Electricity: power supply or regulation systems – In shunt with source or load – Using choke and switch across source
Reexamination Certificate
2000-02-23
2001-09-04
Patel, Rajnikant B. (Department: 2838)
Electricity: power supply or regulation systems
In shunt with source or load
Using choke and switch across source
C363S089000
Reexamination Certificate
active
06285170
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a switching power supply which has a boost converter, and is capable of limiting an inrush current flowing into a smoothing capacitor.
2. Description of the Related Art
In general, a switching power supply incorporating a boost converter has a smoothing capacitor provided therein for smoothing an output voltage from the boost converter. Accordingly, a large inrush current flows into the smoothing converter via the boost converter at the initial stage of power-on. Therefore, this type of switching power supply generally has a power thermistor or a resistor arranged at some location between an AC input and the smoothing capacitor, so as to supply an AC voltage via the power thermistor or the like at the initial stage of power-on, thereby limiting the inrush current to a predetermined value.
A boost power supply
41
shown in
FIG. 6
is among switching power supplies of the above-mentioned type as the related art. The power supply
41
includes a fuse
3
for protection against overcurrent, a rectifying diode stack
4
, a power thermistor
6
connected between the fuse
3
and the diode stack
4
, a boost converter
5
, and a smoothing capacitor
7
. The boost converter
5
is comprised of a choke coil
11
for voltage boost, a switch
12
formed by a switching element such as an FET, and a diode
13
.
In the power supply
41
, at the initial stage of power-on, an AC voltage VAC is input from an AC power source
2
to the diode stack
4
via the fuse
3
and the power thermistor
6
. At this time, the internal temperature of the power thermistor
6
is approximately equal to the ambient temperature, and hence the power thermistor
6
has a larger resistance value (e.g. 10 &OHgr;) than a resistance value (e.g. 1 &OHgr;) it has when heated. Therefore, if an AC voltage VAC of 100 V is input at the initial stage of power-on, with its phase angle of 90 degrees, i.e. at its peak, a peak current IP of the inrush current is limited to approximately 14.1 A since the peak voltage of the AC voltage VAC is approximately 141 V.
On the other hand, when the capacitor
7
is charged to a predetermined voltage, the switch
12
within the boost converter
5
is controlled by a switching control circuit, not shown. At this time, the boost converter
5
outputs excitation energy of the choke coil
11
via the diode
13
to thereby output a voltage which is higher than a pulsating voltage V
1
rectified by the diode stack
4
. In this case, the input current IIN dependent on the AC voltage VAC flows through the power thermistor
6
, whereby the power thermistor
6
heats itself. As a result, the power thermistor
6
has the resistance value thereof reduced to a smaller value as its temperature becomes higher. Further, when the resistance value has been reduced, the amount of heat generated by the power thermistor
6
becomes small. Thus, in a normal state after power-on of the power supply
41
, the parameters consisting of the temperature of the power thermistor
6
increased as it heats itself and the resistance value of the same are stabilized in a predetermined condition, so that in the above example, the resistance value of the power thermistor
6
becomes equal to approximately 1 &OHgr;. In this state, power loss occurring when the input current IIN flows through the power thermistor
6
has been made small, so that the inrush current is limited while attaining the enhancement of the conversion efficiency of the device.
However, the power supply
41
suffers from the following problems: Firstly, there still remains a resistance of approximately 1 &OHgr; in the above example. For this reason, in constructing a switching power supply having a large output, power loss due to the remaining resistance is not negligible, and hence improvement of the conversion efficiency is still desired.
Secondly, another problem occurs when the power is turned on again after restoration of input from momentary interruption. More specifically, in the above example, e.g. when the power supply
41
is continuously operated under an ambient temperature of 40° C., the power thermistor
6
is stable with its resistance value held at approximately 1 &OHgr;. In this state, even if input of the AC voltage VAC is interrupted for a short time (dozens to hundreds of milliseconds, which will cause the charging voltage of the capacitor
7
to drop to a voltage value close to 0 V) due to a power failure or the like, the AC voltage VAC starts to be input again before the temperature of the power thermistor
6
is lowered sufficiently. Assuming that the resistance value of the power thermistor is 1 &OHgr; when the AC voltage is input again, since the peak voltage of the AC voltage VAC is approximately 141 V, the peak current IP of the inrush current is approximately 141 A. A flow of such a large peak current IP not only causes blowing or deterioration of the fuse
3
, momentary interruption of the commercial power line, and tripping of a power breaker for household or commercial use, but also it exceeds rated surge current of various electronic components, such as the diode stack
4
, arranged in a conductive line for the peak current IP, thereby causing breakage or degradation of the electronic components. This problem inevitably occurs so long as the power thermistor
6
is employed as inrush current-limiting means. Therefore, in general, as a countermeasure against the inrush current after restoration of input from momentary interruption, there is employed a method in which a resistor having a resistance value of several ohms is constantly connected to the conductive line of the AC voltage VAC, thereby limit the peak current IP of the inrush current below a predetermined value or a method of selecting for use electronic components which can withstand the peak current IP which will flow after restoration of input from momentary interruption. However, when the former countermeasure is taken, the constantly connected resistor causes constant power loss, which results in a lowered conversion efficiency of the power supply. On the other hand, the problem with the latter countermeasure is that selection of a fuse
3
having a large rated current for use prevents the fuse
3
from properly blowing and that the size of each electronic component is inevitably increased.
It should be noted that if a resistor is used as inrush current-limiting means in place of the power thermistor
6
, large inrush current does not occur after restoration of input from momentary interruption, but the method is basically similar to the above former countermeasure in that power loss is caused by the resistor. Therefore, although the method is somewhat effective in a power supply of a type having a relatively small steady-state current flowing therein, in a power supply of a type having a relatively large steady-state current (e.g. 1 A) flowing therein, large power loss caused by the resistor (e.g. power loss of 10 W when the resistance value is 10 &OHgr;) occurs constantly, which considerably lowers the conversion efficiency of the device.
A power supply
51
shown in
FIG. 7
is among switching power supplies as the related art which can attain high conversion efficiency even with a smaller steady-state current flowing therein. In the power supply
51
, at the initial stage of power-on, a thyristor
52
is controlled to be in an OFF state, which allows a power thermistor
6
to limit the inrush current. Then, when the charging voltage of a capacitor
7
reaches a predetermined voltage, an activation circuit
61
activates switching control circuits
62
and
63
. As a result, a boost converter
5
has its switching operation controlled by the switching control circuit
62
to boost an AC voltage VAC for charging the capacitor
7
. At the same time, a switch
9
formed by a switching element such as an FET or the like is controlled by the switching control circuit
63
to switch the charging voltage of the capacitor
7
. Consequently, a current flows through a primary winding
Matsumoto Akira
Yamazaki Hiroshi
Greenblum & Bernstein P.L.C.
Nagano Japan Radio Co. Ltd.
Patel Rajnikant B.
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