CA2392532A1 - Low input voltage, low cost, micro-power dc-dc converter - Google Patents

Abstract

An apparatus for and method for powering a microprocessor from energy received from a fuel burner at ignition. A thermopile receives heat energy from the flame and generates electrical power to enable operation of a microprocessor.
This electrical power is converted to a higher voltage by a special low voltage DC-to-DC converter which operates shortly after ignition. The special low voltage powers a second DC-to-DC converter which in turn powers the microprocessor.

Description

LOW INPUT VOLTAGE, LOW COST, MICRO-POWER DC-DC CONVERTER
CROSS REFERENCE TO CO-PENDING APPLICATIONS
U. S. Patent Application No. . filed , and entitled, "STEPPER MOTOR
DRIVING A LINEAR ACTUATOR OPERATING A PRESSURE CONTROL REGULATOR";
U. S: Patent Application No. , filed , and entitled, "ELECTRONIC FUEL
CONVERTIBILITY SELECTION"; U.S. Patent Application No. . filed , and entitled, "LOW INPUT VOLTAGE, HIGH EFFICIENCY, DUAL OUTPUT DC TO DC
CONVERTER"; and U. S. Patent Application No. , filed , and entitled, "ELECTRONIC DETECTING OF FLAME LOSS BY SENSING POWER OUTPUT FROM
THERMOPILE" are commonly assigned co-pending applications incorporated herein by reference.

BACKGROUND OF THE INVENTION
1. Field of the Invention: The present invention generally relates to systems for control of an appliance incorporating a flame and more particularly relates to flame management systems.
2. Description of the prior art: It is known in the art to employ various appliances for household and industrial applications which utilize a fuel such as natural gas (i.e., methane), propane, or similar gaseous hydrocarbons. Typically, such appliances have the primary heat supplied by a main burner with a substantial pressurized gas input regulated via a main valve.
Ordinarily, the main burner consumes so much fuel and generates so much heat that the main burner is ignited only as necessary. At other times (e.g., the appliance is not used, etc.), the main valve is closed extinguishing the main burner flame.
A customary approach to reigniting the main burner whenever needed is through the use of a pilot light. The pilot light is a second, much smaller burner, having a small pressurized gas input regulated via a pilot valve. In most installations, the pilot light is intended to burn perpetually. Thus, turning the main valve on provides fuel to the main burner which is quickly ignited by the pilot light flame. Turning the main valve off, extinguishes the main burner, which can readily be reignited by the presence of the pilot light.
These fuels, being toxic and highly flammable, are particularly dangerous in a gaseous state if released into the ambient. Therefore, it is customary to provide certain safety features for ensuring that the pilot valve and main valve are never open when a flame is not present preventing release of the fuel into the atmosphere. A standard approach uses a thermogenerative electrical device (e.g., thermocouple, thermopile, etc.) in close proximity to the properly operating flaifie.
z Whenever the corresponding flame is present, the thermocouple generates a current. A solenoid operated portion of the pilot valve and the main valve require the presence of a current from the thermocouple to maintain the corresponding valve in the open position.
Therefore, if no flame is present and the thermocouples) is cold and not generating current, neither the pilot valve nor the main valve will release any fuel.
In practice, the pilot light is ignited infrequently such as at installation, loss of fuel supply, etc. Ignition is accomplished by manually overriding the safety feature and holding the pilot valve open while the pilot light is lit using a match or piezo igniter. The manual override is held until the heat from the pilot flame is sufficient to cause the thermocouple to generate enough current to hold the safety solenoid: The pilot valve remains open as long as the thermocouple continues to generate sufficient current to actuate the pilot valve solenoid.
The safety thermocouples) can be replaced with a thermopiles) for generation of additional electrical power. This additional power may be desired for operating various indicators or for powering interfaces to equipment external to the appliance. Normally, this requires conversion of the electrical energy produced by the thermopile to a voltage useful to these additional loads. Though not suitable for this application, U.S. Patent No.
5,822,200, issued to Stasz; U.S. Patent No. 5,804,950, issued to Hwang et al.; U.S. Patent No.
5,381,298, issued to Shaw et al.; U.S. Patent No. 4,014,165, issued to Barton; and U.S. Patent No.

3,992,585, issued to Turner et al. all discuss some form of voltage conversion.
Upon loss of flame (e.g., from loss of fuel pressure), the thermocouples) ceases generating electrical current and the pilot valve and main valve are closed.
'~'he delay from loss of flame until closure of the valves depends upon a number of variables. Of greatest concern is the delay caused by heat energy retained in the mass of the thermopile(s). That means that as the size and electrical power generation capacity of the thermopiles) are increased, the system delays are correspondingly increased.
These system delays are found at initial flame on as well as flame out. That means that as the capacity of the thermopile is increased, the length of time between initial electrical output and steady state operation and the length of time between flame out and final electrical output are correspondingly increased. Thus, there is a trade off between desirable high output steady state electrical generation and desirable rapid transition between flame on and steady state and between flame out and steady state.

SUMMARY OF THE INVENTION
The present invention overcomes the disadvantages of the prior art by providing a method of and apparatus for using a relatively large mass, high output thermopile for high electrical power generation with an approach for offering a useful electrical output over a larger portion of the ignition/full operation/flame out cycle. Because the thermopile provides such a very low voltage output even at steady state operation, it is necessary to employ a DC-to-DC
converter to increase the voltage to useful levels.
The preferred embodiment employs a first, low voltage DC-to-DC converter which converts the thermopile output on initial flame ignition to power a second DC-to-DC converter.
As the temperature continues to rise and then stabilize, a second, higher capacity DC-to-DC
converter provides the electrical power for the microprocessor and other electrical equipment.
In accordance with the present invention, a low voltage DC-to-DC converter operates at voltages which are only slightly greater than the base emitter voltage drop.
For silicon devices, I S this is 0.7 volts. For germanium devices, this is 0.3 volts. The thermopile output is directly coupled to a three stage, amplifying oscillator. A resistor/capacitor circuit determines the oscillation frequency A single stage provides the output coupling. The oscillator output is boosted by a single stage booster converter.
The low voltage DC-to-DC converter provides an output sufficient to power the second DC-to-DC converter during the earliest flame on time curve of the thermopile electrical output.
Once started, the second DC-to-DC converter can keep converting even if input voltage drops to a much lower level.

BRIEF DESCRIPTION OF THE DRAWINGS
Other objects of the present invention and many of the attendant advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, in which like reference numerals designate like parts throughout the figures thereof and wherein:
FIG. 1 is a graph showing the thermopile output voltage as a function of time;
Fig. 2 is a simplified schematic electrical diagram of the present invention;
Fig. 3 is a graph, similar to Fig. 1, showing certain key points; and Fig. 4 is a detailed schematic diagram showing the low voltage DC-to-DC
converter of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Fig. 1 is~ a diagram 10 showing the output voltage versus time of the thermopile of the preferred mode of the present invention under various conditions. Shortly after flame on, point 12 is reached whereat the thermopile (not shown) begins generating a measurable voltage. The thermopile output is, of course, a function of the temperature within the pilot (actually, as readily known to those of skill in the art, the output is a function of the temperature differential between the poles; only one of which is thermally coupled to the combustion chamber).
The temperature of the combustion chamber (and hence the thermopile output) continues to rise over time until it reaches a relatively stable level having slight amplitude variations such as the relative minimum at point 14.
Flame out occurs at point 18. Point 20 corresponds to a reduction in combustion chamber temperature at which the thermopile ceases to produce a measurable output. As can be seen by the curve of diagram 10 from point 18 to point 20, a characteristic signature is present. A
microprocessor, powered by the thermopile, continuously and periodically measures the thermopile output such that this flame out signature can be detected well before point 20.
Detecting flame out before loss of thermopile output provides available electrical energy for orderly shut down functions.

Fig. 2 is a very basic electrical diagram 22 of the power circuitry of the present invention.
Thermopile 24 is structured in accordance with the prior art. Resistor 26 represents the internal resistance of the thermopile.
Pilot valve 28 is controlled by switch 30. Similarly, main valve 32 can be turned on or otF
by closing or opening switch 34.
DC-to-DC conversion facility 36 converts the relatively low voltage output of thermopile 24 to a su~ciently large voltage to power the second DC-to-DC converter. In accordance with the preferred mode of the present invention, DC-to-DC conversion facility 36 consists of two DC-to-DC converters. The first converter operates at the extremely low thermopile output voltages experienced during combustion chamber warm up (see also Fig. I ). The other DC-to-DC
converter powers the system during normal operation. A more detailed description of the second device is available in the above identified and incorporated, commonly assigned, co-pending U.S.
Patent Applications.

Fig. 3 is diagram 10 (see also Fig. I ) showing certain additional points of interest concerning the present invention. In accordance with the preferred mode, point 38 represents the point at which tie first DC-to-DC converter (see also Fig. 2) begins producing useful electrical power. This point represents a few tens of millivolts above the basic junction voltage drop of the semiconductors utilized within the system. For silicon transistors, this is 0.7 volts, and for germanium transistors, this is 0.3 volts. DC-to-DC conversion facility 36 contains a low voltage converter which actually produces the power available at point 38. The low voltage converter is described in more detail below.
The output of the low voltage converter begins to power the second DC-to-DC
converter such that it is fully operational at point 40. The time between points 40 and 42 is utilized by the microprocessor to initialize for full operation. This initialization includes setting various status registers and establishing certain initial conditions. A detailed description of the second DC-to-DC converter is available in the above identified, commonly assigned, co-pending U. S. Patent I S Application.

Fig. 4 is a detailed schematic diagram of the low power DC-to-DC converter.
This circuit provides a useable electrical output at sufficient voltage to power the second DC-to-DC converter during power up (e.g., point 38 through point 40 of Fig. 3). This is feasible because the circuit is coupled directly to the thermopile output and because the componentry and architecture are designed to accommodate an extremely low voltage input. As is discussed below, this input voltage need only be a few tens of millivolts above the basic junction voltage drop of the semiconductors used. For silicon transistors at room temperature, this corresponds to 0.7 volts.
For germanium transistors, this corresponds to 0.3 volts.
The input to the circuit is shown at point 50. This corresponds to the output of thermopile 24 (see also Fig. 2). It is understood that the signal ground connections shown within this schematic correspond to the low voltage (i.e., opposite thermopile 24) end of resistor 26.
This input is supplied to an oscillator consisting of three amplifying stages containing transistors 54, 58, and 62. These stages are biased by resistors 52, 56, and 60, respectively. The frequency I 5 of the oscillator is controlled by the RC time constant of resistor 72 and capacitor 70.
The output of the oscillator is coupled via resistor 66 to voltage booster stages 68 and 76, which are biased via resistor 64. These stages amplify the most pronounced portions of the oscillator output signal. In this way, the effective alternating voltage is increased. When transistor 76 is turned on, a current through inductor starts to build up a magnetic field . When transistor 76 turns off, inductor 74 releases the stored magnetic energy via diode 80, to charge capacitor 78 to a much higher voltage level. Schottky diode'80 ensures that the higher voltage stored in capacitor 78 is not discharged while transistor 76 is turned on. The increased voltage output is presented at point 82. This output is coupled via capacitor 78, which smooths the final, higher voltage output used to power the second DC-to-DC converter.
Capacitor 70 is optional. When capacitor 70 is present, the oscillation frequency is lower and more stable; but input voltage needs to be a little higher because the closed loop gain of the S entire oscillator is lower. With a given input voltage and inductor, lower frequency means higher output power. If capacitor 70 is omitted, the oscillation frequency is higher.
To produce the same amount of output power, the inductor can be smaller.

Having thus described the preferred embodiments of the present invention, those of skill in the art will be readily able to adapt the teachings found herein to yet other embodiments within the scope of the claims hereto attached.
WE CLAIM:

Claims (20)

1. In a system having a flame thermally coupled to a device which generates an electrical output in response to heat energy received from said flame, the improvement comprising:
a. A low voltage DC-to-DC converter responsively coupled to said device which converts said electrical output to a higher voltage.

2. The improvement according to claim 1 further comprising a second DC-to-DC
converter responsively coupled to said low voltage DC-to-DC converter.

3. The improvement according to claim 2 wherein said low voltage DC-to-DC
converter powers said second DC-to-DC converter at flame ignition but not during steady state burn.

4. The improvement according to claim 3 wherein said electrical output has a voltage of at least 0.3 volts.

5. The improvement according to claim 3 wherein said electrical output has a voltage of at least 0.7 volts.

6. An apparatus comprising:
a. A low voltage input point;

b. An oscillator responsively coupled to said low voltage input;
c. A voltage booster responsively coupled to said oscillator; and d. A higher voltage output point responsively coupled to said voltage booster.

7. An apparatus according to claim 6 wherein said oscillator further comprises a plurality of transistor amplification stages.

8. An apparatus according to claim 6 wherein said voltage booster further comprises a plurality of transistor amplification stages.

9. An apparatus according to claim 6 wherein said voltage booster further comprises a smoothing inductor.

10. An apparatus according to claim 6 wherein said higher voltage output point further comprises a parallel capacitor.

11. A method of powering a microprocessor comprising:
a. Igniting a flame;
b. Generating an electrical output in response to heat produced by said flame;
c. Coupling a low voltage DC-to-DC converter to said electrical output to produce a higher voltage output; and d. Coupling said microprocessor to said higher voltage output.

12. A method according to claim 11 further comprising:
a. coupling a second DC-to-DC converter to said electrical output to produce a second higher voltage output; and b. Coupling said microprocessor to said second higher voltage output.

13. A method according to claim 12 wherein said low voltage DC-to-DC converter produces said higher voltage output when said electrical output has a voltage slightly above 0.3 volts.

14. A method according to claim 12 wherein said low voltage DC-to-DC converter produces said higher voltage output when said electrical output has a voltage slightly above 0.7 volts.

15. A method according to claim 12 wherein said low voltage DC-to-DC converter powers said microprocessor when said electrical output has a relatively low voltage and said second DC-to-DC converter powers said microprocessor when said electrical output has a relatively high voltage.

16. An apparatus comprising:
a. Means for generating heat energy;
b. Means responsively coupled to said generating means for producing an electrical output in response to said heat energy; and c. Means responsively coupled to said producing means for converting said electrical output to a higher voltage.

17. An apparatus according to claim 16 wherein said converting means further comprises semiconductor means.

18. An apparatus according to claim 17 wherein said converting means converts said electrical output to said higher voltage whenever the voltage of said electrical output is at least slightly higher than the basic junction voltage drop of said semiconductor means.

19. An apparatus according to claim 18 wherein said semiconductor means further comprises a silicon device.

20. An apparatus according to claim 18 wherein said semiconductor means further comprises a germanium device.