JPH0446563A - Switching power supply device - Google Patents

Abstract

PURPOSE:To inhibit the variation of an overcurrent protection starting point due to the fluctuation of input voltage by using a variable capacitance diode in place of a capacitor constituting a time constant circuit. CONSTITUTION:A time constant circuit 10a is composed by using a resistor 12 and a variable capacitance diode 24 connected in series mutually. When input voltage Vi is increased, the voltage Vb of the bias winding 2c of a transformer 2 rises, and voltage Vc applied to the variable capacitance diode 24 also rises and the electrostatic capacity C2 of the diode 24 is reduced, thus lowering the time constant of the time constant circuit 10a. Consequently, a transistor 20 is turned ON earlier, and a MOSFET 6 is turned OFF earlier and a protective mode is acquired. Accordingly, the variation of an overcurrent protection starting point due to the increase of oscillation frequency can be inhibited.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an automatic type flyback converter type (that is, RCC type) switching power supply device.
More specifically, the present invention relates to improvements in overcurrent protection means.

[Background technology]

In a general RCC type switching power supply device, the maximum collector current of the switching transistor is the amplification factor h
By relying on FE, it has an overcurrent protection means, but if the input voltage fluctuates, the overcurrent protection starting point will shift (for example, if the input voltage increases, the overcurrent protection starting point will shift to the larger side). There is a problem with this (misalignment).

Therefore, a switching power supply device that can solve this problem with a simple circuit has been devised by the same applicant.

FIG. 3 shows an example of this, in which a MOS F-E as a switching element is connected to the primary winding 2a of the transformer 2.
T 6 are connected in series and bias winding 2 of transformer 2
The output of c is connected to MO via capacitor 16 and resistor 1.
I am trying to return to the gate of 3F'ET.

Further, a time constant circuit 10 consisting of a resistor 12 and a capacitor 14 connected in series is connected across the bias winding 2C of the transformer 2, and a transistor 20 as a control element is connected in parallel between the gate and source of the MO3FET 6. The voltage of the capacitor 14 is applied to the base of the transistor 20.

To explain the operation, when the input voltage Vi is applied, the gate voltage is applied to the MO3FET 6 through the starting resistor 8, and the MO3FET 6 becomes conductive. As a result, a voltage is applied to the primary winding 2a of the transformer 2, and at the same time, a voltage vb is generated in the bias winding 2C. This is applied to the gate of MO3FET6 through capacitor 16 and resistor 18, and MO3FET6 is quickly turned on. At this time, the voltage across the secondary winding 2b of the transformer 2 is applied to the diode 4 in the opposite direction, so no current flows through the secondary winding 2b, and energy is stored in the transformer 2.

Along with this, a capacitor 14 constituting the time constant circuit 10
A charging current flows through the resistor 12, and the transistor 2
00 base potential gradually rises.

When the voltage of capacitor 14 reaches a predetermined value, transistor 2
0 begins to conduct, the gate voltage of MO3FET6 decreases and MO3FET6 is no longer able to maintain the on state, the voltage of the negative winding 2a decreases, and the bias winding 2c voltage vb
also decreases. Since this is positive feedback, MOS F E
T6 goes off very quickly. By turning off MO3FET6, the energy stored in the transformer 2 is transferred to the secondary winding 2.
b is supplied to the output side through the diode 4.

Thereafter, when the voltage vb of the bias winding 2C becomes 0, the MO3FET 6 is turned on again by the voltage from the starting resistor 8, and the above operation is repeated.

In such an RCC system, as is well known, there is a basic relationship in which the output on the secondary side is proportional to the ON period of MO3FET6. In this switching power supply device, overcurrent protection is performed by using this inversely. That is, the ON period of the MO3FET 6 (this is also the time until the transistor 20 in the OFF state is turned on) is determined by the speed at which the voltage of the capacitor 14 rises, that is, the time constant T (-R・C1
, where R1 is the resistance value of resistor 12, C1 is capacitor 1
4), this time constant T
If the upper limit of the ON period of the MO3FET 6 is determined by , a current exceeding the set value will not be output to the secondary side (that is, overcurrent protection is provided). For example, if the time constant T is made smaller, the transistor 20 turns on sooner and the MO3FET 6 turns off sooner. Therefore, the protection mode is entered with a smaller load current.

[Purpose of the invention]

In the switching power supply device as described above, the overcurrent protection starting point is basically determined by the time constant T of the time constant circuit 10, so it is possible to suppress fluctuations in the overcurrent protection starting point due to fluctuations in the input voltage Vi. .

However, if we look at it more precisely, since it is an automatic type, when the input voltage Vi fluctuates, for example increases, the oscillation frequency increases and works in the direction of producing more output (so from this point of view, overcurrent protection still starts). The points fluctuate, and it can be said that there is still room for improvement in these points.

SUMMARY OF THE INVENTION Therefore, it is an object of the present invention to further improve the switching power supply device as described above so as to be able to more reliably suppress fluctuations in the overcurrent protection starting point due to fluctuations in input voltage.

[Means to achieve the purpose]

In order to achieve the above object, the switching power supply device of the present invention is characterized in that a variable capacitance diode is used instead of the capacitor constituting the time constant circuit as described above.

[Function] A variable capacitance diode has a characteristic that its capacitance decreases as the reverse voltage applied to it increases.

Therefore, when a time constant circuit as described above is constructed using such a variable capacitance diode and a resistor, when the input voltage increases, for example, the reverse voltage applied to the variable capacitance diode also increases and its capacitance increases. Therefore, the time constant of the time constant circuit becomes smaller. As a result, the control element turns on sooner and the switching element turns off sooner to enter protection mode, thereby making it possible to more reliably suppress fluctuations in the overcurrent protection starting point due to input voltage fluctuations. become.

〔Example〕

FIG. 1 is a circuit diagram showing a switching power supply device according to an embodiment of the present invention. Components equivalent to those in the example of FIG. 3 are given the same reference numerals, and the differences from the previous example will be mainly explained below.

In this embodiment, a time constant circuit 10a corresponding to the time constant circuit 10 is constructed using a resistor 12 and a variable capacitance diode 24 that are connected in series with each other. That is, a variable capacitance diode 24 is used instead of the capacitor 14 of the prior example.

For example, as shown in FIG. 2, the variable capacitance diode 24 has the opposite effect when the applied voltage Vc increases.
Its capacitance C2 decreases.

Therefore, when the input voltage Vi fluctuates, for example when the input voltage Vi increases, the voltage vb of the bias winding 2c of the transformer 2 increases, and the voltage Vc applied to the variable capacitance diode 24 also increases, causing the electrostatic Since the capacitance C2 becomes smaller, the time constant T (=R, ・C2
) becomes smaller.

As a result, transistor 20 turns on sooner and MOS
FET6 turns off sooner and enters protection mode. This makes it possible to suppress the fluctuation of the overcurrent protection starting point due to the increase in the oscillation frequency described above. Even when the input voltage Vi decreases, fluctuations in the overcurrent protection starting point can be suppressed by the opposite effect to the above. As a result, fluctuations in the overcurrent protection starting point due to fluctuations in the input voltage Vi can be suppressed more reliably, and the reliability of protection is further improved.

Note that the time constant circuit 10a is combined with a feedback control circuit that includes a photocoupler 26 and the like and stabilizes the output voltage on the secondary side of the transformer 2, as in this embodiment, for example. can be considered separately from the overcurrent protection that is the issue here, so
A detailed explanation thereof will be omitted.

Furthermore, it goes without saying that a transistor may be used in place of the MOSFET 6 as a switching element, and an FET may be used in place of the transistor 20 as a control element.

Further, the input voltage Vi is, for example, a commercial type tA (alternating current) rectified by a rectifier diode.

〔Effect of the invention〕

As described above, according to the present invention, the time constant circuit that defines the upper limit of the on-period of the switching element is configured using a resistor and a variable capacitance diode that are connected in series with each other. It becomes possible to more reliably suppress fluctuations in the overcurrent protection starting point due to fluctuations in input voltage, and as a result, the reliability of protection is further improved.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a switching power supply device according to an embodiment of the present invention. FIG. 2 is a diagram showing an example of characteristics of a variable capacitance diode. FIG. 3 is a circuit diagram showing an example of a switching power supply device which is the background of the present invention. 2... Transformer, 6... MOSFET (switching element), 10a... Time constant circuit, 12... Resistor, 20... Transistor (control element), 2406
．． Variable capacitance diode.

Claims (1)

[Claims] (1) A switching element is connected in series to the primary winding of a transformer, and the output of the bias winding of the transformer is fed back to the control electrode of this switching element. In a switching power supply device that includes a time constant circuit that defines the upper limit of a period, and a control element that forcibly turns off a switching element using the output of this time constant circuit, the time constant circuit is connected to a resistor and a resistor that are connected in series with each other. A switching power supply device characterized in that it is configured using a variable capacitance diode.