EP2999305B1

EP2999305B1 - Induction hob and method for measuring electrical currents of an induction hob

Info

Publication number: EP2999305B1
Application number: EP14185266.5A
Authority: EP
Inventors: Alex Viroli; Laurent Jeanneteau; Massimo Nostro; Massimo Zangoli
Original assignee: Electrolux Appliances AB
Current assignee: Electrolux Appliances AB
Priority date: 2014-09-18
Filing date: 2014-09-18
Publication date: 2017-08-02
Anticipated expiration: 2034-09-18
Also published as: EP2999305A1

Description

The present invention relates generally to the field of induction hobs. More specifically, the present invention is related to an induction hob comprising a current measurement unit for measuring electrical currents of said induction hob.

BACKGROUND OF THE INVENTION

Induction hobs for preparing food are well known in prior art. Induction hobs typically comprise at least one heating zone which is associated with at least one induction element. For heating a piece of cookware placed on the heating zone, the induction element is coupled with electronic driving means for driving an AC current through the induction element. Said AC current generates a time varying magnetic field. Due to the inductive coupling between the induction element and the piece of cookware placed above the induction element, the magnetic field generated by the induction element causes eddy currents circulating in the piece of cookware. The presence of said eddy currents generates heat within the piece of cookware due to the electrical resistance of said piece of cookware.
Induction hobs typically comprise an entity for measuring the electric current provided to one or more induction elements. Frequently, an operational amplifier is used which receives an input signal provided by a shunt resistor. The operational amplifier may implement a signal inversion, amplification and/or an offset addition in order to provide an appropriate measurement signal to a control unit of the induction hob.
Disadvantageously, the usage of an operational amplifier leads to a complex circuit board design because the operational amplifier has to be driven by a power supply providing positive and negative supply voltage (e.g. +5V and -5V). Hence, the manufacturing of the circuit board of the induction hob is quite expen-Document US-A-4 115 676 discloses an induction hob comprising a power stage with at least one switching element for enabling an alternating current flow through an induction element, a control unit for controlling the current flow through the induction element and a current measurement unit.

SUMMARY OF THE INVENTION

It is an objective of the embodiments of the invention to provide an induction hob comprising a current measurement unit which may be manufactured at reasonable cost. The objective is solved by the features of the independent claims. Preferred embodiments are given in the dependent claims. If not explicitly indicated otherwise, embodiments of the invention can be freely combined with each other.
According to an aspect of the invention, the invention relates to an induction hob comprising a power stage with at least one switching element for enabling an alternating current flow through an induction element, a control unit for controlling the current flow through the induction element and a current measurement unit. The current measurement unit comprises a first and a second transistor, wherein the bases or gates of the first and second transistors are directly coupled with each other. The current measurement unit is coupled with a shunt resistor for providing an input signal to the current measurement unit. The shunt resistor is coupled with the emitter path of said first transistor. Furthermore, the base or gate of the first transistor is directly coupled with the collector of said first transistor. Based on the input signal, a measuring signal is obtained at the collector of the second transistor, said measurement signal being indicative for the energy consumption of one or more induction elements of the induction hob. Advantageously, the current measurement unit comprises only discrete components or elements like transistors, resistors and capacities, i.e. the usage of operational amplifiers can be avoided.
According to preferred embodiments, the current measurement unit is adapted to provide amplification between the voltage of the input signal and the voltage of the measuring signal. Thereby, low variations of the input signal may be transferred into significant variations of the measurement signal and therefore detectable by the control unit.
According to preferred embodiments, the current measurement unit comprises a negative amplification characteristic. When arranging the shunt resistor between the negative port of a bridge rectifier, i.e. the node of the bridge rectifier at which the anodes of two adjacent diodes are directly coupled, and ground, the voltage drop measured at the shunt resistor is negative. By using a current measurement unit with a negative amplification characteristic, a non-inverted relationship between the input signal and the measurement signal is achieved, which can be directly processed by the control unit.
According to embodiments, the shunt resistor is arranged between ground and the negative port of a bridge rectifier powering the power stage. Thereby the input signal can be directly derived at the bridge rectifier and the current flowing through all induction elements coupled with said bridge rectifier can be measured. In addition, the emitter of the switching element can be directly coupled with ground thereby improving its driving.
According to embodiments, the shunt resistor is placed in the emitter path of the switching element. Thereby, the electric current flowing through each switching element can be determined separately.
According to embodiments, a capacitor is connected in parallel to the shunt resistor. Thereby, fluctuations of the input signal can be filtered out leading to a smoothing of the measurement signal.
According to embodiments, the collector of the second transistor is coupled with a low pass filter for filtering the measurement signal. Said low pass filter may be a passive first-order low pass filter comprising a resistor and a capacitor. By using a low pass filter at the output of the current measurement unit, the measurement signal is filtered leading to a slowly varying measurement signal without significant signal variations which can be directly processed by the control unit.
According to embodiments, a capacitor is provided between the collector of the first transistor and an input port of the current measurement unit for receiving the input signal. Thereby, impacts of input signal fluctuations on the measurement signal can be further mitigated.
According to a second aspect, the invention relates to a method for measuring the power consumption of at least one induction element of an induction hob, the induction hob comprising a power stage with at least one switching element for enabling an alternating current flow through the induction element, a control unit for controlling the current flow through the induction element and a current measurement unit, the method comprising the steps of:

providing a current measurement unit comprising a first and a second transistor, the bases or gates of the first and second transistors directly coupled with each other,
coupling the current measurement unit with a shunt resistor for providing an input signal to the current measurement unit, the shunt resistor being coupled with the emitter path of said first transistor;

directly coupling the base or gate of the first transistor with the collector of said first transistor; and
obtaining a measuring signal at the collector of the second transistor based on the input signal, said measurement signal being indicative for the energy consumption of one or more induction elements of the induction hob.

The term "essentially" or "approximately" as used in the invention means deviations from the exact value by +/- 10%, preferably by +/- 5% and/or deviations in the form of changes that are insignificant for the function.

BRIEF DESCRIPTION OF THE DRAWINGS

The various aspects of the invention, including its particular features and advantages, will be readily understood from the following detailed description and the accompanying drawings, in which:

Fig. 1: shows an example schematic view of an induction hob according to the current invention;
Fig. 2: shows an example schematic diagram of the electrical components comprised within the induction hob;
Fig. 3: shows an example circuit diagram of the bridge rectifier, the power stage and the driver unit according to Fig. 2; and
Fig. 4: shows an example circuitry of the current measurement unit.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention will now be described more fully with reference to the accompanying drawings, in which example embodiments are shown. However, this invention should not be construed as limited to the embodiments set forth herein. Throughout the following description similar reference numerals have been used to denote similar elements, parts, items or features, when applicable.
Fig. 1 shows a schematic illustration of an induction hob 1 according to the invention. The induction hob 1 may comprise multiple heating zones 2 preferably provided at a common hob plate. Each heating zone is correlated with at least one induction element placed beneath the hop plate. The induction hob 1 further comprises a user interface 3 for receiving user input and/or providing information, specifically graphical information to the user.
Fig. 2 shows a schematic block diagram of an induction hob 1 being adapted to measure the current provided to one or more induction elements comprised within the induction hob 1. The induction hob 1 comprises a power stage 10, a control unit 11 and a user interface 3, said user interface 3 being coupled with the control unit 11 in order to provide information to the user and/or to receive information from the user via the user interface 3. Said control unit is coupled with the power stage 10 in order to control the electrical power provided to the power stage 10, specifically to control the power provided to one or more induction elements comprised within the power stage 10.
Furthermore, the induction hob 1 may comprise a bridge rectifier 13, said bridge rectifier 13 being coupled with the power stage 10 for providing electrical power to the induction element of the power stage 10. The bridge rectifier 13 may be coupled with one or more phases of the mains supply network.
According to embodiments, the control unit 11 is coupled with the power stage 10 via a driver unit 14, said driver unit 14 being adapted to receive a pulsed electrical signal P by the control unit 11, modify said received pulsed electrical signal P and provide said modified pulsed electrical signal P' to the power stage 10. According to other embodiments, the control unit 11 may be directly coupled with the power stage 10, i.e. may provide the pulsed electrical signal P directly to the power stage 10. Said pulsed electrical signal P, respectively, modified pulsed electrical signal P' may be applied to a switching element comprised within the power stage 10 in order to enable an alternating current flow through the induction element.
In order to determine the amount of electrical current flowing through the power stage 10, the induction hob 1 comprises a current measurement unit 12. According to an embodiment, the current measurement unit 12 is coupled with the bridge rectifier 13 for receiving information regarding the amount of current flowing through the power stage 10. According to other embodiments, the current measurement unit 12 is coupled with the power stage 10, specifically with the emitter path of the switching element comprised within the power stage 10 in order to receive information regarding the amount of current flowing through said switching element.
The current measurement unit 12 is configured to receive an input signal IS, said input signal IS being indicative for the amount of current being provided to the power stage 10 and derive a measurement signal MS based on the input signal IS. The current measurement unit 12 is further coupled with the control unit 11 in order to provide said measurement signal MS to the control unit 11. The current measurement unit 12 is configured to derive a measurement signal MS which can be directly processed by the control unit 11, i.e. the values of the measurement signal MS are adapted to the value range directly processible by the control unit 11. For example, the control unit 11 may be adapted to receive voltage values in the range of 0V to 5V. Therefore, the current measurement unit 12 may be adapted to provide measurement signals MS with voltage values within upper-mentioned voltage range.
Fig. 3 shows the driver unit 14, the power stage 10 and the bridge rectifier 13 in closer detail. The driver unit 14 receives the pulsed electrical signal P at the input port I1. The driver unit 14 comprises an electrical circuitry configured to adapt the received pulsed electrical signal P according to the needs of the power stage 10. For example, the driver unit may amplify the received pulsed electrical signal P and/or may change the signal level of the pulsed electrical signal P by adding a certain offset voltage value to said received pulsed electrical signal P in order to derive said modified pulsed electrical signal P'. Said modified pulsed electrical signal P' may be provided to the gate of the switching element 20. Said switching element 20 may be, for example, an IGBT.
The collector of the switching element 20 may be coupled via a filtering circuitry (comprising one or more capacitors) to an oscillating circuit 23, said oscillating circuit 23 comprising the induction element 21 and a capacitor 22. The power stage 10 may comprise a quasi-resonant power stage architecture. On the opposite side of the capacitor 22, the induction element 21 may be coupled with the bridge rectifier 13 in order to power the oscillating circuit 23 by the mains supply network. By enabling a current flow through the switching element 20 by means of the modified pulsed electrical signal P', an alternating current flow through the induction element 21 is obtained which induces eddy currents in a piece of cookware placed above the induction element 21 thereby providing heat to said induction element 21.
In order to determine the amount of current provided by the bridge rectifier 13 to the power stage 10, the induction hob 1 comprises a shunt resistor R_shunt. Said shunt resistor R_shunt is coupled on the one hand with the negative port of the bridge rectifier 13, i.e. the node of the bridge rectifier 13 at which the anodes of two adjacent diodes are directly coupled. On the other hand, the shunt resistor R_shunt is further coupled with ground. The voltage drop over the shunt resistor R_shunt is indicative for the electric current provided by the bridge rectifier 13 to the power stage 10 and might be used as input signal IS of the current measurement unit 12.
Fig. 4 shows an example circuitry comprised within the current measurement unit 12. The current measurement unit 12 solely comprises discrete components like resistors, capacitors and transistors, i.e. there are no integrated circuits, e.g. operational amplifiers etc. The current measurement unit 12 comprises two transistors T1, T2 which are coupled in a current-mirror-circuit-like manner. The current measurement unit 12 further comprises an input for receiving the input signal IS. Said input signal is received at the emitter path of the first transistor T1. The transistors may be, for example, bipolar transistors of an n-p-n type. The current measurement unit 12 is powered by a supply voltage Vcc, wherein Vcc is, for example, 5V.
The collector of the first transistor T1 is coupled with the supply voltage Vcc via a first collector resistor Rc1. The emitter path of said first transistor T1 comprises a first emitter resistor Re1 and an emitter capacity Ce, wherein the first emitter resistor Re1 is coupled at a first contact with the emitter of the first transistor T1 and at a second contact with the emitter capacity Ce. The emitter capacity Ce is coupled at a further contact opposite to the first emitter resistor Re1 with ground. In other words, the first emitter resistor Re1 and the emitter capacity Ce are serially coupled within the emitter path of the first transistor T1.
The second transistor T2 also comprises a collector path and an emitter path. The collector path comprises a second collector resistor Rc2, the second collector resistor Rc2 being coupled with one resistor contact with the supply voltage Vcc and with the further contact with the collector of the second transistor T2. The measurement signal MS may be derived at the collector of the second transistor T2, i.e. at the node between the collector of the second transistor T2 and the second collector resistor Rc2. In the emitter path of the second transistor T2, a second emitter resistor Re2 is arranged wherein the emitter of the second transistor T2 is coupled with ground via said second emitter resistor Re2.
Furthermore, the bases or gates (in case of using field effect transistors) are directly coupled with each other, i.e. coupled via an electrical connection without any electrical device. In addition, there is also a direct electrical connection (without any electrical device) between the collector and the base or gate of the first transistor T1. Thereby, the voltage applied to the collector of the first transistor T1 is equal to the voltage applied to the bases or gates of the first and second transistor T1, T2.
For deriving the measurement signal MS based on the input signal IS, the input of the current measurement unit 12 is coupled with node 25 between the bridge rectifier 13 and the power stage 10.
Keeping in mind the flow direction of the electric current through the shunt resistor R_shunt, the voltage U_Rshunt is negative. Thereby, also the input signal IS comprises a negative voltage with respect to ground level. So, in case that the current flowing through the bridge rectifier 13 is rising, the voltage U_Rshunt is also rising, i.e. the voltage at node 26 between the first emitter resistor Re1 and the emitter capacity Ce is increasing in the negative range. Thereby, also the current flowing through the first transistor T1 is rising.
Due to the upper-mentioned coupling of the first and second transistor T1, T2, the rising of the electric current flowing through the first transistor T1 may cause a rising current flow through the second transistor T2. The rising current flow through the second transistor T2 causes a rising voltage at the collector of said second transistor T2, i.e. the measurement signal MS also shows a rising voltage.
Conversely, a decreasing current flow through the bridge rectifier 13 may cause a reduced current flow through the second transistor T2 and therefore may cause a decreasing voltage at the collector of said second transistor T2, i.e. the measurement signal MS also shows a decreasing voltage.
Due to switching the switching element 20 based on the pulsed electrical signal P and the high currents flowing through the induction element 21, the current measurement within the current measurement unit 12 is very noisy, i.e. the input signal IS may vary due to parasitic side effects which may worsen the measurement results provided by the current measurement unit 12. In order to suppress said noise, the current measurement unit 12 comprises several capacitors which suppress said input signal fluctuations and/or measurement signal fluctuations. At the input of the current measurement unit 12 signal fluctuations of the input signal IS maybe suppressed by the emitter capacitor Ce comprised within the emitter path of the first transistor T1. As already mentioned above, the emitter capacitor Ce may be arranged between the first emitter resistor Re1 and ground. Thereby, the emitter capacitor Ce is connected in parallel to the shunt resistor R_shunt. In addition, at the input of the current measurement unit 12, a collector capacitor Cc may be provided which connects the input of the current measurement unit 12 with the collector of the first transistor T1. Said emitter capacitor Ce and said collector capacitor Cc may lower signal fluctuations of the input signal IS.
Furthermore, the current measurement unit 12 comprises a low pass filter 24, said low pass filter 24 being provided at the output of the current measurement unit 12, i.e. the low pass filter is connected with the collector path of the second transistor T2. For example, the low pass filter 24 comprises a resistor Rf and a capacitor Cf forming a passive first-order low pass filter. Of course, also other low pass filters may be used. By means of said low pass filter 24, a smoothing of the measurement signal MS is achieved. In other words, the low pass filter 24 provides an averaged measurement signal MS thereby filtering out high-frequency signal fluctuations. For example, the low pass filter 24 may be chosen such that the variations of the measurement signal MS are very slow with respect to the timing of the control unit 11.
It should be noted that the description and drawings merely illustrate the principles of the proposed methods and systems. Those skilled in the art will be able to implement various arrangements that, although not explicitly described or shown herein, embody the principles of the invention.

List of reference numerals

1: induction hob
2: heating zone
3: user interface
10: power stage
11: control unit
12: current measurement unit
13: bridge rectifier
14: driver unit
20: switching element
21: induction element
22: capacitor
23: oscillating circuit
24: low-pass filter
25: node
26: node
Cc: collector capacitor
Ce: emitter capacitor
Cf: capacitor
I1: input
IS: input signal
MS: measurement signal
P: pulsed electrical signal
P': modified pulsed electrical signal
Rc1: first collector resistor
Rc2: second collector resistor
Re1: first emitter resistor
Re2: second emitter resistor
Rf: resistor
R_shunt: shunt resistor
T1: first transistor
T2: second transistor
U_Rshunt: voltage over R_shunt
Vcc: supply voltage

Claims

Induction hob comprising a power stage (10) with at least one switching element (20) for enabling an alternating current flow through an induction element (21), a control unit (11) for controlling the current flow through the induction element (21) and a current measurement unit (12), characterised in that,
the current measurement unit (12) comprising a first and a second transistor (T1, T2), the bases or gates of the first and second transistors (T1, T2) directly coupled with each other, the current measurement unit (12) further being coupled with a shunt resistor (R_shunt) for providing an input signal (IS) to the current measurement unit (12), the shunt resistor (R_shunt) being coupled with the emitter path of said first transistor (T1), wherein the base or gate of the first transistor (T1) is directly coupled with the collector of said first transistor (T1) and wherein, based on the input signal (IS), a measuring signal (MS) is obtained at the collector of the second transistor (T2), said measurement signal (MS) being indicative for the energy consumption of one or more induction elements (21) of the induction hob.
Induction hob according to claim 1, wherein the current measurement unit (12) is adapted to provide amplification between the voltage of the input signal (IS) and the voltage of the measuring signal (MS).
Induction hob according to claim 1 or 2, wherein the current measurement unit (12) comprises a negative amplification characteristic.
Induction hob according to anyone of the preceding claims, wherein shunt resistor (R_shunt) is arranged between ground and the negative port of a bridge rectifier (13) powering the power stage (10).
Induction hob according to anyone of the preceding claims 1 to 3, wherein the shunt resistor (R_shunt) is placed in the emitter path of the switching element (20).
Induction hob according to anyone of the preceding claims, wherein a capacitor (Ce) is connected in parallel to the shunt resistor (R_shunt).
Induction hob according to anyone of the preceding claims, wherein the collector of the second transistor (T2) is coupled with a low pass filter (24) for filtering the measuring signal (MS).
Induction hob according to anyone of the preceding claims, wherein a capacitor (Cc) is provided between the collector of the first transistor (T1) and an input port of the current measurement unit (12) for receiving the input signal (IS).
Method for measuring the power consumption of at least one induction element (21) of an induction hob (1), the induction hob (1) comprising a power stage (10) with at least one switching element (20) for enabling an alternating current flow through the induction element (21), a control unit (11) for controlling the current flow through the induction element (21) and a current measurement unit (12), the method comprising the steps of:
- providing a current measurement unit (12) comprising a first and a second transistor (T1, T2), the bases or gates of the first and second transistors (T1, T2) directly coupled with each other,

- coupling the current measurement unit (12) with a shunt resistor (R_shunt) for providing an input signal (IS) to the current measurement unit (12), the shunt resistor (R_shunt) being coupled with the emitter path of said first transistor (T1);

- directly coupling the base or gate of the first transistor (T1) with the collector of said first transistor (T1); and

- obtaining a measuring signal (MS) at the collector of the second transistor (T2) based on the input signal (IS), said measurement signal (MS) being indicative for the energy consumption of one or more induction elements (21) of the induction hob.