Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2000-11-20
2002-10-29
Sterrett, Jeffrey (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S021120, C363S056120
Reexamination Certificate
active
06473318
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of switching power transfer devices, and more particularly to an improved energy recovery method for a flyback converter.
BACKGROUND OF THE INVENTION
The leakage inductance of the transformer in a conventional DC/DC flyback converter causes a voltage spike across the power switch when the power switch turns off Usually a circuit, such as a R-C-D (resistor, capacitor, diode) snubber circuit or an active clamp circuit, is used to absorb this voltage spike. The leakage energy of the transformer is typically dissipated in the R-C-D snubber circuit.
A number of known designs seek to recover this energy. These methods typically require an additional active switch to recover the leakage energy of the transformer.
A well-known conventional DC/DC flyback converter is shown in
FIG. 1
, where L
k
is the leakage inductance of the transformer T. The typical switching waveforms of
FIG. 1
are shown in FIG.
2
. When switch S is turned off at t
2
, the leakage current charges the parasitic output capacitance of switch S (output capacitance of S is not shown in FIG.
1
), which causes a high voltage spike across switch S. After the leakage energy is completely released, the voltage across switch S reaches its steady-state value. As a result, a high voltage rating voltage switch S would be required.
To eliminate this voltage spike, a number of circuit topologies have been reported in the literature. Among them, the R-C-D snubber is one of the most popular ways to minimize the voltage spike as shown in FIG.
3
. The snubber circuit consists of diode D
1
, capacitor C
s
and resistor R
s
. When switch S is turned off, the leakage current flows through diode D
1
and charges capacitance C
s
. If capacitance C
s
is relatively large enough, the voltage across C
s
roughly does not change, so as to clamp the voltage. In this case, the leakage energy of the transformer is first charged to C
s
and is then dissipated by the resistor R
s
. As a result, the voltage clamp is achieved at the expense of low conversion efficiency, i.e., loss of the energy inherent in the spike to heat.
Another prior art circuit is shown in FIG.
4
. See, Moshe Domb, “Nondissipative turn-off snubber alleviates switching power dissipation second-breakdown stress and Vce overshoot: analysis, design procedure and experimental verification,” IEEE Power Electronics Specialists Conference (1982). In this circuit, when switch S is turned off, the leakage energy of the transformer T is transferred to the clamping capacitor C
s
through D
1
. The voltage stress across switch S is the sum of the input voltage V
in
and the clamping voltage V
c
across C
s
, which is expressed as follows
V
ds,max
=V
in
+V
c
.
When switch S is turned on, the clamping capacitor C
s
and inductor L
r
form a resonant tank. The energy stored in capacitor C
s
is transferred to the inductor L
r
and the voltage polarity across capacitor C
s
reverses due to the resonance. When the capacitor voltage v
c
reverses and reaches the input voltage V
in
, diode D
1
conducts. The energy stored in L
r
is delivered to the input source. Therefore, the leakage energy of the transformer is finally feedback to the input source. In this scheme, an additional separate inductor L
r
is required, which increases the cost. See, U.S. Pat. Nos. 4,783,727, “DC/DC Converter”; 6,115,271, “Switching Power Converters With Improved Lossless Snubber Networks”, 5,260,607, “Snubber Circuit For Power Converter”, each of which is incorporated herein by reference.
Another well-known prior art method provides an active clamp, as shown in FIG.
5
. See, R. Watson, F. C. Lee and G. C. Hua, “Utilization of an active clamp circuit to achieve soft-switching in flyback converters” IEEE Power Electronics Specialists Conference (1994). An active switch S
a
and a capacitor C
s
are provided in series and connected in the primary winding N
1
of the transformer T. When switch S is turned off, switch S
a
is turned on. The leakage energy is transferred to the capacitor C
s
through switch S
a
, and the voltage across C
s
is used to reset the transformer. As a result, the voltage across switch S is clamped. However, such a converter requires an additional active switch and its controller. It increases the cost, which is not desirable for manufacturers.
See, U.S. Pat. No. 4,675,796, expressly incorporated herein by reference, discussed below. See also, Farrington, U.S. Pat. No. 5,883,795 and Farrington, U.S. Pat. No. 5,883,793, expressly incorporated herein by reference.
U.S. Pat. No. 6,108,218, “Switching Power Supply with Power Factor Control”, provides two embodiments. In a first embodiment, shown in
FIGS. 1 and 2
thereof, no snubber circuit to recycle the leakage energy of the transformer is shown.
FIGS. 3 and 4
provide an additional active switch as part of the snubber.
U.S. Pat. No. 5,982,638, “Single stage power converter with regenerative snubber and power factor correction” provides a capacitor
44
in
FIG. 1
, which is not only used as a snubber capacitor, but also used to achieve power factor correction, and therefore handles the main power flow from the input to the output. Therefore, the current flowing through this capacitor
44
is very large, which requires a capacitor large in size and value. In this circuit, the recovery of energy from the snubber capacitor
44
occurs by transfer to the input inductor
38
when switch
22
turns on. Since the energy stored in capacitor
44
is large, which causes higher power loss in the circuit. As a result, it has lower power conversion efficiency. The capacitance of capacitor
44
is determined by the input power and satisfies the power factor and input current harmonics requirements.
U.S. Pat. No. 5,991,172, “AC/DC flyback converter with improved power factor and reduced switching loss,” provides a third transformer winding which is not used to recover the leakage energy of the transformer, but rather to reduce the switching loss and improve the power factor. The leakage energy is dissipated by the circuit. Thus, it provides no substantial improvement in efficiency over a dissipative R-C-D snubber.
U.S. Pat. No. 5,999,419, “Non-isolated Boost Converter With Current Steering” relates to a buck boost converter having a tree-winding transformer.
U.S. Pat. No. 5,896,284, “Switching Power Supply Apparatus With a Return Circuit That Provides A Return Energy Ro A Load”, relates to a power supply circuit which utilizes leakage inductance energy to enhance efficiency, for example with a magnetically isolated inductor.
U.S. Pat. No. 5,615,094, “Non-Dissipative Snubber Circuit For A Switched Mode Power Supply”, relates to a snubber circuit for a secondary circuit of a power supply.
U.S. Pat. No. 5,694,304, “High Efficiency Resonant Switching Converters”; and U.S. Pat. No. 5,379,206, “Low Loss Snubber Circuit With Active recovery Switch” each provide a dual active switch architecture converter.
U.S. Pat. No. 5,055,991, “Lossless Snubber”, relates to a converter circuit having an active switch and a transformer with five inductively coupled windings.
U.S. Pat. No. 5,019,957, “Forward Converter Type of Switched Power Supply”, relates to a dual active switch forward power converter.
U.S. Pat. No. 4,805,079, “Switched Voltage Converter”, provides a converter with a snubber circuit.
U.S. Pat. No. 4,760,512, “Circuit for Reducing Transistor Stress and Resetting the Transformer Core of a Power Converter”, relates to a single active switch, triple inductively coupled winding transformer forward converter.
U.S. Pat. No. 4,736,285 relates to a “Demagnetization circuit for Forward Converter”, having two active switches.
U.S. Pat. No. 4,688,160, “Single Ended Forward Converter With Resonant Commutation of Magnetizing Current”, provides a forward converter employing a resonating capacitor to reset the transformer core.
U.S. Pat. No. 4,561,046, “Single Transistor Forward Converter With Lossless magnetic Core Reset and Snubber Network”, relates to a forward converter having a single
Qian Jinrong
Weng Da Feng
Halajian Dicran
Koninklijke Philips Electronics , N.V.
Laxton Gary L.
Sterrett Jeffrey
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