The present invention relates to a control method for controlling a switching power supply circuit that has a delay, for which switching is not conducted, to reduce the power consumption thereof when the load is light. Specifically, the present invention relates to a control method that facilitates reducing the transformer excitation noise and stabilizing the switching operations thereof.
Some conventional switching power supply circuits set a switching delay or pause, to stop the switching operations of the switching devices thereof for reducing the power consumption thereof when the load is light. However, the exciting current that flows through the transformer changes greatly at the changeover between the switching period and the switching delay, for which the switching is stopped, causing transformer excitation noise.
Japanese Patent document JP P 2002-136125 A discloses a switching power supply circuit that is directed to obviate the problems described above. The disclosed switching power supply circuit limits the peak value of the transformer exciting current to a certain value to reduce the transformer excitation noise.
FIG. 10 is a block circuit diagram of the switching power supply circuit disclosed in JP 2002-136125 A.
Referring now to FIG. 10, the disclosed switching power supply circuit includes a DC power supply 1, a first switching device 2, a control circuit 3, an output voltage feedback circuit 5, a transformer 6, a rectifying diode 7, a rectifying capacitor 8, a current detecting resistor 10, and a current limiting resistor 11. The control circuit 3 switches on and off first switching device 2. Output voltage feedback circuit 5 includes resistors 13, 14, 16, 19, and 20; a shunt regulator 15; a capacitor 17; and photocouplers 18a and 18b. Transformer 6 including a primary winding 6a, a secondary winding 6b, and a control winding 6c. Rectifying capacitor 8 and diode 7 smooth the output of the switching power supply circuit. A load 9 is also shown in FIG. 10.
The main circuit shown in FIG. 10 operates in the following manner. A voltage is generated via primary winding 6a across secondary winding 6b in transformer 6 by inducing the first switching device 2 switch on and off the DC voltage from power supply 1. The voltage generated across secondary winding 6b in transformer 6 is rectified by diode 7 and capacitor 8 and converted to a DC voltage. Output voltage feedback circuit 5 lowers the feedback signal thereof when the DC voltage described above rises so that the output voltage may be stabilized at a desired value.
FIG. 11 is a block circuit diagram of the control circuit shown in FIG. 10. Referring now to FIG. 11, control circuit 3 includes a driver circuit 101 that drives switching device 2, a start pulse generator circuit 104, an ON-OFF RS flip-flop 105 for switching ON and OFF, an ON-timing-signal outputting circuit 106, a normal- and waiting-modes-changeover-signal outputting circuit 107 for changing over the normal mode to the waiting mode or vice versa (hereinafter referred to simply as a “changeover-signal outputting circuit”), a waiting mode RS flip-flop 111 used for the waiting mode, a switching stop comparator 113 for stopping switching, a switching start comparator 115 for starting switching, a waiting mode comparator 121 used for the waiting mode, a normal mode comparator 123 used for the normal mode, a constant current supply 117, constant voltage supplies 112, 114, 116, and 122, AND gates 103, 124, and 126, a NOT gate 125, and an OR gate 127.
The control circuit shown in FIG. 11 operates in the following manner. First, the normal mode of operations when the load is light will be described.
As the power supply to control circuit 3 is established, start pulse generator circuit 104 outputs start pulses to set ON-OFF RS flip-flop 105. As ON-OFF RS flip-flop 105 is set, driver circuit 101 makes switching device 2 ON via the output terminal a of control circuit 3. As a current flows through switching device 2, a voltage is generated across current detecting resistor 10 and the voltage proportional to the current value is inputted to the input terminal d of control circuit 3.
Normal mode comparator 123 compares the current value signal inputted from the input terminal d of control circuit 3 with the feedback signal inputted to the input terminal b of control circuit 3. When the current value signal is higher than the feedback signal, normal mode comparator 123 outputs an OFF-signal (H-level). Since changeover-signal outputting circuit 107 is outputting a normal mode signal (L-level) at this timing, the output of AND gate 124 and the output of OR gate 127 are set at the H-level. As the output of OR gate 127 is set at the H-level, ON-OFF RS flip-flop 105 is reset. As ON-OFF RS flip-flop 105 is reset, driver circuit 101 switches off switching device 2 via the output terminal a. As the switching device 2 is switched off, a voltage is generated across secondary winding 6b in transformer 6 and energies are fed to capacitor 8 and load 9 through diode 7.
As the energy discharge is completed, the voltage generated across secondary winding 6b in transformer 6 reduces. At the same time, the voltage generated across control winding 6c in transformer 6 connected to the input terminal c also reduces. As ON-timing-signal outputting circuit 106 detects the trailing edge of the voltage generated across control winding 6c and inputted to the input terminal c, ON-timing-signal outputting circuit 106 sets ON-OFF RS flip-flop 105. As ON-OFF RS flip-flop 105 is set, driver circuit 101 switches on switching device 2 via the output terminal a. As a current flows through switching device 2 and the current value signal exceeds the feedback signal to the higher side, normal mode comparator 123 outputs an OFF-signal.
As the OFF-signal is outputted, ON-OFF RS flip-flop 105 is reset and driver circuit 101 switches off switching device 2. Control circuit 3 switches on switching device 2 in response to the ON-signal from ON-timing-signal outputting circuit 106 or the ON-signal from start pulse generator circuit 104. Control circuit 3 switches off switching device 2 in response to the OFF-signal from normal mode comparator 123. Control circuit 3 controls the ON-period of switching device 2 to stabilize the output voltage of the switching power supply circuit at a desired value.
Now, the waiting mode of operations when the load is light will be described below.
Switching start comparator 115 compares the feedback signal with the voltage from constant voltage supply 114. As the feedback signal exceeds the switching start voltage Vth (H) set by constant voltage supply 114 to the higher side, switching start comparator 115 resets waiting mode RS flip-flop 111. As waiting mode RS flip-flop 111 is reset, ON-OFFRS flip-flop 105 is released from the reset state thereof. As ON-OFF RS flip-flop 105 is released from the reset state thereof, it becomes possible to set ON-OFF RS flip-flop 105 and, therefore, switching is permitted.
As an ON-signal is outputted from start pulse generator circuit 104 or ON-timing-signal outputting circuit 106, ON-OFF RS flip-flop 105 is set. As ON-OFF RS flip-flop 105 is set, driver circuit 101 switches on switching device 2 via the output terminal a. As a current flows through switching device 2, a voltage is generated across current detecting resistor 10 and the voltage proportional to the current value is inputted to the input terminal d. Waiting mode comparator 121 compares the current value signal inputted from the input terminal d with the setting value set in constant voltage supply 122. As the current value signal exceeds the setting value to the higher side, waiting mode comparator 121 outputs a turnoff signal (H-level).
Since changeover-signal outputting circuit 107 is outputting a waiting mode signal (H-level) at this time, the output of AND gate 126 and the output of OR gate 127 are set at the H-level. As the output of OR gate 127 is set at the H-level, ON-OFF RS flip-flop 105 is reset. As ON-OFF RS flip-flop 105 is reset, driver circuit 101 switches off switching device 2 via the output terminal a. During the period, for which waiting mode RS flip-flop 111 is in the reset state thereof, control circuit 3 makes switching device 2 conduct switching operations thereof at a certain peak current value.
Switching stop comparator 113 compares the feedback signal with the setting value of constant voltage supply 112. As the feedback signal exceeds the switching stop voltage Vth (L) set by constant voltage supply 112 to the lower side, switching stop comparator 113 sets waiting mode RS flip-flop 111. As waiting mode RS flip-flop 111 is set, ON-OFF RS flip-flop 105 is reset. As ON-OFF RS flip-flop 105 is reset, it becomes impossible to set ON-OFF RS flip-flop 105 and switching is inhibited. When waiting mode RS flip-flop 111 is in the set state thereof, control circuit 3 does not make switching device 2 conduct switching operations and keeps stopping the switching operations of switching device 2 until the feedback signal exceeds the switching start signal Vth (H) to the higher side.
FIG. 12 is a wave chart describing the operations of the conventional switching power supply circuit shown in FIG. 10. FIG. 12 shows the output voltage waveform when the load is light, the feedback signal waveform, the output waveform of waiting mode RS flip-flop 111, and the exciting current waveform of transformer 6.
As shown in FIG. 12, the variation of the feedback signal changes the state of waiting mode RS flip-flop 111. When waiting mode RS flip-flop 111 is in the set state, control circuit 3 conducts the switching operations thereof at a certain peak current value. Control circuit 3 controls the burst duty determined by the ratio of the switching period and the switching delay to stabilize the output voltage at a desired value.
As described in detail above, the switching power supply circuit disclosed in the Patent Document 1 limits the peak transformer exciting current to be lower than a certain value to reduce the transformer excitation noise in the waiting mode of operations. However, when the load becomes heavier than assumed, the energies fed to the secondary side of the transformer are insufficient, causing a low output voltage.
In view of the foregoing, it would be desirable to provide a control system for controlling a switching power supply circuit that facilitates preventing the output voltage of the switching power supply circuit from lowering even when the load becomes heavier than assumed in the waiting mode of operations.
Further objects and advantages of the invention will be apparent from the following description of the invention.