1. Field of the Invention
The invention relates to the field of switched mode power supplies for apparatus having a run mode and a standby mode of operation, such as a television receiver. In particular, the invention relates to the field of controlling auxiliary power supplies in such apparatus when changing between the run and standby modes of operation.
2. Description of Related Art
In a typical run/standby power supply, for example as used in television receivers, a bridge rectifier and a filter capacitor provide a raw DC voltage (called the B.sup.+ voltage, or raw B.sup.+), whenever the power supply is coupled to the domestic mains. Standby mode loads can be powered directly from the B.sup.+ voltage or from another voltage that is always present. Many run mode loads, however, are powered through a voltage regulating supply such as a switched mode supply, that operates only in the run mode. The run mode power supply for certain loads, such as the deflection circuits and high voltage screen loads, typically employ the flyback transformer that powers beam deflection. A separate or auxiliary power supply also can be operated as a switched mode supply and may provide a regulated B.sup.+ voltage for the flyback transformer as well as other auxiliary supply voltages.
Projection televisions, for example, have particularly demanding power needs because they have three high power cathode ray tubes (CRTs). An auxiliary power supply is useful to power the convergence amplifiers for the tubes, two such amplifiers generally being required for each CRT. These amplifiers require positive and negative polarity voltages and can dissipate substantial power.
In a switched mode supply, an input DC voltage (such as the B.sup.+ voltage in a television) is coupled to one terminal of a primary winding of a transformer and the other terminal of the primary winding is coupled to a switching device, such that current is coupled to the transformer when the switching device conducts. The switching device is alternately turned on and off during the run mode of operation, providing alternating currents in secondary windings of the transformer, which are rectified and filtered to provide run mode supply voltages.
Regulation of the output voltages is achieved by feedback control provided by, for example, a feedback winding of the transformer. The respective secondary windings are closely coupled, so that load variations on any of the secondary windings are reflected on the feedback winding. The feedback control compares a voltage on the feedback winding with a standard or threshold voltage level, which may be provided by the switching device, and modulates the frequency and/or pulse width at which the switching circuit is turned on and off. The switching device is compensated to render it insensitive to variation of the raw B.sup.+ input voltage, while maintaining accurate output voltage levels as current loading varies over a nominal range of power consumption.
The switching device for a power supply as described can be integrated circuit (IC) power supply controller from the Sanyo STK730 series. This controller includes a FET power switching transistor, an error amplifier and driver, and an over-current protection circuit in a single package. When coupled into a switched mode supply and first turned on, current from the B.sup.+ voltage flows to ground through the primary winding of the transformer, the FET and a current sensing resistor. Current increases until the over-current protection circuit in the controller IC is triggered, whereupon the IC controller turns off its FET power transistor. Energy is transferred to the secondary windings of the transformer, where the induced AC current is rectified and charges filter capacitors. After a starting interval of several cycles, the output voltage reaches its regulated level. A threshold comparison circuit provided by the IC controller is coupled to a feedback winding of the transformer and controls the timing of switching by the control IC to maintain the regulated output voltage level. Oscillation stabilizes at a frequency and duty cycle that accommodate the loads coupled to the secondary windings. Many other power supply controllers operate in a similar fashion and may be used instead of the Sanyo STK730 series.
Such an IC controller will attempt to start whenever the raw B.sup.+ voltage is present. Other switched circuits control switching between the standby mode and the run mode.
The FET power transistor in an IC controller such as the STK-730 series can be coupled in part to the voltage or current signals generated on the primary winding by operation of the switching circuit, which advantageously causes the switching transistor to be driven harder as current is built up in the primary winding. The switching transistor needs a positive voltage on its gate drive input in order to begin conducting, and may not conduct dependably or may be switched off if the control voltage is too low. One way for preventing operation of the switching transistor is to provide a means to reduce the voltage at the control input. This may appear to be a possible way to switch between the on/off or run/standby modes of the power supply. However, the raw B.sup.+ voltage is present whether in the run mode or the standby mode (provided the auxiliary power supply is coupled to the domestic mains), and the raw B.sup.+ voltage is the only voltage present during standby.