AC-DC converter

Details AC-DC converter AC-DC converter

2001-04-20

2002-10-29

Patel, Rajnikant B. (Department: 2838)

Electric power conversion systems

Current conversion

With condition responsive means to control the output...

C323S222000

Reexamination Certificate

active

06473322

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to an AC-DC converter comprising
input terminals which are to be connected to the poles of a supply voltage source supplying an alternating voltage and output terminals,
rectifier means coupled to the input terminals for rectifying the alternating voltage,
an inductive element coupled to the rectifier means,
a buffer capacitance coupled to the output terminals,
a unidirectional element coupled between the inductive element and the buffer capacitance,
a switching element coupled to the inductive element for controlling a current through the inductive element,
a control circuit coupled to a control electrode of the switching element for generating a periodic control signal for rendering the switching element alternately conducting and non-conducting at a frequency f, and provided with
a first circuit part for setting a first time interval t-on during which the switching element is conducting in each period of the control signal, and
a second circuit part for setting a second time interval t-off during which the switching element is non-conducting in each period of the control signal.
Such an AC-DC converter is disclosed in U.S. Pat. No. 4,683,529. The control circuit of the known AC-DC converter renders the switching element conducting during a first time interval t-on, which is substantially constant during each half period of the alternating voltage supplied by the supply voltage source. During the first time interval t-on, the current in the inductive element increases substantially linearly. The value of t-on corresponds to the power taken at the output terminals. As the value of t-on is substantially constant during each half period of the alternating voltage, the value of the current taken from the supply voltage source, averaged over a period of the control signal, is substantially proportional to the alternating voltage. It is thus achieved that the power factor of the known AC-DC converter is comparatively high and the THD generated by the AC-DC converter is comparatively low. During the second time interval t-off, the current in the inductive element decreases substantially linearly. In the known AC-DC converter, the control circuit renders the switching element conducting again almost immediately after the current in the inductive element has become substantially equal to zero. This control of the switching element is referred to as “transition mode”. As the current in the inductive element is substantially zero, the same applies to the current through the unidirectional element. It is thus achieved that, when the switching element becomes conducting, only a comparatively small power dissipation occurs in the unidirectional element.
The frequency of the control signal is often chosen to be comparatively high because this enables both the inductive element and an EMI filter, which is often arranged between the input terminals and the rectifier means, to be chosen so as to be comparatively small. As a result, the AC-DC converter is comparatively small and inexpensive. However, if the power taken at the output terminals decreases, or if the amplitude of the alternating voltage supplied by the supply voltage source increases, the value of t-on is reduced by the control circuit. Also at this lower value of the power taken and/or at a higher value of the maximum amplitude of the alternating voltage, the known AC-DC converter operates in the transition mode, as a result of which the frequency of the control signal increases substantially. A drawback of the known AC-DC converter resides in that, at a high frequency, the majority of the known control circuits are insufficiently capable of sufficiently accurately controlling the time interval t-on, so that instabilities in the operation of the AC-DC converter may occur. The amount of power dissipated in the switching element also is comparatively high at a comparatively high frequency of the control signal.
BRIEF SUMMARY OF THE INVENTION
It is an object of the invention to provide an AC-DC converter which can operate in a stable manner over a large range of the power taken at the output terminals and over a large range of the amplitude of the alternating voltage supplied by the supply voltage source, and which has a high power factor, a small THD and a small power dissipation in the components.
To achieve this, an AC-DC converter of the type mentioned in the opening paragraph is characterized in that the second circuit part comprises a third circuit part for setting the second time interval t-off at a value that is equal to the expression
C
2
*(
C
1
*
t
-
on
+(1+
C
1
)*
t
-
d
),
wherein t-d is a third time interval during which the unidirectional element is conducting in each period of the control signal, and C
1
and C
2
are parameters which have a constant value during each half period of the alternating voltage, whereby C
2
>0, C
1
=>0 and t-off>t-d.
The third circuit part brings about that the second time interval t-off lasts longer than the time interval t-d. During the time interval &Dgr;t=t-off−t-d, the inductive element, the unidirectional element and the switching element do not carry current. Consequently, during the time interval At no power is taken from the supply voltage source, and no power is supplied to the buffer capacitance. Such a control of the switching element of an AC-DC converter is referred to as discontinuous mode. In comparison to operation in the transition mode, wherein the duration of a period of the control signal is equal to the sum of t-on and t-d, the duration of a period of the control signal is extended by At. An AC-DC converter in accordance with the invention is frequently dimensioned such that the AC-DC converter operates in the transition mode at a nominal value of the maximum amplitude of the alternating voltage and a nominal value of the power taken at the output terminals. If, for example, the power taken at the output terminals decreases or the maximum amplitude of the alternating voltage increases, then the third circuit part brings about a transition from operation in the transition mode to operation in the discontinuous mode. As a result, the increase of the frequency of the control signal is less than it would have been if the AC-DC converter had continued operating in the transition mode. By virtue thereof, the operation of the AC-DC converter remains stable and the power dissipation in the switching element is limited. It has also been found that the power factor of an AC-DC converter in accordance with the invention is relatively high over a large range of the power taken at the output terminals and over a large range of the amplitude of the alternating voltage supplied by the supply voltage source, while the THD generated by the AC-DC converter is comparatively low. In addition, the frequency of the control signal can be chosen to be comparatively high at a nominal load and a nominal value of the amplitude of the alternating voltage supplied by the supply voltage source. As indicated hereinabove, this has the advantage that the inductive element, and a possible EMI filter, can both be small.
More particularly, favorable results have been achieved with embodiments of an AC-DC converter in accordance with the invention wherein C
1
>0 and C
2
=1. In this case, the time interval &Dgr;t is proportional to the sum of t-on and t-d, as a result of which the ratio between the average values of the current through the inductive element in two successive periods of the control signal during operation in the discontinuous mode is equal to the ratio during operation in the transition mode. In other words, the form of the current taken from the supply voltage source remains substantially unchanged. As a result, the power factor of the AC-DC converter remains comparatively high and the generated THD remains comparatively low if the third circuit part causes the AC-DC converter to change over to operation in the discontinuous mode. Such an embodiment of an AC-DC converter in accordance with t

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