AN1354

When dealing with the current rating for AC semiconductor switches, the rate of decrease of the current must also be checked. This constraint will depend on the chosen topology.

The worst case of turn-off stress appears with a compressor without any start capacitor. In this case, the rise in voltage will not be slowed by the motor capacitor. The higher stress occurs for the "START" winding (where the impedance is lower than the "RUN" winding one) and when the rotor is stalled. These two conditions yield a higher current and therefore, a higher rate of decrease for the ACST current.

Then, for a stalled 1/5 Horse power compressor, supplied with a 264 V RMS voltage, the dI/dtc and dV/dtc equals respectively 2.4 A/ms and 9.6 V/µs through the START ACST (cf. Figure 5, measured with THERM01EVAL board). This is far below the maximum withstanding for ACST6 devices, which is 3.5 A/ms with a 15 V/µs rate.

Up to now, refrigerator compressors have been controlled by electromechanical switches (thermostat or even electronically controlled relays). AN1354 was driven by the high inrush current that can appear when the rotor is stalled. Furthermore, electromechanical relays are advantageous because they are less sensitive to line voltage disturbances. Today, new semiconductor devices feature over-voltage protection and high inrush current capability, allowing them to be used in cold appliances.

AN1354 can so be implemented, allowing the appliance efficiency to be improved by more than 20 W, for 150 W compressors, thanks to better temperature control and the removal of the PTC. Hence, at a similar cost as Electromechanical thermostats, this technical breakthrough could allow refrigerators or freezers to fulfill Class A consumption requirements, bringing the following advantages:

· Better reliability:
    - Higher switching robustness of static switches towards mechanic solutions
    - Higher ACS and ACST overvoltage robustness towards Triacs, which allows Metal-Oxyde varistor removal
· Temperature regulation law flexibility (automatic defrost, Hysteresis threshold adaptation)
· Reduction of the temperature ripple (better food preservation, appliance elements downsizing)
· Possibility to add indication features for the end-user (inside temperature, open door)
· Spark-free operation and EMI reduction (switches can be turned on at Zero Voltage and are turned off at
    Zero Current)
· Overcurrent protection of the motor winding.

This paper outlines the different topologies that can be used for electronic motor control, and lists the electrical constraints AN1354 that result from these different circuits. A comparison is also made between the different performances of electromechanical or electronic thermostats.

All numerical examples are based on the specifications for a 1/5 Horse power compressor, which can be used in 350 L freezers