Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2001-02-05
2002-03-12
Han, Jessica (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S132000
Reexamination Certificate
active
06356462
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to isolated dc/dc converters, and more particularly, to the constant-frequency, isolated dc/dc full-bridge converters that operate with ZVS of the primary-side switches in a wide range of input voltage and load current.
2. Description of the Prior Art
The major factors hindering the operation of conventional (“hard-switched”) pulse-width-modulated (PWM) converters at higher switching frequencies are circuit parasitics such as semiconductor junction capacitances, transformer leakage inductances, and rectifier reverse recovery. Generally, these parasites introduce additional switching losses and increase component stresses, and, consequently, limit the maximum frequency of operation of “hard-switched” converters. To operate converters at higher switching frequencies and, eventually, achieve higher power densities, it is necessary to eliminate, or at least reduce, the detrimental effects of parasitics without a degradation of conversion efficiency. The most effective approach in dealing with parasitics is to incorporate them into the operation of the circuit so that the presence of parasitics does not affect the operation and performance of the circuit. Generally, this incorporation of parasitics can be accomplished by two techniques: the resonant techniques and constant-frequency PWM soft-switching techniques.
The common feature of the resonant techniques is the employment of a resonant tank that is used to shape the current and voltage waveforms of the semiconductor switch (es) to create conditions for either zero-current turn-off, or zero-voltage turn-on. However, zero-current switching (ZCS), or zero-voltage switching (ZVS) in resonant-type converters is achieved at the expense of increased current and/or voltage stresses of semiconductors compared to the stresses in the corresponding “hard-switched” topologies. In addition, the majority of resonant topologies need to circulate a significant amount of energy to create ZCS or ZVS conditions, which increases conduction losses. This strong trade-off between the switching-loss savings and increased conduction losses may result in a lower efficiency and/or larger size of a high-frequency resonant-type converter compared to its PWM counterpart operating at a lower frequency. This is often the case in applications with a wide input-voltage range. In addition, variable frequency of operation is often perceived as a disadvantage of resonant converters. As a result, although resonant converters are used in a number of niche applications such as those with pronounced parasitics, the resonant technique has never gain a wide acceptance in the power-supply industry in high-frequency high-power-density applications.
To overcome some of the deficiencies of the resonant converters, primarily increased current stresses and conduction losses, a number of techniques that enable constant-frequency PWM converters to operate with ZVS, or ZCS have been proposed. In these soft-switching PWM converters that posses the PWM-like square-type current and voltage waveforms, lossless turn-off or turn-on of the switch (es) is achieved without a significant increase of the conduction losses. Due a relatively small amount of the circulating energy required to achieve soft switching, which minimizes conduction losses, these converters have potential of attaining high efficiencies at high frequencies.
One of the most popular soft-switched PWM circuit is the soft-switched, full-bridge (FB) PWM converter shown in FIG.
1
(
a
), which is discussed in the article “Design Considerations for High-Voltage High-Power Full-Bridge Zero-Voltage-Switched PWM Converter,” by J. Sabate et al., published in
IEEE Applied Power Electronics Conf
. (
APEC
)
Proc
., pp. 275-284, 1990. This converter features ZVS of the primary switches at a constant switching frequency with a reduced circulating energy. The control of the output voltage at a constant frequency is achieved by the phase-shift technique. In this technique the turn-on of a switch in the Q
3
-Q
4
leg of the bridge is delayed, i.e., phase shifted, with respect to the turn-on instant of the corresponding switch in the Q
1
-Q
2
leg, as shown in FIG.
1
(
b
). If there is no phase-shift between the legs of the bridge, no voltage is applied across the primary of the transformer and, consequently, the output voltage is zero. On the other hand, if the phase shift is 180°, the maximum volt-second product is applied across the primary winding, which produces the maximum output voltage. In the circuit in FIG.
1
(
a
), the ZVS of the lagging-leg switches Q
3
and Q
4
is achieved primarily by the energy stored in output filter inductor L
F
. Since the inductance of L
F
is relatively large, the energy stored in L
F
is sufficient to discharge output parasitic capacitances C
3
and C
4
of switches Q
3
and Q
4
in the lagging leg and to achieve ZVS even at very light load currents. However, the discharge of the parasitic capacitances C
1
and C
2
of leading-leg switches Q
1
and Q
2
is done by the energy stored in leakage inductance L
LK
of the transformer because during the switching of Q
1
, or Q
2
the transformer primary is shorted by the simultaneous conduction of rectifiers D
1
and D
2
that carry the output filter inductor current. Since leakage inductance L
LK
is small, the energy stored in L
LK
is also small so that ZVS of Q
1
and Q
2
is hard to achieve even at relatively high output currents. The ZVS range of the leading-leg switches can be extended to lower load currents by intentionally increasing the leakage inductance of the transformer and/or by adding a large external inductance in series with the primary of the transformer. If properly sized, the external inductance can store enough energy to achieve ZVS of the leading-leg switches even at low currents. However, a large external inductance also stores an extremely high energy at the full load, which produces a relatively large circulating energy that adversely affects the stress of the semiconductor components, as well as the conversion efficiency.
In addition, a large inductance in series with the primary of the transformer extends the time that is need for the primary current to change direction from positive to negative, and vice verse. This extended commutation time results in a loss of duty cycle on the secondary of the transformer, which further decreases the conversion efficiency. Namely, to provide full power at the output, the secondary-side duty-cycle loss must be compensated by reducing the turns ratio of the transformer. With a smaller transformer's turns ratio, the reflected output current into the primary is increased, which increases the primary-side conduction losses. Moreover, since a smaller turns ratio of the transformer increases the voltage stress on the secondary-side rectifiers, the rectifiers with a higher voltage rating that typically have higher conduction losses may be required.
Finally, it should be noted that one of the major limitations of the circuit in FIG.
1
(
a
) is a severe parasitic ringing at the secondary of the transformer during the turn-off of a rectifier. This ringing is cased by the resonance of the rectifier's junction capacitance with the leakage inductance of the transformer and the external inductance, if any. To control the ringing, a heavy snubber circuit needs to be used on the secondary side, which may significantly lower the conversion efficiency of the circuit.
The ZVS range of the leading-leg switches in the FB ZVS-PWM converter in FIG.
1
(
a
) can be extended to lower load currents without a significant increase of the circulating energy by using a saturable external inductor instead of the linear inductor, as described in the article “An Improved Full-Bridge Zero-Voltage-Switched PWM Converter Using a Saturable Inductor,” by G. Hua et al., published in
IEEE Power Electronics Specialists'Conf. Rec
., pp. 189-194, 1991, and in U.S. Pat. No. 5,132,889, “Resonant-Transition DC-to-DC Converter,” by L. J. Hit
Jang Yungtaek
Jovanovic Milan M.
Delta Electronics , Inc.
Han Jessica
Venable
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