Stabilized power supply unit having a current limiting function

Details Stabilized power supply unit having a current limiting function Stabilized power supply unit having a current limiting function

2003-07-02

2004-11-23

Vu, Bao Q. (Department: 2838)

Electricity: power supply or regulation systems

Output level responsive

Using a three or more terminal semiconductive device as the...

C323S274000, C323S282000

Reexamination Certificate

active

06822428

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a stabilized power supply unit having a current limiting function for maintaining at a constant level the output voltage supplied to a load if the output current to the load has changed, and restricting excessive output current to the load.
BACKGROUND OF THE INVENTION
A stabilized power supply unit having a current limiting function is widely used in a series regulator serving as a convenient power supply and a constant voltage charging apparatus for charging a battery.
FIG. 4
shows a circuit structure of a series regulator having a conventional current limiting function.
The series regulator shown in
FIG. 4
is composed of a voltage control circuit
10
, an output circuit
20
, and a current limiting circuit
30
, integrated on an IC chip.
The voltage control circuit
10
is provided with a differential amplifier Amp and voltage dividing resistors R
11
and R
12
. The differential amplifier Amp is provided at one input thereof (inverting input) with a reference voltage Vref for setting an output voltage, and at another input thereof (non-inverting input) with an output feedback voltage Vfb obtained by dividing the output voltage by the voltage dividing resistors R
11
and R
12
. The difference between the two inputs is amplified by the differential amplifier Amp, and outputted from the voltage control circuit
10
as a control voltage Vc. The differential amplifier Amp is supplied with a constant current from a constant current source
11
.
The output circuit
20
has an output transistor Q
21
consisting of a p-type MOS transistor (hereinafter referred to as p-type transistor) connected between a power source potential Vdd and the output terminal Po of the power supply unit. The control voltage Vc is applied to the gate of the output transistor Q
21
. Connected to the output terminal Po is a load Lo and a condenser Co for stabilizing the output to the load.
The current-limiting circuit
30
includes a p-type current detection transistor Q
31
and a detection resistor R
31
connected in series in the order mentioned, between the power source potential Vdd and the ground. The current limiting circuit
30
is also provided with an n-type MOS transistor (hereinafter referred to as n-type transistor) Q
32
having a gate impressed with the voltage drop across the resistor R
31
. Constant voltage control function of the voltage control circuit
10
is regulated by the operating condition of the n-type transistor Q
32
.
The detection transistor Q
31
is formed together with the output transistor Q
21
on the same IC chip with a predetermined ratio less than 1 in size as compared with the output transistor Q
21
. The gate of the n-type transistor Q
31
is impressed with the same control voltage Vc as the gate voltage of the output transistor Q
21
. As a consequence, a detection current Io′ which is practically proportional (e.g. 1/100) to the output current Io flowing through the output transistor Q
21
flows through the n-type transistor Q
31
. The voltage drop across the detection resistor R
31
by the detection current Io′ determines the operating condition of the n-type transistor Q
32
. The threshold voltage of the n-type transistor Q
32
is set to the voltage that corresponds to the output current (i.e. load current) Io being a preset over-current protection level Is
0
. The threshold voltage is determined by the ratio of the output current Io and the detection current Io′, the resistance of the detection resistor R
31
, and properties of the n-type transistor Q
32
.
Operation of the conventional series regulator will be discussed with reference to
FIG. 5
showing a characteristic relationship between the output voltage Vo and the output current Io of the regulator. Under normal condition in which the output current Io is below the limit of over-current, the voltage control circuit
10
outputs a control voltage Vc so as to equalize the output feedback voltage Vfb with the reference voltage Vref. This control voltage Vc is applied to the gate of the output transistor Q
21
of the output circuit
20
to bring the output voltage Vo to a predetermined set voltage Vs. In this way, the constant voltage control of the regulator can be maintained stable at all times regardless of the magnitude of output current Io, unless the output current Io reaches the over-current protection level Is
0
.
Under such stable condition, the voltage drop by the detection resistor R
31
due to the detection current Io′ does not reach the threshold voltage of the n-type transistor Q
32
. Hence, nothing affects the constant voltage control function of the regulator.
However, as the output current Io reaches the preset over-current protection level Is
0
, the voltage drop across the detection resistor R
31
reaches the operating threshold voltage of the n-type transistor Q
32
. Thus, the n-type transistor Q
32
becomes operative as the output current Io exceeds the over-current protection level Is
0
. In the voltage control circuit
10
, current limiting operation is prioritized, so that the output voltage falls quickly, almost vertically. In this sense, this over-current protection function has a drop-type characteristic. The current level Is
1
at which the output voltage fully drops down to Vo is slightly higher (by the amount of &agr;) than the preset over-current protection level Is
0
, in accordance with the gain (control gain) of the current limiting regulator. The region above the level Is
0
is an over-current region.
In this way, under normal condition the output voltage Vo is controlled to be at a preset voltage Vs. However, if the output current exceeds a predetermined level (over-current protection level Is
0
), the output current Io is automatically limited.
However, the drain voltage of the output transistor Q
21
will be fixed to a predetermined set voltage Vs even if the output current changes, since the drain voltage is controlled to maintain a constant voltage at all times. On the other hand, the drain voltage of the detection transistor Q
31
varies with the detection current Io′, since the drain voltage depends on the product of the detection current Io′ and the resistance of the detection resistance R
31
. Thus, even if the gate voltages Vc of the output transistor Q
21
and of the detection transistor Q
31
are the same, and hence so are the gate-source voltages Vgs, their drain-source voltage Vds can differ.
If the drain-source voltage Vds of the detection transistor Q
31
changes, the detection current Io′ will be changed according to the inclination of the static drain voltage versus drain current characteristic of the detection transistor Q
31
, if the gate of the transistor Q
31
is impressed with the same gate voltage Vc to the output transistor Q
21
.
Therefore, the detection current Io′ is not exactly proportional to the output current Io. Hence, the output current cannot be limited accurately to an over-current protection level Is
0
, to which the current should be limited. For this reason, it is often the case that the over-current protection level Is
0
is set with some margin, or the output transistor Q
21
is provided with a large over-current tolerance.
SUMMARY OF THE INVENTION
It is, therefore, an object of the invention to provide a stabilized power supply unit having a current limiting function, the power supply unit provided with an output transistor and a detection transistor impressed with the same control voltage as the output transistor, making the detection transistor outputting a detection current which is exactly proportional to the output current, thereby enabling accurate detection of the output current.
It is another object of the invention to provide a stabilized power supply unit having a current limiting function, adapted to detect the detection current only if it is necessary, thereby reducing power consumption of the unit.
In accordance with one aspect of the invention, there is provided a stabilized power supply unit for supplying an output voltage, co

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