JPS6024665B2 - switching power supply circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to switching power supply circuits used in various electronic devices, and in particular to preventing abnormal operation of the switching power supply circuits due to fluctuations in the oscillation frequency for pulse width modulation signals.

FIG. 1 shows an example of a conventional switching power supply circuit. In FIG. The output side of the circuit 3 is grounded via the primary winding 4a of the transformer 4 and the main electrode of the switching transistor 5, and the secondary winding 4b of the transformer 4 is used to derive a stabilized DC output voltage to the load. rectifier circuit 6
connected to.

The output side of the rectifier circuit 6 is also connected to an error detection circuit 8 via a detection circuit 7 for detecting a part of its DC output voltage, and in this error detection circuit 8 the reference voltage and the DC voltage from the detection circuit 7 are connected. are compared, and the error signal is supplied as a pulse width control signal to the pulse width modulator 10' through an isolator 9 such as a photocoupler or transformer that insulates the primary and secondary sides of the transformer 4. The pulse width modulator 1 forms, together with the oscillation circuit 11 and the driver 12, a control circuit that controls the switching operation of the switching transistor 5. Further, the pulse width modulator 10 receives a pulse width control signal supplied through the isolator 9 and an oscillation circuit 11.
A pulse width modulation (PWM) signal is supplied to the base of the switching transistor 5 through the driver 12 to control its on-time, thereby increasing the DC voltage obtained at the output side of the rectifier circuit 6. Keep constant.
Furthermore, the pulse width modulator 10, oscillation circuit 11, and driver 12 forming the control circuit are connected to a control circuit power supply 13 connected to the output side of the rectifier circuit 3, respectively, so as to receive the power. Incidentally, an oscillation circuit generally has voltage dependence, and its oscillation frequency changes as the power supply voltage increases.

Therefore, in the conventional switching power supply circuit configured as described above, when the power switch 2 is turned on, the oscillation circuit 11 is also activated immediately, so the switching power supply circuit is operated immediately after the power switch 2 is turned on until the steady state is reached. This caused problems such as abnormal switching operation and stability of circuit operation. Furthermore, it is conceivable to stop the oscillation circuit 11 until the main power supply or the control power supply reaches a steady state, and then start the oscillation circuit 11, but the above-mentioned problem is still difficult to solve, and especially when the oscillation frequency is high, for example There was a drawback that the operation was unstable even at 30kHz to 2nd level from Misaki. The present invention has been made to eliminate these conventional drawbacks, and provides a switching power supply circuit that can prevent abnormal operation when the power is turned on, and can always ensure the stability of the circuit system.

Embodiments of the present invention will be described in detail below with reference to FIGS. 2 to 4.

In each figure, parts corresponding to those in FIG. 1 are denoted by the same reference numerals, and detailed explanation thereof will be omitted. FIG. 2 shows an embodiment of the switching power supply circuit of the present invention.
The difference from the configuration in the figure is that a voltage detection circuit 14 for detecting the output voltage of this power supply and generating a reset pulse after a predetermined time is connected to the output side of the control circuit power supply 13, and the output of the voltage detection circuit 14 is connected to the output side of the control circuit power supply 13. The output terminal of the counter 7 is connected to the reset input terminal of the counter 7, and the output terminal of the counter 7 is connected to the pulse width modulator 10.

Note that parts corresponding to those in FIG. 1 are denoted by the same reference numerals, and repeated explanation thereof will be omitted. Next, the operation of the switching power supply circuit according to the present invention will be explained with reference to the characteristic diagram shown in FIG.

Now, when the power switch 2 is turned on, the main power supply voltage +EB and the voltage 10Bc supplied from the control circuit power supply 13 change as shown in FIG.
B is connected to the control circuit power supply 13 and the transformer 4, and the voltage +E. are also supplied to the oscillation circuit 11, pulse width modulator 10, driver 12, and voltage detection circuit 14, respectively. Therefore, the oscillation circuit 11 starts operating, and the voltage detection circuit 14 also detects the output voltage of the control circuit power supply 13, and outputs the detected output voltage as shown in FIG. 3B to the counter 1.
7, but this counter 17 is reset, so the pulse width modulator 10 and subsequent stages in the next stage remain inoperative. Here, as shown in FIG. 3A, the time required for the oscillation circuit 11 to stabilize after being supplied with the power supply voltage and the time required for the voltage detected by the voltage detection circuit 14 to reach the predetermined voltage 10 Es is determined as L. Assuming that the relationship between time to and t is t>to, the output of the voltage detection circuit 14 becomes a low level at time t, as shown in FIG. 3B, and the reset pulse RP becomes It is not supplied to the reset input of counter 17. Therefore, the pulse from the oscillation circuit 11 is transmitted to the counter 1.
The frequency is divided by 7 and supplied to the next stage pulse width modulator 10'. Therefore, the original pulse width modulation action starts at the point when the pulse width modulator 10 is reached, and the pulse width modulator 10
A pulse width modulated signal is supplied to the base of the switching transistor 5 through the driver 12, and a switching operation is started. As a result, the switching power supply circuit can perform switching operations with the oscillation circuit always in a stable state, preventing abnormal operation when the power switch is turned on, and ensuring the stability of the circuit system itself. Further, the present invention has the advantage that the number of parts does not increase by configuring the counter 7 that frequency-divides the output pulse of the oscillation circuit 11 to be reset until the oscillation circuit 11 becomes stable. In each of the above embodiments, the case where the voltage of the control circuit power supply is detected and the output of the oscillation circuit is controlled is explained.
It goes without saying that the present invention is not limited to this, and similar effects can be obtained by detecting the main power supply voltage, for example.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram showing an example of a conventional switching power supply circuit, FIG. 2 is a configuration diagram showing an embodiment of the present invention, and FIG. 3 is a voltage characteristic diagram for explaining the operation of FIG. 2. . In the figure, 3 is a rectifier circuit, 5 is a switching transistor, 7 is a detection circuit, 8 is an error detection circuit, 9 is an isolator, 10 is a pulse width modulator, 11 is an oscillation circuit, 12 is a driver, and 13 is a control circuit. power supply, 14 is a voltage detection circuit, 1
7 is a counter. Figure 1 Figure 2 Figure 3

Claims (1)

[Claims] 1. In a switching power supply circuit that performs a switching operation by supplying a pulse width modulation signal from a pulse width modulator to a switching element in response to a pulse signal from an oscillation circuit and a pulse width control signal obtained by detecting a DC output voltage. , a voltage detection circuit is provided for frequency-dividing the pulse signal of the oscillation circuit by a counter and supplying it to the pulse width modulator and detecting the input power supply voltage, and when the detection level of the voltage detection circuit is below a predetermined value. A switching power supply circuit characterized in that the counter is reset so that the pulse signal from the oscillation circuit is not supplied to the pulse width modulator.