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TPS7A78: TPS7A78 incluse some circuit protection in the front of Full bridge recitifier or not

Part Number: TPS7A78

Hi Sir,.

please help to check

(1)    What is C6 , R5 voltage rating and what is footprint should I use for this AC/DC application

(2)    Should I incluse some circuit protection in the front of Full bridge recitifier or not.

(3)    For the C3, my calculation is 168uF but I use 220uF here, it is okay for this value

TPS7A78_PIC_project.pdf

  • Hi Philip,

    Thank you for your interest in our TPS7A78 smart AC/DC LDO. The device datasheet and Evolution module, EVM, provide a wealth of information to get you started on your application, however, TI recommends prototyping for your design to get the desirable results for your end application.
    For your questions:


    1. Cap-drop Cs, C6 in you schematic, should be rated to handle the peak VAC supply. As for Surge resistor Rs, R5 in your schematic, should be calculated to be VAC(peak) / 2.5A. Both Cs and Rs values are explained in the datasheet sections 8.1.2 and 8.1.3 respectively.


    2. Cs and Rs provide protection to the device bridge, however, if you are talking about HV transient hazards such as lighting, then an MOV or TVS is recommend at the VAC supply as shown in figures 7 and 8 in the EVM user’s guide and the value of Rs should be calculated based off the peak clamping voltage of such devices as explained in TPS7A78 datasheet section 8.1.3


    3. I see that you are using a HB configuration and the 168 uF cap suggests that your load current ~ 48 mA. If my understanding is correct then yes a 220 uF with more than 35V voltage rating should be ok.

    I also wanted to point out some comments on your schematic;


    a. Your C2 and C1 need voltage ratings of 16V and 10V respectively as mentioned in the datasheet pin function table for those pins. We are in the process of updating the device datasheet for the device full release, so there is a discrepancy in CSC2- pin voltage rating where is should be 10V for both CSC2+ and CSC2- pins.


    b. R1 resistor needs to be 20 kΩ instead of 1MΩ, this is also part of the datasheet update process for the maximum value of RPFD from 1M to 20k. Soon that basis, your calculated R2 value would be in 1.6kΩ range.


    Again thank you for your interest in TPS7A78 device and as mentioned above, hope this help.

  • Yes Haidar,.

    thank you

    besdies, we have definition of TPS7A78 device for the application,  

    1. Cap-drop Cs, C6 in you schematic, should be rated to handle the peak VAC supply. As for Surge resistor Rs, R5 in your schematic, should be calculated to be VAC(peak) /

        2.5A. Both Cs and Rs values are explained in the datasheet sections 8.1.2 and 8.1.3 respectively. 

     ( could you check definition if this is specified by the maximum value then it should just directly specify in the datasheet)

     

    2. Cs and Rs provide protection to the device bridge, however, if you are talking about HV transient hazards such as lighting, then an MOV or TVS is recommend at the VAC

    supply as shown in figures 7 and 8 in the EVM user’s guide and the value of Rs should be calculated based off the peak clamping voltage of such devices as explained in TPS7A78 datasheet section 8.1.3   

    Could I skip this transient component since I guess this chip has the capability of protecting itself and it is not mandatory in the section. Could you ask TI if it can pass the regulation without using MOV or TVS. It could also save the cost and space of my circuit board.

     

    3. I see that you are using a HB configuration and the 168 uF cap suggests that your load current ~ 48 mA. If my understanding is correct then yes a 220 uF with more than

    35V voltage rating should be ok. ( this is full bridge diode MB10S https://www.diodes.com/assets/Datasheets/MB10S.pdf so it should be around 100mA)

  • Hi Philip,

    1. The maximum VAC supply voltage that can be used for an application is determined by the availability of high voltage cap-drop capacitors that can handle such voltage since most of the VAC supply voltage is going to be dropped across this capacitor (Cs) and that's why there was no specific max VAC voltage specified in the datasheet. This was specified in both sections 8.1.2 and 8.1.3 of the datasheet but you are right it needs more clarification and this what we are currently working on for the upcoming revision of the datasheet in the next few months.

    2. Transient components such as TVS, MOV, fuse, ... etc. you are referring to are a system level protection which is outside TI scope for the design and passing regulation. TI recommends prototyping for customers design to get the desirable results. As for Cs and Rs components, the proper sizing and voltage rating of cap-drop Cs ensure the voltage on AC+/AC- pins to stay within the Recommended Operating Conditions similarly for Rs to ensure that surge current typically for 100 us period or less to say with Recommended Operating Conditions.

    3. For full-bridge 100 mA load current with 3.3 V output voltage and according datasheet section 8.1.4, the minimum CSCIN capacitor size shall to be;

    a - 140 uF (if 60Hz VAC is used)
    or
    b- 170 uF for (if 50Hz VAC is used)

    Again thank you for your interests in TPS7A78 device, please let me you should you have further questions.

    BR,

    Haidar