JPH07118912B2 - Forward type DC-DC converter - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a DC-DC converter, and in particular, a suitable forward type DC-applying to a one-stone forward type switching power supply.
Regarding DC converter.

[Conventional technology]

As a conventional protection circuit for this type of converter, there is one described in, for example, "Designing Method of Switching Regulator and Usage of Power Device" (published by Seibundo Shinkosha, Susumu Shiatsu and Jiro Togawa). One of those shown here is a configuration in which a triangular wave oscillating wave and a DC voltage (threshold level) designating a lower limit value are added to a comparison circuit to determine the maximum value of the pulse width (hereinafter referred to as dead time). However, in such a circuit, the required dead time varies over a wide range with respect to variations in the converter section components and variations in the input voltage, so that it is possible to set only a wide dead time value with a sufficient margin. Therefore, in the conventional circuit example, the variable region of the control pulse width is narrowed, and there is a problem that the output voltage cannot be stabilized accurately over a wide range.

[Problems to be solved by the invention]

The above-mentioned prior art does not consider that the variable range of the control pulse width is narrow and the output voltage is stabilized in a wide range with high accuracy. For example, if the dead time is made narrower than necessary in order to increase the control pulse width in the one-stone forward converter, the residual energy of the main transformer cannot be released during the off period of the main switch, and every time the main switch becomes conductive. , Residual energy is accumulated, and finally the main transformer is saturated. When saturated, there was a problem that an excessive current would flow through the transistor of the main switch and the winding of the main transformer, causing burnout.

An object of the present invention is to provide a forward type DC-DC converter which can obtain a stable output voltage even when there are variations in components and variations in input voltage, and does not saturate the transformer.

[Means for solving problems]

The purpose of the above is to detect a voltage (hereinafter, referred to as a reset voltage) that releases the residual energy of the main transformer by providing an error amplifier that compares and detects the voltage generated across the main switch and the input voltage, and detects the detected reset. This is achieved by providing the drive circuit with a semiconductor auxiliary switch that closes the main switch only during the voltage generation period.

[Action]

The voltage across the main switch when the main switch is off is the composite of the input voltage and the reset voltage of the main transformer. Therefore, the error amplifier detects a reset voltage obtained by subtracting the input voltage from the combined voltage, turns on the semiconductor switch with the detected voltage, and turns off the drive circuit with the on signal. As a result, the main switch is closed and the main transformer is not magnetically saturated.

〔Example〕

An embodiment of the present invention will be described below with reference to FIGS. 1 and 3 show a forward type DC according to the present invention.
FIG. 2 and FIG. 4 are examples of a DC converter.
The figure shows the switch waveform of each part in each embodiment.

In FIGS. 1 and 2, 1 is a reset diode for discharging the residual energy of the main transformer 2, 2 is a main transformer for power conversion, 3 is a main switch by a power MOS FET, 4 is a rectifying diode, 5 Is a rectifier diode (flywheel diode) that releases the energy stored in the choke transformer 6 to the output, 6 is a choke transformer for supplying a current to the output when the main switch 3 is off, 7 is a smoothing capacitor, and 8 is a rectangular wave. A pulse width control circuit section including an oscillator, 9 is a drive circuit section of the main switch 3, 10 is an error amplifier for detecting the reset voltage V _r , 11 to 14 are an input voltage V _C and a voltage V _DS across the main switch. A resistor for level conversion, 15 is a transistor that operates based on the signal voltage V _B in the reset generation period detected by the error amplifier 10, and 16, 17
Is a transistor that drives the main switch 3, 18 is an error amplifier that detects the deviation voltage between the specified output voltage and the output voltage V _out as a converter, V _C is the input voltage, V _out is the output voltage, and V _B is the error amplifier 10 Signal voltage of reset occurrence time detected from V _DS, voltage across main switch 3, V _r is reset voltage of main transformer 2, _Tr is occurrence time of reset voltage V _r , N _{1 to} N ₃ are main transformers 2 primary, secondary and tertiary windings, T
_OFF and T _ON indicate the time widths when the main switch 3 is turned on and off, respectively.

In the above configuration, when the main switch 3 is turned on, the input voltage V _C is applied to the primary winding N ₁ of the main transformer 2,
The rectifier diode 4 connected to the secondary side of the main transformer 2 becomes conductive because a forward voltage is applied, and supplies a current to the output terminal. Further, when the main switch 3 is turned off, the rectifier diode 4 is applied with a voltage in the reverse direction, so that the main switch 3 is cut off and power transmission through the main transformer 2 is stopped. However, at the moment when the main switch 3 is turned off, a counter electromotive force is generated in the choke transformer 6 and a current is supplied to the output terminal.
In this operation, when the main switch 3 is turned on, the current flowing through the primary winding N ₁ of the main transformer 2 is the current proportional to the load current and the exciting current for exciting the inductance of the primary winding N _1. Of the exciting current flows, and a component proportional to the load current is transmitted to the secondary side, but the component of the exciting current is directly stored in the main transformer 2 and becomes residual energy. When the stored energy due to this exciting current remains, the next cycle starts, and when the main switch 3 is turned on again, the primary winding N
_The input voltage V _C is applied to ₁ , the residual energy is further accumulated, and the main transformer 2 is finally saturated. When saturated, the main transformer 2 becomes an air-core coil, so the inductance of the primary winding N ₁ becomes a very small value, an excessive current flows, and the main switch 3 and the winding of the main transformer 2 are burned out. . In order to prevent this, a tertiary winding N _{3 in} which a voltage is piled up is provided in the primary winding N ₁ , and when the main switch 3 is turned off, the capacitor 19 on the input side is passed from this tertiary winding N ₃ through a reset diode. By letting residual energy escape, it is possible to prevent saturation of the main transformer 2. Here, since the voltage of the tertiary winding N ₃ is clamped by the input voltage V _{C, the} voltage induced in the winding of N ₁ is
V _r is Therefore, the voltage across the main switch 3 is a combination of the reset voltage V _r and the input voltage V _C. Further, when the main switch 3 is turned off, the amount of change in the magnetic flux of the main transformer 2 is equal, so that V _r × T _r = V _C × T _ON (2) Further, the reset time T _r needs to satisfy the condition of T _r <T _OFF ………… (3).

The problem under these conditions is that, for example, when the input voltage V _C abnormally drops, the DC-DC converter is in an operation mode in which a constant voltage is obtained, so T _ON spreads, and eventually V _C
Although × T _ON has a substantially constant value, the reset time T _r does not change because it is determined by the number of primary windings N ₁ and the number of tertiary windings N ₃ of the main transformer 2. Therefore, the condition of the expression (3) cannot be satisfied, and the main transformer 2 is saturated.

Therefore, with the protection circuit according to the present invention, the reset time
By detecting T _r and closing the main switch 3 with the detected signal voltage V _B , the formula (3) is surely satisfied.

That is, the voltage obtained by converting the level of the input voltage V _C with the resistors 11 and 12 is used as a reference voltage, and the reset voltage V _r and the input voltage
The voltage V _DS across the main switch 3, which is a composite of V _C, is
The generation time width T _r of the reset voltage is detected by applying the voltage level-converted in 4 to the error amplifier 10. Detected T _r turns on the transistor 15 which is newly provided, drop the output voltage from the pulse width control unit to the vicinity of 0V, the drive circuit as well as to close the main switch 3. This allows
Burnout of the main switch 3 and the main transformer winding can be prevented.

Further, according to another embodiment shown in FIGS. 3 and 4, by detecting the voltage of the tertiary winding N ₃ of the main transformer 2,
This is a method of directly detecting the reset time width, and other configurations and operations are the same as those described with reference to FIGS. In addition, the newly installed transistor 15
In principle, by using a MOS FET having no accumulated charge, the main switch 3 can be closed in the absence of storage, so that high-speed switching can be supported.

When the recovery time of the rectifier diode 4 is long, the rectifier diode 4 is short-circuited during the recovery, the main transformer 2 is clamped, and the reset may be delayed for a moment. In particular, according to the present invention, the recovery time is directly detected and controlled even for such unpredictable component variations, which vary greatly depending on the type of diode and have large variations. Therefore, it does not saturate the main transformer.

In the above description of the embodiment, the reset detection of the main transformer 2 is described as the potential of the transformer winding is detected by the error amplifier and the resistor. You may replace with the method.

〔The invention's effect〕

According to the present invention, the reset time can be secured even if the input voltage is abnormally lowered or the parts are varied, and the main switch and the winding of the main transformer can be prevented from being burned.

[Brief description of drawings]

FIG. 1 is a circuit diagram of the first embodiment of the present invention, and FIG.
FIG. 3 is a switch waveform diagram of each part in the embodiment of the figure, FIG. 3 is a circuit diagram of the second embodiment, and FIG. 4 is a switch waveform diagram of each part in FIG. 1 ... Reset diode, 2 ... Main transformer for power conversion, 3 ... Power MOS FET, 4 ... Rectifier diode,
5 ... Rectifying diode, 6 ... Smoothing choke transformer, 7, 19 ... Capacitor, 8 ... Pulse width control unit, 9
...... Drive circuit section, 10,18 …… Error amplifier, 11 to 14…
… Resistors, 15 to 17… Transistors.

Claims (1)

[Claims] 1. An error amplifier for detecting a deviation voltage between a specified output voltage and an output voltage as a converter, and a pulse signal whose pulse width is variably controlled based on the detected deviation voltage from the error amplifier. A pulse width control circuit,
A drive circuit driven by a pulse signal from the pulse width control circuit, a main switch switching-controlled by the drive circuit, and a DC input voltage connected to the main switch as a load and applied to a primary winding. Based on the main winding for generating an AC voltage in the secondary winding, and the tertiary winding connected in series with the primary winding of the main transformer,
A reset diode for discharging the residual energy of the main transformer, a rectifying and smoothing circuit for rectifying and smoothing the output of the secondary winding, and outputting an output voltage as a converter, and a voltage across the main switch. An error amplifier that detects the reset voltage of the main transformer by subtracting the DC input voltage, and the main switch that is provided in the preceding stage of the drive circuit and is in the ON state by the detection reset voltage from the error amplifier. A forward-type DC-DC converter consisting of a semiconductor auxiliary switch for forcibly keeping the switch closed.