JPH066710Y2 - Variable dummy circuit for switching power supply - Google Patents

Description

[Detailed Description of the Invention] <Industrial field of application> The present invention relates to a variable dummy circuit that prevents overvoltage and disturbance that may occur when the switching power supply is lightly loaded, and in particular to an improvement to provide a stable operation circuit. .

<Prior Art> A variable dummy circuit of a switching power supply is described in, for example, “Switching regulator design know-how” (Akira Hasegawa, CQ
Published). A dummy resistor, a swing choke coil or a variable dummy circuit is used to stabilize the operation of the switching power supply at a light load. However, the variable dummy circuit has an advantage that a power loss is small because a dummy current flows only at a light load.

FIG. 4 is a circuit diagram of a conventional variable dummy circuit. In the figure, the rectified switching signal appearing on the secondary side of the switching power supply is supplied to the capacitor C as the input current Iin, and is supplied to the load at the voltage Vout as the smoothed output current Iout. The comparator detects the current flowing into the capacitor C with the resistor Rs, and turns on the transistor Q when compared with the reference voltage Vref and becomes a current corresponding to a light load. Then, the transistor Q has a dummy current ID that is a collector current.
Flow, the input current Iin and the output current Iout and the dummy current I
It was diverted to D.

<Problems to be Solved by the Invention> However, the conventional device has the following problems.

In the case of the load current corresponding to the threshold value of the comparator, the variable dummy circuit repeatedly turns on and off, causing switching loss in the switching element on the primary side, and destabilizing the operation of the entire power supply.

In general, since the load current fluctuates greatly, the output voltage fluctuates accompanying the on / off of the variable dummy circuit, and the function as the stabilized power supply becomes insufficient.

Since the collector current is used as the dummy current ID, a transistor with large collector loss is required,
Parts prices were getting expensive.

<Means for Solving the Problems> In the present invention to achieve such an object, a DC current is supplied to a primary winding of a transformer to be turned on / off by a switching element and induced in a secondary winding of the transformer. A DC power supply unit (10) for rectifying and smoothing a signal and outputting the detected signal; and means (20) for detecting a switching current flowing through the primary winding or the secondary winding and outputting a DC detection signal proportional to the switching current. , When it is determined by the DC detection signal of the current detecting means that the load is light,
A switching power supply having a variable dummy means (30) for extracting a dummy current from the DC power supply unit has the following configuration.

That is, the variable dummy means inputs the voltage signal from the reference voltage generating means and the means (R5, R6) for generating the reference voltage corresponding to the light load state to the plus terminal via the input resistor (R7). Then, the DC detection signal is input to the negative terminal, and the series circuit of the diode (D5) and the output resistance (R9) is connected to the output terminal, and the connection resistance (R8) connecting the output resistance and the positive terminal is connected. A comparator (31) having the dummy resistance (R1), one end of which is connected to the output circuit of the DC power supply unit, and an output signal of the output resistance of the comparator is input to a control terminal to be connected to the other end of the dummy resistance. A switching element (Q2) composed of an FET that opens and closes a connection state with the ground, and one end connected to the output resistance of this comparator and the other end connected to the ground resistance (R10). It is characterized in that.

<Operation> Since the comparator has hysteresis, the operation of the variable dummy circuit is stabilized, and the output voltage stabilizing circuit of the switching power supply operates smoothly even under a light load condition to obtain a stable output voltage. Here, since the series circuit of the diode (D5) and the output resistor (R9) is connected to the output terminal of the comparator, the operation is accurately performed. Also, since the switching element is an FET, the circuit configuration is simpler than that of a bipolar transistor.

<Example> The present invention will be described below with reference to the drawings.

FIG. 1 is a circuit diagram showing an embodiment of the present invention. In the figure, a DC power supply unit 10 is a general DC-DC converter, in which a DC voltage Vin is applied to a primary winding n1 and turned on and off by a switching element Q1. The switching element Q1 is a transistor or an FET, and is turned on / off by a control signal output from a pulse width control circuit for stabilizing the output voltage (not shown). Then the secondary winding n2
Since a switching signal is induced in the diode D
1, D2 for rectification, choke coil L for removing high frequency components, capacitor C1 for smoothing, and DC output voltage V
getting out. When stabilizing the output voltage Vout,
The error signal is obtained by comparing with the reference voltage and fed back to the pulse width control circuit.

The current detection circuit 20 measures the output current of the DC power supply unit 10. Here, the switching signal induced in the secondary winding n4 corresponding to the primary winding n3 connected in series with the primary winding n1 is Rectification is performed by the diodes D3 and D4, smoothing is performed by the capacitor C3, and the time constant is determined by the resistor R4.

The variable dummy circuit 30 (active dummy load) includes resistors R5 to R1 that add hysteresis to the operation of the comparator 31, the switching element Q2, and the comparator 31.
It is 0. The comparator 31 is connected to the positive power supply V _CC and the negative power supply V _EE to obtain the power required for operation. Further, the output signal of the current detection circuit 20 is connected to the negative input terminal, and the reference voltage obtained by dividing the voltage of the positive power supply V _CC by the resistors R5 and R6 is input to the positive input terminal through the resistor R7. At the same time, a series circuit of a diode D5 and an output resistor R9 is connected to the output terminal. Further, the connection resistor R8 connects the output resistor R9 and the positive terminal, and gives the comparator 31 a hysteresis characteristic. The output signal of the comparator 31 is divided by the resistors R9 and R10 and applied to the control terminal of the switching element Q2, and the output voltage V is applied to the input terminal through the dummy resistor R1.
out is applied and the output terminal is grounded.

The operation of the apparatus thus configured will be described below. Second
The figure shows the relationship between the output current and the threshold voltage V _TH of the comparator 31, and FIG. 3 shows the relationship between the output current and the dummy current.

When the DC power supply unit 10 starts operating in the no-load state, the negative input of the comparator 31 is lower than the positive input, the output of the comparator 31 becomes H, and the switching element Q
2 is turned on and a dummy current flows (). As the load current increases, the off threshold voltage V _THOFF is reached, the output of the comparator 31 becomes L, and the switching element Q
2 is turned off, and as a result, the dummy current does not flow, and the output voltage of the current detection circuit 20 is reduced by the dummy current (). Next, when the output current is reduced while the dummy current is off, the on-state threshold voltage V _THON is reached, the output of the comparator 31 becomes H, and the switching element Q2 is turned on. As a result, the dummy current begins to flow. The output voltage of the current detection circuit 20 increases by the dummy current ().

<Effect of Device> As described above, the present invention has the following effects.

Since hysteresis is provided in the operation of the variable dummy circuit 30, repeated on / off is unlikely to occur, and the controllability of the power supply as a whole is increased.

The output voltage is stabilized even if the load current fluctuates greatly.

Since the resistor R1 is used for the dummy current, the switching element Q2 may be an inexpensive ordinary transistor or FET.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, FIG. 2 is a relational diagram of the output current and the threshold voltage V _TH of the comparator 31,
FIG. 3 is a relationship diagram between the output current and the dummy current. FIG. 4 is a circuit diagram of a conventional variable dummy circuit. 10 ... DC power supply section, 20 ... current detection circuit, 30 ...
Variable dummy circuit, 31 ... Comparator, Q2 ... Switching element.

Claims (1)

[Scope of utility model registration request] 1. A direct current is supplied to a primary winding of a transformer,
A DC power supply unit (10), which turns on and off by a switching element, rectifies and smoothes a signal induced in a secondary winding of the transformer, and outputs the signal, and detects a switching current flowing in the primary winding or the secondary winding. , Means for outputting a DC detection signal proportional to this switching current (20)
And a variable dummy means (30) for extracting a dummy current from the DC power supply section when it is determined by the DC detection signal of the current detection means that the variable dummy means is provided. , A means (R5, R6) for generating a reference voltage corresponding to the light load state, and a voltage signal from the reference voltage generating means are input to a plus terminal via an input resistor (R7) to output the DC detection signal. Input to the negative terminal, diode (D5) and output resistance (R9)
Is connected to the output terminal, and a comparator (31) having a connecting resistor (R8) for connecting the output resistor and the positive terminal, and a dummy resistor (R1) having one end connected to the output circuit of the DC power supply unit are connected. ) And the output signal of the output resistance of this comparator are input to the control terminal to open and close the connection state between the other end of the dummy resistance and the ground.
Composed of a switching element (Q2) and an output resistance of the comparator, one end of which is connected to the ground resistance (R1
0) and a variable dummy circuit for a switching power supply.