# US 7,411,406 B2 · Power supply with current measurement circuit for transformer with high frequency output > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 7,411,406 B2 - Document ID: US7411406B2 - Status: Granted - Kind: B2 - Application number: 11/725,393 - Filed: 2007-03-19 - Published: 2007-07-26 - Granted: 2008-08-12 - Earliest claimed: 2005-07-01 - Assignees: Xerox Corporation - Inventors: Hendrikus Adrianus Anthonius Verheijen - Web page: https://patentgrep.com/patent/US7411406B2 ## CPC - G03G 15/0907: {with bias voltage (G03G15/065 takes precedence)} - G03G: ELECTROGRAPHY; ELECTROPHOTOGRAPHY; MAGNETOGRAPHY ## Abstract A current measurement circuit to measure the output of a power supply transformer includes a simulation capacitor having a capacitance proportional to a parasitic capacitance of the transformer. The first electrode of the capacitor is coupled to an output winding of the transformer and the second electrode is coupled to a first node, with a second sense resistor coupled between the first node and ground so that current flowing through the simulation capacitor flows through the second sense resistor. Current flowing through a first sense resistor coupled to a second node and to ground has a component representative of the output current of the power supply and a component representative of the parasitic current. A differential amplifier coupled at an inverting input to the first node and at a non-inverting input to the second node generates an output signal that is proportional to the output current of the power supply. ## Claims ### Claim 1 A current measurement circuit for measuring the output current of a power supply, the current measurement circuit comprising: a simulation capacitor having a capacitance proportional to a parasitic capacitance of a transformer in a power supply, the simulation capacitor having a first electrode and a second electrode, the first electrode being coupled to a hot terminal of an output winding of the power supply transformer and the second electrode being coupled to a first node; a second sense resistor coupled to the first node and to ground so that current flowing through the simulation capacitor flows through the second sense resistor; a first sense resistor coupled to a second node and to ground through which a current flows, the current flowing through the first sense resistor having a component representative of the output current of the power supply and a component representative of the parasitic current; and a differential amplifier coupled at an inverting input to the first node and at a non-inverting input to the second node, the differential amplifier generating an output signal that is proportional to the output current of the power supply. ### Claim 2 (depends on 1) The device of claim 1 wherein the capacitance of the simulation capacitor is related to the parasitic capacitance of the transformer by a proportionality factor. ### Claim 3 (depends on 2) The device of claim 2 wherein the proportionality factor is 1/a, and a is derived to reduce an additional load placed on the transformer by the simulated capacitor and is related to parameters of the power supply and the power supply transformer. ### Claim 4 (depends on 3) The device of claim 3 wherein the resistance of the second sense resistor is (a+1) times the resistance of the first sense resistor. ### Claim 5 (depends on 4) The device of claim 4 wherein the signal generated on the output of the differential amplifier is a voltage proportional to the output current of the power supply times the resistance of the first sense resistor. ### Claim 6 (depends on 5) The device of claim 5 wherein the signal generated on the output of the differential amplifier is a voltage equal to the output current of the power supply times the resistance of the first sense resistor. ### Claim 7 (depends on 4) The device of claim 4 wherein the current present at the non-inverting input of the differential amplifier is proportional to the output current of the power supply plus a parasitic current of the transformer times (a+1)/a. ### Claim 8 (depends on 7) The device of claim 7 wherein the current present at the non-inverting input of the differential amplifier is equal to the output current of the power supply plus a parasitic current of the transformer times (a+1)/a. ### Claim 9 A method for modifying a power supply with a current measurement circuit, the method comprising: coupling a first electrode of a simulation capacitor to a hot terminal of an output winding of a power supply transformer, the simulation capacitor having a capacitance proportional to a parasitic capacitance of the power supply transformer; coupling a second electrode of the simulation capacitor to a first node; coupling a second sense resistor between the first node and electrical ground so that current flowing through the simulation capacitor flows through the second sense resistor; coupling a first sense resistor between a second node and electrical ground to conduct current through the first sense resistor having a component representative of the output current of the power supply and a component representative of the parasitic current; coupling an inverting input of a differential amplifier to the first node; and coupling a non-inverting input of the differential amplifier to the second node so the differential amplifier generates an output signal that is proportional to the output current of the power supply. ### Claim 10 (depends on 9) The method of claim 9 wherein the simulation capacitor coupled to the power supply is related to the parasitic capacitance of the transformer by a proportionality factor. ### Claim 11 (depends on 10) The method of claim 10 wherein the simulation capacitor coupled to the power supply is related to the parasitic capacitance of the transformer by a proportionality factor of 1/a, and a is derived to reduce an additional load placed on the transformer by the simulated capacitor and is related to parameters of the power supply and the power supply transformer. ### Claim 12 (depends on 11) The method of claim 11 wherein the second sense resistor coupled to the power supply has a resistance that is (a+1) times the resistance of the first sense resistor. ### Claim 13 (depends on 12) The method of claim 12, the differential amplifier signal generation including: generating a voltage that is proportional to the output current of the power supply times the resistance of the first sense resistor. ### Claim 14 (depends on 13) The method of claim 13, the differential amplifier signal generation including: generating a voltage equal to the output current of the power supply times the resistance of the first sense resistor. ### Claim 15 (depends on 14) The method of claim 14, the coupling of the non-inverting input of the differential amplifier to the second node provides a current to the non-inverting input that is proportional to the output current of the power supply plus a parasitic current of the transformer times (a+1)/a. ### Claim 16 (depends on 15) The method of claim 15, the coupling of the non-inverting input of the differential amplifier to the second node provides a current to the non-inverting input that is equal to the output current of the power supply plus a parasitic current of the transformer times (a+1)/a.