JP4937958B2 - Switching power supply - Google Patents

Description

  The present invention relates to a switching power supply that supplies a stabilized DC voltage to a load.

  The current supplied from the DC power source to the transformer primary winding is turned on and off by the switching transistor, and the DC voltage obtained by rectifying and smoothing the voltage induced in the transformer secondary winding is supplied to the load, and the DC voltage is set. Switching power supply devices that stabilize the DC voltage supplied to the load by controlling the ON period of the switching transistor according to the error compared to the reference voltage have already been proposed and put into practical use. Yes. For example, FIG. 3 shows a conventional switching power supply apparatus, where DC is a direct current power supply and is shown as a battery, but usually a configuration in which a commercial AC voltage is rectified and supplied with a smoothed DC voltage. Is common. T is a transformer having a primary winding n1, a first secondary winding n2-1, a second secondary winding n2-2, and a control auxiliary winding n3, SW is a power switch, and Q1 is a field effect. Switching transistor such as a transistor (FET), Q2 is a control transistor, PC is a photocoupler for feedback, LD1 and LD2 are loads of a semiconductor integrated circuit (IC), FM is a fan motor for driving a cooling fan, D1 , D2 are diodes, ZD1 is a Zener diode, R1 to R4 are resistors, and C1 to C3 are capacitors. Note that the resistor R1 has a high resistance value for activation and can be disconnected after activation. Capacitors C1 and C2 are capacitors constituting a smoothing circuit for the voltage rectified by the diodes D1 and D2, and the reactor, resistance, and the like are not shown. The capacitor C3 and the resistor R3 correspond to a pulse generation circuit for switching the switching transistor Q1.

  When the power switch SW is turned on, a voltage is applied to the gate of the switching transistor Q1 via the starting resistor R1, the switching transistor Q1 is turned on, and a current flows from the DC power source DC to the primary winding n1 of the transformer T. A voltage is induced in the secondary windings n2-1 and n2-2 and the control auxiliary winding n3. The induced voltage of the control auxiliary winding n3 is applied as a voltage V4 to the gate of the switching transistor Q1 through a series circuit of a resistor R3 and a capacitor C3. This voltage V4 is a pulse voltage according to the time constant by the resistor R3 and the capacitor C3, and the switching operation of the switching transistor Q1 on and off is repeated according to the pulse width and period of this pulse voltage. When the control transistor Q2 is turned on, no pulse voltage is applied to the gate of the switching transistor Q1, and the switching transistor Q1 is turned off.

  The induced voltage of the second secondary winding n2-2 is rectified by the diode D1, smoothed by the capacitor C1, and the DC voltage V2 is supplied to the load LD2. The induced voltage of the first secondary winding n2-1 is rectified by the diode D2, smoothed by the capacitor C2, and the DC voltage V1 is supplied to the load LD1 and the fan motor FM. When the DC voltage V1 exceeds the Zener voltage of the Zener diode ZD1, that is, when the DC voltage V1 exceeds the set value, the photocoupler PC is turned on, and the on-state photocoupler PC and the resistance are connected to the base of the control transistor Q2. The induced voltage of the control auxiliary winding n3 is applied via R2, the control transistor Q2 is turned on, and the switching transistor Q1 is turned off. Thereby, the voltage V1 applied to the load LD1 and the fan motor FM can be stabilized. In this case, the DC voltage V1 supplied to the load LD1 and the DC voltage V2 supplied to the load LD2 can be the same or different.

Also, the induced voltage on the secondary side of the transformer is rectified and smoothed, supplied to the load, the voltage supplied to the load is detected, fed back to the primary side of the transformer via a photocoupler, and the voltage supplied to the load is stabilized A switching power supply device that performs overvoltage protection is also known (see, for example, Patent Document 1).
JP-A-5-146148

  The conventional switching power supply shown in FIG. 3 has a pulse width control circuit including a signal generator for timing control such as a sawtooth wave and a triangular wave, and an output voltage for controlling on / off of the switching transistor Q1. And a reference voltage, and a comparatively simple configuration in which an error amplifier for feedback control that inputs a pulse width control signal of the pulse width control circuit can be omitted, and the first secondary of the transformer T When the DC voltage V1 is supplied only to the load LD1 via the diode D2 to the winding n2-1, that is, when the fan motor FM is not connected, when the power switch SW is turned on and activated, the desired DC voltage Can be supplied. However, depending on various situations, it is necessary to connect a fan motor FM as shown in the figure for driving an air cooling fan or a load through which an inrush current flows when starting a DC / DC converter or the like. In such a case, when the power switch SW is turned on and activated, a large inrush current flows through a load such as the fan motor FM that rectifies and applies the induced voltage of the first secondary winding n2-1 of the transformer T. . Even in this case, the DC voltage V1 obtained by rectifying the induced voltage of the secondary winding n2-1 is fed back to perform switching control of the switching transistor Q1, so that the DC voltage V1 does not rise above the set value. Be controlled. Thus, although the induced voltage of the first secondary winding n2-1 can be stabilized, the induced voltage of the second secondary winding n2-2 does not directly contribute to feedback control. Even if the DC voltage V1 is controlled so as to become the set value, the other DC voltage V2 exceeds the set value, and an overvoltage is applied to the load LD2, and the load LD2 is damaged. There is a possibility problem. In particular, when the load LD2 is a semiconductor integrated circuit or the like, a failure occurs due to overvoltage.

  For example, in FIG. 4, when the DC voltages V1 and V2 applied to the loads LD1 and LD2, respectively, the pulse duty shown as the voltage V4 applied to the gate of the switching transistor Q1, and the current I1 flowing through the fan motor FM are started. The horizontal axis represents time t, and the vertical axis represents voltage V and current I. As time elapses after the power switch SW is turned on, the DC voltages V1 and V2 applied to the loads LD1 and LD2 start to rise, and the current I1 flowing through the fan motor FM also rises, so that the fan motor FM has a predetermined rotational speed. The current I1 becomes a constant value. When the current I1 rises, in order to maintain the DC voltage V1 constant, the duty of the pulse as the voltage V4 applied to the gate of the switching transistor Q1 via the resistor R3 and the capacitor C3 is such that the control transistor Q2 is turned on. Therefore, the maximum value MAX is obtained. Therefore, even if one DC voltage V1 applied to the load LD1 and the fan motor FM is stabilized, there is a problem that the other DC voltage V2 applied to the load LD2 exceeds the allowable maximum value MAX with respect to the load LD2. .

  An object of the present invention is to solve the above-described problems of the conventional example. A load that turns on the power switch and applies a DC voltage obtained by rectifying the induced voltage of the first secondary winding n2-1. However, in the case where the inrush current flows as in the case of the fan motor FM or the DC / DC converter described above, the DC voltage on the second secondary winding n2-2 side that is not feedback controlled does not increase beyond the set value. Thus, it protects at low cost.

  The switching power supply device of the present invention includes a primary winding, a control auxiliary winding, a transformer having a plurality of secondary windings, a power switch and a switching transistor for applying a DC voltage to the primary winding of the transformer, Resistor and capacitor connected in series to apply induced voltage of auxiliary winding for controlling transformer as pulse voltage to gate of switching transistor A rectifying / smoothing circuit that supplies a load through which an inrush current flows when an induced voltage of a first secondary winding among a plurality of secondary windings is applied as a DC voltage, and a DC voltage applied to the load is A first Zener diode connected to turn on the control transistor when the set value is exceeded, and a plurality of secondary windings And a rectifying / smoothing circuit for supplying the induced voltage of the second secondary winding as a DC voltage to the load, wherein the induced voltage of the second secondary winding is applied to the load as the DC voltage. When the output DC voltage of the supplied rectifying / smoothing circuit exceeds a set value, the output DC voltage is provided with a second Zener diode connected so as to be applied to the base of the control transistor, As a load through which an inrush current flows when a DC voltage is applied, there is a fan motor configured to drive an air cooling fan by a DC / DC converter or a DC motor that looks like a capacitive load from the DC power supply side.

  In contrast to the first secondary winding side that stabilizes the output DC voltage by feedback control, the second secondary winding side is not configured to perform conventional feedback control, and thus the DC voltage that exceeds the set DC voltage. However, the load on the second secondary winding side can be obtained by a simple configuration in which the rectified and smoothed DC voltage on the second secondary winding side is supplied to the base of the control transistor via the second Zener diode. The DC voltage applied to can be suppressed to a set value. When overvoltage protection is performed by connecting a second Zener diode in parallel with the load on the second secondary winding side, the power loss increases and the efficiency as a switching power supply device is slightly reduced. With the configuration, the problem caused by the inrush current can be solved with a simple and inexpensive configuration without reducing the efficiency.

The switching power supply apparatus of the present invention, referring to FIG. 1, a transformer having a primary winding (n1) and the control auxiliary winding (n3) and a plurality of secondary windings (n2-1, n2-2) ( and T), the primary winding (power supply switch for applying a DC voltage to n1) (SW) and a switching transistor for applying by switching the DC voltage (Q1), said control auxiliary winding ( resistor in series connection for applying a pulse voltage to the gate of the switching transistor the induced voltage of n3) (Q1) and a capacitor (R3, C3), the pulse voltage applied to the gate of the switching transistor (Q1) collision control transistor for bypassing a (Q2), upon application of a first DC voltage induced voltage obtained by rectifying the secondary winding (n2-1) of the plurality of secondary windings Current flows load the first rectifier smoothing circuit for supplying the DC voltage to the (RD) (D2, C2), when the DC voltage applied the load (RD) exceeds the set value, the control transistor ( a first Zener diode for the Q2) oN (ZD1), supplying said plurality of second secondary winding in the secondary winding an induced voltage of the (n2-2) in the load as a DC voltage The second rectifying / smoothing circuit (C1, D1) and the second rectifying / smoothing circuit for supplying the induced voltage of the second secondary winding (n2-2) to the load (LD2) as a DC voltage. When the output DC voltage of (D1, C1) exceeds a set value, the output DC voltage is applied to the base of the control transistor (Q2) to turn on the control transistor (Q2), and the switching transistor (Q1 ) Is turned off And a configuration in which a zener diode (ZD2).

  FIG. 1 is an explanatory diagram of an embodiment of the present invention, in which DC is a DC power source, T is a transformer, n1 is a primary winding, n2-1 and n2-2 are first and second secondary windings, n3 Is an auxiliary winding for control, SW is a power switch, Q1 is a switching transistor such as a field effect transistor (FET), Q2 is a control transistor, PC is a photocoupler for feedback, LD1 and LD2 are semiconductor integrated circuits (IC), etc. Load, RD is a load through which an inrush current flows when a DC voltage is applied, for example, a load such as a DC / DC converter or an air cooling fan motor, D1, D2 are diodes, ZD1, ZD2 are first and second Zener diodes, R1 ˜R4 is a resistor, and C1 to C3 are capacitors. Note that the resistor R1 has a high resistance value for activation and can be disconnected after activation. Similarly to the configuration of the conventional example, the capacitors C1 and C2 are capacitors that constitute a smoothing circuit for the voltage rectified by the diodes D1 and D2, and the reactor, resistance, and the like are not shown. The capacitor C3 and the resistor R3 correspond to a pulse generation circuit for switching the switching transistor Q1. Further, the DC power source DC can be configured such that a commercial AC voltage is rectified and smoothed as in the conventional example.

  The steady state operation of stabilizing the DC voltages V1 and V2 applied to the loads LD1 and LD2 and the load RD by turning on and off the switching transistor Q1 is the same as the conventional example, and the contents overlapping with the above description are as follows. Omitted. In this embodiment, a DC voltage V2 to be applied to a load LD2 can be input to the base of the control transistor Q2 via a second Zener diode ZD2. The second Zener diode ZD2 is turned on when the DC voltage V2 applied to the load LD2 exceeds the Zener voltage selected corresponding to the set value, and applies the DC voltage V2 to the base of the control transistor Q2. Then, the control transistor Q2 is turned on and the switching transistor Q1 is forcibly turned off. Thereby, an increase in the DC voltage V2 applied to the load LD2 is suppressed.

  FIG. 2 is a diagram for explaining the operation of the first embodiment of the present invention. When the power switch SW is turned on, the DC voltages V1 and V2 applied to the loads LD1 and LD2, respectively, and the voltage V4 applied to the gate of the switching transistor Q1. The outline characteristics of the pulse duty shown and the current I1 flowing through the load RD such as a DC / DC converter or a fan motor are shown, the horizontal axis shows time t, and the vertical axis shows the voltage V, current I, and pulse duty. As time elapses after the power switch SW is turned on, the DC voltages V1 and V2 applied to the loads LD1 and LD2 start to rise, and the current I1 flowing through the additional RD also rises. For example, a fan motor as the load RD is predetermined. When the number of rotations reaches or when the DC / DC converter enters a stable operation state, the current I1 becomes a constant value.

  In this case, if the DC voltage V2 applied to the load LD2 exceeds the set value in the starting process of the load RD through which an inrush current flows such as a fan motor, a DC / DC converter, and a capacitive load, the zener diode ZD2 is used. The control transistor Q2 is turned on and the switching transistor Q1 is turned off. Therefore, the pulse duty shown as V4 does not become the maximum value MAX. Thereby, the DC voltage V2 applied to the load LD2 can be maintained at a predetermined value equal to or less than the maximum value MAX. In this case, the DC voltage V1 applied to the load LD1 and the load such as the fan motor or the DC / DC converter is somewhat longer until it rises to the set value. While suppressing the flowing inrush current I1, it is possible to suppress an abnormal increase in the DC voltage V2 applied to the load LD2 on the second secondary winding n2-2 side.

It is explanatory drawing of Example 1 of this invention. It is operation | movement explanatory drawing of Example 1 of this invention. It is explanatory drawing of a prior art example. It is operation | movement explanatory drawing of a prior art example.

Explanation of symbols

DC DC power supply Q1 Switching transistor Q2 Control transistor T Transformer n1 Primary winding n2-1 First secondary winding n2-2 Second secondary winding n3 Control auxiliary winding SW Power switch PC Photocoupler LD1, LD2 Load RD Load through which inrush current flows D1, D2 Diodes ZD1, ZD2 First and second Zener diodes C1-C3 Capacitors R1-R4 Resistance

Claims (3)

A switching transistor for applying by switching a transformer having a primary winding and the control auxiliary winding and a plurality of secondary windings, the power switch and the DC voltage for applying a DC voltage to the primary winding A resistor and a capacitor connected in series for applying the induced voltage of the auxiliary winding for control as a pulse voltage to the gate of the switching transistor, and a control for bypassing the pulse voltage applied to the gate of the switching transistor A first rectifying / smoothing that supplies the DC voltage to a load through which an inrush current flows when a DC voltage obtained by rectifying an induced voltage of a transistor and a first secondary winding of the plurality of secondary windings is applied. a circuit, a first zener diode for turning on the control transistor when a DC voltage applied to the load exceeds the set value, DC and the second rectifying smoothing circuit for supplying the induced voltage of the second secondary winding an induced voltage of the second secondary winding in a serial plurality of secondary windings to the load as a DC voltage When the output DC voltage of the second rectifying / smoothing circuit supplied to the load as a voltage exceeds a set value, the output DC voltage is applied to the base of the control transistor to turn on the control transistor, and the switching A switching power supply comprising a second Zener diode that turns off a transistor . The induced voltage of the first secondary winding of the transformer is rectified and smoothed by the first rectifying / smoothing circuit, and the load through which an inrush current flows when the DC voltage is applied is defined as DC 2. The switching power supply device according to claim 1, wherein the switching power supply device is a DC / DC converter. And said first DC voltage induced voltage were smoothed rectified by the first rectifier smoothing circuit on the secondary winding of the transformer, the load inrush current flows when applying the dc voltage, 2. The switching power supply device according to claim 1, wherein the fan motor is used for air cooling.

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