JPS591418Y2 - Switching type power supply circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a switching type power supply circuit, in particular, to reduce the load on switching elements when a power switch is turned on.

An example of a conventional switching type power supply circuit is shown in FIG.

1 is an AC power supply, 2 is a power switch S for AC power supply 1
It is a diode bridge for rectification connected via W.

A smoothing capacitor 4 is inserted between output terminals 3 a and 3 b of the diode bridge 2, and the output terminal 3 a is connected to the collector of a transistor 5 as a switching element, and the output terminal 3 b is grounded.

The transistor 5 performs a switching operation in response to a switching pulse supplied from a pulse width modulation circuit 7 via a drive transformer 6.

The emitter of the transistor 5 is connected to the output terminal 9 via a choke coil 8, and a flywheel (current sustaining) diode 10 is inserted between one end of the choke coil 8 and ground, and a smoothing diode 10 is inserted between the other end and ground. A capacitor 11 is inserted.

A detection voltage V1 proportional to the output voltage is taken out to the movable element of the variable resistor 12 inserted between the output terminal 9 and the ground.
By comparing this detection voltage (1) with the reference voltage (2) in the comparison circuit 13, a control voltage (2) is formed, and the duty factor of the switching pulse formed in the width modulation circuit 7 is controlled by the control voltage (V2). Ru.

In such a switching power supply circuit, energy is stored in the choke coil 8 during the ON period of the transistor 5, and the capacitor 11 is also charged, and a current flows through the flywheel diode 10 by the energy stored in the choke coil 8 during the OFF period. As the current flows, the capacitor 11 is charged.

Therefore, an output voltage is generated in which the input voltage is reduced at a rate corresponding to the duty factor of the switching pulse.

The comparison circuit 13 generates a control voltage v2 that changes the duty factor of the switching pulse so as to cancel out fluctuations in the output voltage.

Here, if we consider when the power switch SW is on, the detection voltage
(2) Therefore, the duty factor of the switching pulse becomes maximum.

Furthermore, the peak value of the collector current of transistor 5 also becomes large.

If a large collector current flows for a long on period while a high input voltage is applied to the collector of the transistor 5, the transistor 5 may be destroyed.
For this purpose, a transistor with excellent ASO (safe operating area) must be used as the transistor 5.

Generally, such transistors are expensive, and it is not practical to use such expensive transistors simply for the purpose of turning on the power switch SW.

The present invention solves this conventional problem and prevents an excessive load from being placed on the transistor 5 as a switching element when the power switch SW is turned on.

An embodiment of the present invention will be described below with reference to FIG.

In this example, power switch SW and diode bridge 2
At the same time, the output terminal 3b of the diode bridge 2 is not grounded and is connected to the output terminal 9, so that when the transistor 5 is destroyed and its collector-emitter is short-circuited, the fuse F This prevents the risk of the high input voltage appearing as it is at the output terminal 9 due to melting.

Further, a PNP type transistor indicated by 14 is a comparison transistor.

In order to smooth the input voltage between the collector of the transistor 5 and the ground, a series circuit of a resistor 15 and a capacitor 16 is inserted, and the connection point between the two is connected to the cathode of a Zener diode 18 via a resistor 17. The anode is grounded and a reference voltage 1, formed by a Zener diode 18, is supplied to the emitter of the transistor 14.

A capacitor 19 is connected in parallel with this Zener diode 18.
is connected.

A detection voltage ■1 is applied to the base of the transistor 14 from a variable resistor 12, and a control voltage ■2 is generated at its collector. The signal is supplied to the control terminal 7a.

A series circuit of a smoothing capacitor 20 and a resistor 21 is inserted between the collector of this transistor 14 and the ground, and a Zener diode 22 as a limiter means is inserted.
is inserted.

The operating principle of the circuit shown in FIG. 2 is as follows.

First, when the transistor 5 is turned on, a current flows from the output terminal 3a of the diode bridge 2 to the transistor 5 to the choke coil 8 to the output terminal 3b of the diode bridge 2, and energy is stored in the choke coil 8.

Next, when the transistor 5 is turned off, a current due to the energy stored in the choke coil 8 flows from the ground to the diode 10 to the choke coil 8 to the output terminal 9 to the load (not shown) to the ground. 11 is charged.

Therefore, an operating voltage is supplied to the load.

In steady state, the detection voltage ■1 and the reference voltage ■.

The duty factor of the switching pulse is adjusted to, for example, 50% by the control voltage (2) generated when the magnitude of the output voltage is specified.

When the output voltage becomes larger than the specified value, the control voltage 2 is lowered and the duty factor of the switching pulse becomes smaller, and conversely, when the output voltage becomes smaller than the specified value, the control voltage 2 is increased and the switching pulse is reduced. The duty factor of the pulse is increased.

Such control can be realized by a configuration in which the pulse width modulation circuit 7 is configured with an unstable multi-by-break circuit and the base potential of one of the transistors is controlled.

The control voltage (2) generated during the stabilizing operation in steady state does not exceed the Zener voltage of the Zener diode 22.

Further, when the power switch SW is turned on, no output voltage is generated and the detected voltage ■1 is O■.

Therefore, the control voltage V2 increases.

However, the control voltage (2) cannot exceed the Zener voltage of the Zener diode 22, and is limited to a value that also satisfies the duty factor of the switching pulse.

After passing the transient state when turning on the power switch SW,
The routine moves on to the same steady operation as described above.

As described above, according to the present invention, the tenity factor of the switching pulse when the power switch SW is turned on can be prevented from exceeding a predetermined value.
This has the advantage that the ASO does not need to be especially wide.

Further, once the transient state when the power switch SW is turned on is passed, the operation can be directly shifted to the steady state.

Furthermore, as in the above embodiment, the reference voltage (2) is formed based on the input voltage, and the capacitor 19 is used to connect the power switch SW.
It will be more effective if the reference voltage (2) is not generated immediately when turning on, but is configured to have a rise time constant.

That is, since the level of the control voltage v2 is determined by the difference (V, -V,) between the reference voltage ■ and the detection voltage ■1, the reference voltage Vr has a rise time constant, so that it is essentially the control voltage during a transient period. ■It is possible to limit the level of 2.

[Brief explanation of drawings]

Figure 1 is a connection diagram of a conventional switching type power supply circuit, Figure 2
The figure is a connection diagram of an embodiment of the present invention. SW is a power switch, 2 is a diode bridge, 7 is a pulse width modulation circuit, 9 is an output terminal, 14 is a comparison transistor, and 18.22 is a Zener diode.

Claims (1)

[Scope of utility model registration request] In a switching type power supply circuit, in which an output voltage is formed by smoothing a pulse voltage obtained by intermittent input voltage by a switching element, and the output voltage is stabilized by controlling the above-mentioned intermittent ratio with a control voltage. , a reference voltage having a predetermined rise time constant is supplied to its emitter, a detection voltage proportional to the output voltage is supplied to its base, and a comparison transistor is provided from which the control voltage is taken out from its collector. A switching power supply circuit in which a limiter means for limiting the control voltage is connected to the collector of the transistor for use in the switching mode.