Resistance feedback controlled power supply

A power supply for supplying AC power to a surgical instrument having a resistively-heated heating element, the heating element having a resistance that varies with temperature, the power supply comprising:

• a current supply for supplying a DC sense current to the heating element, the sense current generating a DC voltage signal proportional to the resistance of the heating element;
• an AC voltage supply for receiving a control signal and providing the AC power proportional to the control signal, the AC power provided simultaneously with the DC sense current and having a waveform with a cyclically varying instantaneous amplitude, the AC voltage supply having:
• an input terminal for receiving the control signal, and
• a power output for supplying the AC power to the heating element;
• a blocking capacitor coupled between the power output and the heating element for isolating the AC voltage supply from the DC voltage signal;
• a feedback circuit coupled to the heating element and adapted for receiving a composite signal comprising the AC power and the DC voltage signal, the feedback circuit including means for filtering the composite signal to cancel the AC power from the composite signal, independently of the instantaneous amplitude of the AC power, to provide a feedback signal indicative of the instantaneous resistance of the heating element;
• an error amplifier coupled to the input terminal for providing the AC voltage supply with the control signal, the error amplifier having an error amplifier input;
• a first resistor coupled between the feedback circuit and the error amplifier input for passing the feedback signal from the feedback circuit to the error amplifier;
• and
• a second resistor coupled to the error amplifier input for passing a resistance setpoint voltage to the error amplifier, the control signal provided to the AC voltage supply having a magnitude proportional to a sum of the resistance setpoint voltage and the feedback signal.

A method of supplying AC power to a surgical instrument having a resistively-heated heating element, for maintaining the heating element substantially at a desired operating temperature, the method comprising the steps of:

• applying AC power to the heating element from an AC voltage source, the AC power sufficient to heat the heating element to an operating temperature, the AC power having a waveform with a cyclically Varying instantaneous amplitude;
• applying a DC sense signal simultaneously to the heating element to produce a composite signal having an AC power component and a DC signal component;
• filtering the DC signal component from the AC power component independently of the instantaneous amplitude of the AC power component to produce a DC resistance signal indicative of the instantaneous resistance of the heating element.;
• comparing the DC resistance signal to a resistance setpoint signal, the resistance setpoint signal indicative of the desired operating temperature;
• and
• varying the AC power in response to the comparison of signals, wherein the AC source supplies an AC voltage to maintain the heating element substantially at the desired operating temperature.