• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 62 subscribers
  • Views 445 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

UCC2813-4: Efficiency is Low and It seems current flow through the switch only in part time, not in all of Ton time.

Yansheng Yue
Intellectual 370 points

Part Number: UCC2813-4

Hi, I just designed a fly-back DC-DC converter with UCC2813-4. But it seems not work in high efficiency.

I checked the waveform and found that the duty cycle is only about 29%. At the same time, there's no current flow through the MOSFET at beginning of On-time. The current waveform begins rising at the last one-3rd of the On time. It means the MOSFET is on only a very short time. Maybe that is why the efficiency is very low.

Can you help me to find where is the fault in my design? Thanks.

Desing parameters as follows:

Vin=24V, Vout=20V, Iout=1A

The transformer is EE25-3 with PC40 core. The turns are 14,19 and 12 for pri, sec and aux. The measured Lpri and leakage is 220uH and 1.4uH.

This is the schematic.

Ch1 is the waveform of OUT pin and ch2 is the anode waveform of rectifier  diode of the secondary side.

Here, Ch2 is the waveform of CS pin. It seems that the current flow through the MOSFET only in the second half part of switch-on time.  

  • 0
    TI__Guru 74075 points

    Hi,

    What do you mean "not work in high efficiency"? Do you mean you want to higher efficiency from your converter? Please clarify.

  • 0 in reply to Hong Huang
    Intellectual 370 points

    Hi, thanks for your reply.

    I mean that the total efficiency is only about 62%.The MOSFET and snubber resistor consume a lot of power because these two devices' temperature will rise up to more than 75 degree in 3 minutes if there' not any hot sink mounted on them.

    I don't know where the problem is. I found there's current flow through the MOSFET only during the second half part of switch-on time, which just showed as the waveform attached in the last post. 

  • 0 in reply to Yansheng Yue
    TI__Guru 74075 points

    Hi,

    You need to check Vgs and Vds waveform to see if the switching losses, and your MOSFET Rds-on losses, then to estimate the transformer losses, and the secondary side rectifier diode power losses. These are main losses you need to measure and find out then to see how to reduce each. I do not think 29% duty is the cause, which is related to your transformer turns-ratios, your load current, and your current sense resistor value.