JPS634419B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a power supply device equipped with protection means for limiting output in the event of overload.

1 and 3 are circuit diagrams showing configuration examples of conventional power supply devices, respectively. FIG. 2 shows the output characteristics of the power supply device shown in FIG. 1, and FIG. 4 shows the power supply device shown in FIG. 3. FIG.

In Figures 1 and 3, 1 is the input power supply, 2
is the control power supply, 3 is the main transformer (turns ratio N ₁ :N ₂ ),
4 is a current detection transformer (turn ratio 1:n), 5 is a transistor for the main switch, 6 is a circuit that serves both to suppress the surge voltage applied to the main switch 5 and reset the main transformer 3, and 7 is a rectifier/smoothing circuit. , 8 is a comparator for output current limitation, 9 is a PWM control circuit, 10 is an RS flip-flop, 11 is an AND circuit, 12 is a diode, 13 is a Zener voltage
A constant voltage diode _E1 , 14 a resistor, 15 a resistor with a resistance value R, 16 a resistor, 17 a transistor, and 18 a constant voltage diode with a Zener voltage _E2 .

First, referring to FIG. 1, the basic operation of the power supply device will be explained. The output voltage V is taken into a PWM (pulse width modulation) control circuit 9. The control circuit 9 compares the output voltage V with a certain reference voltage (not shown), outputs a pulse whose pulse width has changed in relation to the magnitude of the error voltage, and outputs a pulse to the base of the transistor 5. (ignoring the AND circuit 11 for now), thereby controlling the on/off period ratio of the transistor 5, and connecting the input power supply 1 to the primary winding of the main transformer 3, transistor 5, and the primary winding of the transformer 4. Controls on/off the current flowing through the line, and output voltage V
is controlled so that it remains constant.

Now, when the positive input voltage E of the comparator 8 does not exceed the Zener voltage _E1 of the voltage regulator diode 13, the output of the comparator 8 is at a low level, the output Q of the flip-flop 10 is at a high level, and the AND circuit 11 is Since it is open, the pulse that is the output of the PWM control circuit 9 passes through the AND circuit 11 and is applied to the base of the transistor 5 to control the output voltage V, as described above.

Next, as the output current I increases, the +input voltage E of the comparator 8 rises and reaches the Zener voltage _E1 , and the output of the comparator 8 becomes high level, which changes the output Q of the RS flip-flop 10 to low level. By closing the AND circuit 11, the pulse width of the pulse applied from the PWM control circuit 9 to the base of the transistor 5 is further controlled to keep the output current at a constant value. Here, the magnitude of the output current I and the comparator 8
The relationship between +input voltage E will be explained. If the current flowing through the resistor 15 is i, then i=E/R. Now, it is assumed that the resistance value of the resistor 14 is sufficiently larger than the resistance value R of the resistor 15, and the current flowing through the resistor 14 can be ignored. Then, since the turns ratio of the transformer 4 is 1:n, the current flowing to the primary side of the current detection transformer 4 is (E/R)×n. The primary current of the transformer can be considered as the primary current of the main transformer 3, so the secondary current of the transformer 3 is (E/R) since the turns ratio of the transformer 3 is N ₁ :N ₂ ×n×N ₁ /N ₂ =nN ₁ E/N ₂ R. Since the secondary current of the transformer 3 can be considered as the output current I, the following equation holds true after all.

I=nN ₁ E/N ₂ R In other words, when the output current I increases and reaches nN ₁ E ₁ /N ₂ R, the output of the comparator 8 changes to low level and the output current is controlled to be constant, as described above. . From the above, it can be said that the Zener voltage E ₁ of the constant voltage diode 13 represents a current reference value for output current control.

It is already clear that the output characteristic of the power supply device shown in FIG. 1 is such that the output current I is controlled to a constant value of nN ₁ E ₁ /N ₂ R, as shown in FIG. 2.

See Figure 3. When the positive input voltage E of the comparator 8 does not exceed the Zener voltage _E1 of the diode 13, its operation is the same as that of the circuit of FIG. Also, the +input voltage E of comparator 8 is
When E ₁ is reached, the output voltage V changes to the voltage V
As long as the sum (V+E ₂ +V _BE ) of the Zener voltage E ₂ of the voltage regulator diode 18 and the base-emitter voltage V _BE of the transistor 17 reaches the Zener voltage E ₁ of the voltage regulator diode 13, , a current control action is performed to keep the output current I constant at a value of nN ₁ E ₁ /N ₂ R, in the same way as described for the circuit of FIG. However, (V+E ₂ +V _BE )
When the output voltage V decreases to the extent that E becomes less than ₁ ,
The current that had previously flowed from the control power source 2 to the constant voltage diode 13 through the resistor 16 no longer flows to the diode 13, but now flows to the transistor 17. In other words, the negative input voltage of comparator 8 has started to drop from _E1 to below, so when the positive input voltage E also reaches a value lower than _E1 , the output of comparator 8 becomes high level. On the other hand, the output Q of the RS flip-flop 10 is set to a low level, the AND circuit 11 is closed, and the pulse width of the output pulse from the PWM control circuit 9 is limited to an earlier period than in the previous case. In other words, this is the same as reducing the current reference value for output current control. Therefore, as shown in FIG. 4, the output characteristics of the power supply device shown _{in FIG} . ₃ are as follows: The current value is also narrowed down and reduced, eventually
This results in a reduction to nN ₁ (E ₂ +V _BE )/N ₂ R.

When using a power supply with the characteristics shown in Figure 2, if a short circuit continues for a long time at the load end, the converter continues to flow current close to the rated output current to the load, causing the short circuit to generate heat. This has the drawback that it may become dangerous and the current of the main switch (transistor 5) in the power supply device increases.

Furthermore, according to the power supply device having the characteristics shown in Fig. 4,
Even if a short circuit continues for a long time at the load end, the output current is reduced and becomes smaller, thereby avoiding the risk of heat generation at the short circuit, and also suppressing an increase in the main switch current in the power supply. but,
When a protective fuse is inserted between the power supply device and the load, there is a drawback that the inserted fuse cannot be blown.

The present invention has been made in order to eliminate the drawbacks of the conventional power supply device as described above, and an object of the present invention is to prevent a fuse from forming between the power supply device and the load when the load side of the power supply device is short-circuited. If it is inserted,
A power supply device that can blow the fuse, suppress heat generation at the short-circuited part of the load in the event of a long-term load short-circuit, and suppress the increase in current in the main switch in the power supply device even if the fuse is not inserted. It is about providing. An embodiment of the present invention will be described in detail below with reference to the drawings.

FIG. 5 shows a circuit showing one embodiment of the present invention, and the circuit components 1 to 18 and their functions are the same as those explained in the conventional circuit (FIGS. 1 and 3). Further, 19 is a comparator that outputs a signal to output c when the a input becomes equal to or less than the b input, 20 is a delay circuit that delays the input signal, and 2
1 is input terminal g when a signal is input to input terminal f
22 is a resistor, 23 is a Zener voltage {E ₁ −(E ₂
+V _BE )} is a constant voltage diode.

Regarding this operation, first, the output voltage V is {E ₁
−(E ₂ +V _BE )} or above, the same operation as in the circuits of FIGS. 1 and 3 is performed. Next, if the output voltage V drops below {E ₁ -(E ₂ +V _BE )} for a certain short period of time within the delay time t of the delay circuit 20, the output h of the analog gate circuit 21 becomes Since it is disconnected from input g, transistor 1
7 is in the off state, and the negative input of the comparator 8 remains at the Zener voltage _E1 of the diode control gate 13.
In order to limit the output pulse of the PWM control circuit 9 as explained above when the positive input reaches E ₁ , the straight line (solid line) of I=nN ₁ E ₁ /(N ₂ R) is shown in FIG. It exhibits constant current drooping characteristics as shown in . Furthermore, if the output voltage V is equal to or less than {E ₁ -(E ₂ +V _BE )} during the delay time t of the delay circuit 20, the output h of the analog gate circuit 21 becomes the input g, that is, the output voltage V of the converter. , the negative input of the comparator 8 becomes (V+E ₂ +V _BE ), and the output current value decreases as the output voltage V decreases, similar to the circuit operation in FIG. The characteristic is shown by the diagonally descending diagonal line (dashed line). Therefore, in this case, after the delay time t has elapsed, the operating characteristic shifts from a drooping characteristic to a diagonal characteristic, as shown by a straight line K.

Because of this operation, in the case of a load short circuit, if a fuse is connected between the power supply and the load, the fuse can be blown, and if the load is not cut off by the fuse (or no fuse is connected), the fuse can be blown. In case of long-term short-circuit accidents, it is effective in preventing heat generation in the short-circuited part.

As explained above, according to the present invention, in the case of a load short circuit, the fuse connected between the power supply device and the load can be blown, and the load that is not cut off by the fuse (or to which the fuse is not connected) In the case of a long-term short circuit, there is an advantage that a highly safe power supply device can be constructed because heat generation at the short circuit point is prevented.

[Brief explanation of the drawing]

Figures 1 and 3 are circuit diagrams showing configuration examples of conventional power supply devices, Figure 2 shows the output characteristics of the circuit in Figure 1, and Figure 4 shows the output characteristics of the circuit in Figure 3, respectively. FIG. 5 is a circuit diagram showing an embodiment of the present invention, and FIG. 6 is a characteristic diagram showing the output characteristics of the circuit shown in FIG. Explanation of symbols 1...Input power supply, 2...Control power supply, 3...
Main transformer, 4... Current detection transformer, 5... Main switch transistor, 6... Circuit that also serves to suppress surge voltage applied to the main switch 5 and reset the main transformer 3, 7... Rectifier/smoothing circuit, 8... Output Current limit comparator, 9...PWM control circuit, 1
0...RS flip-flop, 11...AND circuit,
12... Diode, 13... Constant voltage diode (voltage E ₁ ), 14... Resistor, 15... Resistor (resistance value R),
16... Resistor, 17... Transistor, 18... Constant voltage diode (voltage E ₂ ), 19... Comparator that outputs a signal to output c when the a input becomes lower than the b input, 20... Extension circuit, 21... Input terminal Analog gate circuit that outputs a signal at input terminal g to output terminal h when a signal is input to f, 22...Resistor, 23...
Constant voltage diode (voltage E ₁ − (E ₂ +V _BE ).

Claims (1)

[Claims] 1 A DC-AC inverter section and a first on/off switch that compares its output voltage with a certain first reference value and controls the on/off period ratio of a switch element that controls the inverter section based on the error voltage. - In a power supply device comprising an off control means, when the output current exceeds a reference value (hereinafter referred to as current reference value), the switch element is switched on with priority over the first on/off control means. a second on/off control means for controlling an on/off period ratio; and a current for reducing the current reference value itself when the output voltage falls below a certain second reference value for a certain period of time or more. A power supply device comprising a reference value reducing means.