WEBENCH® Tools/UCC28070: controller design

Hi, Harijarani

I am not quite understand your question, do you mean input rms current increase with input voltage decease and efficiency become lower with lower input voltage?

If yes, I can tell you it is very normal phenomenon. With lower input voltage, because output power is constant, so input rms current increased.

And power loss of power device is increased of Boost,like MOSFET, diode. So efficiency is lower.

If not, please clarify your question.

Regards,

Yunsheng 

Hi yunsheng,

Thanks for your response. I know that input rms current is high at lower input voltages.But in my design rms current is increasing proportionally with input voltage which leads to reduction in the efficiency. This is the problem am facing. can you please help me to resolve this issue. 

Hi Harija

If the output voltage is constant and you have a constant load (either R or Constant current) then the output power from the system is constant and given a constant efficiency the input power will also be constant. If Vin (RMS) increases then Iin (RMS) must decrease.

So: If you are getting increasing input current with increasing input voltage then power is being dissipated somewhere in the system.

I'd check the following

Are Vo and Io constant - there will be some ripple at twice line frequency but if it's less than 7 or 8% of Vo you can ignore it.

Are the Switches running correctly ? What is the V drop across the power MOSFET and Output Diode ?
Double check the forward voltage drop of the diode model you are using - set up a separate sim and simply run 5, 10, 15A DC through it and see that Vf is reasonable in all cases.

Regards

Colin

Dear colin,

your guess is correct. output voltage ripple is very high i.e around 10% and also i had observed that the interleaving not working properly in my design. even though pulses getting fine with 180 degrees phase shift  currents through the boost inductors overlapping and also peak current is higher than the calculated value. Please support me limit this inductor current and also let me know the way of making inductor current continous without any overlapping.

Attached here the inductor current waveform.

Awaiting for your response.

Hello Harija

These inductor currents look ok to me with the following comments
They are 180 deg out of phase -
The Duty cycle is less than 50% so the currents are not 'overlapping' - this behaviour will disappear as the duty cycle increases
Can you calculate the di/dt of the rising edge of the current and then calculate the inductance from V = L di/dt. The result should be the same as the inductor value you have in your sim.
The inductor current drops to zero at the end of the cycle - this means that you don't have enough inductance to maintain CCM or that the load current is too small to maintain CCM. If the load current is correct (your full load condition) then you need to increase L_pfc.

Again, as a diagnostic, I'd suggest you run the system from DC at Vin = 75% of Vout, at 25% of Vout and at 50% of Vout. this will give you duty cycles varying from < 50% to > 50% and allow you to observe the sim behaviour without worrying about line voltage effects. Once you are happy with the DC input behaviour you can then turn on the AC.

Overall - It looks like you are making good progress.

Regards
Colin

Dear colin,

as said earlier my efficiency is going to low values at higher voltages due to proportional increase in the input current. From past 3 weeks am suffering with this problem. Please suggest me the way to debug this proble. I have only 4 more days to complete this project.am expecting support from you.

Awaiting for your response.

Hello Harija

I'm travelling on business all this week and will have almost no opportunity to think about this - or indeed any other issue.

All I can suggest is that you check that Vin max is lower than Vout.

I'd also take a plot of efficiency versus input voltage and a plot of input current against input voltage. Check to see if there is a point at which the system starts to lose efficiency.

I'd also try to identify where the power is going.

A good analysis of the problem is likely to be almost as useful as a  completely working  project - demonstrate your attempts to debug the issue and to understand it. Understanding is a good first step.

Regards

Colin