Electric lamp and discharge devices: systems – Periodic switch in the supply circuit – Impedance or current regulator in the supply circuit
Reexamination Certificate
2002-08-06
2004-03-09
Patel, Rajnikant B. (Department: 2838)
Electric lamp and discharge devices: systems
Periodic switch in the supply circuit
Impedance or current regulator in the supply circuit
C315S219000, C363S089000
Reexamination Certificate
active
06703793
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a switching power unit which is preferably realized as a so-called AC/DC converter, and particularly to reducing a loss in an activating circuit thereof.
BACKGROUND OF THE INVENTION
There is wide use of a switching power unit, used in a portable-compact electronic device, which switches a dc current obtained by rectifying and smoothing a commercial current at a high frequency such as several hundreds kHz, so as to convert the dc current into a desired voltage by means of a compact transformer with high efficiency.
FIG. 14
is an electronic circuit diagram of a typical switching power unit
1
of prior art. A pulse width modulation (PWM) system and of a flyback system are employed in the switching power unit
1
. In the switching power unit
1
, an ac voltage applied to a line between terminals p
1
and p
2
flows via (a) a filter circuit, constituted of a fuse
2
, filter capacitors c
1
and c
2
, a filter coil
1
, which prevents EMI, and (b) a discharging resistor r
1
. Thereafter, the ac voltage is rectified and smoothed by a diode bridge
3
and a smoothing capacitor c
3
. A dc voltage obtained in this manner is applied to a dc circuit constituted of a primary winding n
1
of a transformer n and a main switching element q which is an FET, and a main switching element q is switched at a high frequency by a control circuit
4
as described later, so that an induced voltage occurs in a secondary winding n
2
of the transformer n when the main switching element q is OFF. The induced voltage is rectified and smoothed by a diode d
1
and a smoothing capacitor c
4
so as to be outputted from terminals p
3
and p
4
to a load which is not shown.
When a user pulls an AC plug out from a socket for example, there is a possibility that the user gets an electric shock while filter capacitors c
1
and c
2
are kept to be charged. Thus, there is provided the discharging resistor r
1
so as to discharge the filter capacitors c
1
and c
2
quickly, thus preventing an accident.
An output voltage detecting circuit
5
monitors an output voltage from terminals p
3
and p
4
, and sends the detected output voltage from a photodiode d
2
of a photo-coupler pc to a phototransistor tr, so as to feedback the output voltage to the control circuit
4
on the primary side. The control circuit
4
controls a duty of the main switching element q in accordance with data of the output voltage that has been fedback, so that an output voltage of the switching power unit is stabilized.
While, there is provided a capacitor c
5
as a power source of the control circuit
4
. The dc current that has been rectified and smoothed by the diode bridge
3
and the smoothing capacitor c
3
is applied to the capacitor c
5
via the activating resistors r
2
and r
3
. Thus, when a dc power source is connected to the terminals p
1
and p
2
so as to activate the switching power unit
1
, a charging voltage of the capacitor c
5
gradually increases. When the charging voltage becomes a voltage at which the control circuit
4
begins to operate, the control circuit
4
begins to operate so as to send a control signal to the main switching element q, so that the aforementioned switching operation is started.
Further, a voltage induced in a sub winding n
3
of the transformer n is applied to the capacitor c
5
via the diode d
3
. Also in this manner, the capacitor c
5
is charged. Thus, after the activation, the control circuit
4
keeps on operating by using the current supplied via the diode d
3
as a power source. Furthermore, there is provided a protecting zener diode zd on the capacitor c
5
in parallel to each other so that the charging voltage of the capacitor c
5
applied by the activating resistors r
2
and r
3
does not exceed a rated voltage in a low-power-consumption mode of the control circuit
4
.
The low-power-consumption mode is a mode in which an operation of the control circuit
4
is stopped so that a low charging voltage of the capacitor c
5
does not cause the control circuit
4
to malfunction when power is turned on. In the low-power-consumption mode, a low voltage malfunction preventing circuit that is internally provided in the control circuit
4
stops an operation of the control circuit
4
in a period since power is turned on until the charging voltage of the capacitor c
5
becomes a predetermined voltage at which the control circuit
4
begins to operate. During the period in which the operation is stopped, the control circuit hardly consumes a current. Further, even though a power voltage becomes lower than an operation low limit voltage due to any causes during the steady-state operation of the switching power unit, the low voltage malfunction preventing circuit works, so that the switching power unit becomes in the low-power-consumption mode. Then, the voltage at which the operation is started is set to be higher than the operation low limit voltage, so that a stable operation is ensured.
A current detecting resistor r
4
is connected to the main switching element q in series. A current flowing in the main switching element q is converted into a voltage by the current detecting resistor r
4
. The converted voltage is inputted as a voltage Va, via a noise-removing low pass filter constituted of the resister r
5
and a capacitor c
6
, to a current detecting terminal a of the control circuit
4
which is used to detect an overcurrent. When a peak of the voltage Va exceeds a predetermined overcurrent detecting level, the control circuit
4
performs an overcurrent protecting operation for stopping the switching operation of the main switching element q. This prevents a damage in the main switching element q which is brought about by a reason for which an output current higher than a rated current is drawn from the terminals p
3
and p
4
. At this time, an operation mode of the control circuit
4
shifts to the low-power-consumption mode, so that a power consumption in the control circuit is greatly reduced.
The low pass filter is inserted so as to prevent malfunction of the control circuit
4
which is brought about by a reason for which a sharp current flowing upon turning on the main switching element q causes the voltage Va to exceed a predetermined overcurrent detecting level.
Further, the capacitor c
6
is charged with a dc voltage from the smoothing capacitor c
3
via bias resistors r
6
and r
7
, and the capacitor c
6
is discharged by the resistor r
5
and the current detecting resistor r
4
when the main switching element q is turned off. Thus, the bias resistors r
6
and r
7
charge the capacitor c
6
so as to have a voltage Va calculated by the following expression, so that the current detecting terminal a of the control circuit
4
is biased by the voltage Va.
Va=
{square root over (2)}
Vac
×(
r
5
+
r
4
)/(
r
6
+
r
7
+
r
5
+
r
4
)
Then, a voltage increment of the current detecting circuit r
4
which is brought about by a current of the main switching element q is added to the right side of the foregoing expression, and the voltage is applied, as an increment of the voltage Va, to the current detecting terminal a of the control circuit
4
as described above.
Thus, when the main switching element q remains ON for a time more than a predetermined time due to output short circuit etc. so as to keep on charging the capacitor c
6
, the voltage Va exceeds the overcurrent detecting level due to the charging performed by the bias resistors r
6
and r
7
, so that a short circuit protecting operation, by which the main switching element q is OFF-driven, is performed.
Further, in a case where there are not provided the bias resistors r
6
and r
7
, there occurs the following problem: if the overcurrent detecting level is set with respect to an input ac voltage of 100 V for example, the overcurrent protecting operation is not performed unless the overcurrent detecting level is more than the set overcurrent detecting level when the input ac voltage is changed to 200 V. Thus, the bias resistors r
6
and r
7
are
Patel Rajnikant B.
Sharp Kabushiki Kaisha
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