JPS62163572A - Self-oscillating switching power unit - Google Patents

Abstract

PURPOSE:To keep the output power constant by connecting a Zener diode in series to a resistance of the discharge circuit of a condenser and by varying the change rate of the time of discharge of the condenser against an input voltage. CONSTITUTION:A self-oscillating switching power unit is composed of rectification circuits 1 and 3, a bias circuit 2, a control circuit 4, a switching transformer S, a switching transistor Q1, a photocoupler P1, etc. A condenser C1 is charged and discharged with the voltage induced between a control winding S3 of the transformer S, varying the pulse width by this discharge time constant. On this occasion, a resistance R3, a Zener diode ZD1 inserted in series and a negative resistance element D2 provided side by side with it. Thus, when the input voltage varies, the change rate of the time of discharge of the condenser C1 agrees with this change and the pulse width varies, so that the maximum output power can be kept constant.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a self-oscillation type switching power supply device suitable for use as a power supply for electronic equipment, for example, a relatively inexpensive power supply for electronic equipment such as a personal computer.

Conventional technology Conventionally, this type of self-oscillation type switching power supply device is
The configuration was as shown in FIG. In Figure 6, 1
is a rectifier circuit that rectifies the commercial AC voltage WIN and converts it into a DC voltage. S is a switching transformer with a primary winding S1. It has a base drive winding S2, a control winding 33, and an output winding S4 connected in series.

Ql is a switching transistor whose collector is connected to the positive electrode of the rectifier circuit 1 through the entire primary winding S1, and whose emitter is connected to the negative electrode of the rectifier circuit 1, and which repeatedly turns on and off. R1 is a resistor connected in series to the connection point between the primary winding S1 and the rectifier circuit 1 and the base of the switching transistor Q1 to supply a starting current, and 2 is connected in series to the base of the switching transistor Q1 and one end of the pace drive winding S2. It is connected to a bias circuit that sets the entire base current. Q2 is a reverse bias transistor whose collector is connected to the base of the switching transistor Q1, and whose emitter is connected to one end of the control winding S3.Q3 is a reverse bias transistor whose emitter is connected to the base of the base drive winding S2 and the control winding S3. A control transistor D1 whose collector is connected to the base of the reverse bias transistor Q2 through a resistor R2, and whose base is connected to the same emitter through the capacitor C1 has an anode connected to the control winding S3. A diode connected to the connection point of the emitter of the reverse bias transistor Q2 and connected to the connection point of the cando full capacitor C1 and the base of the control transistor Q3 allows the charging current of the capacitor C1 to flow and cuts the discharging current.

R3 is a resistor connected in parallel with diode D1.

A discharge current of the capacitor C1 is caused to flow.

The primary side and the secondary side are isolated and insulated by the photocoupler P1, and the collector of the negative side transistor is connected to the capacitor C1.
and the connection point of the base of the control transistor Q3,
Emitter control winding S3 and reverse bias transistor Q
The light emitting diode on the secondary side is connected to the control circuit 4. 3 is an output rectifier circuit connected to the output winding S4 and supplies a DC output. DC output.

It is connected to the control circuit 4 to detect and control the output voltage vOUT.

In the conventional circuit configured as described above.

The operation will be explained below.

First, when a commercial voltage of 1xN (hereinafter referred to as input voltage) is applied to the AC input, it becomes a DC voltage in the rectifier circuit 1, and the resistor R1
A current flows through the base of the switching transistor Q1 to turn on the switching transistor Q1, so a voltage is applied to the primary winding S1, and an induced voltage proportional to this voltage is applied to the base drive winding S2. is generated in the same switching transistor Q via bias circuit 2.
The base current IB of 1 is further increased to become an on period.

During the on period, the collector current IC of the switching transistor Q1 increases linearly with a slope determined by the inductance of the primary winding S1, and excites the switching transformer S. On the other hand, an induced voltage proportional to the input voltage is also generated in the control winding S3, and as a discharge current flows through the capacitor C1 and resistor R3, the voltage across the capacitor C1 decreases and the control transistor Q3 This is the voltage that forward biases between the emitter and base of. Due to this voltage, the control transistor Q3 is turned on, and the induced voltage of the control winding S3 is applied to the base of the reverse bias transistor Q2 via the resistor R2, and the reverse bias transistor Q2 is turned on. The base current IB of the switching transistor Q1 is rapidly drawn in, and the base of the switching transistor Q1 is reverse biased to turn it off. During the off period TOFF, the induced voltage generated in the base drive winding S2 reaches the base of the switching transistor Q1! A voltage is generated in the biasing direction, and the excitation energy accumulated in the switching transformer S is released from the output winding S4 to the DC output via the output rectifier circuit 3. When the excitation energy runs out, the ringing voltage generated by the leakage inductance and distributed capacitance of the switching transformer S generates an induced voltage in the base drive winding S2 in the direction of forward biasing the base of the switching transistor Q1 again. The switching transistor Q1 is turned on.

Furthermore, during the off period, the voltage across the control winding S3 is such that an induced voltage is generated in the direction of making the connection point with the base drive winding S2 a negative voltage. The capacitor 01' is charged so as to be reverse biased, and the control transistor Q3 is turned off. The charging voltage of the capacitor C1 is determined by the turns ratio of the output winding S4 and the control winding &, that is, the output voltage.
The voltage is proportional to OUT. Thereafter, the above-described operation is repeated to supply the output voltage. When the output voltage reaches the voltage set by the control circuit 4, the control circuit 4 operates to activate the photocoupler P1, and transfers the electric charge of the capacitor C1 charged during the off period to the resistor Rs and the photocoupler during the on period. Discharge occurs through the parallel circuit of P1. As a result, the discharge time of the capacitor C1 is faster than the discharge time due to only the resistor R3 described above, so the on period is shortened, the duty ratio changes, and the output voltage is kept constant. That is, the output voltage is controlled by the control circuit 4VC.

In FIG. 6, a is the collector-emitter voltage vcx of the switching transistor Q1, and b is the collector current I(H,
c represents the base current I, d represents the voltage across the capacitor C1, vcj, and e represents the base-emitter voltage VBII2k of the reverse bias transistor Q2. As shown in FIG. 6C, it can be seen that the base current of the switching transistor Q1 is sufficiently reverse biased during turn-off from the on period to the off period.

In this way, the output voltage of a power supply device needs to be controlled stably at all times, but even in the event of an abnormality due to an output overcurrent or short circuit, it is necessary to ensure that the power supply device and its load system are not damaged. It is necessary to have drooping characteristics to provide current protection and prevent excessive output current from flowing.

This conventional self-oscillation type switching power supply device also has a protection function against such overcurrent. In other words, when the output is in a constant voltage region (i. By changing the operating state of photocoupler P1, the discharge time constant of capacitor C1 is changed and the entire on period is controlled;
The maximum value of the discharge time constant is limited by the resistor R3 connected in parallel with 1. In other words, even if the output is overcurrent and the photocoupler P1 is turned off, the capacitor C1 is discharged through the resistor R3, so the discharge time constant is equal to the resistor R3, the capacitor C1, and both ends of the capacitor C1 charged during the off period. Since it is fixed at the on-period width determined by the voltage, it is no longer possible to keep the output voltage constant and it enters the overcurrent region, where the output voltage decreases and exhibits drooping characteristics.The following describes the drooping characteristics in the output overcurrent region. state

FIG. 7a shows an equivalent circuit of the control section in the overcurrent region.

b shows the output characteristics. From the equivalent circuit of FIG. 7a, the discharge time of the capacitor C1, that is, the on-period ToNf: is determined.

becomes.

Note that the voltage proportional to the output voltage "OUT" charged in the capacitor C1 during the Voyyu off period, To)l is T.
AC input voltage VX induced in control winding S3 during ol1 period
VRX5 is a voltage proportional to H, and is the forward threshold voltage between the emitter and pace of the control transistor Q3. Furthermore, the figure in parentheses is Ioot Cl-Vt* when the output current ranT is determined from the on-period TON.
X TON y:, K TENow, if the input voltage WIN or 70)l is constant, the output current I. . , ba, VO
It is determined only by the value of FF, that is, the output voltage, and as the same output voltage decreases in the overcurrent region, the output current decreases. . , K increases inversely with VOFF, but TO
N decreases exponentially as the value in parentheses in the relational expression increases in inverse proportion to VOFF, resulting in an output characteristic that shows a so-called foldback characteristic in which the output current rapidly decreases. (It is assumed that the capacitor 01 and the resistor R3 are circuit constants and are constant.) Next, when the input voltage VIN, that is, To)l changes, T
ON is almost constant because both the numerator and denominator in the relational expression change due to VOM, and K in the relational expression for the output current 7012 is also constant since both the numerator and denominator change due to Toll. As a result, the output current r 01
17 is the input voltage vX! It can be seen that the output characteristic varies in proportion to lVc, and the output characteristic becomes a characteristic obtained by completely shifting the above-mentioned figure-7 characteristic in proportion to the input voltage, that is, the output characteristic shown in FIG.

The output overcurrent protection point is the point at which the output changes from the constant voltage region to the overcurrent region, and if the output voltage "011" is approximately constant, it changes depending on the AC input voltage WXN and is set by the resistor R3. Furthermore, it can be seen that the overcurrent drooping characteristic of the output characteristic is mainly influenced by the change in the output voltage vO.

Problems to be Solved by the Invention In such a conventional configuration, the overcurrent protection point of the output characteristics, that is, the constant voltage region, changes greatly depending on the AC input voltage VIN, so it It is necessary to set a sufficiently large overcurrent protection point), and the output power at the time of overcurrent increases, so the switching transistor Q1, switching transformer S, and output rectifier circuit 3 have large capacities, which increases the size and cost. There's a problem. The present invention is intended to solve these problems, and aims to reduce the change At of the overcurrent protection point with respect to the input voltage.

Means for Solving the Problem In order to solve this problem, the present invention provides a switching transformer having a secondary winding, an output winding, a base drive winding, a control winding, and at least a resistor. A circuit in which a parallel circuit of a diode and a capacitor are connected in series is connected between the control windings, and the capacitor is charged and discharged by the voltage induced between the control windings through the resistor and the diode,
The pulse width is varied according to the discharge time of the capacitor, that is, the discharge time constant of a discharge circuit composed of at least the resistor and the capacitor, and the discharge circuit is composed of at least a Zener diode and a negative resistance element in series with the resistor. The rate of change in the discharging time of the capacitor with respect to the input voltage can be varied by connecting a circuit that has the following characteristics.

Effect: With this configuration, it is possible to vary the pulse width and keep the maximum output power constant by matching the rate of change in the discharge time of the capacitor with respect to changes in the input voltage.

Embodiment FIG. 1 is a circuit configuration diagram of a self-excited oscillation type switching power supply according to an embodiment of the present invention. In FIG. 1, the same parts as in FIG. The explanation will be omitted. In Figure 1, 1 and 3 are rectifier circuits, 2 is a bias circuit, 4 is a control circuit, S is a switching transformer, and Q
l is a switching transistor, C2 is a reverse bias transistor, C3 is a control transistor, R1 and R2
゜R3 is a resistor, C1 is a capacitor, DI is a diode, Pl is a photocoupler, and the above structure is the same as that shown in FIG. 6.

ZDl has a cand connected to the resistor R3, an anode connected to the connection point of the control winding S3 and the diode D1, and a resistor R3.
Zener diode inserted in series with.

D2 is a negative resistance element (for example, what is called a lambda-shaped negative resistance element) connected in parallel with the Zener diode, and FIG. 2 shows its voltage-current characteristics.

The operation of the circuit configured as described above will be explained below. The explanation of the operation in the constant voltage region of the output characteristics will be omitted since it is the same as the conventional example, and only the operation in the overcurrent region will be explained using FIG.

FIG. 3a shows an equivalent circuit of the control section in the overcurrent region, and FIG. 3b shows the output characteristics. Here, a case will be described in which the input voltage VXX is sufficiently high, the Zener diode is conductive, and the negative resistance element D2 has a high voltage across it and is in an off state.

The on-period Tou'fr can be calculated from the equivalent circuit shown in Figure 3B. Note that VH is a Zener voltage caused by a Zener diode ZDI. Furthermore, the output current IO2 is
IJr ” vIN X TON
= VON Vz+ +VOFF (!: By appropriately selecting the Zener voltage Vz+, it is possible to set the rate of change in VH5 with respect to change in input voltage, that is, the rate of change in discharge current VR3/R5 of capacitor C1. On-period TOH due to input voltage vXN
By making the change in kVXN as equal as possible to the change in
It is possible to reduce the change in the overcurrent protection point of the output current with respect to the input voltage.

This is the V in parentheses in the relational expression for the on-period TOH.
The purpose is to change the rate of change in the numerator and the rate of change in the denominator with respect to the change in ON using the energizer diode voltage VZi. In this case, the rate of change in the numerator changes significantly compared to the rate of change in the denominator, so the on-period TO
N not only increases the amount of change, but also increases the output current. U
It can be seen that the output current is approximately constant with respect to the input voltage because it changes so as to cancel out the change in Vr)l in the relational expression of T. (However, K in the relational expression is via
Make a relatively small change in proportion to . ) Figure 3 shows the output characteristics when corrected by the Zener diode ZD1, and it can be seen that the change in the overcurrent protection point of the output characteristics with respect to the AC input voltage is small. That is, the output power is approximately constant with respect to the input voltage.

As explained above, when the input voltage WIN is sufficiently high, all of the objectives of the present invention can be achieved by the function of the Zener diode VZI, but when the input voltage WIN decreases and current no longer flows through the Zener diode VZj, its function is impaired. At the same time, transistor Q3 in FIG. 1 is no longer driven and no longer performs the conventional basic operation. In the present invention, the negative resistance element D2 is connected in parallel with the Zener diode ZD1, so that when the input voltage WIN decreases, the negative resistance element D2 becomes conductive and the above problem can be solved. Resistor R3, Zener diode ZDi
FIG. 4 shows the discharge current characteristics of the capacitor C1 due to the negative resistance element D2.

Effects of the Invention As described above, according to the present invention, by connecting a Zener diode in series with the resistor of the discharge circuit of the capacitor, the rate of change of the discharge time of the capacitor with respect to the input voltage can be varied. It is possible to keep the output power constant by matching changes in pulse width, and by connecting a negative resistance element in parallel with the Zener diode, operation at low input voltages can be ensured, making it possible to use switching transistors, etc. The power device does not require an unnecessarily large capacity, and its practical effects are great. It is clear that the same effect as the present invention can be obtained by combining the Zener diode, negative resistance element, and other elements and connecting a circuit having the same characteristics.

[Brief explanation of drawings]

Fig. 1 is a circuit configuration diagram of an embodiment of the self-oscillation type switching power supply device of the present invention, Fig. 2 is a typical characteristic diagram of the negative resistance element adopted in the embodiment of the present invention, and Fig. 3a , b is an equivalent circuit diagram and output characteristic diagram of the control section of the present invention, FIG. 4 is a discharge current characteristic diagram of the control section, FIG. 6 is a conventional circuit configuration diagram, and FIG. 6 is each operating waveform according to the conventional circuit configuration. Figure 7a,
b is an equivalent circuit diagram and an output characteristic diagram of a conventional control section. 1... Rectifier circuit, S... Switching transformer, Sl...-Next winding, 32... Base drive winding, S3... Control winding, 34.・・・・・・
・Output winding, Ql... Switching transistor, 2... Bias circuit, Q2... Reverse bias transistor, Q3... Control transistor, Dl... ...Diode, D2...
・Negative resistance element, Pl...Photocoupler, 3
... Song output rectifier circuit, 4... Control circuit, Zt
N... Zener diode. Name of agent: Patent attorney Toshio Nakao and 1 other person/-
- Ichitateha Kaito No. 2 Figure 3 Our (A) Figure 4 Waji (V,) Figure 5 Figure 6 Figure 7

Claims (1)

[Claims] A switching transformer including a primary winding, an output winding, a base drive winding, and a control winding, and a circuit in which at least a parallel circuit of a resistor and a diode and a capacitor are connected in series are connected between the control windings, The capacitor is charged and discharged by the resistor and the diode due to the voltage induced between the windings, and the pulse width is varied depending on the discharge time of the capacitor, that is, the discharge time constant of a discharge circuit composed of at least the resistor and the capacitor. A self-excited oscillation type switching power supply device, characterized in that a circuit including at least a Zener diode and a negative resistance element is connected in series with the resistor of the discharge circuit.