Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2001-06-18
2003-02-11
Vu, Bao Q. (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S021080, C363S056030, C363S017000
Reexamination Certificate
active
06519164
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a circuit configuration for use with a switched power supply of the forward converter type.
2. Description of the Related Art
“Attention has been focused on the input current total harmonic distortion (THD) due to the increased use of nonlinear loads that tend to degrade AC line quality. THD standards, such as the IEC-1000-3-2 promulgated by the international Electrochemical Commission, have been adopted. Due to requirements associated with cost effectiveness, there are numerous references in the prior art directed to single power stage AC-DC converters which employ such nonlinear loads. The power supply used with these converters usually requires multiple output DC voltages and fast output regulation. In addition, a high frequency power transformer is used to step up/down the output DC voltage. The transformer presents several drawbacks. First, a higher voltage stress occurs across the power switch due to the transformer volts-second reset. Second, the leakage inductance of the transformer, usually causes a voltage spike across the power switch when it turns off. To minimize or reduce the voltage spike, a snubber circuit, such as a R-C-D snubber, is sometimes used to absorb this voltage spike. This is not an optimum solution, however, in that the leakage energy of the transformer is dissipated in the snubber circuit making the circuit inefficient.”
One prior art single power stage AC-DC forward converter is shown in
FIG. 1
, where L
k
is the leakage inductance of the transformer T, D
s
, is the body diode and C
ds
is an internal capacitance of switch S. The inductor L
in
is a boost inductor and is used to shape the input current waveform to achieve a high power factor with low input current harmonics. Winding N
3
, coupled with windings N
1
and N
2
, is used to reset the transformer T.
FIGS. 2
a
-
2
b
illustrate typical switching waveforms associated with the circuit of FIG.
1
. The voltage on DC bus capacitor C
dc
, i.e., V
dc
, is considered to be constant over successive switching cycles as a consequence of the large capacitance value selected for C
dc
in comparison with the switching period. During the time switch S is on, time (t
1
-t
2
), voltage V
dc
is applied to primary winding N
1
and leakage inductor L
k
. Therefore, the voltage V
dc
on capacitor C
dc
is discharged to the load R
o
via transformer coupling N
1
-N
2
. The difference between the reflected voltage across winding N
2
and the voltage across the load R
o
, i.e., V
o
, is applied to the choke inductor L
f
as V
lf
for the period (t
1
-t
2
). As a result, an increased current produced in L
f
is delivered to the output load R
o
. Also, during time interval (t
1
-t
2
), a circuit path is formed from the AC voltage source V
in
, through inductor L
in
, diode D
5
, switch S and back to ground. Energy is stored in inductor L
in
during this time.
Considering the time during which switch S is off, i.e., (t
2
-t
4
). Diode D
5
is nonconducting. The current through L
in
therefore flows through diode D
4
charging capacitor C
dc
. It is noted, however, this charging current is inconsequential given the large capacitance value of C
dc
and the relatively short charging interval. A magnetizing current in winding N
1
, which previously traveled through switch S during the time switch S was on, is now transferred (magnetically coupled) to winding N
3
causing diode D
1
to transition to a conducting state. A voltage developed across winding N
3
by virtue of the coupled magnetizing current ser-cs as additional charging source for capacitor C
dc
. During this time interval (t
2
-t
4
), the winding current in N
3
is effectively reset. A negative voltage which is developed across N
2
in the secondary winding causes diode D
2
to become nonconducting. As such, energy stored in L
f
during the time (t
1
-t
2
) is released through the load R
o
for the time (t
2
-t
4
).
A drawback associated with the circuit of
FIG. 1
is that at time t
2
, the point at which switch S turns off, the energy which was stored in leakage inductor L during the time (t
1
-t
2
), i.e., the time switch S was ON, is released to charge internal switch capacitor C
ds
thereby generating an undesirable voltage spike as shown in
FIG. 2
b
. After the leakage energy is released, the voltage across switch S is equal to the sum of the DC bus voltage plus the tranformers reset voltage. A negative consequence of the higher voltage across switch S is that a higher rated switch S is required, especially for a higher input line voltage, e.g., 277 AC. An associated drawback of using higher voltage rated switches is their increased cost.
A number of circuit topologies have been proposed in the prior art to eliminate the large overvoltages which occur across switching devices at turn off. One method for minimizing the large overvoltages is the use of an R-C-D snubber network. In general, the function of a snubber circuit is to reduce the electrical stresses placed on a device during switching by a power electronics converter to levels that are within the electrical ratings of the device. In the present case, a snubber network is employed to limit voltages applied to a switch during turn-off transients.
FIG. 3
illustrates the prior art circuit of
FIG. 1
with the addition of an R-C-D snubber network. While the snubber network minimizes or reduces the occurrence of voltage spikes, it does so at the expense of reducing the conversion efficiency of the circuit. In addition, snubbers introduce additional complexity and cost to the basic converter circuit. As such, it is a non-optimal solution.
Thus, it is desirable to provide a converter design that limits the maximum voltage stress on the power switch and recycles the leakage energy of the transformer such that the circuits efficiency is enhanced.
SUMMARY OF THE INVENTION
According to the invention, a single power stage AC/DC forward converter with power switch voltage clamping function includes: a switch S; L
k1
and L
k2
representing leakage inductances associated with the primary windings of transformer T, two series clamping and recovery capacitors C
d1
and C
d2
; a transformer T including primary windings N
1
and N
2
having the same number of turns; a boost inductor L
in
; a filter L
f
; a diode D connected in parallel with the switch S and in series with C
d1
and C
d2
.
In one embodiment, the DC bus capacitors C
d1
, and C
d2
, are connected directly to the leakage inductors L
k1
and L
k2
, respectively. In this embodiment, during intervals between conduction by switch S, the capacitors recover leakage energy from the leakage inductors L
k1
, and L
k2
, and during intervals of conduction by switch S, the recovered leakage energy stored in capacitors C
d1
and C
d2
, is provided to the load via transformer T.
A main advantage provided by the circuit of the present invention is the prevention or substantial elimination of voltage spikes which would otherwise occur at each switch transition to the OFF state. Voltage spikes are eliminated in two ways: (1) by configuring the DC bus capacitors C
d1
, and C
d2
to be in parallel with the switch S and, (2) by selecting the capacitance values of capacitors C
d1
and C
d2
to be sufficiently large to clamp the voltage across switch S to a value equal to the DC bus voltage.
A further advantage of the circuit of the present invention is that by recovering the leakage energy in each switching cycle, as opposed to dissipating it in accordance with prior art approaches, the overall circuit efficiency (i.e., power out/power in) is enhanced. An additional advantage of capturing the leakage current is that the voltage rating of switch S is significantly reduced thereby reducing its cost.
Accordingly, it is an object of the invention to provide an AC/DC forward converter in which the voltage across the main switch S is clamped to the DC bus voltage, thereby preventing the occurrence of undesirable voltage spikes.
It is another object of the invention to provide an AC/
Qian Jinrong
Weng DaFeng
Koninklijke Philips Electronics , N.V.
Vu Bao Q.
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