Switching power supply circuit

Details Switching power supply circuit Switching power supply circuit

2003-01-27

2004-09-07

Riley, Shawn (Department: 2838)

Electricity: electrical systems and devices

Safety and protection of systems and devices

With specific current responsive fault sensor

C361S093900

Reexamination Certificate

active

06788513

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a switching power supply circuit, and more particularly, to a switching power supply circuit having an overcurrent detecting function and an output short circuit detecting function.
Moreover, the present invention relates to a switching power supply circuit capable of protection against degradation and breakage, due to an overcurrent or a short circuit current, of a switching element and an inductive load circuit and the like fed by the switching element.
2. Description of the Prior Art
In recent years, the need for protection against degradation and breakage of parts under abnormal conditions has been increasing with decrease in voltage and increase in current in semiconductor integrated circuits.
Referring now to
FIG. 3
, a conventional switching power supply circuit will be described.
The conventional switching power supply circuit comprises as shown in FIG.
3
: an error amplifier
18
detecting the voltage at an output terminal Vout; a switching element
19
whose on/off is controlled by the output of the error amplifier
18
; a constant current source
20
; a capacitor
21
charged by the constant current source
20
; a comparator
22
detecting the charging voltage of the capacitor
21
; a latch circuit
23
receiving as an input the output signal of the comparator
22
; an activating circuit
26
resetting the latch circuit
23
; a control signal generating circuit
24
; an AND circuit
25
serving as an output drive circuit; a MOS transistor M
2
serving as a switching element; a choke coil
29
; a Schottky diode
30
; resistors
31
and
32
for output voltage detection; and a capacitor
33
for output smoothing. Reference numeral
27
represents a direct-current power supply terminal. Reference numeral
28
represents a direct-current power source. The choke coil
29
, the Schottky diode
30
, the output voltage detecting resistors
31
and
32
and the output smoothing capacitor
33
constitute an inductive load circuit.
The operation of the switching power supply circuit structured as described above will be described.
Under normal operation conditions, the MOS transistor M
2
is switched on and off by a pulse width modulation signal generated by the control signal generating circuit
24
in accordance with the voltage at the output terminal Vout. When the MOS transistor M
2
is on, electric power, or energy is supplied from the direct-current power source
28
to the choke coil
29
, the capacitor
33
and the output load. At this time, energy is stored in the choke coil
29
. When the MOS transistor M
2
is switched off by the pulse width modulation signal, a counter electromotive force is caused at the choke coil
29
, so that a regenerative current flows through the Schottky diode
30
. By smoothing by the capacitor
33
the voltage caused at the choke coil
29
at this time, a direct-current voltage is obtained. The direct-current voltage is output to the output terminal Vout.
The control signal generating circuit
24
generally includes a triangular wave generator (not shown) and an error comparator (not shown). The error comparator monitors the normally output voltage, or the voltage at the output terminal Vout and compares the voltage with the output signal of the triangular wave generator, thereby generating the pulse width modulation signal whose pulse width varies according to the voltage at the output terminal Vout. However, since this is not the essence of the invention, description thereof is omitted.
When the inductive load circuit is overloaded or the output terminal Vout is short-circuited under abnormal conditions, the output voltage detecting error amplifier
18
detects that the potential at the output terminal Vout is decreased, and outputs a high level, thereby switching off the switching element
19
. At the same time, the charging of the capacitor
21
is started by the constant current source
20
. This operation is maintained during a period for which the voltage at the output terminal Vout is lower than a predetermined voltage and the error amplifier
18
is generating an inversion signal. During this period, the charging of the capacitor
21
is continued, and when a predetermined time determined by the current value of the constant current source
20
and the capacitance value of the capacitor
21
elapses and the charging voltage of the capacitor
21
exceeds a reference voltage Vx of the comparator
22
, the comparator
22
sets the latch circuit
23
and switches off the MOS transistor M
2
. By doing this, the MOS transistor M
2
and parts of the inductive load circuit and the like are protected from breakage or degradation.
In the above-described conventional structure, when the output of the switching power supply circuit is overloaded or short-circuited to decrease the voltage at the output terminal Vout, a protection function of causing the MOS transistor M
2
to be off for a predetermined period of time works. However, in the overloaded or the short-circuited condition, since no current limitation is imposed, an unlimited overcurrent continuously flows through the MOS transistor M
2
and the inductive load circuit, or the choke coil
29
, the Schottky diode
30
, the resistors
31
and
32
and the capacitor
33
from the start of the timer to the setting of the latch circuit
23
. As a result, there is a possibility that the MOS transistor M
2
or the inductive load circuit is degraded or broken down.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a switching power supply circuit capable of protecting the switching element and the parts constituting the inductive load circuit with higher reliability.
A switching power supply circuit of the present invention comprises: a direct-current power source; an inductive load circuit supplied with electric power from the direct-current power source; a switching element interrupting the electric power supplied from the direct-current power source to the inductive load circuit; a control signal generating circuit generating a control signal periodically bringing the switching element into conduction; an overload detecting circuit generating an overload detection signal when a load current flowing through the switching element exceeds a predetermined current value; a first latch circuit being set by the overload detection signal and being reset in response to a leading edge of the control signal; a timer circuit performing a clocking operation during a period for which the first latch circuit is set, and generating a time-up signal when a predetermined clocking period elapses; a second latch circuit being set in response to the time-up signal of the timer circuit; and an output drive circuit receiving as inputs the control signal and output signals of the first and the second latch circuits, bringing the switching element into conduction in response to the control signal under normal conditions, and shutting off the switching element irrespective of the control signal when at least one of the first and the second latch circuits is set.
According to this structure, the overload detection signal is generated when the load current flowing through the switching element exceeds the predetermined current value, and at this time, the switching element is shut off irrespective of the presence or absence of the control signal, so that the on period of the switching element is reduced. Consequently, the maximum current value can be limited to not more than a predetermined value. Further, at the time of the occurrence of an abnormal current when the overloaded condition continues, the control of the switching element can be stopped so that the switching element is held in the shut-off condition. Consequently, the protection against degradation and breakage of the switching element and parts of the inductive load circuit can be made more reliable.
In the switching power supply circuit having the above-descried structure, for example, it is preferable that the overload detecting circuit c

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