A portion of the disclosure of this patent document contains material which is subject to copyright protection. This patent document may show and/or describe matter which is or may become trade dress of the owner. The copyright and trade dress owner has no objection to the facsimile reproduction by anyone of the patent disclosure as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright and trade dress rights whatsoever.
1. Field
This disclosure relates to electromagnetic interference filters and surge suppressors for use in power converters.
2. Description of the Related Art
Electronic equipment, including power converters, may need to comply with regulations limiting the electromagnetic interference (EMI) that may be radiated or emitted by the equipment. International standards for EMI limits are developed by the Comité International Spécial des Perturbations Radioélectriques (CISPR) and adopted by regional and national authorities. In the U.S., an applicable regulation is FCC Part 15. In Europe the standard for DC power converters is EN 55022.
Since some electromagnetic interference is inherently generated in switching-mode power converters, power converters may incorporate an EMI filter between the input power source and the converter to reduce EMI that is conducted on the power lines. Conducted EMI is generally considered to be comprised of two types of noise: common-mode noise appearing as a voltage between both power supply lines and ground, and differential-mode noise appearing as a voltage between the power supply lines.
Electronic equipment including power converters may also have to comply with various environmental requirements including the ability to withstand input voltage surges or transients. Input voltage surges are also typically divided into two types: common-mode voltage surges appearing as a voltage between power supply lines and ground, and differential-mode voltage surges appearing as a voltage between the power supply lines. In particular, electronic equipment may be required to survive lightning surge tests such as GR-1089-CORE for telecommunications equipment and EN 61000-4-5 in Europe. Some types of equipment may also have to comply with safety standards and requirements which may include requirements for very high impedance and extremely low leakage current between the equipment and ground.
Throughout this description, the embodiments and examples shown should be considered as exemplars, rather than limitations on the apparatus and methods disclosed or claimed.
Description of Apparatus
Referring now to
Capacitors Cx1 and Cx2 may be connected across the AC line on either side of the common-mode inductor Lc1. Cx1 and Cx2 are commonly called “X capacitors” and are adapted specifically for use across the AC line. X capacitors are designed to withstand continuous AC current flow, to have very low loss at the frequency of the AC power input, to have low impedance at the switching frequency of the DC power converter, and to be able to withstand the peak transient voltage that may occur between the AC power conductors.
Capacitors Cy1, Cy2, Cy4, and Cy5 may be connected from the AC power conductors to the ground. These capacitors are commonly called “Y capacitors”. Y capacitors are designed to have very low leakage current, to have low impedance at the switching frequency of the DC power converter, and to be able to withstand the peak transient voltage that may occur between the AC power conductors and ground. The DC power converter 110 may include a bridge rectifier BR that converters the AC line voltage to a DC voltage. Another Y capacitor Cy3 may be connected from one side of the output from the bridge rectifier BR to ground.
Although the examples of
Power converters may be required to withstand voltage surges and transients on the input power lines without damage.
Power converters may incorporate voltage limiting devices, also called transient suppressors or surge suppressors, to absorb input voltage surges without damage to the power converter. Within this description, a voltage limiting device is any component that exhibits an abrupt increase in conductivity when the voltage across the device exceeds a threshold voltage. The threshold voltage of a voltage limiting device is determined during manufacture. Voltage limiting devices are available with thresholds ranging from a few volts to hundreds of volts and higher. Voltage limiting devices include back-to-back connected Zener diodes, silicon transient suppressors, transorbs, voltage variable resistors or varistors, gas discharge tubes, or any other component that exhibits an abrupt increase in conductivity when the voltage across the device exceeds a predetermined threshold voltage.
Differential-mode voltage surges applied between pairs of power input lines may be absorbed and limited by voltage limiting devices connected in parallel with one or more X capacitors in the example filters of
Power converters that do not incorporate common-mode surge limiting components must be designed to withstand common-mode voltage surges without failure and without significant disruption of the power converter's normal function. Although one or more Y capacitors may be the only components physically connected between the circuitry of a power converter and ground, every component and circuit trace within the power converter may have a stray capacitance to ground. The application of a voltage surge between the power input lines and ground will cause current to flow through each of the stray capacitances. The current flow in each stray capacitance will be defined by the well-known formula I=C dV/dt, where I is the current flow, C is the capacitance, and dV/dt is the rate of change of the surge voltage. Assuming the surge voltage rises linearly with time, the magnitude of the current flow will be roughly proportional to the peak amplitude of the voltage surge and inversely proportional to the rise time of the voltage surge. Clearly, limiting the peak amplitude and maximizing the rise time of the surge voltage may simplify the problem of designing the power converter to withstand common-mode voltage surges.
Referring back to
Initially, the inductance of common-mode inductor Lc1 may limit the current that flows through Lc1 to charge the Y capacitors. During this period (see waveform region 320), the voltage across the Y capacitors may rise much slower than the input voltage surge 310. During this period, a substantial voltage may build up across Lc1. At some point, the current flow through the windings of Lc1 may induce a sufficient magnetic field in the core of Lc1 to cause the core to saturate. If the core of Lc1 saturates, the permeability of the core material will drop substantially. The drop in core permeability will cause a corresponding decrease in the inductance of Lc1, and the current flow through the windings of Lc1 will increase precipitously. In response to the increased current flow, the voltage across the Y capacitors will increase rapidly (see waveform region 330). The rise time of the voltage across the Y capacitors may be as little as 0.1 microsecond, more than an order of magnitude less than the rise time of the voltage surge 310.
After the core of common-mode inductor Lc1 saturates, the inductance of Lc1 will drop to a low, but finite, value such that some energy is still stored in Lc1. The energy stored in Lc1 may cause current to flow into the Y capacitors even after the input voltage surge 310 has peaked. Thus the peak voltage across the Y capacitors (see waveform region 340) may exceed, or overshoot, the peak surge voltage by 50% or more.
A first resistor R1 and a voltage limiting device Z1 may be connected in series, and the series combination may be connected in parallel with the first winding of common-mode inductor LC1. The voltage limiting device Z1 may be a Zener diode, a silicon transient suppressor, a transorb, a voltage variable resistor or varistor, a gas discharge tube, or any other component that exhibits an abrupt increase in conductivity when the voltage across the device exceeds some threshold voltage. The voltage limiting device Z1 may have a threshold voltage that is much smaller than the anticipated amplitude of common-mode voltage surges that may be applied to the power input lines 430L/N. The series combination of voltage limiting device Z1 and resistor R1 may limit the voltage that builds up across the first winding of common-mode inductor Lc1 and may provide an alternate path for current to charge Y capacitors, if present. The presence of voltage limiting device Z1 may or may not prevent saturation of the core of common-mode inductor Lc1.
The voltage limiting device Z1 may have a threshold voltage that is larger than the noise voltage developed across the first winding of common-mode inductor during normal operation of the filter 400. The voltage limiting device Z1 may be nonconductive during normal operation of the filter 400.
The filter 400 may include an X capacitor Cx1 connected between the first input to the power converter 415 and the second input to the power converter 420. The filter 400 may include a Y capacitor Cy1 connected between the first input to the power converter 415 and ground. The filter 400 may include a Y capacitor Cy2 connected between the second input to the power converter 420 and ground. The filter 400 may include a Y capacitor Cy3 connected between ground and an output 425 of a bridge rectifier BR within power converter 410.
The resistance of resistor R1 may function to limit the current flow through the voltage limiting device Z1. The resistance of resistor R1 and the capacitance of capacitor Cy1 may be selected such that resistor R1 and capacitor Cy1 have a time constant less than or equal to the rise time of the largest anticipated common-mode voltage surge waveform. Resistor R1 and capacitor Cy1 have a time constant between 33% and 100% of the rise time of the largest anticipated common-mode voltage surge waveform. The resistance of resistor R1 may be selected empirically to minimize the voltage transient measured between the input 415 or 420 to power converter 410 and ground.
A second resistor R2 and a second voltage limiting device Z2 may be connected in series, and the series combination may be connected in parallel with the second winding of common-mode inductor LC1. The voltage limiting device Z2 may be the same or a different type of device from voltage limiting device Z1. The voltage limiting device Z2 may have a threshold voltage that is the same or different from the threshold voltage of voltage limiting device Z1. Resistor R2 may have a resistance that is the same or different from the resistance of resistor R1.
Another voltage limiting device (not shown) may be connected in parallel with capacitor Cx1 to limit and absorb differential-mode voltage surges.
The filter 400 may include additional capacitors, such as capacitor Cx2, Cy4, and Cy5, or fewer capacitors. The filter 400 may including another filter stage including a second common-mode inductor, similar to the second stage of the filter 150 shown in
A first resistor R1 and a voltage limiting device Z1 may be connected in series, and the series combination may be connected in parallel with the first winding of common-mode inductor Lc3. Voltage limiting device Z1 and the threshold of voltage limiting device Z1 may be selected as previously described in conjunction with
The filter 500 may include a plurality of capacitors, and may include more or fewer capacitors than those shown in
A first resistor R1 and a first voltage limiting device Z1 may be connected in series, and the series combination may be connected in parallel with the first winding of the first common-mode inductor Lc1. Voltage limiting device Z1 and the threshold of voltage limiting device Z1 may be selected as previously described in conjunction with
The filter 600 may include a plurality of capacitors, and may include more or fewer capacitors than those shown in
Additional and components or other arrangement of components may be used to achieve the processes and apparatuses described herein.
Closing Comments
The foregoing is merely illustrative and not limiting, having been presented by way of example only. Although examples have been shown and described, it will be apparent to those having ordinary skill in the art that changes, modifications, and/or alterations may be made.
Although many of the examples presented herein involve specific combinations of elements, it should be understood that those elements may be combined in other ways to accomplish the same objectives. Acts, elements and features discussed only in connection with one embodiment are not intended to be excluded from a similar role in other embodiments.
For means-plus-function limitations recited in the claims, the means are not intended to be limited to the means disclosed herein for performing the recited function, but are intended to cover in scope any means, known now or later developed, for performing the recited function.
As used herein, “plurality” means two or more.
As used herein, a “set” of items may include one or more of such items.
As used herein, whether in the written description or the claims, the terms “comprising”, “including”, “carrying”, “having”, “containing”, “involving”, and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of” and “consisting essentially of”, respectively, are closed or semi-closed transitional phrases with respect to claims.
Use of ordinal terms such as “first”, “second”, “third”, etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements.
As used herein, “and/or” means that the listed items are alternatives, but the alternatives also include any combination of the listed items.