The present invention relates generally to electrical power supplies which operate from a main AC line power source. More particularly, the present invention relates to a cost-, weight- and volume-effective discrete circuit arrangement for providing a source of isolated voltage, either low or high potential at low output power levels.
Power converters which operate off of the AC main line are used throughout the world to provide voltages and currents necessary to operate electrical and electronic systems. These power supplies are commonly referred to as main AC line power supplies. Construction of main AC line power supplies is well known in the art, having been in existence since the early 20th century for applications such as battery chargers and welders and later for powering radios, televisions and computers. At present, most AC main line power supplies fall into two different categories. One group converts and isolates the 60 cycle AC waveforms to other voltages using a laminated iron transformer. In this manner the desired output voltage may be galvanically isolated from the main AC line circuit for safety reasons. Into this group fall devices such as the common battery charger for cell phones. It is important for safety considerations that the output voltages from this converter be fully isolated from the AC main line, otherwise a lethal circuit may be set up between the output of the power supply and earth ground that the user may come in contact with (such as a water pipe).
A second type of power converter has emerged within the last two decades which offers lighter weight efficiency than the previous type. This is the switching power supply. It, too, offers isolation from the AC main line and has found uses in computers and entertainment systems. Switching power supplies owe their weight and size efficiency to the use of high frequency ferrite converter transformers which do not rely on laminated iron for a core material. They can be made lightweight and highly efficient due to the absence of eddy current flow which is ever present in laminated devices.
Some AC main line power supplies are used for low power applications. For example, a trickle battery charger, which is to be left on to maintain the charge level in a standby battery needs only to supply a few milliwatts of power to compensate for the natural decline in a battery's condition and keep the battery at ready status. Many high voltage applications require only a low power system. Air purifiers which remove particulate matter in an air stream by electrostatically charging metallic collection plates upon which the foreign matter is deposited, only require a few milliwatts for operation.
A typical prior art AC main line power supply of the first type is shown in
As mentioned earlier, another form of prior art AC main line power supply is the switching supply. Recently these have found use in low power battery chargers for cell phones. Here the bulky and heavy laminated iron transformer is replaced with a low cost miniature transformer whose core is made of ferrite material. The switching power supply converts the AC line voltage to a high DC voltage by direct rectification and filters this rectified waveform with a large energy storage capacitor. From this point on, this design utilizes high voltage switching transistors to drive the high frequency ferrite transformer. The use of high frequency power conversion allows the use of smaller magnetic devices and filtering capacitors at the expense of a relatively complicated control and driver circuitry.
In the prior art circuitry arrangements discussed above, the 60 cycle main AC line iron laminated transformer can account for substantial weight, size and cost of the power supply. This is due to the fact that the cross sectional area of the transformer is inversely proportional to the frequency of operation. Since operation at a fixed line frequency of 50 or 60 Hz is usually the case, the only technique for size reduction in the laminated iron transformer is to increase the number of primary windings, which unfortunately only increases cost and Ohmic power losses. Switching power supplies can use a smaller and lower cost ferrite transformer because their operating frequency is usually set above 20 kHz.
As will be described in the following detailed description, the present invention overcomes many of the cost, size, weight and complexity problems associated with prior art low power AC main isolated power supplies by replacing the costly laminated iron transformer with a lower cost, smaller size, and higher efficiency ferrite transformer and replacing switching converter components with an inexpensive bidirectional two state device. Examples of this device are a SIDAC, DIAC or even a gas plasma lamp. Solid state SIDACs are bi-directional devices primarily intended for use in arc or gas plasma illumination applications. SIDACs are mainly used for spark initiation in high pressure gas discharge lamps. The singular conduction characteristics of a bi-directional two state device may be advantageously adapted to the present invention as will be described in more detail in the following paragraphs. As a result of the replacement of the laminated iron transformer, or replacement of complex drive switching circuitry with the circuitry in this invention, low power AC main line to DC isolated converters, both low and high voltage can be manufactured at significant cost, volume and weight savings.
Because of their unique advantage in generating switching waveforms, SIDACs for use in power supplies are found in prior art designs.
The invention that will be described in the following paragraphs has several advantages over prior art. First, this converter operates directly with incoming AC main line voltage without the need for initial DC conversion as in prior art SIDAC designs. Secondly, it utilizes a ferrite type transformer which offers reduced weight, lower cost, and simplicity of construction over laminated iron transformers. Third, since high conversion frequencies are used in this power supply, it is easy to construct high voltage power supplies with low output ripple and high voltage regulation with large step up ratios utilizing multi-stage voltage multiplier circuits. This is usually precluded in simple 60 Hz designs due to size limitations of high voltage capacitors. Finally, the use of a series capacitor in this design allows for complete commutation of the bi-directional two state device, unlike prior art designs. It cannot latch up if operated into a short circuit.
Circuit arrangements and methods are disclosed for producing low power AC to DC voltage conversion direct and isolated from the AC main line. In the case of a step down converter, the circuitry consists of a high frequency ferrite transformer being driven by waveforms produced by the combination of a bi-directional two state device, e.g. a DIAC or SIDAC, in series with a capacitor. The changing AC main line waveform allows the electronic device to breakover at certain points in time providing high frequency pulses, rich in harmonics to drive the primary of the ferrite transformer. The transformer provides a specific reduction in voltage corresponding to its turns ratio. The secondary of the ferrite transformer drives a rectifier stage and ripple removing capacitor arrangement and provides an output voltage across the terminals as shown.
The operation of this invention may be easily understood by examination of
In a second embodiment, a high voltage converter is shown in
In either case, the AC line voltage is converted to an isolated DC voltage by using the singular properties of the bidirectional two state device, without the use of resistors or rectifier diodes on the AC input primary side of the converter. The high frequency pulses generated by this switching technique drive a compact ferrite transformer which provides power to a rectifier/capacitor output stage.
The present invention discloses circuit arrangements and methods for construction of an AC main line operated isolated power supply utilizing a bi-directional two state device such as a SIDAC, DIAC or gas plasma lamp. In the following description, for purposes of explanation, specific numbers, times, frequencies, dimensions, waveforms, and configurations are set forth in order to provide a through understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced without these specific details.
Reference is now made to
In
With further reference to
In the present invention, it is anticipated that the transformer 16 comprises a miniature ferrite transformer having primary and secondary windings which can couple the voltage pulse developed across the primary to the secondary and achieve the required voltage multiplication or division factor. In the design presently practiced, the capacitor 15 driving the transformer 16 consists of a 0.01 microfarad device. The transformer 16 of this practiced device consists of an 1811 ferrite pot core of material 3C81, having a primary of 24 turns and a secondary of 3 turns offering a voltage reduction factor of 8 to 1. The primary impedance of this transformer 16, in the step down device presently practiced has been measured at approximately 1.0 mH.
Referring to both
Consider point A in time. The AC main line is beginning to increase positively in magnitude from its zero crossing point. The capacitor 15 has an initial negative voltage across it from previous cycles. Thus as time moves on, the AC main line increases in magnitude increasing the voltages across the SIDAC until at time B, the voltage across the SIDAC has reached the breakover voltage Vbo and the device turns on and conducts. When the SIDAC switches on, its impedance drops to a low value and the capacitor charges up to a potential given at point C. The capacitor maintains this voltage and the cycle repeats its operation, only this time with a negative voltage AC wave. Due to this, it is seen that the switching from blocking to conducting occurs twice every AC cycle or at a 120 Hz rate for a 60 Hz driving waveform. By selection of SIDAC parameters, especially breakover voltage, additional pulses may be obtained in the operation of this converter during the course of one sine wave. Referring to
Referring to
Referring to
Unlike prior art embodiments described above and embodied in hardware, the present invention substantially overcomes the cost, weight and volume constraints of prior art that provided isolated DC outputs at low power from the AC main line. Whereas prior art low power converters utilize laminated iron transformers or intricate switching stages to generate the output voltage and current, the bidirectional two state device in combination with the series capacitor and high frequency transformer deliver similar performance at a great savings in cost, volume, and weight. A further benefit of the present invention is the increase in reliability achieved by using fewer parts to accomplish the same result.