JPH0515154A - Switching regulator - Google Patents

Abstract

PURPOSE:To lower the withstand voltage of switching element and to reduce the cost. CONSTITUTION:A series circuit of the primary winding 3 of a transformer and a switching element 4 is connected with a DC power supply. A smoothing and rectifying circuit 10 is connected with the secondary winding 5 of the transformer 2. A surge absorbing and resetting capacitor 19 is connected in parallel with the tertiary winding 18 of the transformer 2 through a parallel circuit of a diode 21 and a transistor 20. The capacitor 19 is connected in parallel with the DC power supply 1 through a diode 22.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching regulator having a circuit for absorbing surge voltage and magnetically resetting a transformer.

[0002]

2. Description of the Related Art A series circuit consisting of a primary winding of a transformer and a switching element is connected to a power source, and a voltage is obtained in a secondary winding by turning on / off the switching element to control on / off of the switching element. The type of switching regulator that stabilizes the output voltage is widely used. When a switching element of this type of switching regulator is turned off, a surge voltage is generated. In order to absorb this surge voltage and to obtain the reset voltage of the transformer, a capacitor and a resistor may be connected to the primary winding via a diode. It is conceivable to provide a circuit having a surge voltage absorbing action and a reset voltage generating action as shown in FIG. 1 on the same principle.

In FIG. 1, a primary winding 3 of a transformer 2 and a first switching element are provided between one end (positive terminal) and the other end (negative terminal) of a DC power supply 1 composed of a rectifying circuit and a smoothing circuit, that is, ground. A series circuit with 4 is connected. The secondary winding 5 of the transformer 2 includes diodes 6, 7 and a reactor 8,
Output terminals 11 and 12 are connected through an output rectifying / smoothing circuit 10 including a capacitor 9. Output terminal 11,
A load 13 that supplies a stabilized voltage is connected to the load 12. The constant voltage control circuit 14 for controlling ON / OFF of the first switching element 4 formed of a transistor to make the voltages of the output terminals 11 and 12 constant includes an error amplifier 15 and an error amplifier 15.
It comprises a reference voltage source 16 and a first control pulse forming circuit 17. One input terminal of the error amplifier 15 is the output terminal 1
1 and the other input terminal is connected to the reference voltage source 16. The first control pulse forming circuit 17 is connected between the error amplifier 15 and the control terminal (base) of the first switching element 4, and connects the triangular wave generating circuit, the triangular wave obtained therefrom and the output of the error amplifier 15. PW in comparison
And a comparator that outputs M pulses. The output voltage may be detected by interposing a voltage dividing resistor,
The error amplifier 15 and the control pulse forming circuit 17 may be coupled by a photo coupler. In order to absorb the surge voltage and obtain the reset voltage, the transformer 2 has a tertiary winding 18,
It has a capacitor 19 and a transistor 20 and a diode 21 as a second switching element. The tertiary winding 18, the capacitor 19 and the diode 21 are connected so as to form a closed circuit, and the transistor 20 is connected in parallel to the diode 21 with an opposite polarity. Further, one end (lower end) of the capacitor 19 is connected to the lower end of the DC power supply 1. Transistor 20 is capacitor 19
And is controlled by the second control pulse forming circuit 22. The second control pulse forming circuit 22 generates an ON control pulse for the transistor 20 during the OFF period of the first switching element 4 in synchronization with the output of the first control pulse forming circuit 17.

When the first control signal V1 shown in FIG. 2 is applied to the switching element 4 and turned on / off, the voltage Ve of the DC power supply 1 is intermittently applied, and this is applied to the primary winding 3. A voltage corresponding to the voltage of the primary winding 3 is obtained at the secondary winding 5, and this voltage is rectified by the rectifying / smoothing circuit 10 and the load 13
Is supplied to. The output voltage is compared with the reference voltage by the error amplifier 15, the error signal is input to the first control pulse forming circuit 17, and the PWM pulse is formed by a known method. Second
In the control pulse forming circuit 22 of FIG. 2, the second pulse including the pulse rising in synchronization with the trailing edge of the pulse of the first control signal V1 in FIG.
Control signal V2 is generated and applied to transistor 20. The pulse of the second control signal V2 corresponds to the end of the off period of the first control signal minus the dead time Td. Therefore, when the OFF period of the first control signal V1 is long, the ON period of the second control signal V2 is long. The transistor 20 is not always turned on during the entire period when the second control signal V2 is generating the ON control pulse. When the diode 21 is conducting, the transistor 20 is kept off. Before the time t2 in FIG. 2, since the load 13 varies little,
No rise in the voltage Vc of the capacitor 19 is seen. When the switching element 4 is turned off at time t1, the energy remaining in the transformer 2 is released in the closed circuit composed of the tertiary winding 18, the capacitor 19 and the diode 21, and the capacitor 19 absorbs the surge voltage. Tertiary winding 1
8 and the capacitor 19 form a resonance circuit, the charging of the capacitor 19 starts at t1, and the voltage Vc rises.
When the charging of the capacitor 19 is completed at time t2, the capacitor 19 is discharged due to resonance operation, and
In the closed circuit of the tertiary winding 18 and the transistor 20, the discharge of the capacitor 19 progresses and the voltage Vc decreases. t1
The voltage Vs of the switching element 4 in the period up to t3 is
It is the sum of the voltage of the primary winding induced corresponding to the power supply voltage Ve and the voltage Vc of the capacitor 19. Now primary winding 3 and 3
If the turn ratio of the next winding 18 is 1: 1, the voltage Vs of the switching element 4 is Ve + Vc. Since the first and second control signals V1 and V2 are both at the low level during the period from t3 to t4, the voltage Vs of the switching element 4 becomes equal to the power supply voltage Ve. During the period from t4 to t5 when the switching element 4 is turned on, both the diode 21 and the transistor 20 as the second switching element are off, so that the capacitor 19 is not charged / discharged and the capacitor voltage Vc becomes the value at the time point t3. To be kept. In the off period of t5 to t6 of the switching element 4, the same operation as in the off period of t1 to t3 occurs. If the current of the load 13 increases and the output voltage decreases at time t6, the ON width of the switching element 4 is widened, and the ON width of t6 to t7 is t4 to t.
Wider than 5 on width. As the ON width becomes wider, the energy supplied to the secondary winding 5 increases, and the energy remaining in the transformer 2 at the turn-off time t7 also increases. As a result, a large voltage is generated in the tertiary winding 18 at turn-off. This voltage causes the capacitor 19
Charging starts and the capacitor voltage Vc rises. By the way, when the ON width of the switching element 4 becomes large and the OFF width becomes narrower and the voltage of the tertiary winding 18 becomes higher, the termination of the charging mode of the capacitor 19 occurs during the OFF period t7 to t8. There are times when you don't. In such a case, even if the ON pulse of the control signal V2 is applied to the transistor 20, the transistor 20 is kept in the OFF state and the capacitor voltage Vc does not decrease. During the ON period t8 to t9 of the switching element 4, the capacitor 1
Since the voltage Vc of 9 holds the value at the time of turn-on,
In the next off period t9 to t10, the capacitor voltage Vc becomes higher. As shown at t11 to t12, the capacitor voltage Vc
Becomes higher, the reset of the transformer 2 is off period t11-
The operation is started within t12 and returns to the stable state.

[0005]

By the way, when the capacitor 19 is only charged and not discharged during the off period of the switching element 4, this voltage Vc becomes the power supply voltage Ve.
As a result, the reverse voltage applied to the transistor 20 and the switching element 4 becomes more than twice the power supply voltage Ve. Therefore, it is necessary to use expensive switching elements 4 and transistors 20 having a high breakdown voltage.

Therefore, an object of the present invention is to provide a switching regulator which can lower the breakdown voltage of a switching element.

[0007]

The present invention for achieving the above object provides a DC power supply, a primary winding of a transformer connected between one end and the other end of the DC power supply, and a first switching. A switching regulator including a series circuit with an element, a rectifying / smoothing circuit connected to a secondary winding of the transformer, and a control circuit for controlling on / off of the first switching element, wherein a tertiary winding of the transformer is provided. A wire, a capacitor connected in series to the tertiary winding and having one end connected to the other end of the DC power source, and a capacitor connected in parallel to the series circuit of the tertiary winding and the capacitor, A second switching element that is turned on during a part or all of the off period of the first switching element, and a diode connected between the other end of the capacitor and one end of the DC power supply. Those related to the switching regulator, characterized in.

The second switching element preferably comprises a diode and a transistor connected in parallel with the diode. Further, the first switching element can be a pair of switching elements connected to one end side and the other end side of the primary winding.

[0009]

The function of the switching regulator is to absorb the surge voltage and generate the reset voltage.
One end of the capacitor is connected to the other end of the DC power supply, and the other end of the capacitor is connected to one end of the DC power supply through the diode.Therefore, when the voltage of the capacitor becomes higher than the power supply voltage, the diode conducts, Suppresses the voltage rise. As a result, the first and second switching elements can have low breakdown voltage and low cost. Also, the lower the voltage, the smaller the power loss. By using the combination of the diode and the transistor as the second switching element, the limitation of the time width of the control signal of the transistor is relaxed. That is, even if the ON control signal is applied to the transistor before the time when the transistor is required to be conductive, the transistor is reverse-biased due to the forward voltage drop of the diode and the conduction is blocked. Therefore, it becomes possible to automatically switch the charging and discharging of the capacitor. If a plurality of first switching elements are provided as described in claim 3, the breakdown voltage per one can be lowered.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, a switching regulator according to an embodiment of the present invention will be described with reference to FIGS. However, in FIG. 3, the same parts as those in FIG. 1 are designated by the same reference numerals and the description thereof will be omitted. The switching regulator of FIG. 3 has a circuit configuration in which a clamping diode 23 is added to the switching regulator of FIG. The clamp diode 23 is connected between the upper terminal of the capacitor 19 and the positive terminal of the power supply 1.

The waveform of each part in FIG. 3 is as shown in FIG. As is clear from the comparison between FIG. 4 and FIG. 2, the basic operation of the circuit of FIG. 3 is the same as that of FIG. The operation of the circuit of FIG. 3 differs from the operation of the circuit of FIG. 1 in that the maximum voltage value of the capacitor 19 is substantially the same as the power supply voltage Ve. That is, as described in detail with reference to FIG. 2, when the current of the load 13 suddenly increases, the capacitor 19 is not discharged and only the charging continues. As a result, when the capacitor voltage Vc becomes higher than the sum of the power supply voltage Ve and the rising voltage of the diode 23, the diode 23 is turned on and the capacitor 19
Of the voltage Vc is limited, and the voltage applied to the transistor 20 is limited. Further, the voltage Vs of the switching element 4 is limited to approximately twice the power supply voltage Ve or less.

[0012]

MODIFICATION The present invention is not limited to the above-mentioned embodiments, and the following modifications are possible. (1) As shown in FIG. 5, a circuit configuration in which a switching element 4a is added to the circuit of FIG. 3 can be used. The additional switching element 4a is connected between the power supply 1 and the upper end of the primary winding 3, and is simultaneously turned on / off at the same time as the other switching element 4. When the pair of switching elements 4 and 4a is provided in this manner, the switching element 4 and
The voltage applied to 4a can be halved in FIG. (2) The diode 21 can be replaced with a transistor, and an ON control signal can be given to this during the OFF period of the switching element 4. A bidirectional control element can be used instead of the diode 21 and the transistor 20. (3) The switching element 4 and the transistor 20 can be field effect transistors.

[0013]

As is apparent from the above, according to the present invention, it is possible to reduce the cost of the switching regulator by using a switching element having a low breakdown voltage and a low cost.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a conventional switching regulator.

FIG. 2 is a waveform diagram showing a state of each part of FIG.

FIG. 3 is a circuit diagram showing a switching regulator according to an embodiment of the present invention.

FIG. 4 is a waveform diagram showing a state of each part of FIG. 3 similarly to FIG.

FIG. 5 is a circuit diagram showing a switching regulator of a modified example.

[Explanation of symbols]

1 power supply 2 transformers 4 switching elements 18 Third winding 19 capacitors 23 Diode

Claims (3)

[Claims] 1. A DC power supply, a series circuit of a primary winding of a transformer connected between one end and the other end of the DC power supply, and a first switching element, and a secondary winding of the transformer. A switching regulator comprising a connected rectifying / smoothing circuit and a control circuit for controlling on / off of the first switching element, comprising: a tertiary winding of the transformer; Is connected in parallel with a capacitor connected to the other end of the DC power source and a series circuit of the tertiary winding and the capacitor, and is turned on during a part or all of the off period of the first switching element. And a diode connected between the other end of the capacitor and one end of the DC power supply. 2. The second switching element is the first switching element.
And a diode having a direction in which the switching element is turned on by a voltage induced in the tertiary winding during the off period of the switching element and a diode having a conduction direction opposite to the conduction direction of the diode and connected in parallel to the diode. The switching regulator according to claim 1, comprising a transistor. 3. The first switching element comprises a pair of switching elements that are turned on / off at the same time, and one of the pair of switching elements is connected between one end of the primary winding and one end of the DC power supply. The other of the pair of switching elements is connected between the other end of the primary winding and the other end of the DC power supply.