Abstract:
A synchronous burst mode power supply includes a power converter for transforming an AC mains from a relatively low frequency to a higher frequency, and a gate circuit responsive to the AC mains supply for enabling the power converter ( 2 ) to initiate a burst of output pulses at the higher frequency each time the AC mains supply occurs within a predetermined range. In an alternative embodiment, the power supply has a regulating circuit for regulating output from the power converter that includes a current feedback loop to the gate circuit for pre-regulating control of the power converter in response to load variations.

Description:
BACKGROUND 
     This invention relates generally to the field of power supplies, and, in particular, to standby mode power supplies for television receivers. 
     The power consumed by electronic equipment in the standby mode is becoming an increasingly visible public policy issue. For example, an article in the Sep. 19, 1997, issue of Europe Energy reports that the European Commission regards reducing the energy consumed by electronic equipment in the standby mode of operation as a priority. The article further states that the Commission has concentrated its initial efforts at reducing the standby power consumption of televisions and VCRs, and that it has elicited voluntary commitments from manufacturers of such products to progressively reduce average standby power consumption. 
     Modern televisions can have a standby power consumption of about 5 to 10 Watts caused by the degaussing circuit and switched mode power supply running in standby mode. Televisions that have an additional standby power supply and disconnect the degaussing circuit can reduce the power consumption to 1 Watt. 
     In a conventional power supply arrangement for a video display apparatus, a primary winding of a standby transformer is coupled to the AC mains. A transformed voltage across a secondary winding of the standby transformer is full-wave rectified and is regulated by some form of linear regulation to provide power for the video display apparatus in a standby mode of operation. This standby power supply consumes power as long as the video display apparatus is connected to the AC mains, and thus also consumes power during the run mode of operation. During standby mode, power losses are incurred partly due to switching losses. U.S. Pat. No. 6,043,994 proposes a power supply for reducing standby power consumption attributable to a start-up resistor of the switched mode power supply controller integrated circuit IC. 
     It is therefore desirable to provide a simple and cost-effective method for reducing the standby power consumption attributable to the switching losses. 
     SUMMARY 
     The present invention is directed to a standby power supply circuit that reduces the standby consumption attributable to switching losses from coupling full AC mains to power conversion circuitry. A synchronous burst mode power supply includes a power converter for transforming an AC mains from a relatively low frequency to a higher frequency; and a gate circuit responsive to the AC mains supply for enabling the power converter to initiate a burst of output pulses at the higher frequency each time the AC mains supply occurs within a predetermined range. A method for providing synchronous burst mode power includes the steps of receiving an AC mains supply at a relatively low frequency, detecting when the AC mains supply occurs within a predetermined range, and initiating a burst of output pulses at a higher frequency than the relatively low frequency in response to the detecting step. 
     The above, and other features, aspects, and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a block diagram and waveforms that illustrate the present invention. 
     FIGS. 2 and 3 show schematic diagrams of standby power supplies that embody the present invention. 
     FIG. 4 is a graph of the range of input power verses output power. 
    
    
     Similar reference characters refer to similar elements in each of the drawings. Resistors values shown are in units of measure indicated as ohms, kilo-ohms (k), or Mega-ohms (M), and capacitor values are in units of measure indicated as microfarads (u) or picofarads (p). 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention reduces power losses associated with circuit switching in a standby switched mode power supply SMPS. The inventive standby SMPS is connected directly to a rippled but rectified mains voltage, which is then gated to the SMPS during periods of low levels of the rectified mains voltage. Gating of the rectified mains voltage to the standby SMPS, which responds by generating burst pulses, is synchronous with a predetermined range in the rectified mains voltage. 
     The invention is illustrated with a block diagram  10  and waveforms  11 - 3  in FIG. 1. A mains voltage Vmains is rectified by a diode D 1  to provide rippled and positive half wave voltages V 1  to a threshold detector  1 . Voltage pulses V 2  are output by the threshold detector  1  when the rise and fall of the positive half wave voltages V 1  are below a threshold level (horizontal line  16  in graph  11  of FIG.  1 ). The voltage pulses V 2  at the relatively low frequency of the mains voltage Vmains, for example 50 or 60 Hz, are converted by the free running oscillator  23  to sawtooth current pulses ITr at a higher frequency. In a sense, the detector acts as a gate with respect to passing part of the positive half-waves to the oscillator circuit  23 . It is noted that the Ac mains voltage alone initiates and terminates the burst pulses V 2 , independent of any external switching control. In the exemplary embodiment of FIG. 1, nine sawtooth pulses ITr are generated for every voltage pulse V 2  output by the detector  1 . This number is related to the free running frequency of the oscillator, for example 25 kHz. Peaks of the sawtooth current pulses decrease in a linear sloping manner, as shown, because the positive half-wave pulse V 1  imposed on the transformer Tr 1  decreases from its peak to zero. Voltage imposed on a transformer follows the relationship (voltage/inductance) multiplied by time. In the present circuit the time factor is constant but the mains sine wave voltage increases from zero to a peak value and then decreases from its peak value to zero. The decrease from peak value to zero causes the linear decaying peaks in the sawtooth current pulses ITr. Conversely, during the rising edge of the mains voltage sine wave the peaks of the sawtooth current pulses ITr rise linearly. 
     The sawtooth current pulses ITr are transformed into a secondary winding voltage VTR 1 , which is then diode D 7  rectified into an unregulated voltage V 3 . The unregulated voltage V 3  is smoothed and regulated by a voltage regulator  3  to an output voltage Vout of 5V DC. 
     An exemplary circuit in FIG. 2 includes a circuit arrangement  20  for controllably coupling the voltage mains Vmains over to a connection point for a run mode power supply (not shown). The voltage mains Vmains is switched across an opto-relay, Triac T 2 , responding to a run control signal through current limiting resistor R 13  from a known type of microcontroller (not shown). Alternative relay switches in lieu of triac driver T 2  can be employed. The voltage Vmains is also coupled across a triac T 1  triggered when the mains voltage is passed by the triac driver T 2  and dropped across a voltage divider made up by resistors R 11  and R 12 . The voltage mains Vmains passed by triac T 1  is coupled across a degaussing circuit  21 , full wave rectified by a diode bridge arrangement D 11 -D 14  and filtered by capacitor C 11  for a run mode power supply. 
     The circuit embodiment of FIG. 2 further includes exemplary circuit embodiments for the threshold detector  1 , free running oscillator  2  and voltage stabilizer  3 . 
     Positive half wave voltages V 1  from the voltage mains Vmains rectified by diode D 1  are voltage divided between resistors R 4  and R 5 , voltage limited by zener diode D 3 , and ripple attenuated by capacitor C 1  to provide +12V to the emitter terminal E of transistor Q 1 . Transistor Q 1  is biased by voltage developed at its base terminal B from the rectifier arrangement of voltage divider resistors R 1  and R 3  and filtering capacitor C 2 . An optional adjustable resistor R 2  allows for fine adjustment of the base terminal B voltage. Transistor Q 1  is protected by diode D 2  against a possible reverse biasing due to the +12V developed at the emitter terminal E of transistor Q. When the input voltage to the base terminal B of transistor Q 1  is below a certain threshold, determined by the emitter E voltage of transistor Q 1  and the voltage divider R 4 , R 5  and D 3 , transistor Q 1  turns on and provides the free running oscillator circuit  23  with a bias voltage. It is noted that resistor R 5  adapts the on-time of the oscillator circuit  23  to different mains voltages. 
     In the threshold detector circuit  22 , +12V at the positive terminal of capacitor C 1  is compared with voltage at the base terminal B of transistor Q 1 . A positive voltage at terminal B of transistor Q 1  greater than 0 and less than about 11.3 volts biases transistor Q 1  on, providing the threshold level  16  of about 11.3 volts. Above 11.3 volts at base terminal B, PNP transistor Q 1  is biased off. The threshold detector or gate circuit  22  provides low voltage level switching which reduces losses otherwise present in a typical switched mode standby power supply. 
     The oscillator  23  in FIG. 2 is a blocking oscillator formed by transformer Tr 1 , resistor R 6 , capacitor C 3 , secondary winding n 3  and transistor Q 2 . The blocking oscillator operates in a conventional manner. It is noted that diodes D 4  and D 5  and resistor R 7  are not necessary for basic operation of the oscillator circuit, but have been included as one form of signal conditioning. The depiction of blocking oscillator circuit  23  is merely exemplary and does not proscribe the use of other oscillator circuits or topologies in the context of the present invention. 
     Positive feedback provided by secondary winding n 3  keeps transistor Q 2  conducting. Current through base terminal B of transistor Q 2  keeps capacitor C 3  discharging until the voltage across the capacitor C 3  is 1.4V, at which point transistor Q 2  stops conducting and power is transferred to the secondary side via winding n 2  in a flyback manner. When there is flyback voltage at the secondary winding n 3  capacitor C 3  is pulled down to negative. At this point current has to be fed through resistor R 6  again to charge up capacitor C 3  and start conduction of another saw tooth current ITR, derived from the positive half-wave pulses V 1 . Capacitor C 4  reduces radiation of the fast switching. 
     The blocking oscillator  23  runs with an almost constant frequency that is dependent on the voltage Vmains, resistor R 6 , capacitor C 3  and the relationship between windings nl and n 3 . The duty cycle of the oscillation can be substantially constant so that the energy transferred to the secondary winding n 2  is substantially constant. This substantially constant energy has two consequences. First, the standby power supply is inherently protected against a short circuit condition on the secondary side of the transformer Tr 1 . Second, parallel voltage regulation techniques can be used to regulate the voltages provided by the secondary windings n 2 . For example, in FIG. 2, the +5V output provided by the secondary winding n 2  can be partly limited by zener diode D 7  and regulated by the voltage regulator IC 1 . The use of voltage regulator IC 1  and diode D 7  is merely illustrative and does not preclude the applicability of other voltage regulation techniques in the context of the present invention. 
     In the embodiment of FIG. 2, the blocking oscillator  23  is advantageously used to transform the relatively low mains voltage frequency, for example 50 to 60 Hz, from which two voltage pulses V 2  appear per cycle to a frequency from which nine sawtooth current pulses are generated for each voltage pulse V 2 . This transformation permits a decrease in the size of standby transformer Tr 1 , which in turn, leads to a decrease in the power consumption by the standby transformer Tr 1 . The secondary winding voltage VTR 1 , reaching 7.2V in the exemplary circuit, is initially rectified by diode D 5 , filtered by capacitor C 5  and then regulated by the voltage regulator IC 1 . In case of reload, diode D 7  prevents capacitor C 5  and voltage regulator IC 1  from too much voltage. Voltage output by the regulator IC is filtered by capacitor C 6  to provide the +5V standby power. 
     The circuit of FIG. 3 is similar to the standby power arrangement of FIG. 2, except for the current feedback loop from an additional opto-coupler IC 2  coupled to the terminal between resistor R 5  and zener diode D 3  of the threshold detector circuit  22 . The circuit embodiment of FIG. 2 is suited for a static load or a relatively small variation in load where resistor R 2  can be adjusted to optimally time the initiation and termination of burst pulses suitable for the load amount. If R 2  is optimally adjusted for a certain load and the actual load is relatively small then the burst pulse frequency will be too high and the power output will be greater than needed for the load, resulting in wasted power. Dynamic load applications are appropriate for the circuit embodiment of FIG. 3, where the current feedback adjusts the initiation and termination of burst pulses V 2  by the gate circuit. The current feedback loop of FIG. 3 eliminates the need for the variable resistor R 2  adjustment of FIG.  2 . 
     The opto-coupler IC 2  conducts whenever secondary voltage V 3  is above a reference voltage developed across D 7 . Conduction by the opto-coupler IC 2  reduces the reference voltage for the emitter of transistor Q 1  via current I 1  in the feedback loop, which reduces the on time of the free running oscillator circuit  22 . As a consequence, the input power is reduced when load decreases, and the voltage controlling potentiometer R 2  in the circuit embodiment of FIG. 2 is unnecessary. 
     FIG. 4 is a graph of the range of input power versus output power demonstrating the increased efficiency provided by the invention. An ordinary power supply will ordinarily consume 1 W to output 200 mW, representing a 20% power conversion efficiency. As the graph of FIG. 4 demonstrates, for example, that with the inventive gating on of momentary low voltage mains an input mains voltage power Pinput of approximately 337 mW is converted to standby power of approximately 115 mW. This represents an increase in power conversion efficiency to approximately 30%. 
     The standby transformer TR 1  may be constructed using an EF16, N67 core with an air gap equal to approximately 0.1 mm. The inductance of the primary winding n 1  of the standby transformer Tr 1  may be equal to approximately 18 mH, using approximately 160 turns, in two layers, of 0.1 mm diameter CuL wire. 
     Approximately one layer of 0.1 mm thickness MYLAR® brand polymeric film may be used to provide electrical isolation between the two layers of wire to reduce parasitic capacitance. The secondary winding n 2  may use 23 turns of 0.315 mm diameter CuL wire, and the secondary winding n 3  may use 16 turns of 0.315 mm diameter wire. Approximately 2 layers of 0.1 mm thickness MYLAR® brand polymeric film may be used to provide electrical isolation between the primary winding n 1  and the secondary windings n 2  and n 3 . 
     It will be apparent to those skilled in the art that, although the invention has been described in terms of specific examples, modifications and changes may be made to the disclosed embodiments without departing from the essence of the invention. For example, in the embodiment discussed portions of positive half-wave voltage levels between 0 and 12V were shown to be passed to the oscillator circuit  23 . However, the inventive Ac mains initiated and termination of burst pulses could be practiced with a threshold range of 2V to 12V. However, the 0 to 12V range is preferable because the lower zero boundary makes the circuit design simpler. Also, in lieu of the preferred gating of positive half-waves from the Ac mains, full wave rectified pulses of the AC mains could be gated to the oscillator circuit  23 . However, gating of full wave rectified AC mains pulses would require dissipating excess power, not needed for standby mode operation, thereby making the power supply circuit less efficient. Accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicating the true scope of the invention.