Transformerless power supply circuit with a switchable...

Details Transformerless power supply circuit with a switchable... Transformerless power supply circuit with a switchable...

2000-07-13

2001-10-09

Riley, Shawn (Department: 2838)

Electricity: power supply or regulation systems

Output level responsive

Using a three or more terminal semiconductive device as the...

C323S285000, C363S126000

Reexamination Certificate

active

06300748

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a power supply circuit, and more particularly, to a DC power supply circuit which converts AC power to DC power.
BACKGROUND OF THE INVENTION
Many consumer and commercial devices require direct current (DC) power. Since alternating current (AC) power is readily available, power supply circuits which convert AC power to DC power are desirable.
A block diagram of a conventional power supply circuit
10
is depicted in FIG.
1
. The power supply circuit consists of a voltage reducing device
11
, rectifier
12
, filter
13
, and regulator
14
. The voltage reducing device
11
steps the AC voltage down since DC-powered devices generally operate at a lower voltage (e.g., less than 12 volts) than commercially-supplied AC power (e.g., 120 volts). Next, the rectifier
12
converts the lower voltage level AC voltage to a pulsating DC voltage. The pulsating DC voltage is then filtered and regulated by the filter
13
and the regulator
14
, respectively, to produce a relatively smooth DC voltage level.
FIG. 2
depicts a known power supply circuit
21
which embodies the functionality depicted in FIG.
1
. The power supply circuit consists of a transformer
22
, a full-wave bridge rectifier
23
, and a capacitor
24
. The transformer
22
steps down the input AC voltage to a usable level. The full-wave bridge rectifier
23
, consisting of diodes D
1
-D
4
, converts the low level AC voltage into a pulsating DC voltage. The capacitor
24
filters and regulates the pulsating DC voltage to achieve a smooth DC output voltage level.
Although the conventional circuits such as the one described above have been used successfully, their design has significant shortcomings. For example, transformers tend to be heavy and bulky and thus unsuitable for miniaturized packaging. Transformers also tend to be relatively expensive given their inherent core material and windings requirements which are unlikely to be eliminated by technological advances. Aside from transformers, conventional circuits also tend to generate significant heat. Heat is generated not only by the normal operation of the circuit's components, but also by DC power generated by the circuit in excess of the load's requirements. To dissipate heat, the components are typically oversized which increases their cost and, again, poses problems in miniaturized packaging. Furthermore, as the temperature of the components rises, their operating characteristics tend to vary and their potential for failure increases.
Attempts have been undertaken to develop transformerless power supply circuits that are more efficient and compact. An example of such a circuit is depicted in FIG.
3
. As shown, the circuit includes a rectifier in the form of a diode D
1
and a current limiting resistor R
1
at the AC input side of the power supply circuit. The circuit has a regulator in the form of a Zener diode D
2
at the output side of the power supply circuit. As the AC input voltage rises, the output voltage supplied to a load increases until the output voltage exceeds the breakdown voltage of the Zener diode D
2
(e.g. 5 Volts), causing the Zener diode D
2
to conduct, thereby limiting the output current and voltage being supplied to the load.
In the power supply circuit of
FIG. 3
, excess power must be dissipated when the output voltage level is above the breakdown voltage of the Zener diode D
2
. The excess power is dissipated by the resistor R
1
, which must be physically large to dissipate the heat resulting from the large voltage drops across it. The power rating of the chosen resistor R
1
will also limit the maximum AC input voltage that may be applied to the power supply circuit. For example, an input voltage of 120 Volts AC (VAC), with a Zener diode having a breakdown voltage of 5 Volts and an average current draw of 15 milliamps (ma), requires resistor R
1
to have a minimum power rating of 1.73 Watts. An input voltage of 240 VAC, requires a resistor with a minimum power rating of 3.6 Watts. Thus, when the power rating of resistor R
1
is chosen, the maximum level of AC input voltage is fixed.
Another known transformerless power supply circuit is disclosed in FIG.
4
. As shown, the output side of the circuit contains a rectifier diode D
1
leading to the load to ensure that only DC power is supplied to the load. The capacitor C
1
is charged during positive portions of the AC cycle when the AC voltage is rising. During declining and negative portions of the AC cycle, the capacitor C
1
supplies the power to the load.
Like the circuit of
FIG. 3
, however, the circuit disclosed in
FIG. 4
requires physically large components to dissipate excess power due to varying levels of current being drawn by the load. For example, excess power must be dissipated when the output current at the output side of the power supply circuit exceeds the load current being drawn by the load. In the configuration shown, a large capacitor C
1
and resistor R
1
are required to handle the relativley large amount of power which must be dissipated. Furthermore, like the circuit of
FIG. 3
, the level of acceptable AC voltage is limited by the power ratings of the individual components.
Therefore, there is a need for a transformerless AC to DC power supply that is compact and efficient but avoids the need for oversized components to dissipate power. There is also a need for a power supply circuit that can accommodate different levels of AC input voltage without requiring its components to be changed out or oversized for the highest expected input power. The present invention fulfills these needs among others.
SUMMARY OF THE INVENTION
The present invention provides for a high-efficiency, transformerless power supply circuit capable of producing DC output from an AC input without the need for oversized components to dissipate excess energy. The circuit operates by coupling an AC input to a DC output terminal and to a capacitive element when the AC input is within a preselected AC voltage range, and uncoupling the AC input from the DC output terminal and capacitive element when the AC input is outside the preselected AC voltage range. Thus, the AC input supplies power to the DC output terminal while charging the capacitive element when within the preselected AC voltage range. When outside the preselected AC voltage range, the AC input is removed from the DC output terminal and power is supplied to the DC output terminal by the capacitive element. Preferably, the AC input is uncoupled from the DC output terminal for high portions of the AC input voltage.
The circuit of the present invention avoids many of the problems faced by conventional AC to DC circuits by connecting the circuit to the AC power supply for only preselected portions of the AC input signal. For example, the circuit avoids the use of a transformer and its attendant shortcomings by coupling the DC output terminal to the AC input terminal only when the AC input voltage level is within a preselected range. Therefore, there is no need to reduce the voltage. Additionally, since only selected portions of the AC input signal are used (which allows voltages to be avoided) components of the DC power supply circuit do not need to be oversized to dissipate excess power. Furthermore, the range of acceptable AC input voltages is not limited by the power ratings of the individual component because the circuit is uncoupled from the AC power supply when the voltage becomes too high. This facilitates the use of components with optimal power ratings (i.e., not overrated) which, in turn, facilitates the circuit's implementation in an integrated circuit. These advantages lead to a compact, efficient, and flexible power supply circuit.


REFERENCES:
patent: 3373344 (1968-03-01), Seer, Jr.
patent: 4060758 (1977-11-01), Wright
patent: 4480201 (1984-10-01), Jaeschke
patent: 4626982 (1986-12-01), Huber
patent: 4893228 (1990-01-01), Orrick et al.
patent: 5132893 (1992-07-01), Klein et al.

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