Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2001-11-05
2003-08-12
Berhane, Adolf Deneke (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S131000
Reexamination Certificate
active
06606257
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed, in general, to the field of DC-to-DC converters. More particularly, the present invention relates to a multiple-output flyback converter having an improved energy cycle sequencing capability.
2. Description of The Related Art
Conventional flyback switching power supplies commonly include a pair of transformers (actually a pair of coupled inductors) and one or more power switches for alternately coupling an unregulated DC or rectified AC voltage across a primary winding of the power transformer in a series of voltage pulses. These pulses are converted into a series of voltage pulses across one or more secondary windings of the power transformer and then rectified and filtered to provide one or more output DC voltages. The output voltage or voltages of the power converter are commonly regulated by controlling the relative amount of time that the power switch is ‘on’ (i.e., the duty cycle).
One common type of switching power supply is the flyback power converter, which is an isolated version of the buck-boost converter. The flyback converter is a very popular power supply topology for use in low-power, multiple-output applications. A flyback power converter works by cyclically storing energy in the power transformer, and then dumping this stored energy into a load. By varying the amount of energy stored and dumped per cycle, the output power can be controlled and regulated. A high-power switching transistor connected in series with the primary winding of the power transformer normally provides such a switching function. That is, the on-time and off-time of this power switch controls the amount of energy coupled across the power transformer. When the power switch is ‘on’, current flows through the primary winding of the power transformer, and energy is stored in the transformer (primary magnetizing inductance). When the power switch is ‘off’, the stored energy is transferred out into a secondary circuit by means of current flowing out of one or more secondary windings of the power transformer. Note that the secondary current does not flow in the power transformer at the same time that the power switch is ‘on’ and the primary current is flowing. The reason for this is that in a conventional flyback power converter, the winding polarity is chosen and a rectifier is coupled to the secondary winding to prevent conduction of current in the secondary winding when the power switch is ‘on’.
Flyback power converters are advantageous at lower power levels over other switching power converters due to the fact that they are generally simpler, they require a reduced number of components, and they allow multiple regulated outputs to be available from a single supply. Common applications for flyback converters are AC adapters, which may, for example, deliver an output voltage in the range of between 9 VDC to 180 VDC at power levels of 20 to 100 Watts, drawing power from a rectified AC line voltage, which may vary between 85 VAC to 270 VAC for universal line voltage inputs.
Flyback converters are generally operated in one of two modes. A first mode of operation, referred to as the Discontinuous Conduction Mode (DCM), is well known in the art, in which the energy stored in the transformer is totally coupled to the output load before the next energy cycle, generally resulting in the secondary current reaching zero before the next drive cycle. The second mode of operation is referred to as the Continuous Conduction Mode (CCM), whereby the next energy cycle begins before all stored magnetic energy is released from the transformer, and, therefore, before the secondary current reaches zero. DCM is more common than CCM because relatively simple control circuitry can be used to maintain output voltage regulation by varying the frequency and/or on-time of the power switch to accommodate heavy or light load conditions.
FIG. 1
illustrates a conventional flyback converter
100
which may be operated in either DCM or CCM. One disadvantage of the flyback converter
100
is that when operated in the CCM, the flyback converter
100
may exhibit potentially unstable operation when used with high bandwidth feedback loops. That is, the converter
100
is susceptible to oscillations when high bandwidth feedback loops are used. Another disadvantage of this circuit topology is that the diode D
1
is hard-switched. That is, in the CCM, current is reversed while the diode D
1
is still conducting.
FIG. 2
is an illustration of another conventional flyback DC-DC converter whose topology is similar to that illustrated in
FIG. 1
except for diode D
1
being replaced by switch S
1
. In the circuit of
FIG. 2
, the body diode of switch S
1
provides the same functionality as the diode D
1
of the circuit of FIG.
1
. The circuit of
FIG. 2
functions in the same manner as the circuit of
FIG. 1
when switch S
1
is held ‘off’. However, when switch S
1
is turned ‘on’, bi-directional current flow is enabled. That is, current can flow in the reverse direction (i.e., out of the filter capacitor C
1
through the secondary winding, nS
1
).
FIG. 3
a
is an illustration of another prior art flyback converter
310
having multiple output circuits
311
,
313
. Each of the respective output circuits
311
,
313
includes a blocking diode (D
1
, D
2
) and a unidirectional switch (S
1
, S
2
). If the unidirectional switch S
1
is in the ‘off’ state, then current is blocked from flowing into the output. Therefore, the ‘on’ time of switch S
1
controls the power flow to the output of circuit
311
.
A drawback of circuit
310
is that when the circuit operates in the discontinuous conduction mode, it is necessary to drain the transformer T of energy in each cycle. This may be problematic in that the unidirectional switch associated with the last or final output circuit in the switching cycle (e.g., switch S
2
) has to remain ‘on’ for a long enough time to fully drain the transformer T in each switching cycle. To ensure that this occurs, in actual practice, switch S
2
is often eliminated.
FIG. 3
b
is an illustration of the circuit of
FIG. 3
a
which eliminates circuit switch S
2
. As discussed above, this ensures that the transformer T will be fully drained in each switching cycle. Elimination of switch S
2
causes output V
2
to be controlled solely by the ‘on’ time of the primary switch S
M
.
FIGS. 3
a
and
3
b
illustrate circuits having two output circuits. Irrespective of the number of output circuits, however, primary side control of the final output circuit occurs. In each case, in DCM, the last output circuit would be controlled by the ‘on’ time of the primary switch S
M
. It may be desirable, in certain cases, to retain the output circuit switch in the last output circuit even though the output is effectively controlled by the ‘on’ time of the primary switch. This is true because retaining the switch enables the implementation of synchronous rectification and primary soft-switching.
FIG. 4
is an illustration of another prior art circuit topology, which is a modification of the circuit topology of
FIG. 3
b
. The circuit of
FIG. 4
includes a switch S
2
in place of blocking diode D
2
of the circuit of
FIG. 3
b
. The circuit topology of
FIG. 4
provides advantages over the topologies of
FIG. 3
b
in that the bi-directional switch S
2
permits synchronous rectification and further permits bi-directional soft switching of primary switch S
M
. However, even though the illustrative prior art topology of
FIG. 4
enables synchronous rectification and soft switching, it shares the drawback or restriction, common to all the prior art circuits illustrated, in that the last output (e.g., output V
2
in
FIG. 4
) has to be cycled last in each energy cycle. From a control or circuit standpoint, this restriction may not always be desirable.
This restriction exists by virtue of the circuit topology. In particular, the cycling restriction exists because switch S
2
of output circuit
413
has no forward blocking capability. As a consequence of t
Berhane Adolf Deneke
Goodman Edward W.
Koninklijke Philips Electronics , N.V.
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