[0001] The present invention relates generally to the field of induction hobs. More specifically,
the present invention is related to an induction hob configured to reduce acoustic
noise during start-up of power stage.
BACKGROUND OF THE INVENTION
[0002] 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 comprising a switching element 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.
[0003] Typically, the switching element receives a pulsed enabling signal in order to enable
a current flow through the induction element according to the enabling signal. When
starting the power stage, the current flowing through the induction element may be
significantly higher than after a certain operating time of powering the power stage.
Said increased switch-on current may lead to significant acoustic noise which might
be disturbing for a user of the induction hob.
SUMMARY OF THE INVENTION
[0004] It is an objective of the embodiments of the invention to provide an induction hob
generating reduced acoustic noise when starting the power stage. 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.
[0005] 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 and a control unit providing an enabling
signal comprising pulses with a variable pulse duration to the switching element for
enabling said alternating current flow through said induction element. The pulse duration
of the pulses is changed between a minimum value and a maximum value in order to adjust
the power provided to the induction element. Said minimum value and maximum value
may be nominal minimum and maximum values defining a nominal range of pulse duration
values. In addition, the control unit is adapted to reduce the pulse duration of a
set of enabling signal pulses to a reduced pulse duration lower than said minimum
value when starting the heating process in order to reduce acoustic noise. Surprisingly,
the Applicant found out that the acoustic noise can be significantly reduced by upper-mentioned
reduction of pulse duration without any significant lowering of the lifetime of the
switching element, although the switching element is driven in hard switching region
due to the reduced pulse duration, i.e. there are hard voltage/current transitions
within the switching element in the switching moment. Thus, acoustic noise can be
avoided or reduced without any additional circuit components for reducing said acoustic
noise.
[0006] According to preferred embodiments, the switching element is an insulated-gate bipolar
transistor. Said insulated-gate bipolar transistor may be arranged in a quasi-resonant
architecture.
[0007] According to embodiments, the power stage comprises a quasi-resonant architecture
comprising a single switching element. In other words, there is no further switching
element needed which may reduce the acoustic noise by lowering the charge of a capacitor
providing electric power to the induction element.
[0008] According to embodiments, the pulse duration is reduced during a first set of half-waves
of mains provided by a bridge rectifier to the power stage immediately after start-up
of the power stage. During said first set of half-waves of mains, the electric charges
accumulated at a capacitor are significantly reduced. Therefore, also the current
provided to the induction element is significantly decreased. Thus, after said first
set of half-waves, the power stage may be driven within the nominal range of pulse
duration, i.e. with a pulse duration between minimum pulse duration and maximum pulse
duration because the risk of causing acoustic noise is eliminated during the period
of said first set of half-waves. According to embodiments, the first set of half-waves
of mains comprises 1 to 20 half-waves.
[0009] According to embodiments, the reduced pulse duration is between half of the minimum
pulse duration and minimum pulse duration. By using a reduction of pulse width in
the upper-mentioned range, a significant reduction of acoustic noise can be obtained.
[0010] According to embodiments, the reduced pulse duration is in the range between 5µs
and 9µs, preferably 7µs, and the minimum pulse duration is at least 10µs.
[0011] According to embodiments, after starting the heating process, the control unit is
configured to increase the pulse duration of the enabling signal pulses with each
enabling signal pulse by a predefined pulse duration step until said minimum value
of pulse duration is reached. So, in other words, starting at a reduced pulse duration
starting value, the pulse duration is stepwise increased until said minimum value
of pulse duration is reached. The step width may be chosen such that said minimum
value of pulse duration is reached as quick as possible without a significant generation
of acoustic noise. Thereby, a return to the nominal range of pulse duration as soon
as possible is achieved.
[0012] According to embodiments, after reaching said minimum value of pulse duration, the
control unit is configured to adapt the pulse duration of the enabling signal pulses
according to the requested power. So, after performing the noise reduction routine,
the control unit is driving the power stage in the normal power regulation mode, wherein
the pulse duration is between minimum and maximum pulse duration value in order to
power the induction element according to the power setting chosen by the user.
[0013] According to a second aspect, the invention relates to a method for operating an
induction hob, the induction hob comprising a power stage with at least one switching
element for enabling an alternating current flow through an induction element and
a control unit providing an enabling signal comprising pulses with a variable pulse
duration to the switching element for enabling said current flow through said induction
element, wherein the pulse duration is changed between a minimum value and a maximum
value in order to adjust the power provided to the induction element. The pulse duration
of a set of enabling signal pulses is reduced to a reduced pulse duration lower than
said minimum value when starting the heating process in order to reduce acoustic noise.
[0014] 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
[0015] 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 a 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;
- Fig. 4
- shows a first section of a signal diagram illustrating the voltage values of the enabling
signal, the collector voltage of the switching element, the capacitor voltage and
the trigger signal; and
- Fig. 5
- shows a second section of a signal diagram according to Fig. 4 wherein the signal
representations of Fig. 5 directly follow after the signal representations of Fig.
4.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0016] 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.
[0017] 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. The induction hob 1 may comprise at least one switching element associated
with a respective induction element for enabling a current flow through said induction
element. The switching element may be controlled by an enabling signal, said enabling
signal enabling a current flow through said induction element in order to induce eddy
currents within the piece of cookware placed above the induction element.
[0018] Fig. 2 shows a schematic block diagram of an induction hob 1 being adapted to perform
a noise reduction routine for reducing acoustic noise after starting a heating process.
[0019] 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. 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 comprised within the power stage 10. The bridge rectifier
13 may be coupled with one or more phases of the mains supply network.
[0020] According to embodiments, the control unit 11 is coupled with the power stage 10
via a driver unit 12, said driver unit 12 being adapted to receive an enabling signal
P comprising electrical pulses provided by the control unit 11, modify said received
enabling signal P and provide a modified enabling 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 enabling signal P directly to the power stage
10. The control unit 11 is adapted to modify the pulse duration ΔP according to the
requested power. The requested power may be provided to the control unit 11 according
to a power demand entered by a user at the user interface 3. The pulse duration ΔP
may vary between a minimum pulse duration ΔP
min, which corresponds to a minimum of requested power, and a maximum pulse duration
ΔP
max, which corresponds to a maximum of requested power. Said minimum pulse duration ΔP
min and maximum pulse duration ΔP
max may be chosen in order to guarantee a certain lifetime of a switching element enabling
an alternating current flow through an induction element based on the enabling signal
P.
[0021] In order to reduce the generation of acoustic noise when providing heat to the piece
of cookware placed above the induction element, the control unit 11 is adapted to
perform a noise reduction mechanism. When performing said noise reduction mechanism,
the pulse duration ΔP is lowered below the minimum pulse duration ΔP
min after start-up of the power stage 10. Thereby, the acoustic noise can be significantly
reduced.
[0022] Fig. 3 shows the driver unit 12, the power stage 10 and the bridge rectifier 13 in
closer detail. The driver unit 12 receives at input I1 the enabling signal P for enabling
an alternating current flow through the power stage 10. The driver unit 12 comprises
an electrical circuitry configured to adapt the received enabling signal P according
to the needs of the power stage 10. For example, the driver unit may amplify the received
enabling signal P and/or may change the signal level of the enabling signal P by adding
a certain offset voltage value to said received enabling signal P in order to derive
a modified enabling signal P'. Said modified electrical pulse P' may be provided to
the gate of the switching element 20. Said switching element 20 may be, for example,
an IGBT.
[0023] The collector (node 23) of the switching element 20 may be coupled via a filtering
circuitry (comprising one or more capacitors) to an oscillating circuit 25, said oscillating
circuit 25 comprising the induction element 21, preferably constituted by an induction
coil, and a capacitor 22. The power stage 10 may comprise 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
25 by the mains supply network.
[0024] During turn-off time of the switching element 20, the capacitor 22 is charged, i.e.
electric energy provided by the bridge rectifier 13 is stored in the capacitor 22.
Thereby, the electric voltage Vcap at node 24 is the rising. For example, the electric
voltage Vcap of node 24 may be up to 1.41* V
mains, wherein V
mains is the mains voltage. When starting the power stage 10 after such turn-off time,
a high current may flow through the induction element 21 thereby causing acoustic
noise. The electric current value flowing at start-up of the power stage 10 may depend
on the electric voltage of node 24 and the pulse duration ΔP of the enabling signal
P. The electric voltage Vcap of node 24 is correlated with the amount of electric
energy stored in the capacitor 22 which may be used for providing current flow through
the induction element 21. The pulse duration ΔP of the enabling signal P may be indicative
for the amount of electric charge which will be used for creating said current flow
through the induction element 21. In other words, at a fixed electric voltage Vcap
of node 24, the longer the pulse duration ΔP is, the higher is the current flowing
through the induction element 21 and the stronger the acoustic noise generated by
said electric current flow. However, it is not possible to decrease pulse duration
ΔP in an arbitrary way below a certain threshold, e.g. below ΔP
min because when reducing pulse duration ΔP below said threshold, the switching element
is driven in hard switching condition which can destroy the switching element. Surprisingly,
the Applicant found out that when driving the switching element below said threshold
pulse duration or below said minimum pulse duration ΔP
min for a limited period of time after starting the power stage 10, the lifetime of the
switching element 21 is not or not significantly reduced.
[0025] For example, the normal range of pulse duration ΔP may be 10µs to 22µs, wherein the
lower value corresponds to the minimum power setting and the upper value corresponds
to the maximum power setting. In order to avoid the occurrence of acoustic noise,
the pulse duration ΔP may be lowered to a reduced pulse duration ΔP
red wherein 0.5·ΔP
min ≤ ΔP
red < ΔP
min. For example, the reduced pulse duration ΔP
red is in the range between 5µs and 9µs, preferably 7µs. Said reduction of pulse duration
ΔP may be only obtained for a short period of time. The pulse duration reduction may
be obtained for a certain number of half wave cycles of the mains supply voltage provided
by the bridge rectifier 13. For example, the pulse duration reduction may be obtained
for 1 to 20 half wave cycles of the mains supply voltage.
[0026] During the first half wave cycles, the voltage drop over capacitor 22 (i.e. voltage
Vcap at node 24) is significantly decreasing. Thus, starting at reduced pulse duration
ΔP
red, the pulse duration ΔP can be stepwise increased without producing more acoustic
noise. For example, the pulse duration ΔP may be increased by a certain step width
every half wave cycle after starting the power stage 10. For example, the step width
may be in the range of 0.2µs to 1µs, specifically 0.5µs. The choosen step width may
depend on the reduction ratio, i.e. the ratio by which the reduced pulse duration
ΔP
red is lower than the minimum pulse duration ΔP
min. The pulse duration ΔP may be increased as long as the minimum power duration ΔP
min (which corresponds to the minimum power setting) is reached.
[0027] After reaching the minimum power duration ΔP
min, the control unit 11 may adapt the power duration ΔP according to the requested power
provided by the user via the user interface 3 in order to heat the piece of cookware
placed above the induction element 21 according to the chosen power setting.
[0028] Fig. 4 and 5 show signal diagrams of the enabling signal P, the capacitor voltage
Vcap at node 24, the collector voltage Vc at node 23 and a trigger signal TS causing
the generation of the next pulse of the enabling signal P. It is worth mentioning
that the signal illustrations of the enabling signal P and the trigger signal TS are
shifted against the collector voltage Vc and the capacitor voltage Vcap for the sake
of a better recognisability. Thereby, fig. 5 shows the signals following on the signals
of fig. 4. As can be seen from the length of the pulses of enabling signal P, the
pulse duration is increasing with every occurrence of a further enabling signal pulse,
wherein the step width for increasing the pulse duration ΔP of the enabling signal
P is 0.5µs in the present embodiment.
[0029] 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
[0030]
- 1
- induction hob
- 2
- heating zone
- 3
- user interface
- 10
- power stage
- 11
- control unit
- 12
- driver unit
- 13
- bridge rectifier
- 20
- switching element
- 21
- induction element
- 22
- capacitor
- 23
- node
- 24
- node
- 25
- oscillating circuit
- I1
- Input
- P
- enabling signal
- P'
- modified enabling signal
- ΔP
- pulse duration
- ΔPmin
- minimum pulse duration
- ΔPmax
- maximum pulse duration
- ΔPred
- reduced pulse duration
- TS
- trigger signal
- Vc
- collector voltage
- Vcap
- capacitor voltage
1. 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) and a control
unit (11) providing enabling signal (P) comprising pulses with a variable pulse duration
to the switching element (20) for enabling said current flow through said induction
element (21), wherein the pulse duration (ΔP) is changed between a minimum value (ΔPmin) and a maximum value (ΔPmax) in order to adjust the power provided to the induction element (21), characterised in that,
the control unit (11) is adapted to reduce the pulse duration (ΔP) of a set of enabling
signal pulses (P) to a reduced pulse duration (ΔPred) lower than minimum value (ΔPmin) when starting the heating process in order to reduce acoustic noise.
2. Induction hob according to claim 1, wherein the switching element (20) is an insulated-gate
bipolar transistor (IGBT).
3. Induction hob according to claim 1 or 2, wherein the power stage (10) comprises a
quasi-resonant architecture comprising a single switching element (20).
4. Induction hob according to anyone of the preceding claims, wherein the pulse duration
(ΔP) is reduced during a set of first half-waves of mains provided by a bridge rectifier
(13) to the power stage (10) immediately after start-up of the power stage (10).
5. Induction hob according to claim 4, wherein the number of first half-waves of mains
is between 1 and 20.
6. Induction hob according to anyone of the preceding claims, wherein the reduced pulse
duration is ΔPred ≥ 0.5·ΔPmin.
7. Induction hob according to anyone of the preceding claims, wherein the reduced pulse
duration (ΔPred) is in the range between 5µs and 9µs, preferably 7µs.
8. Induction hob according to anyone of the preceding claims, wherein, after starting
the heating process, the control unit (11) is configured to increase the pulse duration
(ΔP) of the enabling signal pulses (P) with each enabling signal pulse by a predefined
pulse duration step until said minimum value (ΔPmin) of pulse duration is reached.
9. Induction hob according to claim 8, wherein, after reaching said minimum value (ΔPmin) of pulse duration, the control unit (11) is configured to adapt the pulse duration
(ΔP) of the enabling signal pulses (P) according to the requested power.
10. Method for operating 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 an induction element (21) and a control unit (11) providing enabling
signal (P) comprising pulses with a variable pulse duration (ΔP) to the switching
element (20) for enabling said current flow through said induction element (21), wherein
the pulse duration (ΔP) is changed between a minimum value (ΔPmin) and a maximum value (ΔPmax) in order to adjust the power provided to the induction element (21), characterised in that, the pulse duration (ΔP) of a set of enabling signal pulses (P) is reduced to a reduced
pulse duration (ΔPred) lower than minimum value (ΔPmin) when starting the heating process in order to reduce acoustic noise.