Inverter and method to measure junction temperature for thermal protection

An inverter system to be coupled to three phase windings of an electric machine, the inverter system comprising:

• a first switching branch coupled between a DC supply voltage node and an intermediate node, the first switching branch having a first terminal to be coupled to a first phase winding of the electric machine;
• a second switching branch coupled between the DC supply voltage node and the intermediate node, the second switching branch having a second terminal to be coupled to a second phase winding of die electric machine;
• a third switching branch coupled between the DC supply voltage node and the intermediate node, the third switching branch having a third terminal to be coupled to a third phase winding of the electric machine;
• a shunt resistor coupled between the intermediate node and ground;
• and
• control circuitry configured to apply control signals in accordance with a space vector pulse width modulation (SVPWM) scheme to the first, second, and third switching branches to drive the first, second, and third phase windings of the electric machine with an three phase AC power signal;
• wherein the SVPWM scheme is divided into six sectors, with each sector being divided into a plurality of periods comprising first, second, third, and fourth periods;
• and
• wherein the control circuitry is configured to, during at least some sectors of the SVPWM scheme:
• during the second period of the plurality of periods:
• measure a shunt voltage across the shunt resistor;
• and
• measure a drain to source voltage of a first calibration transistor, the first calibration transistor being a transistor of a given switching branch;
• during the third period of the plurality of periods:
• measure a shunt voltage across the shunt resistor;
• measure a drain to source voltage of a second calibration transistor, the second calibration transistor being a transistor of a next switching branch;
• and
• measure the drain to source voltage of the first calibration transistor;
• during the fourth period of the plurality of periods:
• measure the drain to source voltage of the first calibration transistor;
• measure the drain to source voltage of the second calibration transistor;
• and
• measure a drain to source voltage of a measured transistor, the measured transistor being a transistor of a remaining switching branch;
• determine a drain to source resistance of the first calibration transistor as a function of a resistance of the shunt resistor, the drain to source voltage of the first calibration transistor measured during the second period, and the shunt voltage measured during the second period;
• determine a drain to source resistance of the second calibration transistor as a function of a resistance of the shunt resistor, the drain to source resistance of the first calibration transistor, the drain to source voltage of the first calibration transistor measured during the fourth period, the drain to source voltage of the second calibration transistor measured during the fourth period, and the shunt voltage measured during the fourth period;
• determine a phase current of a given phase winding coupled to the given switching branch as a function of the drain to source voltage of the first calibration transistor measured during the fourth period and the drain to source resistance of the first calibration transistor;
• determine a phase current of a next phase winding coupled to the next switching branch as a function of the drain to source voltage of the second calibration transistor measured during the fourth period and the drain to source resistance of the second calibration transistor;
• and
• determine a phase current of a remaining phase winding coupled to the remaining switching branch as a function of the phase current of the given phase winding and the phase current of the next phase winding.
• wherein the control circuitry is further configured to:
• control driving of the first, second, and third switching branches based upon the phase current of the given phase winding, the phase current of the next phase winding, and the phase current of the remaining phase winding;
• determine a temperature of the first calibration transistor during driving of the electric machine by the inverter system based upon the drain to source resistance of the first calibration transistor;
• modify driving of the given switching branch so as to maintain conductivity of the first calibration transistor at a highest level at which the temperature of the first calibration transistor remains below a threshold temperature;
• cease driving of the given switching branch when the temperature of the first calibration transistor exceeds the threshold temperature;
• determine a temperature of the measured transistor during driving of the electric machine by the inverter system based upon the drain to source resistance of the measured transistor;
• modify driving of the next switching branch so as to maintain conductivity of the measured transistor at a highest level at which the temperature of the measured transistor remains below the threshold temperature;
• and
• cease driving of the next switching branch when the temperature of the measured transistor exceeds the threshold temperature.