Transformer coupled power switching circuit

• US 4,748,532 A
• Filed: 05/06/1987
• Issued: 05/31/1988
• Est. Priority Date: 02/29/1984
• Status: Expired due to Fees

• Current Assignee: International Business Machines Corporation
• Original Assignee: International Business Machines Corporation
• Inventors: Ramage, John G., Fenton, Brian P., Commander, Robert D.
• Primary Examiner(s): Pellinen, A. D.
• Assistant Examiner(s): WILLIAMS, HOWARD L
• Application Number:
  US07/025,914
• Time in Patent Office: 391 Days
• Field of Search: 307/270, 363/56, 363/58, 363/98, 323/289, 361/18, 361/30, 361/31
• US Class Current: 361/31
• CPC Class Codes: H02M 7/5387 in a bridge configuration
• CPC Class Codes: H03K 17/691 using transformer coupling
• CPC Class Codes: H03K 2217/0045 Full bridges, determining the direction of the current through the load

Abstract

A power switching circuit for switching power repetitively to a load at a relatively low frequency employs a power switching device which is driven across an isolating transformer interface. The drive signals are high frequency pulse signals which are modulated with the low frequency switching information, the modulations being detected in the secondary circuit of the transformer. Such an arrangement affords isolated driving with fast switching edges and requires only a small transformer. Power for the detection circuit is derived by rectification of the high frequency signals induced in the transformer secondary.

Claims

1. A power switching circuit for switching power repetitively to a load, comprising a power switching transistor responsive to a low frequency drive signal at a control input thereof to connect a power supply to a load for the duration of the drive signal, the low frequency lying within a predetermined range, and a drive circuit for providing such a drive signal to the control input of said power switching transistor, said drive circuit comprising a transformer and a primary circuit including a primary winding of said transformer, a source of high frequency periodic signals of frequency above said predetermined frequency range, amplitude modulating means arranged to produce from low frequency switching logic signals and said high frequency periodic signals a signal modulated at a frequency within said predetermined range in accordance with the switching logic signals and means for applying the modulated signal to said primary winding, the drive circuit further comprising a secondary circuit including a secondary winding of said transformer, in which a corresponding modulated signal is induced, amplitude detecting means for detecting the modulations by sensing non zero values and producing in response thereto such a low frequency drive signal for application to the control input of said power switching transistor, and rectifying means for rectifying the modulated secondary winding signal, the rectified signal being connected to provide an independent power supply to the detecting means.

2. A power switching circuit as in claim 1 in which the periodic waveform portions of the amplitude modulated signals from the modulating means exhibit cycle voltage excursions which are asymmetric about a mean level, the modulation being effected by inversion of said waveform portions.

3. A power switching circuit as in claim 2 in which the source of periodic signals is an oscillator capable of producing a train of pulses and said modulating means comprises gating logic responsive to pulses from said oscillator and also to said switching logic signals to produce gating signals, and a bridge circuit responsive to the gating signals from the gating logic to produce a modulated signal whose cyclic voltage excursions comprise pulses of one polarity, amplitude and direction alternating with pulses of the opposite polarity, a lower amplitude and a proportionately increased duration.

4. A power switching circuit as in claim 2 in which said detection means includes amplitude comparison means for comparing the voltage excursions of the modulated signal induced in the secondary circuit with at least one predetermined threshold such that the threshold is exceeded by the induced signal in only one of its inverted and non-inverted states.

5. A power switching circuit as in claim 4 in which said amplitude comparison means is arranged to compare the voltage excursions of a signal derived from the modulated secondary signal with an upper threshold and a lower threshold, said detection means further including a bistable device which is set to one of its states when the derived signal exceeds the upper threshold and is set to the other of its states when the derived signal falls below the lower threshold, the output of said bistable device constituting the repetitive drive signal.

6. A power switching circuit as in claim 1 including isolated current sensing means for sensing the current flow through said power switching transistor across an isolated interface and timer means responsive to the sensed current exceeding a reference value to switch off said power switching transistor for a predetermined interval.

7. A power switching circuit as in claim 6 in which said current sensing means comprises a second transformer whose secondary circuit includes a secondary winding connected in parallel with a resistor through which the current to be sensed is arranged to flow and a diode arranged in series with said secondary winding to prevent current flow from said power switching transistor therethrough, the second transformer primary circuit including a primary winding, a controllable current source, responsive to signals from said periodic signal source to apply current pulses to said second transformer primary finding whereby a diode is applied to said secondary winding to induce a proportionate voltage across said primary winding, the second transformer primary circuit further including a charge storage device for storing the induced primary voltage which represents the current flowing through said resistor.

8. A power switching circuit for switching power repetitively to a load, comprising a power switching transistor responsive to a low frequency drive signal at a control input thereof to connect a power supply to a load for the duration of the drive signal, the low frequency lying within a predetermined range, and a drive circuit for providing such a drive signal to the control input of said power switching transistor, said drive circuit comprising a transformer and a primary circuit including a primary winding of said transformer, a source of high frequency periodic signals of frequency above said predetermined frequency range, modulating means arranged to receive low frequency switching logic signals and said high frequency periodic signals and to produce an output signal comprising periodic wave form portions at a frequency above said predetermined range modulated at a frequency within said predetermined range in accordance with the switching logic signals and means for applying the modulated signal to said primary winding, the drive circuit further comprising a secondary circuit including a secondary winding of said transformer, in which a corresponding modulated signal is induced, detection means for detecting the modulations and producing in response thereto such a low frequency drive signal for application to the control input of said power switching transistor, rectifying means for rectifying the modulated secondary winding signal, the rectified signal being connected to provide an independent power supply to the detection means and inhibiting means for inhibiting said detection means from producing the repetitive drive signal if the independent power supply voltage from said rectifying means is below a predetermined minimum.

9. A power switching circuit for switching power repetitively to a load, comprising a power switching device responsive to a drive signal at a control input thereof to connect a power supply to a load, a drive circuit for providing a repetitive drive signal to the control input of said power switching device within a predetermined frequency range, an isolated current sensing means for sensing the current flowing through said power switching device across an isolated interface and timer means responsive to said sensed current exceeding a reference value to switch off said power switching device for a predetermined interval, said drive circuit comprising a transformer and a primary circuit including a primary winding of said transformer, a source of periodic signals of frequency above said predetermined frequency range, modulating means for producing from said periodic signals a signal modulated at a frequency within said predetermined range and means for applying the modulated signal to said primary winding, said drive circuit further comprising a secondary circuit including a secondary winding of said transformer, in which a corresponding modulated signal is induced, and detection means for detecting the modulations and producing in response thereto such a repetitive drive signal for application to said control input of said power switching device, said current sensing means comprising a second transformer whose secondary circuit includes a secondary winding connected in parallel with a resistor through which said current to be sensed is arranged to flow and a diode arranged in series with said second transformer secondary winding to prevent current flow from said power switching device therethrough said second transformer primary circuit including a primary winding, a controllable current source, responsive to signals from said periodic signal source to apply current pulses to said second transformer primary winding whereby said diode is forward biased so that the voltage across said resistor is applied to said second transformer secondary winding to induce a proportionate voltage across said primary winding, said primary circuit further including a charge storage device for storing said induced primary voltage which represents the current flowing through said resistor.