JP3488116B2 - Switching power supply - 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 device having an overcurrent protection function of limiting an output current when an overload or an output short circuit occurs.

[0002]

2. Description of the Related Art An example of a switching power supply device having the above-mentioned overcurrent protection function is shown in FIG. In this switching power supply device, the input voltage vin smoothed by the input stage capacitor c1 is applied to the transistor t.
It is switched by r1. Transistor tr1
While is ON, energy is supplied to the coil l1, the capacitor c2 and the load rl by the voltage vout appearing at the emitter of the transistor tr1. While the transistor tr1 is off, the energy stored in the coil l1 is circulated by the diode d1 and given to the load rl.

The output voltage vo is controlled on the basis of the feedback voltage vadj obtained by dividing the output voltage vo at a predetermined ratio by the resistance values of the resistors r1 and r2 and the reference voltage vref1 of the reference voltage source 2. First, the differential amplifier 1 outputs a voltage corresponding to the difference between the two voltages, and the voltage and the oscillator 3
The comparator 4 compares the triangular wave of 100 [kHz] output from. Then, from the comparator 4,
A PWM signal having a pulse width corresponding to the output level of the differential amplifier 1 is output.

Next, when this PWM signal is applied to the drive circuit 5, the drive circuit 5 controls ON / OFF of the transistor tr1 according to the duty cycle of the PWM signal. As a result, the output voltage vo is controlled to a constant voltage (for example, 5 [V]) determined by the voltage dividing ratio of the reference voltage vref1 and the resistors r1 and r2.

During the above operation, as shown in FIGS. 6A and 6B, the output voltage of the comparator 4,
That is, the PWM signal and the voltage vout have pulse widths shown by the broken line. The duty D of the transistor tr1 is D = t _ON / (t _ON + t _OFF ) = (v _O / v _IN ) × 100 [% if the ON time and the OFF time of the transistor tr1 are t _ON and t _OFF , respectively. ] (1)

However, when the load rl becomes heavy, the coil current il flowing through the coil 11 increases as shown by the broken line to the solid line in FIG. Eventually, coil current il
Exceeds the overcurrent detection level icl, an overcurrent state is detected by the overcurrent detection circuit 6 provided in the input stage,
The set signal is output to the RS flip-flop circuit 7.

The RS flip-flop circuit 7 is set when the set terminal voltage changes to "Low" as shown in FIG. Once the set terminal voltage becomes “Low”, it is latched and the output is held at “Low”. At this time, the reset terminal voltage remains “High”.

As a result, the output voltage of the comparator 4 and the voltage vout have the pulse widths shown by the broken lines in (a) and (b) of FIG. Since it is "Low", the pulse width is narrowed to the pulse width shown by the solid line. In this way, the switching frequency of the transistor tr1 is lowered, so that the output voltage vo is lowered and the increase of the output current is suppressed. Then, as a result, the output current io decreases at point A as shown in FIG.

Further, a reset signal is output from the oscillator 3 to the RS flip-flop circuit 7 when the transistor tr1 is OFF, and as shown in (e) of FIG.
The reset terminal voltage of the flip-flop circuit 7 changes. At this time, the RS flip-flop circuit 7 is latched once the reset terminal voltage becomes “Low”,
Contrary to when the set terminal voltage becomes "Low", the output is held at "High". As a result, the transistor tr1 is turned on at the normal timing at the next turning on.

However, in the above switching power supply device, the switching frequency is increased to reduce the size and weight of the switching power supply (about 50 [kHz]
z] or more), as described below, the operation of the overcurrent protection function is inconvenient.

That is, as shown in FIG. 6, the time td1 until the set terminal voltage becomes "Low" and the transistor t after the set terminal voltage becomes "Low".
There is a delay from the time td2 until r1 is turned off.
The delay time td, which is the sum of both times td1 and td2, is the time from the detection of overcurrent to the turning off of the transistor tr1, that is, the time required for the overcurrent protection function to operate. The delay time td reaches about 1 [μsec], and at the switching frequency of 100 [kHz], that is, 10 [μsec],
If the switching pulse width is narrowed during the overcurrent protection operation, the influence on the protection operation increases and cannot be ignored.

For example, input voltage vin = 40 [V],
When the output voltage vo = 5 [V] and the inductance L of the coil 11 is L = 200 [μH], the current Δi, which is the change in the coil current il during the delay time td, is Δi = [(vin-vo) /L]×td=0.175 [A]. Therefore, the coil current i1 exceeds the overcurrent detection level icl due to the current Δi. This current change Δi is the average current, that is, the output current io.
Will be increased.

As shown in FIG. 7, the output characteristic at this time is such that the emitter current increases as it gets closer to the short-circuited state (vo = 0 [V]), and exceeds the absolute maximum rated value (2.5 [A]). It will not have drooping characteristics. As described above, the above switching power supply device has a problem that the overcurrent protection function does not operate reliably as the switching frequency increases.

Therefore, another conventional technique for solving such a problem is disclosed in Japanese Patent Laid-Open No. 7-46828. This prior art is shown in FIGS. 8 and 9. 8 and 9 correspond to FIG. 5 and FIG. 7 described above, respectively, and corresponding parts are designated by the same reference numerals. It should be noted that this switching power supply device is further provided with a comparator 8, a constant voltage source 9, and an oscillation frequency changing circuit 10 in order to reduce the oscillation frequency when an overcurrent occurs due to an output short circuit or the like. is there.

The voltage vadj obtained by the resistors r1 and r2 is applied to the non-inverting input of the comparator 8, and the constant voltage source 9 is connected to the inverting input. The reference voltage vref1 by the reference voltage source 2 is, for example, 1.25 [V], while the reference voltage vref2 by the constant voltage source 9 is, for example, 0.6 [V]. The feedback voltage vadj to the comparator 8 is 0.6
The output voltage vo when it becomes [V] is vo = 0.6 [V] × (r1 + r2) /r2=2.4 [V] by the following equation. That is, the comparator 8 detects that the output voltage vo has dropped below 2.4 [V], and in response to this, the oscillation frequency changing circuit 10 changes the oscillation frequency of the triangular wave generated by the oscillator 3 to The frequency is lowered from 100 [kHz] to 20 [kHz].

Therefore, a load rl is caused by a load short circuit or the like.
When the collector current of the transistor tr1 increases and the collector current exceeds the overcurrent detection level, the overcurrent detection circuit 6 detects the overcurrent state and the overcurrent state is detected. The protection function starts operating, the RS flip-flop circuit 7 is set by the set signal output from the overcurrent detection circuit 6, the switching pulse width of the transistor tr1 becomes small, and the ON time of the transistor tr1 becomes short, In FIG. 9, the output voltage vo decreases at the point A as described above.

Further, when the resistance value of the load rl decreases, the output voltage vo decreases to point B shown in FIG.
4 [V], and the feedback voltage vadj at this time is 0.
It becomes 6 [V]. From this, the output voltage vo further decreases, and the feedback voltage vadj becomes 0.6 of the reference voltage vref2.
When it becomes lower than [V], the output of the comparator 8 which has been "High" until then becomes "Low", and the oscillation frequency changing circuit 10 outputs a voltage for giving an instruction to the oscillator 3 to change the oscillation frequency, The oscillator 3 reduces the oscillation frequency from 100 [kHz] to 20 [kHz].

By such an operation, the switching pulse width is narrowed by the normal overcurrent protection operation, and approaches the minimum value of the switching pulse width determined by the delay time td from the overcurrent detection to the turning off of the output transistor tr1. Even in this case, the output of the comparator 8 is changed at the point B and the switching frequency is lowered, so that the switching pulse width is widened.

For example, in the normal operation, the transistor tr1 has 5 [V] / 12 [V] .apprxeq.41.7 from the above-mentioned formula 1.
When the switching is performed with the duty D of [%] and the output voltage vo becomes 2.4 [V] at the point B, the expression 1 becomes 20 [%], so that the switching pulse width is 2 Expand from [μsec] to 10 [μsec]. Therefore, the influence of the above-mentioned delay time td in the overcurrent protection operation can be reduced to ⅕ of the conventional one.

Therefore, as shown in FIG. 9, the output current io decreases from the point B, where the switching frequency fs starts to decrease, to the point C, where the switching frequency fs starts to decrease, because it returns to the normal overcurrent point. Since the oscillation frequency is fixed at 20 [kHz] after the point C, the switching pulse width becomes narrower as the load rl becomes smaller, and the influence of the delay time td becomes larger and the output current io increases.

However, since the output current io is lowered from the point B to the point C as described above, the output current i
The increase of o can be suppressed significantly. This allows
The output current io is 2.5 [A] of the above absolute maximum rating value.
Will not exceed. It should be noted that the broken line in the figure shows the overcurrent protection characteristic shown in FIG.

[0022]

There is still a strong demand for miniaturization and cost reduction of power supplies. In order to reduce the size and reduce the cost, it is effective to integrate the switching power supply device including the transistor tr1 into an integrated circuit and reduce the size of the coil l1 and the capacitor c2 externally attached to the integrated circuit. Therefore, switching frequency fs
It is conceivable to further increase the frequency. On the other hand, even if the integrated circuit is realized by a relatively inexpensive bipolar element, the switching frequency fs can be up to about 300 [kHz].

However, also in the prior art shown in FIG. 8, when vin = 24 [V] and vo = 5 [V], the duty D in the normal control state is approximately 20.8 [%] from the above equation 1. ], Fs = 300 [kHz] (switching cycle T = 3.33 [μsec], ON time t _ON
= T × D = 3.33 × 0.208 = 693 [nsec]
Therefore, the delay time td becomes shorter than 1 [μsec].

Therefore, as shown by the phantom line in FIG. 9, even if the overcurrent is detected at the point A and the protective operation is performed, the protective operation is the ON time t _{ON of the} transistor tr1.
Is performed after the lapse of time, the overcurrent protection operation is invalidated, and the output current io increases. Further, the load rl becomes heavy, the output current io increases, and the transistor t
When the capacity of r1 exceeds 3.0 [A], for example, a voltage drop V between the collector and the emitter of the transistor tr1 is generated.
_CE starts to increase, the loss in the transistor tr1 increases, the efficiency decreases, and the output voltage vo begins to decrease.
In this way, when vo = 2.4 [V] at the point D, the switching frequency fs is lowered by the operation of the oscillation frequency changing circuit 10, and the normal overcurrent protection operation is performed to reach the point C.

That is, although the overcurrent protection operation is performed at point C and below after the short-circuit protection operation between points D and C is performed, the overcurrent protection operation is not performed between points A and D. turn into.

An object of the present invention is to provide a switching power supply device capable of reliably performing an overcurrent protection operation even if the switching frequency is increased.

[0027]

According to a first aspect of the present invention, there is provided a switching power supply device in which a switching element switches an input DC voltage in response to an oscillating signal from an oscillating means so that a voltage output of a desired level is obtained. A switching power supply device which can be obtained, and when the overcurrent detection means detects that the output current exceeds a predetermined value, the overcurrent protection circuit narrows the switching pulse width to limit the output current. In response to the overcurrent detection by the overcurrent detection means, the protection operation by the overcurrent protection circuit from the first oscillation frequency in the steady state lower than the first oscillation frequency and the overcurrent detection a first oscillation frequency reduction means for reducing the oscillation frequency of the oscillation means to the oscillation frequency second period longer than the delay time until the implementation of the previous The first oscillation frequency reduction means
When the oscillation frequency of the oscillation means reaches the second oscillation frequency
Both are switching pulses by the overcurrent protection circuit
A second oscillating frequency that detects a decrease in the output voltage of a predetermined level in the narrow width and reduces the oscillating frequency of the oscillating means to a third oscillating frequency lower than the second oscillating frequency. And a lowering means.

According to the above configuration, when the overcurrent detection means detects the overcurrent state at the start of the operation of the overcurrent protection circuit, the first oscillation frequency lowering means causes the oscillation of the oscillation means. The frequency is reduced in advance from the first oscillation frequency in the steady state to the second oscillation frequency. Therefore, even if the overcurrent protection operation is performed and the switching pulse width is narrowed, at the second oscillation frequency, the switching element is turned on more than the delay time from the detection of the overcurrent state to the turning off of the switching element. The period becomes longer, the overcurrent protection operation is effectively performed, and the output current decreases.

Similarly, the second oscillating frequency lowering means further lowers the oscillating frequency of the oscillating means to the third oscillating frequency when detecting that the output voltage has dropped below a predetermined level due to an output short circuit or the like. As a result, after the oscillation frequency is once reduced from the first oscillation frequency to the second oscillation frequency, the switching pulse width narrowed by the overcurrent protection operation is widened again, and the influence of the delay time is reduced. be able to. Thus, it is possible to prevent the output current from increasing due to the influence of the delay time.

Therefore, the first oscillation frequency can be increased to, for example, near the upper limit of the operating frequency of the switching transistor, regardless of the delay time.
For the oscillation means and overcurrent protection circuit that are integrated into a circuit, downsizing externally attached coils and capacitors,
It is possible to further reduce the size and cost of the switching power supply device.

Further, in the switching power supply device according to the invention of claim 2, the overcurrent detection means has a low resistance interposed in series in a power supply line from the input terminal to the switching element, and a terminal voltage of the low resistance. And a comparator for discriminating the level of the drop with a minute threshold voltage.

According to the above arrangement, the low threshold voltage drop between the terminals which is necessary for the overcurrent detection by the comparator due to the minute threshold voltage can be reduced, and the rated current value is relatively large. Also, overcurrent detection can be performed with low loss.

Further, in the switching power supply device according to the invention of claim 3, the oscillating means is a current control type oscillator capable of changing an oscillating frequency in response to an input current, and the oscillating means comprises the first and the first The oscillating frequency lowering means of No. 2 provides a constant current generating circuit for giving a constant current corresponding to the second frequency to the oscillator, a difference between the second frequency and the first frequency, a second frequency and a third frequency. It is characterized in that constant currents corresponding to the respective differences from the frequency are generated, and these constant currents are added or subtracted.

According to the above arrangement, the first and second oscillation frequency lowering means can control the current value by adjusting the emitter area of the transistor, etc., whereby the oscillation frequency of the oscillator can be adjusted to the first oscillation frequency. , Second or third
The frequency can be set relatively accurately.

Therefore, even if the switching pulse width is narrowed by the overcurrent protection operation, the switching pulse width can be surely made longer than the delay time, and stable operation can be realized.

[0036]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described below with reference to FIGS.

FIG. 1 is a block diagram showing an electrical configuration of a switching power supply device according to an embodiment of the present invention. The switching power supply device according to this example is a chopper type,
A coil L1 and smoothing capacitors C1 and C described later.
With the exception of 2, they are integrated circuits. As shown in FIG. 1, this switching power supply device includes an NPN-type bipolar transistor Tr1 that switches an input voltage Vin of 24 [V], for example. In the power line from the input terminal to the collector of the transistor Tr1,
The overcurrent detection circuit 11 is interposed in series, and a capacitor C1 for smoothing the pulsating current is provided in the preceding stage of the overcurrent detection circuit 11. The overcurrent detection circuit 11 detects an overcurrent state when the current flowing between the collector and the emitter of the transistor Tr1 exceeds the overcurrent detection level Icl (corresponding to icl in FIG. 6), which will be described later.
The output signal is output to the S flip-flop circuit 12 as a set signal.

The coil L1 is connected in series to the emitter of the transistor Tr1. The cathode of the diode D1 is connected to one end of the coil L1 on the emitter side, and the anode of the diode D1 is grounded. The other end of the coil L1 has a capacitor C2 for smoothing the output.
Resistor R connected in series with one end of
1 · R2 and a load RL provided in parallel with these resistors R1 · R2 are grounded. The other end of the capacitor C2 is grounded. Also, the resistors R1 and R
2 has resistance values of, for example, 3 [kΩ] and 1 [kΩ], respectively.
Ω], and the output voltage Vo is divided into ¼.

When the input voltage Vin smoothed by the capacitor C1 in the input stage is switched by the transistor Tr1, the voltage Vo appearing at the emitter of the transistor Tr1 during the period in which the transistor Tr1 is ON.
Energy is supplied to the coil L1, the capacitor C2, and the load RL by ut. Transistor Tr
While 1 is OFF, the energy stored in the coil L1 is circulated by the diode D1 and given to the load RL.

The voltage at the connection point between the resistors R1 and R2 is fed to the inverting input of the differential amplifier 13 by the feedback voltage Vadj.
Given as. Further, when the voltage division ratio by the resistors R1 and R2 is 1/4 and the output voltage Vo is 5 [V], the non-inverting input of the differential amplifier 13 is 1.25 [V]. A reference voltage source 14 that generates a first reference voltage Vref1 is connected. Differential amplifier 13
Is the feedback voltage Vadj obtained by dividing the output voltage Vo by the resistors R1 and R2, and the first reference voltage V
A voltage corresponding to the difference from ref1 is output. The output of the differential amplifier 13 is the comparator 15
Connected to the non-inverting input of. In addition, the comparator 1
An oscillator 16 is connected to the inverting input of the signal line 5.

The oscillator 16 generates a triangular wave and a reset signal to be supplied to the reset terminal of the RS flip-flop circuit 12. Further, the oscillator 16 responds to the output of the first oscillation frequency changing circuit 17, which will be described later, with the oscillation frequency of the triangular wave as a first oscillation frequency.
From the oscillation frequency of 300 [kHz],
The second oscillation frequency is lowered to, for example, 100 [kHz], and in response to the output of the second oscillation frequency changing circuit 18, the second oscillation frequency is 100 [kHz]. From the third oscillation frequency,
For example, it is lowered to 20 [kHz].

The comparator 15 compares the triangular wave from the oscillator 16 with the output voltage of the differential amplifier 13, and when the output voltage of the differential amplifier 13 is higher than the level of the triangular wave, "H" is output.
Meanwhile, the comparator 15 outputs "Low" when the output voltage of the differential amplifier 13 is lower than the triangular wave level. That is, the comparator 15 outputs the PWM signal for turning on / off the transistor Tr1. Is output.

The output of the comparator 15 is connected to the drive circuit 19. The drive circuit 19, based on the PWM signal from the comparator 15,
1 is a circuit for driving ON / OFF. The RS flip-flop circuit 12 is set by the set signal from the overcurrent detection circuit 11 and reset by the reset signal from the oscillator 16 to narrow the pulse width of the PWM signal and narrow the switching pulse width. It is like this.

The output voltage Vo is controlled by the feedback voltage vadj obtained by dividing the output voltage Vo by the resistance value of the resistors R1 and R2 and the first reference voltage V from the reference voltage source 14.
It is performed based on ref1 and ref1. First, the differential amplifier 1
A voltage corresponding to the difference between the two voltages is output by 3, and the comparator 15 compares the voltage with the triangular wave output from the oscillator 16. Then, from the comparator 15,
PWM of pulse width according to the output level of the differential amplifier 13
The signal is output.

Next, when this PWM signal is given to the drive circuit 19, the drive circuit 19 controls ON / OFF of the transistor Tr1 according to the duty D of the PWM signal. As a result, the output voltage Vo is controlled to a constant voltage (5 [V]) determined by the voltage dividing ratio of the first reference voltage vref1 and the resistors R1 and R2.

In the present switching power supply device, the overcurrent detection circuit 11 and the RS flip-flop circuit 12 detect the overcurrent state and narrow the switching pulse width to control the operation of the transistor Tr1 to protect the overcurrent. It's designed to work. That is, the overcurrent detection circuit 11 constitutes an overcurrent detection means, and the RS flip-flop circuit 12 constitutes an overcurrent protection circuit.

Further, in the present switching power supply device, when the overcurrent state is detected by the overcurrent detection circuit 11, the first oscillation frequency changing circuit 17 as the first oscillation frequency lowering means transfers to the oscillator 16. "High"
In response to this, the oscillator 16 lowers the oscillation frequency from 300 [kHz] which is the first oscillation frequency to 100 [kHz] which is the second oscillation frequency.

Furthermore, in this switching power supply device, the feedback voltage Vadj at the connection point between the resistors R1 and R2 is applied to the non-inverting input of the comparator 20. A constant voltage source 21 is connected to the inverting input of the comparator 20. The constant voltage source 21 is a circuit that outputs a second reference voltage Vref2 of 0.6 [V], for example. The comparator 20 uses the feedback voltage Vadj.
Is compared with the second reference voltage Vref2 to obtain the feedback voltage Vref.
When adj becomes higher than the second reference voltage Vref2, it outputs "High", and when it becomes lower, it becomes "Lo".
w "is output.

The output voltage Vo when the feedback voltage Vadj becomes 0.6 [V] is given by the following equation: Vo = 0.6 [V] × (R1 + R2) /R2=2.4 [V] Become. That is, the comparator 20 outputs the output voltage Vo.
Is lower than the above-mentioned 2.4 [V].

The second oscillation frequency changing circuit 18 is connected to the output of the comparator 20. The second oscillation frequency changing circuit 18 outputs "High" for reducing the oscillation frequency of the triangular wave to the oscillator 16 when the output of the comparator 20 becomes "Low". In response to this, the oscillator 16 lowers the oscillation frequency from the second oscillation frequency of 100 [kHz] to the third oscillation frequency of 20 [kHz]. Therefore, the second oscillation frequency changing circuit 18, the comparator 20, and the constant voltage source 21 have a function as second oscillation frequency lowering means.

FIG. 2 is a block diagram showing a specific structure of the overcurrent detection circuit 11. This overcurrent detection circuit 11
Is a low resistance RM for converting a load current into a voltage, a comparator CMP, and transistors Q11, Q forming an output circuit.
12 and resistors R11 and R12. The low resistance RM is connected to the transistor Tr from the input terminal.
1 is connected in series to the power supply line L1. The comparator CMP compares the inter-terminal voltage of the low resistance RM with a predetermined threshold voltage, and outputs a high level when the voltage is equal to or higher than the threshold voltage. The threshold voltage of the comparator CMP is, for example, 3
When the load current to the load RL is 2 [A], the resistance value of the low resistance RM is 15 [mV].
Selected as [mΩ]. In this way, the comparator CMP outputs a high level when the negative overcurrent becomes 2 [A] or more.

The output of the comparator CMP is given to the base of the transistor Q11 via the resistor R11 and to the base of the transistor Q12 via the resistor R12. The emitters of the transistors Q11 and Q12 are both grounded. The collector of the transistor Q11 becomes an output to the first oscillation frequency changing circuit 17, and the collector of the transistor Q12 becomes a set output to the RS flip-flop circuit 12. Therefore, in the overcurrent state, the output of the comparator CMP becomes high level,
The first oscillation frequency changing circuit 17 and the RS flip-flop circuit 12 are inverted by the transistors Q11 and Q12.
Given to each.

FIG. 3 shows the first oscillation frequency changing circuit 17
2 is an electric circuit diagram showing a specific configuration of a second oscillation frequency changing circuit 18. FIG. In FIG. 3, the resistor R11 and the transistor Q1 in the overcurrent detection circuit 11 are shown.
1 and the constant current circuit 16a in the oscillator 16 are also shown. Output current I from the constant current circuit 16a
40 is given to the oscillation circuit, and the oscillation circuit receives the current I40
Will oscillate at a frequency corresponding to.

First, the first oscillation frequency changing circuit 17 includes a constant current source F21, a capacitor C21 and a transistor Q.
21 to Q24 and resistors R21 and R22. A series circuit of a constant current source F21 and a capacitor C21 is interposed between the power supply line L2 supplied with the power supply voltage Vs and the ground line L3. A resistor R21 and a transistor Q are also provided between the power supply lines L2 and L3.
21 and a series circuit of a transistor Q23, a resistor R22, and a transistor Q22 that create a constant current I21 are connected. The transistor Q11 is interposed in parallel with the capacitor C21, and the output voltage of the capacitor C21 is given to the base of the transistor Q21. Resistor R21 and transistor Q21
The connection point with is connected to the base of the transistor Q22. The diode-connected transistor Q23 forms a current mirror circuit with the transistor Q24.

Therefore, at the rated load, the transistor Q11 is off, the capacitor C21 is charged by the constant current source F21, the charging voltage turns on the transistor Q21, and the transistor Q22 turns off.
The current I21 becomes 0 due to FF, and the transistor Q24
The output current I22 from is also zero. On the other hand, when the transistor Q11 is turned on in the overcurrent state, the charge stored in the capacitor C21 is discharged, the transistor Q21 is turned off, the transistor Q22 is turned on, and the current I21 flows, and the first oscillation frequency changing circuit is turned on. An output current I22 is sent from 17 to the constant current generating circuit 16a of the oscillator 16.

Here, the emitter area ratio of the transistors Q23 and Q24 is formed to 1: 1, the power supply voltage Vs is selected to be 2.6 [V], and the resistance value of the resistor R22 is 4: 1.
Selected as 6 [kΩ]. Therefore, the I22 = I21 = (Vs-V BE -V SAT) / R22 ... (2). Where V _BE is the base of transistor Q23
The emitter-to-emitter voltage is, for example, 0.65 [V]. Further, V _SAT is a saturation voltage when the transistor Q22 is ON and is, for example, 0.1 [V]. Therefore, the current of I22 = 40 [μA] can be flown out from the equation (2).

The second oscillation frequency changing circuit 18 comprises transistors Q31 to Q34, resistors R31 and R32, and a constant current source F31. Constant current source F31
Is a pair of transistors Q31 and Q3 forming a differential pair.
A constant current I31 is supplied to the second emitter. The feedback voltage Vadj is applied to the base of the transistor Q31,
The collector is grounded. At the base of the transistor Q32, a reference voltage Vre created by voltage dividing resistors R31 and R32 interposed between the power supply lines L2 and L3.
f3, for example, 0.6 [V] is applied. The collector of the transistor Q32 is grounded via the diode-connected transistor Q33. Transistor Q3
3 forms a current mirror circuit with the transistor Q34.

The emitter area ratio of the transistors Q33 and Q34 is selected to be 1: 3, and I31 = 20 [μ
A]. Therefore, Vo in the steady load state
= 5 [V], Vadj = 1.25 [V] and V
ref3 <Vadj, and the transistor Q31 is OF
Then, the transistors Q32 and Q33 are turned on, and the transistor Q34 has a capability of extracting a current of 60 [μA].

The constant current generating circuit 16a includes a transistor Q
41 to Q48, diodes D41 to D44, and a constant current source F41. The power line L
A series circuit of a constant current source F41 and a diode-connected transistor Q41 is interposed between 2 and L3,
The constant current I41 is created by the series circuit. The transistor Q41 is the transistors Q42, Q43, Q4.
4 and a current mirror circuit, and the emitter area ratio of each of the transistors Q41, Q42, Q43, Q44 is formed to be 1: 1: 2: 1.

The transistor Q42 includes a diode-connected transistor Q45 and diodes D41 and D42.
Together, they form a series circuit for producing the output current I40, and are interposed between the power supply lines L2 and L3. In the series circuit of the diode D42 and the transistor Q42, a series circuit of the diode D43 and the transistor Q43 is interposed in parallel. The first oscillation frequency changing circuit 17 is connected to the collector of the transistor Q43.
The output current I22 from is supplied.

The transistor Q44 is connected in series with the diode-connected transistor Q46, and is interposed between the power supply lines L2 and L3. On the other hand, a series circuit of a transistor Q47 and a diode D44 is provided in parallel with the series circuit of the transistor Q45 and the diode D41. The transistor Q46 and the transistor Q47 form a current mirror circuit. Therefore, the I44 flowing through the transistor Q44 is folded back by the transistors Q46 and Q47 and flows into the cathode side of the diode D41. Transistor Q4
The collector of 7 is also connected to the collector of the transistor Q34.

Transistor Q46 and transistor Q
The emitter area ratio with respect to 47 is formed to 1: 0.8. The current I41 supplied to the transistor Q41 by the constant current source F41 is selected to be 10 [μA]. Therefore, the current I4 flowing through the transistors Q42 and Q44 is
2, I44 becomes 10 [μA], and transistor Q43
The current I43 flowing through the transistor Q4 becomes 20 [μA], and the current I45 flowing through the transistor Q47 becomes 8 [μA]. The current I46 flowing through the transistor Q45 is folded back and output as the output current I40 by the transistor Q48 which forms a current mirror circuit with the transistor Q45 and has an emitter area ratio of 1: 1.

Constant current generating circuit 1 configured as described above
6a, in the steady load, the transistor Q2
4 turns off, and I22 = 0 [μA]. At this time, the transistor Q34 turns on and the transistor Q47
The power supply I45 flowing through is sufficiently drawn by the transistor Q34 and bypassed. Therefore, I40 = I46 = I42 + I43 = 30 [A].

Next, when the overcurrent state is detected, the transistor Q24 is turned on and the current I22 is supplied. Since the current I22 which the transistor Q24 can pass is larger than the current I43 which the transistor Q43 can pass, the transistor Q43 can pass the current I43.
The collector potential of the transistor 43 becomes a high level of approximately Vs-V _SAT , and the diode D43 is turned off. As a result, I40 = I46 = I42 = 10 [μA]. At this time, the transistor Q34 remains ON.

Furthermore, the output voltage Vo decreases, and
When it becomes 4 [V] (Vadj = 0.6 [V]) or less, the transistor Q34 is turned off. As a result, the current I45 flowing through the transistor Q47 flows into the cathode side of the diode D41, and I40 = I46 = I42−I45 = 2 [μA].

Here, when the oscillation frequency fs of an oscillation circuit (not shown) is the capacitance Cosc of the oscillation capacitor and the amplitude of the oscillation waveform is Vosc, fs = 1 / T = I40 / 2Cosc × Vosc (3) ) Can be represented.

Therefore, when Cosc = 50 [pF] and Vosc = 1 [V], the oscillation frequency fs is obtained.
Is 300 [kHz] when I40 = 30 [μA]
When I40 = 10 [μA], 100 [kh
z], and when I40 = 2 [μA], 20 [kH
z].

Here, the first oscillation frequency changing circuit 17
In, the current I23 supplied from the constant current source F21
Is selected to be, for example, 1 [μA], and the capacitance of the capacitor C21 is selected to be 150 [pF]. Transistor Q
The passing current of 11 is, for example, [mA] order,
Therefore, the OFF operation of the transistor Q21 is fs =
Approximately 3 which is the switching cycle at 300 [kHz] operation
Sufficiently shorter than [μsec], for example, 20 [nse
c] It ends in about. On the other hand, the transistor Q2
In the ON operation of 1, the constant current source F21 causes the capacitor C21 to turn on the ON voltage of the transistor Q21 (V _{BE = 0.65} [V]).
It takes only the delay time tdf1 to charge the battery. The delay time tdf1 is tdf1 = (C21 * V _BE ) / I23 = (150 [μF] * 0.65 [V]) / 1 [μA] ≈101 [μsec].

Therefore, it is sufficiently longer than the switching period of 10 [μsec] at the time of fs = 100 [kHz] operation. As a result, the switching frequency fs is rapidly reduced when an overcurrent occurs, and when the overcurrent state is released, the transistor Q21 turns off after a lapse of 10 cycles of 100 [kHz] operation, and fs = 30.
Automatically returns to 0 [kHz] operation. The constant current I23 and the capacitance of the capacitor C21 may be appropriately selected according to the switching frequency fs, the desired delay time tdf1, and the like.

FIG. 4 is a graph showing the operating characteristics of the switching power supply device configured as described above. At steady load, I40 = 30 [μA], as described above,
It is operating at fs = 300 [kHz].

The load RL becomes heavy, its resistance value becomes small, the output current Io increases, the collector current of the transistor Tr1 increases, and the collector current at the point A in FIG. A] is exceeded, when the overcurrent detection circuit 11 detects an overcurrent state and the overcurrent protection operation is started, the first oscillation frequency changing circuit 17
As described above, I40 = 10 [μA] is set, and the oscillator 16 is switched to the operation of fs = 100 [kHz]. Further, the RS flip-flop circuit 12 is set, the switching pulse width of the transistor Tr1 is reduced,
The ON time of the transistor Tr1 is shortened, and the output voltage Vo decreases at this point A.

As described above, in performing the normal overcurrent protection operation for narrowing the switching pulse width, the switching pulse width is changed from the overcurrent detection to the output transistor Tr.
The delay time td until 1 is turned off (see FIG. 6) can be once set sufficiently long, and the overcurrent protection operation can be performed.

Further, when the resistance value of the load RL becomes smaller, the output voltage Vo decreases to point B and becomes 2.4 [V], and the feedback voltage Vadj at this time becomes 0.6 [V]. When the output voltage Vo further decreases and the feedback voltage Vadj becomes lower than 0.6 [V] of the reference voltage Vref2, the second oscillation frequency changing circuit 18 sets I40 = 2 [μA] as described above. , Oscillator 16 fs = 2
The operation is switched to 0 [kHz].

By such an operation, the switching pulse width is narrowed by the normal overcurrent protection operation, and even if the switching pulse width approaches the minimum value determined by the delay time td, the switching frequency is restored at the point B. The decrease causes the switching pulse width to increase. As a result, the output current Io is
From the point B at which the switching frequency fs starts decreasing to the point C at which the switching frequency ends, a normal overcurrent point of 2 [A]
Will decrease until it returns to.

After the point C, the oscillation frequency is 20 [kHz
z], the switching pulse width becomes narrower as the load RL becomes smaller, the influence of the delay time td becomes larger, and the output current Io increases. However, since the output current Io is previously decreased from the point B to the point C as described above, the increase in the output current Io can be significantly suppressed. As a result, the output current Io is
The absolute maximum rating value, for example, 2.5 [A] will not be exceeded. The broken line in FIG. 4 indicates that fs = 10.
This is an overcurrent protection characteristic when fixed at 0 [kHz].

As described above, in the switching power supply device according to the present invention, the switching frequency is reduced not only when the output voltage Vo is reduced due to an output short circuit or the like, but also when the overcurrent is detected. The fs can be increased up to about 300 [kHz] which is the upper limit of the operating frequency of the transistor Tr1, the external coil L1 and the capacitor C2 can be downsized, and the switching power supply device can be downsized and the cost can be reduced. .

Further, as shown in FIG. 2, the overcurrent detection circuit 11 has a low resistance R interposed in series with the power supply line L1.
M and a comparator CMP that discriminates the voltage drop across the low resistance by a minute threshold voltage to reduce the voltage drop across the low resistance RM required for overcurrent detection. Therefore, even if the rated current value is relatively large, overcurrent detection can be performed with low loss.

Furthermore, the oscillation circuit of the oscillator 16 is a current control type oscillation circuit capable of changing the oscillation frequency according to the input current I40 from the constant current generating circuit 16a, and the first and second oscillation circuits are provided. Oscillation frequency change circuit 1
7 and 18 are constant currents I42 corresponding to the second frequency.
In the constant current generation circuit 16a, the constant currents I22 and I45 corresponding to the difference between the first frequency and the second frequency and the difference between the second frequency and the third frequency are generated, respectively. Since the constant current is added and subtracted, the current value of the current I40 can be controlled by adjusting the emitter areas of the transistors Q23 and Q24: Q41 to Q47, and the oscillation frequency of the oscillator 16 can be
The frequency can be set to the first, second or third frequency with relatively high accuracy. Therefore, even if the switching pulse width is narrowed by the overcurrent protection operation, the switching pulse width can be surely made longer than the delay time td, and stable operation can be realized.

[0079]

As described above, in the switching power supply device according to the present invention, when the overcurrent is detected, the overcurrent protection circuit narrows the switching pulse width to limit the output current. In the switching power supply device, when the overcurrent detection means detects the overcurrent state at the start of the operation of the overcurrent protection circuit, the first oscillation frequency lowering means changes the oscillation frequency of the oscillation means to a steady state. When the first oscillation frequency is lowered to the second oscillation frequency in advance, and when it is detected that the output voltage has dropped below a predetermined level due to an output short circuit or the like, the second oscillation frequency lowering means changes the oscillation frequency of the oscillation means. It is further lowered to the third oscillation frequency.

Therefore, when the operation of the overcurrent protection circuit is started, the oscillation frequency is lowered from the first oscillation frequency to the second oscillation frequency, and the switching pulse width once widened is narrowed by the overcurrent protection operation. ,
Again, the oscillation frequency is lowered from the second oscillation frequency to the third oscillation frequency and becomes wider, and thus the switching pulse width is always longer than the delay time from the detection of the overcurrent to the realization of the protection operation by the overcurrent protection circuit. Therefore, it is possible to increase the first oscillation frequency to the vicinity of the upper limit of the operating frequency of the switching transistor, which does not depend on the delay time. As a result, it is possible to further reduce the size and cost of the switching power supply device by reducing the size of the externally mounted coil and capacitor.

As described above, in the switching power supply device according to the second aspect of the present invention, the overcurrent detection means has a low resistance interposed in series in the power supply line from the input terminal to the switching element, and the low resistance. And a comparator for discriminating the level of the voltage drop between the terminals by a minute threshold voltage.

Therefore, because of the minute threshold voltage, the voltage drop between terminals of low resistance required for overcurrent detection by the comparator can be made small, and even if the rated current value is relatively large, the loss is low. It is possible to detect overcurrent.

Furthermore, in the switching power supply device according to the invention of claim 3, as described above, the oscillating means is a current control type oscillator capable of changing the oscillating frequency according to the input current. And the second oscillation frequency lowering means generates a constant current corresponding to the difference from the second frequency.

Therefore, by controlling the current value by adjusting the emitter area of the transistor, the oscillation frequency of the oscillator can be set to the first, second or third frequency with relatively high accuracy. Even if the switching pulse width is narrowed by the overcurrent protection operation, the switching pulse width can be reliably made longer than the delay time, and stable operation can be realized.

[Brief description of drawings]

FIG. 1 is a block diagram showing an electrical configuration of a switching power supply device according to an embodiment of the present invention.

FIG. 2 is a block diagram showing a specific configuration of an overcurrent detection circuit in the switching power supply device shown in FIG.

FIG. 3 is a first diagram of the switching power supply device shown in FIG.
FIG. 3 is an electric circuit diagram showing a specific configuration of a second oscillation frequency changing circuit.

FIG. 4 is a graph showing operating characteristics of the switching power supply device shown in FIG.

FIG. 5 is a block diagram showing an electrical configuration of a typical conventional switching power supply device.

6 is a waveform diagram for explaining an overcurrent protection operation of the switching power supply device shown in FIG.

FIG. 7 is a graph showing operating characteristics of the switching power supply device shown in FIG.

FIG. 8 is a block diagram showing an electrical configuration of another conventional switching power supply device.

9 is a graph showing operating characteristics of the switching power supply device shown in FIG.

[Explanation of symbols]

11 overcurrent detection circuit (overcurrent detection means) 12 RS flip-flop circuit (overcurrent protection circuit) 13 differential amplifier 14 reference voltage source 15 comparator 16 oscillator 16a constant current generation circuit 17 first oscillation frequency changing circuit (first Oscillation frequency lowering means) 18 Second oscillation frequency changing circuit (second oscillation frequency lowering means) 19 Drive circuit 20 Comparator (second oscillation frequency lowering means) 21 Constant voltage source (second oscillation frequency lowering means) C1, C2 capacitor CMP comparator (comparator) D1 diode L1 coil Tr1 transistors Q11, Q12; Q21 to Q24; Q31 to Q34; Q
41-Q48 Transistors R1, R2 Resistance R3 Load RM Low resistance Tr1 Transistor

Claims (3)

(57) [Claims] 1. A voltage output of a desired level can be obtained by a switching element switching an input DC voltage in response to an oscillating signal from an oscillating means, and an output current is larger than a predetermined value. In the switching power supply device in which the overcurrent detection circuit limits the output current by narrowing the switching pulse width when detected by the overcurrent detection means, in response to the overcurrent detection by the overcurrent detection means, From the first oscillation frequency in the steady state to the second oscillation frequency lower than the first oscillation frequency and longer than the delay time from the detection of the overcurrent to the realization of the protection operation by the overcurrent protection circuit. First oscillating frequency lowering means for lowering the oscillating frequency of said oscillating means, and said first oscillating frequency lowering means
When the oscillation frequency becomes the second oscillation frequency,
The flow protection circuit narrows the switching pulse width.
A second oscillation frequency lowering means for detecting a lowering of the output voltage of a predetermined level in the state of being present and lowering the oscillation frequency of the oscillation means to a third oscillation frequency lower than the second oscillation frequency. A switching power supply device characterized by the above. 2. The overcurrent detection means includes a low resistance that is interposed in series in a power supply line from an input terminal to a switching element, and a comparator that discriminates a voltage drop between terminals of the low resistance by a minute threshold voltage. The switching power supply device according to claim 1, wherein the switching power supply device is provided. 3. The oscillating means is a current control type oscillator capable of changing the oscillating frequency according to an input current, and the first and second oscillating frequency lowering means are the second oscillator.
Constant current generating circuit for applying a constant current corresponding to the frequency of 1 to the oscillator, and a constant current corresponding to the difference between the second frequency and the first frequency and the difference between the second frequency and the third frequency. 2. The switching power supply device according to claim 1, wherein the switching power supply device generates a constant current and adds or subtracts those constant currents.