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TPS7A78: Efficiency at low output current

Prodigy 70 points

Replies: 2

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Part Number: TPS7A78

Hi Ti

The TPS7A78 looks ideal for an application we have for a wireless device where we have previously used a cap dropper circuit or a cap dropper followed by a buck converter (TPS54601).

I'd just like to clarify the device efficiency.

In a conventional cap dropper a Zener diode clamps the rectified voltage. Current that passes through Cs and is not used by the application is dissipated as heat in the Zener. The power consumed by the circuit is fixed by the value of Cs whether it is used or not.

Am I right in understanding that the TPS7A78 shunts excess current via the controlled bridge back into the AC supply? The apparent power will remain unaffected by this but the real power consumed by the application will be reduced?

Thanks, Nick

  • Hi Nick,

    You are correct in that the active bridge in the TPS7A78's active bridge shunts the current thus removing the power dissipated in the Zener diode in a traditional cap-drop supply.

    I have a calculator that should be coming online soon which helps with basic component selection and it estimates the potential efficiency in the application. 

    I hope this helps.

    John Cummings 蔣康明
    Texas Instruments 德洲儀器
    Johncummings@ti.com

    Please click the "Resolved" link at the bottom of this post if I have answered your question.

    Thank you,

  • In reply to JCHK:

    Hi John

    Thanks for the quick reply and clarification. This is one interesting chip.

    Issue resolved.

    Best regards, Nick

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