Abstract:
A passive component circuit comprising a bridge rectifier and is coupled in parallel to three capacitors; each of the first two capacitors is clamped in series by at least one diode and there in between is disposed another diode and a resistor for shaping the input current waveform such that it falls outside the Class D envelope of the limits on harmonic currents emissions. As such, power supplies incorporating such a passive circuit qualify automatically as Class A devices without further regards to power factor conversion and EMI filtering. Therefore, an economic strategy for complying with harmonic currents emissions limits is realized for low to medium power supplies.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for shaping input waveform of power supply equipment for meeting harmonic current emissions standards. In particular, the present invention pertains to a passive component circuitry for shaping input current waveform for complying with harmonic current emissions regulations in low to medium power supply equipment. 
     BACKGROUND OF THE INVENTION 
     Although harmonic currents emissions standards such as “Electromagnetic Compatibility (EMC)—Part 3: Limits—Section 2, Limits for harmonic current emissions (equipment input current ≦16 A per phase)”, IEC 1000-3-2 Document, First Edition, 1995 (hereinafter IEC-1000-3-2) has been published some time ago, its impact on the ability of power supply manufacturers to export its low to medium range equipment has not been felt until 2000. For the first time, power supplies with input power exceeding 50 W are subjected to the IEC-1000-3-2 regulations in the Europe. A large variety of consumer electrical and electronic devices such as personal computers, printers, scanners and other accessories incorporate power supplies that come under the purview of IEC-1000-3-2. 
     It is well known in the field to rely on active power factor conversion or current conditioning to comply with IEC-1000-3-2. Prior art examples of active power factor conversion include U.S. Pat. Nos. 5,736,842 and 5,757,626. While active power factor conversion limits harmonic current emissions effectively, the power supplies are costly, complex and may be less reliable. Often the electromagnetic interference filters of active power factor conversion at the input stage are complicated. 
     U.S. Pat. No. 5,661,348 shows a typical example of current conditioning with inductors or chokes to meet limits imposed by lEC-1000-3-2. Inductors are simply and reliable devices but their bulk and weight impose considerable design constraints on modem consumer electrical or electronic devices that emphasize compactness and lightweight. 
     OBJECT OF THE INVENTION 
     It is an object of the present invention to shape the input current waveform of a power supply so that it complies with harmonic current emission limits with passive components. 
     It is another object of the present invention to meet the harmonic current emission limits economically without relying on complex and unreliable filters and circuitry. 
     It is yet another object of the present invention to shape the input current waveform of a power supply in complying with harmonic current emission limits without paying excessive overhead for power factor conversion. 
     SUMMARY OF THE INVENTION 
     A passive component circuit comprising a bridge rectifier and is coupled in parallel to three capacitors; each of the first two capacitors is clamped in series by at least one diode and there in between is disposed another diode and a resistor for shaping the input current waveform such that it falls outside the Class D envelope of the limits on harmonic currents emissions. As such, power supplies incorporating such passive components meets automatically the criteria for Class A devices without further regards to power factor conversion and EMI filtering. Therefore, an economic strategy for complying with harmonic currents emissions limits is realised for low to medium power supplies. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is the special wave shape envelope of input current to classify equipment as Class D. 
     FIG. 2 is a prior art power supply circuitry for filtering ripples from an alternating current (AC) source. 
     FIG. 3 is an input waveform sample of the circuit as shown in FIG.  2 . 
     FIG. 4 is a table showing the results of a test for harmonic current emissions with reference to Class D limits for the waveform sample in illustrated in FIG.  3 . 
     FIG. 5 is a power supply circuit for shaping input current waveform according to the present invention. 
     FIG. 6 is an input waveform sample of the circuit of the present invention as shown in FIG.  5 . 
     FIG. 7 is a table showing the results of a test for harmonic current emissions with reference to Class A limits for the waveform sample as shown in FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is the envelope of an input current to define the “special wave shape” and to classify equipment as Class D. Referring to IEC1000-3-2, page 17, equipment shall be deemed to be Class D if two specific conditions are met: (1) the input power is less than or equal to 600 Watts; and (2) the input current wave shape of each half-period—referred to its peak value, i pk —is within the envelope  3  as given in FIG. 1 for at least 95% of the duration of each half-period. This implies that waveforms having small peaks outside the envelope  3  are considered to fall within the envelope. The center line, M, coincides with the peak value of the input current. 
     FIG. 2 is a prior art power supply circuitry for filtering ripples from an alternating current (AC) source. The power supply  5  comprises at least an electromagnetic interference (EMI) filter  10 , a bridge rectifier  15  and a capacitor  20 . The EMI filter  10  receives at its input  30  alternating current (AC) source; the output of the EMI filter is provided as input to the bridge rectifier  15 . The output of the bridge rectifier  15  is coupled in parallel to the filtering capacitor  20  before its output (i.e., unregulated direct current) is provided as input to a power conversion device  25 . It should be understood by one skilled in the art that the power conversion device  25  could be motor driver, electronic ballast or other equivalent devices. It should further be understood by one skilled in the art that the power conversion device is coupled (not shown in FIG. 2) to system or device that require direct current (DC) source to operate such as printer, facsimile machine and personal computer etc. 
     FIG. 3 is an input waveform sample of the circuit as shown in FIG.  2 . The total input power for this sample waveform is 59 Watts. The current waveform of power supply circuit  5  is referenced by a voltage axis  40  on the left, a current axis  45  on the right and a time axis  50 . The input waveform  35  further displays a sinusoidal wave  55  for the input AC voltage, a Class D envelope  60 , and an input current curve  65 . In the preferred embodiment of the present invention, the power supply circuit  5  is tested on a power analyzer featuring IEC 1000-3 Windows Software 2.02. The input current waveform  65  is unshaped and the entire waveform is enveloped by the Class D envelope  60 . It follows that the circuit as shown in FIG. 2 is deemed to be a Class D equipment as It meets the conditions for such. 
     Referring to FIG. 4, a table detailing the even harmonics in column  70 , and odd harmonics in column  72  and the corresponding current readings in columns  75  and  74  is aligned against the Class D limits in columns  80  and  76  respectively. As expected, the input current waveform of power supply circuit  5  does not pass the harmonic current emissions tests. In particular, almost all the odd harmonics exceeds the Class D limits  76  and therefore power supply circuit  5  in FIG. 2 fails the IEC 1000-3-2 test. 
     As mentioned above, prior art power supplies rely on active power factor conversion or current conditioning to comply with IEC-1000-3-2. The present invention illustrates in FIGS. 5 -7 with a new power supply circuit  85  that compliance with IEC 1000-3-2 is achieved by shaping the input current waveform alone. The foundation of the present invention is based on the observation that if two requirements are met, then power supply circuits incorporating the present invention comply with IEC 1000-3-2: (1) the input power is less than or equal to 200 Watts; and (2) the input current waveform is not Class D. With reference to the present invention and will be illustrated below, the input current waveform is shaped to meet Class A limits by increasing the duration of input current whose waveform is outside of the Class D envelope by more than 5% for each half period. 
     FIG. 5 is a power supply circuit for shaping input current waveform according to the present invention. The circuit  85  comprises an EMI filter  90 , a bridge rectifier  95 , capacitors  96 ,  98  and  100 , diodes  97  and  99  and finally a diode  101  and a resistor  102 . The EMI filter  90  is coupled at its input to AC power source  110 . The outputs  91  and  93  from the EMI filter  90  are coupled to the input of the bridge rectifier  95 . The outputs  92  and  94  of the bridge rectifier  95  are coupled in parallel to the bulk capacitors  96 ,  98  and  100  respectively. The capacitor  96 ,  98  are clamped with the diodes  97 ,  99  respectively to form two discharging paths. Additionally, the capacitors  96 ,  98  are coupled in series with the diodes  101  and resistor  102  to form a charging path. Finally, the outputs  92  and  94  is coupled to a power conversion device  105 . 
     FIG. 6 is an input waveform sample of the circuit of the present invention. The total input power of this waveform sample is 57.8 Watts. The waveform  115  is referenced by a voltage axis  120  on the left, a current axis  125  on the right, and a time axis  130 . The input waveform  115  further displays a sinusoidal voltage  135  for the input AC voltage, a Class D envelope  140  and an input current-curve  145 . Like the curve  35  in FIG. 3, the peak value of the input current waveform of each half-period falls within that of the envelope. In FIG. 6, the peak values are points  141  and  143 . Unlike the curve  35  in FIG. 3, the current waveform  145  is shaped by the circuit  85  to have non-Class D envelope waveform. In other words, the duration for the area  142  under the curve  145  and above the Class D envelope  140  is increased by more than 5%. Under IEC 1000-3-2, power supply equipment incorporating the circuit  85  is considered a Class A equipment. 
     Referring again to circuit  85  in FIG.  5  and the waveform  115  in FIG.  6 : at interval  146  to  147 , the input voltage  135  (shown in FIG. 5) is greater than the individual voltage across each of the capacitor  96  and  98  but less than the sum of voltages across the capacitors  96  and  98 . Output lines  92  and  94  provide energy to the application circuit (not shown) and charge the capacitors  100 . 
     At interval  147  to  148 , the input voltage  135  is greater than the sum of voltages across the capacitors  96  and  98 . Output lines  92  and  94  provide energy to the application circuit (not shown) and charge the capacitors  96  and  98  through the diode  101  and the resistor  102 . Here, the resistor  102  suppresses the peak charging current. 
     At interval  149  to  151 , the input voltage  135  is less than the sum of voltages across the capacitors  96  and  98  but higher than the individual voltage across each of the capacitors  96  and  98 . Output lines  92  and  94  provide energy or current to the application circuit (not shown) and charge the capacitor  100 . 
     At interval  147  to  148 , the input voltage  110  is less than the individual voltage across each of the capacitor  96  and  98 . The capacitor  96  and  98 , which are in series with diode  97  and  99  respectively, parallel with capacitor  100  provide energy to the application circuit (not shown). At this moment, the input current will drop to near zero. During the discharge of capacitor  96  and  98 , the diode  101  isolates the two discharging current paths. 
     In order to ensure that the current waveform falls outside the Class D envelope, resistor  102  and capacitor  100  are introduced into the circuit. The resistor  102  which is in series with diode  101  will suppress the peak charging current  141  without affecting the peak area  142 . Whereas the capacitor  100  will further increase the duration of the input current waveform which is outside of the Class D envelope by more than 5% within each half period. 
     This is the shaping process for the waveform and is illustrated by the curve between intervals  146  and  153  on the time axis in FIG.  6 . With the unique arrangement of capacitors, diodes and resistors of the present circuit, shaping of the waveform to comply with Class A will be possible. 
     FIG.7 is a table showing the results of a test for harmonic current emissions for the input waveform  115  in FIG.  6 . The even and odd harmonics are listed in columns  150  and  152 , while the harmonic current readings are shown in columns  155  and  154  respectively. Measured against the Class A harmonic current limits under IEC 1000-3-2 as listed in columns  165  and  156  respectively, the test results confirm that the power supply equipment incorporating the present invention complies with the harmonic current emissions limits. 
     The present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are, therefore, to be embraced therein.