JPH0258868B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a transistor DC converter for use in DC power supply circuits of video equipment, audio equipment, etc.

[Conventional technology]

The transformer has primary, secondary, and tertiary windings,
A switching transistor connected in series to the primary winding is turned on and off by the voltage of the tertiary winding,
Generally obtain a regulated output voltage at the output stage of the next winding
An on/off type switching regulator called RCC is well known.

In this type of switching regulator,
If the power supply voltage increases, the voltage of the primary winding and 3
The voltage and current in the next winding also increases. In order to maintain constant voltage characteristics, all of the current in the tertiary winding must be bypassed to the voltage control transistor without being supplied to the switching transistor. Since the bypass current is unrelated to driving the switching transistor, it results in power loss.
This problem becomes noticeable when the AC power supply voltage to which the equipment is connected changes from, for example, 100V to 220V. The power supply circuit is switched according to changes in the AC power supply voltage so that it can be used with either a 100V AC power source in Japan or a 220V AC power source in a foreign country. Although it is conceivable to configure it as
There is also a risk of forgetting to switch. Such a problem occurs not only when the AC power supply voltage changes, but also in any case where the DC power supply voltage changes for some reason.

In order to solve the above-mentioned problems, the applicant has filed Japanese Patent Application No. 45187-1987 and Japanese Patent Application No. 60-45187.
In No. 221670, we proposed a method in which a capacitor was provided that was charged by the almost constant voltage obtained in the transformer when the switching transistor was off, and the base current of the switching transistor was supplied by the charge of this capacitor. According to this method, efficiency can be significantly improved when the input voltage is high.

[Problem that the invention seeks to solve]

However, the above application does not discuss overcurrent protection. SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a DC converter that can perform both overcurrent protection against load short circuits and prevention of efficiency decline due to input voltage fluctuations using a common circuit.

[Means for solving problems]

To achieve the above object, the present invention will be described with reference to the reference numerals in the drawings showing the embodiments. a switching transistor 6 whose collector is connected to the other end of the primary winding 5 and whose emitter is connected to the other of the pair of DC power supply terminals; and a secondary winding which is electromagnetically coupled to the primary winding 5. line 7, a rectifying diode 8 connected to the secondary winding 7 with a polarity that turns on when the switching transistor 6 is off, and a pair of output lines on the output side of the diode 8. A smoothing capacitor 9 connected thereto, a tertiary winding 11 electromagnetically coupled to the primary winding 5 and the secondary winding 7, and a base drive circuit for supplying base current to the switching transistor 6. a capacitor 16; a charging circuit that charges the base driving capacitor 16 with the voltage of the tertiary winding 11 during the off period of the switching transistor 6; a discharge control circuit that selectively connects the base drive capacitor 16 between the base and emitter of the switching transistor 6 so as to supply a discharge current of and a constant voltage control circuit 26 for controlling the base current of the switching transistor 6 so as to keep the output voltage of the switching transistor 6 constant. 16 between the base and emitter of the switching transistor 6;
When the voltage obtained from the transformer 4 during the on period of the switching transistor 6 has a value within the normal range, the discharge control transistor 20 is controlled to be turned on based on the voltage of the transformer 4, and the voltage from the transformer 4 is controlled to be on. The present invention relates to a transistor direct current converter comprising a discharge control transistor control circuit that turns off the discharge control transistor 20 when the obtained voltage has a value lower than the normal range.

[Effect]

In the above invention, since the discharge control transistor 20 is turned on in the normal voltage range, the base current of the switching transistor 6 can be supplied by the base drive capacitor 16. Since the base driving capacitor 16 is charged with a controlled voltage during the off period, the switching transistor 6 can be driven with little loss regardless of fluctuations in the power supply voltage. On the other hand, when the load is short-circuited, the voltage of the transformer 4 decreases. Therefore, the discharge control transistor 20 is turned off, and the base drive capacitor 16
The supply of base current from the base current stops. As a result,
Excessive current does not flow through the switching transistor 6.

〔Example〕

Next, a DC converter according to an embodiment of the present invention will be described with reference to FIGS. 1 to 3. This DC converter is a ringing chain converter (RCC) type, and one end of the transformer 4 is connected between one end and the other end of the DC power supply 3 connected to the AC power supply terminals 1 and 2.
A switching transistor 6 is connected via the next winding 5. The secondary winding 7 of the transformer 4 has
A rectifying and smoothing circuit 10 consisting of a rectifying diode 8 and a smoothing capacitor 9 is connected. One end of the tertiary winding 11 of the transformer 4 is connected to the base of the switching transistor 6 via a first capacitor 12 and a resistor 13 for driving the base.
The other end is connected to the emitter. Starting resistance 15
is connected between one end of the power supply 3 and the base of the switching transistor 6.

16 is a second capacitor for driving the base provided according to the present invention. As a charging circuit for charging this second capacitor 16 with the voltage obtained at the tertiary winding 11 during the off period of the switching transistor 6, a reverse current is established between one end of the capacitor 16 and one end of the tertiary winding 11. Blocking diode 17
A diode 18 is connected between the other end of the tertiary winding 11 and the other end of the second capacitor 16 .

During the ON period of the switching transistor 6, the second
In order to cause the discharge current of the capacitor 16 to flow as the base current of the switching transistor 6, the second
One end of the second capacitor 16 is connected to the emitter of the switching transistor 6 via a discharge control transistor 20 and a resistor 21, and the other end of the second capacitor 16 is connected to the base of the switching transistor 6 via a resistor 19. has been done.

In order to turn on the discharge control transistor 20 only when the voltage of the transformer 4 is within the normal range (a predetermined value or higher), a Zener diode 2 is connected between the base and one end of the tertiary winding 11 via a resistor 22.
3 is connected. The Zener diode 23 functions as a control circuit for selectively turning on the discharge control transistor 20.

A quaternary winding 24 is provided to obtain a transformer voltage corresponding to the power supply voltage during the ON period of the switching transistor 6, and this is connected to a diode 25.
It is connected in parallel to the resistor 21 via. Note that the series circuit of the first capacitor 12 and the resistor 13 is connected in parallel to the series circuit of the diode 17, the second capacitor 16, and the resistor 19.

26 is a constant voltage control circuit, in order to control the base current value of the switching transistor 6,
It includes a base current bypass transistor 27 connected between the base and emitter, a transistor 28 for supplying an error signal to the transistor 27, and the like. More specifically, a transformer 5th winding 29 is provided as an output voltage detection winding, and a capacitor 31 is connected to this 5th winding 29 via a diode 30. Note that the polarities of the fifth winding 29 and the diode 30 are set so that the capacitor 31 is charged when the switching transistor 6 is off. Therefore, the voltage of capacitor 31 is approximately proportional to the output voltage. resistance 3
2 and 33 are connected between both ends of the capacitor 31,
This voltage division point is connected to the base of transistor 28. The emitter of the transistor 28 is
It is connected to a voltage dividing point between a resistor 34 and a Zener diode 35, which are connected to both end faces of the capacitor 31. Therefore, the transistor 28 has a constant voltage provided by the Zener diode 35 and the resistors 32, 33.
Compare the output voltage detection value obtained at the voltage division point of
This error signal is applied to the base of the bypass transistor 27. As a result, for example, when the output voltage is too high, the resistance value of the bypass transistor 27 decreases, the bypass amount of the base current increases, and the base current of the switching transistor 6 decreases. The on period becomes shorter and the output voltage returns to a constant value.
If the output voltage is too low, the opposite will occur.

〔motion〕

In the circuit shown in FIG. 1, when a power switch (not shown) is turned on, the base current of the switching transistor 6 is supplied through the starting resistor 15, and the switching transistor 6 is turned on. After that, when the switching transistor 6 is saturated and turned off, the diode 8 is turned on, and the energy stored in the transformer 4 when the switching transistor 6 is turned on is released, and when this release is finished, the switching transistor is turned off again. 6 is turned on. In addition to the above-described oscillation operation, a constant voltage output is obtained at the output terminal 36 by constant voltage control by the voltage control circuit 26.

Incidentally, during the off period of the switching transistor 6, a substantially constant voltage is generated in the tertiary winding 11 regardless of changes in the power supply voltage. Since this voltage is opposite to the voltage when the switching transistor 6 is turned on, the diodes 18 and 17 are turned on, and the tertiary winding 11, diode 18, resistor 19, second capacitor 16, and diode 1
A closed charging circuit is formed consisting of capacitor 1
6 is charged at constant voltage. At this off time, 3
The voltage of the next winding 11 is the discharge control transistor 20
However, since the base of the discharge control transistor 20 is connected to the upper end of the tertiary winding 11 via the resistor 22 and the Zener diode 23, the base current does not flow and the discharge control transistor 20 is forward biased. transistor 20 is kept in an off state.

When the switching transistor 6 is turned on after the capacitor 16 is charged, the capacitor 16,
Resistor 19, switching transistor 6, resistor 2
1 and a discharge control transistor 20, a discharge current of the capacitor 16 flows, and a base current of the switching transistor 6 is supplied. At this time, the voltage of the tertiary winding 11 is applied to the base of the discharge control transistor 20 via the Zener diode 23 and the resistor 22.

FIG. 2 shows the waveform of the base current 1 _B of the switching transistor 6 when the input voltage is normal. When the on period begins at t ₁ , an upward voltage is generated in the tertiary winding 11, and a large base current is momentarily supplied by the first base current supply circuit consisting of the first capacitor 12 and resistor 13. At the same time, since the discharge control transistor 20 is turned on, the second base current supply circuit consisting of the discharge circuit of the second capacitor 16 also supplies the base current. Shortly after t ₁ , the base current supply through the first capacitor 12 decreases, while the second capacitor 16
The base current supply continues.

By the way, a resistor 21 is connected to the discharge circuit of the second capacitor 16, and the voltage of the quaternary winding 24 is applied thereto. Therefore, when the power supply voltage increases, the resistance of the discharge circuit of the second capacitor 16 increases, and the current supplied from the second capacitor 16 decreases. This controls the supply of excessive drive current and improves efficiency. In addition, the first
The current passing through the capacitor 12 increases as the power supply voltage increases, but since it is instantaneous, it does not significantly affect power loss.

When the output terminal 36 is short-circuited, the transformer 4
The voltage across each winding of the will drop significantly. As a result,
It becomes impossible to turn on the Zener diode 23 with the voltage of the tertiary winding 11 during the on period, the discharge control transistor 20 does not turn on, and a discharge circuit for the second capacitor 16 is not formed. As a result, switching transistor 6
The base current of the first capacitor 12 and the resistor 1
3, the collector current of the switching transistor 6 is suppressed.

Note that when the voltage of the DC power supply 3 is in the range of 0 to 60V, the Zener diode 23 is kept off during the on period of the switching transistor 6, so the base current is supplied via the first capacitor 12. only. When the power supply voltage exceeds 60V, the Zener diode 23 turns on and the second
The base current can be supplied by the capacitor 16. Since the DC converter of this embodiment is used at 100 to 220 V, the base current is supplied by the second capacitor 16 unless an abnormality such as a load short circuit occurs.

[Modified example]

The present invention is not limited to the embodiments described above, and the following modifications are possible, for example.

(A) As shown in FIG. 4, a control circuit 37 is provided in place of the constant voltage control circuit 26 in FIG. 1 to form an error signal between the voltage at the output terminal 36 and the reference voltage,
The bypass transistor 27 may be controlled by the error output.

(B) As shown in FIG. 5, the voltage control circuit 26 may be composed of a diode 38, a capacitor 39, a Zener diode 40, and a bypass transistor 27.

(C) As shown in FIG. 6, a charging winding 41 is provided in the transformer 4 separately from the tertiary winding 11, and this is connected to the second capacitor 16 via a diode 42, so that the voltage during the off period is It may also be charged.

〔Effect of the invention〕

As is clear from the above, when the voltage of the transformer decreases due to a load short circuit or the like, the discharge control transistor is turned off, so that the base current of the switching transistor is suppressed. Therefore, overcurrent due to load short circuiting or the like is suppressed by sharing the discharge control transistor, which simplifies the circuit configuration. Also, power loss is reduced.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a DC converter according to an embodiment of the present invention, and FIG. 2 is a waveform diagram of the base current of a switching transistor when the voltage of the transformer is normal.
FIG. 3 is a waveform diagram of the base current of the switching transistor when the voltage of the transformer is low, and FIGS. 4, 5, and 6 are circuit diagrams showing a portion of a modified DC converter. 5...Primary winding, 6...Switching transistor, 11...Tertiary winding, 12...First capacitor, 16...Second capacitor, 20...Discharge control transistor, 23... Zener diode.

Claims (1)

[Claims] 1. A primary winding 5 of a transformer 4, one end of which is connected to one of a pair of DC power terminals, and a collector of the primary winding 5 is connected to the other end of the primary winding 5. a switching transistor 6 whose emitter is connected to the other DC power supply terminal; and a secondary winding 7 which is electromagnetically coupled to the primary winding 5.
a rectifying diode 8 connected to the secondary winding 7 and having a polarity that turns on when the switching transistor 6 is off; and a rectifying diode 8 connected between a pair of output lines on the output side of the diode 8. a smoothing capacitor 9, a tertiary winding 11 electromagnetically coupled to the primary winding 5 and the secondary winding 7, and a base driving capacitor 16 for supplying base current to the switching transistor 6.
a charging circuit that charges the base driving capacitor 16 with the voltage of the tertiary winding 11 during the off period of the switching transistor 6; and a charging circuit that charges the base driving capacitor 16 with the voltage of the tertiary winding 11 during the off period of the switching transistor 6; The base driving capacitor 1 is configured to supply current as the base current of the switching transistor 6.
6 is the base of the switching transistor 6.
It has a discharge control circuit that selectively connects the emitters, and a constant voltage control circuit 26 that controls the base current of the switching transistor 6 so as to keep the output voltage of the smoothing capacitor 9 constant. The discharge control transistor 2 is configured to perform DC conversion, and the discharge control circuit selectively connects the base drive capacitor 16 between the base and emitter of the switching transistor 6.
0, and if the voltage obtained from the transformer 4 during the on period of the switching transistor 6 has a value within the normal range, the discharge control transistor 20 is controlled to be turned on based on the voltage of the transformer 4, and the A transistor direct current converter comprising a discharge control transistor control circuit that controls the discharge control transistor 20 to be turned off when the voltage obtained from the transformer 4 has a value lower than the normal range.