Switching power unit

Details Switching power unit Switching power unit

2002-07-24

2004-01-27

Riley, Shawn (Department: 2838)

Electricity: electrical systems and devices

Safety and protection of systems and devices

Voltage regulator protective circuits

C361S094000

Reexamination Certificate

active

06683765

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a switching power unit that has an overcurrent protecting function for restricting an output current in a case where electrical overload or output short circuit occurs.
BACKGROUND OF THE INVENTION
An example of the switching power unit having the overcurrent protecting function is shown in FIG.
11
. As to the switching power unit, an input voltage vin smoothed in a capacitor c
1
of an input stage is switched by a transistor tr
1
. During a period in which the transistor tr
1
is ON, energy is provided to a coil
11
, a capacitor c
2
, and a load r
1
by a voltage vout that appears in an emitter of the transistor tr
1
. During a period in which the transistor tr
1
is OFF, energy accumulated in the coil
11
is refluxed by a diode dl so as to be given to the load r
1
.
An output voltage vo is controlled in accordance with (a) a feedback voltage vadj obtained by dividing the output voltage vo at a predetermined ratio based on resistance values of resistors r
1
and r
2
, and (b) a reference voltage vref
1
of a reference voltage source
2
. First, a voltage corresponding to a difference between both the voltages is outputted by a differential amplifier
1
, and a comparator
4
compares the voltage with a triangular wave of 100 [kHz] outputted from an oscillator
3
. Then, the comparator
4
outputs a PWM signal having a pulse width corresponding to an output level of the differential amplifier
1
.
Next, when the PWM signal is given to a drive circuit
5
, the drive circuit
5
controls the transistor tr
1
so as to be ON/OFF corresponding to a duty cycle of the PWM signal. Thus, the output voltage vo is controlled by a constant voltage (for example, 5 [V]) determined by (a) the reference voltage vref
1
and (b) a dividing ratio based on the resistors r
1
and r
2
.
Upon the foregoing operation, as shown by vcmp and vout in
FIG. 12
, the output voltage of the comparator
4
, that is, the PWM signal and the voltage vout have pulse widths indicated in the figure. When ON time and OFF time of the transistor tr
1
are indicated by tON and tOFF respectively, a duty D of the transistor tr
1
is as follows.
D
=
tON
/
(
tON
+
tOFF
)
×
100
⁢
 
[
%
]
=
(
vO
/
vIN
)
×
100
⁢
 
[
%
]
(
1
)
However, when the load r
1
is under a heavy-loading condition, a coil current i
1
that flows into the coil
11
increases as shown by a difference between the broken line and the continuous line in the figure. When the coil current i
1
exceeds an overcurrent detection level ic
1
, an overcurrent condition is detected by an overcurrent detection circuit
6
provided on the input stage, and a set signal is outputted to an RS flip-flop circuit
7
.
The RS flip-flop circuit
7
is set when a set terminal voltage vset is varied to a high level. Once the set terminal voltage vset becomes a high level, the RS flip-flop circuit
7
is latched, so that the output is retained at a low level. At this time, a reset terminal voltage vrst remains at a low level.
Although the output voltage vcmp and the voltage vout of the comparator
4
have pulse widths shown by the broken line in
FIG. 12
, the pulse widths are shortened as shown by the continuous line because the output of the RS flip-flop circuit
7
remains at a low level since the RS flip-flop circuit
7
has been set. The duty of the transistor tr
1
drops in this manner, so that the output voltage vo drops. Thus, the increase in the output current is restricted. Consequently, the output current io drops at A point as shown in FIG.
13
.
Further, a reset signal is outputted from the oscillator
3
to the RS flip-flop circuit
7
when the transistor tr
1
is OFF, and the reset terminal voltage of the RS flip-flop circuit
7
as shown by vrst in FIG.
12
. At this time, once the reset terminal voltage becomes a high level, the RS flip-flop circuit
7
is latched, and the RS flip-flop circuit retains the output at a high level unlike the case where the set terminal voltage becomes high. Thus, under the next ON condition, the transistor tr
1
becomes ON at an ordinary timing.
However, in the switching power unit, when switching frequency is made higher (not less than approximately 50 [kHz]) so as to miniaturize the switching power unit and realize light weight etc., disadvantages occur in the operation of the overcurrent protecting function as described below.
That is, as shown in
FIG. 12
, there occur delays both in td
1
, a time the set terminal voltage takes to be a high level, and in td
2
, a time the transistor tr
1
takes to turn OFF after the set terminal voltage become a high level. A delay time td, a total of both the td
1
and td
2
, is a time the transistor tr
1
takes to turn OFF after the overcurrent detection has been performed, that is, a time the overcurrent protecting function takes to begin operating. The delay time td comes to approximately 1 [&mgr;sec]. When the switching pulse width is shortened upon overcurrent protecting operation at the switching frequency of 100 [kHz], that is, at a switching cycle of 10 [&mgr;sec], this influences the protecting operation so greatly that this has to be taken into consideration.
For example, supposing that input voltage vin=40[V], and output voltage vo=5[V], and inductance L of coil
11
=200 [&mgr;H], a current &Dgr;i that is variation of the coil current i
1
at the delay time td is as follows.
&Dgr;
i
=[(
vin−vo
)/
L]×td=
0.175
[A]
  (2)
Thus, the coil current i
1
exceeds the overcurrent detection level ic
1
due to the current &Dgr;i. Then, the current variation &Dgr;i increases an average current, that is, the output current io.
An output characteristic at this time, as shown in
FIG. 13
, is such that an emitter current increases as the load approaches a short circuit condition (vo=0[V]), and the emitter current exceeds an absolute maximum rating value (2.5[A]), so that a drooping characteristic is not realized. The foregoing switching power unit has such a problem that: the overcurrent protecting function does not operate exactly as the switching frequency becomes higher.
Japanese Unexamined Patent Publication No. 46828/1995 (Tokukaihei 7-46828)(Publication date: Feb. 14, 1995) discloses another prior art for solving the foregoing problem. FIG.
14
and
FIG. 15
show the prior art. These
FIGS. 14 and 15
correspond to the foregoing
FIGS. 11 and 13
respectively, and the same reference numerals are given to corresponding portions. It is remarkable that the switching power unit is further provided with a comparator
8
, a constant voltage source
9
, and an oscillation frequency changing circuit
10
so as to reduce the oscillation frequency in the case where the overcurrent occurs due to the output short circuit.
The voltage vadj obtained by the resistors r
1
and r
2
is given to a non-inverting input of the comparator
8
, and the constant voltage source
9
is connected to an inverting input of the comparator
8
. The reference voltage vref
1
generated by the reference voltage source
2
is, for example, 1.25[V]. On the other hand, a reference voltage vref
2
generated by the reference voltage source
9
is, for example, 0.6[V]. When the feedback voltage vadj to the comparator
8
is 0.6[V], the output voltage vo is as follows.
Vo=
0.6
[V
]×(r
1
+r
2
)/r
2
=2.4
[V
]  (3)
That is, the comparator
8
detects that the output voltage vo becomes lower than 2.4[V] shown in the foregoing expression. Responding to this, the oscillation frequency changing circuit
10
drops the oscillation frequency of the triangle wave, brought about by the oscillator
3
, from 100[kHz] to 20[kHz].
Thus, a resistance value of the load r
1
is made smaller by load short circuit etc., so that the output current io increases.

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