UCC28070 output voltage goes below setting voltage.

Hi Koji-san,

I will ask one of engineers to look at this applications issue.

Regards

Peter

Did you mean Rimo instead of Riom ?
The resistor on Rimo sets the multiplier gain and if changing the value influences your output voltage then it may be that the correct value is not being chosen.
Please use the calculator tool for the UCC28070 to calculate a value for Rimo
I calculated a value of about 110k for 1.8kW
Regards,
john Griffin

Hi

I am sorry. It was Rimo.

I calculated Rimo with the calculator tool. However the result was Rimo=19.6kohm.
(Condition; Po=1800W, Vo=385V,Fsw=66kHz)

Could you explain to me how you calculated that 100kohm for 1.8kW?

Regards,
Koji Hamamoto

The resistor value on the IMO pin sets the level of the multiplier error signal that's compared to the current signal.
As the power demand increases it is to be expected that the multiplier signal increases. If you use the calculator in any particular application and just increase the power then you can see the effect in the higher value of Rimo. If I recall correctly all I did was increase the power from 1kW to 1.8kW to check this.
If the controller is set up to deliver a lower power then the effect of increasing the power demand would be for the output voltage to drop.
That's is what I thought you were observing

Hi

Thank you for your quick reply.

I understand we should to increase the Rimo if we need to increase the output power.

I would like to know how did you get the value(110kohm). I tried to calculate the Rimo with the calculator tool. However I did not get the same value as you did.

Because we need to explain to our customer about that calculation. 

Regards,

Koji Hamamoto

Hi

I am sorry. I looked wrong cell on that calculation sheet.

It was 113kohm @1.8kW.

Thank you!

Regards,
Koji Hamamoto

Hi John

I got new information from the customer about this issue.

As you mentioned, the Rimo was 110kohm at 1800w.
However the Rimo is 19kohm when the current sens transformer turns ratio is 300 which is customer's condition. And this issue(output voltage goes down) is occurred at this condition.

Do you think the Rimo should be same value(19kohm) at Bridgeless topology?
Or do we need to set the Rimo more than calculated value for bridgeless topology?

Regards,
Koji Hamamoto

Hi Koji,

I would look at the current transformer. Check that it is a high frequency type and capable of delivering a valid waveform at this load point.

Monitor the signal inputs to CSA and CSB.

Regards,

john

Hi John,

I appreciate your kind support.

I talked with the customer about this issue. The phenomenon is occurred when the VAO is clamped to max voltage of this pin.

Also this phenomenon is improved when the Rimo is increased to over 27kohm. 

We think that the calculation tools is not suitable in case of bridgeless topology.

Because the ripple calculation formula on this sheet is not different from bridgeless topology.

Do you think this calculation tool can calculate in case of bridgeless topology as well?

Regards,

Koji Hamamoto

 

Hi Koji,
Can you please contact me at johngriffin@ti.com.
It will be easier to share information with email.

Regards,
John