JPS5836589B2 - Switching regulator protection circuit - Google Patents

Description

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

A switching regulator is a device that eliminates the large and heavy low-frequency power transformer and makes the stabilized power supply circuit smaller and lighter.

FIG. 1 shows the basic circuit of this switching regulator.

In Fig. 1, 1 is a rectifier and smoothing circuit, in which commercial AC power is directly rectified by a bridge type rectifier circuit and smoothed by a capacitor.

Reference numeral 2 denotes a switching transistor serving as a semiconductor element, to which the DC voltage of the rectifying and smoothing circuit 1 is applied between the collector and emitter, and a high frequency control pulse of 20 KHZ to 50 KHZ is applied to the base to perform high-speed switching operation.

Reference numeral 3 denotes a high-frequency ferrite transformer, which is connected as a collector load of the switching transistor 2, and extracts a stabilized voltage from the secondary side and converts it into direct current through a rectifier and smoothing circuit 4.

A protection circuit used in such a switching regulator is shown in FIG.

The protection circuit includes a control circuit T consisting of a current detection resistor R1 connected to the emitter of the switching transistor 2, a pulse oscillation circuit 5 that generates high-frequency control pulses, and a pulse width modulation circuit 6, and a base connected to the resistor R1. It is formed of a connected detection transistor 8 and a resistor R2 whose one end is connected to the base of the detection transistor and whose other end is biased to a negative DC voltage -Vs.

In normal operation, this protection circuit biases the base of the detection transistor 8 negatively through the resistor R2 to shut off the detection transistor 8, and in abnormal operation, it is biased by the positive DC voltage across the resistor R1 generated by the large current flowing through the switching transistor 2. In order to reduce the impedance of the detection transistor 8 by making the detection transistor 8 conductive, the pulse width modulation circuit 6 reduces the pulse width of the high frequency control pulse to perform a protection operation.

Towing...load impedance, η...Conversion efficiency of transformer, ■i ...Average input current ■.

...output voltage, ...input voltage, Ii The following relational expression is established.

In this relational expression, the left side shows the energy on the output side, and the right side shows the energy on the input side.

Next, if the conduction period of the switching transistor 2 is γ and the repetition period is T, what is the above relational expression? Become.

Here ip is the peak current. Now, considering a current limiter that limits ip to a constant value to protect the switching transistor 2, the energy supplied to the secondary side, that is, the output side, also depends on the input voltage Vi, so the third
As shown in the figure, there is a tendency for the power to increase as Vi increases.

For this reason, the power handled by the switching transistor 2 increases, resulting in power loss that is more than necessary in the design.As the switching transistor 2, a high-power transistor with a large degree of surplus is required, and a large heat sink is also required. .

The present invention has been made in view of this point and provides a protection circuit that eliminates the drawbacks of the prior art.One embodiment of the present invention will be described in detail with reference to FIGS. 4 and 5.

A protection circuit for a switching regulator according to the present invention is shown in FIG.

In FIG. 4, the same parts as those in FIG. The protection operation is performed by reducing the pulse width of the high frequency control pulse applied to the switching transistor 2 by both the detection means 10 and 11.

The switching transistor 2 serves as the current detection means 10.
It consists of a resistor R1 connected in series to the emitter of the switching transistor 2.
is detecting the current.

Further, the voltage fluctuation detection means 11 is composed of a resistor R, one end of which is connected to the input voltage Vi.

Both detection means 10.11 are connected to the base of the detection transistor 8, which is connected via a resistor 12 to the negative DC voltage -Vs.

The above-mentioned protection circuit shuts off the detection transistor 8 by a negative DC voltage -Vs during normal operation.
The impedance of the detection transistor 8 becomes extremely large and does not perform the pulse width modulation protection operation.

Next, when a large current flows through the switching transistor 2 due to a load short circuit, etc., the detection transistor 8 becomes conductive due to the positive DC voltage generated in the resistor R8 as described above, and the impedance of the detection transistor 8 decreases, and the pulse width modulation circuit 6
The pulse width is reduced to provide protection.

Furthermore, when the input voltage Vi, which is a feature of the present invention, fluctuates and increases, the detection transistor 8 is made conductive via the resistor R3 to perform a similar protective operation.

That is, the resistor R3 has a compensating effect for input voltage fluctuations.

In FIG. 5, the input voltage dependence characteristics of the conventional protection circuit and the protection circuit of the present invention are shown by dotted lines and solid lines, respectively.

The characteristics of the present invention are constant at a protection level of 3.5A above 90V, while the conventional characteristics vary in an increasing direction for all input voltages.

Therefore, according to the present invention, since the protection level is constant over a wide range of input voltages, an appropriate power transistor can be selected and the design is economical.

As detailed above, according to the present invention, since the input voltage fluctuation detection means is provided, protection can be performed at a constant power level even if the input voltage Vi fluctuates, and a protection circuit that does not depend on the input voltage can be easily constructed. realizable.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram explaining a switching regulator, Figure 2 is a circuit diagram explaining a conventional protection circuit, Figure 3 is a characteristic diagram explaining input voltage dependence of a conventional protection circuit, and Figure 4 is a circuit diagram explaining a conventional protection circuit. FIG. 5 is a circuit diagram illustrating the protection circuit of the present invention. FIG. 5 is a characteristic diagram illustrating the input voltage dependence of the conventional protection circuit and the protection circuit of the present invention. Explanation of the main drawing numbers, 1 and 4 are rectifier and smoothing circuits, 2 is a switching transistor, 3 is a high frequency ferrite transformer, 7
8 is a control circuit, 8 is a detection transistor, 10 is a current detection means, and 11 is a voltage fluctuation detection means.

Claims (1)

[Scope of Claims] 1. A semiconductor device that performs a switching operation based on a high-frequency control pulse output from a rectifying and smoothing circuit connected to an AC power source and a control circuit, and a transformer serving as a load of the device,
Switching to obtain a stable voltage from the secondary side of the transformer
In the regulator, the primary side of the transformer is provided with a current detection means of the semiconductor element and a voltage fluctuation detection means of the input voltage from the rectifying and smoothing circuit, and the pulse width of the high frequency control pulse is modulated by both the detection means. A protection circuit for a switching regulator. 2. In claim 1, the outputs of both of the detection means are applied to a detection transistor, and the pulse width of the high-frequency control pulse is modulated by a change in impedance of the detection transistor. Switching
Regulator protection circuit.