JPS62131759A - Controller for switching power source - Google Patents

Abstract

PURPOSE:To protect against a malfunction by generating a time signal in response to the level of an output signal of a comparator for comparing a DC output signal with an external control signal, and outputting an interruption signal when the time signal becomes a predetermined level. CONSTITUTION:A triangular signal from an oscillator 41 and an output voltage of an output voltage control amplifier 43 are compared by a comparator 42, its output is applied through an AND gate 23 to a switch transistor 46, which is turned ON, OFF. When becoming a malfunction state by an overload, the output voltage of the amplifier 43 drops the output voltage of a voltage comparator 13 becomes a low level, and a transistor 15 is turned OFF. Accordingly, a capacitor C is charged, when this charged voltage becomes a predetermined value, a flip-flop 21 is reset, and a gating operation of the gate 23 is inhibited.

Description

[Detailed description of the invention] Better than 11! The present invention relates to a control circuit for a switching power supply, and in particular,
The present invention relates to a control circuit for a switching power supply that can protect switching control elements and the like from overload caused by abnormal operation.

Switching regulators are often used as power supplies due to their low power consumption. However, in actual use, an abnormality may occur in the load to which the power supply voltage is to be supplied, so it is necessary to prevent damage to the switching control element from such a situation.

BACKGROUND ART A conventional example of a control circuit for a switching power supply that provides such protection is shown in FIG. An example of applying this to a step-down switching power supply is shown in Section 5.
As shown in the figure.

4 and 5, a conventional switching power supply control circuit 40 includes an oscillator (O3C) 41° pulse width control comparator 42, an output voltage control amplifier 43, an overcurrent protection amplifier 44, a voltage adder 45, and Output transistors 46 are connected as shown.

The switching control voltage V c ON output from the output terminal 47 of the control circuit 40 is applied to the switching transistor 5
1 by supplying the base of the transistor 5
1 is turned on and off, and the input voltage Vl11 is interrupted. This intermittent voltage is sent via flywheel diode 53 and smoothed by choke coil 55 and capacitor 57. Therefore, both output terminals 59ou- and 59ou-
9GN+1 has a stabilized DC output voltage V. uT is generated and supplied to the load 61.

For example, if an abnormality occurs in the load 61 and it is short-circuited, the output current will be ■. U□ increases.

However, this output power supply I out is connected to the current detection resistor 63.
The voltage drop that occurs across the current is applied to the overcurrent protection amplifier 44. When the potential of the inverting input terminal 65 becomes lower than that of the non-inverting input terminal 67, the output voltage V44 of the amplifier 44 increases.

On the other hand, the output voltage control amplifier 43 controls the stabilized output voltage
. , T and reference voltage ■, amplified output voltage V43
is occurring. The voltage adder 45 adds the output voltage V s 4 of the amplifier 44 to the output voltage V 43 to thereby increase the added output voltage V a s. Since this output voltage V45 is supplied to the inverting input terminal of the comparator 42, the pulse width of the output voltage of the comparator 42 becomes short. As a result, the pulse density ratio of the switching control voltage V c 011 becomes smaller, and the on period of the switching transistor 51 becomes shorter. As a result, the regulated output voltage ■. LIT will be reduced, and the protection function against overcurrent will be exhibited.

Problems to be Solved by the Invention However, in the conventional example described above, the overcurrent protection amplifier 44 is used, and therefore two additional input terminals 65 and 67 are required. Further, even in the applied device as shown in FIG. 5, it is necessary to externally connect the current detection resistor 63 and both adjustment resistors 69 and 71.

Furthermore, since current always flows through the resistor 63 even in normal conditions, there is a problem in that power loss occurs.

The present invention has been made in view of these problems, and provides a control circuit for a switching power supply that does not require a large number of external connection terminals, has simple connection relationships, and does not consume excessive power. It is an object.

Means for Solving the Problems A control circuit for a switching power supply according to the present invention includes a signal comparator that compares a DC output signal of a switching power supply with an external control signal for controlling the magnitude of the DC output signal; a pulse width control comparator that receives the output of the signal comparator and the output signal of the oscillator to control the pulse width of the output pulse signal; and a signal generating means that generates a time signal in accordance with a predetermined level of the output signal of the signal comparator. and a cutoff signal generating means that generates a cutoff signal and supplies it to the switching control signal supply means when the time signal reaches a predetermined level.

Operation In the above configuration, the output pulse signal obtained by controlling the pulse width by the pulse width control comparator is supplied to the switching power supply as a control pulse. This pulse width control is performed by the magnitude of the comparison output signal corresponding to the DC output signal of the switching power supply. When the comparison output signal reaches a predetermined level, a time signal whose signal level changes over time is generated. When this time signal reaches a predetermined level, a cutoff signal is generated, and the output pulse signal is no longer supplied to the switching power supply in the switching control signal supply means.

Therefore, due to the cutoff of the output pulse signal to the switching power supply, the switching power supply no longer performs its original function, so that protection against abnormalities is provided.

A 31-year-old man FIG. 1 shows an embodiment of the present invention. Note that parts that are the same as or correspond to parts of the conventional example shown in FIG. 4 are given the same reference numerals, and explanations thereof will be omitted. Here, the maximum duty setting voltage Vnut applied to the input terminal 11 for maximum duty setting is supplied to another inverting input terminal of the comparator 42 and an inverting input terminal of the voltage comparator 13, respectively. Further, the non-inverting input terminal of the voltage comparator 13 receives a composite voltage (V43 + V43) of the output voltage V43 of the output voltage control amplifier 43 and the level shift battery voltage Vat
RI) is supplied.

The output voltage V 13 of the voltage comparator 13 is the transistor 15
The emitter of the transistor 15 is grounded, and the collector is connected to a constant current source 17. Further, this collector is connected to the output terminal 19 to which the capacitor C is connected and to the reset input terminal R of the R-S flip-flop 21, respectively. The set input terminal S of this flip-flop 21 is pulled up, and its Q output signal is applied to the AND gate 2 together with the output voltage V a 2 of the comparator 42.
3. The output voltage V23, which is the AND output signal, is supplied to the base of the output transistor 46.

The operation of the embodiment of the present invention constructed in this manner will be described below with reference to FIGS. 2(a) to 2(f) showing signal waveforms at each part thereof.

First, let's look at normal operation. The comparator 42 outputs an output voltage V42 that is at a "high" level during a period in which the triangular wave voltage V41 continuously generated from the oscillator 41 is at a "higher" level than the output voltage V43 of the output voltage control amplifier 43. occurs (see FIGS. 2(a) and (b)).

Therefore, the pulse width of voltage V 42 is controlled by the magnitude of voltage V s 3.

Since the R-S flip-flop 21 is normally in the set state, the AND gate 23 passes the output voltage ■4□ of the comparator 42 as it is, so this is amplified by the output transistor 46 and becomes as shown in FIG. 2(f). ■ Switching control voltage as shown. . , is output. If this is supplied to, for example, a PNP type switching transistor 51 as shown in FIG. 5, the control voltage ■. 01
A "low" level and a "high" level of 1 turn the switching transistor "on" and "off".

Stabilized output voltage of switching power supply ■. A voltage (1) for controlling Ua and its magnitude is supplied to the non-inverting input terminal and the inverting input terminal of the output voltage control amplifier 43.

The voltage comparator 13 has a maximum duty setting input terminal 11
Maximum duty setting voltage applied to Voua and voltage (
Vnz+Va+). Usually, Voua <
(V43+VRI) ... (1), so the output voltage of the voltage comparator 13 V l 3
is at a "high" level (see Figure 2 (C)). Therefore,
Transistor 15 is in an on state, terminal 19 is maintained at ground potential, and capacitor C is not charged.

Next, let's look at a case where an abnormal overload condition occurs.

Assume that an overload occurs at time T1 during normal operation. Accordingly, the output voltage V 4 of the output voltage control amplifier 43 to which the stabilized output voltage (■.UT) is fed back.
s decreases. Here, Vout > (V43+Vll)
” When the output voltage V 4 decreases until the relationship (2) is established, the output voltage vli of the voltage comparator 13 decreases.
l becomes low level and transistor 15 turns off (
Time T,).

Then, the constant current of the constant current source 17 starts charging the capacitor C connected to the terminal 19 (Fig. 2 (C), (d))
reference).

The charging voltage of this capacitor C is ■. is an integrated voltage that changes with time, and the voltage ■. exceeds the high-level threshold voltage of the R-S flip-flop 21 (at time T).
, the flip-flop 21 is reset. As a result, the Q output signal of the flip-flop 21 becomes a "low" level (see FIG. 2(e)), and the gating operation of the AND gate 23 is prohibited.

Therefore, the switching control voltage V c from the output terminal 47
Since the supply of ow to the switching transistor is cut off (see FIG. 2(f)), protection is achieved.

FIG. 3 shows another embodiment of the invention. This embodiment is a so-called dual switching power supply control circuit.

Here, two control circuits 30A and 30B with the same configuration
Each of them is substantially the same as that shown in FIG.

That is, one oscillator 41 is shared, and both output voltages of the voltage comparators 13A and 13B are ANDed by one AND gate 31, and the output voltage is obtained. , to control the charging of the capacitor C.

When the common R-3 flip-flop 21 is reset by this charging voltage Vc, its "low" level Q
The output signal causes both AND gates 23A and 23B to become non-conductive. Therefore, when an abnormal condition such as an overload occurs, the dual switching control voltage ■ that was output from the output terminals 47A and 47B. (I.W.A.
and VC (the supply to the IMB switching transistor (not shown) is cut off. As a result,
The circuit is protected.

With this configuration, a protection function can be provided by adding only one terminal to the switching control circuit. Further, since there is no need to provide special detection means for overcurrent protection, the external connection circuit becomes extremely simple when applied to a switching power supply. Since the conventional current detection resistor 63 is not required, unnecessary power consumption is eliminated, so it can be said that it is efficient in terms of power consumption.

Furthermore, even if a control circuit for a dual switching power supply is formed as shown in FIG. 3, a circuit portion having a protection function can be realized by providing only one terminal in the entire control circuit.

Therefore, the configuration is extremely simple in terms of external connections.

Effects of the Invention As detailed above, according to the present invention, it is possible to realize a control circuit for a switching power supply that has a small number of external connection terminals, a simple connection configuration, no unnecessary power consumption, and a protection function.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the configuration of a switching power supply control circuit according to an embodiment of the present invention. 2(a) to 2(f) are signal waveform diagrams of various parts for explaining the operation of FIG. 1. FIG. 3 is a configuration diagram showing another embodiment of the present invention. FIG. 4 is a configuration block diagram showing a conventional example. FIG. 5 is a wiring diagram showing the configuration of a switching power supply to which a conventional example is applied. (Main reference numbers) 13. 13A, 13B...Voltage comparator, 17... Constant current source, 21... R-S flip-flop, 23. 23A, 23B... AND gate, 41... Oscillator, 42. 42A, 42B...Pulse width control comparator, 43.43A, 43B...Output voltage control amplifier, 4
6.46A, 46B...output transistor, 51...switching transistor, C...capacitor,

Claims (1)

[Claims]: an oscillator; a signal comparator that compares a DC output signal of a switching power supply with an external control signal for controlling the magnitude of the DC output signal; and a signal comparator that compares the output signal of the oscillator with the signal. a pulse width control comparator that receives the output of the signal comparator, a switching control signal supply means that controls the passage of the output pulse signal of the comparator and supplies it to the switching power supply, and is turned on and off at a predetermined level of the output signal of the signal comparator. a state switching element; means for generating a time signal whose signal level changes with time when the state switching element assumes one state; and means for generating a cutoff signal when the time signal reaches a predetermined level. and a cutoff signal generation means for supplying the switching control signal supply means to the switching control signal supply means, and when the cutoff signal is generated, the switching control signal supply means blocks passage of the output pulse signal from the pulse width control comparator. A control circuit for a switching power supply, characterized in that it is configured as follows.