Junction temperature sensing for MOSFET
View Patent ↗A method of protecting a MOSFET switch from overheating. The method is achieved by determining a ratio of ON resistance of the switch to resistance of a sense resistor, the sense resistor being series coupled in the source-drain circuit of the switch. Further, the ratio is compared to a reference signal and an output signal is generated when the ratio exceeds the reference signal.
1. A method of protecting a MOSFET switch from overheating, the method comprising the steps of:
determining a ratio of ON resistance of the switch to resistance of a sense resistor, the sense resistor being series coupled in the source-drain circuit of the switch;
comparing the ratio to a reference signal; and
generating an output signal when the ratio exceeds the reference signal.
2. The method of claim 1 , wherein values of the ratio correspond to junction temperatures of the switch.
3. The method of claim 1 , wherein the reference signal corresponds to a predetermined threshold of a junction temperature of the switch.
4. The method of claim 1 , wherein the output signal indicates overheating of the switch.
5. A circuit for providing an overcurrent protection of a MOSFET switch, the circuit comprising:
a sense resistor coupled in series with the switch,
a first amplifier coupled across the switch,
a second amplifier coupled across the sense resistor,
a divider circuit coupled to outputs of the first and second amplifiers for generating a ratio signal,
a comparator coupled to an output of the divider circuit and to a reference signal,
wherein the comparator generates an output signal when the ratio exceeds the reference signal.
6. The circuit of claim 5 , wherein values of the ratio correspond to junction temperatures of the switch.
7. The circuit of claim 5 , wherein the reference signal corresponds to a predetermined threshold of a junction temperature of the switch.
8. The circuit of claim 5 , wherein the output signal indicates overheating of the switch.
9. The circuit of claim 5 , wherein the divider circuit includes a multiplier having a first input from the second amplifier and a second input from a third amplifier, the third amplifier receiving an input from the first amplifier and another input via a closed loop from the multiplier output, whereby the third amplifier generates the ratio.
10. A circuit for providing an overcurrent protection of a switch, the circuit comprising:
a sense resistor coupled in series with the switch;
a first amplifier coupled across switch; and
a multiplier having a first input coupled to a junction between the sense resistor and the switch and a second input from a second amplifier, the second amplifier receiving an input from an output of the first amplifier and a second input from the multiplier output via a closed loop.
11. The circuit of claim 10 , wherein the second amplifier generates an output signal proportional to a ratio of an ON resistance of the switch to the resistance of the sense resistor.
12. The circuit of claim 11 , further comprising a fault comparator receiving as a first input the output signal and as a second input a reference signal and generating a fault output signal when the ratio exceeds the reference signal.
13. The circuit of claim 10 , further comprising an overcurrent comparator for generating an overcurrent signal if it is determined that a signal proportional to a voltage across the switch has exceeded a predetermined current limit provided to the overcurrent comparator as the second input.