JP3287039B2 - Switching power supply - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply to which a wide input power supply voltage such as a world wide input is applied.

[0002]

2. Description of the Related Art FIG. 2 is a circuit diagram of a conventional switching power supply of a ringing choke converter (RCC) type, which is provided with an overcurrent protection function having a so-called drooping characteristic. Diode bridge 2 for AC power supply 1
Connect the AC-side input terminal is connected to the capacitor C ₀ for smoothing the rectified output end of the diode bridge 2. 1 of the output transformer T constituting the switching circuit
And winding L _P, a switching transistor (main switching element) Q ₁ consisting of FET, the series circuit of the resistor R ₄ is connected in parallel with the capacitor C _0.

The secondary winding L of the output transformer T
A diode D ₁ for rectification and a capacitor C ₃ for smoothing are connected to _O , and power is supplied to the load L from the capacitor C ₃ . The load L
An output voltage detection unit 3 for detecting the output voltage on the side is provided. Further, one end of the feedback winding L _B of the output transformer T, the capacitor C _2, through a resistor R ₃ is connected to the gate of the switching transistor Q _1, the switching transistor Q ₁ gate and between ground transistor Q ₂ for control are connected in parallel. The starting resistor R ₂ is connected to the gate of the switching transistor Q _1.

A time constant circuit including a resistor R ₁ and a capacitor C ₁ for controlling the output voltage and protecting the current from drooping is provided on the base side of the transistor Q _2. Etc. are provided. The phototransistor PT constitutes a photocoupler PC with a light emitting diode PD provided in the output voltage detecting section 3. A signal (light emission amount) from the light emitting diode PD of the photocoupler PC is converted into a phototransistor. P
In response to T, output voltage control and overcurrent protection of the fold-out characteristic are performed.

Next, the operation of FIG. 2 will be described. The switching transistor Q ₁ is turned on and power is turned on via the starting resistor R _2, 1 winding L _P of the output transformer T
Is supplied with a power supply voltage. This also induces a voltage in the feedback winding L _B and the secondary winding L _O of the output transformer T. Since the voltage generated in the secondary winding L _O of the output transformer T is in the opposite direction to the diode D ₁ , the capacitor C
₃ is not charged, and no current flows through the load L.

Accordingly, since the output voltage is close to zero, the phototransistor PT is in a cut-off state and the capacitor C ₁
Is charged via the resistor R ₁ , the capacitor C ₁ is charged in a short time, the control transistor Q ₂ is turned on, and the switching transistor Q ₁ is turned off to turn off the switching transistor Q ₁ . The period is kept short.

[0007] Then, the period of the switching transistor Q ₁ is turned off, is released primary winding L stored magnetic energy _P of the output transformer T to the secondary winding L _O, capacitor C via the diode D _{1 3} is charged. As a result, a current flows to the load L side. The switching transistor Q ₁ is when turned off, the output capacitor C ₁ by the voltage generated in the feedback winding L _B of the transformer T will be charged charges in a direction opposite to the above. Thus, the control transistor Q ₂ is turned off, the switching transistor Q ₁ is turned on. This operation is repeated to continue the oscillation, and power is supplied to the load L.

[0008] Here, after the output voltage has risen, the phototransistor PT also becomes active state from the signal receiving therefore blocked from the light-emitting diode PD, the collector current of the phototransistor PT control the charging time of the capacitor C ₁ to as becomes, so to obtain an oN period of the switching transistor Q ₁ corresponding to the output voltage. When the output current load becomes heavier to increase, in order to output voltage decreases, the amount of light emitted from the light-emitting diode PD is reduced, even less current flowing through the phototransistor PT, the charging time of the capacitor C ₁ becomes longer. Accordingly, the on period of the switching transistor Q ₁ is longer, controls to prevent a decrease in the output voltage.

After the output current increases and the current flowing through the phototransistor PT becomes zero, the resistance R ₁ ,
Can not be increased beyond a value determined by the time constant of the capacitor C _1, the output current is limited. Furthermore, the output current increases, the output voltage begins to drop, the output voltage falls, also decreases the voltage generated in the feedback winding L _B of the output transformer T off period of the switching transistors Q _1, opposite to the capacitor C ₁ reduces the charge stored in the direction, the control transistor Q ₂ is hardly oFF, the oN period of the switching transistor Q ₁ is made shorter.
In this way, until the load L is short-circuited, since the ON period of the switching transistor Q ₁ is continuously shortened, the output voltage Vo to the output current Io, draw a shape curve of full as shown in FIG. 3, a _The overcurrent protection function works from _one point.

[0010]

As described above [0006], in the circuit shown in the prior art of FIG. 2, startup, Yari by passing a starting current through the starting resistor R _2, starting method that I'll turn on the switching transistors Q ₁ Has taken. Such In the conventional example, the starting resistor R ₂ is because it is also related to the intensity of Operation foldback overcurrent protection, it will change the strength of the shaped characteristic of off by the input variation. here,
As an input voltage of the switching power supply device, for example, AC85V to 264V of a world wide input can be considered.

[0011] is set to match the resistance value of the start-up resistor R ₂ at the time of high input, a problem that does not start at the time of low input occurs. Further, by setting the combined resistance value of the starting resistor R ₂ at the time of low input, starting current flowing through the starting resistor R ₂ at the time of high input increases, increased loss in the starting resistor R _2, or cause a decrease in efficiency, There is a problem that the fold-out characteristics during the overcurrent protection operation become invisible.

The present invention has been made in view of the above points, and has as its object to provide a switching power supply device capable of supplying an optimum starting current even in the case of a wide input power supply voltage.

[0013]

SUMMARY OF THE INVENTION The present invention comprises a DC power supply,
A primary winding of an output transformer to which a power supply voltage is applied from the DC power supply; a main switching element connected in series with the primary winding to perform a switching operation; and a starting current supplied to the main switching element from the DC power supply. A starting resistor to be supplied, a control transistor for controlling an ON period of the main switching element, a rectifying / smoothing circuit connected to a secondary winding of the output transformer for supplying power to a load, and an output voltage of the output transformer. An output voltage detecting section for detecting, and a voltage generated in a feedback winding of the output transformer and a signal from the output voltage detecting section, drive the control transistor to perform output voltage control and overcurrent protection of square-shaped characteristics. in the switching power supply and control means for performing, the DC
Detecting an input power supply voltage may be input extensively by being connected to the power supply directly to maintain a substantially constant start current of the main switching element according to the level of that input supply voltage the resistance value of the starting resistor It is provided with a start-up control circuit that automatically switches and changes according to the height.

[0014]

According to the present invention, when the input power supply voltage is input in a wide range , the input power supply voltage is directly detected, and the resistance value of the starting resistor is varied according to the level of the input power supply voltage. That is, even when the input power supply voltage is high, the resistance value of the starting resistor is made variable, that is, increased, thereby preventing an increase in the starting current and preventing a decrease in efficiency. In addition, when the input power supply voltage is low, the startup characteristics can be improved by reducing the resistance value of the startup resistor. Further, since the start-up current is prevented from increasing at the time of high input, it is possible to improve the fold-back characteristic in the overcurrent protection operation at the time of high input.

[0015]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an overall circuit diagram of the switching power supply. The present invention is applied to a case where the input power supply voltage is a world wide input (for example, AC 85 to 264 V) or a single input has a wide input range (for example, AC 170 to 264 V).
The switching characteristic of the switching power supply device is improved. Since the entire configuration is almost the same as the conventional configuration, the description of the same parts will be omitted, and the part of the gist of the present invention will be described.

[0016] That is, as shown in FIG. 1, a starting resistor is composed of two resistors R ₂ and R _5, the starting resistor R _2,
The R ₅ is provided with a startup control circuit 4 for switching and connecting. The start control circuit 4 includes, as shown in FIG.
₂ and transistor Q ₃ connected in parallel, and a resistor R _6, R ₇ connected to the base of the transistor Q _3, a transistor Q ₄ for controlling the transistor Q _3, connected to the base of the transistor Q _4, a Zener diode ZD ₁ having a predetermined Zener voltage, and a resistor R _8, R ₉ to turn on or off by the level of the input voltage of this Zener diode ZD _1.

Next, the operation will be described. Note that the gist of the present invention is to improve the start-up characteristics, and the part of the operation after startup is the same as that of the conventional example. First, when the input power supply voltage is low, the resistor R ₈ and the voltage divided by the R _9, the Zener diode ZD ₁
Cannot be turned on, and the Zener diode ZD ₁
Is off.

[0018] Zener diodes ZD ₁ is off, the transistor Q ₄ is also turned off, the transistor Q ₃ is turned on by the divided voltage between the resistor R ₆ and R _7. When the transistor Q ₃ is turned on, starting resistor R ₂ are shorted,
Start-up resistance is the only R _5. In other words, at low input will initiate the operation only in the starting resistor R _5.

Next, when the input power supply voltage is high, the divided voltage by the resistors R ₈ and R _{9 is changed} by the Zener diode ZD _1.
In order to exceed the Zener voltage of Zener diode Z
D ₁ is turned on. When the Zener diode ZD ₁ is turned on, the transistor Q ₄ is turned on, the transistor Q ₄
Turning off the transistor Q ₃ by the on. Therefore,
There are _two starting resistances, R ₂ and R _5. The resistance value is increased by the _two starting resistances R ₂ and R ₅ , and the switching transistor Q ₁ can be operated without increasing the starting current even at the time of high input. Can be started.

Thus, for example, the starting resistors R ₂ and R ₅
Even if the input power supply voltage fluctuates twice from 100 V to 200 V, the starting current is kept constant, and it can be started well even at low input and high input. There is no change in the fold-back characteristic of the current protection. Also, even at the time of high input, since the starting current does not increase,
There is no increase in loss or decrease in efficiency.

In this embodiment, as shown in FIG. 1, a switching transistor Q _{1 is used} as a main switching element.
Although an example using an FET has been described above, the same effect can be obtained also in the case of the RCC system using a bipolar transistor as the switching element. Further, the circuit configuration for the circuit configuration and overcurrent protection for turning on and off the switching transistor Q ₁ is not limited to those shown in FIG. 1,
It goes without saying that the present invention can be applied to other circuit configurations.

[0022]

According to the present invention, a DC power supply is connected.
The input supply voltage may be input extensively directly by
Detect and, provided a start control circuit for varying automatically switching the starting current substantially match the level of the input supply voltage to the resistance value of the starting resistor to maintain a constant main switching element according to the level of its Therefore, even when the input power supply voltage is high, the startup current can be prevented from increasing by changing, that is, increasing, the resistance value of the startup resistor, thereby preventing a decrease in efficiency. In addition, when the input power supply voltage is low, the startup characteristics can be improved by reducing the resistance value of the startup resistor. Furthermore, since the starting current is prevented from increasing at the time of high input, there is an effect that the square character characteristic in the overcurrent protection operation at the time of high input can be improved.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a switching power supply device according to an embodiment of the present invention.

FIG. 2 is a circuit diagram of a conventional switching power supply device.

FIG. 3 is a characteristic diagram showing a relationship between an output current and an output voltage in a conventional example.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 AC power supply 2 Diode bridge 3 Output voltage detection unit 4 Start-up control circuit Q ₁ Switching transistor Q ₂ Control transistor R ₂ Start-up resistor R ₅ Start-up resistor T Output transformer L _P Primary winding L _O Secondary winding L _B feedback Winding L Load

Continuation of the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H02M 3/28 H02M 3/338

Claims (1)

(57) [Claims] 1. A DC power supply, a primary winding of an output transformer to which a power supply voltage is applied from the DC power supply, a main switching element connected in series with the primary winding to perform a switching operation, and a main switching element A starting resistor for supplying a starting current to the element from the DC power supply; a control transistor for controlling an ON period of the main switching element; and a rectifying / smoothing connected to a secondary winding of the output transformer and supplying power to a load. A circuit, an output voltage detection unit that detects an output voltage of the output transformer, and a voltage generated in a feedback winding of the output transformer and a signal from the output voltage detection unit that drives the control transistor to control output voltage. A switching power supply device having control means for performing overcurrent protection of a square-shaped characteristic, wherein the switching power supply device is connected to the DC power supply.
The input supply voltage is detected directly may be inputted extensively I
Te, in that a start control circuit for varying automatically switching the starting current substantially match the level of the input supply voltage to the resistance value of the starting resistor to maintain a constant main switching element according to the level of its Characteristic switching power supply.