JP3386907B2 - Synchronous rectifier converter - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improved synchronous rectifier converter, and more particularly to a synchronous rectifier converter which can be used in parallel with another DC power supply. It is about. 2. Description of the Related Art Conventionally, a digital rectifier using a synchronous rectifier of this kind has been proposed.
As shown in FIG. 3, a C-DC converter converts a DC voltage of a DC input power supply 1 into a rectangular wave pulse voltage by a switching operation of a semiconductor switch 2, and converts this rectangular wave pulse voltage to a desired voltage by a transformer 3. Rectified and smoothed by a rectifying circuit of semiconductor rectifying elements (synchronous rectifying FET) 4 and (commutation FET) 5 and a smoothing circuit of choke coil 6 and capacitor 7 to obtain an average voltage. I try to take it out. The control of the switching operation of the semiconductor switch 2 is controlled by a voltage detection control circuit 8 for detecting the output voltage of the synchronous rectifier based on the detection status. (2) Generally, in the case of a synchronous rectifier circuit, a synchronous operation is performed using semiconductor switches as the rectifiers 4 and 5 as shown in FIG. In some cases, the present invention focuses on improving the conversion efficiency, but the former circuit, that is, the synchronous rectifying elements 4 and 5 are both field-effect semiconductor switches (F).
ET). Therefore, the semiconductor rectifier 4 is referred to as a synchronous rectifier FET 4 and the semiconductor rectifier 5 is referred to as a commutation FET 5. Since the commutation FET 5 cannot obtain a gate signal over the entire commutation period, a commutation diode 9 is usually added. Such synchronous rectifier converters are available in small to large capacities, and the synchronous rectifier converter according to the load capacity is selected and used. [0004] However, preparing a synchronous rectifier converter according to the load capacity increases the number of models, and requires inventory for each model. Therefore, it is desired to reduce the number of models from the viewpoints of design, production and article management, and a study is being made to cope with large-capacity loads by connecting the same models in parallel. When the synchronous rectifier converters are connected in parallel, or when another DC power supply (battery or the like) is connected in parallel, the following problems occur. That is, when another synchronous rectifier converter of the same type or an external DC power supply 12 such as a battery is connected in parallel to the output terminals 10 and 11 as described above, the other power supply connected in parallel is better. ,
The voltage becomes relatively higher than the output voltage of the synchronous rectifier converter, and the voltage detection control circuit 8 detects this and stops the operation of the semiconductor switch 2, or the synchronous rectifier converter operates in the same manner. When the power supply is stopped due to the operation of the protection device or the like, the external DC power supply 12 is connected to the output terminals 10 and 11.
Is supplied to the synchronous rectification FET 4 and the commutation FE.
This is applied to the gate of T5, and both FETs 4, 5
Means that the conduction state is continued. This is because when the FET (3) is used as a rectifying element and the gate bias is normal with respect to the source potential, the drain-source of both FETs 4 and 5 is increased.
Since current can flow in either direction between the two terminals, current may flow backward from the output terminal and eventually break down. In order to solve the above problem, we have previously proposed a gate signal circuit of a synchronous rectification FET and a commutation FET shown in FIG. 2 (Japanese Patent Application No. 5-148410). This content converts a DC input voltage into a rectangular wave pulse voltage by a switching element and applies it to a primary winding of a transformer. A desired voltage pulse taken out by a secondary winding of the transformer is synchronously rectified by a FET. A synchronous rectifier converter for rectifying and smoothing a DC voltage with an FET, a choke coil, a capacitor, and the like, and outputting the DC voltage. The synchronous rectifier FET operates depending on an external DC power supply connected in parallel to the synchronous rectifier converter. In the synchronous rectifier converter, the current value on the primary side or the secondary side of the transformer is detected, and when the detected current value exceeds a predetermined value, the commutation FET is activated. A switch circuit is provided to prevent the commutation FET from operating by an external DC power supply connected in parallel to the synchronous rectifier converter. In the configuration shown in FIG. 2, as a means for current detection, a pulse current on the primary side of the transformer is converted into a voltage by a current transformer 13 and a resistor connected to its output side. The place where the current transformer 13 is inserted may be a pulse circuit on the secondary side, and other current detecting means may include a method of replacing the current transformer with a low resistance instead of the current transformer. Since a descent occurs, the increase in loss is inevitable. An object of the present invention is to reduce the voltage of the external DC power supply 12 connected in parallel to the output terminals 10 and 11 to a voltage higher than the output voltage of the synchronous rectifier converter of the main body. Even when the synchronous rectification converter is inoperative, the synchronous rectification FET 4 and the commutation FET 5 are not turned on by the external DC power supply 12 (4) and the gate of the commutation FET 5 with a small loss is provided. The present invention provides a synchronous rectifier converter provided with a synchronous signal circuit. According to the present invention, a DC input voltage is converted into a rectangular wave pulse voltage by a semiconductor switch 2 and applied to a primary winding of a transformer. A synchronous rectification converter for rectifying and smoothing a pulse with a synchronous rectification FET 4, a commutation FET 5, a choke coil 6, and a capacitor 7. Synchronous rectifier converter that is configured not to be operated by the power supply 12
A switching circuit for detecting a voltage drop occurring between the source and the drain of the synchronous rectification FET, and activating the commutation FET when the detected voltage exceeds a predetermined value. It is a thing. FIG. 1 is a circuit diagram showing a first embodiment of the present invention.
FIG. 4 is an operation waveform diagram of each part, and the same parts as those in the conventional example are denoted by the same reference numerals. First, an operation state of the synchronous rectification FET 4 in the present embodiment will be described. Synchronous rectification F
Since the gate of the ET 4 is connected to the output side of the choke coil 6 connected to the negative polarity line, a normal rectangular wave pulse is output to the secondary side of the synchronous rectifier converter. Then, the synchronous rectification FET 4 operates normally. However, since the voltage of the external DC power supply 12 connected in parallel is relatively higher than the output voltage of the synchronous rectifier converter, the synchronous rectifier converter is not used.
When the external DC power supply 12 stops operating, even if the external DC power supply 12 is connected to the output terminals 10 and 11, the synchronous rectification FET 4 does not receive an appropriate positive polarity voltage at its gate. Is not working. Next, the operation of the commutation FET 5 will be described. In FIG. 1, the voltage shown in FIG. 4A appears on the secondary side of the transformer 3 with the on / off operation of the semiconductor switch 2. (5) The synchronous rectification FET 4 is turned on by the positive voltage shown in FIG. 4A. At this time, the voltage between the source and the drain of the synchronous rectification FET 4 is immediately after being turned on as shown in FIG. A relatively large spike-like voltage drop occurs, and thereafter, the voltage drops due to the source-drain on-resistance of the FET. The reason why the spike voltage is generated is that the voltage is applied to the source before the gate voltage of the synchronous rectification FET is sufficiently applied. Normal N channel F
It is well known that a pn junction is formed inside the FET between the source and the drain of the ET, but a synchronous rectification FET is connected so that current flows from the source to the drain. , Even if there is no signal at the gate,
Current flows through the junction. However, since the voltage drop at the pn junction is much larger than that caused by the on-resistance between the source and drain of the FET, the voltage drop is as shown in FIG. 4B until a sufficient voltage is applied to the gate. It appears as a spike-like voltage. Since the spike voltage in FIG. 4B is large enough to allow a current to flow through the base of the transistor 28, the voltage of the transistor 2 during the spike is generated.
8 conducts, and makes the voltage across the capacitor 27 of the switch circuit 23 zero. The resistor 25 charges the electric charge of the capacitor 27, and its value is substantially equal to the gate voltage of the switch element 26 (P-channel FET in the example in the figure) over one period.
Selected to keep the switch fully on. FIG. 4C shows the voltage of the capacitor 27 at this time. Thus, when the semiconductor switch 2 is turned on, the flyback voltage of the transformer is directly applied to the gate of the commutation FET 5. This state is shown in FIG. The voltage drop of the synchronous rectification FET 4 shown in FIG. 4B becomes smaller as the circuit current becomes smaller due to the capacitance component between the source and drain, etc., so that the spike-like voltage disappears as shown in FIG. 2
8 cannot operate. Therefore, the voltage of the capacitor 27 remains charged as shown in FIG. 4F, and the switch element 26 remains off. At this time, the commutation F
The voltage applied to the gate of ET5 continues the off state as shown in FIG. The purpose of the diode 24 is to prevent the voltage charged in the capacitor 27 from being discharged through the resistor 25 (6). In this state, the operation of the synchronous rectifier converter is detected by the voltage detection control circuit 8 when the voltage of the external DC power supply 12 becomes relatively high.
When the sending of the switching pulse to the semiconductor switch 2 is stopped, the secondary-side pulse current of the transformer 3 is also eliminated, and the source-drain voltage drop of the synchronous rectification FET 4 does not occur. Thus, the switch circuit 23 that controls the operation of the commutation FET 5 is turned off. in this way,
Control of ON / OFF state of commutation FET5 is synchronous rectification FET
4 is performed by the source-drain voltage drop, even if the external DC power supply 12 is connected to the output terminals 10 and 11, the commutation F
The backflow phenomenon in which current flows into ET5 does not occur. When the load of the synchronous rectifier converter is no load or commutation load, the gate signal of the commutation FET 5 is turned off, and no current flows through the commutation FET 5, but the commutation diode 9 is used. Commutation rectification flows in the forward direction of. Also, the resistor 1 in FIG.
8 is a commutation FE when no voltage is generated in the transformer 3.
It is provided for the purpose of preventing the gate circuit of T5 from becoming high impedance and malfunctioning. As described above, according to the present invention, the voltage drop of the synchronous rectifier FET of the synchronous rectifier converter is detected, and only when the detected voltage is equal to or higher than a predetermined value, the inverter is turned on. The synchronous rectifier FET is operated (the synchronous rectifier FET prevents backflow current of the external DC power supply by other means.) Depending on the external DC power supply connected in parallel to the synchronous rectifier converter, the synchronous rectifier FET and commutation FET are operated. Is not operated, enabling parallel operation of external DC power supply, and in particular, the same type of synchronous rectifier converter can be connected in parallel according to the load capacity. This has the effect of reducing the number of types of synchronous rectifier converters.

[Brief description of the drawings] (7) FIG. 1 is a circuit diagram of an embodiment of the present invention. FIG. 2 is a conventional circuit diagram. FIG. 3 is a conventional circuit diagram. FIG. 4 is an operation waveform diagram of each part of the circuit according to the embodiment of the present invention; [Explanation of symbols] 1 DC input power supply 2 Semiconductor switch 3 Voltage conversion transformer 4 Synchronous rectification FET 5 Commutation FET 6 choke coil 7, 21, 27 capacitors 8 Voltage detection control circuit 9 Diode for commutation 10, 11 output terminals 12 External DC power supply 13 Current transformer 14 Current detection control circuit 15,23 switch circuit 16, 28 transistor 17,26 Control FET 18, 19, 22, 25, 29 resistance 24 Diode

Claims (1)

(57) [Claim 1] A DC input voltage is converted into a rectangular wave pulse voltage by a switching element, applied to a primary winding of a transformer, and extracted by a secondary winding of the transformer. A synchronous rectification converter for rectifying and smoothing a desired voltage pulse with a synchronous rectification FET, a commutation FET, a choke coil, a capacitor, etc., and outputting a DC voltage, wherein the synchronous rectification FET is
Is a synchronous rectifier converter configured so as not to be operated by an external DC power supply connected in parallel to the synchronous rectifier converter, wherein a voltage drop between a source and a drain of the synchronous rectifier FET and a pn connection inside the synchronous rectifier FET are
Voltage drop in the synchronous rectification FET is detected.
During the period when the voltage drop of the n-junction appears, the synchronous rectification is performed.
Conducting the transistor connected to the source of the FET,
The voltage across the capacitor is set to 0, and the
Semiconductor with gate and drain connected to emitter and collector
Turn on the body switch and connect to the gate of the commutation FET.
A synchronous rectifier converter, comprising: a switch circuit that applies a flyback voltage of a transformer and activates the commutation FET when the detected voltage value becomes equal to or greater than a predetermined value.