Adjustable resonant buck converter

A power converter includes first and second circuit modules, a first capacitor, a second diode and a control module. The first circuit module includes a switching element in parallel with a first diode. The second circuit module includes a first inductor and the first circuit module. The inductor is in series with the first circuit module. The first capacitor is in parallel with the second circuit module. The second diode includes a first terminal and a second terminal, where the first terminal is in series with the second circuit module and the first capacitor, and the second terminal is coupled to a second power terminal. The control module varies one or more of the first capacitor and the first inductor based on at least one of a current of a load circuit or an input voltage. A resonating waveform is generated by a resonant circuit of the second circuit and is used by the control module to turn off the switching element under zero-current and zero-voltage conditions.

FIELD OF THE INVENTION

The invention relates generally to minimizing power loss in buck converters, and to apparatuses for and methods of minimizing power loss in resonant buck converter.

BACKGROUND OF THE INVENTION

DC-DC converters are electronic devices used to change DC electrical power from one voltage level to another voltage level. DC-DC converters typically include thyristors, silicon-controlled full-bridge rectifiers with half and full control, and standard buck converters. However, thyristors and silicon-controlled full-bridge rectifiers suffer from high power losses and generate high current ripples. Similarly, standard buck converters are bulky and expensive, because they often include large components. For example, an output inductor of a standard buck converter typically requires high inductance to filter out the high-frequency component of an output waveform. In addition, the freewheeling diode of a standard buck converter typically is mounted on a heat sink because it processes high current. The requirement of a heat sink causes further space constraints. Thus, a standard buck converter is unsuitable for circuits with limited hardware space and cost considerations.

SUMMARY OF THE INVENTION

The invention features a resonant buck converter circuit that is more compact in comparison to a standard buck converter, and includes low power loss and low ripple currents. Because the resonant buck converter of the present invention is based on a resonant topology, the switching loss is negligible. In addition, because the resonant converter can be operated at high switching frequencies, the converter advantageously does not require large components. Furthermore, the resonant converter can dynamically adjust the switching frequency in response to variable circuit characteristics, thereby optimizing switching under zero-current and/or zero-voltage conditions.

In one aspect, a power converter is provided. The power converter includes a first circuit module, a second circuit module, a first capacitor, a second diode and a control module. The first circuit module includes a switching element in parallel with a first diode. The second circuit module includes a first inductor and the first circuit module. The first inductor is in series with the first circuit module. The second circuit module includes a first terminal coupled to a first power terminal. The first capacitor is in parallel with the second circuit module. The first capacitor includes a first terminal coupled to the first terminal of the second circuit module and a second terminal coupled to a second terminal of the second circuit module. The first capacitor is a variable capacitor, the first inductor is a variable inductor, or both the first capacitor and first inductor are variable. The second diode includes a first terminal and a second terminal. The first terminal of the second diode is in series with the second circuit module and the first capacitor. The second terminal of the second diode is coupled to a second power terminal. The control module is adapted to vary one or more of the first capacitor and the first inductor based on at least one of a current of a load circuit or an input voltage. A resonating waveform generated by a resonant circuit of the second circuit is used by the control module to turn off the switching element under zero-current and zero-voltage conditions.

In some embodiments, the first terminal of the second circuit module includes a terminal of the first inductor and the second terminal of the second circuit module includes a terminal of the first circuit module. Alternatively, the first terminal of the second circuit module can include a terminal of the first circuit module and the second terminal of the second circuit module can include a terminal of the first inductor. The power converter can further include a second capacitor in parallel with the first capacitor. The control module can be adapted to disconnect at least one of the first or second capacitors to adjust the resonant capacitance. The power converter can further include a second inductor in parallel with the first inductor. The control module can be adapted to disconnect at least one of the first or second inductors to adjust the resonant inductance. The control module can be adapted to increase the resonant capacitance, decrease the resonant inductance, or increase the resonant capacitance and decrease the resonant inductance if a current of the load circuit is high and the input voltage is low. The control module can be adapted to decrease the resonant capacitance, increase the resonant inductance, or decrease the resonant capacitance and increase the resonant inductance if a current of the load circuit is low and the input voltage is high. The switching element can include a transistor. The first diode can be anti-parallel in polarity with the transistor. The second diode can be connected in parallel with the load circuit. The first and second power terminals can be connected to a DC power source that generates the input voltage. The ratio of the resonant inductance to the resonant capacitance can be less than square of the ratio of the input voltage to the current of the load circuit. In some embodiments, the control module's control of the switching element is not based on pulse-width modulation, pulse-frequency modulation, constant on-time control, or constant off-time control.

In one aspect, a control module for a power converter is provided. The control module includes a first terminal for controlling a switching element. The switching element is in parallel with a first diode defining a first circuit module. The control module also includes a second terminal for controlling a plurality of parallel inductors. The plurality of parallel inductors is in series with the first circuit module. The first circuit module and the plurality of parallel inductors define a second circuit module. The second circuit module includes a first terminal coupled to a first power terminal and a second terminal coupled to a second diode. The control module also includes a third terminal for controlling a plurality of parallel capacitors. Each of the plurality of capacitors include a terminal coupled to the first terminal of the second circuit module and a second terminal coupled to the second terminal of the second circuit module. The control module is adapted to vary the plurality of capacitors and the plurality of inductors based on at least one of a current of a load circuit or an input voltage. The control module is also adapted to turn off the switching element under zero-current and zero-voltage conditions by adjusting a resonating waveform generated by a resonant circuit that is formed by the plurality of capacitors and the plurality of inductors.

In some embodiments, the control module adjusts resonant capacitance and resonant inductance of the resonant circuit such that the ratio of the resonant inductance to the resonant capacitance is less than square of the ratio of an input voltage to a current of a load circuit. The control module can disconnect at least one of the plurality of capacitors to adjust resonant capacitance of the resonant circuit. The control module can disconnect at least one of the plurality of inductors to adjust resonant inductance of the resonant circuit. The control module can turn on the switching element under zero-current conditions. The control module can turn on or off the switching element such that the switching element's on time, off time, or a combination thereof, is variable in a period of operation.

DETAILED DESCRIPTION

FIG. 1illustrates a circuit diagram of an adjustable resonant buck converter circuit100according to some embodiments. The converter100includes a switching element102in series with a positive terminal of a DC power supply (not shown), a resonant inductor108, an inductor switch120, and an output inductor114. The converter100also includes a branch having a capacitor110in series with a capacitor switch122. The branch is parallel to the switching element102, the resonant inductor108, and the inductor switch120. The switching element102includes a transistor104in parallel with a diode106. The transistor104can include, for example, a bipolar junction transistor (BJT) or a field-effect transistor (FET). The diode106can be anti-parallel in polarity with the transistor104. As shown inFIG. 1, the resonant inductor108and inductor switch120are positioned to the right of the switching element102(i.e., on the anode-side of the diode106). However, the resonant inductor108and inductor switch120can be positioned to the left of the switching element102(i.e., on the cathode-side of the diode106).

In some embodiments, the inductance value of resonant inductor108and/or the capacitive value of capacitor110can be varied by a control module as described below.

The converter100further includes a diode112. The cathode of the diode112is electrically coupled to the resonant inductor108, the capacitor110, and the output inductor114. The anode of the diode112is coupled to a negative terminal of a DC power supply (not shown). The converter100can additionally include an output capacitor116coupled between the output inductor114and the negative terminal of the DC power supply.

In some embodiments, the converter100includes a control module118coupled to the base/gate terminal of the transistor104for turning on or off the transistor104based on characteristics of signals measured at various locations of the converter100. Exemplary signals measured include the output current126, the output voltage128, the input voltage148, the current130through the resonant inductor108, the switch node voltage134across the transistor104, and the current142through the output inductor114. In some embodiments, the control module118also monitors the output power calculated based on the output voltage128and the output current126.

The current130can include a resonant current generated by the LC circuit formed by the resonant inductor108and the capacitor110. In some embodiments, the current130is measured by the current sensor132that is positioned between the switching element102and the resonant inductor108. In supplemental or alternative embodiments, the current130can be measured by a sensor (not shown) positioned at other junctions of the converter100—e.g., at the electric junction between the switching element102and the power supply (not shown), or between the resonant inductor108and the electric junction coupled to the output inductor114. The switch node voltage134across the transistor104can be measured between the switching element102and the resonant inductor108. In some embodiments, for purposes of protecting the control module118, the converter100uses a clamping circuit136, which can include a clamp diode138and a clamp capacitor140, to limit the switch node voltage134supplied to the control module118. In some embodiments, the current142of the output inductor114is measured using the current sensor124. In supplemental or alternative embodiments, the current142can be measured by a sensor (not shown) positioned at other junctions of the converter100—e.g., at the electric junction between the output inductor114and the capacitor116. The control module118can also actuate the inductor switch120and/or capacitor switch122such that they are selectively connected or disconnected.

FIGS. 2A-4Eillustrate various modes for operating the buck converter circuit100ofFIG. 1. The portions of the diagrams highlighted by thick lines indicate active components.FIG. 2Ashows an operating mode408of the buck converter100for the time period [t0, t1]. The control module118turns on the transistor104at time t0, causing a current404to cycle through a path defined by the transistor104, the resonant inductor108, the output inductor114, the output capacitor116, the load (not shown), and the power supply402. In addition, a current406flows through the output inductor114, the output capacitor116and the load, and freewheels through the diode112. The voltage across the capacitor110is about the same as the input voltage148of the power supply402. The current404, which linearly rises between the time period [t0, t1], can be expressed as:

ILr=VbLr⁢(t-t0),(Equation⁢⁢1)
where ILrrepresents the current404, Vbrepresents the input voltage148of the power supply402, Lrrepresents the inductance of the resonant inductor108, and t represents time. During the operating mode408, the transistor104is turned on by the control module118under zero-current conditions because the current404that flows through the transistor104is zero at t0and linearly increases thereafter.

In some embodiments, the current406, which linearly decreases between [t0, t1], can be expressed as:

ID=ILO-VbLr⁢(t-t0),(Equation⁢⁢2)
where IDrepresents the current406and ILOrepresents the current of the output inductor114that is measured, for example, by the current sensor124. At the end of operating mode408, the current406reaches zero while the current404is about the same as the output inductor current. In some embodiments, the control module118turns off the diode112when the current406is about zero which can advantageously result in there being substantially no current flowing through the diode112.

FIG. 2Bshows an operating mode410of the buck converter100in the time period [t1, t2], during which the resonant inductor108and the resonant capacitor110form a LC circuit that resonates through the transistor104. As described above, the diode112stops conducting at time t1when the current404reaches the level of the current in the output inductor114.

In some embodiments, resonance current effects between the resonant inductor108and the capacitor110begin to occur at time t1, at which point the current through the capacitor110is zero, the voltage across the capacitor110is about the same as the input voltage148of the power supply402, and the current through the resonant inductor108is about the same as the current of the output inductor114. The resonant current420developed between the resonant inductor108and the capacitor110can be expressed as:

iLr′⁡(t-t1)=Vbω⁢⁢Lr⁢sin⁢⁢ω⁡(t-t1),(Equation⁢⁢3)
where iLr′ represents the resonant current420and ω represents the resonant angular frequency. The resonant angular frequency ω can be expressed as:

ω=1Lr⁢Cr,(Equation⁢⁢4)
where Lrrepresents the inductance of the resonant inductor108and Crrepresents the capacitance of the capacitor110. The voltage across the resonant capacitor110can be expressed as:
νCr(t−t1)=Vbcos ω(t−t1).  (Equation 5)
Equation (5) indicates that, in the operating mode410, the voltage across the capacitor110decreases as the power supply402charges the resonant inductor108, the transistor104, and the output inductor114.

FIG. 2Cshows an operating mode412of the buck converter100in the time period [t2, t3]. At time t2, the resonant current420changes polarity. In such an instance, the resonant current420reverses direction and flows back to the capacitor110via diode106. Therefore, the voltage drop across the transistor104is equal to or close to the ON voltage drop of the diode106. When the resonant current420through the transistor104reaches zero, the control module118can be configured to turn off the transistor104, thereby realizing switching under zero-carried current and/or zero-voltage conditions. After the transistor104is turned off, the current of the output inductor114is provided by the capacitor110, by the power supply402charging the capacitor110. At time t3, which represents the end of the operating mode412, the resonant current420becomes about the same as the current in the output inductor114.

FIG. 2Dshows an operating mode414of the buck converter100in the time period [t3, t4]. As shown, a current422flows through the output inductor114and is charged via the capacitor110by the input voltage of the power supply402. Operating mode414ends at time t4when the capacitor110is fully charged and has a voltage that is equal to or about the same as the input voltage of the power supply.

FIG. 2Eshows an operating mode416of the buck converter100in the time period [t4, t5]. A current424flows through the output inductor114and freewheels through the diode112. The current424linearly decreases until the transistor104is turned on by the control module118at time t5, at which point the buck converter returns to operating mode408. Depending on the duration of the operating mode416, the output inductor current can decrease to zero before the transistor104is turned on. This can create a discontinuous-conduction mode (DCM) or a continuous-conduction mode (CCM) for operating the converter100. The switching frequency is also dependent on the duration of the operating mode416as it controls the amount of time the transistor104is turned off.

FIG. 3illustrates an exemplary timing diagram500for operating the buck converter circuit100ofFIG. 1and the timing conditions corresponding to the operating modes ofFIGS. 2A-2E. The timing diagram500shows the waveforms of a base/gate signal502of the transistor104, the voltage504across the transistor104, the current506of the transistor104, the current508of the diode112, the current510of the output inductor114, the current512of the capacitor110, the voltage514across the capacitor110and the current of the power supply402. In some embodiments, the voltage504across the transistor104is limited by the clamping circuit136before being measured by the control module118. The current506of the transistor104can be measured by the current detector132. The current510of the output inductor114can be measured by the current detector124. The control module118can interact with the base/gate signal502to turn on or off the transistor104. For example, the control module118can trigger the base/gate signal502to logic low, thus shutting down the transistor104. The logic levels for the base/gate signal502that result in turning off the transistor104is a design choice. Accordingly, in some embodiments, logic high of base/gate signal502can turn the transistor104off.

The time period [t0, t1] represents the operating mode408of the buck converter100. The transistor104is turned on at time t0by the control module118, as demonstrated by the base/gate signal502transitioning from logic low to logic high at time t0. This causes the current506of the transistor104to linearly increase from zero to become about equal to the current510of the output inductor114at time t1. In contrast, the current508of the diode112linearly decreases until reaching zero at time t1, at which point the diode112is turned off at zero current. In addition, the voltage514across the resonant capacitor110is about the same as the input voltage of the power supply402during the time period [t0, t1].

The time period [t1, t2] represents the operating mode410of the buck converter100. During this period, the resonant inductor108and the capacitor110begin to resonant through the transistor104, as demonstrated by the resonance in the current506of the transistor104and the current512of the capacitor110. At time t2, the current512of the resonant capacitor110switches its polarity by changing from a positive signal to a negative signal. In addition, the voltage514across the capacitor110decreases during the time period [t1, t2] as the power supply402charges the output inductor114, the resonant inductor108and the transistor104.

The time period [t2, t3] represents the operating mode412of the buck converter100. During this time period, when the current506of the transistor104reaches zero through resonance, the transistor104is turned off by the control module118. This switching is also realized under zero-voltage conditions because the voltage504across the transistor104at the time of switching is zero. In some embodiments, even though the current506through the transistor104reaches zero at time t3, the control module118turns off the transistor104before time t3to guarantee soft switching of the transistor104. For example, as shown in the timing diagram500, the falling edge of the base/gate signal502of the transistor104is between [t2, t3]. In addition, during the time period [t2, t3], the current510of the output inductor114is charged by the power supply402through the capacitor110. At the end of the mode412, the current512of the capacitor110is about equal in magnitude to the current510of the output inductor114, at which point the the anti-parallel diode106is turned off.

The time period [t3, t4] represents the operating mode414of the buck converter100. During this mode414, the transistor104and the diode106remain turned off. The current510of the output inductor114is continuously charged through the capacitor110by the power supply402.

The time period [t4, t5] represents the operating mode416of the buck converter100, during which both the transistor104and the diode106remain turned off. The current510of the output inductor114freewheels through the diode112and linearly decreases until the transistor104is turned on by the control module118at time t5, which initiates the operating mode408.

In general, to achieve zero-current switching when the transistor104is turned off during the time period [t2, t3], the control module118can be configured to ensure that the following criterion is satisfied:

LrCr<(VbILO)2,(Equation⁢⁢6)
where Lrrepresents the resonant inductance, Crrepresents the resonant capacitance, Vbrepresents the input voltage and ILOrepresents the current of the output inductor114, which indicates the heaviness of the load circuit formed by the output inductor114and the output capacitor116.

As represented by Equation (6), a heavier load (high ILO) and a lower input voltage (low Vb) indicates that a higher resonant capacitance (high Cr) and a lower resonant inductance (low Lr) are desired to achieve zero-current switching when the transistor104is turned off during the time period [t2, t3]. In addition, as represented by Equation (6), a lighter load (low ILO) and a higher input voltage (high Vb) indicates that a lower resonant capacitance (low Cr) and a higher resonant inductance (high Lr) are desired to achieve zero-current switching when the transistor104is turned off during the time period [t2, t3]. Hence, it is advantageous if the control module118can adjust the resonant inductance and the resonant capacitance in response to variations in at least one of the load circuit conditions or input voltage.

FIGS. 4A-Billustrate embodiments of variable resonant inductor108and variable capacitor110components of the buck converter100that are used by the control module118to adjust the resonant capacitance and the resonant inductance of the converter100.FIG. 4Ashows an exemplary capacitor structure200for capacitor110having multiple parallel branches. A branch can include a capacitor210in series with a capacitor switch222. In some embodiments, the capacitor structure200includes a branch having only a switch222. The capacitors210can have the same and/or different capacitance values. The capacitor structure200can replace the resonant capacitor110and the switch122ofFIG. 1. Each of the capacitor switches222can be controlled by the control module118such that the control module118selectively connects and/or disconnects any one of the capacitors210to adjust the overall capacitance of the capacitor structure200.

FIG. 4Bshows an exemplary inductor structure250for resonant inductor108having multiple parallel branches. A branch can include a resonant inductor252in series with an inductor switch254. In some embodiments, the inductor structure250includes a branch having only a switch254. The inductors252can have about the same or varying values of inductance. The inductor structure250can replace the resonant inductor108and the switch120ofFIG. 1. Each of the switches254can be controlled by the control module118such that the control module118selectively connects and/or disconnects any one of the resonant inductors252to adjust the overall inductance of the inductor structure250.

The converter100can include one or both of the capacitor structure200, in place of the capacitor110, and the inductor structure250, in place of the resonant inductor108. In operation, the control module118can adjust the resonance frequency of the converter100in response to variations in input voltage and/or load circuit conditions by selectively connecting and/or disconnect one or more capacitors in the structures200and/or one or more inductors in the structure250. In some embodiments, the control module118adjusts the resonant frequency by satisfying Equation (6). For example, if the load is heavy and the input voltage is low, then the control module118can be configured to disconnect all the switches222in the structure200(except the short switch222athat remains closed) to maximize resonant capacitance while configured to connect the switch254ain the structure250to minimize resonant inductance. As another example, under very light load conditions, a standard buck converter is sufficient. Therefore, to disengage the resonant LC circuit from the converter design100, the control module118can be configured to open all the switches in the capacitor structure200to disconnect the corresponding capacitors. The control module118can also be configured to open all but one of the switches254in the inductor structure250. Therefore, by manipulating the resonant frequency, the control module118can dynamically adjust both the on and off times of the transistor104to optimize zero-current switching in response to variations in load and input voltage.

In some embodiments, the switch120ofFIG. 1can be replaced by a saturable-core reactor (not shown) such that the resonant inductor108is connected in series with the reactor. A saturable-core reactor can create a variable inductance as a function of load current. For example, for a light load, the resonant current created by the resonant inductor108and the resonant capacitor110is lower than normal. This can cause the saturable-core reactor to increase its inductance, which increases the overall resonant inductance of the resonant circuit. As a result, the resonant frequency is decreased to facilitate zero-current switching at the light load. In contrast, for a heavy load, the resonant current is higher than normal and this causes the saturable-core reactor to become saturated, thereby reducing its inductance and the overall resonant inductance of the resonant circuit. As a result, the resonant frequency is increased to facilitate zero-current switching at the heavy load.

FIG. 5illustrates an exemplary control module118of the buck converter circuit100ofFIG. 1. Control module118can operate by varying both the ON and OFF time durations of the switching element102. Control module118takes as inputs the output voltage128, the output current126, the output inductor current142, the switch node voltage134, the resonant inductor current130and the input voltage148. In some embodiments, the control module118produces output signals to control the gate/base terminal of the transistor104. In some embodiments, the control module118produces output signals to control one or both of the capacitor switch122and the inductor switch120ofFIG. 1. In some embodiments, the control module118produces output signals to control the switches222of the capacitor structure200inFIG. 4Aand/or the switches254of the inductor structure250inFIG. 4B.

The control module118includes three Proportional-Integral-Derivative (PID) controllers: the output voltage PID controller302, the output current PID controller304and the output power PID controller306. The output current PID controller304can have the greatest bandwidth of all three controllers, followed by the output voltage controller302and the output power PID controller306. Depending on the load conditions, only one of the PID controllers302,304and306is in operation. For example, under normal operating conditions, only output voltage PID controller302controls. However, if the load is shorted, for example, the output current126is adapted to raise sharply, in which case the output current PID controller304takes over control since it has the highest bandwidth. If the output voltage PID controller302is in charge, it uses an internal precision voltage reference308to force the output voltage to a desired value. Similarly, if the output current PID controller304is in control, it uses an internal precision current reference310to force the output current to a desired value. If the output power PID controller306is in control, it uses an internal precision power reference312to force the output power to a desired value. Such value is propagated to the output by using a voltage-controlled oscillator (VCO)314that feeds into the monostable (one-shot) module316, whose output is in turn supplied into a driver circuit module318to drive the gate or base terminal of the transistor104.

In some embodiments, each of the PID controllers302,304and306is associated with a PID output limiter320,322or324, respectively. Each of the PID output limiters320,322and324can, for example, be a clamp. Specifically, in some embodiments, each of the PID output limiters320,322and324is clamped in such a manner that when another PID controller takes over control, the amount of time needed to react from saturation to active mode is minimized. An offset-level adder module326can be coupled to the PID limiters320,322and324to alter the clamp voltage level.

In addition, the control module118includes a zero-crossing and pulse-steering network module328and a conditioning control circuit module330. Each of modules328and330takes as inputs the resonant inductor current130and the input voltage148. The modules328and330use these two inputs to determine when to turn on or off the power stage of the transistor102. For example, if there is no output load, meaning that the output current126is substantially zero, the modules328and330can cause the one-shot module316to output a signal having a minimum pulse width equal to 0.5*Tr, where Tris the resonant time. This pulse width is adapted to turn on the transistor104. If the output load is normal, meaning that the output current126is within the minimum and maximum range of the resonant inductor current130, the modules328and330can cause the one-shot module316to output a signal having a pulse width within the range of 0.5*Trto 0.8*Tr. This pulse width is adapted to turn on the transistor104. If the load is heavy, meaning that the output current126is greater than the maximum resonant inductor current130, the modules328and330can cause the one-shot module316to output a signal having a pulse width equal to about 0.85*Tr. This pulse width is adapted to turn on the transistor104. Therefore, under such a pulse-width modulation scheme, the transistor's on time and/or off time can be variable in a period of operation. In addition, the associated duty cycle and duration of the period can be variable.

In general, control module118is not limited by constraints associated with pulse width modulation (PWM), pulse frequency modulation (PFM), and/or constant-on or -off times. For example, while PWM varies the duty cycle, the period of time is constant. Similarly, while PFM varies the frequency, the duty cycle is constant. With respect to the constant-on time modulation technique, the period of time the signal is on is constant while the time the signal is off is variable. Similarly, the constant-off time modulation technique is constrained by the period of time the signal is off. Control module118is not limited by constant periods, constant duty cycles, constant on times and/or constant off times. In general, control module118can modulate in a hybrid manner by leaving these values variable. As a result, the drive signal to transistor104is advantageously based, in part, on the current state conditions of the circuit100without constraints associated with time periods, frequency, or constant on/off times.

Furthermore, the control module118includes a resonant Lrand Crtuning module332, a Lrdrive circuit334, and a Crdrive circuit336. The modules332,334and336ensure that the control module118operates under zero-current switching (ZCS) conditions when the transistor104is turned on or off.

The control module118also includes a fault detection network module338and a soft-start/soft-stop network module340. The fault detection network module338monitors at least one of the output voltage128, the output inductor current142and the switch node voltage134for abnormalities, such as occurrences of over-voltage, under-voltage or over-current conditions. If an abnormality is detected by the fault detection network module338, the soft-start/soft-stop network module340executes a controlled shutdown of the system and automatic recovery at a later time when the abnormality ceases.

The technology has been described in terms of particular embodiments. The alternatives described herein are examples for illustration only and not to limit the alternatives in any way. The steps of the technology can be performed in a different order and still achieve desirable results. Other embodiments are within the scope of the following claims.