Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2002-05-09
2004-11-23
Nguyen, Matthew V. (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
Reexamination Certificate
active
06822881
ABSTRACT:
DESCRIPTION
The invention relates to a resonant converter comprising a control system, a control method for a resonant converter and a switched-mode power supply.
A switched converter converts a DC voltage on the input side into one or more DC voltages on the output side in that the input-side DC voltage is first chopped, i.e. converted into a switched AC voltage and with this switched AC voltage a resonant circuit comprising at least one capacitor is supplied with power, which capacitor comprises the primary side of a transformer. On the secondary side the transformer comprises one or various windings whose voltages are rectified for generating DC output voltages.
Known switched-mode power supplies comprise a power supply input circuit for connection to the mains and a switched converter. The switched-mode input circuit renders an intermediate-circuit DC voltage available with which the switched converter is supplied. The intermediate-circuit DC voltage is converted into one or more output DC voltages by the converter.
Many circuits for switched converters are known. This comprises in addition to resonant converters also circuits in which no resonant circuit is used. With converters of this type it is possible to manufacture inexpensive, small, lightweight power supply units/switched-mode power supplies, which can advantageously be used in consumer electronics appliances such as set top boxes, satellite receivers, television sets, computer monitors, video recorders and compact audio systems. In these applications there is often a need for converters that generate multiple output voltages on multiple converter outputs from one input DC voltage.
At least one of the output voltages is customarily controlled to a set value. In state-of-the-art converters which produce a plurality of output voltages and each of the output voltages is assigned a secondary winding of the transformer, various output voltages cannot be controlled independently of one another. In such circuits a control device is provided for only one of the output voltages. It is then assumed that the other voltages—which are related to a controlled voltage as regards the number-of-windings ratio—are “controlled along” with these voltages. However, this has considerable disadvantages at the individual outputs in case of strongly differing loads.
A known topology of a converter comprises the so-termed load resonant converter. In a known circuit for this a half bridge supplied with a DC voltage is used as an inverter, which half bridge feeds a series combination of a resonant capacitor and the primary side of a transformer. The resonant capacitor together with the leakage inductance of the transformer as well as further, also secondary-side inductances or capacitances, forms a resonant circuit. On the secondary side the load resonant converter includes one or more secondary windings. In this way a number of output DC voltages is supplied which, after being rectified, are customarily filtered by at least one capacitive filter.
To control the output voltage of such a resonant converter it is known to change the driving of the inverter. The switches of the inverter are then driven so that an AC voltage, in many cases a pulse-width modulated voltage, is generated that has predefined parameters (for example, frequency). By variation of the frequency of this voltage, the value of the output voltage can be controlled. The output voltage then rises the more the frequency comes nearer to the resonant frequency of the resonant circuit. LLC converters commonly operate in the hypercritical domain i.e. supply of the resonant circuit with a voltage whose frequency lies above the resonant frequencies. In this case the output voltage can be increased in that the frequency of the voltage is reduced. In known load resonant converters only one output voltage can be controlled directly. Further output voltages are coupled to the directly controlled output voltage via the number-of-windings ratio and are thus “controlled along”.
The type of converter dominating in consumer electronics appliances is the flyback converter. This is a non-resonant converter. On the primary side generally only one switching element is necessary for the inverter. The flyback converter executes a one-way rectification on each of its outputs. One of the outputs is controlled directly.
If the flyback converter needs to have a second output voltage which must be controlled directly, it is known that a further converter known as a step-down converter or Buck converter is to be connected to one of the outputs of the flyback converter, which further converter is fed with the first output voltage of the flyback converter and produces the second output voltage with a separate control. Such a circuit comprising two converters is very costly, however.
Another extension of the flyback converter topology which renders two controlled output voltages available is the so-termed “double forward-flyback” converter. A respective topology is described, for example, in IEEE-PESC 1988, p. 142 “A Complete Study of the Double Forward-Flyback Converter” by J. Sebastian et al. As with the basic flyback topology, this is not a resonant circuit, but the primary-side AC voltage which is generated via a simple switch directly feeds the primary side of the transformer. On the secondary side there are two secondary units each formed by a secondary winding of the transformer and one one-way rectifier element (diode). The resulting secondary voltages are filtered capacitively by one secondary unit and inductively by the other secondary unit. In this way it is possible to control an (inductively filtered) output voltage via the duty cycle of the pulse-width modulated voltage and the other (capacitively filtered) output voltage via the frequency of the pulse-width modulated voltage. But this “hard switching” topology has considerable switching losses.
In modem consumer electronics appliances it is ever more necessary for two supply voltages can be controlled separately.
It is therefore an object of the invention to provide a resonant converter and a control method which can be realized cost effectively and nevertheless offer the possibility of control of a plurality of output voltages.
This object is achieved by a resonant converter as claimed in claim
1
and a control method as claimed in claim
13
and a switched-mode power supply as claimed in claim
14
. Dependent claims relate to advantageous embodiments of the invention.
According to the invention a resonant topology is proposed i.e. a resonant circuit is fed by an inverter which resonant circuit comprises, for example, a series capacitance and the primary side of a transformer. Also further, secondary-side elements may be part of the resonant circuit. In such a resonant topology the output voltage can be controlled via the frequency of the primary-side AC voltage. By hypercritical operation there may be achieved with such a resonant converter that the resonant circuit at the source behaves as an inductive load, so that a switching without losses is possible (zero-voltage switching).
The separate control of different output voltages is made possible according to the invention in that two types of secondary units are provided, each formed by a secondary winding of the transformer and at least one rectifier element. A first secondary unit (a first type of secondary units, respectively) and a second secondary unit (a second type of secondary units, respectively) here have opposite orientations. The orientation is here to be understood as the direction of winding in conjunction with the wiring with the rectifier element. For example, two secondary units of opposite types may be distinguished in that with otherwise the same circuit the direction of winding on the common transformer core is opposite. It is also possible, when two secondary windings have the same direction of winding, that the secondary unit of the first and second type are distinguished by a respective converted wiring. Wiring is here understood to mean the connection of the rect
Koninklijke Philips Electronics , N.V.
Nguyen Matthew V.
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