Electric power conversion systems – Current conversion – Including automatic or integral protection means
Reexamination Certificate
2002-08-14
2004-08-17
Riley, Shawn (Department: 2838)
Electric power conversion systems
Current conversion
Including automatic or integral protection means
C363S021060, C363S126000, C363S127000, C361S087000
Reexamination Certificate
active
06778412
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to DC-DC converters with synchronous rectifiers, for use in, for example, switching power supply apparatuses and other suitable apparatuses.
2. Description of the Related Art
As is commonly known, DC-DC converters incorporated in switching power supply apparatuses are used to convert a DC input voltage Vin into an AC voltage in response to the switching operation of a switching element, e.g., a MOS-FET (metal oxide semiconductor field-effect transistor), rectify and smooth the AC voltage by a rectifying/smoothing circuit, and output a DC voltage Vout to a load. In these DC-DC converters, the output voltage Vout can be variably controlled by controlling the switching operation of the switching element. In other words, the ratio of the input voltage Vin to the output voltage Vout (input-output conversion ratio) is determined by the switching operation of the switching element. The output voltage Vout is detected and the switching operation of the switching element is controlled on the basis of the detected voltage so that the output voltage Vout can be regulated to a preset voltage. Recently, an increasing number of DC-DC converters using a synchronous rectifier as a rectifier in the rectifying/smoothing circuit to reduce the conduction loss have been used.
Due to a sudden increase in the input voltage Vin or a decrease in the current flowing in the load, a voltage (overshoot voltage) that is greater than the output voltage Vout, which is supplied from the DC-DC converter to the load, may be applied at the output side of the DC-DC converter.
In such a case, due to the application of the overshoot voltage, a smoothing capacitor of the rectifying/smoothing circuit is charged by the overshoot voltage. Subsequently, when the application of overshoot voltage is removed, the voltage at the output side of the DC-DC converter returns to a steady voltage. At this moment, the charge in the charged smoothing capacitor is released. In response to the application of the overshoot voltage, the switching operation of the switching element is controlled to reduce the output voltage Vout. As a result, a voltage obtained by multiplying the input voltage Vin by the input-output conversion ratio becomes smaller than the voltage across the smoothing capacitor. Thus, the charge released by the smoothing capacitor flows toward the input side of the DC-DC converter, thus generating a reverse current flowing from the output side to the input side of the DC-DC converter.
The amount of reverse current flowing becomes very large even when the overshoot voltage is only slightly higher than the steady output voltage Vout. Many problems may occur due to this large reverse current flowing.
Such a large amount of reverse current flows because the DC-DC converter with the known synchronous rectifier has regulation characteristics such as those shown in FIG.
12
A. Specifically, the known DC-DC converter for rectifying/smoothing a current by the synchronous rectifier and for outputting a DC output voltage Vout has, as shown in
FIG. 12A
, regulation characteristics in which the amount of change in the output voltage Vout in the increasing direction (slope) is gradual compared with the amount of change in the output current in the decreasing direction, even in a reverse current flowing region. In a region where the output current shown in
FIG. 12A
is positive (+), the output current flows from the input side to the output side of the DC-DC converter. A region where the output current is negative (−) is the reverse current flowing region in which the reverse current flows from the output side to the input side of the DC-DC converter.
Since the known DC-DC converter has regulation characteristics such as those shown in
FIG. 12A
, for example, the application of an overshoot voltage Vx, which is very slight, at the output side of the DC-DC converter causes a high reverse current Ix to flow through the DC-DC converter.
Since this high reverse current flows, a high stress is placed on the components of the DC-DC converter, and the components may be damaged. In a DC-DC converter including a transformer, a rectifying/smoothing choke coil, and a synchronous rectifier, a large amount of electromagnetic energy generated by the reverse current flowing in a period during which the switching element is ON is accumulated in the choke coil and the transformer. When the switching element is turned OFF, a large voltage due to the accumulated energy is applied to the switching element and the rectifying/smoothing synchronous rectifier, and the switching element and the synchronous rectifier may be damaged. The components of the DC-DC converter may be damaged by the reverse current flowing.
FIG. 11
shows a parallel running configuration including a plurality of DC-DC converters (two in
FIG. 11
, namely, DC-DC converters A and B) connected in parallel to a load. When such parallel running is performed, the parallel-connected DC-DC converters A and B may have different output voltages Vout. In such a case, a reverse current flows from the DC-DC converter A that has a higher output voltage Vout toward the DC-DC converter B that has a lower output voltage Vout. Due to the reverse current, a circulating current is generated between the DC-DC converters A and B.
For example, the DC-DC converter A that has the higher output voltage Vout has regulation characteristics indicated by the solid line A in
FIG. 12B
, and the DC-DC converter B that has the lower output voltage Vout has regulation characteristics indicated by the solid line B in FIG.
12
B. The parallel-running DC-DC converters A and B, as a whole, supply a current Ic to the load. In this case, a reverse current Ib based on the output voltage Vout of the DC-DC converter A flows from the DC-DC converter A to the DC-DC converter B having the lower output voltage Vout. Due to this reverse current flowing, the DC-DC converter B has a loss Ib.
In contrast, the DC-DC converter A is required to output a current Ia (Ia=Ib+Ic) in order to compensate for the loss Ib due to the reverse current flowing relative to the amount of current supplied to the load Ic. As a result, the amount of current flowing in the DC-DC converter A is increased, and the loss is increased. As described above, when parallel running is performed, if the parallel connected DC-DC converters A and B have different output values Vout, the DC-DC converter A having the higher output voltage and the DC-DC converter B having the lower output voltage both have an increased amount of loss. As a result, the circuit efficiency is deteriorated.
SUMMARY OF THE INVENTION
In order to overcome the problems described above, preferred embodiments of the present invention provide a DC-DC converter which has a synchronous rectifier and which is capable of minimizing the amount of reverse current flowing, preventing the components from being damaged by the reverse current flowing, and preventing an increase in loss due to the reverse current flowing while parallel running is performed.
According to a preferred embodiment of the present invention, a DC-DC converter for converting an input voltage at an input-output conversion ratio and for outputting the converted voltage to a load includes a switching element, the input voltage being converted in response to the switching operation of the switching element, and the input-output conversion ratio being determined by the switching operation of the switching element, a choke coil through which the converted voltage is applied to the load, a synchronous rectifier, a reverse current detector for detecting a reverse current flowing from the output side to the input side of the DC-DC converter, and a reverse current suppressor for suppressing the amount of reverse current flowing when a reverse current is detected. The choke coil has a function of increasing the inductance when the current is within a current operating range that is less than or equal to a predetermined value and decre
Matsumoto Tadahiko
Nagai Jun
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
Riley Shawn
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