JPH0150187B2 - - Google Patents

Description

Detailed Description of the Invention Technical Field The present invention relates to a DC-DC converter called an on-off single-stone transistor converter, and more specifically to a self-excited DC-DC transistor having an overcurrent detection resistor on the primary side of a transformer. Concerning converters.

Prior Art In Fig. 1 showing a self-excited on-off type (flyback type) DC-DC converter including a conventional current limiting circuit, 1 is a DC power supply, 2 is an output transformer, and 3 is a switching transistor. One end of the primary winding 5 of the switching transistor 2 is connected to one end (positive terminal) of the DC power supply 1, the other end is connected to the collector of the switching transistor 3, and the emitter of the switching transistor 3 is connected to the collector of the switching transistor 3 via the overcurrent detection resistor 4. and is connected to the other end (negative terminal) of the DC power supply 1.

In addition to the secondary winding 6, the transformer 2 also has a tertiary winding 7, and a rectifying and smoothing circuit 8 consisting of a rectifying diode 8a and a capacitor 9 is connected to the secondary winding 6.
A load 10 is connected to this output stage.

The tertiary winding 7 is a base drive winding, and one end (upper end) thereof is connected to the base of the switching transistor 3 by the base circuit 16, and the other end (lower end) is connected to the emitter. 15 is a constant voltage control circuit provided in the base circuit 16, which includes a current limiting resistor 11 connected in series to the base circuit 16, a constant voltage control capacitor 12, a rectifier diode 13 for capacitor charging, and a constant voltage control circuit connected in series to the base circuit 16. It consists of a Zener diode 14 as a diode. One end of a capacitor 12 in this constant voltage control circuit 15 is connected to the lower end of the tertiary winding 7 , and the other end is connected to the upper end of the tertiary winding 7 via a rectifier diode 13 . The rectifier diode 13 has a tertiary winding 7 oriented to reverse bias the transistor 3.
The capacitor 12 is charged with the voltage of the tertiary winding 7 during the off-period of the transistor 3, that is, the regulated voltage. A Zener diode 14 as a constant voltage diode is connected between the other end of the capacitor 12 and the base of the transistor 3. The Zener diode 14 is connected in such a way that it breaks down and becomes a constant voltage when the transistor 3 is forward biased.

Reference numeral 17 denotes a starting resistor, which is connected between the base of the switching transistor 3 and the DC power supply 1. 18 is an overcurrent limiting transistor, the base of which is connected to one end (upper end) of the overcurrent detection resistor 4 via the base current limiting resistor 16;
Its emitter is the other end (lower end) of overcurrent detection resistor 4.
, and its collector is connected to the base of the switching transistor 3.

The main circuit portion of the converter shown in FIG. 1 except for the overcurrent protection circuit is substantially the same as the circuit disclosed in Japanese Patent Publication No. 52-47524.

Next, the operation of the circuit shown in FIG. 1 will be explained. First, when the power supply 1 of the converter is turned on, the base current of the switching transistor 3 flows through the starting resistor 17, and this transistor 3 is turned on and starts oscillation. During the ON period of the switching transistor 3, a power supply voltage is applied to the primary winding 5, and a voltage is also obtained in the tertiary winding 7 in response to this, and the switch is activated as shown in FIG. 2A. A base current I _B is supplied to the switching transistor 3. As a result, the conduction of the transistor 3 is maintained, and the collector current I _C gradually increases as shown in FIG. 2B. At this time, a voltage is generated in the secondary winding 6 that turns off the diode 8a, and the release of energy is prevented. After that, when h _FE · I _B (where h _FE is the current amplification factor of transistor 3) is reached and transistor 3 is saturated, it becomes impossible to increase the collector current I _C and transistor 3 shifts to an unsaturated state. do. As a result, the voltage across the primary winding 5 decreases, the base current also decreases, and the transistor 3 suddenly turns off. Then, the on period of the transistor 3 (t ₁ ~
The energy stored in the transformer 2 during the period t ₂ ) is released through the rectifier diode 8 during the period when the transistor 3 is off (t ₂ to _{t 3} ). While the transformer 2 is discharging energy, a voltage is generated in the tertiary winding 7 that reverse biases the transistor 3.
Transistor 3 does not turn on, but when the energy release is finished, transistor 3 turns on again.

The capacitor 12 of the constant voltage control circuit 15 is charged with the voltage of the tertiary winding 7 during the off period of the transistor 3. Since the voltage of the tertiary winding 7 during this off period is a voltage corresponding to the output voltage V ₀ , the capacitor 12 is charged with a voltage corresponding to fluctuations in the output voltage V ₀ . Note that the capacitor 12 is charged in a direction that reverse biases the transistor 3. transistor 3
In parallel to the emitter-base junction of , that is, the base circuit 1
A series circuit of a Zener diode 14 and a capacitor 12 is connected in parallel to the Zener diode 14, and since the Zener diode 14 has the directionality of breaking down at the voltage that turns on the transistor 3, the constant voltage obtained across the Zener diode 14 is That is, the base bias, that is, the base current of the transistor 3 is controlled by the voltage difference between the reference voltage and the charging voltage of the capacitor 12, and a constant voltage characteristic is obtained. If the output voltage increases now, the capacitor 12 is charged to a high voltage via the diode 13 during the off period.
As a result, when the base current is supplied to the transistor 3 using the voltage generated in the tertiary winding 7 during the ON period of the transistor 3, the current flowing through the Zener diode 14 increases and the base current flowing to the transistor 3 decreases. As a result, the peak of the collector current I _C determined by h _FE I _B is suppressed to a low level, and the energy stored in the transformer 2 is reduced.
The output voltage V ₀ is returned to the reference voltage. Output voltage V ₀
When is lower than the reference value, the operation is opposite to the above.

When the emitter current I _E exceeds a certain value due to a short circuit in the load 10, etc., the voltage drop across the overcurrent detection resistor 4 exceeds the threshold of the overcurrent limiting transistor 15, and the transistor 15 enters a conductive state corresponding to the emitter current. becomes. Therefore, a part of the base current of the switching transistor 3 is shunted to the overcurrent limiting transistor 15, and the base current I _B decreases.
The collector current I _C determined by h _FE・I _B also decreases.
For this reason, the amount of energy released through the secondary winding 6 of the transformer 2 also decreases, making it impossible to maintain constant voltage characteristics, resulting in a decrease in the output voltage V ₀ as shown in FIG. I ₀ is restricted from increasing towards infinity, resulting in drooping behavior. However, the output current I ₀ corresponding to the supplied energy continues to flow, and this current value becomes larger than the current value at the switching point from the constant voltage characteristic line to the drooping characteristic line, that is, the starting point of the drooping operation.

Furthermore, when the voltage V ₁ of the power supply 1 changes, the drooping start point in the characteristic line of output current I ₀ vs. output voltage V ₀ shown in Fig. 3 changes significantly as shown in P ₁ , P ₂ , and P ₃ . Change. This is the overcurrent limiting transistor 15.
This is because the operation start point of is determined only by the emitter current I _E on the primary side and is not directly controlled by the output current I ₀ on the secondary side. To explain this in more detail, the emitter current I _E at the start of operation of the overcurrent limiting transistor 15 is always a constant value regardless of the magnitude of the input voltage V ₁ . Therefore, when the drooping operation starts when the input voltage _V1 is low, the energy supplied by the transformer 2 is small, and the current value at the drooping start point _P1 is also as shown by characteristic line A in Fig. 3. This results in a relatively small I ₁ . On the other hand, when the input voltage V ₁ is medium or high, the energy supplied to the transformer 2 becomes large, and the drooping operation starting points P ₂ , P ₃
As shown by characteristic lines B and C, the current values of I ₂ and I ₃ also increase successively. Therefore, it is impossible to start the drooping operation at a predetermined value of the output current I ₀ . The former problem of increase in current during drooping operation and the latter problem of variation in the drooping start point can be solved by directly detecting the output current I ₀ and performing overcurrent control. However, the circuit configuration inevitably becomes complicated.

OBJECT OF THE INVENTION Therefore, an object of the present invention is to provide a DC-DC converter that can easily limit the increase in output current during drooping operation in a system that performs overcurrent detection on the primary side. be.

Structure of the Invention To achieve the above object, the present invention will be described with reference to the reference numerals in the drawings showing the embodiments for ease of understanding. 5, a switching transistor 3 connected in series to the switching transistor 3;
Overcurrent detection resistor 4 connected in series with the emitter of
and a DC power source 1 that supplies a DC voltage to a series circuit consisting of the primary winding 5, the switching transistor 3, and the overcurrent detection resistor 4, and an electromagnetic coupling to the primary winding 5 of the transformer 2. The output of the secondary winding 6 is blocked during the ON period of the switching transistor 3, and the stored energy of the transformer 2 is obtained during the OFF period of the switching transistor 3. Next winding 6
an output rectifying and smoothing circuit 8 connected to the switching transistor 3; a starting resistor 17 connected between the power supply line from the DC power supply 1 to the collector of the switching transistor 3 and the base of the switching transistor 3; The switch is electromagnetically coupled to the primary winding 5 and the secondary winding 6 of the switching transistor 2, and is configured to supply a base current to the switching transistor 3 by an induced voltage generated during the ON period of the switching transistor 3. 3 connected between the base and emitter of the switching transistor 3
a constant voltage control circuit 15 that controls the base current of the switching transistor so as to obtain a constant output voltage based on a voltage corresponding to the output voltage and a reference voltage; It is connected to one end of the overcurrent detection resistor 4 through a resistor 19, and the emitter is connected to the overcurrent detection resistor 4.
In a self-excited on-off type transistor DC-DC converter consisting of an overcurrent limiting transistor 18 connected to the other end and whose collector is connected to the base of the switching transistor 3, charging corresponding to the output voltage is performed. One end is connected to one end of the overcurrent detection resistor 4 in order to apply a reverse bias to the overcurrent limiting transistor 18 based on the voltage, and the other end is connected to the base of the overcurrent limiting transistor 18 directly or through the resistor 23. and a reverse bias capacitor 12 connected to the voltage of the tertiary winding 7 during the off period of the switching transistor 3 or a voltage detecting quaternary winding provided in the transformer 2. The tertiary winding 7 or the 4
The present invention relates to a DC-DC converter characterized by comprising a rectifier diode 13 connected between the next winding and the other end of the reverse bias capacitor 12.

In order to simplify the circuit configuration, the constant voltage control circuit 15 is composed of a constant voltage diode, that is, a Zener diode 14, and a capacitor 1, as shown in FIG.
2, a rectifier diode 13, and a resistor 11, and one end of the capacitor 12 is connected to one end of the current detection resistor 4 in order to reverse bias the overcurrent limiting transistor 18 by the charging voltage of the capacitor 12. , the other end may be connected to the base of the overcurrent limiting transistor 18 directly or via the resistor 23.

Effects of the Invention According to the above invention, the capacitor 12 is charged with a voltage corresponding to the output voltage, and this is applied to the overcurrent limiting transistor 18 as a reverse bias. The bias component also becomes smaller, making it easier for the overcurrent limiting transistor 18 to conduct. Therefore, the base current of the switching transistor 3 is limited, and its collector current I _C and output current I ₀ are also significantly limited.

Embodiment Next, a self-excited on-off type DC-DC converter according to an embodiment of the present invention will be described with reference to FIGS. 4 and 5. However, in FIGS. 4 and 5, parts common to those in FIGS. 1 to 3 are given the same reference numerals, and their explanations will be omitted. In the converter shown in FIG. 4, the first capacitor 12 for constant voltage control is
In addition, a second capacitor 20 for droop control is provided, and in addition to the first rectifier diode 13, a second capacitor 20 is provided.
A rectifier diode 21 is provided, two resistors 22 and 23 are further added, and the second capacitor 20 and the second rectifier diode 21 are connected in series, and this series circuit is connected to the tertiary winding 7. connected in parallel. Since the anode of the second diode 21 is connected to one end of the tertiary winding 7,
When a voltage that turns on the switching transistor 3 is induced in the tertiary winding 7, the second diode 21 turns on, and the second capacitor 20 for controlling the drooping characteristics is charged to the voltage during the on period. be done. One end of the capacitor 20 is connected to the lower end of the tertiary winding 7 and one end of the resistor 19.
The other end of the capacitor 20 is connected to the base of the transistor 18, that is, the other end of the resistor 19 through the resistor 22, so the voltage of the capacitor 20 acts to forward bias the current limiting transistor 18, causing the transistor 18 to become conductive. Control the state. Furthermore, since the resistor 23 is connected between the other end of the constant voltage control capacitor 12 and the base of the transistor 18, the charging voltage of the capacitor 12 acts on the current limiting transistor 18 as a reverse bias. Therefore, the first capacitor 12 can also be called a droop control capacitor or a reverse bias capacitor.

Since the normal operation of the circuit in FIG. 4 is the same as that in FIG. 1, the explanation will be omitted, and the overcurrent operation will be explained. While the switching transistor 3 is on, the voltage of the power supply 1, that is, the input voltage _V1 is applied to the primary winding 5 almost as is. As a result, the tertiary winding 7
A voltage corresponding to the input voltage V ₁ is also obtained, and the second capacitor 20 for forward bias is connected to the input voltage V ₁
The battery will be charged to the voltage corresponding to the voltage. On the other hand, the conventionally provided first capacitor 12 is charged to a voltage during the off-period of the transistor 3, that is, a voltage corresponding to the output current _I0 . Note that the second capacitor 20 is charged positively with reference to the emitter potential of the switching transistor 3, and the first capacitor 12 is charged negatively.

Therefore, the operation of the current limiting transistor 18 is determined by the combination of the voltage across the overcurrent detection resistor 4, the forward DC bias supplied from the capacitor 20, and the reverse DC bias supplied from the capacitor 12. It is determined. By the way, under constant voltage control, the charging voltage of the first capacitor 12 is kept almost constant regardless of fluctuations in the input voltage. Therefore, the first capacitor 12 is substantially independent of the relationship between the input voltage variation and the droop starting point variation. On the other hand, since the second capacitor 20 has a charging voltage corresponding to the input voltage, it is directly related to the starting point of the drooping operation. That is, the charging voltage of the second capacitor 20 is
2, the base-emitter junction of the transistor 18, the overcurrent detection resistor 4, and the second capacitor 20, and the closed circuit composed of the resistor 22, the resistor 19, and the second capacitor 20. is applied to the overcurrent limiting transistor 18. If the forward bias based on the capacitor 20 is large in response to the high input voltage, the overcurrent limiting transistor 18 starts conducting while the voltage across the overcurrent detection resistor 4 based on the emitter current _IE is small. On the other hand, if the forward bias based on the capacitor 20 is small in response to a low input voltage, the overcurrent detection resistor 4 based on the emitter current _IE
The transistor 18 becomes conductive when the voltage is high. That is, when the input voltage is high, the overcurrent limiting transistor 18 is conductive when the emitter current I _E of the switching transistor 3 is small, and when the input voltage is low, the overcurrent is suppressed when the emitter current I _E of the switching transistor 3 is large. Limiting transistor 18 becomes conductive. Since the stored energy of the transformer 2 is approximately equal to the product of the input voltage V ₁ and the emitter current I _E , the input voltage V ₁ and the emitter current I _E influence the start of conduction of the overcurrent limiting transistor 18 according to the above-mentioned relationship. This means that the energy stored in the transformer 2 at the time when the transistor 18 starts conducting can be kept approximately constant regardless of fluctuations in the input voltage _V1 . Therefore, as shown by characteristic lines A, B, and C in Fig. 5, the input voltage _V1 is low,
Even if the voltage changes sequentially from medium to high, the drooping start points P ₁ , P ₂ , and P ₃ of the load current I ₀ vs. output voltage V ₀ characteristic do not change significantly. That is, even if the input voltage fluctuates, the drooping operation can be started at a substantially constant load current _I0 .

As mentioned above, when the overcurrent limiting transistor 18 starts conducting, a shunt circuit is formed by the transistor 18, so the base current I _B of the switching transistor 3 becomes small, and is ultimately determined by h _FE · I _B The peak value of I _C also decreases, the energy stored in the transformer 2 decreases, and the output voltage V ₀ decreases.

When the output voltage V ₀ starts to drop due to drooping operation,
The charging voltage of the reverse bias capacitor, that is, the first capacitor 12 for constant voltage control, also decreases following the decrease in the output voltage V ₀ . Therefore, the determination is made based on a closed circuit consisting of the first capacitor 12, the resistor 4, the emitter-base junction of the transistor 18, and the resistor 23, and a closed circuit consisting of the first capacitor 12, the resistor 19, and the resistor 23. The amount of reverse bias applied to the transistor 18 decreases, and the transistor 18 becomes easily conductive. Therefore, the base current of the switching transistor 3 is significantly reduced, and the collector current I _C becomes small.
Since the stored energy also decreases, an increase in the output current I ₀ is suppressed. In other words, the positive feedback operation suppresses the increase in the collector current I _C , and as shown in Fig. 5, the output current I ₀ is suppressed near the current value at the start of drooping, resulting in a constant current type or fold-back type drooping characteristic. is obtained. Note that if the collector current I _C decreases, the voltage across the overcurrent detection resistor 4 will also decrease, but the reverse bias based on the first capacitor 12 will be significantly reduced.
The overcurrent limiting transistor 18 is maintained on.

As is clear from the above, according to this embodiment, it is possible to suppress fluctuations in the starting point of drooping operation based on fluctuations in input voltage with a simple circuit.

Furthermore, since the drooping current is determined by the combination of the reverse bias voltage from the capacitor 12 and the voltage across the resistor 4 due to the emitter current, it is possible to easily suppress an increase in the output current _I0 .

Modification (A) If it is not necessary to suppress fluctuations in the drooping start point due to fluctuations in input voltage, a capacitor 20, a diode 21, and a resistor 2 are used as shown in FIG.
It is also possible to remove the circuit No. 2 and obtain a drooping characteristic as shown in FIG.

(B) In order to charge the capacitors 12 and 20, the transformer 2 is provided with a quaternary winding, one end of the quaternary winding is commonly connected to the emitter of the transistor 3, and the quaternary winding and the capacitors 12, 20 are connected together.
The capacitors 12 and 20 may be charged with the voltage of the quartic winding that provides the same voltage as the tertiary winding 7 by connecting diodes 13 and 21 between the tertiary winding and the tertiary winding.

(C) In Figures 4 and 6, the resistor 11 is connected to the lower end line of the tertiary winding 7, and the capacitor 12
may be connected to the upper end of the tertiary winding 7, and the rectifier diode 13 and Zener diode 14 may be connected between the lower end line and the capacitor 12.

(D) The constant voltage control circuit 15 is not limited to the circuit shown in FIG. 4, but can adjust the base current of the switching transistor 3 based on the output voltage and the reference voltage to obtain constant voltage characteristics. Any circuit configuration may be used as long as it is possible.
For example, a transistor may be connected in parallel to the base circuit 16, and the conduction state of this transistor may be changed according to the output voltage to obtain constant voltage characteristics.

(E) Although the capacitor 12 is commonly used for constant voltage control and constant current droop control (positive feedback control) shown in FIG. 5, a separate capacitor for droop control using reverse bias may be provided.

(F) The starting resistor 17 may be connected to the collector of the transistor 3. Further, in order to continue oscillation after startup, a capacitor, a resistor, a diode, etc. may be connected in parallel to the primary winding 5, and a ringing chain converter system may be used, for example.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram of a conventional converter, Fig. 2 is a waveform diagram showing the base current and collector current of the switching transistor of the circuit of Fig. 1, and Fig. 3 is an output current-voltage diagram of the circuit of Fig. 1. It is a characteristic diagram. FIG. 4 is a circuit diagram showing a converter according to an embodiment of the present invention, and FIG. 5 is an output current-voltage characteristic diagram of the circuit shown in FIG. 4. FIG. 6 is a circuit diagram showing a modified converter, and FIG. 7 is an output current-voltage characteristic diagram of FIG. 6. 1...DC power supply, 2...Transformer, 3...Switching transistor, 4...Overcurrent detection resistor,
5...Primary winding, 6...Secondary winding, 7...Tertiary winding, 8...Rectifier smoothing circuit, 11...Resistor, 12...
... Capacitor for constant voltage control, 13 ... Rectifier diode, 14 ... Zener diode for constant voltage control, 15 ... Constant voltage control circuit, 17 ... Starting resistor, 18 ... Transistor for overcurrent limiting, 19 ...
... Resistor for base current adjustment, 20 ... Capacitor for droop control, 21 ... Rectifier diode, 22, 23
……resistance.

Claims (1)

[Claims] 1. A primary winding 5 of a transformer 2, a switching transistor 3 connected in series to the primary winding 5, and an overcurrent connected in series to the emitter of the switching transistor 3. a detection resistor 4; a DC power supply 1 that supplies a DC voltage to a series circuit consisting of the primary winding 5, the switching transistor 3, and the overcurrent detection resistor 4; and the primary winding 5 of the transformer 2. a secondary winding 6 that is electromagnetically coupled to the secondary winding 6; and a secondary winding 6 which is electromagnetically coupled to an output rectifying and smoothing circuit 8 connected to the secondary winding 6; and a starting circuit connected between the power supply line from the DC power supply 1 to the collector of the switching transistor 3 and the base of the switching transistor 3. A base current is applied to the switching transistor 3 by an induced voltage that is electromagnetically coupled to the primary winding 5 and the secondary winding 6 of the transformer 2 and generated during the ON period of the switching transistor 3. the switching transistor 3 so as to supply
a tertiary winding 7 connected between the base and the emitter of the switching transistor; and controlling the base current of the switching transistor so as to obtain a constant output voltage based on a voltage corresponding to the output voltage and a reference voltage. The constant voltage control circuit 15 has a base connected to one end of the overcurrent detection resistor 4 via a base current adjustment resistor 19, an emitter connected to the other end of the overcurrent detection resistor 4, and a collector connected to the switching transistor. an overcurrent limiting transistor 18 connected to the base of the overcurrent limiting transistor 18; a reverse bias capacitor 12 having one end connected to one end of the overcurrent detection resistor 4 and the other end connected to the base of the overcurrent limiting transistor 18 directly or through a resistor 23 to provide a directional bias; The bias capacitor 12 is connected to the switching transistor 3 during the off period of the switching transistor 3.
In order to charge the voltage of the secondary winding 7 or the voltage of the voltage detection quaternary winding provided in the transformer 2, the tertiary winding 7 or the quaternary winding is connected to the other end of the reverse bias capacitor 12. A DC-DC converter comprising: a rectifier diode 13 connected between the DC-DC converter; 2 a primary winding 5 of the transformer 2; a switching transistor 3 connected in series to the primary winding 5; an overcurrent detection resistor 4 connected in series to the emitter of the switching transistor 3; a DC power source 1 that supplies a DC voltage to a series circuit consisting of a primary winding 5, the switching transistor 3, and the overcurrent detection resistor 4; a secondary winding 6; and a secondary winding configured to block the output of the secondary winding 6 during the ON period of the switching transistor 3 and obtain the stored energy of the transformer 2 during the OFF period of the switching transistor 3. an output rectifying and smoothing circuit 8 connected to the switching transistor 6; a starting resistor 17 connected between the power supply line from the DC power supply 1 to the collector of the switching transistor 3 and the base of the switching transistor 3; The coil is electromagnetically coupled to the primary winding 5 and the secondary winding 6 of the transformer 2, and is configured to supply a base current to the switching transistor 3 by an induced voltage generated during the ON period of the switching transistor 3. switching transistor 3
The tertiary winding 7 is connected between the base and the emitter of the tertiary winding 7, the base is connected to one end of the overcurrent detection resistor 4 via the base current adjustment resistor 19, and the emitter is connected to the other end of the overcurrent detection resistor 4. A constant voltage is obtained by the overcurrent limiting transistor 18, which is connected to the overcurrent limiting transistor 18 whose collector is connected to the base of the switching transistor 3, and the voltage of the tertiary winding 7 during the ON period of the switching transistor 3. a constant voltage diode 14 having one end connected to the base or emitter of the switching transistor 3 and having the directionality of the switching transistor 3;
A constant voltage control/reverse bias capacitor 12 arranged in parallel with the base-to-base junction; and the constant voltage control/reverse bias capacitor 12.
is connected between the tertiary winding 7 and the constant voltage control/reverse bias capacitor 12 so as to charge the voltage induced in the tertiary winding 7 during the off period of the switching transistor 3. A rectifier diode 13 for charging the capacitor, connected in series with the base circuit from the tertiary winding 7 to the switching transistor 3, and arranged between the constant voltage diode 14 and the tertiary winding 7. a current limiting resistor 11; and the constant voltage control for applying a reverse bias to the overcurrent limiting transistor 18 based on a charging voltage corresponding to the output voltage of the constant voltage controlling/reverse biasing capacitor 12. One end of the reverse bias capacitor 12 is connected to the overcurrent detection resistor 4.
A DC-DC converter characterized in that one end of the constant voltage control/reverse bias capacitor 12 is connected to the base of the overcurrent limiting transistor 18 either directly or through a resistor 23. .