Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Reexamination Certificate
2002-08-02
2004-08-17
Berhane, Adolf (Department: 2838)
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
C323S222000, C323S265000, C323S299000
Reexamination Certificate
active
06778416
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the field of electrical power converters, and more specifically, to AC-DC conversion circuits using a principle of intermittently charging a capacitor.
BACKGROUND OF THE INVENTION
Such converters are notably used directly for motorized products in buildings, such as small screens for solar protection, venetian blinds, etc. They make it possible to convert the AC voltage from the power mains (“mains”) into a lower virtually DC voltage capable of being used to supply an actuator with a low-power DC motor, for example of the CONCEPT-25 type product manufactured by the applicant. They may also supply only the control electronics of a more powerful AC motor, for example, for roller shutters, or a single sensor combined with a radio frequency receiver or transmitter.
These converters are termed “unregulated” because they operate without their output voltage being tied to a fixed reference. This simplifies the structure of the converter and lowers the cost thereof At the input, these converters receive an AC current, which may or may not be rectified; the half-wave or full-wave rectification of an AC current is considered in the present description as known in the art and is not described. At the output, these converters provide a virtual DC voltage with an amplitude less than the peak amplitude of the input voltage. In the remainder of the description, it is assumed that the input voltage is the mains voltage. This assumption is the most common for applications in buildings. It is clear, however, that other applications are possible, and that the input AC current of the converter, whether or not rectified, is not limited to the current supplied by the mains. These converters are called low-power converters, in that it is possible to tolerate therein a lower conversion efficiency than in a high-power plant, and therefore proportionally larger losses. The output power is typically less than 50 W.
In principle, as explained for example in FIG. 1 of U.S. Pat. No. 4,641,233, the converters use the intermittent charging of a high-capacitance capacitor, hereinafter called C. In this context, the term “high capacitance” refers to capacitances which are often greater than 100 &mgr;F. However, it will be understood that the capacitance of the capacitor used is simply sufficient for maintaining enough output voltage between the periods of intermittently charging the capacitor. As a result, the capacitance of the capacitor depends on the variation in voltage accepted at the output of the voltage converter, on the output current needed, and on the charging frequency, which itself depends on the input voltage frequency. For low output currents or for a high frequency of the input voltage, a capacitor with a lower capacitance at this threshold of 100 &mgr;F could be used. A switching element, hereinafter called Q, is placed between the half-wave or full-wave rectified mains and this capacitor C, so as to charge the latter only during time periods when the mains voltage remains below a given threshold. For a given load, it is thus possible to dimension C and to choose the threshold voltage so as to comply with a given service voltage and a given waveform at the output of the converter.
U.S. Pat. No. 4,001,668 (Lewis 1973) describes a device of this sort in FIG. 4. This patent emphasizes the very high tolerance to variations in the mains voltage, allowing the use of such a circuit to supply a motor with DC current whether power is provided by 110-120 V AC mains or from 220-240 V AC mains.
The conducting and non-conducting states of the switching element Q (
47
) are completely determined by comparison of a voltage, which is an image of the rectified voltage, with a threshold (
50
,
51
,
45
). It should be noted that the use of a current limiter formed by a resistor RP (
48
) and placed upstream of the voltage measuring point makes it possible to benefit from a cumulative effect favoring fast switching of Q from an off state to a conducting state or vice-versa. In
FIGS. 5 and 6
, this patent clearly indicates the pulsed shape of the line current (
61
,
65
) and of the downstream voltage waveform at the terminals of the load (therefore at the terminals of C) (
62
,
66
). Patents filed subsequently constitute particular embodiments making it easier to control the switch Q, or improvements relating to the reduction of harmonics generated by current spikes necessary for the periodic recharging of the capacitor.
GB-A-2 203 003 (Sanderson 1987) describes, in generic terms, a topology which is slightly different but based on the same principle. In
FIG. 2
, a current limiting device (
11
) is inserted between a switching element formed by a field effect transistor and the capacitor (
6
), the voltage measurement for controlling the switching element being carried out on C (by means of the block referenced
10
). This arrangement may remove the possibility of a cumulative effect for the effective control of Q. A device (
12
) is intended to take the switching element out of service when the voltage at its terminals exceeds a maximum threshold, however, this device is not described.
EP-A-0 763 878 (Helfrich 1995) describes, in
FIG. 1
, a converter topology using a current limiting device RP (R
1
), a switching element Q (Q
1
) and a capacitor C (C
2
) with composite control of Q both by the voltage upstream of RP (R
6
) and by the downstream voltage (R
6
). However, the voltage threshold is the same for the conducting and non-conducting state of the switching element.
Other embodiments of converters using this intermittent charge principle are described in DE-A-44 44612, DE-A-31 44742, EP-A-0 399 598, DE-A-32 45238, EP-A-0 249 259, FR-A-2 672 448, EP-A-0 500 113.
EP-A-0 622 889 (Wong 1994) describes a series topology (Q, C) without a current limiter. The input voltage V
in
—the voltage of the rectified mains—is applied to the series circuit of the capacitor and of the switching element. This circuit is intended to allow, as in the previous cases, double charging of C per half-cycle, and therefore to reduce the waveform upstream of a regulator. The device analyzes both the voltage at the terminals of C (
15
) and the voltage at the terminals of Q (
16
). Q becomes conducting if the voltage V
cap
at the terminals of the capacitor is less than a first given threshold. Q also becomes conducting if the voltage V
ds
at the terminals of Q, which is the difference between the rectified input voltage and the voltage at the terminals of the capacitor, becomes less than a second given threshold. In one half-cycle, this allows the capacitor to be charged when the voltage increases from zero, then the capacitor to be charged when the voltage decreases to zero.
U.S. Pat. No. 4,641,233 (Roy 1985) describes, with reference to FIG. 1, a similar topology. The switching device, a bipolar transistor, becomes conducting if the output voltage of the converter (downstream voltage) is less than the chosen reference voltage. However, the switching device only becomes conducting if the voltage upstream of the switching element is less than a voltage, which is about twenty volts for an output voltage of five volts. This is obtained using a second transistor, which is switched on according to the input voltage, and which switches off the first transistor when the input voltage is too high. According to the patent, the objective of the second transistor is to prevent damaging the components as a result of switching at a high voltage.
A drawback of this device is that the threshold values for the input voltage and the output voltage are only approximately set. It is known that the conducting threshold voltage of a bipolar transistor or of a single p-n junction are not accurately defined. The proposed solution involves replacing the second transistor by a precision reference-voltage source.
There is therefore a need for a converter, which makes it possible for the switching element to be accurately controlled, allowing the capacitor to be charged by the input voltage.
A second drawba
Berhane Adolf
Somfy SAS
Welsh & Katz Ltd.
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