JPH04121056A - Converter - Google Patents

Abstract

PURPOSE:To enable use of parts with low dielectric strength voltage characteristics and to reduce the cost, by providing a clamping circuit in a self-reset-type converter, and by suppressing the rise of resonance voltage in transient conditions such as a load short circuit, etc. CONSTITUTION:A clamping circuit 4 is composed of a diode D3 and a zener diode ZD, and is connected to the secondary winding of a transformer 1. The clamping circuit 4 does not clamp resonance voltage when normal, but clamps the induced voltage of the secondary winding beginning to rise by the resonance voltage in transient conditions where a switching element (transistor) 2 has a maximum 'on' width by a load short circuit etc. It also clamps the resonance voltage on the primary winding side, and makes it possible to protect the switching element (transistor) 2 and a diode D1 of a rectifying and smoothing circuit 3. Namely, it becomes possible to use parts not having high dielectric strength characteristics.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a self-resetting converter.

A converter excites the primary winding of a transformer with direct current through a switching element such as a transistor, and rectifies and smoothes the induced voltage in the secondary winding. For example, in the case of a one-stone forward type, the transformer In order to avoid magnetic saturation, a configuration with a reset winding was common. However, by providing a reset winding in the transformer, it has been difficult to reduce the size of the transformer. Therefore, a self-resetting type is adopted in which resonant current is used to perform resetting using a resonant circuit including the excitation inductance and capacitance of the transformer and the output capacitance of the switching element.

The withstand voltage of each part in such a self-resetting converter is a problem, and there is a desire to reduce costs by making it possible to use parts with low withstand voltages.

(Prior Art) A conventional self-resetting one-stone forward converter has, for example, the configuration shown in FIG.
A transistor (switching element) 2 such as a bipolar transistor or a field effect transistor is connected to the primary winding of the transformer 1, and this transistor 2 is controlled on and off by a control circuit 5 to receive direct current from a direct current power source 6. The voltage induced in the secondary winding of the transformer 1 is rectified and smoothed by the rectifier and smoothing circuit 3, and a stabilized DC voltage is supplied to the load 7. The rectifying and smoothing circuit 3 includes diodes Di, D2, an inductance L, and a capacitor C.
This shows the case where it is composed of.

When the transistor 2 is turned on, current flows from the DC power supply 6 to the primary winding of the transformer 1, and when the transistor 2 is then turned off, the primary side excitation inductance Lm of the transformer 1
A resonant circuit consisting of the capacitance Cm of the primary winding, the output capacitance Csw of the transistor 2, etc. generates a resonant voltage due to the stored energy of the transformer 1, and the resulting current flows to the primary winding of the transformer 1. , transformer 1 is reset. In this case, when a field effect transistor is used as a switching element, its output capacitance Cs
Since w is relatively large, it is easy to form a resonant circuit.

The voltage Vsw applied across the transistor 2 in such a self-resetting converter is as shown in FIG. 4, for example. That is, the switching period T of the transistor 2 by the control circuit 5 is constant, and the on period T
On is controlled to stabilize the DC voltage output through the rectifier and smoothing circuit 3. After the transistor 2 is turned off, a resonance voltage Vr that reaches its maximum value occurs after a period of Tr, and this resonance voltage A current due to Vr flows through the primary winding, and the transformer 1 is reset.

Therefore, the reset winding can be omitted, and loss in the reset loop including the reset winding is eliminated, making it possible to realize a compact and highly efficient converter. Furthermore, since it is possible to extend the on-period Ton to a period of (T-Tr), it is possible to stabilize the output DC voltage supplied to the load 7 even if the input voltage on the primary side of the transformer 1 is varied over a wide range. It becomes possible.

[Problem to be solved by the invention]

As the on-period Ton of the transistor 2 increases, the resonant voltage Vr for self-resetting increases; however, under normal control conditions, the resonant voltage Vr should not exceed the withstand voltage of the transistor 2 and the diode D1. An on period Ton is set in .

However, in a transient state where the load 7 is short-circuited, the on-period Ton of the transistor 2 reaches its maximum width in order to maintain the output DC voltage at the set value, and as a result,
The resonant voltage Vr may rise to several times the primary side input voltage of the transformer 1, and a voltage corresponding to the turns ratio is also induced in the secondary winding of the transformer l. Therefore, in order to prevent the transistor 2 and diode D1 from being destroyed, it is necessary to use a high voltage transistor 2 and diode D1, which causes an increase in cost.

The present invention aims to reduce costs by suppressing the maximum value of the resonant voltage during load short-circuits and the like.

[Means for Solving the Problems] The converter of the present invention suppresses the maximum value of the resonant voltage using a clamp circuit, and will be described with reference to FIG. 1.

A switching element (for example, a transistor) 2 is connected to the primary winding of the transformer l, the on-width is controlled by pulse width control by a control circuit 5, and a rectifying and smoothing circuit 3 is connected to the secondary winding of the transformer 1. The output DC voltage is supplied to the load 7, and the primary-side equivalent combined capacitance of the transformer 1, which includes the capacitance of the winding of the transformer 1 and the output capacitance of the switching element (transistor) 2, is In a self-resetting converter that resets the transformer 1 by resonance with the primary side excitation inductance,
Reset 7 in which BH occurs in a transient state such as when the on-width of the switching element (transistor) 2 exceeds the setting, such as when a load is short-circuited.
- A clamp circuit 4 is provided to clamp the overvoltage applied to the switching element (transformer 1; star) 2 and rectifying and smoothing circuit 3 at the time of operation.

1 Effect] The clamp circuit 4 does not clamp the resonant voltage during normal operation, but in a transient state where the switching element (transistor) 2 reaches its maximum ON width such as when a load is short-circuited, the resonant voltage is The induced voltage in the secondary winding of the transformer 1 also rises, so if the clamp circuit 4 is connected to this secondary winding as shown in the figure, the induced voltage in the secondary winding, which tends to rise due to the resonant voltage, will increase. This will clamp the induced voltage.

As a result, the resonant voltage on the primary winding side is also clamped, and the switching element (transistor) 2 and the diode D1 of the rectifying and smoothing circuit 3 can be protected. That is, it can be used even if it is not a high-voltage component, and costs can be reduced.

Embodiment 1: An embodiment of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a circuit diagram of a main part of an embodiment of the present invention, where the same reference numerals as in FIG.
The case where it is connected to the secondary winding of transformer 1 is shown. The Zener diode ZD of this clamp circuit 4 has a Zener voltage higher than the induced voltage of the secondary winding of the transformer 1 during normal operation, and the diode D3 has a polarity such that forward current of the Zener diode ZD does not flow. connected with.

The induced voltage in the secondary winding of the transformer 1 due to the resonant voltage for resetting the transformer 1 during normal operation is in the forward direction of the diode D3, but since it is less than the Zener voltage of the Zener diode ZD, the clamp circuit 4 It will not work.

However, in a transient state when the load 7 is short-circuited, the ON width of the transistor (switching element) 2 becomes maximum, and the resonant voltage increases accordingly when the transistor (switching element) 2 turns OFF. , the induced voltage in the secondary winding of the transformer 1 also increases. Even if the induced voltage in that case exceeds the Zener voltage of the Zener diode I''ZD, a current flows through the Zener diode ZD and the diode D3, and the induced voltage of the secondary winding is increased by the Zener voltage and the diode D3. The voltage will be clamped to the forward voltage.

By clamping the induced voltage of the secondary winding of the transformer 1 by the second clamp circuit 4, the voltage of the primary winding of the transformer 1 is clamped to a value corresponding to the turns ratio, thereby clamping the resonance voltage. I can do it. Therefore, the transistor (switching element) 2 can be protected, and the diode D1 of the rectifying and smoothing circuit 3 can be protected. That is,
It becomes possible to use the transistor (switching element) 2 and diode D1 with low breakdown voltage. Further, since only a demagnetizing current corresponding to the excitation current of the transformer 1 flows through the clamp circuit 4, it is sufficient that the diode D3 and the Zener diode ZD have a relatively small current capacity.

FIG. 2 is a diagram of the applied voltage waveform of the transistor according to the embodiment of the present invention, in which the voltage Vsw applied to the transistor 2 is measured when the load 7 is short-circuited immediately after time L1 in a normal state before time L1. FIG. That is, the applied voltage Vs
w is zero during the on period, and takes a value formed by the input voltage and the resonance voltage during the off period.

When the load 7 is short-circuited immediately after time L1, the ON width of the transistor 2 becomes maximum (the period when the applied voltage Vsw is zero is the maximum), and the resonant voltage increases correspondingly, but the clamp circuit 4 lowers the applied voltage Vsw to Vc. is clamped. Therefore, even if the transistor 2 has a low breakdown voltage, it will not be destroyed.

On the other hand, in the conventional example in which the clamp circuit 4 is not provided, the applied voltage Vsw of the transistor 2 near time t2 after the switching period T from time to further increases, and in the case of a low voltage transistor, it is destroyed. will be done.

In the second embodiment described above, the clamp circuit 4 is connected to the secondary winding of the transformer l, but if a tertiary winding is provided, the clamp circuit can also be connected to this tertiary winding. It is also possible to connect a clamp circuit to the primary side. Further, the clamp circuit 4 can use various configurations other than the above-mentioned embodiments. For example, it is also possible to connect a current limiting resistor in series with the Zener diode ZD'.

〔Effect of the invention〕

As explained above, the present invention provides a clamp circuit 4 in a self-recent type converter to clamp the converter so as to suppress an increase in resonant voltage in a transient state such as a load short circuit. Thereby, even when the switching element 2 reaches its maximum ON width, the resonant voltage applied to the switching element 2 can be suppressed and the switching element 2 can be protected from high voltage. Also, the diode D1 of the rectifying and smoothing circuit 3 connected to the secondary winding of the transformer 1,
Can be protected from high voltage. Therefore, it is possible to use the switching element 2 and the diodes DI and D2, which are low in voltage and inexpensive, so that there is an advantage that the cost of the converter can be reduced.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of the main part of the embodiment of the present invention, Fig. 2 is a voltage waveform diagram of the applied voltage of the transistor of the embodiment of the invention, Fig. 3 is a circuit diagram of the main part of the conventional example, and Fig. 4 is the voltage waveform. It is an explanatory diagram. 1 is a transformer, 2 is a switching element, 3 is a rectifier and smoothing circuit, 4 is a clamp circuit, 5 is a control circuit, 6 is a DC power supply,
7 is a load, Di, D2. D3 is a diode, and ZD is a Zener diode. Patent applicant: Fujitsu Denso Co., Ltd. Representative patent attorney: Shoji Aitani

Claims (1)

[Claims] A switching element (2) is connected to the primary winding of the transformer (1), a rectifying and smoothing circuit (3) is connected to the secondary winding, and the electrostatic charge of the winding of the transformer (1) is A converter that resets the transformer (1) by resonance between a primary-side equivalent combined capacitance including a capacitance and an output capacitance of the switching element (2), and a primary-side excitation inductance of the transformer (1). In this case, the overvoltage applied to the switching element (2) and the rectifying and smoothing circuit (3) at the time of reset in a transient state such as a load short circuit in which the ON width of the switching element (2) exceeds a setting is A converter characterized in that a clamp circuit (4) for clamping is provided.