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# TPS7A78: Addition of bidirectional TVS clamp at AC+/- pins to reduce Rs value

Part Number: TPS7A78

Hi Ti

I understand that the resistor Rs is required to limit the surge current into the AC+/- pins when the device is first powered from the AC supply. The limiting current is 2.5A from the datasheet.

Our application will run from 230Vac so the value of Rs is Vac (max) / 2.5A. Or (230 x 1.414)/2.5 = 130 ohms. Happy with that!

If we add an MOV to protect against surges I expect the clamp voltage to be about 1000V. Re-calculating the value of Rs for this scenario yields an Rs value of 400 ohms. If our dropper capacitor is 330nF then our rms current through this 400 ohm resistor is 23mA resulting in a power consumption of 210mW. (Rough figures) Not so happy with that - would like to save a few mW as the application is all about saving power.

Is an alternative approach to add a TVS bidirectional clamp across the device AC+/- pins to deal with surges and keep the value of Rs at 130 ohms for normal running conditions. I was thinking of a 16 or 18V TVS which will clamp at less than 30V with a peak current of 20A. The resistor Rs is a lower cost option but we'd like the lowest power consumption achievable.

I realise this may only be suitable for 3.3V output.

Thanks, Nick

• Hi Nick,

A long as the absolute maximum ratings of the device are not exceeded (2.5A into the VAC pins), the TPS7A78 can work with any external surge scheme you come up with. Please note for 3.3Vout, the SCIN pin could be between 16-17V depending on load so if the TVS clamps too low, this could also be dissipating power.

I hope this helps.

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