Compact heating module with soft start

Abstract

An electronic heating module includes a heating transistor operated in its linear region as a current source. The temperature of the heating transistor is detected and used to control whether the heating transistor is turned on or off. Further control of the heating transistor is provided via a feedback control loop that monitors a voltage at one terminal of the heating transistor.

Description

FIELD OF THE INVENTION

[0001] The present invention relates to an electrical heating apparatus. More specifically, the present invention relates to a compact heating module having a soft start feature.

BACKGROUND OF THE INVENTION

[0002] A conventional electrical heating system typically comprises a heating element and a separate current source. Commonly, a temperature control circuit is also used to control the temperature of the heating element. With the use of separate components, however, the conventional system is not cost-effective to manufacture and results in inefficient utilization of generated heat.

[0003] It is thus desirable to provide a compact heating module that regulates the temperature of a heating element by controlling a current supplied to the heating element with fewer circuit components, and which utilizes heat generated by the components efficiently.

SUMMARY OF THE INVENTION

[0004] It is an object of the present invention to provide a compact heating module that regulates a temperature of a heating element by controlling a current supplied to the heating element with few circuit components, and which utilizes heat generated by the components efficiently.

[0005] The above and other objectives are achieved according to the invention by the provision of a compact heating module comprising: a power supply; a transistor coupled between the power supply and ground for operation in a linear-region to generate heat; a temperature detecting unit thermally coupled to the heating transistor to detect a temperature of the heating transistor; and a control unit coupled to the power supply and to the temperature detecting unit to apply a control voltage to the heating transistor to turn the heating transistor on or off based on the detected temperature.

[0006] According to a preferred embodiment of the invention, the control unit may comprise a first resistive element having a first node coupled to the power supply and a second node coupled to the temperature detecting unit and the heating transistor. The heating module may further comprise a second resistive element coupled between the heating transistor and ground, a third resistive element having a first electrode coupled to the second resistive element, and another transistor having its control electrode coupled to a second electrode of the third resistive element and having a main current conducting electrode coupled to a control electrode of the heating transistor. The heating module may further comprise a capacitive element having a first electrode coupled to a control electrode of the heating transistor for receiving the control voltage. Preferably, the temperature detecting unit used in the heating module may be a thermistor.

[0007] In a further embodiment of the invention, the heating module may be equipped with means for moving air across the heating transistor. For example, a fan may be used to force air across the heating transistor or across a heat sink thermally coupled to the heating transistor. In so doing, heat is efficiently distributed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The invention is better understood by reading the following detailed description with reference to the accompanying figures, in which like reference numerals refer to like elements throughout, and in which:

[0009] FIG. 1 is a circuit schematic showing an exemplary illustration of a heating module according to an embodiment of the present invention; and

[0010] FIG. 2 shows a further example of a block diagram of a heating module according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0011] FIG. 1 illustrates an example of a heating module according to an exemplary embodiment of the present invention. The constant current source of the heating module includes a heating transistor Q-1. The heating transistor Q-1 is used as both a heating element and a controlling element to control the amount of heat generated. The heating transistor Q-1 of the present invention may be of any suitable type, for example, a BJT or a FET, and may be arranged on a heat sink (shown, for example, in FIG. 2). The conductivity type of the transistor shown in FIG. 1 may be changed, and the same overall function of the circuit can be maintained, for example, by reversing the polarity of the power supply. The heating transistor Q-1 has a control electrode Q-1c and first and second electrodes Q-1a and Q-1b. The DC power supply may be of any reasonable magnitude for a heating transistor, e.g., 5V or 12V. The first and second electrodes Q-1a and Q-1b are coupled between the DC power supply and ground to enable current flow through the transistor. The heating transistor Q-1 may also be coupled to a resistive element, which in turn is connected to the ground. The resistance of the resistive element may be, for example, about 0.1&OHgr;. The resistive element may be formed by any single resistor or any combination of resistors or resistive elements whose overall resistance provides the desired resistance value. For example, FIG. 1 shows the resistive element being comprised of parallel connected resistive elements R-4 and R-5, which may, for example, comprise two parallel-connected 0.1&OHgr;, 3 W resistors.

[0012] The constant current source also includes a feedback control circuit. The feedback control is provided via a second transistor Q-2. The second transistor Q-2 may be of any type, for example, a BJT or a FET, and typically includes first and second electrodes Q-2a and Q-2b and a control electrode Q-2c. A resistive element R-3 is connected between the control electrode Q-2c of the second transistor Q-2 and an intervening node N-1 between the heating transistor Q-1 and the resistive elements R-4 and R-5. The first and second electrodes Q-2a and Q-2b are coupled between the control electrode Q-1c of the heating transistor Q-1 and ground. The resistive element R-3 may be of about 100&OHgr;, such that the voltage at the control electrode Q-2c of the second transistor Q-2 may be about 0.6V when the second transistor Q-2 is turned on.

[0013] A voltage divider comprising a resistive element R-1 and a thermistor T-1 produces an output to control the turn-on and turn-off of heating transistor Q-1 and, in turn, the current flow through heating transistor Q-1. The thermistor T-1 is in thermal contact with the heating transistor Q-1 to detect the temperature thereof. The thermistor T-1 may be a negative-temperature-coefficient (NTC) type, in which resistance decreases as temperature rises. R-1 is chosen in accordance with the resistance of thermistor T-1 and in accordance with desired functionality, as will become apparent from the discussion below. An output of the voltage divider is coupled to the control electrode Q-1c of the heating transistor Q-1 via a resistive element R-2. The resistive element R-2 may be of about 47 k&OHgr;.

[0014] The heating module may be provided with a soft start feature. When a soft start feature is provided, the control gate Q-1c of the heating transistor Q-1 is coupled to a capacitive element C-1 to “soften” (i.e., make gradual, rather than abrupt) the initial turning-on of the heating transistor Q-1 during a power-up of the heating module. The capacitive element C-1 may be a 16V capacitor with a capacitance of about 1 &mgr;F.

[0015] One of ordinary skill in the art would appreciate that the discussed values of resistive and capacitive components are exemplary and can be adjusted depending on design needs and the particular environments in which the heating module is going to be used. Moreover, the thermistor T-1 may also be changed to a positive-temperature-coefficient type by making accompanying changes to the rest of the circuitry in FIG. 1. The conductivity type of the second transistor Q-2 may be changed from one type (e.g., NPN) to another type (e.g., PNP) with accompanying changes to the rest of the circuitry in FIG. 1.

[0016] An example of the operation of the above-described heating module is now provided. When power is provided from a power supply to the heating module during a power-up of the heating module, the voltage divider R-1 and T-1 divides the power supply voltage based on a ratio of the resistances therein and provides the divided voltage to the control electrode Q-1c of the heating transistor Q-1, via the resistive element R-2. The capacitive element C-1 acts as a short circuit when the divided voltage is first applied to the control electrode and slowly raises the voltage at the control electrode Q-1c of the heating transistor Q-1 to gradually turn on the transistor. When the heating transistor Q-1 is turned on, it operates in a linear region, where the transistor gain is small compared to its saturation region and a large amount of heat is generated because of the inefficiency in transistor gain. Most of the heat generated by the heating module comes from this linear operating region of the heating transistor Q-1.

[0017] After the heating transistor Q-1 is turned on, the second transistor Q-2 turns on in response to a rise in the voltage applied to its control electrode Q-2c via resistive elements R-3, R-4 and R-5. When the second transistor Q-2 turns on, it initially lowers the voltage at the control electrode Q-1c of the heating transistor Q-1 to slightly reduce the current through the heating transistor Q-1. However, the current is quickly stabilized at a constant value by virtue of the operation of a feedback loop formed via the heating transistor Q-1 and transistor Q-2. The constant current through the heating transistor Q-1 keeps heating transistor Q-1 in its linear region to generate heat, which may then be drawn off, for example, using a heat sink, and utilized. The temperature of the heating transistor Q-1 increases gradually because of its operation in the linear region. As temperature gradually increases, the resistance of the thermistor T-1 gradually decreases. When the temperature of the heating transistor exceeds a predetermined threshold temperature, the resistance of the thermistor falls below a predetermined resistance threshold, which corresponds to the temperature threshold. When the resistance of the thermistor T-1 drops below the resistance threshold, the output of the voltage divider becomes low enough to turn the heating transistor Q-1 off. After the transistor Q-1 turns off, the second transistor Q-2, in turn, turns off. Consequently, there is no current flow through the heating transistor Q-1, and its temperature slowly drops. When the temperature of the heating transistor Q-1 drops below the predetermined threshold temperature by a certain predetermined temperature margin, which may be chosen to be any number of degrees or zero, as desired, the resistance of the thermistor T-1 increases, and the output of the voltage divider turns the heating transistor Q-1 on again. Hence, the temperature of the heating transistor Q-1 is regulated to be within a certain margin of the predetermined temperature.

[0018] In the present embodiment, by using the heating transistor Q-1 as both a heating element and a current source, the number of components required in the heating module is reduced, and any heat generated by the current source is utilized to generate the heat output. Thus, the heating module of the present invention is efficient and cost-effective.

[0019] FIG. 2 shows a further embodiment of the invention, in block diagram form. Heating transistor 1 is shown, along with its associated circuitry (e.g., as shown in FIG. 1) 3. Heating transistor 1 is shown thermally coupled to a heat sink 2; however, heat sink 2 may be omitted, with an attendant loss in efficiency. FIG. 2 further shows air moving means 4, which moves air across heat sink 2, from which heat from heating transistor 1 is radiated. When air moving means 4 moves air across heat sink 2, the air is heated by heat radiated from heat sink 2 and may then be directed as desired. Note that air moving means 4 may comprise a fan or any other suitable means, including passive means, for moving air across a heat source.

[0020] The embodiments illustrated and discussed in this specification are intended only to teach those skilled in the art the best way known to the inventors to make and use the invention. Nothing in this specification should be considered as limiting the scope of the present invention. The above-described embodiments of the invention may be modified or varied, and elements added or amended, without departing from the invention as appreciated by those skilled in the art in light of the above teachings. It is thus to be understood, within the scope of the claims and their equivalence, the invention may be practiced otherwise than as specifically described.

Claims

1. An electronic heating module comprising:

a heating transistor having terminals coupled to a power supply and to ground and configured to operate as a current source;

a temperature detecting unit arranged to detect a temperature of the heating transistor; and

a control unit arranged to receive a signal from the temperature detecting unit and to provide a control voltage to a control electrode of the heating transistor to cause the heating transistor to alternately operate in a linear operating region and to not operate, depending on the detected temperature.

2. The electronic heating module according to claim 1, further comprising:

a soft-start means that causes the control voltage provided from the control unit to gradually approach a value that causes the heating transistor to operate in its linear operating region.

3. The electronic heating module according to claim 2, wherein the soft-start means comprises:

a capacitive element coupled between the control electrode of the heating transistor and ground.

4. The electronic heating module according to claim 1, wherein the control unit comprises:

a first resistive element having a first electrode coupled to the power supply and a second electrode coupled to both the temperature detecting unit and the control electrode of the heating transistor.

5. The electronic heating module according to claim 1, wherein the control unit includes:

a second resistive element coupled between the heating transistor and ground;

a third resistive element having a first electrode coupled to the second resistive element; and

a control transistor having a control electrode coupled to a second electrode of the third resistive element and having a main electrode coupled to the control electrode of the heating transistor.

6. The electronic heating module according to claim 5, wherein the control transistor comprises a BJT.

7. The electronic heating module according to claim 5, wherein the control transistor comprises a FET.

8. The electronic heating module according to claim 1, wherein the temperature detecting unit comprises a thermistor.

9. The electronic heating module according to claim 8, wherein the control means includes a first resistive element coupled to the power supply and to the thermistor, thereby forming a voltage divider.

10. The electronic heating module according to claim 1, wherein the heating transistor comprises a FET.

11. The electronic heating module according to claim 1, wherein the heating transistor comprises a BJT.

12. The electronic heating module according to claim 1, further comprising:

a heat sink coupled to the heating transistor for drawing off heat from the heating transistor.

13. The electronic heating module according to claim 1, further comprising:

air moving means arranged to move air across the heating transistor to thereby absorb heat from the heating transistor.

14. The electronic heating module according to claim 13, wherein the air moving means comprises a fan.

15. An electronic heating module comprising:

a heating transistor having terminals coupled to a power supply and to ground and configured to operate as a current source;

a temperature detecting unit arranged to detect a temperature of the heating transistor;

a control unit arranged to receive a signal from the temperature detecting unit and to provide a control voltage to a control electrode of the heating transistor so as to cause it to alternately operate in a linear operating region and to not operate, depending on the detected temperature;

air moving means arranged to move air across the heating transistor to thereby absorb heat from the heating transistor; and

a soft-start means that causes the control voltage provided from the control unit to gradually approach a value that causes the heating transistor to operate in its linear operating region.

16. A method of controlling a temperature of an electronic heating module comprising the steps of:

operating a heating transistor in a linear region of the transistor;

detecting a temperature of the heating transistor; and

applying a control voltage to a control electrode of the heating transistor to turn the heating transistor on or off based on the detected temperature.

17. The method according to claim 16, wherein the step of applying a control voltage comprises a step of dividing a power supply voltage based on the detected temperature and supplying the divided voltage to the control electrode of the heating transistor.

18. The method according to claim 16, further comprising a step of operating the heating transistor as a current source.

19. The method according to claim 16, further comprising a step of preventing an abrupt rise of the control voltage when the heating transistor is turned on during a power-up of the heating module.

20. The method according to claim 16, further comprising a step of causing air to flow over the heating transistor and to thereby absorb heat from the heating transistor.

21. The method according to claim 16, wherein the step of applying a control voltage comprises a step of applying a feedback control voltage to the control node of the heating transistor.