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TPS7A78: AC+, SC_IN waveform

Part Number: TPS7A78


My customer have some questions regarding TPS7A78EVM waveform.

We change the AC input voltage from 80VAC to 240VAC and find the value of SC IN and AC+ voltage is constant.

however, the [H] duty of SCIN begins to decrease when we increase the AC input voltage as follows.

Regarding on the waveform as follows we have some question, please give us an explanation.

1. Why is the AC+ voltage constant ? even if we change the AC INPUT voltage. whether there is a clamp circuit ?

2. IS there a minimum on DUTY spec for SC IN PIN? if we increase AC INPUT voltage above 300VAC or higher,

     the SCIN pin or AC+ PIN will be broken because of overcurrent ?


  • Hi Mukuno,

    1. Yes, the device full-bridge rectifier has a clamp to maintain SCIN pin voltage as specified in the datasheet section 7.3.1 regardless on the AC supply input voltage and frequency.

    2. AC supply voltage is going to be dropped across the cap-drop capacitors Cs and the surge resistor Rs is going to limit the hot-plug current to AC+ and AC- pins. As long as you cap-drop capacitor Cs is rated for the peak AC supply voltage and you have calculated your surge resistor Rs according to peak AC voltage, then SCIN, AC+, and AC- pins voltages will be fine.

    As mentioned in the datasheet section 8.1.2 and section 8.1.3, the max AC supply voltage that can be used is determined by the availability of high voltage Cs capacitor and to limit the hot-plug current, the min value of the surge resistor Rs must be calculated based off the peak AC supply voltage.

    Please click on verified answer if I have answered your questions.