Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2002-10-28
2004-01-06
Vu, Bao Q. (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S021180, C363S041000
Reexamination Certificate
active
06674656
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of the Invention
The present invention relates to a power supply. More particularly, the present invention relates to a pulse width modulation (PWM) controller of a switching power supply.
2. Background of the Invention
The PWM is a conventional technology used in the switching power supply to control and regulate the output power. Various protection functions, such as over-voltage and over-current protections are built-in in the power supply to protect the power supply and circuits connected thereto from permanent damage. The function of output power limit is generally used for overload and short circuit protections. Referring to
FIG. 1
, a traditional PWM power supply circuit uses a PWM controller
100
, such as PWM-control integrated circuit
3842
, which has been widely used for power supplies. A resistor
230
that is connected serially with the power MOSFET
300
determines the maximum output power of the power supply. The method is to connect the voltage of the resistor
230
to the current-sense input (VS) of PWM controller
100
. If the voltage on the VS is greater than the maximum current-sense voltage VM (e.g. 1V), PWM controller
100
will disable the output of its OUT pin, and it will also restrict the maximum power output. As the energy stored in an inductor is given by:
&egr;=½
×L×I
2
=P×T
In the above equation, P is the maximum output power and can be expressed as:
I
p
=
V
IN
L
p
×
t
ON
(
1
)
P
=
L
p
2
×
T
×
I
p
2
=
V
IN
2
×
t
ON
2
2
×
L
p
×
T
(
2
)
In Equations (1) and (2), Ip and Lp are the primary current and the primary inductance of a transformer
400
, t
ON
is the switch-on period of the PWM controller in which the power MOSFET
300
is switched on, T is the switching period of the PWM controller. From Equation (2), we find that the output power will vary as the input voltage V
IN
varies. When the safety regulations are taken into consideration, the range of the input voltage is AC 90V~264V, wherein the output power limit of the power supply in high line voltage is many times higher than the output power limit in low line voltage. Although the output voltage (power) will be kept constant by automatically adjusting t
ON
through a feedback loop, the maximum t
ON
is restricted when the VS≧VM (Ip×Rs≧VM, where Rs is the resistor
230
). Furthermore, the maximum output power is also affected by the delay time t
D
of the PWM controller. From the moment that the voltage in VS pin is higher than the upper limit voltage (Ip×Rs≦1V) to the moment that PWM controller
100
's OUT pin is actually turned off, there is a delay time t
D
. Within this delay time t
D
, the power MOSFET is still on, and it will continue delivering power. Therefore, the actual turn-on time of the PWM signal is equal to t
ON
+t
D
, and the actual output power becomes as follows:
P
=
V
IN
2
×
(
t
ON
+
t
D
)
2
2
×
L
p
×
T
(
3
)
Although the tD time is short, generally within the range of 200 nsec~350 nsec, the higher the operating frequency is, the more impact is caused bytD. Because the switching period T is short, tD becomes relatively more important. The input voltage VIN should be compensated properly, such that the input voltage will not affect the maximum output power. Referring to
FIG. 1
, a bias resistor
220
is added between VIN and the VS pin for compensation. The function of bias resistor
220
can compensate the difference caused by the input voltage VIN and the delay time tD. By properly selecting the value of bias resistor
220
, an identical output power limit for the low line and high line voltage input can result. The following analysis illustrates how to determine the bias resistor
220
reaching an identical output power limit.
With the incorporation of the bias resistor, the VS can be shown as
V
S
=
(
V
IN
L
p
×
R
S
×
t
ON
)
+
(
V
IN
×
B
p
)
.
(
4
)
Bp=R
225
/(R
225
+R
220
), R
220
and R
225
are the resistance of resistor
220
and
225
.
The PWM controller
100
is turned off when VS=VM, take in t
D
and t=t
ON
+t
D
, the equation (4) can be expressed as
V
M
=
[
V
IN
L
p
×
R
S
×
(
t
-
t
D
)
]
+
(
V
IN
×
B
p
)
(
5
)
The expression of “V
IN
×Bp=(V
IN
/L
P
)×R
S
×t
D
” is set to achieve the identical output power limit. We finally get
B
p
=
R
S
L
p
×
t
D
(
6
)
However, the bias resistor
220
causes significant power consumption, especially in high line voltage input. The power consumption can be shown as:
P
R
=
V
IN
2
R
(
7
)
Besides, the high voltage existing in the resistor
220
causes inconvenience for the component selection and PCB layout.
SUMMARY OF INVENTION
The invention provides a PWM controller having a saw-limiter for power limit to achieve an identical output power limit for low line and high line voltage input. The saw-limiter comprises an adder, a reference voltage, a scaler and a saw-tooth signal that is generated by a PWM oscillator.
The input of the scaler is the saw-tooth signal, in which the saw-tooth signal is attenuated and maximum voltage of the saw-tooth signal is clamped. The adder sums the output of the scaler with the reference voltage, and produces a saw-limited voltage for the output power limit. The PWM controller will turn off its output when the current-sense input signal of the PWM controller is higher than the saw-limited voltage. The saw-limited voltage is equal to the reference voltage while a PWM switching period starts. After that, the amplitude of the saw-limited voltage will gradually increase until it reaches its maximum voltage. Subsequently, a saw-tooth like waveform is generated for the saw-limited voltage. The ramp of the current-sense input signal is proportional to the variation of the line voltage. If the line voltage is higher, the slope of the current-sense input signal will be steep and its related saw-limited voltage will be lower. In terms of power limit, using the saw-limiter, the power limit will be lower when the line voltage is higher. By properly selecting the value of the scaler in the saw-limiter an identical output power limit for the low line and high line voltage input can be achieved.
Advantageously, the PWM controller having a saw-limiter for output power limit of the present invention can provide compensation for a power supply's output power limit. Furthermore, no resistor is applied to sense the line voltage, which saves the power consumption, eases the PCB layout, and shrinks the size of power supply.
REFERENCES:
patent: 4764856 (1988-08-01), Rausch
patent: 5170333 (1992-12-01), Niwayama
patent: 5189599 (1993-02-01), Messman
patent: 5465201 (1995-11-01), Cohen
patent: 5583752 (1996-12-01), Sugimoto et al.
patent: 6411119 (2002-06-01), Feldtkeller
patent: 6583994 (2003-06-01), Clayton et al.
Lin Jenn-yu G.
Yang Ta-yung
Jianq Chyun IP Office
System General Corporation
Vu Bao Q.
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