SEPIC converter not working

[rragupathiraja]

I am designing a SEPIC converter. (Schematic enclosed). I am using a 6 - 15V as input to get a fixed 12V output. I am giving the input through driver. Though i am increasing the supply voltage i am not getting any output..

IS there any problem in the circuit.. I am using two separate inductors and not Coupled inductors..
View attachment SEPIC.bmp

thanks

 

[Orson Cart]

Can you post a sketch of the gate drive waveform? Also you need to automatically vary the duty cycle down to near zero if the load is removed, and to allow for your varying input voltage.
Regards, Orson Cart.

 

[rragupathiraja]

Can you post a sketch of the gate drive waveform? Also you need to automatically vary the duty cycle down to near zero if the load is removed, and to allow for your varying input voltage.
Regards, Orson Cart.

I have checked the gate signal, its giving 67% duty @100kHZ. when i was trying to check the switching waveform across the (2SK3817) Drain and source, the circuit gets shorted.

Do i selected right components? I am using a 1N5408 Diode..

 

[Orson Cart]

1N5408 is no good - too slow, try to find a schottky diode, 45V 5A for example,

It sounds like you are shorting something when you try to look across D-S, check the 0V gnd of your 'scope is not connected via earth to the wrong polarity of your power source.

Regards, Orson Cart.

 

[rragupathiraja]

thank you Orson,

I have changed the diode and its now working fine.. But If i am placing inductive loads like lamp the circuit gets shorted.. I can do with the resistive loads only.. what may be the problem?

 

[Orson Cart]

Possibly a soft start is needed to ramp the PWM from 0 - normal level, A low value inductance on the output will change the characteristics of the power converter, so be sure you have enough output capacitance. What is DC resistance of the inductive load? If it is too low you may be trying to drag too much current through your circuit and out of your source.
Regards, Orson Cart.

 

[rragupathiraja]

the converter is now working for inductive loads also after i changed the inductor to toroidal core (previously it was rectagular iron core).. Thank you for the reply.


thanks..