JPH089634A - Switching power unit - Google Patents

Abstract

PURPOSE:To provide a switching power unit which can exert a reasonable overcurrent protective function against both a maximum load and a steady load. CONSTITUTION:A switching power unit is provided with a constant-voltage element 55 connected in series with an overcurrent protective element 16, current value switching element 56 connected in parallel with the constant-voltage element 55, and frequency detecting circuit 50 which outputs a signal for turning off the switching element 56 upon detecting a large load having a low oscillation frequency. The power unit is constituted so that a set current value for overcurrent protection can be automatically switched in two steps of a high set current value corresponding to the maximum load at the starting time and low set current value corresponding to a light load during a steady load in accordance with actual loads.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a primary DC power supply circuit and a secondary DC power supply circuit.
The present invention relates to a switching power supply device which is connected and coupled to a next DC power supply circuit via a transformer and has an overcurrent protection function.

[0002]

2. Description of the Related Art In FIG. 3, a primary DC power supply circuit 10 and a secondary DC power supply circuit 30 are connected and coupled via a transformer 20. The primary DC power supply circuit 10 is an AC power supply 1
1, switch 12, noise filter 13, rectifier circuit 1
4, the smoothing capacitor C10, the switching element 15, and the voltage control element (transistor) 40 are formed, and the switching element 15 is connected between the primary winding L11 of the transformer and the negative electric path Ln of the rectifier circuit 14. Also,
The control power supply (voltage Vc) is the auxiliary winding L12 of the transformer 20.
Is oscillated by the diode D10 and is supplied through the resistor R10.

The secondary DC power supply circuit 30 includes a diode D21 connected to the secondary winding L21 of the transformer 20,
The load 3 is formed by including the smoothing capacitors C21 and C22.
Connected to 5. Reference numeral 45 denotes a stabilizing circuit which inputs a voltage detection signal for generating and outputting a feedback signal to a feedback signal generating circuit 46 including a photo coupler.

Thus, in such an RCC type self-excited oscillation type (frequency fluctuation type) transformer coupling type switching power supply device, the voltage control element 40 is fed back with a feedback signal as shown in FIG.
ON-OFF at the cycle T shown in, that is, the switching element 15 is turned ON-OF by using the control power supply (Vc).
F control is performed to supply electric power energy from the primary winding L11 to the secondary winding L21. There is a half cycle difference between the primary DC power supply voltage and the secondary DC power supply voltage.

In the secondary DC power supply circuit 30, the diode D
21 is rectified and smoothed by smoothing capacitors C21 and C22 to generate a secondary DC power supply and supply it to the load 35. The fluctuation of the load 35 is detected as the fluctuation of the secondary DC power supply voltage by the stabilizing circuit 45 and is output to the feedback signal generating circuit 46. This stabilizes the secondary DC power supply voltage.

Therefore, when the load 35 is light, the current I flowing through the switching element 15 decreases and the oscillation frequency f1 increases as shown in FIG. 4 (A). The cycle T1 is short. On the other hand, when the load 35 becomes heavier, the current I flowing through the switching element 15 increases and the oscillation frequency f2 decreases as shown in (B). The cycle T2 is long. Voltage control element 40
This is because the ON duty of the switching element 15 is constant.

By the way, the secondary DC power supply voltage is smoothed by the smoothing capacitors C21 and C22, and is stabilized by the stabilizing circuit 45.
The voltage is controlled to a constant voltage value in cooperation with the voltage control element 40. However, the fluctuation of the load 35 and the AC power supply 11
There may be a case where the primary side DC power supply circuit 10 is overloaded due to the voltage fluctuation of the above. Therefore, an overcurrent protection circuit is provided to deal with this.

That is, the conventional overcurrent protection circuit is shown in FIG.
As shown in, a current detection resistor 18 connected in series with the switching element 15 and a switching element (transistor)
A current I flowing through the switching element 15 has a set current value DI, that is, a set current value equivalent, which is formed of an overcurrent protection element (MOS / FET) 16 connected between the base of the negative electrode 15 and the negative electrode circuit Ln. When the drive voltage Vd of the voltage is detected, the overcurrent protection element 16 is turned on, and the current Ib flowing into the switching element 15 is changed to the negative electrode line Ln.
It is formed so as to forcibly turn off the switching element 15 by dropping it to the side and attenuating it.

[0009]

By the way, such a switching power supply is no exception, and there is a strong demand for smaller size, lighter weight, and lower cost. Therefore, the overcurrent protection circuit includes one current detection resistor 18 and one current detection resistor 18. And one overcurrent protection element 16. Therefore, the set current value D
I is carefully selected and set as a value according to the actual situation. Generally, the steady load (light load) shown in FIG.
Since the transient current Ip at the time of startup (maximum load) after power-on shown in (B) is larger than the steady-state current In at that time, the set current value slightly higher than the maximum value based on this transient current Ip. Selected as DI. This set current value equivalent voltage (Vd) is equal to the drive voltage Vd of the overcurrent protection element 16.

By the way, due to the diversity of the loads 35, there is an increasing demand for application to larger load fluctuations. On the other hand, there is also an increasing demand to further improve the quality of secondary DC power supply voltage such as ripple.

Therefore, the maximum load is the smoothing capacitors (output capacitors) C21, C of the secondary DC power supply circuit 30.
What happens when 22 is not fully charged, 2
Since the capacities of the smoothing capacitors C21 and C22 of the next DC power supply circuit 30 tend to be large, the ratio of the maximum load to the steady load is becoming higher. Thus,
If the set current value DI is selected for the maximum load (transient current Ip), there is a possibility that necessary and sufficient overcurrent protection at the time of steady load cannot be performed.

An object of the present invention is to provide a switching power supply device capable of exhibiting an appropriate overcurrent protection function for both maximum load and steady load.

[0013]

The present invention provides cost reduction,
From the viewpoint of large current consumption and prevention of overheating, the current detection resistor is formed from one resistor as in the case of the conventional example, and a constant voltage element, a current value switching element, and a frequency detection circuit are provided while maintaining a fixed resistance value. There are two levels of set current value for over current protection for load and set current value for maximum load according to actual load (set high current value, set low current value)
The above-mentioned object is achieved by enabling automatic switching.

That is, in the switching power supply device according to the present invention, a primary DC power supply circuit including a rectifier circuit and a switching element and a secondary DC power supply circuit to which a load is connected are connected and coupled via a transformer to connect the switching element. ON-OF
It is formed so as to supply power from the primary side to the secondary side by F control, and for overcurrent protection when it is detected that the current flowing through the switching element exceeds the set current value using the current detection resistor. Switching element is turned off via the element
In a switching power supply device provided with an overcurrent protection circuit, a constant voltage element for increasing an apparent operating voltage of the overcurrent protection element is connected between the overcurrent protection element and a negative electrode circuit of the rectifier circuit. And a current value switching element is connected to the constant voltage element, and a charging resistor and a charging capacitor connected in series between the positive side of the auxiliary winding of the transformer and the negative electric path of the rectifying circuit, and this charging A frequency detecting circuit formed of a discharging resistor connected in parallel with the charging resistor and in series with the charging resistor, and connecting the charging resistor and the charging capacitor and the drive terminal of the current value switching element. , Is characterized.

[0015]

In the case of the present invention having the above-described structure, a large current flows at the time of turning on the power supply of the primary DC power supply circuit and at the time of starting thereafter since the smoothing capacitor on the secondary DC power supply circuit side has not been charged. The maximum load seen from the primary DC power supply circuit side. Therefore, the current flowing through the switching element increases and the oscillation frequency decreases. That is, the oscillation frequency of the control power supply is lowered and its cycle becomes longer.

When this cycle becomes longer than the charging time constant equivalent time forming the frequency detecting circuit, the charging voltage becomes high and the current value switching element is turned off. Therefore, since the constant voltage element is substantially connected in series to the overcurrent protection element,
The apparent operating voltage of the overcurrent protection element increases by the operating voltage of the constant voltage element. That is, the set current value for overcurrent protection is switched to a high set current value. Therefore, a smooth start can be performed.

When the load becomes steady, the current flowing through the switching element decreases, the oscillation frequency increases, and the cycle becomes short. When this cycle becomes shorter than the time corresponding to the charging time constant of the frequency detection circuit, the current value switching element is turned on. Therefore, since the constant voltage element is bypassed, the predetermined operating voltage of the overcurrent protection element is obtained. That is, since the set current value can be switched to a low set low current value, necessary and sufficient overcurrent protection can be performed even for a light load during a steady load.

[0018]

Embodiments of the present invention will be described below with reference to the drawings. As shown in FIG. 1, the present switching power supply device is of the same RCC type self-excited oscillation type (frequency fluctuation type) as that of the conventional example (FIG. 3) and has a constant voltage element 55.
A current value switching element 56 and a frequency detection circuit 50 are provided, and the set current value for overcurrent protection corresponds to the set high current value DI2 corresponding to the maximum load at startup and the light load during steady load. It is configured so that it can be automatically switched between two stages, the set low current value DI1 and the actual load.

The same parts as those in the conventional example (FIG. 4) are designated by the same reference numerals, and their description will be simplified or omitted.

In FIG. 1, a constant voltage element 55 composed of a Zener diode is connected between the drain of the overcurrent protection element 16 composed of a MOS.FET and the negative electrode circuit Ln of the rectifier circuit 14. The constant voltage element 55 apparently raises the drive voltage (operating voltage) Vd of the overcurrent protection element 16 by the operating voltage Vz. That is, the apparent operating voltage is (Vd + Vz). That is, the set current value for overcurrent protection is switched to the set high current value DI2 shown in FIG.

A current value switching element (transistor) 56 is connected in parallel with the constant voltage element 55 between the drain of the overcurrent protection element 16 and the negative electrode circuit Ln.
Therefore, if the current value switching element 56 is turned on, the constant voltage element 55 can be bypassed, so that the operating voltage (Vd) of the overcurrent protection element 16 can be switched to a predetermined value. That is, the set current value can be switched to the set low current value DI1 shown in FIG.

Next, the frequency detection circuit 50 is connected to the transformer 2
It is connected between the diode R10 that oscillates and generates the control power source Vc on the positive side of the auxiliary winding L12 of 0 and the resistor R10, and between the negative electrode circuit Ln of the rectifier circuit 14.

That is, the charging resistor 51 and the charging capacitor C51 are connected in series, and the charging capacitor C51 is connected.
A discharge resistor 52 is connected in parallel with 51.
The charging resistor R51 and the charging capacitor C51 are electrically connected to the drive terminal (base) of the current value switching element (transistor) 56.

The voltage applied to the switching element 15 and the flowing current are the same as those in the conventional example (FIG. 4) when the load 35 is a light steady load, as shown in FIG. Although the oscillation frequency f1 (cycle T1) is high (short), the oscillation frequency f2 (cycle T2) is low (long) under the transient current Ip at the maximum load at startup.

Therefore, if the charging time constant (R51 × C51) of the frequency detection circuit 50 is set to be equal to or longer than the cycle T2 at the maximum load, the charging voltage Va is established at the maximum load and is less than that including the steady load. Charging voltage Va at load
Is not established. That is, the set current value (DI2, DI
The switching point of 1) (frequency f2 or higher) can be detected.

In the embodiment having such a configuration, the AC power source 1
When 1 is turned on and the primary DC power supply circuit 10 is started, a large current flows for charging the smoothing capacitors C21 and C22 of the secondary DC power supply circuit 30, and thus the maximum load is achieved.
That is, the voltage waveform and the current waveform of the switching element 15 are as shown in FIG. The voltage is made constant by the voltage control element 40, but the transient current Ip is very large, the oscillation frequency f2 is low, and the cycle T2 is long.

Therefore, a control power supply (V
c) flows into the frequency detection circuit 50, and its cycle T2 becomes equal to or greater than the charging time constant, so that the charging voltage Va is established. As a result, the current value switching element 56 is turned off, and the series connection of the constant voltage element 55 and the overcurrent protection element 16 is enabled.

Thus, the operating voltage (driving voltage Vd) of the overcurrent protection element 16 is equal to the operating voltage Vz of the constant voltage element 55.
Apparently increased by the amount. That is, the set current value for overcurrent protection is switched to the set high current value DI2. That is, even if the large transient current Ip flowing through the switching element 15 is detected by the current detection resistor 18, the large set high current value DI2
The overcurrent protection element 16 is turned on
do not do. Therefore, it is possible to prevent the unnecessary overcurrent protection operation from being started, and it is possible to smoothly start even at the maximum load.

In FIG. 2, after the time t, the smoothing capacitors C21 and C22 of the secondary DC power supply circuit 30 are sufficiently charged, and the load 35 becomes a light steady load. Therefore, the steady-state current In flowing through the switching element 15 is small and the voltage is constant. At this time, the oscillation frequency f1 is high and the cycle T1 is short.

Since the period T1 does not exceed the charging time constant of the frequency detecting circuit 50, the charging voltage Va is not established.
That is, the current value switching element 56 is turned on and the constant voltage element 55 is bypassed. That is, the set current value can be switched to the set low current value DI1. Therefore,
It is possible to perform appropriate overcurrent protection such as reducing current stress of each component and reducing power consumption.

According to this embodiment, however, the constant voltage element 55, the current value switching element 56 and the frequency detection circuit 50 are provided so that the set current value for overcurrent protection corresponds to the maximum load at the time of starting. Since it is configured to be automatically switchable according to the actual load in two stages of the set high current value DI2 and the set low current value DI1 corresponding to the light load at the time of steady load, either of the maximum load and the steady load It is also possible to exert a reasonable overcurrent protection function. Therefore, it is possible to avoid an unnecessary overcurrent protection operation at the time of start-up, reduce the current stress of the component due to the overcurrent protection in the steady state and reduce the power consumption, and moreover, the setting of the current detection resistor 18 is easy and the applicability is expanded. it can.

Further, since the set current value is switched to two stages (DI2, DI2) depending on whether or not the constant voltage element 55 connected in series to the overcurrent protection element 16 is bypassed, the circuit structure is formed. Is very easy and
Since it is not necessary to increase the resistance value of the current detection resistor 18 or to provide a plurality of resistors, it is possible to further reduce power consumption and prevent overheating.

Further, the frequency detection circuit 50 includes the transformer 20.
Since it is formed of the charging resistor 56 and the charging capacitor C51 connected to the auxiliary winding L12, the selection setting of the detection frequency (time) is simple and the applicability is wide. Further, no special power supply device is required.

Further, since the overcurrent protection is performed with the different set current values DI2 and DI1 for the maximum load and the steady load, the smoothing capacitors C1 and C2 on the secondary side are provided.
The flexibility in selecting the capacity of the capacitor can be greatly expanded, and load fluctuation characteristics and high voltage quality can be further improved.

[0035]

According to the present invention, a constant voltage element, a current value switching element, and a frequency detection circuit are provided, and a set current value for overcurrent protection is set to a set high current value corresponding to the maximum load at startup. Since it is configured so that it can be automatically switched between two stages, a set low current value corresponding to a light load during a steady load, according to the actual load, a constant voltage element, a current value switching element, and a frequency detection circuit are provided. , Automatic switching of the set current value for overcurrent protection into two stages, a set high current value corresponding to the maximum load at startup and a set low current value corresponding to a light load at steady load, according to the actual load Since it is configured to be possible, it is possible to exert a valid overcurrent protection function for both the maximum load and the steady load. Therefore, unnecessary overcurrent protection operation at startup can be avoided, current stress of components due to overcurrent protection during steady state can be reduced and power consumption can be reduced, and the current detection resistor can be easily set and the applicability can be expanded. .

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention.

FIG. 2 is a timing chart for explaining the operation.

FIG. 3 is a circuit diagram for explaining a conventional example.

FIG. 4 is also a timing chart for explaining the operation.

[Explanation of symbols]

 10 Primary DC power supply circuit 11 AC power supply 12 Switch 13 Noise filter 14 Rectifier circuit 15 Switching element 16 Overcurrent protection element 18 Current detection resistor 20 Transformer 30 Secondary DC power supply circuit 35 Load 40 Voltage control element 45 Stabilization circuit 46 Feedback Signal generation circuit 50 Frequency detection circuit 55 Constant voltage element 56 Current value switching element C10 Smoothing capacitor C51 Charging capacitor DI Setting current value DI1 Setting low current value DI2 Setting high current value I current L12 Auxiliary winding Ln Negative electric circuit R51 Charging resistance R52 Discharge resistance Va Charging voltage Vc Control voltage Vd Drive voltage Vz Operating voltage

Claims (1)

[Claims] 1. A primary DC power supply circuit including a rectifier circuit and a switching element and a secondary DC power supply circuit to which a load is connected are connected and coupled via a transformer to turn on the switching element.
It is formed to supply power from the primary side to the secondary side by OFF control, and overcurrent protection is performed when it is detected that the current flowing through the switching element exceeds the set current value using the current detection resistor. In a switching power supply device provided with an overcurrent protection circuit for turning off a switching element via a protection element, an apparent operating voltage of the overcurrent protection element is provided between the overcurrent protection element and a negative electrode circuit of the rectifier circuit. And a current value switching element connected to this constant voltage element, and charging connected in series between the positive electrode side of the auxiliary winding of the transformer and the negative electrode circuit of the rectifier circuit. A frequency detection circuit formed of a resistor and a charging capacitor and a discharging resistor connected in parallel with the charging capacitor and in series with the charging capacitor is provided, and the charging resistor is charged with the frequency detecting circuit. A switching power supply device characterized in that a current-switching element and a drive terminal of the current value switching element are connected to each other.