AN3213 datasheet(47/64 Pages) STMICROELECTRONICS

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AN3213

ST7570 demonstration board description

Doc ID 17429 Rev 1

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7.6

Surge and burst protection

The specific structure of the coupling interface circuit of the application is a weak point

against high voltage disturbances which may come from the external environment. In fact,

an efficient coupling circuit with low insertion losses realizes, consequently, a low

impedance path from the mains to the power line interface of the device.

For this reason it is recommended to add some specific protections on the mains coupling

path, to prevent high energy disturbances coming from the mains from damaging the

internal power circuitry of the ST7570.

The possible environments for this kind of application can be both indoor and outdoor;

residential, commercial and light-industrial locations. To verify the immunity of the system to

environmental electrical phenomena, a series of immunity specification standards and tests

must be applied to the power line application.

The immunity requirements for any PLC metering application, communicating in the

European A band (9-95 kHz), are listed in the EN50065-2-3 document, which refers to

EN61000 and ENV50204 for tests to be applied (References 3).

These standards include surge tests, both common mode and differential mode (+/- 4 kV

peak, tR = 1.2 µs, tN = 50 µs) and fast transient (burst) tests (+/- 2 kV peak, tR = 5 ns, tH = 50

ns, repetition frequency 5 kHz).

For the application to be able to withstand such a severe electrical overstress, the coupling

capacitor C4 must be a Y2 type part, rated for 5 kV pulses.

In the case of non-metering applications, communicating outside the A band, the

requirements are listed in the EN50065-2-1 document, which set lower pulse levels. In such

a case, an X1 capacitor (e.g. Epcos B32912A3224K) can be used instead of a Y2 part.

Protection devices must also be included in the board design, as described below. Figure 43

and 44 show the protection against common mode disturbances. D4 and D2 low-drop

Schottky diodes are able to quickly absorb fast transient disturbances exceeding the supply

rails. Figure 45 describes the protection intervention in case of differential mode

disturbances. A differential voltage higher than 15 V p-p is clamped by the D1 bi-directional

Transil diode. D1 is the most robust protection and also the one which is able to absorb most

of the energy of any incoming disturbance.

Figure 43.

Common mode disturbance protection - positive disturbance

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