The present invention relates to a switching power supply which has a high efficiency and improves the cross-regulation and regulation of an output voltage.
Recent electronic equipment often requires a multi-output type power supply as a power supply to be incorporated therein. This is because various kinds of recently-developed electronic devices and active parts often require a plurality of driving voltages or control voltages having different voltage values.
Assume a fluorescent display tube as an electronic device requiring a plurality of driving voltages or control voltages for its operations.
A multi-output type switching power supply having the circuitry shown in FIG. 1 has been available as a power supply for supplying power to the fluorescent display tube.
The switching power supply PS shown in FIG. 1 generally operates as described below.
When an input voltage V.sub.IN is applied to a series circuit composed of a primary winding N1 of a transformer T1 and a switching transistor Q1 via input terminals 1a and 1b, the circuit shown in FIG. 1 starts operating. At this time, the switching transistor Ql turns on or off alternately in response to a pulse signal sent from a control circuit CONT. This causes a current passing through the primary winding N1 to be conducted or cut off. Repetition of conduction and cutoff of a current induces a voltage in each winding of the transformer T1.
A synthetic voltage of the voltage induced in the primary winding N1 and the input voltage V.sub.IN arises at a node between the primary winding N1 and an anode of a diode D1. The synthetic voltage is rectified and smoothed by the combination of the diode D1 and a capacitor C2. A dc voltage present at the terminals of the capacitor C2 is conducted as a first output voltage V.sub.01 to first output terminals 2a and 2b. Likewise, a voltage induced in a secondary winding N2 is rectified and smoothed by the combination of the diode D2 and a capacitor C3. A dc voltage present at the terminals of the capacitor C3 is conducted as a second output voltage V.sub.02 to second output terminals 3a and 3b. The first output voltage V.sub.01 and second output voltage V.sub.02 are supplied to a drive circuit DR for display-controlling a fluorescent display tube FIT and to the filament of the fluorescent display tube FIT.
The first output voltage V.sub.01 is controlled by the control circuit CONT for detecting a dividing voltage of the voltage V.sub.01 arising at a node between resistors R1 and R2 and varying the on-duty of the switching transistor Q1 according to the dividing voltage so that the first output voltage V.sub.01 will remain constant. By contrast, the second output voltage V.sub.02 is theoretically held constant due to magnetic coupling of the transformer T1 composed of the primary winding N1 and secondary winding N2 as long as the first output voltage V.sub.01 remains constant.
On lighting, the fluorescent display tube FIT requires potential differences between the filament and anode and between the filament and grid, respectively. The circuit shown in FIG. 1 includes a circuit portion in which a resistor R3 and constant voltage diode DZ are connected in series with each other between the first output terminals 2a and 2b, and the cathode of the constant voltage diode DZ is connected to one terminal of the filament. Due to the circuit portion, a potential difference corresponding to a Zener voltage for the constant voltage diode DZ is obtained between the filament and anode and between the filament and grid, respectively.
In the circuit shown in FIG. 1, when the fluorescent display tube FIT is in operation, the first output voltage V.sub.01 of the switching power supply PS is supplied to the drive circuit DR, and the second output voltage V.sub.02 is supplied to the filament of the fluorescent display tube FIT. At this time, a current flowing into the drive circuit DR shall be a current I.sub.01 , and a current flowing into the filament of the fluorescent display tube FIT shall be a current I.sub.02.
Herein, we will explain a status of the circuit in which the states of loads that are coupled to the output voltages of the switching power supply PS are changed.
The filament of the fluorescent display tube FIT is applied to a load that remains nearly constant even when a display state is changed, while the drive circuit DR is applied to a load whose magnitude varies with a change in the display state. In particular, in controlling the display of the fluorescent display tube, a state in which the luminance of the display surface is degraded, which is referred to as "dimmer mode", may take place depending on ambient brightness. At this time, the drive circuit DR serves as a quite light load. If the state of the load changes, the current I.sub.01 varies. The value of the current I.sub.01 in the dimmer mode becomes very small compared with the one in the display state in which luminance is high.
As mentioned previously, the second output voltage V.sub.02 is theoretically held constant owing to magnetic coupling between the windings of a transformer as long as the first output voltage V.sub.01 remains constant. However, in practice, the phenomenon takes place in which even when the first output voltage V.sub.01 is controlled to remain constant by means of the control circuit CONT, if the value of the current I.sub.01 becomes very small, the second output voltage V.sub.02 decreases. This phenomenon is realized to change as described below according to a coupling coefficient observed between the first winding N1 and secondary winding N2 of the transformer T.
First, as the coupling coefficient falls below 1.0, the second output voltage V.sub.02 decreases to have a much smaller voltage value than an original voltage value.
However, when the coupling coefficient falls below a certain value, as the coupling coefficient decreases, the second output voltage V.sub.02 rises gradually and the voltage value thereof approaches the original voltage value.
Incidentally, the stability of the output voltage V.sub.02 against the variation of the output current I.sub.01 flowing into output terminals (2a, 2b) other than the output terminals (3a, 3b) to which the output voltage V.sub.02 is applied is referred to as cross-regulation. The larger a magnitude of variation of a voltage (magnitude of decrease) is, the poorer the cross-regulation.
Experimentally, it is known that cross-regulation exhibited in the switching power supply can be improved by setting the coupling coefficient observed between the windings of a transformer employed in the power supply to a value very close to 1.0 (for example, 0.99) or to a value far away from 1.0 (for example, 0.85). However, it is almost impossible to set the coupling coefficient observed between the windings of an actual transformer to 1.0. It is also very hard to set the coupling coefficient to a value very close to 1.0. Incidentally, a coupling coefficient observed between the windings of a typical transformer that is available is lower than 1.0 (about 0.95). Typically, when a switching power supply exhibiting high cross-regulation is needed, a coupling coefficient observed between the windings of a transformer has been set low.
However, a transformer which has a low coupling coefficient observed between the windings thereof has a drawback that a power loss of the transformer is larger than the one of a transformer which has a high observed coupling coefficient. This poses a first problem that a switching power supply using a transformer which has a low coupling coefficient observed between the windings thereof is poor in efficiency.
Moreover, the state of a load connected between the output terminals 3a and 3b of the power supply PS is, differing from the filament, not always constant. In the case that the load connected between the output terminals 3a and 3b is not a fluorescent display tube, the state of the load may change greatly. In this case, there arises a second problem that in a switching power supply using a transformer which has a low coupling coefficient observed between the windings thereof, the second output voltage V.sub.02 varies greatly with the variation of the current I.sub.02.
Incidentally, the stability of the output voltage V.sub.02 against the variation of the output current I.sub.02 flowing into the same output terminals as the output terminals to which the output voltage V.sub.02 is applied is referred to as regulation. The larger a magnitude of variation of a voltage is, the poorer the regulation is.
In the case of a multi-output type switching power supply having the circuitry shown in FIG. 1, at least one of the cross-regulation, efficiency, and regulation is poor according to the coupling coefficient observed between the windings of a transformer employed.