Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2001-10-23
2004-04-27
Han, Jessica (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S021160, C363S021170, C363S021180
Reexamination Certificate
active
06728117
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates generally to a power supply, more particularly, to a self-oscillating switching power supply (SOP) adapted to supply DC current to wide range of loads.
2. Related Art
A switch-mode power supply (SMPS) can operate in or between two current-conduction modes, continuous conduction mode (CCM), and discontinuous conduction mode (DCM). By controlling the power switch with a flyback (feedback signal) to monitor the energy (e.g., current) remaining in an inductor coil, a self-oscillating switching power supply (SOP) can operate at the critical-conduction point between the continuous and discontinuous conduction modes, wherein the power supply begins a new switching cycle at the exact point in time when an output-current inductor coil's (e.g., a transformer's secondary coil's) current (i.e., energy) falls to zero (i.e., approaches zero or is zero). A self-oscillating (flyback-driven) switching power supply (SOP) will include an input-current inductor coil and an output-current inductor coil, but may be implemented with or without a transformer. In a transformer-less (i.e., no transformer) SOP, the input-current inductor coil will also be the output-current inductor coil (e.g., a there will be only a single inductor coil for energy input and output).
FIG. 1A
is circuit diagram depicting a typical topology of a transformer (T
1
)-based self-oscillating (i.e., flyback) switching power supply (SOP)
100
of the related art. The SOP
100
includes a power switch SW
1
for interrupting a current I
1
through an input-current inductor coil (e.g., primary winding L
1
of transformer T
1
). The power switch SW
1
may be implemented as a metal oxide semiconductor field effect transistor (MOSFET) or a insulated gate bipolar transistor (IGBT), or a mechanical switch, etc, or by any suitable presently know or future electrical current-switching device. The power switch SW
1
has two states, an “ON” state characterized by a low impedance, and an “OFF” state characterized by a high impedance. The power switch SW
1
is generally cyclically turned ON and OFF in a periodic manner, such that the power switch SW
1
is ON during a first “ON-time” period and then OFF during a first “OFF-time” period, and then ON again during a second “ON-time” period (t
ON
) and then OFF during a second “OFF-time” period (t
OFF
), and so forth. The switching frequency F
SW
of the SOP
100
is calculated as the inverse of the sum of the ON-time plus the subsequent OFF-time (i.e., F
SW
=1/(“ON-time”+“OFF-time”). The duty cycle (Q
S
) of the SOP
100
is calculated as the ratio of the ON-time to the sum of the ON-time plus the subsequent OFF-time (i.e., Q
S
=“ON-time”/(“ON-time”+“OFF-time”)).
In general, because there is inductive energy storage in the SOP
100
, and a capacitance associated with the GATE terminal of the power switch SW
1
, a “minimum ON-time” (t
ONMIN
) will be characterized by the characteristics of the power switch SW
1
and other characteristics of the SOP
100
. During normal operation (e.g., critical conduction mode operation) of the SOP
100
, the OFF-time will be characterized by (and equal to) the time it takes for the current (i.e., energy) in an output inductor coil (e.g., a transformer secondary coil L
2
and/or transformer auxiliary secondary coil L
3
) to fall to zero (i.e., to approach zero or to be zero). During any discontinuous conduction mode (DCM) operation of the SOP
100
, the OFF-time will be longer than the time it takes for the current (i.e., energy) in an output inductor coil (e.g., a transformer secondary coil L
2
and/or transformer auxiliary secondary coil L
3
) to fall to zero. During any continuous conduction mode (CCM) operation of the SOP
100
, the OFF-time will be substantially less than the time it would otherwise take for the current (i.e., energy) in an output inductor coil (e.g., a transformer secondary coil L
2
and/or transformer auxiliary secondary coil L
3
) to fall to zero, and the current will not fall all the way to zero.
The power switch SW
1
is gated (i.e. controlled ON and OFF) by a switch-control signal asserted on the GATE node of the power switch SW
1
by a switch-driver circuit, such as the Frequency Clamped Flyback Driver
110
. Frequency clamped flyback switch driver
110
can be implemented with an integrated circuit chip manufactured by Motorola Corp. known as an MC33364 critical-conduction mode controller chip (See, e.g., FIG.
1
C).
The power switch SW
1
alternately opens and closes, alternately passing and interrupting an input current (I
1
) which is driven through the transformer's primary coil L
1
by the voltage potential difference (V
1
) between a power source input voltage V
IN
and power switch SW
1
. (In most real circuits, the ON resistance of the power switch SW
1
will be negligibly small, such that V
1
is approximately equal to voltage V
IN
when the power switch SW
1
is closed). Power source voltage V
IN
may be a fixed DC voltage or a variable DC voltage (e.g., a DC voltage having a ripple due to lack of filtering of a rectified AC). Persons skilled in the art will recognize that V
IN
may be provided as a substantially direct current (DC) voltage produced from an alternating current (AC) input voltage (i.e., a line voltage) source via a diode bridge rectifier (not shown) that full-wave rectifies the alternating current and a filter capacitor (not shown) that filters and smooths current pulses received from the bridge rectifier.
The SOP
100
includes an input-current inductor coil (e.g., primary winding L
1
) connected in series with a power switch SW
1
and between a power source (V
IN
) and a reference potential (ground). As is commonly known, closing and opening of the power switch SW
1
causes energy to be stored as a magnetic field in the input-current inductor coil (e.g., in the primary winding L
1
) which is transferred to an output-current inductor coil (e.g, the magnetically coupled secondary winding L
2
) and thereupon output substantially as an output current (I
2
) driven at a secondary voltage V
2
and dissipated through a load associated with an impedance, and/or with a resistance (R
LOADEQ
). A very small, (i.e., negligible) amount of the input energy is output as an auxiliary output current (I
AUX
) and dissipated through a sensing circuit within or operatively coupled to the switch-driver circuit (e.g,
110
). Because the transformer-based SOP
100
operates by transferring energy between the primary and secondary windings L
1
and L
2
, the turns ratio N
T
of the windings L
1
and L
2
may be adjusted to either increase or decrease the output voltage (V
OUT
) associated with the power source V
IN
, as needed for a particular application. A rectifier diode D
1
, and a filter capacitor C
1
are connected to output-current inductor coil (e.g, secondary winding L
2
) as shown in FIG.
1
A. The rectifier diode D
1
rectifies the current pulses (I
2
) provided by the output-current inductor coil (e.g, secondary winding L
2
) and the filter capacitor C
2
filters and smooths the rectified current pulses to form a substantially direct current (DC) output voltage V
OUT
.
The transformer T
1
includes a primary winding (L
1
) (connected in series to the power switch SW
1
), and at least one secondary winding (e.g., L
2
and/or L
3
). A first secondary winding L
2
is provided to output at voltage V
2
all, or substantially all, of the energy input to the transformer T
1
(e.g., energy input as current I
1
in the primary winding L
1
at voltage V
1
). The voltages V
1
and V
2
are generally related by the equation V
2
=N
T
*V
1
. An auxiliary secondary winding L
3
is provided to output, at voltage V
AUX
, a very small portion, (i.e., a negligible amount or none) of the energy input to the transformer T
1
. The voltage V
AUX
across the auxiliary secondary winding L
3
is related to the voltage across the first secondary winding L
2
by the ratio of turns in each of coils L
2
and L
3
(when the curr
Chappell Lee
Schemmann Marcel F. C.
Belk Michael E.
Han Jessica
Koninklijke Philips Electronics , N.V.
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