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UCC28742: Relationship between tstr and Rstr

Shinichi Inoue
Guru 19931 points
Part Number: UCC28742
Other Parts Discussed in Thread: , UCC28740

Dear Specialists,

My customer is considering UCC28742 and has a question.

I would be grateful if you could advise.

---

I'm considering whether tstart time is <80ms.

Could you please let me know, as our system needs to be within 80ms.

According to the UCC28742EVM-001, start up time is almost 800ms

This value is calculated by equation(7). on the UCC28742 datasheet P.24, 1.09s.

In this case, Rstr is 15Mohm and CDD is 0.33uF.

In order to be tstart within 80ms, the product of RC must be 1/10 or less.

I gave two points that can be considered when the product of RC was reduced to 1/10.

(1)The power consumption of the resistor increases as the resistance value decreases.

(2)The cut-off frequency of the RC filter rises and the influence of noise increases.

(3)Is there any other possible disadvantage?

(4)Generally, I think that RSTR uses MΩ order, why is 100kΩ order not used?

ｰｰｰ

I appreciate your great help in advance.

Best regards,

Shinichi

  • 0
    TI__Mastermind 28890 points

    Hello Shinichi-san,

     

    Thank you for your interest in the UCC28742 flyback controller.

     

    Your point #1 is correct and power consumption is really at the root of the issue.

     

    For point #2, RC filtering is not a function of these components.

    For point #3, the entire disadvantage is in the power consumption.

    For point #4, RSTR is in Mohm order to minimize power consumption.

     

    The UCC28742 uses RSTR to charge up Cdd from the bulk voltage. RSTR is always in place, so is always providing current to VDD even after start-up. At high AC-line input voltage this current can be relatively high. It contributes to the no-load stand-by power consumption (Pstby), which often is required to be less than 75mW (based on European Code of Conduct Tier-2 efficiency requirements).

     

    Usually, RSTR is designed as high as possible to help minimize Pstby. The disadvantage is that it takes a long time to start up, especially if Cdd is large.

     

    It is possible to start up much faster by reducing RSTR, but then Pstby will be too high. The solution for fast start and low Pstby, then, is to have an active switch in series to turn off RSTR current after start-up. The UCC28740 controller has this switch built-in and does not need an external RSTR. Current is limited to ~250uA, so start-up into 0.33uF can be done in <30ms, but often Cdd must be larger than 0.33uF for other reasons.

     

    I suggest for you to review the UCC28740 for possible application for fast start-up. If it is not fast enough, then the UCC28742 can work with an external high-voltage switch with switch-control to shut off the current after start-up is complete.

     

    Regards,

    Ulrich

  • 0 in reply to Ulrich Goerke
    Guru 19931 points

    Hi Ulrich,

    Thank you for your reply.

    I understand Power loss is the problem when RSTR is lower.

    Also UCC28740 has a good function faster start up and low power consumption.

    I'll share these information with the customer.

    If they have an additional question, I cousult you again.

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi