Sundstrand Corporation

Sampat S Shekhawat

P John Dhyanchand

John J Dhyanchand

Pierre S Thollot

Chai-Nam Ng

Mahesh J Shah

Jayant G Vaidya

Alex Cook

John Horowy

An inverter in accordance with the present invention includes a plurality of bipolar transistors (102) with each bipolar transistor having a freewheeling diode (108) poled in parallel with outputs of the bipolar transistor, a current transformer (112) in a positive feedback circuit (110) associated with each bipolar transistor causing a positive feedback to be applied from an output of the inverter to a base of each bipolar transistor to provide a base drive proportional to current flowing in a load coupled to the bipolar transistors, a rectifier (120), disposed in each of the positive feedback circuits for permitting current to flow to the base of the bipolar transistor when the bipolar transistor is conductive and blocking flow of current from the base to the emitter when the bipolar transistor is not forward biased.

Inverter with proportional base drive controlled by a current transformer

The problem of sensing current through inverter switches is solved by a current transformer which has windings connected with the flyback diodes to demagnetize or reset the current transformer core. Size and weight of the transformer are minimized while avoiding a buildup of flux which would saturate the core.

Inverter switch current sensor with shoot-through current limiting

Prior base drive circuits for Darlington-connected transistors have accomplished a significant increase in the rate at which such transistors can be turned off and on. However, such circuits have utilized relatively numerous circuit elements or components to accomplish this result. In order to overcome this problem, a drive circuit 26 for rapidly switching driver and driven transistors Q3, Q4 connected in a Darlington configuration includes a first controllable switch Q1 coupled to a control electrode of the driver transistor Q3, an inductor L1 coupled to the first controllable switch Q1 and the control electrode of the driver transistor Q3 and a second controllable switch Q2 coupled to the control electrode of at least one of the transistors. A driving stage 32 is coupled to the controllable switches and closes same at the beginning of a turn on sequence to establish current flow through the inductor. The driving stage subsequently opens the second controllable switch while maintaining the first switch closed so that the current flow through the inductor is provided to the driver transistor to turn on same. The drive circuit utilizes fewer circuit components than prior drive circuits.

Base drive circuit for Darlington-connected transistors

A load is controlled by a pulse waveform from a source. The source and load are electrically isolated by using two monostable circuits to detect the leading and trailing edge of the pulse waveform, two pulse transformers to isolate the source and the load, and a bistable circuit to reconstruct the original pulse waveform.

Isolation circuit for pulse waveforms

A Darlington connected switching circuit (20) having a bipolar driver transistor (22) and at least one driven transistor (24) having a rapid and reliable turn-off characteristic with low losses suitable for use in power applications such as inverters. The collector to emitter potential of the at least one driven transistor is sensed by a comparator (26) to produce an output signal for controlling the conduction of minority carriers from the base of the driven transistor in response to sensing that the emitter to collector potential has become greater in magnitude than a reference potential (Vref) applied to the comparator. The reference potential is chosen to be at least a function of the switching characteristic of the driven transistor. Appropriate selection of the reference potential applied to the comparator insures that the driver transistor is sufficiently turned off to prevent false turn-offs under light load conditions while additionally ensuring that the at least one driven transistor is rapidly turned off after the driver transistor has been turned sufficiently off to prevent possible false turn-offs.

Darlington connected switch having base drive with active turn-off

A Darlington-type amplifier (10) having an input field effect transistor and an output bipolar transistor (T.sub.2) is disclosed with improved turn-off time. Turnoff is enhanced by short-circuiting the gate-to-source or drain capacitance of the input field effect transistor (T.sub.1) and after turnoff is complete of the input field effect transistor prior to turn off of the output bipolar transistor (T.sub.2) coupling the base of the output bipolar transistor to a reference potential to discharge minority carriers therein to complete turnoff.

Darlington amplifier with high speed turnoff

A three phase inverter (40) is disclosed for use with loads (100) which are unbalanced. A transformer (80) having primary windings (82), (84) and (86) which are respectively connected to phase outputs (52, 54 and 56) has a secondary having individual windings (88, 90 and 92) connected in a delta configuration which minimizes the flow of unbalanced current in the unbalanced load. A second embodiment utilizes a transformer in an interconnected configuration. A neutral connecting the conductive switches to the load is unnecessary with the invention.

Three phase inverter power supply with balancing transformer

A DC to DC power converter having first and second input terminals and an output terminal for providing an output voltage at the output terminal which is at a potential halfway between a DC potential applied by a DC power supply across the first and second input terminal is disclosed. The converter comprises first and second switches connected at a switch junction in series across the first and second input terminals, first and second series-coupled diodes coupled at a diode junction across the first and second input terminals in reverse-bias relationship with respect to the DC potential applied across the first and second input terminals, a transformer comprising two windings inductively linked and having first and second ends and a center-tap, the ends coupled between the switch junction and the diode junction and the center-tap coupled to the output terminal and means for alternately switching the first and the second switches at an equal duty factor.

DC to DC power converter

Prior circuits for rapidly switching Darlington-connected transistors between on and off states have accomplished relatively fast switching by applying reverse base drive to the transistors to quickly sweep the excess carriers therefrom. However, such circuits have not accomplished the required degree of reduction of turn off time and have induced localized 'hot spots' in the base-collector junctions of the transistors. In order to overcome these problems, a drive circuit according to the present invention includes first and second power sources of first and second polarities, a turn-on network coupled between the first power source and the control electrode of a driver transistor of a Darlington-connected pair for selectively turning on the transistor pair, turn-off capacitors coupled between the turn-on network and the control electrodes of each of the transistors and a turn-off network coupled between the capacitors and the second power source for coupling the second power source to the capacitors to rapidly turn off the transistors. Means are included for providing a dwell interval between operation of the turn-on network and operation of the turn-off network to allow recombination of excess carriers in the transistors to minimize the incidence of localized hot spots.

Drive circuit for fast switching of Darlington-connected transistors

Bi-directional switches for use with neutral point clamped PWM inverters have utilized clamping switches which are operated alternately with main power switches in the inverter to periodically connect an output terminal of the inverter to a neutral voltage. However, a delay period is typically interposed between turn-off of a main power switch and turn-on of the clamping switch to minimize the possibility of a potentially destructive shoot-through condition. This delay adversely affects the quality of the power developed by the inverter and leads to an undesired increase in the size of snubber circuits coupled across the power switches. In order to overcome these problems, a bi-directional switch according to the present invention includes circuitry for providing a high impedance path in series circuits including the main power switches and the clamping switch so that shoot-through currents are blocked even when one of the main power switches and the clamping switch are simultaneously on. The delays between turn-off of a main power switch and turn-on of the clamping switch can thereby be reduced or eliminated, thereby resulting in improved quality of the output power of the inverter and a reduction in the size of snubber circuits used therein.

Bi-directional switch for neutral point clamped PWM inverter

Prior types of neutral point clamped PWM inverters have included memories for storing a plurality of PWM waveforms, one of which is utilized to control switches in the inverter to maintain an output parameter within prescribed limits. Such types of inverter controls, however, cannot provide the necessary degree of regulation in some applications. To overcome this problem, an inverter control according to the present invention permits on-line generation of PWM patterns for a neutral point clamped PWM inverter. The control includes circuitry for generating a depth of modulation signal representing the deviation of the output of the inverter from a desired level. A microprocessor and memory are coupled to the generating circuitry for calculating switching points for the inverter switches during operation of the inverter based upon the depth of modulation signal for a given number of pulses to be produced during each half cycle of the output. Timer modules are coupled to the microprocessor for developing switch control signals for each switch from the switching points so that the switches are operated to reduce the deviation of the output from the desired level.

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Rockford, IL, United States of America

Sampat S Shekhawat