Electricity: power supply or regulation systems – Output level responsive – Using a three or more terminal semiconductive device as the...
Reexamination Certificate
2002-02-20
2004-01-27
Berhane, Adolf D. (Department: 2838)
Electricity: power supply or regulation systems
Output level responsive
Using a three or more terminal semiconductive device as the...
C323S902000, C323S901000
Reexamination Certificate
active
06683443
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a regulated power supply controller, and in particular to a feedback circuit for the controller of an isolated switching power supply.
2. Description of the Related Art
Power supplies are used in electronic devices to provide power to portions of the device's circuit at specific voltage levels. Regulated power supplies are required where the voltage levels must be tightly controlled for proper operation of the circuit. A regulated power supply has a primary side to which electrical power, such as from an AC line or a higher DC voltage, is connected. The primary side is connected to the primary windings of a power transformer, and the secondary windings of the transformer are connected to the secondary side of the circuit to provide the regulated output voltage of the power supply. The output voltage may be, for example, a smaller DC voltage. The transformer provides an isolation barrier between the primary side and the secondary side.
Feedback is required to ensure the tight regulation necessary for controlling the output voltage level, and the feedback signal must be provided across the isolation barrier. An optical isolator is utilized to communicate the feedback signal across the isolation barrier.
A common type of regulated power supply is a switching power supply, in which regulation is performed by a square wave signal having a duty cycle that changes depending on the voltage to be output. The transmission of power from the primary side to the secondary side is switched on and off using the square wave, and the averaged value of the changing duty cycle rests in the regulated output voltage.
FIG. 1
shows a power supply circuit including a feedback circuit utilized to communicate the feedback signal across the isolation barrier between the primary side and the secondary side of the regulated power supply circuit. The feedback circuit includes a voltage divider made up of a resistor
10
and a resistor
12
which is provided across the output voltage V
out
. The divided voltage is provided at a tap that is connected to a control lead of a reference element
14
. In one embodiment, the reference element is a TL431 precision adjustable three terminal shunt voltage regulator, made by Texas Instruments. The reference element
14
is connected to a LED part
16
of an optical isolator
18
, also referred to as an optical coupler, so that variations in the signal applied to the control lead result in changes in the light output by the LED part
16
. The LED part
16
typically emits light in the infrared region, although visible light or other wavelengths are of course also possible,
Also linked to the control lead of the reference element
14
is an RC combination of a resistor
20
and a capacitor
22
linked to the output voltage V
out
, and an RC combination of a resistor
24
and a capacitor
26
linked to the LED part
16
. A resistor
28
is provided in series with the LED part
16
between the LED part
16
and the output voltage V
out
.
The optical isolator
18
provides signal transmission across the isolation barrier
30
, and the light, such as infrared light, from the LED part
16
is transmitted to a photodiode
40
of the optical coupler
18
. The photodiode
40
is connected to a base of a transistor
32
within the optical isolator
18
. The level of received light by the photodiode
40
is translated to current flow through the transistor
32
and fed to an output resistor
34
. An output
36
to a control circuit
38
for the power supply is taken at the connection between the transistor
32
and the resistor
34
.
The power supply that utilizes the feedback circuit is shown in simplified form, including the controller, or control circuit
38
, which is a pulse width modulation controller. The output of the controller
38
is connected to the gate lead of a power FET
42
. The FET
42
is connected to the primary of a transformer
43
which receives the input voltage for the power supply. The secondary side of the transformer
43
receives the transformed voltage switched by the pulse width modulator
38
, forwards it through diodes
44
and
45
, through an inductor
46
and through a capacitor
47
to produce the output voltage of the power supply at the outputs
48
and
49
.
The feedback circuit, thus, operates through the control circuit
38
to cause regulation of the power supply to generate a desired output voltage. The output voltage is provided as the voltage V
out
that is provided to the LED
16
of the optical isolator
18
.
At startup of the circuit, the reference element
14
does not respond until the operating voltage has reached the desired operating point. The startup response of the reference element
14
causes an overshoot in the feedback voltage so that the output voltage overshoots the desired output voltage level. The circuit then overcorrects the output voltage, which results in ringing in the circuit during startup.
SUMMARY OF THE INVENTION
The present invention provides a power supply feedback circuit with a gradual startup to avoid ringing in the circuit. The present circuit includes a control connected to the reference element to reduce the output voltage to a lower level during startup. The control then gradually raises the output voltage to the desired operating level. In one example, the control circuit includes an active element and an RC combination connected to the control lead of the reference element.
REFERENCES:
patent: 4692687 (1987-09-01), Yokoyama
patent: 4761724 (1988-08-01), Brown et al.
patent: 5124630 (1992-06-01), Tsutsumi
patent: 5515263 (1996-05-01), Otake et al.
patent: 5880950 (1999-03-01), Kim
patent: 5995384 (1999-11-01), Majid et al.
patent: 6385061 (2002-05-01), Turchi et al.
Neild Kristopher K.
Williams David A.
Berhane Adolf D.
Brady W. James
Swayze, Jr. W. Daniel
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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