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# ISO1212: When current goes over Rsense?

Part Number: ISO1212

Dear,

In related question you wrote that 500V surge gives 0.5A through ISO1212 https://e2e.ti.com/support/isolation-group/isolation/f/isolation-forum/768338/iso1212-recommended-cin-for-surge-protection

However, it is not clear when the device starts to swallow 0.5A when it can work up to 60V with current selected by Rsense value, for example, 2mA.
What is inside ISO1212 to start taking more than the selected current value above 60V and what is the exact value of this breakdown voltage?

It is also related to one more thread https://e2e.ti.com/support/isolation-group/isolation/f/isolation-forum/852396/iso1212-voltage-rating-of-cin where the voltage limit of the capacitor is recommended to be 60V without any surge *note.
So something is contradictory or the circuit has additional capabilities that are not described in the datasheet.

Best regards,
Miroslav

• Hi Miroslav,

Welcome to TI E2E forum and thanks for showing interest in ISO1212.

However, it is not clear when the device starts to swallow 0.5A when it can work up to 60V with current selected by Rsense value, for example, 2mA.
What is inside ISO1212 to start taking more than the selected current value above 60V and what is the exact value of this breakdown voltage?

ISO1212 is designed to limit the input current using RSENSE for line voltages of upto 60V, for higher voltages the current is no longer limited/regulated as device is not designed to support higher voltages than 60V directly. Hence, when a surge voltage is applied that is much larger than 60V, the input sinks in current according to the series resistance.

For surge transient currents, the device can handle upto 0.5A. This needs to be considered into account while designing the input for surge specifications.

It is also related to one more thread https://e2e.ti.com/support/isolation-group/isolation/f/isolation-forum/852396/iso1212-voltage-rating-of-cin where the voltage limit of the capacitor is recommended to be 60V without any surge *note.

Please note that the voltage on ISO1212 pins directly cannot exceed 60V as that is the absolute maximum rating of device pins. Since the CIN capacitor is also connected directly onto device pins, it will also needed to be rated 60V only and maybe some margin. All the excess surge voltage is made to appear across the series RTHR resistor by choosing its value such that the peak input current into device is not exceeding 0.5A.

I hope this answers your questions, thanks.

Regards,
Koteshwar Rao

• Thanx Koteshwar!