OFF-LINE SWITCHER WITH BATTERY RESERVE

Abstract: An off-line switcher includes an auxiliary winding (22) on the secondary side of the power transformer (20) coupled by a power switch (14) to a secondary voltage source (12) in order to provide uninterruptible power to the output (10) from either a primary AC (1) or the secondary DC source. The power switches (13, 14) coupling both primary and secondary source voltages to the power transformers are driven at identical duty cycles by a common regulation control (30). Differing transformation ratios between primary and secondary sources of power define different operational ranges and determine flow of power based on the magnitude of the primary source of power.

Description: This invention relates to switching type power supplies. Uninterruptible power supplies have generally assumed either a serial or parallel architecture. In a serial architecture format, a rectifier, battery and inverter are connected in tandem with the AC line, with the battery floating when the AC source input to the rectifier is satisfactory. This arrangement is generally satisfactory, but is inefficient since the overall efficiency can be no higher than the product of the efficiencies of the tandem connected components. In addition, failure of any of the tandem connected components may cause the entire power supply to fail. The parallel architecture format avoids many of the disadvantages of the serial architecture format, since the two sources of energy are fully independent of each other. While efficiency and reliability is improved, the control arrangement for substituting the load from one power source to the other is more complex; especially in those applications where a load transfer must be transient free and transparent to the load. The control arrangement must maintain synchronism and a definite phase relation between the two power processing paths. While these parallel architecture arrangements can be made cost effective at relative high power levels, the added control circuitry and complex transformer design requirements mitigate against their widespread use at relatively low power levels. US-A-4564767 discloses an uninterruptible switching power supply having a first primary input energized by rectified AC line voltage and a second reserve input energized by a battery voltage. The power modulating switch associated with the second input is operative as a power switch only when the AC voltage has failed. Separate circuitry is needed to sense the failure of the primary input and to engage the power switch processing the reserve battery voltage. The present invention seeks to provide a simplified control arrangement for parallel architecture arrangements. According to this invention there is provided a switching type power supply as claimed in claim 1. The respective winding turns of the primary winding, the secondary and the auxiliary winding may be selected so that output voltage regulation for the primary and secondary voltage sources is effective over substantially different input voltage ranges that include a small common overlap range. Normally the regulated output voltage is derived solely from the primary AC voltage source. If this line voltage source is degraded in magnitude, regulation control slips into the overlap range in which both primary and secondary voltage sources contribute to the output power. Should primary power fail entirely, the output power is supplied solely from the secondary voltage source. The invention will now be described with reference to the accompanying drawings, in which: FIG. 1 is a block schematic of a power supply embodying the invention; FIG. 2 discloses idealized switching waveforms of the power supply of FIG. 1; FIG. 3 discloses a graph of operative voltage ranges of the power supply of FIG. 1; and FIG. 4 is a schematic of a power train of the power supply of FIG. 1. An off-line switcher designed to be energized by AC line voltage is shown in FIG. 1. It includes provisions for supplying power to a load at output terminal 10 from one or both of two independent sources of power. Source 1 representing a commercial AC line is coupled to a rectifier filter circuit 11. Circuit 11 is coupled to a diode 2 which is operative to prevent reverse power flow from winding 21 when the input voltage is low. The secondary or reserve power source 12 is illustratively shown as a battery and is coupled to winding 22 through diode 3, which prevents reverse power flow into battery 12. Power flow from either the AC source 1, or source 12, is through power transformer 20 to one or more outputs, such as terminal 10. Power flow from the primary AC input source 1 is coupled to a primary winding 21 by power switch 13, whereas power flow from the secondary source 12 is coupled to an auxiliary winding 22 by power switch 14. Voltage regulation at output 10 is controlled by a pulse width modulation control (PWM) 30. The output voltage is sensed at output terminal 10 and a pulse control signal shown by waveform 101 in FIG. 2 is coupled, via a primary to secondary isolation coupler 31, to logic circuit 32. Coupler 31 may comprise an optical coupler, a pulse transformer, or any other transmission medium capable of maintaining ground isolation. This pulse control signal is utilized by logic circuit 32 to supply timing signals to base drive 33 to control the duty cycle of the power switch 13 as shown by waveform 103 in FIG. 2. Power switch 13 may comprise a bipolar power transistor or a FET power transistor, but is not necessarily limited to these devices. Voltage pulse waveform 103 used to control the drive of power switch 13 has variable pulse width. This pulse width is controlled by PWM control 30 in order to regulate the voltage output at output terminal 10. The output of PWM control 30 is also applied to base drive 34 to control a duty cycle of power switch 14 (either a bipolar or FET, or the like) which couples the reserve voltage source 12 to the auxiliary winding 22. The duty cycle of power switch 14 is shown by waveform 102 in FIG. 2. The different duty cycles of waveforms 102 and 103 are disclosed to illustratively show typical duty cycles at which the AC voltage source 1 and DC voltage source 12 normally supply power to the output. Both power switches 13 and 14 are simultaneously driven at a common duty cycle, and for a certain range, as shown by waveform 101 between edges 8 and 9, both switches couple power to the output. To the left of edge 8 only the switch 13 couples power to the output and to the right of edge 9 only the power switch 14 couples power to the output. While the two power switches 13 and 14 are operated with the same duty cycles, the resultant voltage output 10, as derived from the AC primary input 1 and the secondary source 12, is derived from different voltage sources of differing output voltage ranges, such as shown in FIG. 3, wherein lines 301 and 302 represent voltage level boundaries of the rectified DC operative input voltage range supplied from AC primary voltage source 1 via rectifier 11, and lines 303 and 304 represent boundaries of an operative input voltage range supplied by the secondary source 12. As shown, the voltage ranges are different except for a small overlap between lines 302 and 303. Line 305 represents an alarm level at which an alarm indicating low AC line voltage is signaled if the voltage drops below this level. Since power switches 13 and 14 operate at identical duty cycles, the differing voltage ranges are achieved by control of source voltage levels and by differing transformation ratios between primary winding 21 to secondary winding 23 and between auxiliary winding 22 to secondary winding 23. Hence, under normal operating conditions where output is supplied from the AC line voltage, the output voltage is defined by the equation: where: V O is the output voltage, V P is the rectified AC line voltage, and D is the duty cycle at which the power switches 13 and 14 are operated. In the illustrative embodiment of a forward converter, this value is normally less that 1/2. However, the invention may be embodied in other types of converters such as flyback converters, for example. Under emergency conditions where the AC line has degraded or failed, the output voltage is defined by the equation: where: V O and D are as defined above; V B is the reserve voltage; and It is apparent, from the foregoing, that at a given duty cycle, the output voltage is dependent upon the turns ratio in the transformer coupled power flow path from source to the output if both primary and secondary voltage sources are identical. The turns ratio in the primary power path is selected so that the control 30 regulates over a generally higher input voltage range than that for the reserve power path. A drop in AC line voltage first causes control 30 to increase the duty cycle of both power switches to maintain the regulated output voltage. If the AC line voltage continues dropping, the output voltage is partially maintained by power from the reserve voltage source in the overlap region between lines 302 and 303 in FIG. 3, and when the AC line voltage drops below a threshold level, as shown by line 302, corresponding to a critical duty ratio D, the output voltage is sustained entirely by the reserve voltage source in the regions defined by voltage levels 304 to 303. It is readily apparent that by control of the relative transformation ratios of the nominal AC line voltage and the selected battery voltage, an uninterruptible power supply can be implemented which utilizes a common regulation control system for controlling both primary and reserve power. An off-line switcher operating as a forward type converter and providing reserve battery power is schematically disclosed in FIG. 4. Primary power, supplied from an AC line source, is coupled to input terminals 401 and 402 and is rectified and filtered by rectifier/filter circuitry 404. A connection to the primary winding 405 of transformer 406 is periodically completed by the power switching transistor 403. The rectified DC output at leads 411 and 412 is derived from secondary winding 407 and rectified by diode 408. Diode 409 provides a flyback path for the output filter inductor 410. The output voltage is sensed across the filter capacitor 413 by a pulse width modulation controller 430, from which control signals are transmitted via coupler 431 to a logic circuit 432 which, in turn, controls timing of a drive circuit 433 to control the duty cycle of power switching transistor 403 in order to regulate the output voltage across the filter capacitor 413. A battery 451 is utilized as a reserve energy source and is coupled to an auxiliary winding 415 of power transformer 406 by a Darlington connected power transistor switch 453 (or power FET) whose switching is under control of base drive transistor 455 which is in turn responsive to an output of the PWM control 430. The duty cycle of power switch 453 is in synchronism with and substantially identical to the duty cycle of power switch 403. While power switches 403 and 453 are switched in unison, power is supplied to the output within one of the differing regulation ranges which are established by the relative turns ratio of windings 405 and 415 with respect to the secondary winding 407. The reserve energy supplied by battery 451 is current limited. A current transformer 456 is utilized to sense the peak discharge current flowing from the battery 451 to the transistor switch 453. This current signal is rectified by diode 457 and filter 458 to derive a DC control signal. This control signal is coupled to the PWM controller 430 via leads 461 and 462 and is utilized through the controller 430 to provide overload protection.

Claims: A switching type power supply including a power transformer (20) including a primary winding (21), a secondary winding (23) and an auxiliary winding (22), and having a first transformation ratio between the primary winding and the secondary winding and a second transformation ratio between the auxiliary winding and the secondary winding, first input means (11,13) for accepting a primary source of energy, second input means (3,14) for accepting a secondary source of energy (12), and output means (10) for coupling energy from the secondary winding to a load, the first input means including a first power switch (13) for continuously periodically connecting the primary source of energy to the primary winding, and CHARACTERIZED BY the second input means including a second power switch (14) for continuously periodically connecting the secondary source of energy to the auxiliary winding, and by regulation control means (30) responsive to a magnitude of a signal generated at the output means by the power supply for continuously and concurrently driving the first and second power switches at a common duty cycle, whereby, in operation, power signals are continuously supplied to the output means through the first and second power switches, as controlled by the first and second transformation ratios within first and second input voltage ranges, so that energy is automatically supplied to the output means from the primary source of energy within the first input voltage range, and energy is automatically supplied to the output means from the secondary source of energy within the second input voltage range when the primary source of energy is below its desired operating limit. A switching type power supply as claimed in claim 1, including a battery (FIG. 4, 451) as the secondary source of energy, and monitoring circuitry including a sensing transformer (456) for sensing a discharge current from the battery.

Inventors: ENG, WING KEUNG, SCHROEDER, ROBERT EDWARD
Applicants: AMERICAN TELEPHONE AND TELEGRAPH COMPANY
Application year: 1987
Main classification: H02J9
IPC codes: H02J9