AC to DC conversion using LNK305DG

[Srihari P]

I am using the LNK305DG IC to convert AC to 6.4V DC. The components (resistors,capacitors and inductors) are selected according to the datasheet. I am getting an output voltage of 6.4V. When this 6.4V is isolated and the power is measured with the help of a wattmeter, the power is shown as 0.17W and the reactive power is -1.78. I have also tried using slightly different values of capacitors than those mentioned in the datasheet but the reactive power is at -1.78, power factor being 0.34 in both the above cases. How can I reduce the reactive power, as the reactive power consumed is more than the real power? Please help.

 

[picgak]

Hi Srihari P,
Try increasing the value of resistor in series with the FB pin. we can solve your problem if you post the circuit you have constructed.
regards ani

 

[Srihari P]

Hi Srihari P,
Try increasing the value of resistor in series with the FB pin. we can solve your problem if you post the circuit you have constructed.
regards ani

Hi,

I have uploaded the image file of the circuit. I'll increase the resistor value and post the result.

 

[picgak]

Hi Srihari P,
According to your circuit you can try increasing R1

 

[Srihari P]

Hi Srihari P,
According to your circuit you can try increasing R1

Hi,

Could you please explain, how increasing r1 might reduce reactive power?

 

[picgak]

During normal operation, switching of the power MOSFET is controlled by the pin FB. MOSFET switching is terminated when a current greater than 49 μA is delivered into this pin. so if you increase R1 feed back voltage will be reduced this will result in more on time and more voltage at the output. Hope this helped you if so dont forget to click the helpedme button below.
regards ani

 

[mtwieg]

I am using the LNK305DG IC to convert AC to 6.4V DC. The components (resistors,capacitors and inductors) are selected according to the datasheet. I am getting an output voltage of 6.4V. When this 6.4V is isolated and the power is measured with the help of a wattmeter, the power is shown as 0.17W and the reactive power is -1.78. I have also tried using slightly different values of capacitors than those mentioned in the datasheet but the reactive power is at -1.78, power factor being 0.34 in both the above cases. How can I reduce the reactive power, as the reactive power consumed is more than the real power? Please help.

The low PF is due to the bridge rectifier which converters the line AC to high voltage DC. Poor PF is inherent in such circuits, and there's really not much you can do about it. Lowering the input bypass capacitors might help slightly (by increasing the conduction angle of the bridge), but this will likely result in more ripple.

If you really want good PF, then you should look at active or passive PF schemes.

During normal operation, switching of the power MOSFET is controlled by the pin FB. MOSFET switching is terminated when a current greater than 49 μA is delivered into this pin. so if you increase R1 feed back voltage will be reduced this will result in more on time and more voltage at the output. Hope this helped you if so dont forget to click the helpedme button below.
regards ani

He wants to change his PF, not his output voltage.

 

[Srihari P]

During normal operation, switching of the power MOSFET is controlled by the pin FB. MOSFET switching is terminated when a current greater than 49 μA is delivered into this pin. so if you increase R1 feed back voltage will be reduced this will result in more on time and more voltage at the output. Hope this helped you if so dont forget to click the helpedme button below.
regards ani

I increased the value of R1 resistor to 6.2K. Now the wattmeter is not showing a stable value of active or reactive power. The power factor is varying from 0.34 to 0.95 continuosly. The reactive power has actually increased and varying.

 

[picgak]

The low PF is due to the bridge rectifier which converters the line AC to high voltage DC.
.

Hi Srihari P,
To attain a good power factor as mtwieg has pointed out you have to implement a good PFC circuit. you can get good details from faichild application note 42047.
regards ani