JPS6033745Y2 - switching regulator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a switching regulator having a transformer.

In this type of switching regulator, as shown in FIG. 1, an input voltage E is applied to a series circuit of a primary winding n1 of a transformer 1 and a transistor 2, which is a switching element.

When the transistor 2 is turned on, the primary current 11 flows, and when the transistor 2 is turned off, the energy stored in the iron core of the transformer 1 by i□ is transferred to the secondary winding ~, the secondary current i2? on the n3 side.
It is released as 13.

The secondary current 12 on the side of the secondary winding n2 is rectified and smoothed by the rectifying and smoothing circuit 3, and is applied to the load RL as an output voltage EO.

In addition, the secondary current i3 on the side of the secondary winding is rectified and smoothed by the rectifying and smoothing circuit 4 to become a feedback voltage Er with respect to EO, and the resistor R1
, R2 and applied to the error amplifier 5.

The error amplifier 5 amplifies the deviation between the feedback voltage E and the reference voltage EEES, and the output thereof is compared with the triangular comparison voltage Er from the oscillator 7 in the comparator 6.

Based on the comparison result, drive circuit 8 turns transistor 2 on or off.

In this way, the transistor 2 is repeatedly turned on and off so that Ef=Es, and the output voltage Eo is controlled to a constant value.

Incidentally, in a switching regulator having such a configuration, an overcurrent protection circuit is required because the switching regulator operates to maintain a constant voltage even in the event of an overload, and an excessive current flows.

For this reason, conventionally, the output current is monitored and when the output current becomes excessive, the transistor 2 is turned off, thereby stopping the increase in the output current and providing overcurrent protection.

Since conventional methods detect the output current in this way, it is not possible to suppress the peak value of the negative primary current, which causes an excessive primary current to flow, especially when the power is turned on or in response to a sudden load short circuit.
There was a risk that the iron core of transformer 1 would become saturated and damage transistor 2.

Further, when the feedback voltage Ef is obtained from a separate winding from the output voltage Eo to provide input/output insulation as shown in FIG. 1, the overcurrent protection circuit must also be of an insulated type, resulting in a complicated configuration.

In addition, when providing multiple secondary windings to create a multi-output type,
Overcurrent detection means is required for each output, making the configuration even more complicated.

In this invention, when the voltage based on the primary current of the transformer exceeds the limit value, the input of the difference amplifier on the side to which the reference voltage is applied is set to a low level, thereby shortening the ON time of the switching element. This realizes a switching regulator without any noise.

Figure 2 is an electrical connection diagram showing one embodiment of the present invention.
The same parts as in the figures are given the same reference numerals.

In Fig. 2, the difference from the conventional example shown in Fig. 1 is that the primary current detection resistor R5 is connected to the emitter side of the transistor 2, and the voltage drop TE, C of this resistor R6 is applied to the overcurrent through the resistor R6. It is added to the base of the detection transistor Q.

In addition, the reference voltage Es is connected to the base of the transistor Q by a resistor R.
It has been added via 7.

The transistor Q has its collector connected to the inverting input terminal (-) of the error amplifier 5, and its emitter connected to the reference point.

Note that the reference voltage E is connected to the inverting input terminal (-) of the error amplifier 5.
s is applied through a resistor R3, and the output of the error amplifier 5 is applied through a parallel circuit of a resistor R4 and a capacitor C.

In the present invention configured in this manner, under normal conditions, the voltage drop Ec across the resistor R5 based on the primary current 11 is determined by the limit value El ( The amount of dirt does not exceed Bo-&E5), the transistor 7R7 is off, and the error amplifier 5 amplifies the deviation between the feedback voltage Er and the reference voltage Es, and the voltage of the transistor 2 is adjusted so that E, = ES. Controls on/off.

By the way, when the primary current i□ increases due to a short-circuit accident in the load RL, and the voltage drop Ec across the resistor R5 exceeds the limit value E1, the transistor Q is turned on.

As a result, the inverting input terminal (-) of the error amplifier 5 becomes low level, and the output of the error amplifier 5 becomes bilevel.
Transistor 2 is turned off to stop the increase in negative voltage and provide overcurrent protection.

At this time, if the transistor 2 is turned on and off multiple times during one cycle, the transistor 2 will generate heat, so this is prevented by providing a capacitor C in the feedback circuit of the error amplifier 5 and adjusting the response speed of the error amplifier. .

Further, in the present invention, when the power is turned on, the transistor Q starts operating from a short on time, and the on time is lengthened every cycle to transition to a normal state, so a soft start is possible.

As explained above, in the present invention, an overcurrent detection transistor is used, and when the primary current exceeds the limit value, the input on the side to which the reference voltage of the error amplifier is applied is set to a low level, thereby shortening the on time of the switching element. In addition to suppressing the peak value of the primary current, a capacitor is provided in the feedback circuit of the error amplifier to adjust the response speed of the error amplifier, resulting in a switching regulator that can provide overcurrent protection with a simple configuration. It will be done.

[Brief explanation of the drawing]

FIG. 1 is a connection diagram showing an example of a conventional switching regulator, and FIG. 2 is a connection diagram showing an embodiment of the switching regulator of the present invention. 1...Transformer, 2...Switching element, 3.4...Rectifier smoothing circuit, BL...
...Load, 5...Error amplifier, 6...
Comparator, 7...Oscillator, Es...Reference voltage, Q...Transistor for overcurrent detection, R□
, R2, R3, R4, R5, R6, R7...
Resistance, C... Capacitor.

Claims (1)

[Scope of utility model registration request] an error amplifier that amplifies the deviation between the feedback voltage and the reference voltage related to the output voltage; a comparator that compares the output of the error amplifier with a comparison voltage; and a switching element that turns on and off based on the output of the comparator. In a switching regulator having means for controlling a primary current of a transformer, an overcurrent detection transistor connected between an input terminal of the error amplifier to which a reference voltage is applied and a reference point, and a transistor for detecting the primary current of the transformer. a resistor, a first resistor that applies a voltage drop across the resistor to the base of the overcurrent detection transistor, a second resistor that applies the reference voltage to the base of the overcurrent detection transistor, and an error amplifier. A capacitor connected to a feedback circuit for adjusting its response speed is provided, and when the voltage drop occurring in the secondary current detection resistor exceeds a limit value, the overcurrent detection transistor is turned on, and the error is detected. A switching regulator that provides overcurrent protection by setting the input on the side where the reference voltage of the amplifier is applied to a low level.