AN2744 datasheet(39/56 Pages) STMICROELECTRONICS

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AN2744

Board description

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5.5

Surge and burst protection

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

against high voltage disturbances that can come from the external environment. In fact an

efficient coupling circuit with low insertion losses realizes consequently a very low

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

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

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

the internal power circuitry of the ST7538Q.

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 requirements for ac-connected ports include EN610000-4-4 (electric fast transients),

EN610000-4-5 (surges), EN610000-4-6 (RF out-of-band disturbances), EN610000-4-11

(voltage dips). All these tests are listed in the EN50065-2-3 document (part 7, immunity

specifications).

In particular, surge tests are specified as both common and differential mode at level +/- 4

kV, with pulse shape 1.2 x 50 µs. Fast transient burst tests are specified at level +/- 2 kV,

with pulse shape 5 x 50 ns and pulse frequency 5 kHz.

Figure 37

and Figure 38 illustrate the protection criteria implemented in the ST7538Q

reference design.

Figure 37

shows the protection against common mode disturbances. The ESD Transil™

protection diodes are able to absorb quickly fast transient disturbances starting from their

6.1 V threshold voltage.

Figure 38

describes the protection intervention in case of differential mode disturbances. A

differential voltage higher than 6.8 V is shorted by the bidirectional power Transil™, which is

the most robust protection and also the one that is able to absorb most of the energy of any

incoming disturbance.

Figure 37.

Common mode disturbances protection

ATOP1

ATOP2

ATOP1

ATOP2

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