JPS644307Y2 - - Google Patents

Description

[Detailed Description of the Invention] Industrial Application Field The present invention relates to a control circuit for a switching power supply.

Prior Art FIG. 1 is a circuit diagram showing a known basic configuration of a control circuit for a switching power supply. To briefly explain the configuration and operation of such a control circuit, a switching transformer 5 is constructed in which power supplied from an AC power supply 1 is rectified by a rectifier 2, smoothed by a capacitor 3, and connected in series with a switching element 4 such as an FET. is supplied to the primary coil 51 of. The switching element 4 is repeatedly turned on and off by a gate drive pulse signal generated by a pulse width control circuit 82. The pulse current generated in the secondary coil 52 of the switching transformer 5 by turning on and off the switching element 4 is smoothed by the rectifying filter circuit 6 composed of diodes 61 and 62, a reactor 63, and a capacitor 64, and then passed to the load 7.
supplied to 81 is a constant voltage control circuit for keeping the voltage applied to the load 7 constant;
is a current limiting circuit that uses a current detection resistor 83 to protect the switching power supply from short circuits and overloads of the load 7. 85 is a control power source for the current limiting circuit 80. The pulse width control circuit 82
The pulse width of the pulse signal is controlled according to each output signal of the constant voltage control circuit 81 and the current limiting circuit 80, and thereby the voltage and current supplied to the load 7 are controlled. In addition, resistor 101, capacitor 10
2. The circuit composed of the diode 103 is an excitation energy processing circuit for the switching transformer 5. Here, resistor 91 and Zener diode 92
is a circuit for supplying voltage to the pulse width control circuit 82, and a constant voltage obtained by the Zener diode 92 is supplied to the pulse width control circuit 82, but in such a circuit, the voltage of the capacitor 3 is Then, the voltage of Zener diode 92 is low,
For this reason, the resistor 91 has a disadvantage in that it consumes a large amount of power.

The following circuits are known as circuits that overcome these drawbacks. Figure 2 is JP-A-55-
This circuit is disclosed in Japanese Patent Laid-Open No. 53182 and Japanese Patent Application Laid-Open No. 183268/1983. To briefly describe the configuration and operation of this circuit, in FIG. , 9 is a rectifier circuit. At startup, the voltage of the Zener diode 92 obtained from the rectifier 2 and the capacitor 3 via the resistor 96 is supplied as the base voltage of the transistor 93 via the resistor 95 to turn on the transistor 93.
As a result, the voltage of the capacitor 3 is reduced by the resistor 91 and the internal impedance between the collector and emitter of the transistor 93, and is supplied to the pulse width control circuit 82. When the pulse width control circuit 82 generates a pulse signal and the switching element 4 starts operating, a voltage is generated in the tertiary coil 53, this voltage is rectified by the diode 97 and smoothed by the capacitor 98, and then the pulse width control circuit 82.
When the voltage of capacitor 98 increases, capacitor 9
8 to the Zener diode 92 via the diode 94 and the resistor 95, the transistor 93 is turned off, and the pulse width control circuit 82
Only the power from the tertiary coil 53 is supplied to the tertiary coil 53. In this example, the configuration of the rectifier filter circuit 6 is a capacitor input type, so the load 7
If the voltage of the capacitor 98 is controlled at a constant voltage, the voltage of the capacitor 98 becomes a substantially constant voltage, and a stable voltage can be supplied to the pulse width control circuit 82. By the way, the capacitor input type rectifier filter circuit 6 is not economical for a switching power supply with a large capacity, and when a switching power supply with a large capacity is used, diodes 61, 62, reactance 63, and A circuit consisting of a capacitor 64 (called a forward converter),
Bridge configuration circuits (not shown) having a plurality of switching elements are known to be advantageous. However, when such a rectifying filter circuit is used, there is no proportional relationship between the voltage of the load 7 and the voltage of the capacitor 98, so even if the voltage of the load 7 is controlled to a constant value, the voltage of the capacitor 3 and the size of the load 7 will change. As a result, the voltage of the capacitor 98 changes greatly, and as a result, it becomes difficult to supply a stable operating voltage to the pulse width control circuit 82.

Purpose of the invention The present invention was made under these circumstances, and is an economically advantageous switching power supply control circuit that can always supply a stable operating voltage to the pulse width control circuit 82. This is what we are trying to provide.

Summary of the invention The feature of the invention is that a diode for stabilizing the voltage of the smoothing capacitor 98 is provided between the output end of the rectifier circuit 9 and the cathode of the Zener diode 92, and the voltage of the capacitor 98 is The voltage is limited to the sum of the Zener diode 92 and the voltage of the diode, and a resistor is further provided on the output side of the rectifier of the rectifier circuit 9 to suppress the peak value of the charging current of the capacitor 98.

Embodiments Examples of the present invention will be described below with reference to the drawings.

FIG. 3 is a circuit diagram showing an embodiment of the present invention, and the same reference numerals as in FIG. 2 indicate the same or equivalent parts. To explain the main part of this embodiment, 20 is a first rectifier circuit, and between this rectifier circuit 20 and a pulse width control circuit 82, an operating voltage is supplied to the pulse width control circuit 82 at the time of starting. A constant voltage circuit 90 is provided, and this constant voltage circuit 90 includes a resistor 96 and a Zener diode 9.
2 in series, and a starting NPN transistor 93 whose base is connected to the connection point of the resistor 96 and the cathode of the Zener diode 92, and whose emitter is connected to the pulse width control circuit 82 via the power supply line 10A. . The tertiary coil 53 of the switching transformer 5 is connected as a rectifier to the line 10A via a diode 97 and a resistor 99, and a second rectifier circuit 9 is connected together with the diode 97 between the line 10A and the ground line 10B.
A smoothing capacitor 98 is provided. The resistor 99 is for suppressing the peak value of the charging current of the capacitor 98. A diode 94 for stabilizing the voltage of the capacitor 98 is provided between the connection point of the capacitor 98 and the resistor 99 and the cathode of the Zener diode 92 so that its cathode is connected to the cathode of the Zener diode 92.

To explain the operation of the embodiment with such a configuration, the voltage supplied from the AC power supply 1 is rectified by the rectifier 2, smoothed by the capacitor 3 to become a DC voltage,
This DC voltage is applied to the primary coil 51 and the switching element 4. On the other hand, this DC voltage is connected to the resistor 96
The voltage obtained by the Zener diode 92 is applied between the base and emitter of the transistor 93. Since no voltage is generated in the tertiary coil 53 at the time of starting, the transistor 93 is turned on, and the voltage of the capacitor 3 is stepped down by the internal impedance between the collector and emitter of the transistor 93 and is supplied to the pulse width control circuit 82.
As a result, when the pulse width control circuit 82 generates a pulse signal for gate drive, the switching element 4 performs an on/off operation, and a voltage is generated in the tertiary coil 53. This voltage is rectified by a diode 97, stepped down by a resistor 99, and smoothed by a capacitor 98. In this way, the voltage of the capacitor 98 increases, but by setting the voltage of the capacitor 98 after steady operation to exceed the voltage of the Zener diode 92, the transistor 93 is turned off and the pulse width control circuit 82 is turned off. is supplied with a voltage smoothed by a capacitor 98. Since the diode 94 is provided, the capacitor 98
When the voltage increases, current from the capacitor 98 flows to the Zener diode 92 via the diode 94. As a result, the pulse width control circuit 8
2 is supplied with a voltage equal to the sum of the voltage of Zener diode 92 and the forward drop voltage of diode 94. On the other hand, switching transformer 5-2
The pulse current generated in the next coil 52 is transferred to the rectifier circuit 6.
The rectified and smoothed signal is supplied to the load 7. Then, the pulse width of the pulse signal from the pulse width control circuit 82 is controlled according to each output signal of the constant voltage control circuit 81 and the current limiting circuit 80, and the voltage and current supplied to the load 7 are controlled.

Experimental Results Fig. 4 shows how the voltage of capacitor 98 changes with respect to the voltage change of capacitor 3 for the conventional circuit shown in Fig. 2 and the circuit of the present invention shown in Fig. 3. This shows experimental results, with the voltage of capacitor 98 plotted on the vertical axis and the voltage of capacitor 3 plotted on the horizontal axis. In the same figure, the solid line A indicates the characteristics related to the conventional circuit, and the dotted line B
shows the characteristics of the circuit of the present invention. From these results, it can be seen that in the present invention, the voltage of the capacitor 3 is stabilized against changes in the voltage of the capacitor 3, and the operating voltage of the pulse width control circuit 82 is stabilized, compared to the conventional one.

Furthermore, when comparing the power consumption of the conventional circuit shown in FIG. 1 and the circuit of the present invention shown in FIG. When , the first
In the circuit shown in the figure, if the current in the Zener diode 92 is zero, power equivalent to 150-15 = 135 (V) is consumed in the resistor 91, whereas in the circuit shown in FIG. For example, if the tertiary coil is selected so that the voltage of the capacitor 98 is 15V, the power loss will be the loss corresponding to the drop voltage of the diode 97 and the resistor 99, and the tertiary coil. The loss corresponds to the generated voltage of 53, which is considerably smaller than the power loss in the circuit of FIG.

Effects of the invention According to the invention, the Zener diode 9 constituting the output terminal of the second rectifier circuit 9 and the constant voltage circuit 90
A diode 94 for voltage stabilization of the capacitor 98 is provided between the capacitor 2 and the capacitor 98, and when the voltage of the capacitor 98 rises, the current from here flows through the diode 94 to the Zener diode 92. Since the voltage at 98 is limited to the sum of the voltage at Zener diode 92 and the forward drop voltage at diode 94, a stable operating voltage can be supplied to pulse width control circuit 82, as is clear from the results shown in FIG. Moreover, the rectifier circuit 9
Since a resistor 99 is provided between the output terminal of the circuit and the diode 97 as a rectifier to suppress the peak value of the charging current of the capacitor 98, an excessive voltage is supplied to the pulse width control circuit 82. Never. Furthermore, as mentioned earlier, the power loss is small, so it is economically advantageous.

In the above, capacitor 9 during steady operation
The resistor 99 and the tertiary coil 5 are connected in advance so that the voltage of the Zener diode 92 is slightly lower than the voltage of the Zener diode 92.
If the number of turns is 3, the sum of the current from the transistor 93 and the current from the capacitor 98 will be supplied to the pulse width control circuit 82. In this way, the constant voltage circuit 90 and the rectifier circuit 9 will be operated in parallel, and the switching mode of the control power source will not be entered between startup and steady operation, so the pulse width control circuit 82 will be operated in parallel. The advantage is that it is not affected by voltage fluctuations.

[Brief explanation of drawings]

1 and 2 are circuit diagrams each showing a control circuit of a switching power supply, FIG. 3 is a circuit diagram showing a control circuit of a switching power supply according to an embodiment of the present invention, and FIG. 4 is a circuit diagram showing a control circuit of a switching power supply according to an embodiment of the present invention. 3 is a graph showing the results of comparing changes in operating voltage between a conventional device and a device according to the present invention. 20... First rectifier circuit, 4... Switching element, 5... Switching coil, 6... Rectifier filter circuit, 7... Load, 81... Constant voltage control circuit, 82... Pulse width control circuit, 9... Second rectifier circuit, 90... Constant voltage circuit, 92... Zener diode, 93... Starting transistor, 97
... Diode as a rectifier, 98 ... Smoothing capacitor, 99 ... Resistor, 94 ... Diode for voltage stabilization of smoothing capacitor, 10A ...
power line.

Claims (1)

[Scope of utility model registration request] A DC voltage obtained by rectifying and smoothing an AC voltage in a first rectifier circuit is supplied to a series circuit of a switching transformer and a switching element, and a voltage on the secondary side of the switching transformer generated by switching of the switching element is rectified and smoothed. In a control circuit for a switching power supply that is supplied to a load and controls a pulse signal for gate drive of a switching element generated by a pulse width control circuit in response to a load voltage, the first rectifier circuit and the pulse width control circuit A constant voltage circuit including a starting transistor and a Zener diode for supplying a base current to the starting transistor is provided between the power supply line, a tertiary coil is provided in the switching transformer, and the tertiary coil is connected to a rectifier. and a smoothing capacitor, the second rectifying circuit is connected to the power supply line, and a resistor is provided on the output side of the rectifier to suppress the peak value of the charging current of the smoothing capacitor. A diode for stabilizing the voltage of the smoothing capacitor is provided between the output end of the rectifier circuit and the cathode of the Zener diode so that the anode is connected to the output end of the rectifier circuit. After the transistor is turned on and the DC voltage from the first rectifier circuit is supplied to the pulse width control circuit as an operating voltage, and the switching element starts the switching operation, the voltage from the second rectifier circuit is supplied to the pulse width control circuit. 1. A control circuit for a switching power supply, characterized in that the voltage of a smoothing capacitor is supplied as an operating voltage to the Zener diode and the sum of the voltages of the diode.