Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2003-02-10
2004-09-07
Riley, Shawn (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S021080, C363S056110, C307S103000
Reexamination Certificate
active
06788557
ABSTRACT:
FIELD OF INVENTION
The present invention relates to power converters, and more particularly, to a power converter that efficiently provides the required hold-up time during power line disturbance conditions.
BACKGROUND OF THE INVENTION
Many electronic devices require one or more regulated DC voltages. The power for such electronic devices is ordinarily supplied by a power converter that converts an input voltage into the regulated DC voltages required by the devices. Many types of power converters can operate over a wide input voltage range. If the input voltage falls below the minimum permissible voltage and adversely affects the converter operation, the electronic devices that rely on the converter for power could experience critical failures such as the loss of data. The length of time that the power converter can continue to operate in the absence of line voltage is referred to as the “hold-up” time. One known way that converters address this problem is to connect a bulk capacitor in parallel with the input power source. During normal operation, energy can be stored in the bulk capacitor to provide this hold-up time. The hold-up time depends upon the size of the bulk capacitor and the available duty cycle for the converter.
FIG. 1
shows a prior art AC-DC power converter
10
that includes a bulk capacitor for hold-up time. The power converter
10
includes an AC-to-DC boost converter
8
at the front end followed by a DC-to-DC converter
30
stage. A bridge rectifier
20
is arranged to convert an AC voltage applied at AC input terminals
14
and
16
to unregulated rectified DC pulses between terminals
15
and
13
. This unregulated DC, which may be unsmoothed DC, is switched by a switch
12
through a boost inductor
24
. The switch
12
is typically a MOSFET having a control signal input to its gate. The drive for the control signal input to switch
12
can be either a variable frequency or fixed frequency type, such that the input current is also sinusoidal with a minimum harmnonic distortion. Various integrated control circuits are known in the art for providing this drive (e.g., L4981, UC3854, and L6561). The boost converter
8
can operate as either a continuous current mode type or a discontinuous current mode type converter. Boost converter
8
develops a regulated DC bulk voltage across a bulk capacitor
18
. The boost ratio provided by boost converter
8
is such that this DC bulk voltage is marginally higher than the highest peak of the input AC voltage. The DC bulk voltage is regulated by means of the boost converter
8
. Converter
30
operates directly on this DC bulk voltage to provide the required isolation and secondary regulated voltage at DC output terminals
36
and
38
.
Upon failure of the line input AC voltage, the energy stored in the bulk capacitor
18
will keep the DC to DC converter
30
in an operational state for a period of time, the hold-up time, following this interruption of input power. For converter
10
, this hold-up time depends upon the size of the bulk capacitor
18
and the available duty cycle for the converter. Boost converter
8
typically has a wide regulation range due to the ability to operate at a nearly 100% duty cycle. The DC to DC converter
30
has limited operational duty cycle range and cannot operate over a very wide input voltage range. As a result, a larger bulk capacitor
18
is required to meet the hold-up time required to keep the DC output of the converter within acceptable limits.
Power converter
10
is presently commonly used and provides high performance characteristics. The supplied output voltage has line frequency ripple rejection. At low power levels, however, power converter
10
is expensive and has a high component count. Many low power applications exist which do not require fast transient response because of the nature of the load or the presence of fast post-regulators, at the outputs of converter
10
. A need therefore exists for a lower cost, lower component solution for low power applications.
FIG. 2A
shows a circuit diagram for a prior art AC-DC power converter
100
. Power converter
100
comprises a power factor corrected flyback converter that switches directly on the rectified AC input pulses. AC input power is applied at terminals
114
,
116
and is conventionally used to produce unsmoothed DC at terminals
113
,
115
through the use of a conventional bridge rectifier
20
. A capacitor
118
is connected in series with a diode
152
across the terminals
113
,
115
. Power converter
100
includes a transformer
128
having a primary winding
140
, a secondary winding
142
, and an auxiliary winding
144
, each having a first and second end. In power converter
100
, the auxiliary winding
144
provides the energy for recharging the capacitor
118
during each flyback cycle of the flyback converter
100
.
Primary winding
140
is conventionally switched on and off at a predetermined frequency by a first switch
112
. First switch
112
is typically a MOSFET having a control signal input at its control gate. The control signal input to switch
112
is typically a conventional pulse width modulation (PWM) or power factor correction (PFC) type drive signal (details not shown). Secondary winding
142
is connected to a rectifying and filter circuit comprising a diode
132
and a capacitor
134
, to produce the rated DC output voltage at terminals
136
and
138
.
The charging of capacitor
118
to a predetermined voltage is controlled by the circuit comprising auxiliary winding
144
, a resistor
154
connected in series with a diode
126
between one end of auxiliary winding
144
and one terminal of capacitor
118
, and a second switch
156
connected between the second end of auxiliary winding
144
and the other terminal of capacitor
118
.
In operation, when switch
112
of converter
100
closes, current flows in the transformer primary
140
and energy is stored therein. When the first switch
112
is opened during the flyback period of converter
100
, the polarity on the transformer
128
windings changes and rectifier diode
132
becomes forward biased. Diode
132
provides power to a load connected at DC output terminals
136
,
138
and stores energy in output capacitor
134
. During this flyback period when the first switch
112
is open, switch
156
is turned on and capacitor
118
is charged to a predetermined voltage determined by the turns ratio between primary winding
140
and auxiliary winding
144
.
The voltage on capacitor
118
is usually selected low (around 50V or so). In normal operation, when the instantaneous voltage of the rectified AC pulse across terminals
115
,
113
is higher than the voltage at which capacitor
118
is charged, diode
152
is reverse biased. Capacitor
118
will continue to hold its charge during this time. When this instantaneous voltage falls below the capacitor
118
voltage near the “valley point” of the rectified AC pulse, diode
152
becomes forward biased. As a result, capacitor
118
provides energy to transformer
128
to continue operation during this time. Capacitor
118
thus provides hold-up time during this period. Switch
156
can also be held off when the charge on capacitor
118
is being used by converter
100
in, order to reduce the peak currents in the transformer
128
.
One drawback of the circuit in
FIG. 2A
is that capacitor
118
fails to provide the larger hold-up time required in most applications. Moreover, this also impacts the power factor since current is not drawn near the bottom of the rectified AC pulse. If capacitor
118
is to provide a large hold up time, then a huge capacitor will be needed, since the voltage charge on the capacitor
118
is very close to the voltage that exists at the bottom of the rectified pulse. As a result, there is poor energy utilization.
FIG. 2B
shows a circuit diagram for another prior art flyback power converter that provides line harmonic correction. Power converter
110
comprises a fly back converter that switches directly on the rectified pulse and provides a DC out
Astec International Limited
Coudert Brothers LLP
Riley Shawn
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