AN2716

Electromagnetic compatibility (EMC) tests

Doc ID 14460 Rev 2

19/28

Figure 16.

AVF signal detection

The demonstration board and mains wires are placed 10 cm above the ground reference.

The mains wire is shorter than 1 m. Each operating cycle has been tested (load off and on).

The demonstration board supports burst levels higher than 4 kV without spurious triggering

of the ACS or ST7Lite3 MCU loss whatever the coupling mode (to L, N, etc.).

5.2.3

Advice to improve the application immunity

To improve the application performance in term of EMC, the software must be EMC oriented

(for more information please refer to the application note AN1015):

●

Auto-recovery routine. At each reset interrupt, the program must check if the data

stored in the RAM are as scheduled or not. A reset can occur without the supply

is not necessary, and the program can keep working with the previous RAM data. This

is helpful to avoid missing load control and the AC switch status when a reset occurs

due to an EMI problem, for example. If the checked RAM registers are not as expected,

then a complete initialization procedure is launched. If the RAM area is adequate, then

a “smart reset” can be performed. Only the registers that are used to store internal sub-

routine variables are cleared. Only the main registers keep their previous values (AC

switch status, AC switch control …).

●

Using the watchdog properly. Enable the watchdog as soon as possible after reset

and never refresh the watchdog in an interrupt routine.

●

Secure the unused program memory area. Fill the unused memory locations with

code that forces a watchdog reset or jumps to a known program

location if you do not

want to generate a reset.

●

Input filtering. It is recommended that the AVF signal be read during several AC line

cycles.

VAC

Timer

ZVS detection => Timer on

ZVS delay

> T/4 ms

T/2 ms

Read AVF and timer off

Read AVF and timer of

ZVS detection => Timer on

ZVS delay