Switching power source

Details Switching power source Switching power source

1999-02-24

2001-01-23

Han, Jessica (Department: 2838)

Electric power conversion systems

Current conversion

Including d.c.-a.c.-d.c. converter

C363S097000, C363S049000

Reexamination Certificate

active

06178100

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a switching power source which is desirably applied as so-called AC-DC converters and DC-DC converters and other devices.
BACKGROUND OF THE INVENTION
A switching power source, which is used in portable small-size electronic apparatuses and which switches a dc obtained by rectifying and smoothing a commercial ac or a dc from a battery by using a high frequency of, for example, approximately several hundreds kHz so as to convert it to a desired voltage efficiently by using a small-size transformer, has been widely used.
As a typical construction of such a switching power source, a switching power source of a pulse-width modulation (PWM) system has been widely used, in which the secondary-side output voltage is detected by a voltage detection circuit, and a control circuit controls the switching pulse width of the main switching element in accordance with the results of detection so as to obtain a desired secondary-side output voltage.
Moreover, as another typical construction of the switching power source, a switching power source of a ringing choke converter (RCC) system has been widely used, in which excited energy, which has been accumulated in a transformer during the on-period of the main switching element, is outputted to a secondary-side circuit during the off-period, and upon completion of the output, a ringing pulse generated in the control coil of the transformer is fed back to the control terminal of the main switching element through a dc-cut capacitor so that the main switching element is again activated.
In the above-mentioned switching power source of the RCC system, as the load becomes higher, the above-mentioned off-period and on-period are automatically lengthened, that is, the switching frequency is reduced, so that the secondary-side output voltage is maintained at a predetermined constant voltage; therefore, a complex control circuit, such as required for the switching power source of the PWM system, is not necessary, and a power-source circuit for generating a voltage forming the basis for the pulse width also is not necessary, both contributing to achieve a low-cost power source.
FIG. 27
shows an electric circuit diagram of a typical prior-art switching power source
1
of the RCC system. A dc, obtained by rectifying a commercial ac by a main power-source circuit not shown, is inputted between input terminals p
1
and p
2
. This dc is smoothed by smoothing capacitor c
11
, and a main power-source voltage is outputted between a main power-source line
2
on the high-level side and a main power-source line
3
on the low-level side from smoothing capacitor c
11
.
A series circuit consisting of the primary coil nil of a transformer n and a main switching element q is connected between the above-mentioned power-source lines
2
and
3
. The above-mentioned main switching element q is realized by, for example, a bipolar transistor and a field-effect transistor, and
FIG. 27
shows a case in which a field-effect transistor is used. A starter circuit
4
, which consists of voltage-dividing resistors r
3
and r
5
, is connected between the main power-source lines
2
and
3
.
Upon application of power, that is, when a power-source voltage is applied between input terminals p
1
and p
2
, the output voltage of smoothing capacitor c
11
, that is, the main power-source voltage, increases, and when the voltage-divided value due to voltage-dividing resistors r
3
and r
5
becomes not less than the threshold voltage of the main switching element q, for example, not less than 3 V, the main switching element q is turned off; thus, a voltage in the upward direction in
FIG. 27
is applied to the primary coil n
11
so that excited energy is accumulated therein. When the main switching element q is turned off in a manner as described later, a voltage in the upward direction is induced in the secondary coil n
21
by the above-mentioned excited energy. Moreover, vibration, generated by leakage inductance between the primary coil n
11
and the other coils n
21
and n
12
at the time of turning off, is absorbed and eliminated by a snubber circuit
5
that consists of a series circuit of resistor r
11
and capacitor c
12
and that is parallel-connected between the drain and source of the main switching element q.
The dc, induced in the above-mentioned secondary coil n
21
, is given to smoothing capacitor c
13
through diode d
12
, and after having been smoothed by smoothing capacitor c
13
, it is outputted to a load circuit, not shown, from output terminals p
3
and p
4
through the output power-source lines
6
and
7
. A voltage detection circuit
8
is interpolated between the above-mentioned output power-source lines
6
and
7
. The voltage detection circuit
8
is constituted by voltage-dividing resistors, photo-coupler pc
1
, etc., and light-emitting diode d
13
of the photo-coupler pc
1
is driven so as to light up with a luminance corresponding to the output voltage, and the value of the output voltage is fed back to the primary side.
Upon turning the main switching element q on, a voltage is induced in the control coil n
12
in the same upward direction as that in the primary coil n
11
, and its induced current is given to the gate of the main switching element q through capacitor cl for cutting dc and bias resistor r
2
; thus, the gate potential of the main switching element q is further raised so that the main switching element q is maintained in the ON state.
Moreover, the current induced in the control coil n
12
upon turning the main switching element q on is given to one of the terminals of capacitor c
14
from capacitor c
1
and bias resistor r
2
through photo-transistor tr
11
of the above-mentioned photo-coupler pc
1
. The other terminal of capacitor c
14
is connected to the aforementioned main power-source line
3
in the low level; therefore, the higher the secondary-side output voltage becomes, the greater the charging current, thereby allowing the terminal voltage of capacitor c
14
to increase rapidly. The charging voltage of capacitor c
14
is supplied to the base of control transistor tr
12
that is interpolated between the gate and source of the main switching element q, and when the output voltage goes beyond the threshold voltage of control transistor tr
12
, for example, not less than 0.6 V, control transistor tr
12
is allowed to conduct, making the gate voltage of the main switching element q drop abruptly, with the result that the main switching element q is off-driven.
Therefore, the higher the secondary-side output voltage becomes, that is, the lighter the load, the quicker the charging voltage of capacitor c
14
increases, with the result that the main switching element q is off-driven more quickly. Moreover, the current induced in the control coil n
12
is supplied to capacitor c
14
through resistor r
12
. The series circuit of these resistor r
12
and capacitor c
14
is connected in parallel with control coin n
12
so as to form an overcurrent protection circuit. With this overcurrent protection circuit, even if the output voltage of smoothing capacitor c
13
on the secondary side is low due to shortcircuiting between output terminals p
3
and p
4
, etc., the on-time of the main switching element q is limited to a predetermined period, thereby making it possible to protect the main switching element q.
Here, supposing that the numbers of coil of the control coil n
12
and the secondary coil n
21
are represented by the same numbers as the reference numerals and the output voltage on the secondary side is vo, the voltage (n
12
/ n
21
) vo is induced in the control coil n
12
in the downward direction of
FIG. 27
upon turning the main switching element q off; thus, since the induced current is allowed to flow resistor r
12
, the charge of capacitor c
14
is drawn, and a resetting operation for the next on-operation of the main switching element q is carried out.
When, after turning the main switching element q off, the excited energy, accumulated in the primary main coil n
11
, has

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