Abstract:
A power supply electromagnetic interference (EMI) filter circuit and technique. In one embodiment, a method of filtering EMI in a power supply includes rectifying an AC signal from an AC source, smoothing the rectified AC signal with a bulk storage capacitor to provide a DC output as an input to a power conversion circuit and filtering the EMI generated by the power conversion circuit from reaching the AC source by using the bulk capacitor and one or more inductors in combination with the AC source capacitance as an EMI filter. In one embodiment the method also includes the use of one or more of the inductors as a fusing element.

Description:
REFERENCE TO PRIOR APPLICATION 
   This application is a continuation of and claims priority to U.S. application Ser. No. 10/299,175, filed Nov. 18, 2002, now U.S. Pat. No. 6,813,168. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to power supplies and, more specifically, the present invention relates to a switched mode power supply with an input electromagnetic interference (EMI) filter circuit. 
   2. Background Information 
   Electronic devices use power to operate. Switched mode power supplies or adapters are widely used to power electronic products as well as charge batteries used to power mobile products such as for example wireless phones, palm top computers, toys, etc. Switched mode power supplies generate EMI, which must be filtered to allow the power supply to meet national and international standards stipulating acceptable levels of EMI. This requires that the switched mode power supply include components at the input of the power supply that filter EMI in order to meet these standards. Furthermore, an input fuse is required to meet national and international safety standards. 
   Known power supply techniques employ input EMI filter circuits of varying complexity. The simplest form of input EMI filter is known as a pi filter and is used in low-power power supplies to reduce power supply cost. The fuse is a separate component, which is typically either designed solely for use as a fuse or as a resistor specifically designed to meet national and international safety standards as a fusible component. 
   SUMMARY OF THE INVENTION 
   A power supply input EMI filter circuit is disclosed. In one embodiment, a method of filtering EMI in a power supply includes rectifying an AC signal with a rectifier, smoothing the rectified signal with a bulk storage capacitor to provide a DC output as an input to a power conversion circuit and filtering the EMI generated by the power conversion circuit from reaching the AC source by using the bulk capacitor and one or more inductors in combination with the AC source capacitance as an EMI filter. In one embodiment the method also includes the use of one or more of the inductors as a fusing element to meet safety requirements. Additional features and benefits of the present invention will become apparent from the detailed description and figures set forth below. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention detailed illustrated by way of example and not limitation in the accompanying figures. 
       FIG. 1  is a schematic illustrating a power supply input circuit including a fuse and input EMI filter circuit. 
       FIG. 2  is a schematic illustrating one embodiment of a power supply with a simplified input EMI filter circuit and fuse in accordance with the teachings of the present invention. 
       FIG. 3  is a schematic illustrating another embodiment of a power supply with a simplified input EMI filter and fuse circuit in accordance with the teachings of the present invention. 
       FIG. 4A  provides an illustration of one embodiment of a full wave rectifier utilizing a diode bridge in accordance with the teachings of the present invention. 
       FIG. 4B  provides an illustration of one embodiment of a half wave rectifier utilizing a single diode on one of the rails in accordance with the teachings of the present invention. 
       FIG. 4C  provides an illustration of one embodiment of a half wave rectifier utilizing a single diode on each of the rails in accordance with the teachings of the present invention. 
       FIG. 4D  provides an illustration of one embodiment of a half wave rectifier utilizing a plurality of diodes on at least one of the rails in accordance with the teachings of the present invention. 
       FIG. 5  is a schematic illustrating yet another embodiment of a power supply with a simplified input EMI filter and fuse circuit in accordance with the teachings of the present invention. 
       FIG. 6  is a schematic illustrating still another embodiment of a power supply with a simplified input EMI filter and fuse circuit in accordance with the teachings of the present invention. 
   

   DETAILED DESCRIPTION 
   A novel technique to reduce the cost of input EMI filter circuitry in switched mode power supplies is disclosed. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one having ordinary skill in the art that the specific detail need not be employed to practice the present invention. In other instances, well-known materials or methods have not been described in detail in order to avoid obscuring the present invention. 
   In one embodiment, the present invention provides a simplified input EMI circuit, which therefore reduces the cost and complexity of input EMI filter circuitry and the fuse function. 
   To illustrate,  FIG. 1  shows a schematic of a power supply input stage including a fuse  101 , a rectifier circuit  100  and EMI filter circuitry including capacitors  103  and  104  and an inductor  105 . The input EMI filter circuitry is coupled in a configuration that is known as a pi filter, which can be appreciated to one skilled in the art. As can be appreciated by one skilled in the art, the rectifier circuit  100  can be either a well-known full or half wave rectifier circuit. If a full wave rectifier circuit is used, the rectification bridge of rectifier circuit  100  can be constructed using either discrete diodes or a single component bridge rectifier. If a half wave rectifier circuit is used, the rectifier circuit  100  can be constructed using a single diode or multiple diodes coupled in series. The latter construction is used in certain embodiments to reduce EMI and or to increase the amount of input voltage surges that rectifier circuit  100  can withstand, as will be appreciated to one skilled in the art. 
   The circuitry shown in  FIG. 1  has an AC source  102  at the input and provides a rectified and smoothed or filtered DC output voltage at DC output  106 . Such a configuration is typical of low power AC to DC power supply circuits such as those employed in low power (&lt;10 Watts output) adapters and chargers for consumer electronics items and the like. As well as forming part of the pi filter, capacitors  103  and  104  also provide bulk storage of charge derived from the AC source  102  when the voltage across the AC source  102  is greater than the DC voltage across capacitors  103  and  104 . When this condition is met, current flows from the AC source  102  through capacitors  103  and  104 . The charge thus stored on capacitors  103  and  104  provides a relatively stable DC output voltage at DC output  106  as required for efficient operation of a power conversion circuit that is to be coupled to receive the DC output voltage at DC output  106 . 
   The fuse  101  shown in  FIG. 1 , which is coupled between AC source  102  and rectifier circuit  100  is a fusible resistor. In order for a resistor to fulfill the international standards required of a fuse component, the resistor is normally of a wore wound construction and covered in a flame retardant or heatshrink material that prevents pieces of the resistor being scattered when the fuse  101  is blown during a fault condition. The position of the fuse  101  is also important to ensure compliance of the power supply with international safety standards. 
   To comply with international safety standards, the fuse  101  must be positioned such that it becomes an open circuit in the event of an abnormally high current being drawn from AC source  102  due to a fault on any component in the rectifier circuit  100 , EMI filter consisting of components  103 ,  104  and  105  or any component in the power supply circuit coupled to DC output  106 .  FIG. 1  therefore shows the most obvious position for fuse  101  to ensure that it limits an abnormal current in the case of a fault in any component coming after fuse  101 . 
     FIG. 2  shows a schematic illustrating one embodiment of a power supply input stage utilized in a power supply in accordance with the teachings of the present invention. In the illustrated embodiment, an AC signal from an AC source  102  is rectified by rectifier circuit  200  and is then smoothed at DC output  106  by capacitor  203 . The circuit shown is a simplification of that shown in  FIG. 1  through the elimination of the fuse  101  and also one of the bulk storage capacitors  103  and  104 . In the circuit embodiment of  FIG. 2 , the input fuse function is provided by inductor  202 , which also forms part of an EMI filter as described later. As can be appreciated to one skilled in the art, typical input inductor components used in power supplies, such as for example below 10 Watts output, are typically constructed of a fine wire wound on a ferrite core. As such the construction is similar to the fusible resistor  101  described above. As such, by the correct choice of wire gauge (diameter), inductor  202  can be designed to become an open circuit under specific conditions of abnormal current flow such as those due to a fault in one of the power supply components. In one embodiment, the position of inductor  202 , in common with the fuse  101  in  FIG. 1 , is on the AC source  102  side of the rectifier circuit  200  to ensure circuit protection under any of the abnormal current conditions as described earlier. 
   The EMI filter configuration of the circuit in  FIG. 2  forms a pi filter with the single bulk storage capacitor  203 , inductor  202  and AC source capacitance  201  of AC source  102 . It is appreciated that AC source capacitance  201  is a distributed source capacitance of AC source  102 , rather than a specific component. This AC source capacitance  201  is present in every AC source  102  and is represented by an equivalent capacitance in the equipment used to make EMI measurements. This equipment is called a Line Impedance Stabilization Network (LISN), as will be familiar to one skilled in the art. The AC source or LISN capacitance can therefore be specifically used to form a pi filter with the simplified input circuitry of  FIG. 2  in accordance with the teachings of the present invention. Since EMI measurements are made under standardized conditions of input cable length and LISN circuitry, the AC source capacitance  201  is deterministic and provides repeatable measurements to ensure consistent EMI filter performance. 
   As shown in the depicted embodiment, rectifier circuit  200  is coupled between inductor  202  and capacitor  203 . In one embodiment, the capacitance value of capacitor  203  is not necessarily equal to the value of capacitance  104  of  FIG. 1 . In the depicted embodiment, capacitor  203  is a single capacitor and is adapted to provide the bulk storage function and is therefore usually a larger value of capacitance value than that of capacitor  104  to achieve the same average DC output voltage at DC output  106 . However, despite typically being a larger capacitor, the elimination of one bulk storage capacitor typically provides a significant cost saving over the configuration in  FIG. 1  since the cost of each capacitor component is strongly influenced by the packaging itself, which is reduced using a single component. Furthermore, the elimination of one bulk storage capacitor reduces the cost of circuit assembly in production by reducing component count. In one embodiment, inductor  202  also does not necessarily the same inductance value as inductor  105  and is chosen in each case to optimize the pi filter performance with the AC source capacitance  201  and bulk storage capacitor  203 . In operation, the rectified and smoothed or filtered DC voltage at DC output  106  is received by power conversion circuit  208 , which generates an output voltage at power conversion circuit output  210 . In one embodiment, power conversion circuit  208  is a switched mode power converter and the EMI filter circuit of  FIG. 2  is employed to filter the EMI in accordance with the teaching of the present invention. 
     FIG. 3  shows a schematic illustrating another embodiment of a power supply input stage employed in a power supply in accordance with the teachings of the present invention. The circuit shown is again a simplification of that shown in  FIG. 1  through the elimination of the fuse  101  and also one of the bulk storage capacitors  103  and  104 . In the circuit of  FIG. 3 , the input fuse function is provided by inductor  302 , which also forms part of the EMI filter. 
   In this configuration however, the inductor  302  is placed on the output side of the rectifier circuit  300 . That is, rectifier circuit  300  is between AC source  102  and inductor  302 . In one embodiment, this places a limitation on the type of input rectifier circuit  300  that can be used if the inductor is also required to perform the fuse function. This limitation arises since connection of inductor  302  can only provide a fuse function in compliance with international safety standards described earlier if the input rectifier circuit  300  is a half wave rectifier circuit. As can be appreciated by one skilled in the art, if input rectifier circuit  300  is a full wave bridge rectifier circuit, the short circuit failure of any one of the diodes in the rectification bridge will cause abnormally high current to be drawn from AC source  102  without this current flowing through inductor  302 . In one embodiment, the circuit shown in  FIG. 3  is therefore limited to use when input rectifier circuit  300  is a half wave rectifier circuit if the inductor is also required to function as a fuse. 
   To illustrate,  FIGS. 4A ,  4 B,  4 C and  4 D provide various example schematics of various embodiments of rectifiers that may be utilized in accordance with the teachings of the present invention. In particular,  FIG. 4A  provides an illustration of one embodiment of a full wave rectifier utilizing a diode bridge.  FIG. 4B  provides an illustration of one embodiment of a half wave rectifier utilizing a single diode on one of the rails.  FIG. 4C  provides an illustration of one embodiment of a half wave rectifier utilizing a single diode on each of the rails.  FIG. 4D  provides an illustration of one embodiment of a half wave rectifier utilizing a plurality of diodes on at least one of the rails. It is appreciated that other suitable variations of the schematics illustrated in  FIGS. 4A ,  4 B,  4 C and  4 D may be utilized in accordance with the teachings of the present invention. Any one of the rectifier circuits shown could be used in the circuits of  FIG. 2 ,  3  or the circuits described below though their application is not limited to only those configurations shown in  FIGS. 4A ,  4 B,  4 C and  4 D. 
   As illustrated in  FIGS. 4B ,  4 C and  4 D, embodiments of the half wave rectifier circuit can be constructed using a single diode or multiple diodes coupled in series. When using multiple diodes, one or more diodes can be connected on either or both AC input rails. Using one or more diodes on both rails of the AC input reduces EMI by blocking the EMI generated by the power conversion circuit from being coupled to the AC source on both rails during times when the diodes are not conducting. 
   As will be appreciated by one skilled in the art, in one embodiment, the configuration of  FIG. 3  can be used with rectifier circuit  300  as a full wave rectifier circuit, such as for example the rectifier illustrated in  FIG. 4A , if a separate fuse component is used coupled in series with the AC source  102  on the input side of rectifier circuit  300 . In this embodiment, inductor  302  no longer provides the fuse function and the circuit complies with international safety standards. 
   Referring back to the embodiment of  FIG. 3 , an AC signal is provided by AC source  102  and is rectified by rectifier circuit  300  and smoothed by capacitor  303  such that a rectified and smoothed or filtered DC signal is generated at DC output  106 . The rectified and smoothed DC signal at DC output  106  is coupled to be received by power conversion circuit  208  such that an output is generated at power conversion output  210 . In one embodiment, power conversion circuit  208  is a switched mode power converter. 
   The EMI filter function in the embodiment of  FIG. 3  is made up of inductor  302 , bulk storage capacitor  303  and AC source capacitance  201  of AC source  102 . In common with the embodiment of  FIG. 2 , the AC source or LISN capacitance is used to form a pi filter with the simplified input circuitry of  FIG. 3 . Other benefits of the simplified input circuitry of  FIG. 3  are common with those of the circuit illustrated in  FIG. 2  as described previously. 
     FIG. 5  shows a schematic illustrating yet another embodiment of a power supply input stage of a power supply in accordance with the teachings of the present invention. In the embodiment depicted in  FIG. 5 , the general configuration and functionality share similarities with the embodiment shown and described with respect to  FIG. 2 . An AC signal is output by AC source  102  and it is rectified by rectifier  500  and smoothed or filtered by capacitor  503  to provide a DC signal at DC output  106  at capacitor  503 . Power conversion circuit  208  receives the rectified and smoothed DC signal at DC output  106  such that an output is provided at power conversion circuit output  210 . 
   Two inductors  502  and  504 , however, have replaced the input inductor  202  of the embodiment of  FIG. 2 . In the illustrated embodiment, inductors  502  and  504  are coupled between AC source  102  and rectifier circuit  500 . In one embodiment, the total inductance of inductors  502  and  504  is not necessarily equal to the inductance of inductor  202 . For instance, in one embodiment, one of the inductors  502  and  504  could be designed specifically to have different impedance versus frequency characteristics than the other in order to filter specific EMI frequencies more efficiently. In this embodiment either one or both of inductors  502  and  504  can act as a fuse. Typically the lowest cost solution is to design only one of the inductors  502  and  504  to act as a fuse in order that only one has the flame retardant or heatshrink covering to comply with international safety standards as a fuse component. This can be accomplished, for example, by using a thinner wire on the inductor that is to function as fuse than the wire used on the other inductor. 
     FIG. 6  shows a schematic illustrating still another embodiment of a power supply input stage used in a power supply in accordance with the teachings of the present invention. In  FIG. 6 , the general configuration and functionality share similarities with the circuit shown in  FIG. 3 . In the depicted embodiment, an AC signal is generated by AC source  102  and rectifier circuit  600  rectifies the AC signal and capacitor  603  smoothes the signal such that a rectified and smoothed or filtered DC signal is generated at DC output  106  at capacitor  603 . The rectified and smoothed DC signal at DC output  106  is received by power conversion circuit  208  such that an output is generated at power conversion circuit output  210 . 
   The input inductor  302  of embodiment  FIG. 3 , however, has replaced by two inductors  602  and  604 . As shown, rectifier circuit  600  is coupled between AC source  102  and inductors  602  and  604 . In one embodiment, the total inductance of inductors  602  and  604  is not necessarily equal to the inductance of inductor  302 . For instance, in one embodiment, one of the inductors  602  and  604  could be designed specifically to have different impedance versus frequency characteristics than the other in order to filter specific EMI frequencies more efficiently. In this embodiment either one or both of inductors  602  and  604  can act as a fuse. Typically the lowest cost solution is to design only one of the inductors  602  and  604  to act as a fuse in order that only one of the inductors  602  or  604  includes flame retardant or heatshrink covering to comply with international safety standards as a fuse component. 
   In one embodiment, the circuit embodiment of  FIG. 6  shares similar limitations as that of the circuit embodiment of  FIG. 3  in terms of the input rectifier circuit  600 . For instance, in one embodiment, rectifier circuit  600  must be a half wave rectifier circuit in order for the use of either inductor  602  or  604  as a fuse to meet international safety standards as previously described. Rectifier circuit  600  can, however, be a full wave rectifier circuit if a separate fuse component is coupled between the AC source  102  and the input to rectifier circuit  600 . 
   In the foregoing detailed description, the present invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the present invention. The present specification and figures are accordingly to be regarded as illustrative rather than restrictive.