Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2000-11-15
2002-08-13
Berhane, Adolf Deneke (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S097000
Reexamination Certificate
active
06434021
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains generally to the field of power conversion and, more particularly, to switching power supplies with feedback control.
BACKGROUND
Compact and efficient power supplies are an increasing concern to users and manufacturers of electronics. Switching power supplies with pulse width modulated (“PWM”) controllers offer both compactness and efficiency in a number of different topologies. Boost and buck switching power supply topologies are efficient, but do not isolate the power input from the power output. Other topologies, such as the flyback, do isolate the power input from the power output by using a transformer. In such topologies, feedback from the secondary (power output) side of the transformer is needed to adjust the pulse width modulation duty cycle of the power switch. PWM control for a switching power supply may be provided from a single integrated circuit chip or package having some number of external connection pins or terminals. As with many other types of integrated circuit chips or packages, limiting the number of external connection terminals of a power supply package can be advantageous.
For example, U.S. Pat. No. 5,313,381 to Balakrishnan (the “'381 patent”), which is fully incorporated by reference, discloses a three-terminal switching power supply control chip for use with a flyback converter.
FIG. 1
illustrates a flyback converter
20
according to the '381 patent. The converter
20
employs a three-pin control chip
22
to supply current from a rectified DC source (V
bb
)
28
across an isolating transformer
24
to supply power for a load
26
. The power supply chip
22
includes a first terminal
30
coupled to a primary winding
32
of the transformer
24
, a second (“ground”) terminal
36
coupled to a primary side ground reference, and a third terminal
40
for accepting a combined feedback control signal (I
FB
) and a bias supply voltage (V
cc
) to operate the control chip
22
.
Within the power supply chip
22
, the first terminal
30
is alternately coupled to the ground terminal
36
by a power transistor switch
42
. PWM control circuitry
44
drives the power switch
42
at a variable duty cycle. When the power switch
42
is ON, current flows through the primary winding
32
and energy is stored in the magnetic core
45
of the transformer
24
. When the switch
42
is OFF, a secondary diode
46
is forward biased and the stored energy in the transformer core
45
is released through a secondary winding
48
to a filter/storage capacitor
47
and the load
26
. After the transformer
24
is reset, the ON/OFF cycle is repeated.
An error amplifier
50
compares the output voltage V
out
across the load
26
with a reference voltage to generate the feedback control signal I
FB
. The bias supply voltage V
cc
is supplied from an auxiliary secondary winding
52
of the transformer
24
. The bias supply voltage V
cc
is modulated with the feedback control signal I
FB
in an opto-isolator
54
to create the combined bias voltage, feedback signal V
cc
/I
FB
. A feedback extraction circuit (not shown) in the chip
22
separates the feedback signal I
FB
from the bias voltage V
cc
by sensing the excess current flowing through a shunt regulator. The extracted feedback signal I
FB
is used to control the output of the PWM circuitry
44
to constantly adjust the duty cycle of the power switch
42
so as to transfer greater or lesser current to the secondary.
Notably, to properly compensate the PWM controller based on feedback from the secondary requires extra components and often involves expensive re-design, depending upon the particular application. Yet, prior art isolated power supplies that used feedback only from the primary side of the transformer do not account for power losses encountered on the secondary side of the transformer. See, e.g., U.S. Pat. No. 5,982,644, (the '644 patent), which discloses a pulse-width-modulated boost converter coupled to a high voltage converter, which in turn is coupled to the primary side of a transformer. The modulation of the boost converter is adjusted according to an amplified error signal representing the difference between the boost converter's output voltage and the voltage from a current sensing circuit sensing the current through the primary winding. This error signal has no way of sensing and accounting for the losses on the secondary side of the transformer. Thus, the power supply disclosed in the '644 patent employs a linear regulator on the secondary side of the transformer to maintain a constant voltage over the load. Although this power supply avoids the use of feedback from the secondary side of the transformer, it introduces the expense and loss associated with installing an additional regulator at the load.
Thus, it would be desirable to provide minimal terminal power supply packages for controlling more complex power converter topologies, including packages that isolate the input and output through a transformer without requiring feedback from the secondary side of the transformer, thereby easing design and reducing the component count.
SUMMARY OF THE INVENTION
In accordance with a first aspect of the invention, a three-terminal power supply package is provided for controlling delivery of power from a source to a load. In a preferred embodiment, the three-terminal package has a first terminal for coupling to a primary winding of a transformer, a second terminal for coupling to a ground reference, a third terminal for coupling to a source of operating power, and an internal power switch. The internal power switch has an input coupled to the first terminal, an output coupled to the second terminal, and an activation gate. The control package further includes pulse train control circuitry coupled to the power switch activation gate and responsive to an error signal for driving the power switch. The error signal is derived from an internally generated compensation signal corresponding to an expected voltage loss between the source and the load. The control package may be used to control a number of single switch, transformer coupled power converter topologies, including (by way of example only) flyback and combined forward-flyback converters. Depending on design considerations, the internal power switch and pulse train control circuitry may be formed as part of a single integrated circuit.
In accordance with another aspect of the invention, a four-terminal power supply package is provided for controlling delivery of power from a source to a load. In a preferred embodiment, the control package has a first terminal for coupling to a primary winding of a transformer, a second terminal for coupling to a ground reference, a third terminal for coupling to a source of operating power, and a fourth terminal for coupling to an external compensation circuit for generating a compensation signal corresponding to an expected voltage loss between the source and the load. The package includes an internal power switch having an input coupled to the first terminal, an output coupled to the second terminal, and an activation gate. The control package further includes pulse train control circuitry coupled to the internal power switch activation gate and responsive an error signal for driving the power switch, the error signal derived from the compensation signal received on the fourth terminal. The control package may be used to control a number of single switch, transformer coupled power converter topologies, including (by way of example only) flyback and combined forward-flyback converters. Depending on design considerations, the internal power switch and pulse train control circuitry may be formed as part of a single integrated circuit.
In accordance with yet another aspect of the invention, a four-terminal power supply package is provided for controlling delivery of power from a source to a load. In a preferred embodiment, the control package has a first terminal for coupling to a primary winding of a transformer, a second terminal for coupling to an
Collmeyer Arthur J.
Manner David B.
Telefus Mark D.
Wong Dickson T.
Berhane Adolf Deneke
IWatt
Lyon & Lyon LLP
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