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1. WO2020139994 - LEAKAGE DETECTION IN A FLAME SENSE CIRCUIT

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[ EN ]

What is claimed is:

1. A flame detection system comprising:

a flame sensor for sensing a flame, the flame sensor drawing a flame sense current when a flame is present;

an amplifier operatively coupled to the flame sensor for amplifying the flame sense current and drawing an amplified flame sense current from an amplifier output; a negative voltage supply generator for supplying a negative supply voltage to the amplifier;

a detection circuit operatively coupled to the amplifier output for detecting the amplified flame sense current;

a microcontroller operatively coupled to the negative voltage supply generator and the detection circuit, wherein the microcontroller is configured to:

change the negative supply voltage from a nominal negative

supply voltage to a boosted negative supply voltage;

determine a leakage current condition in the flame detection

system when the amplified flame sense current detected by the detection circuit changes by more than a

threshold amount when the negative supply voltage is changed from the nominal negative supply voltage to the boosted negative supply voltage;

providing a shutdown signal to shut down the flame when the

leakage current condition is determined.

2. The flame detection system of claim 1, wherein the

microcontroller is further configured to change the negative supply voltage back from the boosted negative supply voltage to the nominal negative supply voltage.

3. The flame detection system of claim 2, wherein the

microcontroller is configured to change the negative supply voltage from the nominal negative supply voltage to the boosted negative supply voltage for less than a second before changing the negative supply voltage back from the boosted negative supply voltage to the nominal negative supply voltage.

4. The flame detection system of claim 3, wherein after changing the negative supply voltage back from the boosted negative supply voltage to the nominal negative supply voltage, the microcontroller waiting for a period of time before again changing the negative supply voltage from the nominal negative supply voltage to the boosted negative supply voltage for less than a second before changing the negative supply voltage back from the boosted

negative supply voltage to the nominal negative supply voltage.

5. The flame detection system of claim 1, wherein the detection circuit comprises:

a capacitor having a first end operatively coupled to the amplifier output; a first resistor having a first end operatively coupled to the amplifier output, the first resistor having a first resistance value; and

a second resistor having a first end operatively coupled to the amplifier output, the second resistor having a second resistance value that is different from the first resistance value.

6. The flame detection system of claim 5, wherein the microcontroller is configured to:

charge the capacitor through the first resistor from a first lower

threshold voltage to a first upper threshold voltage, and then allow the amplified flame sense current to

discharge the capacitor down to the first lower threshold voltage;

determine a first duty cycle of the charging of the capacitor

through the first resistor and subsequent discharge of the capacitor;

charge the capacitor through the second resistor from a second

lower threshold voltage to a second upper threshold

voltage, and then allow the amplified flame sense

current to discharge the capacitor down to the second lower threshold voltage;

determine a second duty cycle of the charging of the capacitor through the second resistor and subsequent discharge of the capacitor; and

determine a leakage current condition in the flame detection system based at least in part on the first duty cycle, the second duty cycle, the first resistance value and the second resistance value; and

providing a shutdown signal to shut down the flame when the leakage current condition is determined.

7. The flame detection system of claim 6, wherein first upper threshold voltage and the second upper threshold voltage are the same, and the first lower threshold voltage and the second lower threshold voltage are the same.

8. The flame detection system of claim 6, wherein the capacitor has a second end, and the second end is operatively coupled to ground.

9. The flame detection system of claim 6, wherein both the first upper threshold voltage and the second upper threshold voltage have a magnitude and are positive, and both the first lower threshold voltage and the second lower threshold voltage have the magnitude and are negative.

10. The flame detection system of claim 6, wherein the

microcontroller is configured to determine the first duty cycle of the charging of the capacitor through the first resistor and subsequent discharge of the capacitor by monitoring a voltage of the first end of the capacitor and clock how long it takes to charge the capacitor through the first resistor from the first lower threshold voltage to the first upper threshold voltage (ChargeRlTime), and to clock how long it takes for the amplified flame sense current to discharge the capacitor down to the first lower threshold voltage

(DischargeFCTime), and calculate the first duty cycle using the relation ChargeRlTime/(ChargeRlTime+DischargeFCTime).

11. The flame detection system of claim 10, wherein the

ChargeRlTime and DischargeFCTime are average values taken over a

plurality of charging and discharging cycles of the capacitor.

12. The flame detection system of claim 8, wherein the

microcontroller determines the leakage current condition in the flame

detection system when the ratio of the first duty cycle to the second duty cycle is not within a predetermined margin of the ratio of the first resistance value to the second resistance value.

13. A method for detecting a leakage current condition in a flame detection system, the method comprising:

amplifying with an amplifier a flame sense current provided by a flame sensor, resulting in an amplified flame sense current;

supplying the amplified flame sense current to the amplifier via charge storage device;

charging the charge storage device with a first charging circuit that produces a first charging rate;

subsequently charging the charge storage device with a second charging circuit that produces a second charging rate, wherein the second charging rate is different from the first charging rate;

determine a leakage current condition in the flame detection system

based at least in part on a comparison of the charging of the charge storage

device with the first charging circuit and the charging of the charge storage device with the second charging circuit; and

providing a shutdown signal to shut down the flame when the leakage current condition is determined.

14. The method of claim 13, further comprises:

providing a negative supply voltage to the amplifier;

changing the negative supply voltage from a nominal negative supply voltage to a boosted negative supply voltage; and

determine the leakage current condition in the flame detection system when the amplified flame sense current changes by more than a threshold

amount when the negative supply voltage is changed from the nominal negative supply voltage to the boosted negative supply voltage.