SEPIC converter possible design?

Thread Starter

Senorjohnny

Joined Feb 18, 2018
47
Specifications:
Vin = 9V
Vout = 1.5V - 12V

I've built a prototype based on the LTSPICE model. It doesn't seem to work as intended, my inductors are getting 18V each instead of 9V and Vout = 0V.
Any possible solutions?

Thanks in advance.
 

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ericgibbs

Joined Jan 29, 2010
18,872
hi j,
I have added a Load resistor 100R and I get +10Vout.
E

EDIT:

This what I get by adding a simple 25K Pot for basic control.
Looks OK.
E
 

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Thread Starter

Senorjohnny

Joined Feb 18, 2018
47
hi j,
I have added a Load resistor 100R and I get +10Vout.
E

EDIT:

This what I get by adding a simple 25K Pot for basic control.
Looks OK.
E
Hello,

Thank you. I seem to have solved my Vout problem, i had the pins on the Schottky diode wrong. However now, I am getting too high of a Vout(40+V) . What could be the problem?
 

ericgibbs

Joined Jan 29, 2010
18,872
hi johnny,
Is the Vout not controllable ie: stuck at 40V.?
It is possible that the LT1370 IC has blown.
Is the circuit the same as that LTSpice circuit diagram you posted .
A clear photo of the psu may give us an idea of what is wrong.
E
 

Thread Starter

Senorjohnny

Joined Feb 18, 2018
47
hi johnny,
Is the Vout not controllable ie: stuck at 40V.?
It is possible that the LT1370 IC has blown.
Is the circuit the same as that LTSpice circuit diagram you posted .
A clear photo of the psu may give us an idea of what is wrong.
E
Hi Eric,

Yeah, the Vout is not controllable. The circuit is exactly the same as the LTSpice circuit diagram.
Thank you for the quick response.

Regards,
J
 

Thread Starter

Senorjohnny

Joined Feb 18, 2018
47
Hi J.
Try a range of load resistor values, lets know what results you get.
E
Hey Eric,
I managed to get it working. So far i've tested on 100R, 500R, 1K and 2K. Also, i was able to fix the feedback control, I did not connect the wiper to the pin.

@47K ( Potentiometer )
Vout @ 100R = 9.98V
@ 500R = 10.17V
@ 1K = 10.21V
@ 2K = 10.21V

@1K
Vout @ 100R = 1.42V
@ 500R = 1.42V
@ 1K = 1.42V
@ 2K = 1.42V

I think i should swap out the potentiometer for a higher value one, that should give me the 12V i want.
What would you recommend me to do next? In term of data collection.

Thank you,
John
 

ericgibbs

Joined Jan 29, 2010
18,872
hi John,
What value resistor do you have connected from the Vout to the top end of the 47k pot.?
E

EDIT:

This should give the required Vout range.
 

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Thread Starter

Senorjohnny

Joined Feb 18, 2018
47
hi John,
What value resistor do you have connected from the Vout to the top end of the 47k pot.?
E

EDIT:

This should give the required Vout range.
Hey Eric,
That resistor is 1K.
How do i know much current is the LT1370 chip drawing? Is it the "Quiescent Current: 4.5mA" mentioned in the datasheet?

Thanks
J
 

ericgibbs

Joined Jan 29, 2010
18,872
hi J,
Off Load the LT1370 would draw only 4.5mA from the power supply.
To measure the LT1370 you would connect an Amp meter is series with power pin of the LT1370.
E
 

Thread Starter

Senorjohnny

Joined Feb 18, 2018
47
hi J,
Off Load the LT1370 would draw only 4.5mA from the power supply.
To measure the LT1370 you would connect an Amp meter is series with power pin of the LT1370.
E
Morning Eric,
How should i test the efficiency of the circuit?

Would obtaining the Vout while varying the load(output A) be correct?
I'm thinking of varying output A from 0 to 0.8A, by using resistors in series as load.

John
 

ericgibbs

Joined Jan 29, 2010
18,872
hi j,
Measure the input current from the 9V battery and the current thru the Load resistor at different Vout voltages.
Post what you measure.
E
 

ericgibbs

Joined Jan 29, 2010
18,872
hi Johnny,
This sim is for 9Vin and 9V out, with a 100R Load, 90mA
Try it with the actual project.
NOTE: the input current will fluctuate due to the switching action of the IC/MOSFET.

The images of the pop up menu show the LTS values for Power.
Using the two mWatt values you should be able to calculate the Eff%, at those V and I values.
It will change as the load changes.
E
 

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Thread Starter

Senorjohnny

Joined Feb 18, 2018
47
hi Johnny,
This sim is for 9Vin and 9V out, with a 100R Load, 90mA
Try it with the actual project.
NOTE: the input current will fluctuate due to the switching action of the IC/MOSFET.

The images of the pop up menu show the LTS values for Power.
Using the two mWatt values you should be able to calculate the Eff%, at those V and I values.
It will change as the load changes.
E
Hi Eric,

Sorry it took so long, my order for the resistors just arrived. This is what I have gotten from my project. Does it look right? The load values were calculated using V=IR , V=12, I = 0 - 1 .

Thank you,
J
 

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ericgibbs

Joined Jan 29, 2010
18,872
hi Johnny,
Downloaded your results image, will look them over, get back to you.
E

EDIT:
Your table of results is incorrect.
Look at my image Red columns .
Note: the Input current will be 'chopped' due to the switching action of the converter, so a regular ammeter will give incorrect results.

Update:
Added a LTS sim for the circuit when Rload =12R and Vout =3.7v [ your last table reading]
 

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