JPH08280172A - Dummy load circuit for switching power unit - Google Patents

Abstract

PURPOSE: To make ideal the rise characteristic of the output voltage of a switching power unit and, at the same time, to reduce the loss of the power unit. CONSTITUTION: A load current iL flows to a switching power unit through a resistor 24 and a transistor 25 until the output voltage Vout of the power unit is stabilized after power supply is made. Since the load current iL flows to the power unit after the power supply is made under any load condition, the rise characteristic of the output voltage Vout becomes ideal. In addition, the loss of the power unit becomes zero at steady state, because, when the output voltage Vout is stabilized, the load current iL does not flow and the transistor 25 is turned off due to a charged capacitor 23.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dummy load circuit of a switching power supply device having a soft start circuit for the purpose of improving the rising characteristics of the output voltage, especially under no load or light load.

[0002]

2. Description of the Related Art Generally, a switching power supply device of this type switches a switching element by a control circuit, intermittently applies a DC input voltage to a primary winding of a transformer, and induces it from a secondary winding of the transformer. The rectified and smoothed voltage is rectified and smoothed to obtain a predetermined DC output voltage.However, when the power is turned on, the charging current of the output capacitor may damage the switching element or the output voltage rising waveform may overshoot. To prevent
There is a transformer provided with a soft start circuit for controlling the duty of the drive signal from the control circuit, that is, the pulse conduction width, on the primary side of the transformer. This soft start circuit controls the control circuit to gradually increase the pulse conduction width of the drive signal to the switching element until the output voltage is stabilized by feedback control by the control circuit after the power is turned on, and the output is controlled. The voltage is ramped up substantially linearly.

[0003]

Problems of the switching power supply device in the above-mentioned prior art will be described with reference to the waveform diagram of FIG. FIG. 3 shows the rising characteristics of the output voltage Vout under no load and under load. In the soft start region until the output voltage Vout stabilizes, when an appropriate load is connected, Although the output voltage Vout can be raised linearly with a substantially ideal characteristic, when the load is not connected, the energy sent out is applied only to the output capacitor, so the charging speed of the output capacitor increases. It becomes faster than when the load is connected, and as shown in the waveform diagram of FIG. 3, the output voltage Vout rises stepwise or overshoots. Especially, the rated output voltage is 2V to 3.3V
When the output voltage Vout is low, the output voltage Vout fluctuates to 80% or more of the rated output voltage at the step-like rising point, which is unacceptable.

In order to deal with such a problem, conventionally, a dummy fixed resistor is connected between the output terminals and a load current is forcibly generated when the power is turned on, so that the output voltage at no load or at light load is increased. There is a method of improving the rising characteristic of Vout. However, with such fixed resistance,
There is a drawback in that the efficiency of the power supply device is reduced due to the loss of the power supply device even in a constant time.

Therefore, in view of the above problems, the present invention is to obtain a dummy load circuit of a switching power supply device which can obtain an ideal rising characteristic of an output voltage under any load condition and has less loss. To that end.

[0006]

In order to achieve the above object, a dummy load circuit of a switching power supply device according to the present invention switches a switching element by a drive signal from a control circuit, and a direct current is applied to a primary winding of a transformer. An input voltage is applied intermittently, the voltage induced from the secondary winding of this transformer is rectified and smoothed to obtain a predetermined DC output voltage, and the duty of the drive signal from the control circuit is controlled when the power is turned on. In a switching power supply device including a soft start circuit, a series circuit including a diode and a capacitor and a series circuit including a resistor and a transistor are connected between output terminals, and the base of the transistor is connected to the diode and the capacitor. It is constructed by connecting points.

[0007]

With the above structure, the dummy load circuit causes the load current to flow through the resistor and the transistor after the power is turned on until the output voltage is stabilized, so that the load current flows in any load state. As a result, ideal rising characteristics of the output voltage can be obtained. Moreover, when the output voltage is stabilized, the transistor is turned off by charging the capacitor and the load current does not flow. Therefore, despite the provision of the dummy load circuit, there is no loss in the power supply device in the steady state.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to the accompanying drawings, FIG. 1 and FIG. First, the circuit configuration of the switching power supply device according to the present embodiment is shown in FIG. In the figure, 1 is a transformer that insulates the primary side from the secondary side, and 2 is a MOS type F connected to the primary winding of the transformer 1.
ET is a switching element, and by switching the switching element 2 by a drive signal from the control IC 3 corresponding to the control circuit, the input terminal + V
A DC input voltage Vin between i and -Vi is intermittently applied to the primary winding of the transformer 1. Further, 4 is a rectifying / smoothing circuit composed of, for example, a rectifying diode, an output capacitor for smoothing, and the like, which rectifies and smoothes the voltage induced in the secondary winding of the transformer 1 to load the load. Between the output terminals + Vo and -Vo connecting (not shown),
It outputs a predetermined DC output voltage Vout. An output voltage detection circuit 5 forming a feedback circuit is connected between the output terminals + Vo and -Vo, and a voltage detection signal of the output voltage Vout is fed back to the control IC 3.

Reference numeral 11 is a soft start circuit provided on the primary side of the transformer 1. This soft start circuit 11
Is for monitoring whether or not the power is turned on by the input voltage Vin and controlling the duty so that the pulse conduction width of the drive signal from the control IC 3 gradually expands when the power is turned on. In FIG. 1, the soft start circuit 11 is provided separately from the control IC, but the soft start function built into the control IC 3 may be used as it is as the soft start circuit 11. Reference numeral 21 is a dummy load circuit which is located at the subsequent stage of the rectifying and smoothing circuit 4 and is provided between the output terminals + Vo and -Vo. In this dummy load circuit 21, the cathode of a diode 22 is connected to one output terminal + Vo, the anode of this diode 23 is connected to one end of a capacitor 23, and one end of a resistor 24 is connected to the one output terminal + Vo. The emitter of the PNP transistor 25 is connected to the other end of the resistor 24. The base of the transistor 25 is connected to the connection point between the diode 22 and the capacitor 23, and the other end of the capacitor 23 and the collector of the transistor 25 are connected to the other output terminal -Vo. That is, the first series circuit including the diode 22 and the capacitor 23 and the second series circuit including the resistor 24 and the PNP transistor 25 are connected between the output terminals + Vo and -Vo, respectively, and the base of the transistor 25 is connected to the diode. A connection point between the capacitor 22 and the capacitor 23 is connected to form a dummy load circuit 21.

Next, the operation of the above structure will be described with reference to the waveform diagram of FIG. First, when the power is turned on, a drive signal having a pulse conduction width such that the output voltage Vout becomes a minimum state is output from the control IC 3 to the switching element 2.
Control signal is output from the soft start circuit 11 to the control IC 3 so as to be supplied to the control IC 3. As a result, the switching element 2 starts the switching operation, and the input voltage Vin is intermittently applied to the primary winding of the transformer 1.

The voltage induced in the secondary winding of the transformer 1 is output to the output terminals + Vo and -Vo as the output voltage Vout through the rectifying / smoothing circuit 4, but the output voltage Vout immediately after the power is turned on is low. , The transistor 25 of the dummy load circuit 21 is turned on, and the load current iL flows in between the resistor 24 and the emitter / collector of the transistor 25. At this time, as is clear from the waveform diagram at the time of no load in FIG. 2, even when the load is not connected between the output terminals + Vo and −Vo, the load current iL generated in the dummy load circuit 21
Thus, the stepped rising point A of the output voltage Vout is limited to low voltage level fluctuations.

After that, based on the control signal from the soft start circuit 11, the pulse conduction width of the drive signal from the control IC 3 to the switching element 2 gradually widens, and the output voltage Vout also rises accordingly. In addition, the dummy load circuit 21
The base current of the transistor 25 causes the capacitor 23
Is charged, the load current iL flowing into the load 24 decreases as the voltage across the capacitor 23 rises. And finally, the emitter of the transistor 25
When the collectors are turned off, the inflow of the load current iL is cut off, and thereafter, the loss of the power supply device due to the dummy load circuit 21 becomes zero. At this point, the control IC 3 also performs feedback control based on the detection signal from the output voltage detection circuit 5 instead of the control signal from the soft start circuit 11, so that the output voltage Vout is stable as shown at point B in FIG. Switch to a normalized state.

On the other hand, the power supply to the power supply is cut off,
When the output voltage Vout goes down, the capacitor 23
The electric charge stored in is quickly sent to the output terminal + Vo side via the diode 22. As a result, the capacitor 23 of the dummy load circuit 21 is discharged to wait for the next power-on.

As described above, in the dummy load circuit 21 of the above embodiment, after the power is turned on and before the output voltage Vo becomes stable, the load current i is passed through the resistor 24 and the transistor 25.
Since L is made to flow, the load current iL flows in any load state, and ideal rising characteristics of the output voltage Vout can be obtained. Moreover, the output voltage V
When out is stabilized, the transistor 25 is turned off by charging the capacitor 23, and the load current iL does not flow. Therefore, despite the provision of the dummy load circuit 21, the power supply device loses no power in a steady state. . Further, the dummy load circuit 21 includes a first series circuit including a diode 22 and a capacitor 23, a resistor 24 and a transistor 25.
Since it can be obtained only by connecting the second series circuit consisting of 2 to the output terminals + Vo and -Vo, various problems in the conventional example can be eliminated even though the configuration is extremely simple.

The present invention is not limited to the above embodiment, but various modifications can be made within the scope of the present invention. For example, although the forward type converter has been described as an example in the above embodiment, the present invention can be applied to other types of converters such as a flyback type converter. The switching element is a MOS type F
A transistor may be used instead of ET.

[0016]

The dummy load circuit of the switching power supply according to the present invention switches the switching element by the drive signal from the control circuit to intermittently apply the DC input voltage to the primary winding of the transformer. In a switching power supply device equipped with a soft start circuit for rectifying and smoothing the voltage induced from the secondary winding to obtain a predetermined DC output voltage and controlling the duty of the drive signal from the control circuit when the power is turned on, a diode And a series circuit composed of a capacitor and a series circuit composed of a resistor and a transistor are respectively connected between output terminals,
It is configured by connecting the connection point of the diode and the capacitor to the base of the transistor, and in a simple configuration, in any load state,
It is possible to provide a dummy load circuit of a switching power supply device which can obtain an ideal rising characteristic of an output voltage and has less loss.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a switching power supply device showing an embodiment of the present invention.

FIG. 2 is a waveform diagram of an output voltage when the power is turned on with no load.

FIG. 3 is a waveform diagram of a conventional output voltage when the power is turned on with no load and with a load connected.

[Explanation of symbols]

 1 transformer 2 switching element 3 control IC (control circuit) 11 soft start circuit 21 dummy load circuit 22 diode 23 capacitor 24 resistor 25 transistor + Vo, -Vo output terminal

Claims (1)

[Claims] 1. A switching element is switched by a drive signal from a control circuit, a DC input voltage is intermittently applied to a primary winding of a transformer, and a voltage induced from a secondary winding of the transformer is rectified and smoothed. In a switching power supply device having a soft start circuit that controls the duty of the drive signal from the control circuit when the power is turned on, a predetermined direct current output voltage is obtained, and a series circuit including a diode and a capacitor, and a resistor and a transistor are used. And a series circuit connected between the output terminals, and a connection point between the diode and the capacitor is connected to the base of the transistor.