Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2001-05-04
2003-05-06
Sterrett, Jeffrey (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S098000
Reexamination Certificate
active
06560127
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to power conversion circuitry and, more particularly, to an apparatus having an efficient power conversion to accomplish zero voltage switching using a novel switch timing technique.
BACKGROUND OF THE INVENTION
A known full bridge forward DC-to-DC converter having zero voltage switching typically includes a DC-to-AC converter circuit and an AC-to-DC converter circuit linked together by a high frequency AC link, where isolation is provided on the intermediate AC link. This type of converter is a common circuit topology used to transform electric energy from a source at a given potential to a destination load at a different potential. It typically includes four switches, typically power metal-oxide semiconductor field-effect transistors (MOSFETs), operated in alternating pairs, an input/output isolation and step-up/step-down transformer, an output rectifier, and an output filter. A feedback regulator or controller is included to control the switches.
The main advantages of this converter topology include: constant frequency operation, which allows optimum design of magnetic filter components, pulse width modulation (PWM) control, minimum voltage and current (VA) stresses, and good control range and controllability. Power converters are typically employed in applications that require conversion of an input DC voltage to various other DC voltages, higher or lower than the input DC voltage. Examples include telecommunications and computer systems wherein high voltages are converted down to lower voltages needed to operate the systems. Power converters generally suffer from problems such as switching losses, switching noise and common-mode power transformer noise. Switching losses reduce system efficiency, resulting in greater input power requirements for the same output power. Switching and transformer noise, both conducted and radiated, require filtering to prevent or reduce interference with other sensitive electronic equipment.
When switching devices turn on and off, there is a power loss associated with this action. The power loss relates to the current through the switch and the voltage across the switch during the switching transition. The greatest loss is associated with the turn on of the switch. Zero voltage switching, however, provides a means for eliminating switching losses particularly in higher line voltages. The resulting converter will be more efficient by dissipating less heat. Zero voltage switching is achieved by adding a controlled dead time at the turn on of each stage.
A full bridge converter of this type operates generally as follows. The switches are arranged in two diagonal pairs that are alternately turned on for a fraction of a switching period to apply opposite polarities of the input DC voltage across the primary of the transformer. The operation of the switches produce a zero voltage across the transformer by turning off only one switch of the pair. A switch from the alternate pair is then turned on, allowing the current in the primary circuit to circulate at zero voltage through the two switches. The two switches clamp the voltage across the transformer at zero, thereby eliminating the ringing behavior suffered by the conventional bridge when the switches are off. Thus the switches operate to convert the input DC voltage into an AC voltage required to properly operate the transformer.
Different schemes have been developed to reduce the additional switching losses caused by high frequency switching of conventional converters. For example, semiconductor switching losses can be reduced by using reactive snubber elements. In
FIG. 1. a
first snubber circuit is implemented in zero voltage switching converter
40
. As illustrated, a snubber capacitor
64
may be connected in parallel with a converter semiconductor switch
56
, having an anti-parallel connected diode
60
. This snubber element
64
tends to limit the rate of rise of voltage experienced by the switching device
56
. Thus, snubber element
64
provides an easy method to divert the energy that would be dissipated in the switching device
56
during switching. However, the energy stored in the snubber element
64
needs to be dissipated during a subsequent part of the switching cycle. Each converter semiconductor switch
57
-
59
are connected in parallel with a snubber capacitor
65
-
67
and in anti-parallel with a diode
61
-
63
, respectively.
Converters that allow lossless resetting of the reactive snubber energy are referred to as “soft-switching” converters. Soft-switching converters may be broadly categorized as zero voltage switching. Various zero voltage switching schemes and converter topologies have been proposed in an attempt to achieve increased performance over conventional hard-switching converters. Many are disclosed in U.S. Pat. No. 5,781,419 which is incorporated herein.
An exemplary known soft-switching converter circuit topology is the full-bridge PWM converter shown at
40
in FIG.
1
. This converter topology
40
achieves PWM control with resonant switching of the converter semiconductor switches. The basic DC-to-DC converter circuit topology
40
includes an input side circuit
42
and an output side circuit
44
with the input circuit
42
and output circuit
44
linked by a transformer
46
. The transformer
46
includes a primary winding
48
, a secondary winding
50
, and is characterized by a leakage inductance
52
. The primary
48
of the transformer
46
is connected to a DC input voltage source
54
by a bridge of converter switches that forms the input circuit
42
. Four semi-conductor switching devices
56
-
59
, e.g., transistors, form the input side circuit
42
converter bridge. Each switching device
56
-
59
includes an anti-parallel connected diode
60
-
63
and parallel connected capacitor
64
-
67
. The output side circuit
44
connects the secondary winding
50
of the transformer
46
to a load, shown here as a resistance load
68
, by a diode bridge including four diodes
70
-
73
. An output side filter inductor
74
is connected in series between the diode bridge and the load
68
. An output side capacitor
76
is connected in parallel with the load
68
. In operation, a PWM controller is used to switch the input side circuit switching devices
56
-
59
in a sequence to generate an AC signal from the DC voltage source
54
across the primary winding
48
of the transformer
46
. The resulting AC signal appearing on the secondary winding
50
of the transformer
46
is rectified by the diodes
70
-
73
of the output side circuit
44
to provide a DC output voltage to the load
68
. The output side inductor
74
and capacitor
76
filter high frequency and transient voltages from the output voltage applied to the load
68
. The magnitude of the DC output voltage applied to the load
68
is determined by the magnitude of the DC source voltage, the duty cycle of the PWM controller, and the turns ratio of the transformer
46
.
In the DC-to-DC converter topology
40
, the leakage
52
and magnetizing inductance's of the transformer
46
are effectively utilized to achieve zero voltage switching of the switching devices
56
-
59
. The operation of the full-bridge PWM converter
40
, to achieve zero voltage switching, is as follows. With input circuit switching devices
56
and
59
initially turned on and conducting, the voltage applied across the primary winding
48
of the transformer
46
will be the voltage level of the voltage source
54
, V
in
. A corresponding voltage will appear on the secondary winding
50
of the transformer
46
, causing an output current to flow through diodes
70
and
73
. When switching device
59
in the input side circuit
42
is turned off, the input voltage
54
is disconnected from the primary winding
48
. With the input voltage Vin no longer applied to the primary winding
48
of the transformer
46
, the current in the output side circuit
44
will free wheel through all of the output side diodes
70
-
73
. A current thus continues to flow through the outpu
Brady III Wade James
Mosby April M.
Sterrett Jeffrey
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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