Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2000-06-29
2002-09-24
Nguyen, Matthew (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S049000
Reexamination Certificate
active
06456511
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a switching mode power supply. More particularly, the present invention relates to a transformer-based flyback converter employing secondary pulse width modulation control and having a primary side start-up circuit powered by voltage supplied from the secondary side.
2. Introduction to the Invention
The present invention relates to electronic switching power supplies in high input voltage, low power applications, such as off-line battery charging circuits that require self-contained bias power derived from the input-side AC mains. For safety reasons it is necessary to provide electrical isolation between the input mains and the output power of a switching power converter. In AC mains powered switching power converters, output isolation is conventionally accomplished by providing a transformer between the input side and the output side of the converter. The high voltage switching element and the pulse width modulation (PWM) control circuit are typically implemented on the primary side of the transformer. To regulate the output voltage or output current, or both, one or more feedback loops are provided for coupling control values from the output side to the input side control circuit. Due to the need for isolation, the feedback paths from output side to input side also have to be isolated. Isolation of the control values is frequently achieved by employing optical coupling via an optical isolator assembly, or by induction via a control transformer. The signal transmitted across the isolation barrier is usually an analog signal, and as such, is susceptible to noise and parameter drift due to temperature variation, distortion due to isolation circuit nonlinearities, and bandwidth limits of the isolation circuit or component.
Based upon the foregoing reasons, a secondary-side control circuit may be incorporated into a switching power supply. In using secondary-side control, the PWM control circuit is implemented entirely on the secondary side, while the electronic switch element is on the primary side. Since all output voltage or current sensing is carried out on the secondary side, there is no need to transfer analog control signals across the isolation barrier. Rather, the control circuit generates an on-off pulse-width-modulated control sequence which is coupled to the primary side switch element through a pulse transformer, for example. Because direct connection is made to the AC mains on the primary side, there is no power readily available at the secondary side PWM control circuit at start-up. Thus, special provision must be made to ensure that the power supply will begin switching when power is first applied via the AC mains.
FIG. 1
illustrates an example of a conventional switching power supply
20
having a secondary side control. The supply
20
includes an input side
21
and an output side
22
, separated by a switching power transformer
17
having a primary winding
4
and two secondary windings
5
and
6
. The primary winding
4
is connected to a high frequency inverter
2
, which in turn is connected to an input filter and polarity protection (rectifier) circuit
1
in direct connection with the AC mains. During operation of the supply
20
, a switching element within the converter circuit
2
causes an alternating current to flow through the primary winding
4
, and currents are induced in secondary windings
5
and
6
. An output rectifier and filter circuit
7
is connected to the secondary side
6
and rectifies the induced AC power in order to provide DC power output at desired voltage and current levels.
In order to regulate the output of the circuit
7
to the desired levels a control circuit
15
is provided. In the
FIG. 1
example, the control circuit
15
includes a primary side control circuit
12
which generates a startup switch waveform, and a secondary control circuit
14
which generates a PWM control signal regulated by feedback control. A pulse transformer
16
provides primary/secondary side isolation and couples the PWM control signal from the secondary control circuit
14
to the high frequency inverter circuit
2
via a control path
13
. A primary side on-off switch
10
bypasses the primary control startup circuit
12
, and/or a secondary side on-off switch
11
bypasses the secondary control circuit
14
. Switches
10
and/or
11
may be provided to control startup and shutdown operations of the supply
20
.
In order to provide initial startup, the primary control startup circuit
12
derives operating power through a resistor R
1
from a DC bus between rectifier
1
and inverter
2
. The primary control startup circuit
12
puts out square wave switching control signals over a path
3
to the inverter
2
which bypasses the pulse transformer
16
in order to control the high frequency inverter circuit
2
during startup. After startup, a feedback signal from the secondary winding
5
will cause the primary control circuit
12
to stop sending the square wave switching signals when sufficient energy is being transferred to the secondary winding
6
to operate the secondary control circuit
14
. From this point on, the secondary control circuit
14
will take over all switching control of inverter
2
via control path
13
and feedback isolation pulse transformer
16
. The secondary control circuit
14
performs conventional voltage regulation by comparing output voltage level with a predetermined reference in order to adjust the on-off duty cycle of the switching element of the high frequency inverter
2
. Power transformer
17
is typically, although not necessarily, a step-down transformer. A low voltage induced in secondary winding
6
provides power to the output rectifier and filter circuit
7
which in turn provides a smooth, regulated DC voltage at the output.
Since there is no isolation component in a feedback control line
8
from the output to the secondary PWM control circuit
14
, the limitations noted above with analog signal isolation are not present. However, startup power for the secondary control circuit
14
is more difficult to acquire, as compared with the conventional primary side control scheme, where the entire control circuit is present on the primary side of the power transformer. One typical approach is to include an electronics circuit to generate a PWM signal with a fixed frequency and duty cycle, or a square wave, in order to cause transfer of start-up power to the secondary control circuit
14
. Since this start-up electronics circuit
12
is on the primary side, the components may be subject to high voltage stress from the AC mains, and a high voltage silicon integrated circuit process may be required to implement the start-up circuit
12
.
From a reliability standpoint, it is desirable to limit silicon components on the primary side to rectifiers and the switching element in inverter
2
. Other concerns and drawbacks include added cost and complexity to provide effective startup circuitry.
SUMMARY OF THE INVENTION
A general object of the present invention is to provide an isolated output, switching mode power supply which includes a simplified input side starting circuit and a low voltage output side integrated control circuit which overcomes limitations and drawbacks of prior approaches.
One more general object of the present invention is to provide an isolated output, switching mode power supply which includes a starting circuit employing self-oscillation during an initial startup interval and a low voltage output side integrated control circuit which takes over control of the starting circuit as soon as secondary side power becomes available, in a manner overcoming limitations and drawbacks of prior approaches.
A third general object of the present invention is to provide a switching mode battery charger circuit which starts up and operates reliably over a wide variety of AC mains voltages present throughout the world.
Yet a fourth general object of the present invention is to provide a low voltage integrated circuit
Nguyen Matthew
Tyco Electronics Corporation
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