Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Reexamination Certificate
2001-09-24
2003-10-07
Berhane, Adolf D. (Department: 2838)
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
C363S021030
Reexamination Certificate
active
06631082
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a switching power supply unit such as a DC-DC converter, and more particularly to a switching power supply unit that is capable of reducing noise and energy loss.
2. Description of Related Art
As a conventional power supply, a switching regulator is widely used which is constructed such that a switching device serially-connected to a transformer is turned on/off to convert a direct-current voltage into an alternating-current voltage, which is then applied to a primary winding of the transformer to cause a secondary winding thereof to generate an alternating-current voltage stepped down according to a turn ratio thereof, and the alternating-current voltage is rectified and smoothed to obtain a direct-current voltage.
A one-chip type forward converter as shown in
FIG. 12
is known as an example of the switching regulator.
The one-chip type forward converter shown in FIG.
12
is constructed such that direct-current voltage Vin obtained by rectifying and smoothing a commercial alternating-current power supply input by a rectifier circuit, not shown, is applied to a serial circuit that is comprised of an insulating transformer T and a semiconductor switching device Q
1
.
A reset circuit that is comprised of a diode D
1
, a capacitor C
1
and a resistance R
1
is connected to both ends of a primary winding N
1
of the insulating transformer T.
A pair of a rectifier diode Ds
1
and a flywheel diode Ds
2
is connected to a secondary winding N
2
of the insulating transformer T. The respective cathodes of the diodes Ds
1
and Ds
2
are connected to each other and are connected to the positive end of a smoothing capacitor C
2
via a choke coil Lo. The negative end of the capacitor C
2
is connected to an anode of the diode Ds
2
and the positive end of the secondary winding N
2
. These circuits constitute a secondary rectifier/smoothing circuit.
There will now be outlined the operations of the forward converter in FIG.
12
.
The semiconductor switching device Q
1
is turned on/off in synchronism with a drive signal. This drive signal is generated by a conventional PWM control circuit (not shown) that controls the ON/OFF period ratio of the semiconductor switching device Q
1
while monitoring a direct-current output voltage from the converter so that the direct-current output voltage becomes equal to a desired constant voltage.
Under the control of the PWM control circuit, the semiconductor switching device Q
1
is switched at a substantially higher frequency than that of a commercial alternating-current power supply (50 Hz or 60 Hz). This causes the direct-current input voltage Vin to be applied to the primary winding N
1
of the insulating transformer T only while the semiconductor switching device Q
1
is ON, and causes the secondary winding N
2
of the insulating transformer T to generate an alternating-current voltage according to a turn ratio of the transformer T. The generated alternating-current voltage is rectified by the diodes Ds
1
and Ds
2
, and smoothed by the choke coil Lo and the smoothing capacitor C
2
, and a direct-current output of a predetermined voltage is acquired between the terminals of the smoothing capacitors C
2
.
When the semiconductor switching device Q
1
is turned off, excitation energy accumulated in the insulating transformer T during the ON period is converted into thermal energy and consumed by the resistance R
1
in the reset circuit comprised of the diode D
1
, the capacitor C
1
and the resistance R
1
, so that the excitation energy is reset. This enables absorption of a surge voltage.
In this case, the maximum duty of the PWM control circuit is set to be not greater than 50% in terms of the time required for resetting the excitation energy accumulated in the transformer T.
Thus, in the case of the forward converter, the insulating transformer T becomes saturated unless the excitation energy therein is reset.
The CRD circuit serving as a conventional reset circuit causes its resistance to consume the excitation energy, and this results in energy loss.
The switching power supply unit that acquires a direct-current output by interrupting a voltage by the switching device causes power loss since changes in current and voltage overlap with each other in the turn-on period and turn-off period of the switching device (see
FIGS. 13A
to
13
D).
To eliminate such energy loss, a partial resonance type power supply has been proposed in which a capacitor is connected in parallel with a switching device, and when the switching device is turned off, a surge voltage is absorbed by resonance and a terminal voltage across the capacitor is gradually raised so as to reduce switching loss such as the above-mentioned energy loss and power loss, and then energy accumulated in the capacitor is regenerated to an input side.
In such a partial resonance type power supply, a capacitor Ccv is connected in parallel with a main switching device Q
1
via an auxiliary switching device Q
2
, and the ON/OFF timing of the auxiliary switching device Q
2
is shifted from that of the main switching device Q
1
as shown in
FIG. 3
such that excitation energy is once accumulated in the capacitor Ccv and then the excitation energy is regenerated to an input side, and also the main switching device Q
1
is switched after the terminal voltage across the main switching device Q
1
is lowered to zero.
The partial resonance type power supply has the following advantages, for example: (1) it can be controlled by PWM; (2) the zero-voltage switching enables a reduction in switching loss and noise; and (3) the regeneration of excitation energy reduces reactive power. In particular, the partial resonance type power supply controlled by PWM has the following advantages: (4) it is possible to use an inexpensive PWM control IC; and (5) it is easy to design the transformer and reduce noise since the driving frequency is fixed.
To reduce the loss in the regeneration of electric current in the partial resonance type power supply, a variety of methods have been proposed: e.g. an inductance is inserted in parallel with the secondary winding, or a tertiary winding is used.
With any of these methods, however, if a forward converter is used as the partial resonance type power supply, a rectifier diode and a flywheel diode are made to conduct at the same time for a long period of time in a rectifier diode section on the secondary side as shown in FIG.
14
. Therefore, at the same input voltage, the power transmission efficiency is deteriorated compared with the conventional power supply unit although the loss is reduced.
If the conventional forward converter controlled by PWM is modified directly into a partial resonance type forward converter, since the maximum duty of the main switching device is not greater than 50% due to the reset period, if the load current is increased, regenerative current flowing to the input side during the OFF period is larger than current flowing during the ON period during which power is transmitted to the secondary side. This may cause excitation of the insulating transformer T in the negative direction (negative excitation).
Even if the restraint on the maximum duty can be released, since the timing of driving the auxiliary switching device is determined according to the time period that is required for discharging regenerative current in a light load state, in the case of a power supply unit installed in an apparatus such as a copying machine whose load fluctuates in a very wide range because most of the load is applied by a motor, in a heavy load state regenerative current during the OFF period is larger than the current flowing during the ON period during which power is transmitted to the secondary side as in the case where the maximum duty is restrained. This results in the negative excitation of the transformer T. Therefore, the resonance has to be made in a limited load range. Consequently, the effects of low loss and low noise obtained by zero-voltage switching cannot be achieved in a lig
Berhane Adolf D.
Laxton Gary L.
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