JPH0884468A - Dc-dc converter - Google Patents

Abstract

PURPOSE: To efficiently interrupt the output of a control circuit with reliability by detecting overloading of a DC-DC converter. CONSTITUTION: A transformer 9 is provided with a third winding 9c that takes out voltage when a switching element 2 is in the off state, and a fourth winding 9d that takes out voltage when the switching element 2 is in the on state. The third and fourth windings 9c, 9d are connected with a rectifier circuit 6a composed of a diode 6 and a capacitor 7 and with a rectifier circuit 16a composed of a diode 16 and a capacitor 17, respectively. At the time of operation with constant voltage output, the fourth winding 9d is virtually in the off state and a power supply terminal 40 is efficiently supplied with control power from the third winding 9c. At the time of overloading, no winding voltage is generated in the third winding 9c, and the power supply terminal 40 is stably supplied with voltage through the winding voltage of the fourth winding 9d, which can interrupt the output of a control circuit 4 with reliability.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to converters, especially DC-
The present invention relates to an auxiliary power supply for a control circuit used in a DC converter.

[0002]

2. Description of the Related Art FIG. 2 shows a conventional DC-DC converter. In this DC-DC converter, the DC power supply 1 charges the capacitor 7 via the starting resistor 5. When the charging voltage of the capacitor 7 reaches a predetermined level, the control circuit 4
Generates a pulse width modulated (PWM) control output from the output terminal 41. The switching element 2 starts the on / off operation by the control output, and intermittently applies the DC voltage of the DC power supply 1 to the first winding 9a of the transformer 9. The voltage generated in the second winding 9b of the transformer 9 is rectified by the diode 10 and smoothed by the capacitor 11, and DC power is supplied to the load 12. At the same time, the voltage generated in the third winding 9c of the transformer 9 is rectified by the diode 6 and smoothed by the capacitor 7, so that stable DC power is supplied to the power supply terminal 40 of the control circuit 4. The voltage generated in the third winding 9c of the transformer 9 is the off-state voltage of the switching element 2 and is substantially proportional to the output voltage. Therefore, if the output voltage is kept constant, the voltage of the third winding 9c is also kept substantially constant.

The operational amplifier 13 compares the DC voltage supplied to the load 12 with the first reference voltage 15 to cause the light emitting diode 8b of the photocoupler 8 to emit light. As a result, a control current flows through the light receiving transistor 8a of the photocoupler 8,
The output is input to the FB terminal 43 of the control circuit 4 and pulse width modulation control is performed. Therefore, the DC voltage supplied to the load 12 is kept constant.

FIG. 3 shows a detailed circuit of the control circuit 4. When a predetermined voltage is applied to the input terminal 40 of the control circuit 4, the input drop detection circuit 23 operates, the bias power supply 39 is turned on by the low level signal of the NOR gate 32, and the operation of the control circuit 4 is started. The triangular wave oscillator 24 generates a triangular wave for pulse width control shown by A in FIG. 4 and a reset signal of the flip-flop 25 for overcurrent latch shown by B in FIG. The pulse width control comparator 28 has a triangular wave oscillator 24, a voltage of the fifth reference voltage 29 (E in FIG. 4) that determines the dead time, a voltage of the FB terminal 43 (D in FIG. 4), and a soft start function. Capacitor 3
The voltage of 6 (C in FIG. 4) is compared to generate the pulse width control pulse (F in FIG. 4). Pulse width control comparator 2
The pulse width control pulse 8 is applied from the output terminal 41 to the gate of the switching element 2 through the NOR gate 22 (I in FIG. 4) and the amplifier 21, and the switching element 2 operates. The bias current of the photocoupler 8 connected to the FB terminal 43 flows through the diode 44 and the resistor 45.

When the photocoupler 8 is in the ON state, the comparator 37 operates to detect the voltage of the FB terminal 43 and the third reference voltage 38.
Is compared to limit the voltage of the capacitor 36 to a predetermined level. When the photo coupler 8 is off, the constant current source 3
The capacitor 36 is charged by 3 and the capacitor 36
Voltage rises. The comparator 27 is a capacitor 36
Is compared with the fourth reference voltage 26, and the capacitor 3
When the voltage of 6 is higher than the fourth reference voltage 26, a high level signal is output and the bias power supply 39 is turned off through the NOR gate 32. At this time, the control circuit 4 is kept off and the power source is kept off.

When a predetermined voltage is applied to the IS terminal 42, the comparator 30 is turned on (G in FIG. 4) and the flip-flop 25 is set (H in FIG. 4).
As a result, the output from the output terminal 41 is instantaneously cut off to perform overload protection. The flip-flop 25 is reset by the triangular wave oscillator 24 for each pulse.

When the load 12 is overloaded, the current flowing through the switching element 2 increases and is detected by the current detecting resistor 3. This detected current value is detected by the IS terminal 42 of the control circuit 4, and limits the ON drive width of the switching element 2. Therefore, the switching current decreases and the output voltage also decreases, and each element can be protected.

When the output voltage drops due to an overload condition, the voltage appearing in the third winding 9c also drops, so that FIG.
The power supply voltage of the control circuit 4 also decreases. When the power supply voltage of the control circuit 4 drops to a predetermined level, the control circuit 4 is turned off. Therefore, the power supply to the load 12 is stopped. When the power supply to the load 12 is stopped, the starting resistor 5 charges the capacitor 7 again. When the voltage of the capacitor 7 reaches the predetermined level of the control circuit 4, the control circuit 4 starts operating again, and the power supply is started in the overload state. However, as long as the overload state is maintained, the control power cannot be obtained from the third winding 9c of the transformer 9, so the power is turned off again, and the on / off operation is repeated thereafter.

[0009]

As described above, the control circuit 4 detects the overload by detecting that the photocoupler 8 is in the OFF state by the comparator 27, the NOR gate 32 and the bias power supply 39, and detects the power supply. On the other hand, the time constant is set by the capacitor 36 and the constant current source 33 while preventing the erroneous operation in the overload state such as the start-up. For this reason, the voltage of the power supply terminal 40 decreases before the control circuit 4 enters the cutoff state upon detection of the overload state, and thus the cutoff function does not work well. Furthermore, load 1
If a large capacitor is added to 2 or if a strong soft start function is activated, the third transformer 9
However, there is a drawback that the control power cannot be obtained from the winding 9c and the startup cannot be performed normally.

An improved circuit for eliminating the above drawbacks is shown in FIG. Among the reference numerals shown in FIG. 5, reference numerals 1 to 15 are the same as the reference numerals in FIG. When the switching element 2 is in the ON state, power is supplied to the power supply terminal 40 from the fourth winding 9d via the rectifier circuit 16a and the constant voltage circuit 20a. The rectifier circuit 16a includes a diode 16 and a capacitor 1.
The constant voltage circuit 20a includes a transistor 20 having a collector / emitter connected between the rectifier circuit 16a and the power supply terminal 40, and is connected between the base of the transistor 20 and the negative terminal of the DC power supply 1. And a resistor 18 connected between the collector and the base of the transistor 20. The operation of the circuit of FIG. 5 is substantially the same as that of FIG. 2, but only the way of taking the auxiliary power source is different. In FIG. 2, since a voltage substantially proportional to the output voltage is used as the control power supply when the switching element 2 is in the off state, the voltage is stable and no special constant voltage circuit is required. On the other hand, in FIG. 5, when the switching element 2 is in the ON state, a voltage substantially proportional to the input voltage is used as the control power supply, so that a stable control power supply can be obtained even in the case of overload, and the interruption function operates normally. is there. On the other hand, since the voltage changes in proportion to the input voltage, the constant voltage circuit 20a is required and there is a drawback that the constant voltage power source 20a has a large power loss.

Therefore, in order to eliminate the above-mentioned drawbacks, it is an object of the present invention to provide a DC-DC converter which can detect an overload and shut off the output of a control circuit reliably and efficiently.

[0012]

SUMMARY OF THE INVENTION DC-DC according to the present invention
The converter has a DC power supply, a switching element, and a first winding of a transformer connected in series, and a rectifying circuit is provided in the second winding of the transformer so that a DC current is supplied from the second winding to a load. Supply power. As a power supply of the control circuit for driving the switching element, there are provided a third winding for extracting a voltage when the switching element is in an off state and a fourth winding for extracting a voltage when the switching element is in an on state. It is provided in the transformer. In the embodiment of the present invention, a rectifier circuit including a diode and a capacitor is connected to the third winding and the fourth winding to supply power to the control circuit and connect to the fourth winding. A constant voltage circuit is connected between the rectifying circuit and the control circuit. The output voltage of the constant voltage circuit connected to the fourth winding via the rectifying circuit is smaller than the output voltage of the rectifying circuit of the third winding.

[0013]

When operating with the constant voltage output, the fourth winding is practically turned off, and the control power is efficiently supplied from the third winding to the power supply terminal of the control circuit. Further, when the overload is generated, the winding voltage is not generated in the third winding, and the voltage is stably applied to the power supply terminal of the control circuit by the winding voltage of the fourth winding.
The output of the control circuit can be reliably cut off.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a DC-DC converter according to the present invention will be described below with reference to FIG. In FIG. 1, the same parts as those shown in FIGS. 2 and 5 are designated by the same reference numerals, and the description thereof will be omitted.

As shown in FIG. 1, the switching element 2 is used as a power source of the control circuit 4 for driving the switching element 2.
A third winding 9c for extracting a voltage when is off,
The transformer 9 is provided with a fourth winding 9d for extracting a voltage when the switching element 2 is in the ON state. A rectifying circuit 6a including a diode 6 and a capacitor 7 and a rectifying circuit 16a including a diode 16 and a capacitor 17 are connected to the third winding 9c and the fourth winding 9d, respectively. Third
Power is supplied from the winding 9c and the fourth winding 9d to the power supply terminal 40 of the control circuit 4. The constant voltage circuit 20a is connected between the rectifier circuit 16a connected to the fourth winding 9d and the power supply terminal 40 of the control circuit 4. Rectifier circuit 1 on the fourth winding 9d
The output voltage of the constant voltage circuit 20a connected through 6a is
It is slightly smaller than the output voltage of the rectifier circuit 6a of the third winding 9c. The third winding 9c is set so as to generate a control voltage of a suitable level when operating with a constant voltage output. Further, the voltage of the fourth winding 9d is set to a level that can sufficiently supply the voltage to the control circuit 4 when the output is short-circuited.

As a result, when operating with a constant voltage output,
Control power is supplied to the power supply terminal 40 from the third winding 9c, and the fourth winding 9d is substantially turned off, which is efficient. Further, at the time of overload, the winding voltage of the third winding 9c disappears, but since the voltage is stably supplied to the power supply terminal 40 by the winding voltage of the fourth winding 9d, it is possible to ensure the reliability at the time of overload. In addition, the output of the control circuit 4 can be cut off.

[0017]

As described above, the output of the control circuit can be reliably cut off without lowering the power supply level of the control circuit at the time of overload.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a DC-DC converter according to the present invention.

FIG. 2 is a circuit diagram showing a conventional DC-DC converter.

FIG. 3 is a circuit diagram showing details of the control circuit shown in FIG.

FIG. 4 is a time chart showing the output of each part shown in FIGS. 2 and 3.

FIG. 5 is a circuit diagram showing another example of a conventional DC-DC converter.

[Explanation of symbols]

1 ... DC power supply, 2 ... Switching element, 4 ...
Control circuit, 6, 10, 16 ... Diode, 6a,
10a, 16a ... Rectifier circuit, 7, 11, 17 ... Capacitor, 9 ... Transformer, 9a ... First winding,
9b ... second winding, 9c ... third winding, 9d
..Fourth winding, 12 ... Load, 20a ... Constant voltage circuit,

Claims (4)

[Claims] 1. A DC power supply, a switching element, and a first winding of a transformer are connected in series, and a rectifier circuit is provided in the second winding of the transformer, and a load is applied from the second winding. In a DC-DC converter that supplies DC power to a third winding as a power source of a control circuit that drives the switching element, a third winding that extracts a voltage when the switching element is in an off state, and a switching element that is in an on state. A DC-DC converter characterized in that the transformer is provided with a fourth winding for extracting a voltage at times. 2. The D according to claim 1, wherein a rectifying circuit including a diode and a capacitor is connected to each of the third winding and the fourth winding to supply power to the control circuit.
C-DC converter. 3. The DC- according to claim 1, wherein a constant voltage circuit is connected between the rectifier circuit connected to the fourth winding and the control circuit. DC converter. 4. The output voltage of a constant voltage circuit connected to the fourth winding via a rectification circuit is smaller than the output voltage of the rectification circuit of the third winding, according to claim 3. DC-
DC converter.