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Original question:

SN6507-Q1: SN6507-Q1: Design with duty control and Half-wave rectification

SN6507-Q1: Unable to operate the Power supply in Duty cycle control for better line regulation.

Venkatesh Naidu
Prodigy 20 points
Part Number: SN6507-Q1
Other Parts Discussed in Thread: SN6507, SN6507DGQEVM

I am trying to design a power supply using SN6507 with the following specification. 

  1. Input – 8V to 16V
  2. Output Voltage – 15V
  3. Output Current ~20mA

 I tried testing this concept using SN6507DGQEVM Evaluation board.

First step was to modify the transformer to a value which supports this requirement. For which I found the following reference :-

 

Considering the turns ratio of the above options I had to reverse(Primary <--> Secondary) the transformer present on the board i.e. Coilcraft TX1-ZB1459-BE.

 

Rds was calculated as per the following formula.:-

 

The slew rate and the frequency were kept the same. Output inductor bypass jumpers were removed.

 

The Testing result was disappointing, it had poor regulation. The PS doesn’t go into Duty cycle control for the required range of input.

Here the horizontal is the Vin and vertical is the Vout.

 

Can you figure out what can be issue? Let me know soon.

  • +1
    TI__Guru 55815 points

    Hi Venkatesh,

    Thank you for reaching out and for sharing details related to your issue.

    Venkatesh Naidu said:
    Considering the turns ratio of the above options I had to reverse(Primary <--> Secondary) the transformer present on the board i.e. Coilcraft TX1-ZB1459-BE

    Please note that when duty cycle control is used, the typical duty cycle is to be set to 25% as against the default duty cycle of 48% with no duty cycle control. This is done to allow increasing and decreasing of duty cycle with input voltage variation such that output voltage stays in regulation.

    Since the duty cycle is nearly halved, the turns-ratio of the transformer needs to be doubled to achieve normal operation and make duty cycle control work. Due to this, transformer Coilcraft TX1-ZB1459-BE might not be suitable for your input / output requirements even when the transformer is reversed. The suitable transformer for your application would be TX1-ZC1892-AED with primary and secondary swapped. This transformer produces 30V output from 12V input when used at 48% duty cycle while it produces 15V output from 12V input when duty cycle is set to 25%. As the 12V input varies between 8V and 16V, the output will vary proportionally.

    Venkatesh Naidu said:
     

    When I substituted values according to your applications, I calculated RDC to be 25kΩ.

    Venkatesh Naidu said:
    Output inductor bypass jumpers were removed.

    The filtering works best for higher loads, 20mA is too low a current for the filter inductor or duty cycle control to work. Using the equation for LMIN, I calculated the minimum value to be 117µH while the EVM has 33µH. You will have to replace the inductors with 117µH or more to achieve needed output filtering.

    Let me know if the above inputs help you modify the circuit to suit your application needs, thanks.


    Regards,
    Koteshwar Rao