JP2597099Y2 - Switching power supply circuit - Google Patents

Description

[Detailed description of the invention]

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply circuit having a soft start circuit and a short-circuit protection circuit, and performing pulse width modulation of a DC power supply voltage.

[0002]

2. Description of the Related Art As a switching power supply circuit of this kind, there is a circuit known as model number FA7612. FIG. 2 is a conventional switching power supply circuit diagram.

In FIG. 1, an IC is a PWM (pulse width modulation) switching power supply control IC containing a basic circuit of the switching power supply, and includes terminals indicated by terminal numbers 1 to 8. REF is a reference voltage section constituting a DC constant-voltage power supply of the IC, and receives a DC power supply voltage VCC, and its reference voltage VREF is stabilized at 2.5 V ± 5%. C1 is a capacitor for smoothing the DC power supply voltage VCC. OSC is an oscillator unit that continuously generates a sawtooth wave of a predetermined frequency. ER. AMP is an error amplifier that inverts and amplifies a detection voltage formed of a divided voltage at an output terminal OUT, which will be described later, with reference to 0.5 V, and outputs the result. PW
M is a PWM comparison unit, which includes an error amplifier ER. C. AMP output level and constant current circuit C. The lower level of C is compared with the output level of the oscillator section OSC, and when these corresponding levels are higher than the level of the oscillator section OSC, an H level signal is output. G is an output selection gate, which is usually an error amplifier ER. An output control signal consisting of the output of the AMP is passed, and the passage of the output control signal is prohibited when a prohibition signal OFF described later is received. Q1 is a transistor that inverts and takes out the output control signal. RC1 is a first resistor, RC2 is a second resistor, and the resistors RC1 and RC2 are connected in series with each other. The resistor RC1 receives the reference voltage VREF, and the resistor RC2 is grounded. CS is a capacitor charged for soft start and short circuit protection; C is a constant current circuit for supplying a charging current to the capacitor CS, and the capacitor CS is connected to the connection point between the resistors RC1 and RC2 by a resistor R
C2 is connected in parallel with the constant current circuit C. C, or a reference voltage VREF via a resistor RC1, the voltage is divided by resistors RC1 and RC2 and charged to a divided voltage. In response to the charging current from C, the charging current is charged to a voltage sufficiently higher than the divided voltage. SCP is a comparator for detecting a short circuit, and includes an error amplifier ER. When the output level of the AMP reaches a predetermined abnormal level, the constant current circuit C.P. Activate C. LO is a comparator for short-circuit protection, and when the charging voltage VCS of the capacitor CS reaches a predetermined short-circuit-corresponding potential, an inhibition signal OFF is supplied to the output selection gate G. UVLO is a constant voltage malfunction prevention circuit. When the reference voltage VREF falls to a predetermined abnormal voltage, the transistor Q2 is activated to discharge the capacitor CS, stop the operation of the PWM comparison unit PWM and the like, and connect the output selection gate G to the output selection gate G. The prohibition signal OFF is given. Q3 is an output transistor, and the transistor Q1
Is conducted for a period during which the DC power supply voltage is VCC.
And outputs a chopping voltage signal having a level according to the following. L
Is a choke coil for smoothing the chopping voltage signal and outputting it to the output terminal OUT, and C2 is a smoothing capacitor for smoothing the output. R1 and R2 are resistors for dividing the voltage of the output terminal OUT, VR is a variable resistor for adjusting the voltage dividing ratio and taken out, and the detection voltage by the variable resistor VR is the error amplifier ER. Given to AMP.

Next, the soft start and short-circuit protection operation in the switching power supply circuit of FIG. 2 will be described. FIG.
4 is a timing chart showing the operation of the PWM comparison unit PWM, and FIG. 4 is a diagram showing the charging voltage VCS characteristics of the capacitor CS in each operation state.

Referring to FIG. 3, after the DC power supply voltage VCC is supplied and the reference voltage VREF is activated to a predetermined voltage,
The oscillator section OSC continuously generates a sawtooth wave VOSC of a predetermined frequency. When the DC power supply voltage VCC is initially supplied,
Error amplifier ER. The output VER of the AMP is sufficiently larger than VOSC, the charging voltage VCS of the capacitor CS is smaller than VOSC, and the output VPWM of the PWM comparator PWM is L.
Level. In addition, a comparator SCP for short-circuit detection
Is at H level and the constant current circuit C. C is working. The capacitor CS has a constant current circuit C.S. C is charged by receiving a constant current by C and a reference voltage VREF via a resistor RC1, and the charged voltage VCS rises with time. And when VCS crosses VOSC, the output VPWM becomes VCS
Soft start by setting the period exceeding
The voltage at the output terminal OUT increases. Therefore, VER drops and the constant current circuit C.V. C stops operating. Subsequently, the capacitor CS receives the reference voltage VREF via the resistor RC1 and is charged until the voltage reaches the divided voltage VCS0 obtained by dividing the voltage by the resistors RC1 and RC2. When VER becomes lower than VCS, output VPWM
Changes the period during which VER exceeds VOSC to the H level, and the voltage at the output terminal OUT is adjusted according to the H level period. In a steady state, VCS is a partial voltage VCS0 which is sufficiently higher than VOSC.

Referring to FIG. 4, when DC power supply voltage VCC is supplied and activated at time t1, constant current circuit C.V. C operates, and the charging voltage VCS of the capacitor CS rises and reaches the steady voltage VCS0 at time t2. Thereafter, at time t3, when the load circuit connected to the output terminal OUT is short-circuited and the voltage at the output terminal OUT drops abnormally, the error amplifier ER. When the output level of the AMP rises and reaches a predetermined abnormal level, the comparator S for short-circuit detection
When the output of CP becomes H level, constant current circuit C.P. C operates. The capacitor CS is started to be charged by the constant current. When the charging voltage VCS reaches a predetermined short-circuit-corresponding potential VU at time t4, the output of the comparator LO for short-circuit protection becomes high.
At this time, the output selection gate G is supplied with the inhibition signal OFF, and the output at the output terminal OUT is cut off. Thereafter, VER and VCS become higher than VOSC, and the cutoff state is maintained.

As shown in FIGS. 3 and 4, the point at which the transition from the soft start to the steady state is set to the point at which the duty ratio of the steady operation state is reached. DC power supply voltage VCC
Soft start time ts from supply to steady state
Is the time from when the charging voltage VCS of the capacitor CS reaches 0 V to almost the average potential of the sawtooth wave OSC, the capacitance of the capacitor CS and the average potential of the sawtooth wave VOSC, and the constant current circuit C. It is determined by the constant current of C and the value of the resistor RC1. On the other hand, in the state where VCS = VC0, the load is short-circuited and the constant current circuit C.V. The short-circuit response time tp from the time t3 when C starts to operate to the time t4 when the charging voltage VCS reaches the predetermined short-circuit-corresponding potential VU is determined by the above values and the difference between VU and VCS0. This VU is constrained by the protection conditions of the circuit, and VCS0 is used to connect the reference voltage VREF to the resistors RC1 and RC1 so that the soft start circuit does not operate in a steady state.
The pressure is set appropriately with C2. Therefore, tp / t
S becomes almost constant.

[0008]

[Problems to be Solved by the Invention] However, the above t
Since p / ts is almost constant, there is a problem that it is not possible to individually adjust the times tp and ts.

An object of the present invention is to provide a soft start time tS.
Is relatively short, and a short-circuit response time tp from occurrence of a short-circuit to activation of short-circuit protection is relatively long.

[0010]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention provides a method for connecting a first resistor and a second resistor connected in series to each other and receiving a constant reference voltage. A capacitor connected in parallel with the second resistor, a soft-start circuit that starts operation by starting a reference voltage and shifts to a stable output control in a process of charging the capacitor to a predetermined low voltage; And a constant current circuit that supplies a constant current to the connection point to charge the capacitor to a predetermined high voltage, and a short-circuit protection circuit that cuts off the output when the voltage of the capacitor exceeds a predetermined high voltage. A switching power supply circuit that converts the DC power supply voltage to a predetermined constant voltage by pulse width modulation and outputs the same, and connected in series with each other and in parallel with a series circuit of the first resistor and the second resistor. Connection The third resistor and the fourth resistor, and the respective connection points in the forward direction from the connection point between the third and fourth resistors to the connection point between the first resistor and the second resistor. And a diode connected between them.

[0011]

According to the present invention, when the reference voltage is activated, the capacitor is charged through the parallel circuit of the first resistor and the third resistor, and is soft-started when charged to a predetermined low voltage. You. When the load is short-circuited, the constant current circuit operates to supply a constant current to the capacitor. At this time, the third and fourth resistor circuits are separated from the capacitor by the diode, and are charged to a low voltage first. When the battery is charged to a predetermined high voltage, the output is cut off.

[0012]

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

In FIG. 2, the same parts as those in FIG. 2 are denoted by the same reference numerals, and different parts will be described below.

RA is a third resistor, RB is a fourth resistor,
They are connected in series with each other and are connected in parallel with a series circuit of a first resistor RC1 and a second resistor RC2.

A diode DA is connected between the connection points of the resistors RA and RB in a forward direction from the connection point of the resistors RC1 and RC2 to the connection point of the resistors RC1 and RC2. The values of the resistors RA and RB are set so that the potential at the connection point thereof is the potential obtained by adding the forward voltage of the diode DA to the potential at the connection point between the resistors RC1 and RC2.

In the switching power supply circuit shown in FIG. 1, when the DC power supply voltage VCC is supplied and the reference voltage VREF is activated to a predetermined voltage, the oscillator section OSC generates a sawtooth wave of a predetermined frequency as in FIG. VOSC is generated continuously, and when the DC power supply voltage VCC is initially supplied,
The output VPWM of the PWM comparison unit PWM is at the L level, and the constant current circuit C.V. C is working. The capacitor CS has a constant current circuit C.S. While receiving a constant current by C, the resistance Rc1
And the reference voltage VREF is charged through a parallel circuit composed of R1 and RC2, and the charging voltage VCS rises with time.
And when VCS crosses VOSC, the output VPWM becomes VCS
The period during which the voltage exceeds OSC is set to the H level to perform soft start, and the voltage of the output terminal OUT rises. Therefore, VER drops and the constant current circuit C.V. C stops operating. Subsequently, the capacitor CS receives the reference voltage VREF through a parallel circuit of the resistors RC1 and RA, and is charged up to a divided voltage VCS0 obtained by dividing the voltage by the resistors RC1 and RC2. When VER becomes lower than VCS, the output VPWM changes to the H level during the period when VER exceeds VOSC, and the voltage at the output terminal OUT is adjusted according to the H level period. In a steady state, VCS is a partial voltage VCS0 which is sufficiently higher than VOSC. In this circuit, the capacitor CS is connected to the resistor Rc
Since it is charged through a parallel circuit of 1 and RA, the resistor Rc1
The charging time is reduced as compared with the circuit of FIG. 2 which is charged via the circuit, and the slope of VCS shown in FIG. 3 becomes steeper, and the soft start time ts shown in FIG. 4 becomes shorter.

Thereafter, when the load is short-circuited and the voltage at the output terminal OUT drops abnormally, as in FIG. When C operates, charging of the capacitor CS is started by the constant current. At this time, the circuit of the resistors RA and RB is separated from the capacitor CS by the diode DA, and is further charged from the state where it has been charged to a low voltage first. The output of the output comparator OUT is turned off, and the output of the output terminal OUT is cut off. After that, VER and VCS are VOS
C and the cutoff state is maintained. The charging conditions based on the above short-circuit are the same as those in the circuit of FIG. 2 because the circuit of the resistors RA and RB is disconnected from the capacitor CS. 2 is the same as in the second circuit.

[0018]

As described above, according to the present invention, the second resistor is connected to the connection point between the first resistor and the second resistor connected in series and receiving a constant reference voltage. A capacitor is connected in parallel with this, a constant current circuit that operates by a load short circuit is connected to the connection point, and a series circuit of a third resistor and a fourth resistor is connected in parallel with the series circuit of each resistor. From the connection point between the third and fourth resistors to the first and second resistors.
Since each connection point is connected by a diode in the forward direction to the connection point with the resistor, when the reference voltage is started, the capacitor is connected via a parallel circuit of the first resistance and the third resistance. When the battery is charged to a predetermined low voltage, a soft start is performed. Therefore, the charging time is reduced as compared with the conventional circuit, and the soft start time is shortened. When the load is short-circuited, the constant current circuit is activated to supply a constant current to the capacitor.The battery is charged further from a state where it has been charged to a low voltage first, and the output is cut off when charged to a predetermined high voltage. You. At this time, the circuit of the third and fourth resistors is separated from the capacitor by the diode, and the charging condition is the same as in the conventional circuit, so that the time from the occurrence of a short circuit to the activation of the short-circuit protection is the same as in the conventional circuit. become.

[Brief description of the drawings]

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

FIG. 2 is a conventional switching power supply circuit diagram.

FIG. 3 is a timing chart showing the operation of a PWM comparison unit of a conventional switching power supply circuit.

FIG. 4 shows a capacitor CS of a conventional switching power supply circuit.
Of charging voltage characteristics

[Explanation of symbols]

REF: Reference voltage section, OSC: Oscillator section, ER. AMP
... Error amplifier, PWM ... PWM comparator, G ... Output selection gate, Q1 ... Transistor, RC1 ... First resistor, RC2 ...
Second resistor, RA: third resistor, RB: fourth resistor, C
S: Capacitor, C.I. C: constant current circuit, SCP: comparator for short-circuit detection, LO: comparator for short-circuit protection, DA: diode.

Claims (1)

(57) [Scope of request for utility model registration] A capacitor is connected in parallel with a second resistor at a connection point between a first resistor and a second resistor connected in series and receiving a reference voltage of a constant voltage. A soft-start circuit that starts operation by starting and shifts to output stabilization control in a process in which the capacitor is charged to a predetermined low voltage; and a capacitor that operates by supplying a short circuit to supply a constant current to the connection point. A constant current circuit that charges the DC power supply to a predetermined high voltage, and a short-circuit protection circuit that shuts off the output when the voltage of the capacitor becomes higher than the predetermined high voltage. A switching power supply circuit that converts the voltage into a voltage and outputs the third and fourth resistors connected in series with each other and connected in parallel with a series circuit of the first resistor and the second resistor; 3rd and 4th resistors Switching power supply circuit, characterized in that, comprising a first resistor and toward the forward direction to the connection point between the second resistor formed by connecting between respective connection points diode from a connection point between.