JPS6349107Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to an overcurrent protection circuit for a switching regulator that performs overcurrent protection by detecting a primary side peak current that changes in proportion to the output current.

FIG. 1 shows a conventional example of a switching regulator equipped with an overcurrent protection circuit. In the figure, V _IN
is the input AC power supply, RF is the rectifier, _C0 is the smoothing capacitor, T is the transformer, _Q1 is the switching transistor, _R1 is the resistor for detecting the primary side peak voltage I _P , and the output of the rectifier RF is A series circuit consisting of the primary winding of the transformer T, a transistor Q ₁ and a resistor R ₁ is connected to the end.

CN is a control unit that applies a switching pulse to the transistor _Q1 , and CP is a comparator.The current detection voltage _VF is input to the comparator CP and compared with the reference voltage _VR . The voltage V _F is obtained by rectifying the voltage V _D across the resistor R ₁ with the diode D ₁ and connecting it to the capacitor.
After smoothing with C ₁ , divide the voltage with resistors R ₂ and R ₃ to obtain
Further, the voltage V _R is obtained by dividing the voltage V _E , which is the power supply voltage of the control unit CN, by resistors R ₄ and R ₅ ,
When V _R < V _F , the output of the comparator CP controls the control unit CN so that the output current I ₀ decreases, that is, the transistor
Control so that the on time of _Q1 is shortened.

The voltage V _E is obtained via a transistor Q ₂ from the Zener voltage of a Zener diode ZD ₁ connected across the output terminals of the rectifier RF via a resistor R ₆ ;
Approximately equal to the Zener voltage, a constant voltage is maintained even if the voltage of the input power supply V _IN fluctuates.

D ₂ is a diode that rectifies the secondary voltage of the transformer T, C ₂ is a smoothing capacitor, and R ₀ is the output voltage V ₀
The detected value is fed back to control the width of the switching panel that drives the control unit CN and the switching transistor _Q1 so that the output voltage _V0 is constant. Normally, this operation maintains the output voltage V ₀ at a constant voltage.

In this way, the switching operation of the switching transistor Q ₁ is controlled by the control unit CN, and when the output voltage V 0 and the current I ₀ are within the rated value range, the switching operation is controlled by the control unit CN, and when the output voltage V ₀ and the current I 0 are within the rated value range, the switching operation is controlled by the control unit CN. The output voltage is controlled to be constant even when

Also, if the output current I ₀ exceeds the rated value,
An increase in the peak current I _P on the primary side of the transformer T is detected by the resistor R ₁ and compared with the reference voltage V _R in the comparator CP. Then, if V _R <V _F , the switching operation of the switching transistor Q ₁ is controlled by the control unit CN, and the output voltage drops. That is, damage to the switching regulator due to overcurrent is prevented.

However, in the above circuit configuration, the emitter voltage V _E of the transistor Q ₂ is always maintained at a constant voltage even if the power supply voltage _{V IN fluctuates .} The divided reference voltage V _R is also a constant voltage, and as the voltage of the input power supply V _IN increases, the peak current I _P increases, and its detection voltage, and therefore the voltages V _F and V _D , increase. When V _R <V _F , the output of the comparator CP is applied to the control unit CN,
As the width of the switching panel of the switching transistor _Q1 becomes narrower, the output voltage begins to drop.

The droop point of this output voltage V ₀ is set to be point a in Figure 2 when the input voltage (voltage at the output end of rectifier RF) V _C is the rated voltage, but as mentioned above, the voltage V Since _R is a constant voltage,
If the input voltage increases and the output current I ₀ increases,
The voltage V _F becomes higher and higher, and soon the state of V _F > V _R is reached. As a result, the drooping point becomes point b on the left side of the set point at the rated voltage. In addition, when the input voltage V _C decreases, the drooping point shifts to the left,
This will be point c. That is, the range of the output current I ₀ before drooping becomes considerably wide, and as a result, components that can sufficiently withstand a large output current are required, resulting in an uneconomical design.

The present invention was developed in order to eliminate the above-mentioned drawbacks, and by varying the reference voltage of the comparator according to the input power supply voltage, a switch with high overcurrent setting accuracy and contributing to economical design was created. The purpose of the present invention is to provide an overcurrent protection circuit for an angular regulator.

Hereinafter, the present invention will be explained in detail based on illustrated embodiments.

Figure 3 shows an embodiment of the present invention, in which a resistor
A Zener diode ZD ₂ and a resistor R ₇ are further connected in series to the series circuit of R ₆ and a Zener diode ZD ₁ , and one end of a resistor R ₄ is connected to the connection point of both Zener diodes ZD ₁ and ZD ₂ (conventionally, one end of the transistor Q ₂ is It differs from the vertical one in that the end (which was connected to the emitter) is connected. That is, the voltage V _B (Zena voltage of Zener diode ZD ₂ + R ₇ · I _ZD ) generated across the series circuit of Zener diode ZD ₂ and resistor R ₇ is divided by resistors R ₄ and R ₅ and the voltage of comparator CP is divided. The reference voltage V _R is used.

Note that the other parts are the same as those of the conventional device (FIG. 1), and the same components are given the same reference numerals and their explanations will be omitted.

With this configuration, if the voltage of the input power supply V _IN fluctuates, the DC voltage V _C will also fluctuate, and the current flowing through the resistor R ₆ and the Zener diode will change accordingly.
The current I _ZD flowing through ZD ₂ and resistor R ₇ begins to change. In this case, the reference voltage V _R is obtained by dividing the voltage V _B by resistors R ₄ and R ₅ , and the voltage V _B is
As it fluctuates due to changes in I _ZD・R ₇ , it also fluctuates. That is, it varies depending on the voltage of the input power source.

For example, when the output current is a certain constant current,
When the voltage on the input power supply V _IN becomes low, the transistor
The width of the switching pulse of _Q1 becomes wider, and the input peak current I _P becomes lower, so that the detection voltage V _F of the comparator CP decreases. However, in this case, the DC voltage V _C decreases, the current I _ZD decreases, R ₇・I _ZD becomes smaller, and the voltage V _B decreases, causing the reference voltage V _R
will also be lower.

In this way, the voltage V _F decreases when the voltage of the input power supply V _IN becomes low, and increases when the voltage of the input power supply V IN increases, but the reference voltage V _R also fluctuates in the same way, so even if there is a voltage fluctuation in the input power supply, The overcurrent setting value of the output current is now corrected to the same value, allowing highly accurate setting.

FIG. 4 shows another embodiment of the present invention, in which the reference voltage V _R is obtained by dividing the emitter voltage V _E of the transistor Q ₂ by resistors R ₄ and R ₅ in the same way as in the conventional case. Resistor R ₇ is connected in series with ZD ₁ .

Even if this is done, as the DC voltage V _C fluctuates,
Since the values of R ₇ and I _ZD change, the voltage V _E changes,
Therefore, the reference voltage V _R also comes to vary depending on the voltage of the input power supply.

In addition, in each of the above embodiments, as the values of R ₇ and I _ZD change, the emitter voltage of transistor Q ₂ and
In other words, the power supply voltage of the control unit CN changes, but the proportion of I _ZD and R ₇ in the emitter voltage is small.
There are no practical problems.

As described above, according to the present invention, since the reference voltage is changed according to the voltage fluctuation of the input power supply, the primary side peak current, that is, compared with the reference voltage _VR , is determined by the fluctuation of the input power supply voltage. Detection voltage V _F
Even if the voltage fluctuates, it is corrected by the fluctuation of the reference voltage, and the overcurrent protection set point is always constant regardless of the voltage fluctuation of the input power supply. Furthermore, as a result, components such as diodes on the secondary side can have less margin in current capacity, making it possible to reduce size, weight, and cost. Additionally, since the overcurrent protection set point is stable, safety on the load side is increased.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing a conventional example of a switching regulator equipped with an overcurrent protection circuit, Fig. 2 is an overcurrent protection characteristic diagram, and Fig. 3 is a circuit diagram showing an embodiment of the present invention. FIG. 4 is a circuit diagram showing another embodiment (main part) of the present invention. V _IN ...Input AC power supply, RF...Rectifier, T...
Transformer, Q ₁ ... Switching transistor, CN
...Control unit, CP...Comparator, _R0 ... _R7 ...Resistor,
C ₀ ~ C ₂ ... Capacitor, ZD ₁ , ZD ₂ ... Zener diode, V ₀ ... Output voltage, I ₀ ... Output current, I _P
...Primary side peak current, V _F ...Detection voltage, V _R ...
...Reference voltage.

Claims (1)

[Scope of utility model registration request] A switching regulator in which a transformer is connected to a DC power source via a switching element, and a constant DC voltage is obtained on the secondary side of the transformer by controlling switching intervals of the switching element, etc. The primary side peak current flowing through the converter is detected and converted into a voltage, and this is compared with a reference voltage that fluctuates depending on the input power supply voltage, and the switching element is controlled to drop the output voltage in the event of an overcurrent. An overcurrent protection circuit for a switching regulator.