1. Field of the Invention
The present invention relates to a stabilized power circuit, and, more particularly to a switching regulator in a pulse width control system.
2. Description of the Prior Art
Prior switching regulators of this type, which operate in a line operation system, adopt the following control methods: one method, the so-called variable frequency method wherein a duty cycle is changed while the pulse width is kept unchanged; and another method, the so-called pulse width control method wherein a pulse width is changed while the frequency is kept unchanged. Circuits to execute these methods are illustrated in FIGS. 6 and 9, respectively.
Both circuits are basically arranged as follows. A fraction of a DC output from the circuit using either the variable frequency method or the pulse width control method is fedback to a control circuit 1. In this control circuit 1, the fraction of the DC output is compared in an error amplifier 7 with a reference voltage and outputted to a V/F converter 9 (in the case of the variable frequency method as illustrated in FIG. 6) or a pulse width converter 8 (in case of the pulse width control method as illustrated in FIG. 9). The V/F converter 9 or the pulse width converter 8 modulates an output signal from an oscillator (not shown) with respect to its frequency (in case of the former method) or with respect to its pulse width (in case of the latter) in response to the output from the error amplifier 7 or 8. (The oscillator circuit, in the above description, may sometimes be omitted in a self-exciting system). The output signal from this control circuit 1 is then fed to a driver circuit 2 composed of a driver and a current transformer to permit the driver circuit 2 to drive for error correction a switching part 3 composed of high voltage, high speed switching transistors, called a main switcher in their combination, and of a pulse transformer. As a result, a main signal is fed from an input circuit 6, which serves to supply energy to this apparatus, to the pulse transformer of the switching part 3 and is error corrected. Thereafter, the error corrected main signal is boosted to a higher pulsed voltage by the pulse transformer, the high frequency transformer, and is outputted from a DC output terminal after passing through a rectifier part 4, which serves to rectify the pulsed voltage, and through a filter part 5 for smoothing the rectified/voltage.
The variable frequency method illustrated in FIG. 6 provides the V/F converter 9 in the control circuit 1, i.e., an A/D converter part, which permits the main switcher (composed of power transistors) to have its on-time kept unchanged but to have its switching frequency changing in comformity with the magnitude of the load used, for the purpose of keeping the output from the switching regulator constant. This is shown in FIG. 8 wherein the number of on-pulses for each unit of time, i.e., the repetition frequency of those pulses is increased as the load on the output becomes heavy, while the number of pulses is reduced as the load becomes light, to thereby keep the output constant. As a result, the operating frequency may fall into an audio range of frequency at no load.
In contrast with this variable frequency method, the pulse width control method illustrated in FIG. 9 provides the pulse width converter circuit in the control circuit 1, i.e., in the A/D converter part, which keeps the operating frequency constant at all times, and changes the on-time in the period T, thereby making the output constant. This is shown in FIG. 11 wherein the width of the on-pulse is increased as the load on the output becomes heavy, while the pulse width is made narrow at a light load, to thereby keep the output voltage constant.
In the following discussion, the operation of the switching regulator in the line operation system described above will be described with reference to FIGS. 7 and 10.
AC voltage is input into the input circuit 6, rectified through a rectifier, and smoothed through a filter (capacitor input type). The smoothed high DC voltage serves to bias the switching part 3.
The bias voltage from the input circuit 6 is converted to a high AC voltage with a frequency of from 20 KHz to 40 KHz by the switching transistors of the switching part 3 and transmitted to the rectifier part 4 on the secondary side through the pulse transformer.
The rectifier part 4 rectifies the high frequency AC voltage through fast recovery time rectifying diodes and supplies it to a load as DC voltage with a reduced ripple fraction through the filter part 5.
The following is a description as to how to stabilize the output from the switching regulator. The error amplifier 1 connected to the output terminal senses constantly the output voltage at the output terminal. The error amplifier 7 compares the output voltage with the reference to detect a fraction of error and amplifies the error signal. The error signal thus amplified is transmitted to the next pulse width converter 8 (in the pulse width control system) or to the V/F converter 9 (in the frequency control system), both belonging to the control circuit 1, to provide a control signal. These converters include a fixed frequency oscillator in the pulse width control system and a variable frequency oscillator in the frequency control system.
The control signals output from these converters are divided respectively into two phases through a two-phase divider circuit when the switching part 3 is of a push-pull type, while it is transmitted to the driver circuit 2 when the switching part 3 is single-ended.
The driver circuit 2 drives the switching transistors of the switching part 3, and mostly serves to insulate the primary side (input side) from the secondary side (output side). The switching transistors of the switching part 3 driven by the driver circuit 2 control the DC output using the control signal for the stabilization thereof.
The flow of the control signal forms a closed loop as described above, and the input power from the input circuit flows to the output side.
The following is a discussion of another example of the switching regulator in the pulse width control system and will be described with reference to a circuit block diagram of FIG. 12. The system controls the DC output from the switching regulator by detecting an error voltage from a third winding wound around the same transformer core as that of the secondary winding.
A switching element 10, a MOS FET in this embodiment, is switched on by allowing a voltage divided by a resistor 11 for starting the power circuit together with a resistor 12 to be applied to the drain and source thereof, to thereby start the conduction of a current through the primary winding 13 of a transformer 32. A resistor 14 detects the current. Resistors 15, 16 and transistor 17 constitute an overcurrent protector. The overcurrent protector serves to switch the switching element 10 off by switching the transistor 17 on using the voltage across the resistor 14 upon the appearance of the overcurrent. Thus, the switching element 10 has its gate potential lowered and is switched off.
Resistors 18, 19, a diode 20, and capacitors 21, 22 constitute a driver circuit for the switching element 10. The driver circuit serves to rapidly switch the switching element 10 on, upon the current starting to flow through the third winching 23 by supplying the voltage produced across the third winding 23 to the gate of the switching element 10 through a differentiation circuit composed of a capacitor 22 and a resistor 12. The driver circuit thereafter continues to supply the voltage and current produced across and through the third winding 23 to the gate of the switching element 10 via the resistors 19, 18, the diode 20, and the capacitor 21.
The switching element 10 is switched off by allowing the gate potential thereof to be lowered owing to the drop of the voltage across the tertiary third winding 23 the drop being produced by the reduction of a change in the current flowing through the primary winding 13 caused by a change in output impedance of the switching element 10 defined by the overcurrent protector circuit or the gate potential of the switching element 10.
A control part for effecting the stabilization of the output voltage includes resistors 24, 25, capacitors 26, 27, and a shunt regulator 28. The control part serves to make the tertiary winding voltage (the voltage across a capacitor 30) produced by a diode 29 and the capacitor 30 constant. The reason why the tertiary voltage is made constant is that the tertiary winding 23 is wound around the same transformer core as that of the secondary wiring 31 to couple with the latter magnetically to result in the output voltage being kept constant.
The control part in operation forces the shunt regulator 28 to change its cathode current such that voltage divided by the resistors 24 and 25 becomes constant. It accordingly lowers the tertiary voltage when it is high, by dropping the gate potential of the switching element by increasing the current absorption by the gate of the switching element 10 to thereby drop the gate potential of the same and hence by lowering the current flowing through the switching element 10. In opposition, it raises the tertiary voltage when it is low, by reducing the current absorption by the gate of the switching element 10 to thereby raise the gate potential of the same and hence by increasing the current flowing through the same.
The capacitor 27 here serves as an integrating factor. That is, it forces the control circuit to operate with the average of the tertiary voltage. The resistor 25 thereupon acts as a differentiation factor for compensating for the phase characteristic of the control circuit delayed by the capacitor 27.
Now, energy stored in the transformer 32 during the time the switching element 10 is on forwardly biasses a diode 33 on the side of the secondary winding 31 to charge the capacitor 30 and likewise forwardly biases a diode 29 on the side of the tertiary winding 23 to charge the capacitor 30, by allowing the switching element 10 to be switched off. Simultaneously, the energy backwardly biases the gate potential of the switching element 10 to keep the switching element 10 off through the resistor 19, and the capacitors 21, 22.
When the transformer 32 releases completely the energy stored therein during the off period of the switching element 10, the back bias to the gate electrode of the switching element 10 is removed to permit the switching element 10 to enter the on period thereof. Thus, the voltage across the tertiary winding 23 produced owing to the current flowing through the primary winding 13 raises the gate potential of the switching element 10 via the capacitor 22 to rapidly switch the switching element 10 on. This operation is repeated thereafter.
The circuit described above is operated in the variable frequency mode which allows the switching element to have in its on-period changed depending upon the load capacity as well having its on-time changed of the same rate as that of the on-period. FIGS. 13 and 14 illustrate relationships between the on-periods and the on-times respectively in the pulse width control and variable frequency methods, and between load frequencies and gains of the switching regulator in the same methods.
Those prior methods described above however suffer from the following difficulties when the load varies widely:
(1) The pulse width control method is fixed in the frequency within the range of a high frequency, the output becomes impulsive with a light load, which the control element can not follow to result in increased loss and severely reduced efficiency.
(2) With the load made further light, the switching element might sometimes not be switched on to cause intermittent oscillation with its period being lower than an audio frequency (lower than 20 KHz) followed by an oscillation sound.
(3) The frequency fixed on the side of a lower frequency in the pulse width control method prolongs the on-time with a heavy load, and hence requires large-sized transformers and filters, etc., to make it difficult to provide a compact power supply device.
(4) In the variable frequency method, reduction of the on-period with a light load shortens the on-time to cause the foregoing difficulties (1) and (2).
(5) Further, in the variable frequency method, a heavy load causes a frequency lowering to an audio frequency with an oscillation sound produced thereby.
(6) The control method shown in FIG. 12 effects the control on an average since the error voltage taken from the tertiary winding voltage is fedback using an integrated waveform. It accordingly allows disadvantageously the stability of the output voltage to be deteriorated for transient variations in the load.
(7) The control circuit composed of analog elements makes it difficult to yield them in the form of an LSI device, and thus is prevented from being small-sized.