Electricity: power supply or regulation systems – In shunt with source or load – Using choke and switch across source
Reexamination Certificate
2002-03-12
2004-02-10
Nguyen, Matthew V. (Department: 2838)
Electricity: power supply or regulation systems
In shunt with source or load
Using choke and switch across source
Reexamination Certificate
active
06690142
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a DC—DC converter for generating a higher, second DC voltage from a first DC voltage, which converter comprises
a first input terminal which is to be connected to the positive pole of a voltage source supplying the first DC voltage,
a second input terminal which is to be connected to the negative pole of the voltage source supplying the first DC voltage,
a first branch which interconnects the input terminals and comprises a series arrangement of an inductive element L and a switching element S
1
,
a control circuit, which is coupled to a control electrode of the switching element S
1
, and which is intended to generate a control signal for rendering the switching element S
1
conducting and non-conducting,
a second branch which connects a point of the first branch between the inductive element L and the switching element S
1
to the second input terminal, and which comprises a series arrangement of a unidirectional element D
1
and an output capacitor C
1
.
The invention also relates to a circuit arrangement for feeding a discharge lamp.
A DC—DC converter as mentioned in the opening paragraph is commonly referred to as up-converter or boost-converter and is frequently applied in, for example, electronic ballast circuits for feeding a discharge lamp. The known DC—DC converter is frequently operated in the so-termed “critical mode”. This means that, in each period of the control signal, the switching element is rendered conducting during a constant time interval. Subsequently, the switching element is rendered non-conducting, after which it is rendered conducting again as soon as the current through the inductive element has become equal to zero. Operating said converter in the “critical mode” has the advantage that the power dissipation in the unidirectional element is comparatively small while, at the same time, the frequency of the control signal is comparatively high, so that the inductive element L can be chosen to be comparatively compact.
The known DC—DC converter is frequently applied in, for example, electronic ballast circuits for feeding a discharge lamp. In this application, and in many other applications, the first DC voltage is generally formed by a full-wave rectified low-frequency AC voltage supplied by the mains. To satisfy statutory requirements regarding power factor and THD, it is necessary for the current drawn from the mains by the DC—DC converter to be an alternating current having the same frequency and approximately the same shape as the low-frequency AC voltage. In addition, this current must be in phase with the low-frequency AC voltage. In the case of the known DC—DC converter, which is operated in the “critical mode”, these two requirements cause the frequency with which the control signal renders the switching element conducting and non-conducting to depend very substantially on the instantaneous amplitude of the low-frequency AC voltage and the power drawn from the output capacitor. In practice it has been found that the integrated circuits that are generally applied in the control circuits of the known DC—DC converter set a practical limit to the frequency of the control signal. At frequencies above this limit, instabilities occur in the operation of the DC—DC converter that may lead to damage. As a result of this limit the DC—DC converter must be designed such that, even in the vicinity of the zero crossings of the low frequency AC voltage and at a small value of the power supplied by the DC—DC converter, the frequency of the control signal does not go beyond this limit. As a result, the frequency of the control signal is comparatively low at a comparatively high instantaneous amplitude of the low-frequency AC voltage and a comparatively high value of the power supplied by the DC—DC converter. This means that, for example, the inductive element L must be chosen to be comparatively large, which causes the DC—DC converter to become voluminous and expensive. Another drawback of the known DC—DC converter resides in that “hard switching” occurs if the instantaneous amplitude of the first DC voltage is higher than half the voltage present across the output capacitor. Hard switching means that the switching element is rendered conducting by the control signal while a comparatively high voltage is present across the switching element. This leads to a comparatively high power dissipation in the switching element, which may also cause damage to the switching element.
RELATED ART
U.S. Pat. No. 6,194,880 (Fraidlin et al.) discloses a DC—DC converter in which current flows bi-directionally through a boost inductor.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a DC—DC converter wherein hard switching is effectively counteracted, and which can be embodied so as to be comparatively compact and inexpensive.
To achieve this, a DC—DC converter as mentioned in the opening paragraph is characterized in accordance with the invention in that the DC—DC converter is further provided with
a third branch which comprises a further unidirectional element D
2
, and which connects the second input terminal to a point of the first branch between the inductive element L and the switching element S
1
,
a fourth branch which shunts the unidirectional element D
1
, and which comprises a further switching element S
2
a control electrode of which is coupled to the control circuit,
and in that the control circuit is provided with means for alternately rendering the switching element S
1
and the further switching element S
2
conducting and non-conducting with a frequency f, thereby causing the current through the inductive element L to change direction twice in each period of the control signal.
By virtue of the presence of the further switching element, it is possible to design a DC—DC converter in accordance with the invention in such a manner that hard switching is precluded at any value of the power supplied by the DC—DC converter and at any ratio between the instantaneous amplitude of the first DC voltage and the voltage across the output capacitor. It has also been found that if the DC—DC converter is fed from a supply voltage source supplying a low-frequency AC voltage, the frequency of the control signal does not have to be changed over a large range of both the power supplied by the DC—DC converter and the instantaneous amplitude of the first DC voltage to meet high requirements regarding power factor and THD. By virtue thereof, the frequency of the control signal can be chosen to be at a constant high value so that, for example, the inductive element L can be chosen to be small and the DC—DC converter can be embodied so as to be compact and inexpensive.
Said advantages are used in a preferred embodiment of a DC—DC converter in accordance with the invention, wherein the DC—DC converter is additionally provided with
a fifth branch which comprises an input capacitance and interconnects the first and the second input terminal,
a rectifier respective output terminals of which are connected to the first and the second input terminal, and which is provided with rectifier input terminals which are to be connected to a supply voltage source supplying a low-frequency AC voltage,
and the control circuit is provided with means for controlling each one of the switching elements in each period of the control signal in such a manner that the current drawn from the supply voltage source is an alternating current of the same frequency as the low-frequency AC voltage, and is in phase with the low-frequency AC voltage.
This preferred embodiment can very suitably be used in circuit arrangements that are fed from the mains that supply a low-frequency sine-shaped AC voltage.
In a further embodiment of a DC—DC converter in accordance with the invention, the control circuit is provided with
a microprocessor for generating, in dependence on the instantaneous amplitude of the low-frequency AC voltage and the power drawn from the output capacitor, a first signal that is a measure of the maximum amplitude Ipos of the curren
Koninklijke Philips Electronics , N.V.
Nguyen Matthew V.
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