential current from the panel drops. As soon as the current out drops below 12A the PWM demands more current but the panel can not make more current. So the voltage drops causing the PWM to demand even more current. In a fraction of a second the voltage from the panel will be at 3V headed to zero. The PWM shuts down and pulls no current. Now with no load the panel will output voltage. The voltage will go up to 3V very fast. The PWM will try to start but needs 50A (more than) but can not get it. It is very likely that you will need a very bright day to get the 50A needed to start the PWM. Maybe you can never start the PWM from a solar panel.

From experience, this whole project is not going to work. (you can build the PWM but it will not work with a solar panel)

If you want to see this; make a solar panel using a constant current source and a 19V limit on the voltage. (add a large capacitor) Use that to drive the PWM. Slowly reduce the current from the panel and see what happens.

RonS.

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#### Ian0

Joined Aug 7, 2020
10,277

#### ronsimpson

Joined Oct 7, 2019
3,221
Product of too many years in the British school stem. (actually many systems)
First thing, we must memorize, is a poem about doing what you are told with out questioning even if it gets you killed, and the war is lost. "Do what your betters say" (also in the Samurai system) In the American system, we learned that they are not your betters, and the war-lost is more important than that is said. Then when you get old enough to die, you learn to do what it takes.

#### Ian0

Joined Aug 7, 2020
10,277
Just had to check whether I went to the same school as Tennyson (he was born 5½ miles from here), but no - he went to the same school as Captain John Smith (of Pocahontas fame).
If I may correct your slight misquote "Theirs not to reason why" I see it as more of an illustration of what happens if you just follow instructions without questioning them. One thing is for certain, our education systems at both sides of the Atlantic should teach critical thinking, and not just "teach to the test".

#### ericgibbs

Joined Jan 29, 2010
19,121
hi Guys.
Let's keep on the TS's Topic.
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#### ericgibbs

Joined Jan 29, 2010
19,121
hi Peter,
As pointed out in earlier posts, using a Solar panel to drive the SEPIC over that voltage range of 3V thru 19V, at those currents is not a viable option.

Have you done any research into Solar panel specifications and performance.?

If Yes, which Solar panel have you considered for the project.?

E

#### Peter523

Joined Aug 15, 2021
97
hi Peter,
As pointed out in earlier posts, using a Solar panel to drive the SEPIC over that voltage range of 3V thru 19V, at those currents is not a viable option.

Have you done any research into Solar panel specifications and performance.?

If Yes, which Solar panel have you considered for the project.?

E
I did a small research. The solar panel that i took as a reference is this : https://www.cesi.it/app/uploads/2020/03/Datasheet-CTJ30-Thin.pdf . Suppose many of them connected in series (that's why the range ~3-19V).

#### ericgibbs

Joined Jan 29, 2010
19,121
hi Perter,
How many individual Solar panels do you calculate are required, say, connected in series/parallel do you estimate.?

E

#### Peter523

Joined Aug 15, 2021
97
hi Perter,
How many individual Solar panels do you calculate are required, say, connected in series/parallel do you estimate.?

E
View attachment 246349
I estimate about 6-7 in series

#### ericgibbs

Joined Jan 29, 2010
19,121
hi P,
Seven in series, with ideal 100% illumination will give 18.27V at 0.454Amps. say 8.3Watts

E

#### Peter523

Joined Aug 15, 2021
97
hi P,
Seven in series, with ideal 100% illumination will give 18.27V at 0.454Amps. say 8.3Watts

E

View attachment 246350
Sorry i don't understand. Whar do you mean "If loaded with your Sepic, virtually nothing"?

#### ericgibbs

Joined Jan 29, 2010
19,121
hi P,
I should have been more clear, the output voltage of the Solar array will be close to zero volts.
E