WO2002096155A1

WO2002096155A1 - Heater with overheating protection

Info

Publication number: WO2002096155A1
Application number: PCT/IB2002/001864
Authority: WO
Inventors: Rene H. Van Der Woude; Thijs De Haan; Tang P. Har
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2001-05-23
Filing date: 2002-05-21
Publication date: 2002-11-28

Abstract

The invention relates to an overheating-protected heater (2) for an appliance having a substrate (6) which is to be heated. The heater comprises a heating element (10), a temperature sensor (11) in thermal contact with the heating element, and a control circuit for controlling the heating element in response to temperature measurement signals from the temperature sensor. The invention particularly relates to a configuration in which the temperature sensor comprises in that order a first conductive layer (10) having a resistance, an insulator layer (14) having temperature-dependent dielectric characteristics, and a second conductive layer (16) so as to form the equivalent of a parallel configuration of a resistor and a capacitor. The temperature-dependent leakage current through the parallel configuration provides the temperature measurement signal for the temperature control circuit.

Description

Heater with overheating protection

The present invention relates to an overheating-protected heater for an appliance. Such an appliance may be an electric iron, a water cooker, a hot-plate, etc. The heater comprises a heating element which is selectively connectable to a power supply and a temperature control circuit having a temperature sensor in thermal contact with the heating element, which control circuit is arranged so as to control the heating element in response to temperature measurement signals from the temperature sensor.

Such an overheating-protected heater is known, for example, from US-A-6,046,438, where tracks of a heating resistor are arranged on a surface of a substrate and a thermal sensor is located in or surrounded by a thinner portion of the substrate. The present invention has for its object to provide a simplified configuration of an overheating-protected heater in which the substrate, which belongs to an appliance, does not require further processing for implementing the overheating-protected heater in the appliance.

Another object of the present invention is to provide an overheating-protected heater with an improved degree of reliability and transient response.

A further object of the present invention is to provide an overheating-protected heater with which the required temperatures can be reliably reached, but are not exceeded.

The above and other objects of the present invention are achieved by providing an overheating-protected heater which is characterized in that the temperature sensor comprises in that order a first conductive layer having a resistance, an insulator layer having temperature-dependent dielectric characteristics, and a second conductive layer so as to form the equivalent of a parallel configuration of a resistor and a capacitor, wherein the temperature-dependent leakage current through the parallel configuration provides the temperature measurement signal for the temperature control circuit. The present invention thus utilizes leakage currents. Leakage currents used in the above-mentioned way were found to increase exponentially at high temperatures, i.e. in dependence upon the actual temperature, the temperature-dependent resistivity properties of the conductive layers, and/or the temperature-dependent dielectric properties of the insulating layer. The leakage currents in combination with a previously gained knowledge of the temperature-dependent properties provide a reliable indication of the actual temperature. Thus the temperature control circuit can reliably respond to the temperature measurement signal and control the power supply to the heating element so as to achieve the desired temperature. In a preferred embodiment, the heating element comprises a heating resistor, and the first conductive layer forms the heating resistor. In such an embodiment the number of components is reduced and the configuration is further simplified. Especially in such a configuration, but also when the temperature sensor is arranged at a distance from the heating element, the insulating layer of the temperature sensor may be made to extend as a protective layer over the heating element. The robustness and reliability of the heater can be increased thereby in that the heating element is protected by the insulating layer.

It is noted here, however, that the sensor preferably extends over a large area of the heating element. In such an embodiment, a very local overheating can also lead to an adaptive control of the power supply. Furthermore, a highly preferable embodiment of the present invention is one in which separate components of the heater are formed from a similar material as a basic material, and the separate components are formed by the provision of a specific filler material in the basic material so as to provide the desired properties corresponding to the actual component being formed therefrom. As a specific, non-limiting example, polyimide resin and silicone are mentioned as basic materials. The insulating layer having the desired temperature-dependent dielectric properties may be chosen from a group of suitable materials such as SiO₂, Al₂O₃, A1N, SiN, Bn, TiO, BaTiO₃, semiconductors, glass materials, or polymer resins such as poly-imide and silicones. A filler material such as graphite or carbon may be added in a predetermined amount to polyimide as a binder material for the heating resistor and/or the conductive layer so as to provide the desired conductive properties. Contact elements may also be made from polyimide as a basic material, to which a filler material such as silver may be added to obtain the desired contacting and power-distributing properties for contact elements. By using the same basic material, such as polyimide, to which the present invention is, however, not limited, a good bonding of the layers can be achieved in spite of high temperature fluctuations. Such temperature fluctuations occur during rapid heating or cooling down of the heater and could, with an inappropriate choice of materials, result in disintegration of the heating resistor, the insulating layer, the resistive layer, and the contact elements.

Features, properties, characteristics, advantages, etc. of the present invention will be made apparent below by a description of non-limitative embodiments of the present invention, referring to the accompanying drawings, in which the same or similar components are identified by the same reference numbers, and wherein:

Fig. 1 shows a water cooker having a heater according to the present invention; Fig. 2 diagrammatically shows an electric iron having a heater according to the present invention;

Fig. 3 shows a heater according to the present invention on a substrate, partly broken away, in more detail than in Figs. 1 and 2;

Fig. 4 is a graph of leakage currents plotted against temperature in an overheating-protected heater according to the present invention;

Fig. 5 shows an electrical circuit which is equivalent to the configuration of Fig. 3; and

Figs. 6 and 7 show embodiments of control circuits, to be used for overheating protection in a heater according to the present invention. Fig. 1 schematically shows a water cooker 1 having a heater 2 according to the present invention incorporated therein. The novel and inventive heater 2 is arranged in a conventional manner on a substrate 6, of which the face opposite the heater 2 is directed toward an inner space 7 of the water cooker 1 for transferring heat from the heater 2 to water in the interior space 7 so as to heat this water. The heater 2 comprises a control circuit 5 which analyzes temperature measurement signals from the heater 2 and controls a relay 8 in response thereto. The relay 8 is arranged between a power supply 4 and connectors 9 leading to the heater 2.

The heater 2 comprises a heating resistor 10 and a temperature sensor 11 which extends over a considerable portion of the surface of the heating resistor 10. The sensor 11 will be described in more detail below.

Fig. 2 shows another embodiment of an appliance according to the present invention. The appliance in Fig. 2 is an electric iron 3 of which the ironing sole may be considered the substrate 6. The heater 2 according to the present invention is arranged on the surface of the substrate 6, opposite the actual sole of the iron 3, which can be brought into functional contact with garments to be ironed.

It is noted that many other appliances having heaters may be considered to be embodiments of the present invention, such as cooking hot-plates, etc., and the present invention is therefore not limited to water cookers or irons. Fig. 3 shows a heater 2 according to the present invention arranged on a substrate 6 of an appliance, where two insulating layers 12, 13 are arranged between the substrate 6 and the heater 2. The first insulating layer 13, closest to the substrate, may be made, for example, from sol-gel, and the second layer 12 may be made, for example, from polyimide. The first insulating layer 13 provides a bonding for the polyimide second insulating layer 12. Furthermore, the second insulating layer 12 may provide a good bonding for the heater 2.

The heater 2 itself comprises, according to the present invention, a heating resistor 10 having contacts 15 at opposite sides of the heating resistor 10. The heating resistor 10 is made, for example, from polyimide as a basic material, to which a filler material is added so as to provide the desired heating resistor properties, i.e. to generate heat when a voltage is applied across the contacts 15. The contacts themselves may also be made, for example, from polyimide as a basic material, to which silver may be added as a filler material so as to obtain the desired contacting and power-distributing properties for the contacts 15. An insulating layer 14 having temperature-dependent dielectric properties is arranged over the heating resistor 10. The insulating layer 14 is partly broken away in Fig. 3, but it may extend over a large surface area. It may furthermore be made, for example, from polyimide as a basic material, to which a suitable filler material or materials may be added, such as SiO₂, Al₂O₃, A1N, SiN, BN, TiO₂, BaTiO₃, semiconductor materials etc. in order to provide the desired temperature-dependent dielectric properties. The added amount of the filler materials can be used to determine the actually achieved temperature-dependent dielectric properties of the insulating layer 14. It is shown clearly in Fig. 3 that the insulating layer 14 extends over a considerable portion, if not all, of the heating resistor 10, thus forming a protective layer over the heating resistor. Furthermore, a conductive layer 16 is arranged over a considerable portion of the surface of the insulating layer 14. The conductive layer 16 may be made, for example, from polyimide as a basic material, to which a filler material is added, such as graphite or carbon, to provide the desired conductive properties to the conductive layer 16.

In the configuration described above, the use of the same or a similar basic material for each of the components of the configuration provides an optimal bonding.

Fig. 4 is a graph of leakage currents LC plotted on the vertical axis against temperature T plotted on the horizontal axis. Curve 17 shows the AC leakage current, and curve 18 shows the DC leakage current. Both leakage currents increase exponentially at temperatures above 250° C. Any appropriate choice of the material for the insulating layer 14 having the temperature-dependent dielectric characteristics renders it possible to define the temperature at which this insulating layer 14 begins to exhibit its exponential increase. In a configuration in which a basic material is used such as the one described above with reference to Fig. 3 together with a filler material to achieve the desired properties, the amounts of this filler material may be varied in order to obtain a adesired temperature- dependent characteristic, such as the one shown in Fig. 4.

Fig. 5 shows an electrical circuit which is equivalent to the configuration of Fig. 3. It is apparent from Fig. 5 that the control circuit 5 shown is connected to the conductive layer 16 in Fig. 3, whereby a parallel configuration of a capacitor 21 and a resistor 22 is formed between the control circuit and the electrical path between the contacts 15 in Fig. 3.

A similar configuration 19 of a capacitor and a resistor in parallel is formed between the electrical paths from one of the contacts 15 to the other and the substrate. This parallel configuration 19, however, is of less importance to the present invention than the parallel configuration of the capacitor 21 and the resistor 22.

In Fig. 5, the control circuit 5 is connected to the power supply 4. The control circuit 5 is further connected to the parallel configuration of the capacitor 21 and the resistor 22 to obtain therefrom leakage currents from which the ambient temperature of the heating resistor 10 can be derived. The control circuit 5 is further connected to the relay 8 to open the switch 20 whenever the received leakage currents indicate an excess of temperature. The leakage current through the latter parallel configuration is thus used as the temperature measurement signal for the control circuit 5. As in Fig. 1, the relay 8 is shown to be connected to the control circuit 5, so that the relay 8 can open the switch 20 in the electrical path for power supply to the heating resistor in response to a higher than desired temperature. The switch 20 forms the switch means in the sense of the present invention.

A possible embodiment of the control circuit 5 is shown in Fig. 6. Fig. 6 shows a capacitive sensor and the control circuit 5 in accordance with the invention. A first inverting amplifier 23, in the form of a Schmitt trigger, has its input connected to its output via a feedback resistor 24. Two capacitors are situated between the input and a reference terminal 25, which functions as signal ground for the sensor circuit. A first capacitor is formed by a parasitic input capacitor 26 having a value C_pι and a second capacitor is formed by the capacitance of value C_s of a sensing capacitor 21 in Fig. 5, having a first plate formed by the conductive layer 16 forming a sensor electrode connected to the input. The second plate is formed by the heating resistor 10. The temperature measurement signal is diagrammatically shown as an AC source 27. The inverting amplifier 23, the feedback resistor 24 and the two capacitors 26 and 21 (the latter capacitor being shown in Fig. 5) form an oscillator whose oscillation frequency F_s decreases as the overall summed capacitance C_p] + Cs of the capacitors 26 and 21 increases. In the aforementioned heater with heating resistor 10, the oscillation frequency F_s will be comparatively low if the heater is cool and it will be comparatively high if the heater is hot. When the detected voltage of voltage source 27 is very high there will no longer be a free oscillation and the oscillation frequency F_s will be pulled to the mains frequency. The output of the first inverting amplifier 23 is connected to the input of a second inverting amplifier 28, also formed by a Schmitt trigger, via a series resistor 29 having a value R₂. This input is connected to the reference terminal 25 via a parasitic capacitor 30 having a value C_p2 and a reference capacitor 31 having a value C_r.

In order to simplify a rough calculation of the oscillation frequency F_Sι it will be assumed, although this is not essential for the operation of the capacitive sensor and control circuit 5, that the average value of the low threshold voltage Vi and the high threshold voltage Vj, lies at half the supply voltage N_cc and that their difference voltage is N„:

V_h = 1/2 * (V_cc + V_n); V, = 1/2 * (V_cc - V_n) (1)

During oscillation the voltage V(t) across the overall capacitance at the input of the first inverting amplifier 23 will vary between the low threshold voltage Vj and the high threshold voltage V_h within one period T. The voltage across the overall capacitance will vary in accordance with:

V(t) = No * exp (-t/τ) (2)

Here, N₀ is the initial voltage, τ =

* (C_s+C_pl), and Ri is the value of the feedback resistor 24. When t = T/2, V(T/2) = N, and V₀ = V_h, the following will be valid for one half period:

T/2 = τ * In {(V_cc + V_n)/(V_CC - V_n)} = τ * In {(l+α)/(l-α)} (3)

where α = V_n/V_cc. For a small value of α, equation (3) can be reduced to the following approximation:

T = 4 * τ * α = 4 * R₁ * (C_s+C_pl) * V_n/V_cc (4) The oscillation frequency F_s is therefore approximately equal to:

F_s = V_cc / {4 * V_n * R₁ * (C_s + C_pl)} (5)

At the output of the first Schmitt trigger 23 a square-wave voltage appears which charges and discharges the capacitors 30 and 31 via the series resistor 29. This results in a ripple voltage across the capacitors 30 and 31, whose peak-to-peak voltage V_pp is approximately equal to:

V_pp = V_cc / {4 * F_s * R₂ * (C_r + C_p2)} (6)

If resistor 29 is now selected to be equal to resistor 24, and equation (5) is substituted in equation (6), it follows that:

V_pp = V_n * (C_s + C_pl) / (C_r + C_p2) (7)

Now if a Schmitt trigger having the same voltage difference V_n between the high and the low threshold voltage as that applied to the first inverting amplifier 23 is used for the second inverting amplifier 28, the following will be achieved. V_pp is smaller than V_n if C_s is smaller than C_r, so that the two trigger thresholds V_n and V_h of the second inverting amplifier 28 are not exceeded and a DC level appears at the output of this amplifier 28. V_pp is greater than V_n if C_s is greater than C_r, so that the two trigger thresholds V_n and V_h are exceeded periodically and an AC signal with the oscillation frequency F_s appears at the output of amplifier 28. The voltage difference V_n is substantially equal for the inverting amplifiers 23 and 28 if the amplifiers are identical and are integrated on one semiconductor body. This has the additional advantage that the parasitic capacitors C_pl and C_p2 are then also substantially equal and the influence of these parasitic capacitors on the amplitude of V_pp is eliminated. The resistors 24 and 29 have equal values. A parasitic capacitance parallel to these resistors will then have no effect on the ripple voltage V_pp either because a strictly symmetrical load is seen from the output of the first amplifier 23 to the reference terminal 25.

It is achieved thereby that the output of the second amplifier 28 supplies an AC signal if C_s is greater than C_r and a DC signal if C_s is smaller than C_r, inaccuracies as a result of component spreads being largely eliminated owing to the symmetry of the circuit. A comparator 32, which is also formed by an inverting Schmitt trigger and which has an input and an output, can detect whether this output carries an AC or a DC signal said input receiving a signal from the output of the second inverting amplifier 28 via a charge pump 33. The charge pump 33 comprises a first capacitor 34 connected between the output of the second amplifier 28 and a node 35, a first diode 36 having its cathode connected to the node 35 and its anode to the reference terminal 25, a second diode 37 having its anode connected to the node 35 and its cathode to the input of comparator 32, and a resistor 38 and a capacitor 39, which are connected between the input of the comparator 32 and the reference terminal 25.

The charging current i through the capacitor 34 per oscillation period 1/F is approximately equal to:

i = Ci * (V_cc - 2*V_j - V₂) * F = V₂ / R₃ (8)

In this equation, Ci is the value of the capacitor 34, V_j is the junction voltage of the diodes 36 and 37, V₂ is the voltage across the capacitor 39, and R₃ is the value of the resistor 38. If the threshold voltages of the comparator 32 lie approximately at half the supply voltage V_cc> the voltage V₂ must be equal to V_cc/2 at the minimum frequency Fi_, and the following equation is valid:

F, = V_cc / {R₃ * C, * (V_cc - 4*V_j)} (9)

As was stated above, the oscillation frequency may be equal to the detected mains frequency. If Fi = 20 Hz, V_cc = 5 V, and Vj = 0.7 V, it follows from equation (9) that the time constant R₃*Cι has a value of approximately 114 ms. Fig. 7 shows the capacitive sensor circuit of Figure 6 used in an electrical apparatus. The inverters 23, 28 and 32 are implemented by means of dual-input Schmitt trigger NANDs. The output of the inverter 32 drives the base of an NPN switching transistor 40 via a fourth dual-input Schmitt trigger NAND 41 and a current-limiting resistor 42, the emitter of said switching transistor being connected to the reference terminal 25 via a light- emitting diode (LED) 43 and the collector of this transistor driving the energizing coil 44 of a relay 8 via an interrupter switch 46. The relay and the NANDs receive their supply voltage from a supply voltage source (not shown), which may comprise a rectifier circuit (not shown). The relay actuates a switch 20. At comparatively low oscillation frequencies, i.e. at low temperature of heater 2, when the transistor 40 is turned on via the inverter 41, the energizing coil 44 of the relay 8 is energized, the switch 20 is closed and the heating resistor 10 receives mains voltage. The interrupter switch 46 can interrupt the power supply to the energising coil 44 to render the electrical heater 2 inoperative. The LED 43 signals that the load is connected to the mains via the switch 20. When the temperature of heater 2 rises, the oscillation frequency is increased, the output of the second amplifier 28 is then continually increased, and the input of the inverter 32 is low, so that the output thereof is high and the transistor 40 receives no base current. Now the relay 8 is not energized and the heating resistor 10 is disconnected from the mains voltage. Thus the relay 8 is energized in response to the AC/DC signal at the output of the second inverting amplifier 28.

It will be apparent to those skilled in the art that many additional and alternative embodiments are possible within the scope of the present invention other than those already explicitly described above. For example, it is possible to use materials other than polyimide as a base material for the several components making up the heater according to the invention, such as resin, silicone, etc. Also, other configurations for the control circuit than those explicitly described with reference to Figs. 6 and 7 are conceivable, in as far as they are capable of sensing the leakage current and controlling the heating resistor in response to a temperature measurement signal, reflecting the actual temperature of the heater or at least of the sensor. Also, other materials for influencing the temperature-dependent dielectric properties of the insulator layer are possible in addition to those explicitly described above, and the same holds for the filler materials mentioned in relation to the conductive layer, the heating resistor, and the contact elements. Therefore, the scope of protection is only limited by the definitions according to the accompanying claims, which do not reflect any limitation to a specific control circuit, specific materials, etc., and therefore alternatives within the scope of these definitions are possible, for example, where the first conductive layer having a resistance of the R C equivalent configuration is a component separate from the heating resistor, i.e. the sensor is arranged at a distance from (and not on) the heating element.

Claims

CLAIMS:

1. An overheating-protected heater (2) for an appliance (1,3) having a substrate

(6) to be heated, comprising: a heating element (10) which is selectively connectable to a power supply (4); and - a temperature control circuit (5) having a temperature sensor (11) in thermal contact with the heating element, which control circuit is arranged so as to control the heating element in response to temperature measurement signals from the temperature sensor, wherein the temperature sensor comprises in order that a first conductive layer (10) having a resistance, an insulator layer (14) having temperature-dependent dielectric characteristics, and a second conductive layer (16) so as to form the equivalent of a parallel configuration of a resistor (22) and a capacitor (21), wherein the temperature-dependent leakage current through the parallel configuration provides the temperature measurement signal for the temperature control circuit.

2. A heater as claimed in claim 1, wherein the heating element (10) comprises a heating resistor (10), and the first conductive layer (10) forms the heating resistor (10).

3. A heater as claimed in claim 1, wherein the insulating layer (14) comprises a material chosen from a group of suitable materials comprising: SiO₂₎ Al₂O₃, TiO, BaTiO₃, A1N, SiN, BN, semiconductors, and glass materials.

4. A heater as claimed in claim 1, wherein the insulating layer (14) comprises a basic material, such as a silicone or polyimide resin.

5. A heater as claimed in claims 3 and 4, wherein the insulating layer (14) comprises the basic material as a binder and at least one of the materials from said group as a filler material in a predetermined amount so as to obtain desired temperature-dependent dielectric characteristics.

6. A heater as claimed in claim 1 or 2, wherein the temperature control circuit (5) is connected to the second conductive layer (16).

7. A heater as claimed in claim 1, wherein at least the second conductive layer (16) extends over a large area of the heating element (10).

8. A heater as claimed in claim 1, wherein the insulator layer (14) extends as a protective layer over the heating element (10).

9. A heater as claimed in claim 1, wherein the heating element (10) and/or the conductive layer (10) is/are made from a basic material, such as polyimide, and a filler material, such as graphite and carbon, in a predetermined amount so as to provide the desired conductive properties.

10. A heater as claimed in claim 1, wherein contact elements (15) of the heating resistor (10) are made from a basic material, such as polyimide, and a filler material, such as silver, to obtain the desired contacting and power distributing properties.

11. An appliance (1, 3), in particular a thermal domestic appliance such as a water cooker, an electric iron, a hot-plate, etc., comprising a substrate (6) having a surface which is to be heated and a heater (2) as claimed in at least one of the preceding claims.