Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2001-12-28
2004-09-07
Han, Jessica (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S021140, C363S126000
Reexamination Certificate
active
06788553
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of the Invention
The present invention relates generally to electrical power conversion, and more particularly, to switching-mode power converters having complementary synchronous rectification.
2. Description of the Related Art
DC-to-DC power converters are commonly used to convert power from one DC level to another.
FIG. 1
shows a simplified single-ended converter signified by the reference numeral
2
. The converter
2
includes a transformer
4
having a core
5
. There are primary and secondary windings
6
and
8
wound around the core
5
. The primary winding
6
is connected to a DC power source
3
through a switch
7
. Attached across the secondary winding
8
is an inductor
10
connected in series with a capacitor
12
via a rectifier
9
. In this case, the rectifier
9
is a diode
14
. There is also a free-wheel diode
15
connected across the secondary winding
8
as shown in FIG.
1
.
The operation of the converter
2
, in conjunction with some basic terms relating to rectification are herein described. Reference is now directed to
FIGS. 1 and 2
. Suppose the DC power source
3
supplies a DC voltage V
IN
. The switch
7
is turned on and off periodically. As a consequence, a periodic step-pulse voltage v
P
is generated and is applied across the primary winding
6
. In this specification, lower case alphabets are used to denote parameters that vary with time. For example, v
P
designates a time-varying voltage signal. During the first half-cycle t
1
(FIG.
2
), the primary voltage v
P
is positive, from basic electromagnetic theory, a positive secondary voltage v
S
is induced in the secondary winding
8
. The positive-going secondary voltage v
S
forward biases the rectifying diode
14
. Consequently, the capacitor
12
is charged via the forward-biased diode
14
through the inductor
10
. The resultant current path is denoted by the reference numeral
11
, as shown in FIG.
1
. The converter
2
is said to be in a forward rectification mode.
At the end of the half-cycle t
1
, the supply voltage v
P
begins to switch polarities and approaches the zero potential. However, at this juncture, the stored energy in the transformer
4
, such as in the windings
6
and
8
, releases and sends spurred signals of opposite polarity to original voltage v
S
. Take the secondary winding
8
as an example. The spurred signal is in the form of a spike
16
as shown in FIG.
2
. Phrased differently, in accordance with Lenz's law, the sudden cessation of current supply i
S
in the secondary winding
8
provokes the winding
8
to generate a voltage spike
16
of opposite polarity to that of the secondary voltage v
S
which occurred during the forward rectification mode. However, with the spike
16
having negative polarity impinging upon the secondary winding
8
, the diode
14
is reversely biased. At the same time, as is well known, inductors always maintain current continuity and attempt to sustain the original current flows. Thus, with the reverse-biased diode
14
acting as an open circuit, the stored energy in the windings
6
and
8
goes nowhere but as spurious current charging the parasitic elements in its path. The current discharge is in the form of a damped oscillation until all the stored energy is dissipated. The converter
2
is said to be in a resetting mode. The current path of the resetting mode is identified by the reference numeral
13
.
In the same manner as the windings
6
and
8
of the transformer
5
, at the end of the half-cycle at t=t
1
, the stored energy in the inductor
10
also releases itself. In this case, the discharge is through the capacitor
12
and the freewheel diode
15
. The inductor
10
is normally designed with a large inductive value. The freewheeling current normally continues until the onset of the next switching cycle. The path of the freewheeling current flow is identified by the reference numeral
17
shown in FIG.
1
. The converter
2
is said to be in a freewheeling mode.
Attention is now directed to the rectifier
9
in FIG.
1
. The diode
9
poses considerable Ohmic drop during the forward rectification mode. In operation, the p-n junction of the diode
9
can consume approximately 0.7 Volt of voltage level. To rectify this shortfall, attempts have been made to insert a Schottky diode
18
as a replacement for the regular diode
14
, as shown in FIG.
3
. Still, the Schottky diode
18
can adsorb close to 0.5 Volt of voltage level.
An efficient design of the converter
2
is to have the resetting current totally discharged swiftly and efficiently with minimal disturbance to the normal operation of the entire circuit
2
. A slow decay of the resetting current in comparison to the switching frequency of the switch
7
can distort the periodic waveform feeding the primary winding
6
, causing the “staircase-DC-bias” effect. The staircase-DC-bias effect is to be avoided and is especially crucial in modern-day switching mode power converter with compact sizes operating at high frequencies. There is still another undesirable effect for not efficiently discharging the resetting current. Specifically, if the resetting current is discharged through a high-impedance discharge path, excessive Joule heat can be generated. The generated heat not only undercuts the power efficiency by unnecessarily consuming power as wasteful heat. Excessive heat generated, if not properly controlled, can also detrimentally effect reliability.
Modern-day converter designs require compactness, low power consumption, and efficiency. For special applications such as high-speed data communications and computing, circuits are operated at very low voltage levels yet demanding high current outputs. Too high a voltage drop consumed by the converter is undesirable and sometimes impractical. To further curtail the Ohmic drop, FETs (Field Effect Transistors) have been adopted to substitute the diodes in the rectifying circuit
9
. As shown in
FIG. 4
, a FET
20
is disposed to take the place of the diode
14
. However, the FET
20
must be controlled by a control circuit
22
to provide proper timing signals to the FET
20
such that the FET
20
turns on and off appropriately. That is, the control circuit
22
has to operate in synchronization with the timing of the switch
7
(FIG.
1
). Accordingly, the rectifier
9
shown in
FIG. 4
is called a synchronous rectifier, and the process is called synchronous rectification. Due to the various operating modes as mentioned above, the control circuit
22
must operate with precise timing. If the FET
20
is turned on incorrectly, a circuit short may occur. Likewise, if the FET
20
is turned off at the wrong time, a unacceptable high voltage drop may result causing significant decline in operating efficiency and overheating.
The converter
2
shown in
FIG. 1
is a single-ended converter. For usage at high power levels, double-ended circuit schemes, such as push-pull, half-bridge, and full-bridge designs are common. Because of the relatively complex current traffic of the various modes of operation of the converter
2
as described above, providing a control circuit
22
with proper timing is quite elaborate. Heretofore, there has not been any practical scheme that works satisfactorily.
In light of the above, there is a need to provide efficient switching-mode power converters utilizing synchronous rectification.
SUMMARY OF INVENTION
It is accordingly the object of the invention to provide a DC-to-DC power converter with synchronous rectification that substantially optimizes current flows during the various modes of operation. It is also another object of the invention to provide such a converter at low cost and high operational efficiency.
The switching-mode power conversion circuit of the invention includes input and output circuits. A transformer having primary and secondary windings which are respectively coupled to the input and output circuits. The secondary winding includes two end terminals respectively connected to first and second switching circuits, whi
Fan Jian Ping
Jin Xiao Ping
Broadband Telcom Power, Inc.
Han Jessica
Tam Kam T.
LandOfFree
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