JPH072009B2 - Switching regulator - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a self-excited on / off type switching regulator having an overcurrent protection function outside a control region.

[Prior Art and Problems Thereof] Switching regulators are roughly classified into a self-excited type and a separately excited type. The separately excited type has a large degree of freedom in design and can meet all specifications, but is expensive. On the other hand, the self-excited type has a limited degree of freedom in design but is suitable for reducing the price. FIG. 5 shows a Japanese Patent Application No. 60-2 filed by the same inventor as the present invention.
It is a circuit diagram of an on-on type switching regulator using the technical content filed as No. 44997.

The blocking oscillator circuit is the collector winding L1, the secondary winding L2,
It comprises a transformer T1 having a base winding L3, and an oscillating transistor Q1 connected to the collector winding L1 and connected between the base and the emitter and the base winding L3 so as to form a closed circuit. The emitter of the transistor Q1 is connected to one end of the base winding L3, but the base is connected to the other end of the base winding L3 via the series circuit of the diode D1 and the resistor R2 and the series circuit of the diode D2 and the resistor R3 which are connected in parallel. Connecting. These two series circuits constitute one voltage drop circuit R, which blocks both forward and reverse currents in a minute voltage range and prevents the secondary winding.
The transistor Q1 remains in the "off" state until the choke coil L connected to L2 has completely discharged the excitation energy. D3 is a rectifying diode, D4 is a flywheel diode, and C1 is a smoothing capacitor. Together with the choke coil L, the secondary side of the transformer T2 constitutes a rectifying and smoothing circuit.
E1 is a DC source, and 1A and 1B are output terminals from which a DC output can be obtained.

Such a configuration can prevent an excessive surge current due to magnetic saturation of the choke coil L from flowing between the collector and the emitter of the transistor Q1 when the transistor Q1 is “on”, and can prevent the damage. Energized and on
The practicality of the on-type switching regulator can be improved.

The emitter of the transistor Q1 is grounded via the resistor R1 that detects the collector current. The DC source E2, the resistor R6, and the variable resistor VR form a base bias circuit of the transistor Q2 and the transistor Q3 that controls the "on" time of the transistor Q1 while "turning it off". The resistor R1 detects the collector current as a voltage and compares it with the base-emitter voltage V _{BE2 of the} transistor Q2 that serves as a reference voltage.
When the voltage corresponding to the detected collector current exceeds the sum of the voltage V _BE2 and its base voltage, the transistors Q2 and Q3 are sequentially turned on and the transistor Q1 is turned off. The voltage generated by the resistor R1 is set to 0 by the variable resistor VR.
_Is configured to control the "on" time of transistor Q1 by varying from to a voltage V _BE2 . In the overload condition beyond the control range, the variable resistor VR becomes maximum, the base voltage of the transistor Q2 becomes equal to the ground potential, the voltage detected by the resistor R1 becomes the voltage V _BE2 , and the transistor Q1 turns off. "is doing. In that case, the "on" time is constant and the "off" time is defined by the output voltage.

The characteristics of the transistor Q1 such as h _fe constitute a blocking oscillator circuit whose operation is not affected.

However, such a self-excited ON / ON type switching regulator has a characteristic as shown in the characteristic diagram of the relationship between the output voltage V ₀ and the output current I _{0 in} FIG. 6 even if the load exceeds the control range and the load becomes heavy. , The output current I ₀ still increases.

This is because the waveform of the current of the choke coil L is a continuous triangular wave, and the average value of the output current I ₀ is represented by the equation (1) as 1/2 of the maximum value of the triangular wave.

I ₀ = {(V ₂ −V ₀ ) T _ON / L} / 2 (1) In the formula (1), V ₂ is the voltage of the secondary winding L 2, L is the inductance of the choke coil L, and T _ON is the transistor. This is the "on" time of Q1.

Therefore, at the time of overload, the amount of heat generated by the oscillating transistor Q1 increases, and it is still necessary to allow the current capacity to have a considerable margin to ensure reliability.

〔Purpose〕

An object of the present invention is to provide a self-excited, on-on type switching regulator in which a current transformer is used to detect the collector current of an oscillating transistor to “off” the transistor so that the load is heavy outside the control range. Even so, it is to prevent the output current from increasing.

[Technical means for solving problems]

The present invention configures a blocking oscillator circuit using a transformer having a collector winding, a base winding, a secondary winding, and a transistor, and a rectifying diode, a flywheel diode, a choke coil, and a smoothing capacitor are rectified in the secondary winding. In an on-on type switching regulator to which a smoothing circuit is connected, a closed circuit formed between the base winding and the base-emitter of a transistor is provided with a circuit until the excitation energy of the choke coil is released. To connect a voltage drop circuit for continuing the "off" state of the transistor, convert the collector current of the transistor into a voltage, and turn the transistor "off" when the voltage on the positive side exceeds a reference voltage. It has a current transformer of.

〔Example〕

A circuit diagram of FIG. 1 showing an embodiment of a switching regulator of the present invention, a state of a voltage V _T2 corresponding to a collector current I _c of an oscillating transistor Q1 and a collector current I _c detected by a current transformer T2 in FIG. 2 is a characteristic diagram showing the collector current I _c of the oscillating transistor Q1,
This will be described with reference to FIG. 3, which is a characteristic diagram showing the states of the collector voltage V _c , the magnetic flux Φ _{T1 of the} transformer T1, and the magnetic flux Φ _L of the choke coil L. In FIG. 1, the same parts as those in FIG. 5 are designated by the same reference numerals.

The blocking oscillator circuit is the collector winding L1, the secondary winding L2,
It is mainly composed of a transformer T1 having a base winding L3 and an oscillating transistor Q1 which is connected to the collector winding L1 and is connected between the base and the emitter and the base winding L3 so as to form a closed circuit. The emitter of transistor Q1 is the base winding
The base is connected to one end of L3, and the base is connected to the other end of the base winding L3 through a series circuit of a diode D1 and a resistor R2 and a series circuit of a diode D2 and a resistor R3 which are connected in parallel. The diode D1, the resistor R2, the diode D2, and the resistor R3 constitute one voltage drop circuit R, and the forward direction in the minute voltage range,
It blocks any current in the opposite direction.

The base of the transistor Q1 is connected to the anode of the DC source E1 via the resistor R4 and the primary winding L11 of the current transformer T2.
The primary winding L11 is connected to one end of the collector winding L1, one end of the secondary winding L22 is connected to the base of the transistor Q5 via the emitter and collector of the transistor Q4, and the other end is connected to one end of the base winding L3. Connecting. A resistor R is placed between both ends of the secondary winding L22.
5 is connected. The collector of the transistor Q5 is connected to the base of the transistor Q1 and the emitter is connected to the cathode of the bias DC source E2. The anode of the DC source E2 is connected to one end of the base winding L3.

The base of the transistor Q4 is connected to the connection point of the series circuit of the resistor R6 and the variable resistor VR between the anode and cathode of the DC source E2. The cathode of the DC source E1 and the anode of the DC source E2 are grounded. A rectifying / smoothing circuit similar to that shown in FIG. 5 is connected to the secondary winding L2 of the transformer T1.

The voltage V _RNB across the base winding L3 of the voltage drop circuit R in the base direction of the transistor Q1 is set by the following equation (2).

Forward voltage V ₃ for rectifying diode D3 to conduct, forward voltage for flywheel diode D4 to conduct
V _4, when the transistor Q1 is the number of turns N ₂ turns N _3, 2 winding L2 'on' to the base-emitter voltage V _BE1 for the base winding _{L3, V RNB> {(V} 3 -V ₄ ) N ₃ / N ₂ } -V _BE1 (2) The (V ₃ −V ₄ ) N ₃ / N ₂ of the first term on the right side of the equation (2) is the current flowing through the choke coil L and the flywheel diode D 4. Is an induced voltage generated in the base winding L3 when the current flows.

Further, the voltage V _RBN across the base of the transistor Q1 of the voltage drop circuit R in the direction of the base winding L3 is set by the equations (3) and (5).

Equation (4) is obtained from equation (3).

V _BE1 <V _BE1 − {(V ₃ −V ₄ ) N ₃ / N ₂ } (3) V _BE1 > {(V ₃ −V ₄ ) N ₃ / N ₂ } + V _RBN (4) V _RBN > V _BE1 (5) Next, the operation of the switching regulator configured as described above will be explained. First, regarding the advantage when the oscillation transistor Q1 in the oscillation state changes from the “off” state to the “on” state, the time common to the horizontal axis is shown. Description will be given with reference to FIG.

First, due to the small base current flowing through the resistor R4,
The collector current I _c flows. An induced voltage is generated in the base winding L3 by mutual inductance with the collector winding L1, and the base potential is biased in the forward direction. Therefore collector current
I _c increases from before. Increased collector current I _c increases the induced voltage of the base winding L3, the base current increases, further increasing the collector current I _c. Such a regenerative action enters the "on" state the transistor Q1 is at time t _1, the collector current I _c is ever-increasing. A voltage corresponding to eventually resistor R5 collector current I _c detected in both ends of, the transistors Q4, and was sequentially "on" transistor Q5, the base current of the transistor Q1 transistor Q5 is the collector current I _c to bypass The increase stops and the induced voltage in the base winding L3 starts to decrease this time because the change of the magnetic flux Φ _T1 disappears. When the base current decreases, also the collector current I _c decreases, before and by reverse regenerative action transistor Q1 rapidly "off" at time t _2.

The flow exciting current to the collector winding L1 immediately before the time t ₂ when the transistor Q1 is "off", the magnetic flux [Phi _T1 is accumulated, it is reverse biased abruptly "off".

The reverse bias state continues until time t ₃ when the magnetic flux Φ _T1 disappears.

A current for charging the smoothing capacitor C1 flows through the secondary winding L2 through the rectifier diode D3 and the choke coil L when the transistor Q1 is "on". During this time, the magnetic flux Φ _L is accumulated in the choke coil L as excitation energy.

The time t ₃ after that, the choke coil L and a smoothing capacitor C1, current is sustained by the loop of the flywheel diode D4. The smoothing capacitor C1 continuously supplies the DC output from the output terminals 1A and 1B to the load.

By the way, the blocking oscillator circuit is the magnetic flux Φ _{T1 of the} transformer _T1.
Until time t ₃ disappears, the same state as before time t ₁
Transistor Q1 attempts to "turn on" but still remains "off". This is because current flows through the flywheel diode D4 until the choke coil L completely discharges the excitation energy, and the base winding L3 has (2)
This is because the voltage of the first term on the right side of the equation is generated, and further, the voltage drop circuit R is present.

That is, the following expression (6) obtained by modifying the expression (2) is satisfied, and the base of the transistor Q1 is not supplied with a voltage equal to or higher than the voltage V _BE1 .

V _BE1 > {(V ₃ −V ₄ ) N ₃ / N ₂ } −V _RNB (6) Here, the dot side of the base winding L3 has a low potential.

Then, as described above, the voltage V _RBN of the voltage drop circuit R in the direction from the base of the transistor Q1 to the base winding L3 is set by the expressions (3) and (4).

Therefore, the starting current flowing from the resistor R4 is the diode D1,
It cannot flow into the base of the transistor Q1 as well as the resistance R2 and the base L3.

The voltage drop circuit R acts in this way, and the transistor Q1
The "off" state of continues.

Then, when the exciting energy of the choke coil L is completely released and the voltage of the first term on the right side of the equation (2) and the voltage of the second term on the right side of the equation (3) disappear in the base winding L3, the above-mentioned is performed. Thus, the equation (5) is established.

As a result, equation (3) no longer holds at time t ₄ ,
A voltage is applied from the resistor R4 to the base of the transistor Q1,
Transistor Q1 turns "on".

Next, the oscillating transistor Q1 goes from the "on" state to the "off" state.
Advantages in the case of the state will be described with reference to FIG.

In the transistor Q4, the voltage V _T2 corresponding to the collector current I _c of the transistor Q1 detected in the secondary winding L22 of the current transformer _T2 is compared with the base-emitter voltage V _BE4 as a reference voltage, and the transistor Q4 and the transistor Q4 are compared. The transistor Q1 is turned off by sequentially turning on Q5. Then, a stable “on” time T _ON is obtained which is not affected by the characteristics of the transistor Q1 such as h _fe . The DC output of the output terminals 1A and 1B can be easily controlled by changing the reference voltage.

By the way, the same reference numerals are given to the times corresponding to FIG. 3, and the duty ratio D of the transistor Q1 is 50%. As is apparent from FIG. 2, the secondary winding of the transformer T2 is shown.
The voltage V _T2 of L22 changes similarly to the collector current I _c , but the direct current component is removed and only the alternating current component is detected across the resistor R5.

Transistor Q4 compares the positive sides of voltage V _BE4 and voltage V _T2 , and transistor Q1 is "off" when voltage V _T2 exceeds voltage V _BE4 . The time of turning "on" is determined by the discharge speed of the magnetic flux ΦL of the choke coil L as described above. Therefore, the "off" time varies with the output voltage.

Then, assuming that the amplitude of the voltage V _{T2 on} the positive side is A ₁ and the amplitude of the negative side is A ₂ , the equation (7) is established.

In the equation (7), A ₁ + A ₂ = A _p and D = t ₂ / t ₄ , and D is the duty ratio.

Further, the expression (8) is established from the expression (7).

A _p = (A _p D / 2) + A ₁ = A ₁ / (1-D / 2) (8) In the formula (8), A _p corresponds to the amplitude when the collector current I _c is detected by the resistor. , A ₁ is the amplitude detected and compared by the current transformer T 2 in the present invention. The relationship between the amplitude A _p and the amplitude A ₁ changes depending on the duty ratio D. For example, when the duty ratio D is 50% as shown in FIG. 2, A _p = 4
A _1/3, and the value of A ₁ is 75% of A _p. When the duty ratio D is 0, it becomes 100% of A _p .

Therefore, if the duty ratio D is designed to be about 50% when the load is normal in the control region, the duty ratio D approaches 0 due to a decrease in the output voltage outside the control region in the overloaded state. In the control region, 75% of the amplitude A _p is compared with the reference voltage, but when the duty ratio D becomes small outside the control region and approaches 0, it is compared with almost 100% of the amplitude A _p. . By rapidly increasing the value to be compared, it is possible to quickly stop the increase of the output current I ₀ .
Then, an overcurrent protection function by so-called drooping characteristics as shown in the characteristic diagram of FIG. 4 can be exhibited.

[Effect] As described above, the switching regulator of the present invention can prevent an excessive surge current when the oscillating transistor is "on", and detect the collector current when the transistor is "off" with a current transformer for comparison. By doing so, when the load becomes heavy outside the control region and the duty ratio becomes small, the increase of the output current can be stopped more quickly by the overcurrent protection function as described above than by using the resistor.

In this way, it is possible to obtain a self-excited, on-on type switching regulator that can be configured by using an oscillating transistor having a small current capacity, and further improve its practicality.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a switching regulator of the present invention, FIGS. 2, 3, and 4 are characteristic diagrams of the switching regulator of FIG. 1, and FIG. 5 is a circuit diagram of a conventional switching regulator. , FIG. 6 is a characteristic diagram of the switching regulator of FIG. T: ... transformer, T2: current transformer, L11: primary winding, L2
2: Secondary winding, L: Choke coil, Q1: Oscillation transistor,
D3: Rectifying diode, D4: Flywheel diode, R:
Voltage drop circuit

Claims (1)

[Claims] 1. A blocking oscillation circuit is constructed using a transformer having a collector winding, a base winding, a secondary winding, and a transistor, and a rectifying diode, a flywheel diode, a choke coil, and a smoothing capacitor are provided in the secondary winding. In an on-on type switching regulator in which a rectifying and smoothing circuit is connected, in a closed circuit formed between the base winding and the base-emitter of a transistor,
The voltage across the base winding in the direction of the base of the transistor
V _RNB , where V _RBN is the voltage across the transistor from the base to the base winding direction, V _RNB > {(V ₃ −V ₄ ) N ₃ / N ₂ } −V _BE1 V _RBN <V _BE1 − {(V ₃ −V ₄ ) N ₃ / N ₂ } V _RBN −V _{BE1 In} accordance with the condition of connecting the voltage drop circuit that keeps the “off” state of the transistor until the excitation energy of the choke coil is released, and A switching regulator having a current transformer for converting a collector current of the transistor into a voltage and turning the transistor "off" when the voltage on the positive side exceeds a reference voltage. (However, V ₃ is the forward voltage of the rectifier diode, V ₄ is the forward voltage of the flywheel diode, V _BE1 is the base-emitter voltage of the transistor, and N ₃ and N ₂ are the secondary windings of the base winding and the transformer, respectively. It is the number of turns of the winding.)