Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2003-01-22
2004-09-07
Riley, Shawn (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S021160, C363S021020, C363S056100
Reexamination Certificate
active
06788556
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a switching power source device, and more particularly, to a self-excited oscillation type switching power source device provided with an overcurrent protection circuit.
2. Description of the Related Art
FIG. 14
is a circuit diagram of a switching power source device (example 1) disclosed in U.S. patent application Ser. No. 10/232,901 filed on Sep. 3, 2002.
A first switching circuit S
1
and an input power source Vin are connected in series with a series circuit of a primary winding T
1
and an inductor L of a transformer T.
A series circuit including a second switching circuit S
2
and a capacitor C is connected in parallel to a series circuit including a primary wiring T
1
and an inductor L of a transformer T.
A series circuit including a second switching circuit S
2
and a capacitor is connected in parallel with the series circuit including the first switching circuit S
1
and the inductor L.
A rectification-smoothing circuit including a rectification element Ds is provided for a secondary winding T
2
of the transformer T.
The first switching circuit S
1
is a circuit including a first switching element Q
1
, a first diode D
1
, and a first capacitor C
1
connected in parallel to each other. The second switching circuit S
2
is a circuit including a second switching element Q
2
, a second diode D
2
, and a second capacitor C
2
connected in parallel to each other.
A switching control circuit is connected between a first driving winding T
3
provided for the transformer T and the control terminal of the first switching element Q
1
, and also between a second driving winding T
4
and the control terminal of the second switching element Q
2
, respectively.
The switching control circuit controls such that the first and second switching elements Q
1
and Q
2
are alternately turned on/off while the off-period when both of the first and second switching elements Q
1
and Q
2
are turned off is interposed between the on/off operations. Energy is stored in the primary winding T
1
and the inductor L during the on-period of the first switching element Q
1
, and the energy is released during the off-period of the first switching element Q
1
, and thus, the first and second switching elements Q
1
and Q
2
are self-excited-oscillated.
Referring to the above-described configuration, the inductor L and the capacitor C constitute a resonance circuit which resonates during the off-period of the first switching element Q
1
.
The above-described switching control circuit includes an on-period control circuit set at a time constant at which the first switching element Q
1
is turned off at a predetermined time after the first switching element Q
1
is turned on, and a second on-period control circuit set at a time constant at which the second switching element Q
2
is turned off, such that the resonance current flows through the series circuit, including the second switching element Q
2
and the inductor L, after the second switching element Q
2
is turned on and before the energy release from the second winding is completed. Thereby, the switching control circuit is operated in a continuous current mode.
Furthermore, an overcurrent protection circuit is provided. The overcurrent protection circuit includes a resistor R which is a current-detecting device connected in series with the first switching element Q
1
, and limits the on-period of the first switching element Q
1
when the current detected by the resistor R reaches a threshold.
The operation of the overcurrent protection circuit is as follows. The transistor Tr
2
is connected to the control terminal of the first switching element Q
1
. A voltage developed across the above-described current-detecting device (the resistor R) is applied to the control terminal of the transistor Tr
2
via a resistor R
6
. When the current flowing through the first switching element Q
1
reaches a predetermined value, the control terminal voltage of the transistor Tr
2
reaches a threshold to be turned on, causing the first switching element Q
1
to turn off so that the peak current flowing through the first switching element Q
1
is limited.
FIG. 16
shows the waveform of current Id
1
flowing through first switching element Q
1
which is generated when the output voltage is reduced. As seen in
FIG. 16
, when the output voltage is reduced, the off-period of the first switching element Q
1
is kept substantially constant and the on-period is reduced. Accordingly, the switching frequency is increased, which increases the switching loss, and also, the output current is increased.
FIG. 15
shows an example of a ringing choke converter provided with a peak current limiting circuit (an example 2). When the peak current flowing through the first switching element Q
1
reaches a predetermined value, the transistor Tr
4
is turned on, and the first switching element Q
1
is turned off.
FIG. 17
shows the waveform of current Id
1
, flowing through the first switching element Q
1
, which is generated when the output voltage is reduced. As seen in
FIG. 17
, as the off-period of the first switching element Q
1
is increased, the switching frequency is decreased in correspondence with the reduction of the output voltage. Therefore, the increase of the switching loss is suppressed, and the output current is increased.
The above-described switching power source device and ringing choke converter have the following defects.
(1) The delay time from the time when the current flowing through the first switching element Q
1
is detected and the detected voltage reaches the base-emitter threshold voltage of the transistor Tr
2
to the time when the transistor Tr
2
is turned on, is long. Therefore, the on-time cannot be reduced to be short, the secondary current is increased and a secondary rectification diode and so forth may be broken.
One of the reasons for this is that the transistor TR
2
cannot be turned on immediately after the detection voltage reaches the base-emitter threshold voltage of the transistor Tr
2
. Sufficient base current is required to turn on the transistor Tr
2
. The time required for securing the base current is the delay time. Thus, the on-period cannot be reduced to be sufficiently short, and the output power is increased. Moreover, when the first switching element Q
1
is turned off by the transistor TR
2
, the current flowing through the first switching element Q
1
is reduced, and the voltage applied across both ends of the resistor R is reduced. When this voltage becomes less than the base-emitter threshold voltage of the transistor Tr
2
, the transistor Tr
2
cannot be turned on. Thus, the on-speed is rapidly reduced. When the on speed of the transistor Tr
2
is low, and the delay time is long, the turn-off speed of the first switching element Q
1
becomes low. Thus, the switching loss is increased, and also, the on-period cannot be reduced when an overcurrent exists. The increase of the output current is more than that caused when the output voltage of the device is reduced. When the output voltage of the device is increased, inconveniences such as breaking of a primary diode and so forth are caused. Therefore, it is indispensable to rapidly turn off the first switching element Q
1
.
(2) When the primary peak current is limited to a predetermined value, the output power is restricted to have a substantially constant value. When the output voltage is reduced, the output current is increased. Thus, a secondary rectification diode and so forth may be broken.
(3) When the output is shortcircuited, the shortcircuit current is increased. Thus, a secondary rectification diode and so forth may be broken.
SUMMARY OF THE INVENTION
In order to overcome the problems described above, preferred embodiments of the present invention provide a self-excitation oscillation type switching power source device having an overcurrent protection circuit, in which a transistor circuit for controlling a first switching element Q
1
has an improved configuration in which when
Hosotani Tatsuya
Takemura Hiroshi
Murata Manufacturing Co. Ltd.
Riley Shawn
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