501 results about "Zero voltage" patented technology

The invention discloses a TCM-mode-based Si-SiC hybrid power multi-phase staggered H-bridge converter structure and a control method thereof, according to the scheme, a three-phase staggered parallel topology of a two-phase low-frequency Si-IGBT bridge arm and a one-phase high-frequency SiC-MOSFET bridge arm is adopted, and a TCM control strategy is combined, so that zero voltage switching (ZVS) of a Si device is successfully realized, the switching loss is remarkably reduced, and the switching efficiency is improved. The system efficiency is greatly improved; and meanwhile, through pilot frequency interleaving and ripple compensation technologies, total output current ripples are greatly reduced, and optimal balance between system performance and manufacturing cost is realized while the power capacity of the converter is remarkably expanded and the dynamic response speed and the output electric energy quality are improved.

A two-transistor flyback topology in which a portion of leakage inductance energy in the transformer is extracted and used to obtain zero voltage switching on both switching elements in any operating conditions, and in which a third low power switch eliminates parasitic oscillations and allows an operation at a determined frequency. In this topology, there are no switching losses and there are no spikes or glitches in any switching node, eliminating the need for snubbers. This topology is an “ideal two transistor flyback topology.”

The invention discloses a three-port soft switching optical storage servo all-in-one machine based on device multiplexing and a modulation method. The all-in-one machine comprises a DC-AC servo driver, a low-voltage DC energy storage module, an auxiliary unit, a bus capacitor and a photovoltaic module. The DC-AC servo driver comprises three bridge arms, and each bridge arm is provided with a bridge arm upper tube and a bridge arm lower tube; the low-voltage direct-current energy storage module comprises an energy storage battery, an energy storage switch tube S7 and a direct-current energy storage inductor; the photovoltaic module comprises a photovoltaic power generation panel, a photovoltaic inductor, a photovoltaic switch tube S8 and a diode D1; the auxiliary unit comprises an auxiliary switch tube Sa, an auxiliary resonant inductor Lr and an auxiliary capacitor Cr. According to the invention, an integrated resonant soft switching architecture is innovatively adopted, zero-voltage / zero-current switching of all power switching devices is realized, and the problem of loss caused by a traditional hard switch is thoroughly solved; and by optimizing a device multiplexing technology, the number of switching tubes is greatly reduced while the soft switching performance is ensured, so that the system efficiency is improved in a breakthrough manner.

The invention discloses an extended double-side asymmetric phase shift modulation method for a dual-active bridge converter, and belongs to the technical field of isolated high-frequency power conversion in a power electronic technology. According to the symmetric phase shift modulation method, under the condition that the voltage conversion ratio is not matched, the zero voltage switching range of a switching tube is small, the light load efficiency is low, and variable abrupt change can occur during mode switching to cause the uncontrollable oscillation problem of inductive current. The invention provides an extended double-side asymmetric phase shift modulation method for a dual-active bridge converter in a wide voltage range. A multi-objective optimization scheme is established based on the provided modulation method, and an approximation method is provided to solve a full power range analytical solution. A smooth switching method between modes is realized, and uncontrollable oscillation of inductive current can be effectively avoided. And the conversion efficiency in a full-power range is improved.

The invention provides a zero-voltage ride-through control method for a self-synchronizing voltage source doubly-fed wind turbine generator, which comprises the following steps that: when the self-synchronizing voltage source doubly-fed wind turbine generator is in a transient state, a grid-side converter adopts vector control, and a rotor-side converter adopts additional virtual impedance and annular current limiting; the transient overcurrent of the grid-side converter and the rotor-side converter can be effectively suppressed, so that the continuous operation capability of the wind turbine generator in the zero-voltage ride-through period can be maintained; meanwhile, by improving the transient capacity and the transient reactive current amplitude of the converter, the transient voltage supporting capacity of the wind turbine generator during the zero voltage ride-through period can be effectively provided; when the self-synchronizing voltage source doubly-fed wind turbine generator set is in a transient state, overvoltage of direct-current bus voltage is limited by a Chopper circuit, active-priority current amplitude limiting control is adopted by a grid-side converter, parameters of a current inner ring PI controller of a rotor-side converter are optimized, energy balance of a direct-current side can be achieved in the zero-voltage ride-through period, and stability of the direct-current bus voltage is maintained.

The invention discloses a zero-voltage turn-on circuit and a zero-voltage turn-on method of a switching tube in a switching power supply, and belongs to the technical field of circuits. In a critical conduction mode, when a low-side power tube is turned off, the rising edge zero-voltage starting circuit compares a switch node voltage with an input voltage or compares the switch node voltage with an output voltage under the control of a low-side gate driving voltage, outputs a high level to a high-side gate driving end when the switch node voltage is equal to the input voltage or the output voltage, and outputs a low level to a low-side gate driving end when the high level is equal to the output voltage. A high-side power tube is controlled through a high level to realize zero-voltage starting; in a critical conduction mode, when the high-side power tube is turned off, the falling-edge zero-voltage starting circuit compares an induced voltage with a pull-up resistor voltage under the control of a high-side gate driving voltage, outputs a high level to a low-side gate driving end when the induced voltage is equal to the pull-up resistor voltage, and controls the low-side power tube to realize zero-voltage starting through the high level. The induced voltage is equal to the sum of the pull-up resistor voltage and the switch node voltage. According to the invention, the transmission delay can be reduced.

Disclosed is a system for zero voltage switching in a power converter with a first half-bridge and a second half-bridge. The first half-bridge has a first set of transistors and output inductor. The second half-bridge has a second set of transistors and auxiliary inductor. A current sensing means measures, at switching cycle scale, average and maximum current values of output inductor and average, minimum and maximum current values of auxiliary inductor. A voltage sensing means measures input and output voltages of first and second half-bridge. A controller is configured to compute dead times of pulse width modulation control signal of second half-bridge, generate pulse width modulation control signal of second half-bridge and to adapt dead times of first half-bridge.

The invention discloses a detection device and a detection system for harmonic oscillator capacitors of a hemispherical resonator gyroscope, and relates to the technical field of capacitor detection, the detection device comprises a signal detection circuit, a modulation compensation circuit, a differential amplification circuit and a control circuit, the signal detection circuit is connected with the output ends of the two harmonic oscillator capacitors; the modulation compensation circuit is correspondingly connected with two input ends of the signal detection circuit; the differential amplification circuit is correspondingly connected with two output ends of the signal detection circuit; the control circuit is configured to output modulation voltage to the two input ends of the modulation compensation circuit according to a voltage signal of the output end of the differential amplification circuit, and zero setting is carried out on voltage at the connecting positions of the two output ends of the modulation compensation circuit and the two input ends of the signal detection circuit. According to the invention, the modulation compensation circuit is adopted to enable the output ends of the two harmonic oscillator capacitors to work near zero voltage, the circuit has the characteristics of ultrahigh linearity, extremely low temperature drift, rapidness and stability, errors caused by power supply drift can be eliminated, and the detection precision is improved.

The invention discloses a half-bridge ZVS Buck-Boost converter and a control method thereof, and belongs to the technical field of power converters, the half-bridge ZVS Buck-Boost converter comprises a primary side conversion module, a voltage transformation module and a secondary side conversion module, and the primary side conversion module, the voltage transformation module and the secondary side conversion module are connected in sequence; the primary side conversion module comprises a first bridge arm, a second bridge arm, a first inductor and a bidirectional switch, the first bridge arm and the second bridge arm are connected in parallel, a bridge arm midpoint of the first bridge arm is connected with a first end of the first inductor, a second end of the first inductor is connected with a first end of the voltage transformation module, and a second end of the voltage transformation module is connected with a second end of the voltage transformation module. The second end of the voltage transformation module is connected with the bridge arm midpoint of the second bridge arm, the bridge arm midpoint of the first bridge arm is connected with the first end of the bidirectional switch, and the second end of the bidirectional switch is connected with the bridge arm midpoint of the second bridge arm. According to the invention, zero-voltage switching-on of the half-bridge circuit switching tube in a full-input, full-output and full-load range under a constant switching frequency can be realized.

The invention relates to the field of electronic power, particularly discloses a bridge resonant converter based on forward and reverse shock coordination of double transformers, and belongs to the technical field of power electronic conversion. Aiming at the problems of over-high peak current, high magnetic core saturation risk, remarkable switching loss and the like of a traditional LLC resonant converter under the requirements of a wide load range and high power density, the invention provides the following innovative schemes: a dual-transformer forward and reverse excitation collaborative structure, a zero voltage switching (ZVS) driving technology and resonant network parameter optimization.

The invention belongs to the technical field of electrochemical energy storage, and relates to a zero-voltage attenuation lithium-rich positive electrode material, a preparation method and application thereof, the chemical general formula is Lix (Li < 1-y-z > Mn < y > Ni < z >) O < 2 >, xlt; 0.85, 0.5 lt; yt; Yt; 0.7, 0.1 lt; zlt, zlt; 0.3 part; the preparation method comprises the following steps: 1, preparing a sodium ion positive electrode precursor; 2, screening a sodium ion positive electrode precursor for the first time; 3, preparing a lithium-rich positive electrode material by combining a molten salt ion exchange method with secondary screening; according to the method, the structure rearrangement reaction occurring in situ during ion exchange is maximized through two screening procedures, so that the finally obtained lithium-rich positive electrode has the zero-voltage attenuation characteristic while having high capacity retention rate, and the long-term voltage attenuation problem of the lithium-rich positive electrode is effectively solved.

The invention relates to an LLC resonant soft switching converter based on self-detection synchronous rectification and a control method thereof. The converter comprises an LLC resonance circuit, a secondary transformer with a center tap, a self-detection synchronous rectification circuit, an input filter circuit, an output filter circuit, an LLC resonance control circuit, a primary PFM drive circuit, an optocoupler isolation circuit, an output voltage sampling circuit and an operational amplifier compensation circuit. Switching tubes S1 and S2 in the primary PFM drive circuit adopt high-voltage MOS tubes, secondary synchronous rectifier tubes S3 and S4 adopt low-voltage MOS tubes, output voltage feedback control and secondary synchronous rectifier tube self-detection drive control are adopted, and the resonant soft switching converter with the 240V-300V input voltage range, the constant output of 28V / 300W and the full-load efficiency of 92% is realized. According to the invention, zero-voltage switching-on of the primary switch tube, zero-current switching-off of the secondary synchronous rectifier tube and accurate synchronous rectification control are realized, the loss of the rectifier circuit is reduced, the efficiency of the converter is improved, and the electromagnetic compatibility characteristic of the converter is optimized.

The invention relates to the technical field of electric energy conversion, and discloses a high-frequency resonant topology-driven isolation power conversion system, which comprises a power conversion circuit, a driving unit, a feature sensing unit and a logic regulation and control unit, and is characterized in that the logic regulation and control unit extracts a parasitic ringing extreme point of a voltage evolution waveform in dead time; a charge release balance point is determined by calculating an arithmetic mean value of the first wave crest and the first wave trough, and high-frequency parasitic parameter interference is stripped; and predicting a time domain window of a return-to-zero potential based on a charge release balance point, performing phase compensation on a driving pulse in the time domain window according to a voltage waveform second derivative feature, and locking the starting moment of a switching tube at a zero voltage point. Interference of high-frequency distribution electromagnetic parameters on conduction judgment is eliminated, and soft switching self-healing in a full-load domain is achieved.

The invention discloses a control method, and particularly relates to an energy storage power converter self-adaptive control method based on digital twinning, which comprises the following steps: step 1, digital twinning model synchronization and state sensing; step 2, multi-target collaborative optimization based on prediction; and step 3, optimizing instruction issuing and physical system execution. Through real-time prediction and optimization of the digital twinborn model, it is ensured that the power switch tube always works in a zero-voltage switching state, switching loss is effectively reduced, and overall efficiency is improved. Dynamic thermal balance management is realized by combining thermal model prediction, local overheating is avoided, the service life of devices is prolonged, and the reliability of long-term operation of the system is enhanced.

The invention provides a phase current reconstruction method and system, computer equipment and a readable storage medium, and belongs to the technical field of phase current reconstruction, and the method comprises the steps: installing a single current sensor on two branches of an inverter, so as to construct a dual-branch single current sensor circuit topological structure; dividing six sectors formed by the non-zero voltage vector to form twelve sub-sectors; and for each sub-sector, sampling the single current sensor in the action time of the non-zero voltage vector and the zero voltage vector of one PWM period to obtain current information of two different phases, and reconstructing a third-phase current of the motor according to the current information of the two different phases. The method has the advantages that the zero voltage vector and the non-zero voltage vector are introduced for each sub-sector in the twelve sub-sectors to perform current sampling and reconstruction, the problem of current reconstruction blind areas of a sector boundary area and a low modulation ratio area is solved, meanwhile, the current reconstruction mode is single, and the complexity of an algorithm during actual reconstruction is reduced.

The invention discloses an inductive current peak control method for a high-efficiency four-switch Buck-Boost converter capable of smoothly switching between a DCM mode and a CCM mode, which belongs to the technical field of a direct-current power transformation control system and is characterized in that smooth switching between the DCM mode and the CCM mode is determined on the basis of energy storage time and direct power transmission time of the four-switch Buck-Boost converter in the DCM mode and the CCM mode; a negative current value-IZVS turned on under zero voltage of a switching tube of the four-switch Buck-Boost converter and a current value of an inductor of the four-switch Buck-Boost converter are compared through a comparator, and follow current time T4 of the inductor current is output; and calculating and outputting control signals of four switches of the four-switch Buck-Boost converter on the basis of the determined duty ratio of the energy storage time in the DCM mode or the CCM mode, the duty ratio of the power direct transmission time, the phase shift delta D between two bridge arms of the converter and the follow current time T4 of the inductive current, so as to realize the high-efficiency control of the peak value of the inductive current of the four-switch Buck-Boost converter.

The invention provides a fixed-frequency inductive-current-sensor-free four-switch buck-boost soft switching control method, which comprises the following steps of: generating a regulating variable based on a deviation between an output voltage and a reference value, and determining a time sequence parameter of each stage of the period; charging and discharging in opposite directions are carried out on output junction capacitors of two switch elements of the same bridge arm by utilizing inductive current in a commutation dead zone; when it is detected that the duration of the free circulation stage of the current period does not exceed a preset lower limit, adjusting the time sequence of at least one stage to execute power expansion under a fixed switching frequency, and meanwhile, keeping on under a zero voltage condition; identifying a bias component of the inductive current based on indirect characterization of the output current to the inductive current, and performing correction by adjusting a timing sequence of at least one stage; and combining the outputs of the time sequence generation, the power expansion and the offset correction obtained in the previous steps into a time sequence parameter of the current period. According to the method, the microgrid conversion requirements of a wide voltage adaptation scene on high efficiency, high power density and high robustness are effectively met.

The invention discloses a high-power modular radio frequency power amplifier capable of realizing wide-range power regulation. The high-power modular radio frequency power amplifier comprises a radio frequency load RL and m power amplification modules, the m power amplification modules are connected in parallel at a direct current side and are connected with an adjustable direct current bus voltage source; and the m power amplification modules are connected in parallel at the alternating current side and are superposed at the alternating current side. The output current of all the power amplification modules is transmitted to the radio frequency load RL; each power amplification module comprises a high-frequency switch power amplification unit, an LCL resonance network and an isolation transformer. According to the invention, through the configuration of the number of the power amplification modules and the adjustable cooperative control of the DC bus voltage source, the continuous adjustment of the output power is realized, meanwhile, the stable operation of each power amplification module under the resonance condition is ensured, the zero-voltage switching state is maintained, and the overall operation efficiency of the architecture and the adaptability of the radio frequency load are effectively improved.

The invention discloses a trapezoidal wave current soft switching method of an interleaving DC-DC converter, which is used for a two-phase interleaving bidirectional DC-DC converter based on a coupling inductor and a multi-phase interleaving converter thereof, and is characterized in that an inductive current adopts a trapezoidal wave mode, so that the converter works in an approximate critical conduction mode; a fixed switching frequency control or hybrid control method is adopted, so that the device can work in a Buck mode and can also work in a Boost mode. The method for realizing the soft switching is characterized in that the parasitic capacitance or the external resonant capacitor of the switching tube is discharged to be zero under the resonance action of the inductor and the capacitor in the dead zone time when the upper bridge arm switch and the lower bridge arm switch are alternately switched on, so that the zero-voltage switching-on of all the switching tubes is realized. And each phase current sharing control is adopted in the multi-phase interleaving parallel topological structure. According to the invention, the problems of large current peak value and high turn-off loss of a traditional triangular wave current mode are solved, the output ripple current is reduced, a soft switching method is provided, the switching loss is reduced, and the conversion efficiency of the converter is improved.

Techniques for valley detection in flyback power converters. In an example, circuitry implementing the techniques is configured to detect a positive slew at the onset of the leakage reset portion of the switching terminal signal, and to blank out the leakage reset portion. The circuitry is also configured to detect a negative slew of the leakage ringing portion of the switching terminal signal, and to blank out any false valley detections that occur during the leakage ringing portion. The blanking periods may be fixed or variable. The circuitry may also be configured to discern the difference between a true valley that reaches zero voltage within the magnetizing ringing portion and a false valley within the leakage ringing portion.

The application provides a driving method of an inverter bridge, a driving circuit, a servo driver and a motor device. In the driving method of the inverter bridge, voltage vector output time and dead time are set, so that a power switch tube of a jth bridge arm is kept in a conduction state in two adjacent voltage vector output times before switching, the power switch tubes kept in the conduction state are changed in turn and staggeredly, in each dead time, two other power switch tubes kept in the conduction state in a previous voltage vector output time are switched off, the corresponding switching bridge arm and winding complete resonance work, the end voltage of the power switch tube to be turned on is clamped to zero potential, zero voltage conduction is realized, a soft switching mode is formed, and the switching loss is reduced and the power conversion efficiency is improved.

The invention discloses a high-power-density modular B2G converter soft switching control method, system and device and a medium, and belongs to the technical field of converter control, and the method comprises the steps: constructing a two-stage converter topological structure; a frequency conversion and phase shift mixed soft start strategy is introduced, and soft start is carried out on the two-stage converter topological structure; according to the current transmission direction of the two-stage converter topological structure, selecting a control strategy according to the current transmission direction; soft switching control operation is carried out on a front-stage unit and a rear-stage unit in the two-stage converter topological structure, so that zero-voltage switching-on of an auxiliary switching tube is realized and the zero-voltage switching-on is maintained; an auxiliary resonance circuit is introduced, zero-voltage switching-on of a high-frequency bridge arm switching tube and an auxiliary switching tube is achieved through two times of resonance, and a rear-stage unit is designed to be an isolated bidirectional symmetrical CLLLC resonance type converter topology to be matched with closed-loop frequency adjustment; during soft start, a frequency conversion and phase shift mixed strategy is adopted, system energy is smoothly established through phase shift control, and starting current impact is restrained.

This invention discloses an adaptive fault-tolerant control method for a three-level ANPC inverter. Specifically, based on the influence of a faulty switching transistor on the output voltage vector, the inverter's fault-tolerant modes are classified into zero-level loss and zero-level retention fault-tolerant modes. The maximum modulation index of the inverter's output voltage vector is calculated under both fault-tolerant modes to determine whether the inverter system needs derating. For the zero-level retention fault-tolerant mode, a model predictive current control strategy with duty cycle modulation is adopted, utilizing a virtual zero-voltage vector synthesis method to solve the inverter's midpoint voltage imbalance problem. For the zero-level loss fault-tolerant mode, a cost function representing the fault-tolerant control performance index is designed, and a neural network is used to optimize the weight factors of the cost function. This invention achieves adaptive fault-tolerant control for open-circuit faults in inverters by adopting corresponding model predictive control optimization methods for different fault-tolerant modes, thereby improving the reliability of the inverter system.

The application provides a circuit control method and a circuit control device thereof, which are applied to a hybrid flyback circuit. The method comprises the following steps: determining a collection time point according to the alternating time length of the conduction of a first MOS switch and a second MOS switch on the main side of the hybrid flyback circuit and a preset time coefficient; collecting a midpoint voltage between the first MOS switch and the second MOS switch according to the collection time point to obtain a first voltage signal; and adjusting a field excitation negative current in the hybrid flyback circuit according to a comparison result of the first voltage signal and a preset voltage value, so that the field excitation negative current meets the zero-voltage turn-on of a primary measurement switch of the hybrid flyback circuit.

The application relates to the technical field of switching power supply, and discloses a bridge type low-stress passive soft switching boost power factor correction circuit, which comprises a direct current input power supply, a main power switching tube, an output filter capacitor and a coupling inductor comprising a primary side and a secondary side; a passive soft switching buffer network is connected to a main circuit of the circuit, the network comprises a diode full-bridge structure and a buffer capacitor, and the leakage inductance of the secondary side of the coupling inductor is connected between the midpoints of two bridge arms of the full-bridge structure; the application utilizes the ratio difference of the coupling inductor to generate a forward potential difference, drives the buffer capacitor to reversely charge to establish a negative voltage, and further provides a discharge circuit for the parasitic junction capacitance of the main power switching tube, so that zero voltage turn-on is realized, and high-frequency switching loss is reduced; meanwhile, the highest voltage stress of the semiconductor device is clamped in the range of the direct current output voltage through the potential restriction of the full-bridge network. The application does not need auxiliary active devices, and improves conversion efficiency and reliability while simplifying the system structure.

The invention relates to a quasi-resonance parameter optimization method for an IPT system with a wide power range, and belongs to the technical field of wireless power transmission. The problems that in the prior art, a battery load model is not accurate, quasi-resonance characteristic research is insufficient, and power adjustment and zero-voltage switching are difficult to consider are solved. The method includes equalizing a battery load to a variable resistance that meets voltage constraints; determining a quasi-resonance range of the detuning scale factor g based on the power regulation ratio and the maximum resonance angle tangent offset value; solving an optimal compensation inductance parameter; and continuously adjusting the output power by adjusting g, and keeping the operation under a weak inductive zero-voltage switching condition. The system includes a secondary side switch control capacitance and a controller. The technical effects are that wide-range power regulation is realized, high-efficiency zero-voltage switching operation is maintained, control logic is simplified, and engineering applicability is high.

Provided is a current resonance type switching power supply device with a simple configuration that has a wide variable range of output voltage and can also achieve ZVS of the main switching element. 【Solution means】The switching power supply device 38 includes a ZVS auxiliary circuit 40 formed by a series circuit of an auxiliary diode 42, an auxiliary inductor 44, and an auxiliary DC power supply 46, which is connected in parallel across both ends of the main switching element 14(2). The switching control circuit 36 determines first and second on-times ton1 and ton2 based on the control voltage Vs of the output by the error amplification circuit 34, and provides a dead time Td to turn on and off the main switching elements 14(1) and 14(2) complementarily. When the first on-time ton1 is ton1 ≧ Tk, control of the first operation mode is performed to set the second on-time ton2 to ton2 = ton1. When the first on-time ton1 becomes ton1 < Tk, control of the second operation mode is performed to set the second on-time ton2 to ton2 > ton1.

The invention discloses a bidirectional DC-AC converter control method considering a soft switching condition and current optimization. According to the invention, the switching loss of the converter is reduced by optimizing the phase shift angle and the switching condition of the converter, and the energy transmission efficiency is optimized by minimizing the effective value of the current, so that the performance of the whole sodium battery energy storage system is improved. Specifically, based on the relation between the excitation inductance Lm and the current effective value Iprms, the phase shift angle of the converter is accurately controlled, the current transmission efficiency is optimized, and it is ensured that the zero-voltage switching condition of a primary side bridge arm and a secondary side bridge arm can be achieved in the working process, so that the loss of the converter is remarkably reduced, and the energy conversion efficiency is improved.

A circuit is disclosed. The circuit includes a transformer having a primary winding extending between a first terminal and a second terminal, and further including a secondary winding extending between a third terminal and a first output terminal; a first switch having a first gate terminal, a first source terminal and a first drain terminal, the first drain terminal coupled to the second terminal and the first source terminal coupled to a power source; a second switch having a second gate terminal, a second source terminal and a second drain terminal, the second source terminal coupled to the second terminal, and the second drain terminal coupled to the power source; and a third switch having a third gate terminal, a third source terminal and a third drain terminal, the third source terminal coupled to the third terminal and the third drain terminal coupled to a second output terminal.

An example apparatus can include one or more of a transformer, a primary circuit electrically coupled to the transformer, and a first auxiliary circuit and a second auxiliary circuit, wherein the primary circuit includes a first bridge leg having a first pair of switches and a second bridge leg having a second pair of switches, the first auxiliary circuit is electrically coupled to the first bridge leg of the primary circuit, the second auxiliary circuit is electrically coupled to the second bridge leg of the primary circuit, wherein each auxiliary circuit is configured to store energy when one switch of a respective pair of switches is on and the other switch of the respective pair of switches is off, and provide the stored energy to recharge all attached capacitances in the respective bridge leg when both switches of the respective pair of switches are off.