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SN6507DGQEVM: The SN6507 wide range input function does not work properly.

Hongseok Kim
Prodigy 30 points
Part Number: SN6507DGQEVM
Other Parts Discussed in Thread: SN6507,

Hello

I want to design a +-15V circuit with 18~30V(24V) input using SN6507.

SN6507DGQEVM is used, all functions except DC function are set to default, and j7 and j12 jumpers are removed and the installed L is used.(1Mhz, DC_ADJ, EN_H)

Currently, I am checking the input by applying a load using a resistor without connecting the LDO circuit. (The resistance value is 680ohm.)

After changing the J13 Jumper to DC adjust, the Rdc value was calculated and used according to the contents of the DataSheet.

RDC = 0.816 x D x VCC x (RCLK + 1) - 1

RDC = 0.816 x D x 24V x (9600 + 1) -1

RDC = 47Kohm 

According to the design, +15V output should come out, but the output voltage from each voltage is as follows.

Input 18V -> Output 9.39V

Input 24V -> Output 11.61V

Input 30V -> Output 14.31V

Since the diode has not been changed yet, it is the initial purchase state of the development board that only outputs +15V.

I want to know what went wrong.

  • 0
    TI__Guru 54195 points

    Hi Kim,

    Thanks for reaching out and sorry to hear about the issue.

    We expect the SN6507 to support the wide range input voltage in DC control mode without any issues. The EVM should already be tested for this configuration, please allow me to check on this and come back to you. Thanks.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    TI__Guru 54195 points

    Hi Kim,

    There is an application note listed on SN6507 product page under technical documents, please see the link below. This application note has SN6507DGQEVM tested with 16V to 32V with 15V output, please refer to Figure 4 (also copied below).

    Optimize 24-V Isolated Power Supply Designs in PLC I/O Modules with SN6507

    The following are the reasons why the device was not working as expected in your case,

    1. RCLK value to be used is 9.6 and not 9600
      1. RDC = 0.816 x D x VCC x (RCLK + 1) - 1 = 0.816 x D x 24V x (9.6 + 1) -1 = 51kΩ
    2. Transformer turns-ratio needs to be doubled for DC control mode of operation as duty cycle gets reduced from 0.48 to 0.25.
      1. i.e., instead of 750319696, please use 750319948. The transformer used in the above application note is actually 750319948 while it was mistakenly referred as 750319696. Sorry about that, we will get this correct.
    3. The minimum inductance value (LMIN) needs to be changed from 33µH on default EVM to 100µH due to low load.

      After making the above 3 changes, you should see your design work fine for DC control mode with wide input voltage range. Let me know if you have any further questions, thanks.


      Regards,
      Koteshwar Rao

    4. 0 in reply to Koteshwar Rao
      Prodigy 30 points

      Thanks for the reply Koteswar Rao.

      If the turns ratio of the transformer used in the EVM is 1:0.7, if I input it in the 0.7 part, the turns ratio will be about 1:1.4

      Is it possible to test using this?

    5. 0 in reply to Hongseok Kim
      TI__Guru 54195 points

      Hi Kim,

      Yes, connecting the transformer in a reverse orientation does double the turns-ratio which happens to work in our favor. Although it lowers the primary inductance when reversed, it should be okay for testing. Make sure you do not use connecting wires to connecting the transformer which otherwise could lead to other issues. Thanks.


      Regards,
      Koteshwar Rao

    6. 0 in reply to Koteshwar Rao
      Prodigy 30 points

      Thanks for the reply Koteswar Rao.

      As advised, I changed to RDC: 51Kohm, L: 100uH, and connected the transformer in the reverse direction, and the test result came out as follows.

      Based on the above calculation formula in the data sheet, 100uH should operate normally at a minimum load of about 20mA.

      However, as a result, even under a load of 100mA and 24V, the duty is greater than 25%, and as the load is reduced, it becomes more distant from the design.

      Rather, in the initial setup, only the reverse connection of the transformer gives better results than after the change.

      I would like to know if this is a problem with the transformer or if additional work is needed to improve it.

      In addition, the transformer gets extremely hot in every operation.

    7. 0 in reply to Hongseok Kim
      TI__Guru 54195 points

      Hi Kim,

      Thanks for testing the SN6507 with the suggested changes and sharing the update.

      Please note that when duty cycle control is not used, the device operates with fixed duty cycle of 48%. When input voltage varies, the output voltage will also vary proportionally as the duty cycle is fixed.

      In duty cycle control mode, the typical duty cycle is set to 25% by external resistor and when input voltage varies, the duty cycle is proportionally varies between 10% to 43% to maintain a constant output voltage. Hence, what you are observing is expected and in duty cycle control mode, duty cycle is varied to maintain line regulation.

      Duty cycle mode works best for more than 20% load and for it to work good for low loads, the Lmin needs to increased further. Otherwise, all the measurements you have seen seem expected.

      Regarding the transformer heating, do you have any specific values of temperatures that you see? Since, the transformer was not optimum to be used in duty cycle control mode, maybe that is leading to heating.

      Let me know if you have any further questions, thanks.


      Regards,
      Koteshwar Rao