Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2000-08-22
2002-04-30
Wong, Peter S. (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S021110, C363S097000, C363S131000
Reexamination Certificate
active
06381151
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to switching power supplies and, more particularly, the invention relates to a high efficiency switching controller for use in switching power supplies.
2. Description of Related Technology
Generally speaking, a switching power supply (SPS) provides a cost effective and energy efficient device for converting energy from a single direct current (DC) supply voltage into one or more DC output voltages that have a greater or lesser magnitude than the supply voltage. Traditionally, a SPS has an integrated control circuit that modulates the duty cycle of a transistor switch, which controls the flow of energy into the primary of a transformer to produce one or more desired output voltages that are derived from the secondary of the transformer. As is well known, the energy (i.e., the time integral of power) supplied to the primary of the transformer minus efficiency losses equals the energy transferred to the secondary of the transformer. Thus, if more energy is needed by the secondary, then the control circuit increases the duty cycle of the transistor switch to provide more energy to the primary of the transformer. Conversely, if less energy is needed by the secondary, then the control circuit decreases the duty cycle of the transistor switch.
FIG. 1
is an exemplary schematic block diagram of a conventional SPS, which includes a DC voltage supply block
10
, a voltage output block
20
, a feedback block
30
, and a switching control circuit
40
. The DC voltage supply block
10
includes a full wave bridge rectifier
1
and a filter capacitor C
1
. The bridge rectifier
1
rectifies alternating current (AC) line voltage to produce current pulses which are substantially smoothed to a DC supply voltage by the filter capacitor C
1
. For example, if the AC line voltage is 110 volts AC, then the smoothed DC supply voltage across capacitor C
1
may be approximately 155 volts DC.
The output voltage block
20
includes a switching transformer
22
having a primary winding L
1
and secondary windings L
2
and L
3
and switching rectifier diodes D
5
and D
6
that receive current pulses from the respective secondary windings L
2
and L
3
to provide rectified current pulses to respective filter capacitors C
2
and C
3
. The filter capacitors C
2
and C
3
smooth the rectified current pulses to substantially DC voltages.
The feedback block
30
includes a feedback voltage amplifier
31
and a photo-coupler
32
. The feedback voltage amplifier
31
detects the DC voltage across the filter capacitor C
2
and provides a proportional current to the photo-coupler
32
.
The switching control circuit
40
includes a pulse width modulated (PWM) signal generator
41
, a switching transistor M
1
, a flyback diode D
7
, and a feedback capacitor C
4
. The switching transistor M
1
is connected to the primary L
1
of the transformer
22
and is switched on and off by the PWM signal generator
41
at a duty cycle that is based on the magnitude of a feedback voltage Vfb provided by the feedback capacitor C
4
.
Initially, when AC line voltage is first provided to the bridge rectifier
1
, a supply voltage Vcc applied to the PWM signal generator
41
is substantially near zero volts DC and the PWM signal generator
41
is off. Additionally, because the PWM signal generator
41
is off, the switching transistor M
1
is off, energy is not provided to the primary winding L
1
, and the output voltages across the filter capacitors C
2
and C
3
are substantially near zero volts DC.
As is generally known, the PWM signal generator
41
is typically fabricated using conventional integrated circuit technologies and requires a relatively low DC supply voltage, which may be, for example, between 4 volts DC and 12 volts DC. Typically, the low supply voltage required by the PWM signal generator
41
is derived from the output voltage block
20
. Thus, as shown in
FIG. 1
, the supply voltage Vcc for the PWM signal generator
41
is provided by the voltage across the filter capacitor C
3
. Additionally, because the voltage across the filter capacitor C
3
is initially substantially near zero volts DC, a start up resistor R is connected between the filter capacitors C
1
and C
3
. The start up resistor R provides an initial charging current to the filter capacitor C
3
that causes the voltage across the filter capacitor C
3
to increase. When the voltage on the filter capacitor C
3
reaches a level sufficient to cause the PWM signal generator
41
to begin functioning, the PWM signal generator
41
regulates the voltage across the filter capacitor C
3
and the current flowing through the start up resistor R no longer increases the voltage across the filter capacitor C
3
.
Although the start up resistor R is needed to the start the operation of the PWM signal generator
41
, the start up resistor R becomes a significant source of energy inefficiency once the PWM signal generator
41
is operational. More specifically, a large voltage differential exists across the start up resistor R because the difference between the output voltage of the DC voltage supply block
10
is substantially greater than the low voltage supply Vcc for the PWM signal generator
41
. For example, the output voltage of the DC voltage supply block
10
may be 155 volts DC while the low voltage supply Vcc is about 5 volts DC. This large voltage drop across the start up resistor R during continuous operation of the SPS results in a significant source of energy inefficiency.
SUMMARY OF THE INVENTION
In accordance with one aspect of the invention, a high efficiency switching controller for use in a switching power supply having a voltage source, a transformer with a primary winding coupled to the voltage source and a secondary winding, a switching transistor coupled to the primary winding, an output voltage circuit coupled to the secondary winding, and a feedback circuit coupled to the output voltage circuit. The high efficiency switching controller includes a current control device coupled to the voltage source and a switch connected between the current control device and the output voltage circuit. The high efficiency switching controller may also include an under voltage lockout regulator coupled to the output voltage circuit and the switch that controls the state of the switch based on a voltage of the output voltage circuit.
The high efficiency switching controller may further include a bias unit coupled to the under voltage lockout regulator that provides current to circuitry within the switching controller based on the voltage of the output voltage circuit, a source/sink unit coupled to the feedback circuit, a first comparator coupled to the source/sink unit, an oscillator coupled to the first comparator, and a pulse width generator coupled to the oscillator and an output of the first comparator that generates a gate drive signal having a duty cycle based on an output of the feedback circuit.
Additionally, the high efficiency switching controller may include a protector coupled to the feedback circuit, the under voltage lockout regulator, and the pulse width generator and an adjuster coupled to the under voltage lockout regulator and the protector. The protector may provide a control signal to the pulse width generator that controls the gate drive signal in response to an operating condition of the switching controller. The operating condition of the switching controller may be associated with a thermal condition or, alternatively, may be associated with an excessive load on the switching controller. Also, the control signal may periodically enable the pulse width generator in response to a voltage of the output voltage circuit so that the gate drive signal includes groups of gate drive pulses.
The high efficiency switching controller may still further include a leading edge blanking unit coupled to the pulse width generator, a second comparator having a first input that monitors a current flowing through the switching transistor and a second input that receives a
Fairfield Korea Semiconductor Ltd.
Laxton Gary L.
Marshall Gerstein & Borun
Wong Peter S.
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