JP2532203Y2 - Switching power supply - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a self-excited switching power supply.

2. Description of the Related Art A switching regulator of a type in which a series circuit of a DC power transformer and a switching transistor is connected and the switching transistor is driven in a positive feedback manner by a voltage of a driving winding coupled to the transformer is widely used.

[Problem to be Solved by the Invention] By the way, there is a demand for a reduction in switching loss of a switching transistor. If this current flows in a state where there is a voltage between both terminals when the switching transistor is turned on, switching loss occurs. In particular, when a capacitor is connected in parallel with the switching transistor, the electric charge of the capacitor is discharged via the switching transistor when the switching transistor is turned on, so that the power loss increases.

Therefore, an object of the present invention is to provide a switching power supply device capable of reducing a switching loss when a switching element is turned on.

Means for Solving the Problems The present invention for achieving the above object has a main winding of a transformer connected to one end of a DC power supply, first and second main terminals, and a control terminal. A switching element having the first main terminal connected to one end of the DC power supply via the main winding and the second main terminal connected to the other end of the DC power supply; and a switching element connected to the transformer. Rectifying and smoothing circuit,
The switching element is electromagnetically coupled to the main winding, and is connected between the control terminal and the second main terminal of the switching element so as to perform positive feedback driving of the switching element. A switching power supply device comprising: a drive winding having a polarity for supplying a positive voltage to a control terminal; and a starting circuit connected to the control terminal of the switching element, the switching power supply device having a collector, an emitter, and a base. An NPN-type delay control transistor having the collector connected to the control terminal of the switching element, the emitter connected to the second main terminal, and the anode connected to the emitter of the delay control transistor A first diode having a cathode connected to the base of the delay control transistor, and one end connected to the delay diode. A capacitor connected to the base of the control transistor and the cathode of the first diode, an anode connected to the other end of the capacitor, and a cathode connected to the control terminal of the switching element of the drive winding And a second diode connected to a terminal connected to the second terminal, and a resistor connected between the other end of the capacitor and the emitter of the delay control transistor. It relates to the power supply.

[Operation and effect of the invention] The invention has the following operation and effect.

(A) The capacitor is charged by the voltage generated in the drive winding during the off period. When the direction of the voltage of the drive winding is reversed for the transition from the off period to the on period, the delay control transistor is turned on based on the discharge of the capacitor. The charge of the capacitor is not immediately discharged at all, but is discharged through a resistor with a predetermined time constant. Therefore, the ON of the delay control transistor is maintained for a predetermined time after the voltage for turning on the switching element is generated in the drive winding, and the switching of the switching element to ON is prevented. As a result, the turn-on time of the switching element is delayed, and this current flows after the voltage between both terminals of the switching element decreases, so that the switching loss when the switching element is turned on is reduced.

(B) Since the first diode is connected in anti-parallel between the base and the emitter of the delay control transistor, the first diode not only functions as a rectifier diode for charging a capacitor, but also functions as a delay connection transistor. Also functions as a protection diode.

[Embodiment] Next, an RCC (on / off type) switching power supply according to an embodiment of the present invention will be described with reference to FIGS. 1 to 4.

A switching element for conversion is connected between one terminal 2 and the other terminal 3 of a DC power supply 1 composed of a rectifier connected to an AC power supply and a smoothing circuit via a primary winding (main winding) 5 of a transformer 4. Main transistor 6 is connected. That is, the collector (first main terminal) of the main transistor 6 is 1
The power supply terminal 2 is connected to the power supply terminal 2 via the secondary winding 5, and the emitter (second main terminal) is connected to the other power supply terminal 3.

A rectifying / smoothing circuit 10 including a diode 8 and a capacitor 9 is connected to a secondary winding 7 which is an output winding of the transformer 4. A load 13 is connected to output terminals 11 and 12 of the rectifying and smoothing circuit 10.

One end of the tertiary winding of the transformer 4, that is, the driving winding 14, is connected to the base (control terminal) of the main transistor 6 via the diode 15 and the resistor 16, and the other end is connected to the emitter of the main transistor 6. . Note that diode 15 and resistor 16
, A resistor 17 and a capacitor 18 are connected in parallel. A transistor 20 as a voltage control element is connected between the base and the emitter of the main transistor 6, and a well-known constant voltage control circuit 21 is connected to the base.

A switching loss reducing capacitor (first capacitor) 21 at the time of turn-off is connected between the collector and the emitter of the main transistor 6.

The control transistor 23 for delay and current control is connected between the base and the emitter of the main transistor 6. That is, the collector of the control transistor 23 is connected to the base of the main transistor 6, and this emitter is connected to the emitter of the main switching transistor 6.

A delay capacitor (second capacitor) 24 having one end connected to the base of the control transistor 23 has a function of delaying the turning-off time of the control transistor 23 and delaying the turn-off of the main transistor 6. The delay capacitor 24 is connected to the drive winding 14 during the off period of the main transistor 6.
A diode 25 is connected between the lower end of the drive winding 14 and one end of the delay capacitor 24, and a diode 26 is connected between the other end of the capacitor 24 and the upper end of the drive winding 14. Is connected. The delay capacitor 24
A resistor 27 is connected between the other end of the capacitor 24 and the emitter of the control transistor 23 in order to form a discharge circuit.

The capacitor 28 is used to limit the intermittent oscillation current.One end of the capacitor 28 is connected to the emitter of the control transistor 23 and the lower end of the drive winding 14, and the other end is connected to the drive winding 14 via voltage dividing resistors 29 and 30. Connected to the upper end. This lower end is connected to the upper end of the drive winding 14 via the resistor 31 and the diode 26 in order to charge the intermittent oscillation limiting capacitor 28 with the voltage during the off period of the drive winding 14. Therefore, the diode 26 functions as a charging circuit for the two capacitors 24 and 28.

The resistor 32 connected in parallel to the current limiting capacitor 28 functions as a discharge circuit.

A diode 33 connected between the voltage dividing point P of the voltage dividing resistors 29 and 30 and the base of the transistor 23 functions as a backflow preventing element.

A resistor 34 connected between the power terminal 2 and the base of the switching transistor 6 supplies a starting current to the switching transistor 6.

[Operation] When power supply from the DC power supply 1 is started, a base current flows to the main transistor 6 through the starting resistor 34, and this is turned on. As a result, a power supply voltage is applied to the primary winding 5, a positive feedback voltage is obtained in the drive winding 14, and the main transistor 6 is driven by the voltage of the drive winding 14. Since the primary winding 5 has an inductance, the collector current I _c of the main transistor 6 increases with time. However, when the collector current becomes twice the current amplification factor of the base current, it saturates and turns off. The diode 8 of the rectifying / smoothing circuit 10 has a polarity that turns off during the on period of the transistor 6 and turns on during the off period, and discharges the energy stored in the transformer 4 to the capacitor 9 and the load 13 during the off period. .

When the release of the stored energy is completed, an oscillating voltage for turning on the main transistor 6 is generated in the drive winding 14,
The main transistor 6 turns on.

The output voltage is controlled by bypassing the base current 20 of the main transistor 6.

By the way, the flyback voltage generated when the main transistor 6 is turned off is absorbed by the capacitor 21. As a result, when the collector of the transistor 6 is turned off,
It is possible to delay the rise of the voltage between emitters,
Switching loss at turn-off is reduced. If the delay capacitor 24 is not provided, the discharge current of the capacitor 21 flows through the transistor 6 simultaneously with the fall of the collector-emitter voltage V CE of the main transistor 6 as shown in FIG. Switching loss occurs.

On the other hand, if the delay capacitor 24 is provided, the turn-off of the main transistor 6 is delayed, and as shown in FIG. 3, when the collector-emitter voltage V CE decreases, the capacitor 21 and / or the floating The discharge current of the capacitor flows through the main transistor 6, and the switching loss here is reduced. Further, regardless of the capacitor 21, the current flowing through the transistor 6 also starts to flow after the voltage between the terminals of the transistor 6 decreases, so that the power loss due to this current is reduced.

To explain the turn-off delay control in more detail, the delay capacitor 24 is charged by a closed circuit including the drive winding 14, the diode 25, the capacitor 24, and the diode 26 during the off period. The voltage of the delay capacitor 24 has a polarity for forward-biasing the base of the control transistor 23, but turns on because the control transistor 23 is reverse-biased with the voltage of the drive winding 14 during the off period. Never. In the ON period, a positive voltage is generated in the drive winding 14, and the control transistor 23 is turned on because the control transistor 23 is forward-biased. Therefore, a bypass circuit of the base current supplied from the drive winding 14 to the main transistor 6 is formed, and the main transistor 6 is not immediately turned on. The discharge of the delay capacitor 24 is performed in a closed circuit including the delay capacitor 24, the base and the emitter of the control transistor 23, and the resistor 27, and the voltage of the delay capacitor 24 gradually decreases. 23 turns off and the main transistor 6 turns on.

When the turn-on of the main transistor 6 is delayed, the third
Discharge current and the collector current I _c of the capacitor 21 flows after the collector-emitter voltage V CE of the main transistor 6 drops as shown in FIG., The switching loss is reduced.

In FIG. 1, when the load 13 is reduced or becomes no load, intermittent oscillation as shown in FIGS. 4A and 4B occurs. In FIG. 4, a section T1 indicates an oscillation period, and a section T2
Indicates a non-oscillation period. Oscillation at a frequency of about 200 kHz occurs in the section T1, and the section T1 occurs intermittently with a repetition frequency of about 1 to 5 kHz. If the current limiting capacitor 28 is not provided, the fourth
Flowing a large collector current I _c in the main transistor 6, as shown in FIG. (B). As a result, the transformer 4 vibrates to generate an audible sound (abnormal sound). However, when the current limiting capacitor 28 is provided, the collector current I _{c of the} main transistor 6 is increased.
Is greatly limited as shown in FIG. 4 (C), and the occurrence of abnormal sound of the transformer 4 can be prevented.

The intermittent current limiting operation will be described in detail. During the off period, a closed circuit including the drive winding 14, the current limiting capacitor 28, the resistor 31 and the diode 26 is established, and the current limiting capacitor 28 is turned off during the off period. Charged at almost constant voltage obtained in 14. By the way, when the load 13 is operating normally with an appropriate size, the current limiting capacitor 28 is charged to a relatively large value because the main transistor 6 is operating on and off at a frequency of about 25 to 60 kHz. At this time, the lower side of the current limiting capacitor 28 becomes negative,
33 is kept off and the current limiting capacitor 28 is independent of the control transistor 23. When the load 13 becomes light or no load, the oscillation of the main transistor 6 at about 200 kHz occurs intermittently, so that the voltage of the capacitor 28 hardly rises. That is, even if the charging of the current limiting capacitor 28 is started by the voltage of the drive winding 14 at the time of oscillation, the current limiting capacitor 28 is discharged through the resistor 32 during the oscillation suspension period.
28 voltage does not rise. As a result, the voltage component for canceling the voltage of the drive winding 14 during the ON period of the current limiting capacitor 28 becomes zero or small, and the voltage at the voltage dividing point P passes through the diode 33 during the ON period, and the voltage of the control transistor 23 is reduced. applied to the base, the control transistor 23 is turned on, the main bypass the base current of the transistor 6 occurs, the collector current I _c of the main transistor 6 is limited Fig. 4 substantially as shown in (C), trans The generation of the abnormal sound of No. 4 is suppressed.

[Modifications] The present invention is not limited to the above-described embodiment, and for example, the following modifications are possible.

(1) The turn-on delay circuit by the delay capacitor 24 and the current limiting circuit at the time of intermittent oscillation by the current limiting capacitor 28 can be formed completely independently.

(2) The main transistor 6 can be an FET (field effect transistor).

(3) Instead of charging the capacitor 28 with the voltage of the drive winding 14, a quaternary winding is provided in the transformer 4 and can be charged with this voltage.

(4) Instead of providing the secondary winding 7, an output voltage may be obtained as a single-turn transformer configuration.

(5) Instead of detecting the voltage of the load 13, a voltage detection winding may be provided in the transformer 4, and the control circuit 21 may be controlled by this voltage during the off period.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a switching power supply according to an embodiment of the present invention, FIG. 2 is a waveform diagram showing the relationship between collector-emitter voltage and collector current of a main transistor of a conventional switching power supply, FIG. 4 is a waveform diagram showing the relationship between the collector-emitter voltage and the collector current of the main transistor in FIG. 1, and FIG. 4 shows the intermittent oscillation operation. FIG. 4 (A) shows the collector-emitter voltage of the main transistor. Waveform diagram,
FIG. 2B is a waveform diagram showing the collector current of the main transistor of the conventional switching power supply device, and FIG. 2C is a waveform diagram showing the collector current of the main transistor of FIG. 4 Transformer 6 Main transistor 14 Drive winding 23 Control transistor 24 Delay capacitor 28 Intermittent oscillation current limiting capacitor

Claims (1)

(57) [Scope of request for utility model registration] 1. A main winding of a transformer connected to one end of a DC power supply, first and second main terminals, and a control terminal, wherein the first main terminal is connected via the main winding. A switching element connected to one end of the DC power supply and the second main terminal connected to the other end of the DC power supply; a rectifying and smoothing circuit connected to the transformer; and electromagnetically coupled to the main winding. And a positive voltage is connected between the control terminal of the switching element and the second main terminal so as to drive the switching element in a positive feedback manner, and a positive voltage is supplied to the control terminal during an ON period of the switching element. A switching winding having a polarity, and a starting circuit connected to the control terminal of the switching element, comprising: a collector, an emitter, and a base, wherein the collector is An NPN-type delay control transistor connected to the control terminal of the switching element and having the emitter connected to the second main terminal; an anode connected to the emitter of the delay control transistor; A first diode connected to the base of the delay control transistor; a capacitor having one end connected to the base of the delay control transistor and the cathode of the first diode, respectively; A second diode connected to the other end of the capacitor, the cathode of which is connected to a terminal of the drive winding connected to the control terminal of the switching element; the other end of the capacitor and the delay control And a resistor connected between the emitter and the transistor. Switching power supply.