JPS5844248B2 - switching regulator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a switching regulator.

FIG. 1 is a circuit diagram showing a conventional switching regulator.

The DC voltage of the DC power supply 1 is interrupted by a switch 2, and the interrupted DC voltage is again smoothed by a filter consisting of a choke 3, a capacitor 4, and a flywheel diode 5, and is supplied to a load 6.

The control circuit 7 controls the on/off state of the switch 2 so that the load voltage supplied to the load 6 is constant.

Since the above-mentioned filter is a choke input type smoothing circuit, the current flowing through the choke 3 is intermittently critical current (2). In the following load current range, the load voltage tends to rise according to the characteristics shown in FIG. 2, but the control circuit 7 The load voltage is kept constant.

However, since the control circuit 7 does not operate to make the intermittent current flowing through the choke 3 continuous, the ripple increases in the operating range below the critical current (2).

This will be further explained using FIG.

FIG. 3 shows the base voltages of transistors 10 and 11 when the load current is below the critical current IK.

The broken line A is the base voltage of the transistor 10, which is the value obtained by dividing the load voltage by the resistors 8 and 9, and the solid line B is the base voltage of the transistor 10.
1 base voltage.

A high level Bh appears when the switch 2 is turned on, and the DC power supply voltage of the power supply 1 is connected to the resistors 13, 14 and the Chener diode 12.
This is the value divided by .

The low level Bt appears when the flywheel diode 5 is on due to the energy stored in the choke 3 when the switch 2 is on, and the Zener voltage vD is applied to the resistor 14.
This is the value divided by 13 degrees.

The middle level Bm does not appear when the load current is greater than the critical current ■, but when the load current is less than the critical current ■, the switch 2 is turned off even if no current flows through the flywheel diode 5. During this period, the base voltage of the transistor 11 is a value obtained by dividing the load voltage by the resistors 13 and 14 and the Zener diode 12.

Transistors 10 and 11 are turned on and off alternately, and switch 2 is on when transistor 11 is on and off when transistor 10 is on.

Note that the resistor 15 is used to turn on the transistor 11 and turn on the switch 2 at startup.

Further, the resistor 14 is used to prevent the base voltage of the transistor 12 from being fixed to the Zener voltage, and to supply the base of the transistor 11 with a voltage corresponding to whether the switch 2 is turned on or off.

This provides a switching control effect that stably suppresses fluctuations in load voltage.

When the load current is small, less energy is stored in the choke 3 and the on period of the flywheel diode 5 is shorter than when the load current is large.

Therefore, since the transistor 11 does not turn on immediately after the flywheel diode 5 turns off, the switch 2
It doesn't turn on either.

That is, there is a period in which both the switch 2 and the flywheel diode 5 are off, so that the current flowing through the choke 3 is intermittent and the ripple increases.

On the other hand, if the inductance of the choke 3 is increased, the energy stored in the choke 3 increases, but such a choke is expensive, and has a large shape and weight.

Therefore, it is not desirable to use a choke with large inductance.

SUMMARY OF THE INVENTION In view of the above drawbacks, an object of the present invention is to provide a switching regulator in which the current flowing through the choke does not intermittent even when the load current becomes small.

The present invention provides a switch that allows the DC power source to intermittent the charging current that charges the choke, regardless of the magnitude of the load current.
In order to immediately turn on the flywheel diode that discharges the charge in the choke after turning off, the switch is switched from off to on by detecting the voltage across the flywheel diode.

The present invention will be explained below with reference to FIG.

FIG. 4 is a circuit diagram showing one embodiment of the switching regulator according to the present invention.

In FIG. 4, since the connection point between the resistor 13' and the Zener diode 12 is connected to the base of the transistor 11, the base voltage of the transistor 11 is in the forward direction of the Zener diode 5 when the flywheel diode 5 is on. When the voltage Vf is a negative voltage and the flywheel diode 5 is off, it is a Zener voltage VD.

Even when the load current is small, the base voltage of the transistor 11 has two states, high level Bh' and low level B, ff', as shown in FIG. 5, so the transistor 11 is turned on and off depending on these two states. can be done.

That is, even when the load current is small, there is no middle level Bm state, so the switch 2 can be turned on immediately after the flywheel diode 5 is turned off.

In FIG. 5, A' is the base voltage of transistor 10.
13/ is the base voltage of the transistor 11.

The resistor 15' is for supplying the base current of the transistor 11 at startup, turning on the transistor 11, and turning on the switch 2.

In order to change the base voltage of the transistor 11 according to the on/off state of the flywheel diode 5, in the circuit of FIG.
Since the other end of 5 is connected to the Chechi diode 12 side, the resistor 14 is required. However, in the circuit of FIG. 2, the starting resistor 15' is connected in parallel with the switch 2, so the resistor 14 is not necessary.

Therefore, by directly connecting the connection point between the resistor 13' and the Zener diode 12 to the base of the transistor 11, the transistor 11 can be turned on immediately after the flywheel diode 5 is turned off.

In the circuit shown in FIG. 4, when the flywheel diode 5 is turned on, a differential voltage between the base voltage of the transistor 10 and the voltage (VF) is applied between the base and emitter of the transistor 11 in the opposite direction to the conduction direction. Therefore, if this voltage difference is larger than the base-emitter reverse withstand voltage,
Transistor 11 is destroyed.

To prevent this, a diode 16 may be inserted in series with the Zener diode 12 to prevent the forward current from flowing through the Zener diode 12, as shown in FIG.

As described above, since the switch 2 can be turned off or on according to the on or off state of the flywheel diode 5, a stable DC voltage can be obtained even if the load current is small.

Furthermore, in the conventional circuit, when the load current is large, the base voltage of the transistor 10 when the transistor 11 is switched from on to off is different from the base voltage of the transistor 10 when the transistor 11 is switched from off to on. Therefore, ripples occur in the load voltage.

However, in the present invention, the base voltage of the transistor 10 when the transistor 11 is turned on and off is the same when it changes from on to off and when it changes from off to on, so the above-mentioned ripple is eliminated.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram showing a conventional switching regulator, Figure 2 is a characteristic diagram showing its output characteristics, Figure 3 is a waveform diagram showing the base voltage of the transistor that constitutes the control circuit in Figure 1, and Figure 4. 5 is a circuit diagram showing one embodiment of the switching regulator according to the present invention, FIG. 5 is a waveform diagram showing the base voltage of the transistor forming the control circuit in FIG. 4, and FIG. 6 is a diagram showing another embodiment of the present invention. It is a circuit diagram. DESCRIPTION OF SYMBOLS 1... DC power supply, 2... Switch, 3... Choke, 4... Capacitor, 5... Flywheel diode, 6... Load circuit, 12... Zener diode, 13' . 15'...Resistance.

Claims (1)

[Scope of Claims] 1. A DC power supply, a load circuit, a switch that connects and connects the DC power supply voltage, a filter that smooths the intermittent DC power supply voltage and supplies it to the load, and a control that controls whether the switch is turned on or off. In the switching regulator, the filter is provided with a choke that is charged during the on-time period of the switch, and a flywheel diode that discharges the charge stored in the choke during the off-time period of the switch, and the control circuit is connected in parallel with the flywheel diode. A series circuit consisting of a resistor and a Zener diode connected to the resistor is provided, and a voltage corresponding to whether the Zener diode is turned on or off is generated at the connection point between the resistor and the Zener diode, and the switch is connected to the connection point. A switching regulator characterized by being turned on and off according to voltage. 2. The switching regulator according to claim 1, further comprising a diode inserted in series with the Zener diode to prevent the Zener diode from turning on in the forward direction.