JPH06106020B2 - Switching regulator - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a switching regulator.

[Conventional technology]

As shown in FIG. 4, a typical conventional separately-excited switching regulator is a switching element 4 connected to a DC power source 1 via a primary winding 3 of a transistor 2 and a secondary element of a transformer 2. Rectifying and smoothing circuit 10 including diodes 6, 7 connected to winding 5, reactor 8 and capacitor 9
And a control circuit 12 for controlling the switching element 4 at a constant voltage based on the outputs of the DC output terminals 11a and 11b, and an overcurrent protection circuit including a current detection resistor 13 connected in series to the switching element 4. There is. More specifically, the voltage control circuit 12 includes a voltage detection resistor connected between the output terminals 11a and 11b.
14 and 15, the voltage detected here is input to the error amplifier 17 together with the reference voltage of the reference voltage source 16, the light emitting diode 18 emits light based on the output corresponding to the difference between the two, and the light emitting diode 18 The output of the optically coupled phototransistor 19 becomes the input of the voltage comparator 21 via the amplifier 20. The voltage comparator 21 compares the triangular wave supplied from the triangular wave generator 22 with the output of the error amplifier 17, forms a PWM pulse, and supplies it to the switching element 4.

To perform overcurrent protection, the voltage across the current detection resistor 13 is applied to the capacitor 23 via the sampling switch 22. The voltage V _C held by the capacitor 23 is
Is input and is compared with the reference voltage of the reference voltage source 25.
Since the reference voltage source 25 is set to correspond to the overcurrent level, if the voltage of the capacitor 23 becomes higher than the reference voltage due to the overcurrent, the overcurrent detection voltage comparator
The output state of 24 is inverted, whereby the PWM voltage comparator 21 is controlled via the diode 26 and the switching element 4 is turned off. Since the voltage comparator 21 is also used for voltage control, a reverse current blocking diode 27 is connected between the amplifier 20 and the PWM voltage comparator 21.

[Problems to be Solved by the Invention]

By the way, in the switching regulator of Fig. 4, PWM
It was necessary to use a relatively expensive voltage comparator 21 to form the pulse. Further, as shown in FIG. 4, a current detection resistor 13 is connected in series with the switching element 4.
Is connected and the current is detected on the primary side of the transformer 2,
Compared with the case where a current detector is provided on the secondary side, it has an advantage that an insulating means between the primary and the secondary in the current detection system is unnecessary, but on the other hand, the primary side current has a pulse waveform, As shown in FIG. 4, it is necessary to turn on the sampling switch 22 in synchronization with the ON period of the switching element 4, sample the current, and hold it by the capacitor 23.

As another overcurrent protection circuit, there is one that provides an overcurrent latch flip-flop at the output stage of the overcurrent detection voltage comparator 24 and controls the switching element 4 by latching the overcurrent. The circuit structure inevitably becomes complicated and expensive to provide.

Therefore, an object of the present invention is to provide a switching regulator that can perform voltage control or voltage control and overcurrent protection with a simple and inexpensive circuit.

[Means for Solving the Problems]

The present invention for achieving the above object includes a series circuit of a winding connected to a DC power source and a switching element, and a controlled DC output voltage is obtained from an output rectifying / smoothing circuit by connecting and disconnecting the switching element. In the switching regulator configured as described above, a voltage detection unit that detects the DC output voltage, and an error signal formation circuit that forms an error signal between the detection value obtained from the voltage detection unit and the constant voltage control reference voltage. Controlling a capacitor charged with a current determined by the error signal, a discharging switch connected in parallel with the capacitor to periodically discharge the capacitor, and a discharging switch in a non-discharging state. Generates a square wave having a first voltage level and a second voltage level for controlling the discharge state with a predetermined period and a predetermined pulse width. A square wave generator for generating an output for turning on the switching element in response to the square wave being at the first voltage level and the voltage of the capacitor being lower than a predetermined voltage level. And a gate circuit that generates an output for controlling off of the switching element during a period when the wave is at the second voltage level and a period when the voltage of the capacitor is equal to or higher than the predetermined voltage level. Related to.

As described in claim 2, the capacitor is configured to be charged based on the output of the comparator for detecting the overcurrent, and one capacitor can be used for both the voltage control and the overcurrent protection.

[Operation and Effect of the Invention] In any one of claims 1 and 2, the control signal (PWM pulse) for turning on / off the switching element can be formed by a gate circuit which is cheaper than the comparator, and Cost reduction is achieved.

According to the invention of claim 2, since the capacitor is used for both the voltage control and the overcurrent protection control, the circuit configuration is simplified.

〔Example〕

Next, a switching regulator according to an embodiment of the present invention will be described with reference to FIGS. However, in FIG. 1, the same parts as those in FIG. 4 are designated by the same reference numerals, and the description thereof will be omitted.

The capacitor 30 connected between the overcurrent detection voltage comparator 24 and the ground generates a triangular wave for voltage control during normal operation, and holds the output of the comparator 24 during overcurrent. A phototransistor 19 as a variable impedance element is connected between the power supply terminal 31 and the capacitor 30 in order to control the charging of the capacitor 30 in response to the output of the error amplifier 17 for voltage control to change the slope of the triangular wave. ing.

The overcurrent detection voltage comparator 24 of FIG. 1 generates a high voltage (high level voltage) similar to the power supply voltage Vcc of the terminal 31 when an overcurrent occurs, and a low level voltage (zero volt) when a nonovercurrent occurs. Configured to occur and the terminal
It is configured to have a function of preventing current from flowing back from the capacitor 31 or the capacitor 30 to the comparator 24. Of course, instead of incorporating the backflow prevention function in the comparator 24 of FIG. 1, a diode 26 can be provided as in FIG.

A switch 32 connected in parallel with the capacitor 30 is for periodically discharging the capacitor 30, and is on / off controlled by the output of the square wave generator 33.

The square wave generator 33 generates a square wave at a constant cycle equal to the on / off cycle of the switching element 4 composed of an FET. In this embodiment, the square wave generator 33 has a first voltage level (low level).
The output of the switch 32 controls the switch 32 to be turned off, and the output of the second voltage level (high level) controls the switch 32 to be turned on.

One input terminal of the NOR gate circuit 34 is connected to the capacitor 30, the other input terminal is connected to the square wave generator 33,
The output terminal is connected to the control terminal or gate of the switching element 4.

When the circuit of FIG. 1 is in the non-overcurrent state, the overcurrent detection voltage comparator 24 is disconnected from the capacitor 30. Therefore, the capacitor 30 is charged with the current limited by the phototransistor 19. Phototransistor
The incident light amount of 19 changes according to the output of the error amplifier 17. Now, when the voltage of the output terminals 11a and 11b becomes high due to the rise of the voltage of the power source 1 or the reduction of the load current, the error amplifier
The output voltage of 17 also becomes high as shown in FIG. As a result, the output light amount of the light emitting diode 18 increases, the resistance value of the phototransistor 19 decreases, the time until the voltage of the capacitor 30 reaches the threshold value Vth is shortened, and the switching element generated from the NOR gate circuit 34 is generated. The width of the ON control pulse of 4 becomes narrow. Contrary to the above, when the output voltage drops below the reference value, the output of the error amplifier 17 also drops, the light emission amount of the light emitting diode 18 decreases, the resistance of the phototransistor 19 increases, and the capacitor 30 is charged. Since the CR time constant increases, the charging speed of the capacitor 30 decreases, and the pulse width obtained from the NOR gate circuit 34 decreases. To explain this in more detail, the discharging switch 32 is turned on during the high level period t _{0 to} t ₁ of the square wave of FIG. 2 (B), and the voltage of the capacitor 30 is as shown in FIG. 2 (C). It will be zero volts. When the square wave becomes low level at time t ₁ , the discharging switch 32 is turned off, and the charging of the capacitor 30 is started. The capacitor 30 is charged via the phototransistor 19. The phototransistor 19 has a low resistance value when the error output shown in FIG. 2A is large, and has a high resistance value when the error output is small. Therefore, the voltage of the capacitor 30, that is, the slope of the triangular wave, changes according to the error output. t ₁
When the charging of the capacitor 30 is started from the time point with the time constant controlled by the phototransistor 19, the voltage of the capacitor 30 gradually increases as shown in FIG. 2 (C). Since the NOR gate circuit 34 has the threshold value V _th , when the capacitor voltage becomes equal to or higher than the threshold value V _th , the logical operation is started. t ₁
Since both inputs of the NOR gate circuit 34 are at the low level in the period of up to t ₂ , a high level output is obtained as shown in FIG. 2 (D). t ₂ ~t ₃ in the period one input is high, since the other input goes low, the output of the NOR gate circuit 34 becomes low level. Since the second view becomes the t ₃ time connexion discharging switch 32, such a square wave is at a high level as shown in (B) is turned on, the capacitor voltage of the second view (C) becomes zero. In the period of t _{3 to} t ₄ , one input is low level and the other input is high level, so the output of the NOR gate circuit 34 is low level. When both inputs become low level again in the period of t _4, the output of the NOR gate circuit 34 becomes high level. The width of the output pulse of the NOR gate circuit 34 shown in FIG. 2 (D) changes in inverse proportion to the error output of FIG. 2 (A).
That is, when the DC output voltage becomes high, the switching element 4
The ON period of is shortened and a constant voltage is achieved. In addition,
The maximum on-time width of the switching element 4 is equal to the low level period of the square wave.

FIG. 3 shows the waveform of each part when an overcurrent state occurs. t ₀
If an overcurrent condition such as a load short circuit occurs at this point, the output voltage drops and the output of the error amplifier 17 shown in FIG. 3 (A) also drops. On the other hand, since the current flowing through the current detection resistor 13 increases, the detection voltage V _d obtained from the current detection resistor 13 becomes higher than the reference voltage V _r of the reference voltage source 25 as shown in FIG. 3 (E), The output of the comparator 24 becomes high level as shown in FIG. The capacitor 30 is rapidly charged by the output voltage of the comparator 24 and becomes a value exceeding the threshold value V _th of the NOR gate circuit 34 as shown in FIG. 3 (C).
As a result, since one input of the NOR gate circuit 34 is at high level and the other input is at low level in the period of t _{0 to} t ₁ , the output becomes low level and the switching element 4 is controlled to be off. At time t ₁ , the square wave shown in FIG. 3 (B) becomes high level and the discharge switch 32 is turned on, so that the voltage of the capacitor 30 becomes zero. Even if the voltage of the capacitor 30 becomes zero, the NOR gate circuit 3
The output of 4 remains low until time t ₂ . When square wave at t ₂ time goes low, both inputs of NOR gate 34 goes low, because its output is at a high level, switching-element 4 is turned on again. If the overcurrent state of the load is not eliminated, the output of the comparator 24 becomes high level again at the time point t ₃ , and the switching element 4 is turned on at the time point t _0.
Turns off.

As shown in FIG. 3 (E), as the amplitude of the detection voltage V _d corresponding to the current flowing through the switching element 4 increases, the output pulse width of the NOR gate circuit 34, that is, as shown in FIG. 3 (D), The on-time width of the switching element 4 becomes narrow.

In the switching regulator of the present embodiment, overcurrent can be prevented with a simple circuit including the comparator 24, the capacitor 30, the discharging switch 32, the square wave generator 33, and the NOR gate circuit 34. it can. Further, since the NOR gate circuit 34 is provided instead of the comparator 21 of FIG. 4, the cost can be reduced. Further, since the capacitor 30 functions not only for holding the overcurrent detection but also for generating the triangular wave for voltage control, the circuit configuration is simplified.

[Modification]

The present invention is not limited to the above embodiments, but can be modified. For example, the discharge switch 32 may be configured to be on-controlled during the low level period of the square wave, and the NOR gate circuit 34 may be replaced with an exclusive OR gate.

Also, a voltage control circuit may be provided independently, and the capacitor 30 and the like may be used only for overcurrent protection.

Further, the present invention can be applied to various switching regulators having different main circuits.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a switching regulator according to an embodiment of the present invention, and FIG. 2 is a waveform diagram showing voltages at points A to D during voltage control of the switching regulator shown in FIG. FIG. 3 is a waveform diagram showing the state of each portion of the switching regulator shown in FIG. 1 during overcurrent, and FIG. 4 is a circuit diagram showing a conventional switching regulator. 1 ... Power supply, 4 ... Switching element, 13 ... Current detection resistor,
24 ... Comparator, 30 ... Capacitor, 32 ... Discharge switch, 33 ... Square wave generator, 34 ... NOR gate circuit.

Claims (2)

[Claims] 1. A switching circuit comprising a series circuit of a winding connected to a DC power source and a switching element, wherein the switching element is intermittently connected to obtain a controlled DC output voltage from an output rectifying / smoothing circuit. In the regulator, a voltage detection unit that detects the DC output voltage, an error signal forming circuit that forms an error signal between the detection value obtained from the voltage detection unit and a constant voltage control reference voltage, and the error signal A capacitor that is charged with a charged current, a discharging switch that is connected in parallel with the capacitor to periodically discharge the capacitor, and a first voltage level that controls the discharging switch to a non-discharging state. A square wave having a second voltage level for controlling the discharge state is generated with a predetermined period and a predetermined pulse width, and A square wave generator connected to the discharge switch; a capacitor connected to the square wave generator; and the square wave is at the first voltage level and the voltage of the capacitor is higher than a predetermined voltage level. Generating an output that controls the switching element to turn on in response to a low voltage,
A gate circuit that generates an output for controlling the switching element to be turned off during the period when the square wave is at the second voltage level and the period when the voltage of the capacitor is equal to or higher than the predetermined voltage level. Switching regulator. 2. A switching circuit comprising a series circuit of a winding connected to a DC power source and a switching element, wherein the switching element is intermittently connected to obtain a controlled DC output voltage from an output rectifying / smoothing circuit. In the regulator, a current detector for detecting a current flowing through the switching element, a reference voltage source for generating a reference voltage corresponding to an overcurrent level of a current flowing through the switching element, and a detection obtained from the current detector A voltage comparator that compares a voltage and the reference voltage, a voltage detection unit that detects the DC output voltage, and an error that forms an error signal between the detection value obtained from the voltage detection unit and the constant voltage control reference voltage. The signal forming circuit and the voltage comparator are determined by the error signal when the overcurrent is not detected. Current, the capacitor being charged by the output of the voltage comparator at a speed faster than the charging by the error signal when the voltage comparator detects an overcurrent, and the capacitor for periodically discharging the capacitor. A square wave composed of a discharge switch connected in parallel and a first voltage level for controlling the discharge switch to a non-discharge state and a second voltage level for controlling the discharge switch to have a predetermined period and a predetermined pulse width. A square wave generator configured to generate and connected to the discharge switch; a square wave generator connected to the capacitor and the square wave generator, wherein the square wave is at the first voltage level and the capacitor; Generates an output for controlling the switching element to turn on in response to the voltage of the switch being lower than a predetermined voltage level. Switching regulator but which is characterized in that a gate circuit which generates an output voltage of period and said capacitor in said second voltage level is turned off controlling the switching element during a period of more than the predetermined voltage level.