OBJECT OF THE INVENTION
The object of the present invention is the optimal control and the
management of the power provided by a flexible and reconfigurable
topology based on the three-phase bridge with non-symmetric legs. This
flexible and reconfigurable topology constitutes a module which is able
to drive two loads, which in this preferred embodiment, are the two
heating plates of an induction cooking hob (Figure 1). The goals
achieved with the optimal control proposed are:
a) The proposed control drives both coils independently with
regulated power up to their power ratings. b) The proposed control allows the utilisation of all the installed power
and using it for driving anyone of the coils providing ultra-fast heating
ability. This performance provides a power capacity higher than the rated
power of the coil without increasing the rating characteristics of the
system. c) The proposed control matches the installed power to the demands
in order to lower the temperature of the electronic power devices,
improving in the system reliability and integrability, and lowering the
heat sinking demands.
FIELD OF THE INVENTION
This invention relates to induction heating and more specifically to
the optimal control of the installed power in induction cooking apparatus
with re-configurable structure topology, which provides ultra-fast heating
ability and optimised semiconductor switches power losses.
BACKGROUND OF THE INVENTION
The principle of induction cooking are the eddy current and hysteretic
losses in the surface of a metallic object therein, it has several
advantages over heating by conventional techniques as convection or
conduction. Induction heating is usually faster than convection or
conduction heating because lower thermal mass is associated with
induction heating systems. In addition, the induction heating focuses the
heat within the heated object yielding higher energy transfer efficiency in
contrast to convection or conduction heating wherein the heat is
produced outside the heated object.
The domestic induction heating hobs are driven by an alternative
current of medium frequency (25-65 kHz) applied to an induction coil
which heats by induction a pot or a pan placed on the coil. This current
is generated by a power converter based on a solid-state power devices.
In what follows a new concept of the control strategy for the power
converter used for induction heating hobs is presented. This control
strategy lower the power losses in the electronic power switches of the
converter and provides ultra-fast heating ability.
The owner of this patent also owners the Patent of Invention n°
539.790 wherein an electronic system drives by a high frequency pulsed
current a thermal plate as those used in an electric hob. This system
uses a full-bridge inverter with MOS transistors which drives a flat coil
housed inside a thermal plate by a serial of high frequency power pulses
heating ferromagnetic pots or pans.
The transistorised full-bridge described in the afore mentioned Patent
is activated by a control circuit which achieves the regulation and the
self adaptation of the firing time for each leg of the transistorised full-bridge,
scheduled in relation with the inductive-energy time-recovering
of the flat coil, and stopping the powering of the coil when there is non-ferromagnetic
load. In that way, when a ferromagnetic pot or pan is
placed on the thermal plate, a thermal with self-firing ability is achieved.
Although the induction heating method is fast, the rising time
necessary to reach the stationary thermal state can be shortened if the
spare power drive capacity of an idle bridge which normally drives the
induction heating plate are used to boost the bridge which drives the
active induction plate. Another application of the idle bridge is to help in
the reduction of the overall conduction losses of the power
semiconductor switches of the active bridge.
To achieve these goals it is necessary a bridge inverter topology with
a variable structure able to perform these tasks. The owner of this patent
also owners the Patent of Invention which describes the foregoing
topology with variable structure. The control of this topology with
variable structure is the core of this invention.
DESCRIPTION OF THE INVENTION
This invention provides several control strategies for driving the
electronic power switches which feed the induction coils of an induction
cooking hob. These strategies aim to perform a regulated power in the
pot or pan placed on the coil and lowering the power losses in the
electronic power switches. In this way, the working temperature of the
electronic power devices is lowered, thus, either the heat sinking
demands are reduced or the devices can work in an environment with
higher room temperature.
The topology where these control strategies are designed for is the
three-phase bridge with non-symmetric legs including one or two
switches with a single-pole two-positions each. This topology is obtained
by modifying another one which is well-known as the tree-phase bridge.
The later is frequently used as DC-AC converter to drive three-phase
loads, but in this case, it has been modified by adding one or two
switches with a single-pole two-positions each. In this way, a flexible and
reconfigurable structure topology is obtained depending on the position
of the switches. A bridge is not symmetric if all their legs are not
identical concerning the controllability of the power switches, the
working zone of the voltagelcurrent quadrant or the handled power. The
modification introduced in the bridge consists in rating individually two
of the legs with a power handling capacity fitted to the respective coil
rating. These coils constitute two independent heating plates of an
induction cooking hob. The third leg, named as common leg, is rated
with a power handling capacity equal to the added ratings of the other
two legs.
The control strategy implemented allows separated driving of each of
the two coils with regulated power up to their rated values.
In the case wherein a single coil demands for power, all the installed
power can be used for driving this coil, and thus, performing ultra-fast
heating ability with a power capacity higher than the rated power of the
coil without increasing the rating characteristics of the system.
The control strategy proposed determines the switch states as a
function of the power demands yielding four cases: 1) Both coils off, 2)
The first coil demands ultra-fast heating, 3) The second coil demands
ultra-fast heating, and 4) Both coils demand power equal to or less than
their rated power.
The ultra-fast heating ability consists in driving a single coil with a
regulated power higher than its rated power and feeding it with constant
frequency. To provide it, the control strategy proposed states a current-controlled
voltage-polarity switching.
When driving a single coil with a power equal to or less than its rated
power two control strategies are stated depending on the demanded
power level. These strategies are either current-controlled voltage-cancellation
or duty ratio control. The later is also named energy control.
It is set a power level so called boundary power. This is the maximum
power that can be delivered to a load by using the energy control
strategy, taking into account the limitations imposed by the
Electromagnetic Compatibility Standard concerning the voltage
fluctuations and flicker in low voltage supply systems.
The current-controlled voltage-cancellation is used for power
demands within the boundary power and the rated power. The energy
control is used for power demands below the boundary power.
When the current-controlled voltage-cancellation strategy imposes
large voltage-cancellation and the power demand is higher than the
boundary power, an alternative control method is proposed. This method
maintains constant the voltage-cancellation time and varies the time
without voltage-cancellation. This alternative method works with variable
frequency and allows the snubbers networks maintaining their
functionality. Consequently, the power losses in the electronic power
devices can be maintained low.
The proposed control strategy allows driving the two coils with power
equal to or less than their respective rated values. If the demanded
power is in both coils higher than the boundary power, it is applied the
current-controlled voltage-cancellation strategy. The activation time of
the electronic power devices are co-ordinated for the sake of optimising
their power losses. This co-ordination applies a bipolar voltage to one of
the coil at the start of a half-period and to the other coil at the end of a
half-period. With this co-ordination it is avoided the simultaneous coil-current
overlapping through the electronic power devices, thus
minimising the root mean square (RMS) current circulating through the
common leg. Consequently, the working temperature of the power
electronic devices belonging to the common leg is lowered, improving
the reliability and integrability of the system.
The foregoing control strategy allows simultaneously driving of both
coils with power levels equal to or lower than their rated values. When
the power demand in any coil is lower than the boundary power, the
control strategy applied to that coil is the energy control, and for the
other coil, is applied the current-controlled voltage-cancellation. For
power demands lower than the boundary power in both coils, the energy
control applied yields lower switching losses in the electronic power
devices when compared to the current-controlled voltage-cancellation
strategy.
When both coils demand power equal to or lower than their rated
power, the proposed control strategy states a common working
frequency for both coils, consequently, there are not intemodulation or
sub harmonic frequencies, thus, audible noise is avoided.
Other advantage provided by the foregoing control strategy is that
they are able to be implemented in integrated programmable logic
devices, thus, allowing maximum integrability, flexibility,
programmability and reliability of the control system.
BRIEF DESCRIPTION OF THE DESIGNS
The features of the invention believed to be novel are set forth with
particularity in the appended claims. The invention itself, however,
together with further objects and advantages thereof, may be understood
by reference to the following description taken in conjunction with the
accompanying drawings in which:
Figure 1.- Shows the re-configurable and multi-bridge topology
with two switches and with power boost in both coils B1 and B2. In this
Figure it is named:
- S1 to S6 as the electronic power semiconductor switches.
- B1 and B2 are the load coils.
- The leg formed by S3 and S4 are the so called common leg of the
three-phase bridge.
- The leg formed by S1 and S2 are dimensioned for the rated power
of B2.
- The leg formed by S5 and S6 are dimensioned for the rated power
of B1.
- R1 and R2 are the single-pole two positions switches.
- Vd is a generic DC current or voltage power source, obtained by
rectifying an AC power source, either filtered or not, being the filter
active or passive whether applicable, without any limitation introduced
by the nature of the power source.
The switches positions shown in Figure 1 corresponds both to non-activated
state. In this situation the common pole of each switch is
electrically connected to its normally-closed (nc) output pole, paralleling
together both legs S1-S2 and S5-S6. If only the switch R1 is activated, all
the installed power is available for coil B1. If only the switch R2 is
activated, all the installed power is available for coil B2. When both
switches are activated, each coil uses its own installed rated power.
- Table 1 shows the power available for each coil B1 an B2 versus
the activation state represented as "1" or no-activated state represented
as "0", of both switches R1 and R2.
- Figure 2 is used for explanation of the current-controlled voltage-polarity
switching applied to ultra-fast heating of coil B2.
- Figures 2a, 2b, 2c, and 2d show the control signals for driving the
electronic power switches S3, S4, S2-S6 and S1-S5, respectively. The
electronic power devices S2-S6 and S1-S5 are activated as two switch
pairs. The activation of S3 and S2-S6 determines a current through the
coil B2 which comes out of the centre tap of leg S3-S4, as shown in
Figure 1, and flows into the coil.
- Figure 2e shows the shape of the pulsed bipolar voltage applied
to coil B2.
- Figure 2f depicts the current shape iB2 flowing through the coil B2
when the voltage shape of Figure 2e is applied. The peak current level
reached is limited to the reference value iP2. The voltage applied is
inverted when the current iB2 of coil B2 reaches the level iP2. That is the
reason because is so called current-controlled voltage-polarity
switching.
- The power level applied as ultra-fast heating is controlled by the
current amplitude iP2 used for the current-controlled voltage-polarity
switching. This power level can be set to values within the rating of B2
and the whole installed power corresponding to adding the ratings of B1
and B2.
- The half periods T are both equal and are determined by the
power level indirectly derived from iP2.
- Figure 3 is used for explaining the control applied when any of the
coils is powered alone. The situation shown in this figure corresponds to
the case in which only the coil B2 is powered.
- Figure 3a shows the control signal for driving the electronic
power switch S3. This signal has a 50% of duty ratio and constant half-period
T.
- Figure 3b shows the control signal which drives the electronic
power switch S4. This control signal is the logically complement to
foregoing control signal.
- The control signal of Figure 3c drives both S2 and S6 because all
the installed power is applied to B2.
- In the same way S1 and S5 are driven by the control signal in
figure 3d. The activation time of S2, S6, S1 and S5 is t2, being t2 less than
the half period T. The time t2 determines the width of the bipolar pulsed
voltage wave applied to B2, as shown in Figure 3d.
- The time t2 ends when the current iB2 through the coil reaches
the reference level iP2 shown in figure 3f. If this level is variable, the
voltage wave shape applied to B2 is also variable, thus the power
delivered by B2 is variable. When the level of iP2 determines that t2
enlarges up to T then B2 delivers the maximum power which is equal to
its rated power. By a continuous or discrete control of iP2, a continuous
or discrete control of the delivered power by B2 is obtained.
- Figure 4 is used for explaining the control to be applied when both
coils are powered below the boundary power. this control strategy is so
called duty ratio control and also energy control.
- To implement it, a time-base of half period Tb is adopted.
- From this half period, times tb1 and tb2 are derived with
Tb=tb1+tb2.
- During tb1 the coil is powered with the boundary power level.
During tb2 the coil is not powered. In this way, a continuous average
power can be obtained by controlling the tb1/Tb ratio.
- Figure 5 is used for explaining the co-ordination of coils controls
applied when both coils are powered within the boundary power level
and their rated power.
- Figures 5a, 5b, 5c, 5d, 5e, and 5f show the control signals for the
electronic power switches S3, S4, S6, S2, S5, and S1, respectively.
- The state of the electronic power switches S6, S5 determine the
bipolar wave shape in figure 5g, which drives the coil B1.
- The state of the electronic power switches S2, S1 determine the
bipolar wave shape in figure 5h, which drives the coil B2.
- Figure 5i shows the currents iB1 and iB2 flowing through the coils
B1 and B2, respectively.
- On both coils are applied a current-controlled voltage-cancellation
as described in the third control strategy described below. However, as
both coils are powered simultaneously, the activation time of the
electronic power devices are co-ordinated for the sake of optimising
their power losses.
- From a time-base with half period T1 common for both coils, the
activation times t1 and t2 are arranged to be separated as much as
possible within T1. With this arrangement a bipolar voltage as in Figure
5g is applied to coil B1 during t1 placed at the start of the half period T1.
In the same way, the bipolar voltage as in Figure 5h is applied to coil B2
during t2 placed at the end of the half period T1. This situation provides
current wave shapes iB1 and iB2 flowing through each coil which do not
completely overlap in time each other, thus its addition yields lower root
mean square current as compared when they fully overlap.
Consequently, the power losses in the electronic power devices
belonging to the common leg S3-S4 are lowered to minimum, the
working temperature of the devices is lowered and the reliability and
integrability of the system is enhanced.
- The power level of the coil B1 is determined with the peak value
iP1 of the current depicted in figure 5i. In the same way, iP2 limits the
current value in the coil B2 and determines its delivered power.
DESCRIPTION OF A PREFERRED EMBODIMENT
This invention provides a control method for the power delivered by
the flexible topology in Figure 1. The control strategy is based on the
power demand information requested for each load coil B1 and B2 which
are used as heating plates of an induction cooking hob.
The positions of the switches R1 and R2 determines the power
handling ability of each coil B1 and B2.
The position depicted in Figure corresponds to both switches in
non-activated state. If only R1 is activated, all the installed power is
available for coil B1. If only R2 is activated, all the installed power is
available for coil B2. If Both switches are activated, each coil draws its
own rated power.
Table 1 shows the power availability for each coil B1 and B2 versus
the activated or non-activated state of the switches R1 and R2. The
activated state is represented as "1" and the non-activated state
represented as "0".
In what follows four control strategies providing in the one hand, the
external functionality of attending the demanded power and in the other
hand, the internal functionality of producing the minimum power losses
in the electronic power devices.
The first control strategy for the flexible topology depicted in Figure 1
consists in determining the positions of switches R1 and R2, thus, their
respective activated or non-activated state versus the maximum power
availability demanded. All possibilities are constrained to four cases
stated in each of the four columns in Table 1.
The second control strategy is used for performing the ultra-fast
heating and consist in applying to a single coil a regulated power higher
than rated. To get it, it is applied a current-controlled voltage-polarity
switching. This control is explained in Figure 2.
Assuming that coil B2 is used for implementing the ultra-fast heating,
the first strategy in Table 1 determines that the switch R2 must be
activated and the switch R1 must be non activated. The second strategy
shown in Figure 2 applies periodically a pulsed bipolar voltage to coil B2.
Figure 2e shows the wave shape of the pulsed bipolar voltage applied to
B2. Figures 2a, 2b, 2c, and 2d show schematically the control signals for
the electronic power devices S3, S4, S2-S6 and S1-S5, respectively. The
devices S2-S4 and S1-S5 are activated as two switch pairs. The
activation of S3 and S2-S6 determines the current setting up through the
coil B2 which comes out of the centre tap of leg S3-S4, as shown in
Figure 1, and flows into the coil. As Figure 2f shows, this applied voltage
periodically inverts when the current peak iB2 through coil B2 reaches
the maximum iP2. That is the reason because this control is named as
current-controlled voltage-polarity switching.
The power level applied as ultra-fast heating is controlled by the
amplitude value of the current iP2 used for implementing the current-controlled
voltage-polarity switching. This power level can be stated
controllable with values within the rated power of B2 and the installed
power obtained by adding the rated power of B1 and B2. The half periods
T are both equal and are determined by the working frequency specified.
Once the frequency is selected, it is maintained constant. The situation
shown in Figure 1 corresponds to a fast-heating of B2 with a power
corresponding to the power boost stated. The period T and the selected
power boost in the load coil B2 indirectly determines iP2.
The third control strategy is used for powering any of the coils alone
with regulated power equal to or less than its rated power. Taking the
demanded power level as reference, it will be applied two control
strategies. These strategies are either current-controlled voltage-cancellation
or duty ratio control. The later is also named energy control.
The energy control is later explained using Figure 4.
The current-controlled voltage-cancellation is used for power
demands within the boundary power and the rated power. The energy
control is used for power demands below the boundary power.
Assuming as an example that this strategy is applied to coil B2,
Figure 3 shows the current-controlled voltage-cancellation applied to coil
B2. The power electronic device S3 is activated with the control signal in
Figure 3a which has a 50% duty ratio and a half period T1. The device S4
belonging to the same leg is activated with the complementary of the
foregoing signal as shown in Figure 3b. As it is only a coil involved, all
the installed power is applied to it by activating both S2 and S6 with the
control signal in Figure 3c. In the same way, S1 and S5 are both activated
with the control signal in Figure 3d.
The activation time of S2, S6, S1 and S5 is t2 and is lower than the half
period T1. The time t2 ends when the current through the coil iB2
reaches the reference level iP2 shown in Figure 3f. If this level varies, it is
obtained a variable voltage wave shape applied to B2, and thus, a
variable power.
Alternatively, the same objective can be achieved by increasing the
reference level iP2, maintaining T1-t2 constant and increasing
simultaneously both T1 and t1. In other words, by lowering the frequency
and increasing simultaneously the reference iP2 to rise the power
delivered. The opposite action will be taken for lowering the power
delivered. This method is proposed when the current-controlled voltage-cancellation
strategy imposes large voltage-cancellation and the power
demand is higher than the boundary power. This method maintains
constant the voltage-cancellation time and varies the time without
voltage-cancellation. This alternative method works with variable
frequency and allows the snubbers networks maintaining their
functionality. Consequently, the power losses in the electronic power
devices can be optimised and maintained low.
Assuming that the working frequency is maintained constant, with the
current peak level iP2 of coil B2 is derived the time t2 by which the
voltage applied to coil is cancelled. Thus, it is a current-controlled
voltage-cancellation.
When iP2 is such that t2 equals to T1 the maximum power is applied
to B2 and this is stated as its rated power. By the continuous or discrete
control of iP2 it can be obtained a continuous or discrete control of the
power delivered by B2.
Although a continuous power control can be obtained following the
foregoing procedure, from the device losses point of view, this
procedure can be improved with low power demands. Thus, this
procedure is not used for power demands below the boundary power.
For power demands below the boundary power it is used the duty ratio
control method also named as energy control. This control is explained
using Figure 4.
The energy control uses the afore mentioned current-control voltage
cancellation but setting iP2 to that value corresponding to the boundary
power.
To implement it, it is adopted a time base with period Tb lower than
the thermal time constant of the heating system constituted by any of the
coils B1, B2, and the heated pot or pan. From this period, times tb1 and
tb2 are derived with Tb=tb1+tb2 as shown in Figure 4. If during Tb is
delivered energy, the system is delivering the boundary power (BP) as
power level. If it is only delivering energy within tb1 and the system is
not activated within tb2, the average power (AP) level delivered is :
AP=BP(tb1/Tb)
In this way, it is obtained a continuous regulation of the average
power delivered by setting the tb1/Tb ratio.
The boundary power is selected lower than the maximum allowed by
the Electromagnetic Compatibility Standard concerning the voltage
fluctuations and flicker in low voltage supply systems, when the energy
control strategy is applied. This Standard limits the switched power
versus the switching ratio. In our case, the boundary power level
depends on the selected value for 1/Tb.
The fourth control strategy is applied when both coils B1 and B2 are
simultaneously powered with power levels equal to or lower than their
rated power. If the power demands of both coils are higher than the
boundary power, it will be applied the powering guidelines depicted in
Figure 5. Figures 5a, 5b, 5c, 5d and 5f show the control signal for the
electronic power devices S3, S4, S6, S2, S5 and S1, respectively. The
device states of S6 and S5 determine the bipolar wave shape in Figure
5g used for powering the coil B1. The device states of S2 and S1
determine the bipolar wave shape in Figure 5h used for powering the coil
B2. Figure 5i shows the currents iB1 and iB2 flowing through the coils B1
and B2, respectively. On both coils are implemented the current-controlled
voltage-cancellation as described in the third control strategy.
As two coils are involved, the activation time of the electronic power
devices are co-ordinated for the sake of optimising their power losses.
To cope with this objective, on a time base with half period T1, the
bipolar voltage in Figure 5g is applied to coil B1 within a time t1 placed at
the start of the half period T1. In the same way, the bipolar voltage in
Figure 5h is applied to coil B2 within a time t2 placed at the end of the
half period T1. With this co-ordination it is avoided the simultaneous coil-current
overlapping through the electronic power devices, thus
minimising the root mean square (RMS) current circulating through the
common leg. A full wave shape overlap would happen if both coils were
powered at the start of every half period. As the sum of both currents
flows through the common leg S3-S4, when the currents do not fully
overlap the root mean square (RMS) current circulating through the
common leg is lower. Consequently, the working temperature of the
power electronic devices belonging to the common leg is lowered,
improving the reliability and integrability of the system.
The power level for coil B1 is set by the current peak iP1 depicted in
figure 5i. In the same way, iP2 limits the current level through the coil B2
and determines its power delivered.
If the demanded power of any coil is lower than the boundary power it
will be applied to it the energy control described in Figure 4.
If on a time base Tb in Figure 4, the coil B1 delivers the boundary
power (BP_B1) within time tb1, the average power (AP_B1) delivered by
B1 is:
AP_B1= BP_B1(tb1/Tb)
In the same way, if on the same time base Tb the coil B2 delivers the
boundary power (BP_B1) within time tc1, the average power (AP_B2)
delivered by B2 is:
AP_B2= BP_B2(tc1/Tb)
If any of the coils demands a power higher than the boundary power
and the other coil demands a power lower than the boundary power, that
coil demanding higher than boundary power will follow the current-control
voltage-cancellation strategy depicted in Figure 3 but that coil
demanding lower than boundary power will follow the energy control
depicted in figure 4.
As it has been described in the fourth control strategy, the working
frequency is the same for both coils, thus, there are not intemodulation
or sub harmonic frequencies and audible noise is avoided.
Finally, the control strategies proposed are able to be implemented in
integrated programmable logic devices, thus, allowing maximum
integrability, flexibility, programmability and reliability of the control
system.
While the invention has been particularly shown and described with
reference to several preferred embodiments thereof, it will be understood
by those skilled in the art that various changes in form and detail may be
made therein without departing from the true spirit and scope of the
invention as defined by the appended claims.