Abstract: The invention relates to a stabilized high voltage source the output voltage of which may be regulated in wide range, whereby the adjusted output current is constant at variable load. The high voltage supply source comprises a high voltage rectifier fed by a push-pull driving circuit, a control circuit supplying push-pull width modulated control signals of the driving circuit and an error signal processing circuit working up the divided voltage and current signals taken from the output of the high voltage source. The output signal of the error signal processing circuit is applied to the control circuit as a control signal. The supply voltage of auxiliary circuits of the supply source is provided by a stabilized low voltage supply unit.

Abstract: A power supplier for providing a regulated high-frequency AC output with a bucking and boosting section and a push-pull output section in which a common switching element is used in both the bucking and boosting section and in the push-pull output section.

Abstract: A method and apparatus for controlling the magnitude of an analog current through a load to be proportional to the magnitude of an input voltage. A sensing resistor is connected in series with the load. A controllable voltage is applied across the series connection of the load and sensing resistor for driving a current therethrough. A pulse width modulator receives the input voltage and a feedback voltage corresponding to the voltage drop across the sensing resistors when current is conducted thereby and produces square wave pulses each having a pulse width which is adjusted in response to a difference between the magnitudes of the input voltage and the feedback voltage.

Abstract: A pulse-width-modulated inverter-converter for generating a direct current output from a direct current supply which utilizes a power inverter having smoothed trapezoidal waveform at the inverter output. A nonlinear inductive element, e.g. a reactor, is provided in series with the center tap of the primary winding of the power transformer which is connected to the power transistors of the inverter to allow storage in a capacitor upstream of this inductor of energy during the recovery times of the transistors.

Abstract: A DC-AC converter circuit for a power supply contains a parallel resonant circuit topology. Accordingly, stray capacitance associated with the converter forms a part of the resonant circuit and assists in setting the resonant frequency thereof. The energy in the stray capacitance which is normally lost is recovered by the resonant circuitry. A coupled inductor is connected in series with a primary winding of an output transformer of the converter. The coupled inductor includes a resonating inductor winding and an energy recovery winding which are tightly wound out of phase with one another. Energy from the resonant inductor winding is coupled to the recovery winding when the switching elements of the converter are switched on and off. Therefore, energy otherwise dissipated in switching operation is conserved by being returned to its source. Further, the switching elements of the converter are connected in series through diodes to the return line.

Abstract: A power supply for converting a low DC voltage, in particular a DC voltage delivered from a sea water galvanic cell (or solar cell), into a DC voltage useful for normal electronic equipment includes a number of MOS-FET transistors connected in parallel to an output transformer. The transformer has a number of primary windings distributed on a toroidal ferrite core so as to interact with the secondary winding of the transformer.

Abstract: A current fed inverter circuit is provided in which there are two interconnected circuits, each of which shares a common inductor and transformer primary. These interconnected circuits are arranged so that an operator selector switch can connect the two circuits in a first configuration to form a current fed half bridge circuit and in a second, different configuration, to form a current fed push-pull circuit.

Abstract: This regulating system for an array of solar panels or the like consists of a tap connection into the array, and a boost switching regulator connected to the tapped array. The tap connection divides the array into first and second portions, and the switching regulator is connected so that its power-input terminals are across only one of the two portions of the array. The power-output terminals of the switching regulator are connected across the load (i.e., across the entire array), and the sensing or feedback terminals of the regulator receive a signal which acts as a measure of voltage or current at the load.Boost regulators do not dump overvoltage into a dissipative load; hence the invention prevents local heat generation and resulting spacecraft heat-balance problems of conventional dissipative regulators.

Abstract: A system for converting three-phase AC to DC voltage employs three separate converter circuits having their outputs connected in parallel to reduce current harmonics. Each converter circuit includes a full wave rectifier feeding a pulse width current modulator which modulates the DC current flow to the load in a manner to maintain the magnitude of the DC current proportional to the instantaneous magnitude of the DC voltage. The current modulators act as substantially resistive loads to reduce source current harmonics and correct the power factor. The modulators include a pair of transistors connected in a push-pull circuit which are controlled by an oscillator circuit to modulate the DC current at a frequency which is many orders of magnitude greater than the line frequency of the AC source. The DC current outputs of the modulators are coupled through DC/DC isolation transformers to the load.

Abstract: A stabilizing power source apparatus includes a transformer with one primary and two secondary coils, a switching transistor, first and second differential amplifiers, an oscillator, and a pulse modulator. The oscillator generates a triangular wave of variable frequency. The modulator changes the pulse width and frequency of its output in accordance with a shift of a crossing point between the leading ramp of the triangular wave and the signal from the first differential amplifier. The pulse drives the switching transistor, so that the duty factor of the transistor stabilizes a DC output voltage and a switching frequency thereof stabilizes an AC output voltage. Another stabilizing power source apparatus includes a saturable reactor for stabilizing the DC output voltage and a resonance type inverter to generate a sinusoidal wave voltage.

Abstract: Current converter comprising a rectifier circuit and a converter transformer at the primary side of which a damping network is arranged, the damping network comprising a blocking oscillator circuit.

Abstract: A single regulation power supply providing load compensation of an auxiliary voltage output, wherein power is transferred from the primary winding of a transformer to a first and a second secondary windings each coupled to a rectifying and filter network respectively providing a main and an auxiliary voltage output, wherein a control loop senses the algebraic sum of the main output voltage and the voltage drop developed in a resistor by current drawn by a load on the auxiliary voltage output and controls current pulses applied to the primary winding so as to keep constant such sum, thereby rendering the auxiliary voltage insensitive to load changes at the auxiliary output, to any desired extent determined by the value of such resistor, with the trade off of a limited degradation in the control of the main voltage output.

Abstract: A switched power supply of the self-balancing push-pull square-wave converter type which includes a pair of switching transistors connected to the primary of a transformer and which are driven by a square-wave oscillator, and which also includes a pre-regulator chopper connected to the transistors. A ripple voltage appears on the output of the pre-regulator which is a function of the current flow through the individual transistors, and this voltage is coupled through a capacitor and a series resistor to the square-wave oscillator to terminate the drive current through each transistor for each cycle in which the current flow through the transistor exceeds a predetermined maximum.

Abstract: A multiple output switching power supply wherein the voltage at a main output which couples to a main secondary winding of a transformer is regulated by pulse width modulation of the current flowing in the primary winding of the transformer and wherein the voltage at an auxiliary output which couples to an auxiliary secondary winding of the transformer is regulated by pulse width modulation of the current flowing in the auxiliary winding. The pulse width modulation is performed in synchronism with a preestablished turn-on delay of the control switches which control current pulses in the primary winding as to the turn-on of the switches which control current pulses in the auxiliary winding. Besides avoiding the need for a filter between the auxiliary winding and the related control switch, this form of synchronization reduces switching power losses, current spikes and induced radio frequency emission.

Abstract: A single power converter is capable of drawing low distortion current from an AC line and delivering DC or AC power to a load. The single power converter is used to independently control the input current and the output current. The power converter transformer has an output winding which is controlled by pulse-width modulation (PWM) and a frequency controlled boost winding for controlling input current.

Abstract: A voltage inverter is disclosed. A clock circuit provides timing signals to the electronics of the system. Electronics include a circuit for controlling the phase relationship between a pair of square waves, an overcurrent protection circuit, an auto-on circuit, a low voltage shutdown circuit, and output driver transistors for driving a primary transformer. A power transformer is connected to the primary transformer, and the output of the inverter is taken from the secondary transformer. Voltage and current magnitude indicative feedback signals are provided from the output of the inverter with the voltage signal being monitored by the phase varying circuit and the current feedback signal being monitored by the overcurrent protection circuit. Control of the off-time between alternating pulses during a cycle of the output is controlled by overlapping the square waves controlling the driver transistors for exitation of the primary transformer.

Abstract: A power converter has a power switching stage which includes an output transformer containing a primary winding and a secondary winding, as well as structure for switching the current flowing through the primary from a first direction to a second direction. A feedback circuit is provided for feeding back a voltage proportional to a portion of the voltage across the primary winding on said output transformer to the structure for switching, to regeneratively turn on the switch and maintain the current flowing through the primary in the direction to which the current has been switched until the current is turned off. To help start the current through the primary in one direction or the other direction, structure is provided which supplies a pulse to the structure for switching, to positively start the current flowing in the desired direction through the primary.

Abstract: A single DC power switching converter feeds two DC load circuits from two transformer secondary windings. One winding is tightly coupled to the primary and its output voltage is controlled using pulse-width modulation. The other secondary winding is loosely coupled to the primary so that its leakage inductance resonates with a secondary capacitor such that its output is controlled by converter frequency adjustment. Thus, both output voltages are controlled using a single power switching stage.

Abstract: A switching regulator including a control circuit including a deviation detector for detecting a deviation between the detection value of the controlled variable and the reference value, a phase difference signal generator for generating a phase difference signal having a pulse width corresponding to the deviation, a reference signal shaper for shaping two reference signals of the same waveform having a phase difference corresponding to the pulse width of the phase difference signal, and a control signal generator for ON/OFF control of switching elements corresponding to the reference signals for controlling a switching circuit having transistors connected as the switching elements in a bridge formation, the switching elements are all driven with an equal ON/OFF ratio, the phase difference between the ON instructions to the two sets of the switching elements is controlled to control the output so that the control of the switching regulator is accomplished not by the pulse width for driving the switching eleme

Abstract: A switching power supply for converting an input voltage to a different output voltage includes a power switch for producing a sequence of pulses from the input voltage, an output filter for filtering said pulses and for producing an output voltage from the filtered pulses, and means for sensing the output voltage and for controlling the value of the output voltage to a specified value. The power supply includes means for responding to a reduction in the input voltage beneath a normal value and for increasing the duty cycle of the sequence of pulses from the power switch in response thereto. Structure is provided for automatically controlling the frequency of the sequence of pulses to a selected value and for overriding the automatic control of this frequency so as to decrease this frequency in the event the load connected to the power supply draws too much current. Circuitry is provided to minimize off time in each period of the pulse sequence under "brown-out" conditions.

Abstract: In a power supply having transformer coupling for regulation of voltage by a pulsing of primary current, a control circuit integrates the output voltage to obtain a measure of flux and primary current. A comparator monitors the integrated voltage to terminate a pulse of primary current prior to saturation of a core of the transformer. This insures linear operation and efficient transfer of energy from the primary to the secondary windings of the transformer. A sensor of secondary current initiates a new pulse of primary current when the secondary current has decayed to a fractional value of the peak secondary current.

Abstract: The present invention relates to a circuit for producing a switching modulating signal for switching a pulse duration modulator, so that the pulse duration modulator produces an output signal which is independent of the supply voltage. The circuit is comprised of an amplifier having at least one input port and an output port. The input port is connected to a modulating signal via a first resistor. A multiplier is provided which has first and second input ports and an output port. The first port of the multiplier is connected to the supply voltage, the second input port is connected to the output port of the amplifier, and the output port of the multiplier is connected via a second resistor to at least one input port of the amplifier. The switching modulating signal appears at the output port of the amplifier. This switching modulating signal, when used to switch a pulse duration modulator, provides a signal at the output of the modulator which is independent of the supply voltage.

Abstract: A two quadrant power modulator using bi-directional and uni-directional flyback converters in which output waveshape is determined by the modulation of the pulse repetition rate and width. The output voltage is equal to the square root of the driving function. A large number of converters may be paralleled with sequential switching to raise the output power and raise the output ripple frequency, thereby simplifying filtering. Output voltage is constant with varying load condition irrespective of the reactive or non-linear nature of the load.A two quadrant power modulator using flyback converters to generate a time varying voltage. Dynamic regulation and low output impedance are achieved without use of closed loop control. Input voltage is not distorted by reactive and/or non-linear loads.

Abstract: A switched-mode power supply provides a plurality of directly regulated DC output voltages. The power supply includes a first inverter-coupled transformer for providing first and second DC output voltages, with a feedback loop provided from a first output to the inverter for regulating the first DC output voltage. A second transformer is coupled across the first and second output lines of the first transformer and is further coupled across the second output lines by means of a switching regulator circuit including a second feedback loop for regulating the second DC output voltage. By proper selection of the number of turns of the second transformer, the regulator is capable of either substantially increasing the second output voltage when the switching device is on while lowering the output voltage very little when the switching device is off, or increasing the second output voltage very little with the switching device on while substantially lowering the output voltage when the switching device is off.

Abstract: A method for generating pulse width modulated (PWM) control signals from a reference waveform uses a unipolar timing waveform to modulate the reference waveform thereby avoiding the need for lock-out intervals to prevent shoot-through in power inverters supplied by the PWM control signals. Further, a sinusoidal reference waveform is approximated by a trapezoid, simplifying the computation of switching points located at intersections between the reference and timing waveforms.

Abstract: The a. c. power converter forms an electronic alternating voltage transformer for transforming an essentially sinousoidal input alternating voltage of a predetermined frequency, e.g. power line frequency, and a first voltage into a second voltage of the same frequency and essentially the same wave shape. A chopper circuit (52) is coupled between input terminals (50a, 50b) and the primary winding of a transformer (54), dimensioned for the operating frequency of the chopper, e.g. 50-150 kHz. A secondary of the transformer is coupled to a rectifier circuit (60) having two output connections, on each of which one of two unipolar voltages of opposite polarities appear. The output terminals of the rectifier circuit (60) are each coupled through a low-pass filter (73a, 73b) to alternately conductive electronic switches (64a, 64b) and to a first (62a) of two output terminals. The second output terminal (66b) is coupled to a common terminal, e.g. a transformer center tap (56) of the rectifier circuit.

Abstract: An inverter circuit is provided for use with a DC power supply and includes a transformer having a center-tapped primary coil and a secondary coil as well as a switch and a switch controller operating to maintain a constant RMS output voltage. Improvements in the inverter circuit include a switch formed of individual discrete and Darlington transistors connected in parallel so that the discrete transistors predominantly carry the load during low load operation and the Darlington transistor during high load operation.

Abstract: An alternating current power source which controls its own alternating current output through a feedback circuit which monitors the alternating current output is disclosed. The power source can be used as a backup to a primary power supply which provides power to a load. The present invention continuously monitors such primary power supply to detect power interruptions in the primary power supply. When an interruption is detected, monitoring logic circuitry of the alternating current power source disconnects the primary power supply from the load and energizes line driver circuitry of the present invention which provides the alternating current output. When the power interruption ceases, the monitoring logic circuitry reconnects the primary power supply to the load and disables the line driver circuitry so that alternating current is not provided by the present invention to the load.

Abstract: A high integrity multiple output power conditioner operating from redundant dc sources utilizes circuits to combine the sources to share a load and provide source isolation in event of component failure in any one of plural converters employed. Multiple ground planes permit high integrity checking of individual operation of multiple power converters employed in the system.

Abstract: A pulse width modulated power converter having a single time multiplexed symmetry correction circuit operative to vary the pulse width of output drive pulses applied to an output transformer. As a result, the average current and thus the volt second product seen by each half of the transformer primary winding will be equal under steady state conditions. The transformer drive remains balanced over the entire dynamic range to prevent unwanted saturation of the transformer core.

Abstract: A DC-to-DC conversion circuit for supplying dual positive and negative DC output voltages of the same level in absolute value to a load circuit such as a class B power amplifier. An alternative fluctuation in the positive and negative DC output voltages is regulated, making it unnecessary to regulate both simultaneously.

Abstract: A push-pull converter has transistor power switches which conduct alternately to connect a DC source with the primary windings of an output transformer. A turn-off snubber circuit is connected with each power switch. When a power switch is turned on, snubber current pulses flow in the circuits connected with both switches. A switch current sensor circuit has a current transformer with an output winding inductively coupled with each of the conductors connected between the power switches and the primary windings of the output transformer. The snubber current pulses through the conductors are 180.degree. out of phase and are suppressed in the current transformer output. The switch current signal from the secondary of the current transformer is free of snubber current pulses and is used in a flux balance circuit and in a pulse-by-pulse current limiter.

Abstract: A multiwinding inductor is disclosed which inductor is employed in a power control circuit. The power control circuit is of the type having a transformer which has a center tapped primary winding coupled to and associated with a plurality of secondary windings. The primary winding of the transformer is switched by means of switching devices during a power control operation. The inductor has a first winding in the primary which winding has one terminal coupled to the center tap and a second terminal coupled to a generator and a filter arrangement. Another winding of the inductor designated as a free wheeling winding is in series with an auxiliary impedance and has one terminal coupled to the common terminal of the switching devices and a second terminal coupled through a rectifier to the generator. At least two additional windings of the inductor are coupled to suitable rectifier circuits associated with the secondary windings of the transformer.

Abstract: A power converter circuit, which is voltage-regulated and current-limited by a peak current control circuit, significantly reduces the current tailout effect by decreasing the frequency of operation of the converter during low voltage output current-limited operation by increasing the nonconduction interval of the power switch while maintaining a fixed conduction interval.The peak current control circuit periodically initiates current conduction in the power switch in response to a clock pulse, and terminates conduction when a peak current level is attained. The peak current control includes circuitry to compare decaying current in the filter circuit during nonconducting intervals with a preset current tailout threshold level. The periodic triggering of the power switch into conduction is inhibited until current has decayed to this current tailout threshold level.

Abstract: A power supply circuit having a higher power factor and decreased current crest factor. The circuit preferably comprises an input circuit for receiving an input AC voltage and rectifying this voltage and an output circuit for providing a DC output voltage. A pulse-width-modulation circuit intercouples between the input and output circuits and includes a controlled circuit adapted to pass input AC current in pulse-wide increments with the pulse-width varying in inverse proportion to the AC voltage. In this manner, when the instantaneous AC voltage is low, the pulse widths are wider and conversely when the voltage is high, the pulse widths are narrower. In one version of the invention, the pulse width modulation is carried out by a programmed variable ratio transformer.

Abstract: An AC-DC power converter includes a rectifying circuit for rectifying the AC signal. The AC signal is supplied to a switching mode power supply for regulation and final filtering to obtain the output DC signal. A baseline generator in series between the rectifying circuit and the switching mode power supply ensures that there is always a minimum voltage level present in the signal that is applied to the switching node power supply.

Abstract: A switching regulator includes a first full-wave rectifier for rectifying an input a.c. voltage, at least one power switch connected between the first rectifier and the primary winding of a pulse transformer whose secondary winding is connected through a second full-wave rectifier and a filter to a pair of output terminals across which a regulated d.c. output voltage appears, and a control circuit comprised of a level-shift circuit, a comparator and a pulse width modulator for controlling the on/off condition of the power switch, whereby the pulse width modulator supplies a pulse modulated signal to turn the power switch on only when the comparator supplies a high level signal to the pulse width modulator.

Abstract: A portable, battery-powered high capacity dc-dc voltage converter suitable for charging capacitor-based devices such as electronic camera flash lamps is disclosed. The voltage converter utilizes two high frequency metal oxide semiconductor field effect power transistors which are driven in a push-pull arrangement by an emitter-follower full wave bridge circuit. The transistors drive a low leakage inductance toroidal ferrite-core power transformer at a frequency which is sufficiently high to avoid saturation of the transformer, resulting in high efficiency and low power consumption. The output of the transformer is rectified to produce a high capacity dc charging signal on the order of 300 volts with a maximum current of 28 amps. The voltage converter is capable of recharging commercially available inexpensive electronic 100-watt flash lamps at a rate of once every 1.8 seconds, compared with a rate of 15 seconds for comparable prior art self-contained charging units.

Abstract: A pulsed high voltage variable duty cycle current generator is described. One embodiment comprises a constant current power supply cyclically driving either one or both halves of a transformer primary. The transformer primary is polarized so that there is a secondary current output pulse proportional to the value of primary current only when one half of the primary is driven. The secondary current is rectified, and the average dc secondary current valve is used to control the primary driver duty cycle so that the secondary average current is maintained at a predetermined level. Thus, the output is a series of current pulses where the output peak value is determined by the amount of constant primary current and the output average value is determined by the output duty cycle.

Abstract: A direct current to alternating current inverter with an output transformer is regulated by a pulse width modulator which alternately closes one of two switches connected with the center tapped primary winding of the output transformer. A synchronizing circuit for the pulse width modulator has means for gating synchronizing pulses to the clock of the modulator, blocking synchronizing signals which would cause an unbalance in the duty cycles of the switches and saturation of the output transformer.

Abstract: A closed-loop core saturation control circuit for use in pulse width modulated power supplies is disclosed. The circuit uses a single transistor as a unity gain, low impedance track and hold amplifier to sense the current in the primary winding of the transformer and supply a related voltage to the power supply comparator, where it sums with the linear ramp voltage. The on-times of the switching transistors are therefore individually controlled and varied such that both switching transistors will see substantially equal peak currents.

Abstract: An overlap control system is provided in a push-pull switching mode inverter power supply which prevents a second switching transistor from being turned on after a first switching transistor has been turned off until the end of the storage time of the transistor being turned off so as to prevent the conduction of through currents from one transistor to the other which would otherwise occur if the switch-over took place during the storage time, and which would cause high stress and failure of the switching transistors because there is no inherent limit to the magnitude of the through currents, and because the through currents do not pass through the load circuit of the power supply. The overlap control circuit of the invention permits operation of the power supply with up to 100% duty cycle without any possibility of damaging cross-conduction of the switching transistors to occur.

Abstract: A circuit which prevents the flow of excess spikes of magnetization current in the primary windings of a transformer used in a switch mode power supply is disclosed. The circuit comprises an AC coupled, peak detecting, negative feedback path from a current sensing resistor in the primary windings of the transformer to the input of the voltage regulator so that as excess primary winding current spikes begins to flow the driving switches on the transformer primary are rapidly turned off thereby preventing any further rise in transformer primary current.

Abstract: In a switching regulator including a pulse-width modulator for controlling the pulse width of a base input applied to a pair of transistors which are alternately turned on and off by the base input, the pulse-width modulator is provided, by an error amplifier, with an input corresponding to the magnitude of a detected output voltage, and variations in the input applied to the pulse-width modulator for increasing the pulse-width of the aforementioned base input are moderated by means of a time-constant circuit and a circuit for causing the time-constant circuit to be charged and discharged.

Abstract: A DC to DC switching regulator includes a first pulse width modulator (PWM) (26) and two switching transistors (48) and (50) that are coupled to a transformer (18) to generate an output signal that has pulse widths variable in response to the PWM (26). The output of the transformer (18) is rectified to provide a regulated DC output. A second PWM (78) is provided to convert the amplitude of the regulated DC output to a stream of pulses that are input to an isolation transformer (112) by a push-pull arrangement of transistors (118) and (120). The pulse width of the pulses input to the isolation transformer (112) is variable as a function of the level of the regulated DC and the amplitude thereof is a constant referenced to the internal reference of the PWM (78). The output of the isolation transformer (112) is rectified and filtered to provide an error signal that varies in response to the pulse width of the output pulses from the PWM (78).

Abstract: A power supply system that utilizes a plurality of inverter power supplies connect in common to drive a common load with control circuitry for forcing the power supplies to share the load equally is described. Control circuitry associated with each inverter power supply senses the current level provided to the load and compares to the average current provided by all other power supplies in the system. The control circuitry includes circuitry for controlling the pulse width modulator circuitry in response to the sensed condition that the power supply is supplying more than its equal share of the load and causes it to adjust the duty cycle of power switches in the power supply downward to reduce the output current level.

Abstract: Method and apparatus are disclosed for use with switching power transformer circuits for balancing and compensating for core flux caused by unswitched D.C. current flowing in the transformer. A method and apparatus for adjusting the duty cycle of the switching portion of the power transformer circuit serves to balance the flux present in the transformer core during each half cycle of the operation of the circuit.

Abstract: In a system of parallel switching regulators sharing a common load, each regulator generates controllable pulses from an unregulated d-c. voltage source and then smoothing those pulses to form a d-c. output that varies according to changes in the pulses. A comparator controls the pulses according to the differential between a reference signal and a feedback signal, and an output resistor is connected between the comparator and the load. A reference signal source generates a reference signal for the comparator in a first one of the regulators, and the load side of the output resistor in the first regulator is connected to the feedback signal input to the comparator in that regulator so that the load voltage is determined by the reference signal for the first regulator.

Abstract: A digital controller includes a digital feedback circuit for use in generating a pulse-width modulated control signal. The signal is derived from binary signals representing a standard pulse-width signal for the rated output of the power supply plus a correction pulse-width signal reflecting the change in the output level of the power supply. Logic circuits alter the correction pulse-width signals to provide a balancing correction. Soft start operation of the power supply together with a selection of operating clock frequencies are also provided.

Abstract: A digital controller for a power supply which includes a transformer having primary and secondary windings and switching transistors for switching a current from a D.C. input means through the winding of the transformer to an output circuit. Pulse-width modulated control signals are used to control the switching operation of the transistors. In order to correct for an imbalance in the operation of the transistors, an error signal is developed which alters the value of the duty cycle of the transistors forcing the B-H curve of the transformer to oscillate between upper and lower threshold values.