Thermal protection of a switch

1. A method for detecting an overheating of an electronic switch of power supply of a load by an A.C. voltage, wherein a voltage representative of the temperature in the vicinity of the switch is compared with a threshold, the result of this comparison being sampled at zero values of the A.C. power supply voltage and at a frequency corresponding to an even multiple of the frequency of the A.C. power supply voltage, to provide a signal indicative of the fact that a temperature threshold has been exceeded.

2. The method of claim 1 , wherein the sampling frequency corresponds to twice the frequency of the power supply voltage.

3. The method of claim 1 , wherein a deactivation of said sampled signal is delayed by an odd number of zero crossings of the A.C. power supply voltage.

4. The method of claim 3 , wherein said odd number is 3.

5. A circuit for detecting a possible overheating of an A.C. switch, comprising:

a comparator of an A.C. voltage proportional to the current temperature with respect to a threshold; and

a flip-flop sampled at zero values of the A.C. voltage and at a frequency corresponding to an even multiple of the frequency of the A.C. voltage.

6. The circuit of claim 5 , wherein a counter of an odd number of zero crossings of the A.C. voltage is activated by the output signal of said flip-flop and is sampled at the same frequency as the flip-flop.

7. The circuit of claim 5 , wherein said flip-flop is controlled by a signal for detecting the zero crossing of the A.C. supply voltage.

8. The circuit of claim 5 , wherein said flip-flop is controlled by a signal for detecting the zero crossing of the current through the switch.