Active filter for a converter having a DC line

[0002] The present invention relates to electrical power supplies. More specifically, the invention relates to a converter having a dc line.

[0003] A down chopper of a dc-dc converter receives dc current from a dc power supply, modulates or “chops” the current to reduce current amplitude, and provides the chopped current on a dc line. The reduction in current is proportional to duty cycle of the chopping.

[0004] On-off action of the down chopper creates a pulse train-type pattern in the chopped current. Consequently, the chopped current on the dc line contains harmonic frequency content.

[0005] The dc-dc converter may also include an EMI filter for reducing the harmonic content to acceptable limits. A conventional EMI filter includes passive inductance, capacitance and resistance elements, the values of which are determined by power rating, chopping frequency and filter attenuation requirements. The EMI filter typically constitutes a significant amount of the overall weight of the dc-dc converter.

[0006] Reducing the weight of EMI filters for dc-dc converters used in aircraft would be very desirable. Reducing the weight would lower aircraft fuel consumption. Lowering the fuel consumption, in turn, would lower the cost of flying the aircraft.

[0007] According to one aspect of the present invention, an apparatus includes a converter having a dc line, and an active filter coupled to the dc line. The converter is operable to provide a pulsed current on the dc line at a chopping frequency; and the active filter is operable to trap a fundamental of the pulsed current. The active filter allows a substantially smaller passive filter to remove remaining harmonic frequency components.

[0008] FIG. 1 is an illustration of a dc-dc converter including an active filter;

[0009] FIG. 1ais an illustration of a switch of the active filter;

[0010] FIG. 2 is an illustration of a fundamental frequency component of resonant current and a symmetrical deadband;

[0011] FIG. 3 is an illustration of phase and amplitude controls for the dc-dc converter of FIG. 1;

[0012] FIGS. 4ato 4dare illustrations of chopper duty cycle, chopper fundamental current, a sine wave reference signal, and a cosine wave reference signal, respectively; and

[0013] FIG. 5 is an illustration of alternative phase and amplitude controls for the dc-dc converter of FIG. 1.

[0014] Reference is made to FIG. 1, which illustrates a dc-dc converter 100 including a conventional down chopper 102. A dc power source 104 is on the “source” side of the chopper 102, and a dc line 106 is also on the source side of the chopper. The chopper 102 modulates or “chops” the current from the dc source 104 and thereby reduces the average amplitude of the chopped current on the dc line 106. Current flowing through the chopper 102 is regulated by a chopper control 108. The chopper control 108 generates a DC signal that modulates a solid-state power switch 107 in the chopper 102. The chopper switch 107 is modulated such that (a) current flows from the dc source 104 onto the dc line 106 while the chopper switch 107 is closed; and (b) the current circulates through a freewheeling diode 110 while the chopper switch 107 is open. The reduction in current LINE on the dc line 106 is proportional to duty cycle of the chopping. An inductor 112 smoothes the current-ripple in the output of the chopper 102.

[0015] The on-off action of the chopper 102 creates a pulse-train type flow pattern in the chopper current I_CHOPPER(see FIG. 4a). The interval T between pulses is proportional to the pulse width modulated (“PWM”) or chopping frequency f_chop. That is, T=1/f_chop.

[0016] The chopper current I_CHOPPERhas a dc content and a harmonic frequency content. The harmonic frequency content includes a fundamental frequency component and its associated higher harmonic frequency components. The frequency of the fundamental is equal to the chopping frequency f_chopof the chopper 102. To assure a small ripple current in the current on the dc line 106, the chopping frequency is relatively high (e.g., 20 kHz).

[0017] The dc-dc converter 100 further includes an active filter 114 for trapping (by canceling or at least by reducing) the fundamental frequency component that would otherwise appear in the line current I_LINEon the dc line 106 without the active filter 114. The active filter 114 includes an adjustable power dissipating element 116, a tunable trap circuit 118 and a bidirectional first switch 124 coupled between the dc line 106 and the trap circuit 118. The trap circuit 118 includes a capacitor 126 and an inductor 128. The power-dissipating element 116 includes a second switch 130 and a resistor 132.

[0018] The bi-directional switch 124 may have an H-bridge configuration operated during positive and negative resonant half-cycles, as shown in FIG. 1a.The bi-directional switch 124 includes transistor-type switches 124aand 124b,blocking diodes 124cand 124d,and reverse-current diodes 124eand 124f.

[0019] The first switch 124 is closed to couple the trap circuit 118 to the dc line 106. The second switch 130 is closed to discharge the capacitor 126 in the trap circuit 118.

[0020] The fundamental frequency component in the line current I_LINEis minimized by two independent decoupled controls: a phase control 120 and an amplitude control 122. The phase control 122 modulates the first switch 124 to produce a resonant current I_RES. The amplitude control 120 modulates the second switch 130 to modulate the amplitude of the resonant current I_RESin the trap circuit 118. This is accomplished by increasing or decreasing the charge on the capacitor 126 relative to the dc line 106.

[0021] Appropriate operation of the switches 124 and 130 produces a resonant current I_RESthat is in phase and equal in magnitude with the fundamental frequency component of the chopper current I_CHOPPER. To achieve this, phase and amplitude of the resonant current I_RESare controlled. First, the phase of the resonant current I_RESis controlled with respect to the fundamental frequency current component produced by the chopper 102. Second, the amplitude of the resonant current I_RESis adjusted to match the amplitude of the fundamental frequency current component produced by the chopper 102 as the duty cycle of the chopper 102 is varied. As long as the resonant current I_RESis in phase and has about the same amplitude as the fundamental frequency component of the chopper current I_CHOPPER, the fundamental frequency component will not appear in the current I_LINEon dc line 106.

[0022] Additional reference is made to FIG. 2. The phase of the resonant current I_RESmay be adjusted by creating a first dead-band DB1 before the first switch 124 is closed and a second dead band DB2 after the first switch is closed. During a deadband DB1 or DB2, little to no current flows through the trap circuit 118. The first deadband DBl may be created by holding-off the gating of the first switch 124 (which is in series with the resonant LC components 126 and 128 of the trap circuit 118) every half cycle. The time during which the gating is held off may be adjusted so that the first and second deadbands DB1 and DB2 are symmetrical or asymmetrical.

[0023] The amplitude of the resonant current I_RESmay be adjusted by controlling the voltage of the capacitor 126 just prior to closing the first switch 124. This is accomplished by closing and then opening the second switch 130. Consider two cases: (1) the voltage Vcap across the capacitor 126 is less than the dc line voltage Vdc across a capacitor 142 (that is, Vcap <Vdc), and (2) the capacitor voltage Vcap is greater than the dc line voltage Vdc (that is, Vcap>Vdc). When Vcap<Vdc just prior to closing the first switch 124, the momentary closing of second switch 130 results in the capacitor 126 being (partially) discharged through the resistor 132 to a grounded negative potential and the peak of the subsequent resonant current I_REShalf-cycle will increase. Similarly, when Vcap>Vdc just prior to closing the switch 124, the momentary closing of second switch 130 (partially) discharges the capacitor 124 but, this time, the peak of the resonant current I_RESduring the subsequent resonant half-cycle will decrease.

[0024] The frequency of the resonant current I_RESis selected to be higher (e.g., approximately 10% higher) than that of the chopper fundamental current I_FUNDproduced by the chopper 12 to accommodate a range of amplitude and phase values that are associated with changes in the duty cycle of the chopper 12. That is, the higher frequency allows space for phase-shifting using the aforementioned deadbands DB1 and DB2.

[0025] The dc-dc converter 100 further includes a conventional EMI filter 134 coupled to the dc line 106. The EMI filter 134, which may have a standard construction (e.g., passive elements 136, 138, 140 and 142), is designed to filter the remaining harmonic components from the current I_LINEon dc line 106. Since the active filter 114 has already trapped the fundamental frequency component from the line current _LINE, the EMI filter 134 does not have to deal with the fundamental frequency component. Therefore, the size of inductors 138 and 140 in the EMI filter 134 can be reduced. Reducing the inductances of the EMI filter 134 can result in a significant decrease in the overall weight, since the reduction in the EMI filter inductor weight is greater than the corresponding weight increase due to the addition of the active filter 114. Thus, the combined weight of the active filter 114 and the EMI filter 134 is lower than the weight of a corresponding conventional EMI filter.

[0026] FIG. 1 shows that the three currents I_LINE, I_RESand I_CHOPPERare measured by three sensors 144, 146 and 148. However, the two sensors that measure the residual and chopper currents I_RESand I_CHOPPERare not used by the controls 120 and 122 of the active filter 114 and are shown only for descriptive purposes.

[0027] FIG. 3 shows the amplitude and phase controls 120 and 122 in greater detail. The fundamental in-phase and quadrature components of the line current I_LINEare used to control the phase and amplitude of the resonant current I_RESThe fundamental in-phase and quadrature components of the line current I_LINEare extracted by a synchronous demodulator 200, which heterodynes the measured line current I_LINEwith respect to two reference signals: (1) a sine wave S_IPfor phase control and (2) a cosine wave C_Qfor amplitude control. Both reference signals S_IPand C_Qare synchronized with respect to the PWM frequency of the chopper 102. FIG. 4ashows the duty cycle of the chopper 12, FIG. 4bshows the chopper fundamental current, FIG. 4cshows the sine wave reference signal S_IP, and FIG. 4dshows the cosine wave reference signal C_Q.

[0028] Output signals S1 and C1 of the synchronous demodulator 200 are passed through low pass filters 202 and 204 to produce two filtered signals S2 and C2. Due to the heterodyning action of the synchronous demodulator 200, the filtered signals S2 and C2 are decoupled dc signals that represent the in-phase and quadrature components of the fundamental frequency content of the line current I_LINE.

[0029] The filtered signal S2 representing the in-phase component may be used as amplitude feedback, and the filtered signal C2 representing the quadrature component may be used as phase feedback. A first summing element 206 generates an in-phase error signal P_ERRrepresenting the difference between the phase feedback C2 and a reference phase signal P_REF. The error signal P_ERRis integrated over a complete “sampling period” by a phase shift regulator 208, where the sampling period equals a chopping frequency cycle. The output of the phase shift regulator 208 is used to determine when to close the first switch 124 and thereby adjust the symmetry of the dead-band until the phase error signal P_ERRvanishes. When the reference phase signal P_REF=0 and the phase error signal P_ERRvanishes, the deadband should be symmetrical.

[0030] A second summing element 210 generates an amplitude error signal A_ERRrepresenting the difference between the amplitude feedback S2 and a reference amplitude signal A_REF. The amplitude error signal A_ERRis integrated over a complete sampling period by an amplitude control regulator 212. An output of the amplitude control regulator 212 provides the signal S3. The magnitude and the polarity (sign) of the signal S3 is provided to selector logic 214 which is used to modulate the second switch 130 such that the amplitude error signal A_ERRvanishes. The polarity of the signal S3 determines the resonant half-cycle in which the second switch 130 closed, and the magnitude of the signal S3 determines how long the second switch 130 shall be closed. For example, the second switch 130 remains closed while the magnitude is negative, but opens once the amplitude transitions from negative to positive.

[0031] Thus, the reference signals P_REFand A_REFindicate the desired amount of phase offset and amplitude. The reference signals P_REF=0 and A_REF=0 cause the complete trapping of the fundamental frequency component of the chopper current I_CHOPPERby the active filter 14.

[0032] The invention is not limited to a synchronous demodulator for creating two independent, decoupled control signals that are orthogonal to each other with respect to a common synchronizing signal. Such orthogonal signals may be created by a Park-vector based decoupler.

[0033] Referring now to FIG. 5, a Park-vector based decoupler 300 is shown in a reference frame synchronized with respect to the chopper current I_CHOPPER. In this reference frame the d-components and the q-components of the Park-vectors are inherently decoupled. A Park vector inherently contains information on both the instantaneous magnitudes and phase relationship of a rotating field with respect to a reference coordinate system. A Park vector, in general, is a mathematical representation that describes the locus of an electrical quantity in the complex space domain (where time is a parameter). The current Park vector is defined with the vector's amplitude and the vector's direction in spatial relation to the reference coordinate system. A general discussion of Park vectors can be found in P. K. Kovacs, “Transient Phenomena in Electrical Machines,” Elsevier Science Publishing Co. (1984).

[0034] A synchronization signal SYNC may be generated by the gating of the chopper switch 107 or by equivalent converter switching. A second block 304 converts the synchronization signal SYNC using a high frequency (e.g., greater than 100 MHz) clock (e.g. counter) to an equivalent angular value with respect to the repetition rate of the synchronization signal SYNC, where one repetition equals 360 degrees. A first block 302 represents a hardware or software implementation of a Park vector algorithm. A third block 306 represents a hardware or software implementation of a transformation algorithm which converts the Park vector signal (in stationary coordinates) to a complex vector (in a synchronous coordinates) with respect to the synchronization signal SYNC.

[0035] A fourth block 308 represents a hardware or software implementation for performing a d-q transformation. The d-q transformation includes a decomposition of the current Park vector with respect to the current voltage into in-phase and out-of-phase orthogonal components.

[0036] A first low pass filter 310 extracts the quadrature component C2 from the imaginary portion of the decomposition, and a second low pass filter 312 extracts the in-phase component S2 from real portion of the decomposition. Summing junctions 314 and 316 sum these components C2 and S2 with phase and amplitude reference signals P_REFand A_REFto produce error signals A_ERRand P_ERR.

[0037] The error signals P_ERRand A_ERRare supplied to phase and amplitude regulators 318 and 320. An output of the phase regulator 318 is used to control the first switch 124. Sign and magnitude of an output of the amplitude regulator 320 are used to control the second switch 130.

[0038] The invention has been described above in connection with a dc-dc converter, and particularly in connection with a dc-dc converter including a down-chopper (buck-chopper). However, the invention is not so limited. The invention may also be applied to a dc-dc converter including an up-chopper (boost-chopper).

[0039] The invention may be used for dc-ac conversion, where the chopper down-chopper (buck-chopper). However, the invention is not so limited. The invention may also be applied to a dc-dc converter including an up-chopper (boost-chopper).

[0040] The invention may be used for dc-ac conversion, where the chopper is replaced by a three-phase inverter, and the duty cycle control is replaced by a PWM frequency commutation logic. The commutation logic generates commutation commands that cause the inverter to modulate the dc line current at the commutation frequency. The three-phase ac current may be supplied to a load such as permanent magnet motor, induction motor or switched reluctance motor.

[0041] The invention may also be used in combination with a switched reluctance machine that is used as a switched reluctance generator. The switched reluctance generator supplies ac current to an ac-dc converter. The ac-dc converter, in turn, supplies pulsed current on a dc line. An active filter removes the fundamental frequency component on the dc line. The active filter can reduce capacitor size at the output terminals of the dc line.

[0042] The common element in all of these embodiments is that the dc line current has a high frequency content at the commutation frequency and at the associated higher harmonics. The phase and amplitude controls just described are capable of filtering the fundamental frequency harmonic component since the fundamental frequency is an integer multiple of the PWM frequency.

[0043] The invention is not limited to an active filter having a single trap circuit. Additional trap circuits may be used to sink additional harmonic frequency components

[0044] Accordingly, the invention is not limited to the specific embodiments described above. Instead, the invention is construed according to the claims that follow.