Abstract:
Circuitry provides various protection mechanisms to an external circuit using actively controlled elements. The controlled elements may by controlled to provide overcurrent, overvoltage, or undervoltage protection to an external circuit.

Description:
BACKGROUND 
   This invention relates to electronic components having actively controlled circuit elements. 
   The interconnected electronic components that form many electronic circuits include simple, passive elements, like resistors and capacitors, and more complex actively controlled circuit elements that provide logic and control functions. One example of an actively controlled circuit element is a MOSFET, which can he controlled to perform a switching function (e.g., turned on and off) or controlled in a linear fashion (e.g., the voltage across the MOSFET and/or the current flowing in the MOSFET are controlled over a continuous range of values). 
   The elements of an electronic component may be formed as regions of material on a substrate as part of an integrated circuit. Or the elements may be commercially available discrete devices (both passive and active) mounted on a circuit board using either conventional soldered leads or surface mounted contact pads. 
   The resulting electronic components may be packaged in housings or cases that have terminals for making electrical connection to electronic circuits. The terminals of the components can be in the form of leads or surface mounting contact pads. 
   Micro-lead packaging (MLP) techniques can be used to house integrated circuits and discrete devices in tiny, inexpensive electronic packages that are not much larger than the devices within them and that are easily mounted on circuit boards. Heat can easily and economically be removed from power-dissipating devices that are packaged in MLP packages (see, e.g., U.S. patent application Ser. No. 09/643,159, “Power Converter Assembly”, filed Aug. 21, 2000, incorporated by reference). 
   Some electronic components can be thought of as serving secondary or service functions for other, primary circuits. For example, a power converter circuit may be considered a primary circuit while a ripple filter component connected to the output of the converter may be viewed as providing a secondary or service function. 
   Sometimes the service functions are provided by including them directly in the primary circuit. In other cases, when the primary circuits are sold as commercial products without inclusion of the service functions, the service functions may be provided by components that are sold and mounted separately. 
   For example, a commercially available DC-to-DC power converter will typically include ripple filtering circuitry. However, certain applications require very low ripple, and the additional filtering requirements may be met by providing an add-on commercial product that is connected to the output terminal of the converter. One example of such a secondary product is the VI-RAM Ripple Attenuator Module (“RAM”) available from Vicor Corporation of Andover, Mass., which serves as an active ripple filter at the output of a switching power converter, such as the VI-200 and VI-J00 families of converters sold by Vicor. The filter function of the RAM is provided by a combination of a linear MOSFET element connected between the output of the power converter and the load and an integrated circuit that actively controls the MOSFET to cancel the ripple at the output of the power converter as a way of reducing ripple at the load. 
   SUMMARY 
   In general, in one aspect, the invention features apparatus that includes two or more electronic components, each of the components having (a) an internal circuit having a controlled element and a control element, and (b) terminals coupled to the internal circuit and adapted for surface mounting on a circuit board. The internal circuits of the components are adapted to be connected in parallel through one of the terminals of each of the internal circuits to a common point of an external circuit and to cooperatively protect the external circuit against occurrence of an adverse electrical event. None of the electronic components has ratings sufficient by itself to protect the external circuit. 
   Implementations of the invention may include one or more of the following features. The event is a loss of a source of power for the external circuit or a sudden change in a voltage at a point of load of the external circuit. The controlled element is a FET. The internal circuit is adapted to detect a current reversal in a path between a power source and the external circuit, and the controlled element is controlled to disconnect the power source from the external circuit in response to the detection. The internal circuits are connected in parallel between a single power source and the external circuit. 
   Each of the internal circuits includes a voltage generator adapted to derive power from an external source and to provide a voltage to drive the internal circuit. Each of the internal circuits includes a comparator that compares the voltages at the common point and at another point to determine when a current has reversed. The FET and a control circuit are formed on a single integrated substrate, or they may comprise discrete components mounted on a single substrate. The FET, the control circuit, and the terminals are part of a micro-lead package. The internal circuit includes elements adapted to pull up a voltage at one of the terminals when the voltage at the terminal drops and elements adapted to pull down the voltage at the one of the terminals when the voltage at the terminal rises. The elements include a DC-to-DC converter. 
   Implementations of the invention include one or more of the following features. The external circuit comprises a power converter, and the filtering function comprises a ripple filtering of a power converter. The filtering function comprises attenuating the ripple generated at the output or input of the converter. The controlled element comprises a FET the conductivity of which is controlled to provide the filtering function. The control element includes elements adapted to detect a component of ripple at one of the terminals. The controlled element comprises a MOSFET, and the average voltage across the MOSFET is controlled to be greater than the peak-to-peak variation in the ripple. The control regime includes regulating the voltage variations across the FET to effect ripple attenuation. The apparatus includes terminals coupled to the apparatus and adapted for surface mounting on a circuit board. 
   In general, in another aspect, the invention features apparatus that includes (a) a protection circuit, and (b) terminals for connecting the protection circuit respectively to a power source and to an external circuit that is to be powered by the source and protected by the protection circuit against an occurrence of an electrical event. The protection circuit is connected to provide two different kinds of protection for the external circuit using two controlled elements. 
   Implementations of the invention may include one or more of the following features. The protection circuit includes two protection mechanisms connected in series between the source and the external circuit. The protection mechanisms include two FETs connected in series in a common drain configuration or a common source configuration. The protection circuit comprises two protection mechanisms connected across an external circuit. The protection mechanisms include a FET. The protection mechanism shunts current away from the external circuit. A protection mechanism delivers current to the external circuit. The protection mechanism shunts current to ground. The apparatus comprises an energy reservoir at a predetermined voltage, and the protection mechanism shunts current to the energy reservoir or delivers current to the energy reservoir. 
   In general, in another aspect, the invention features a method that includes setting an average voltage across a series pass element of an active filter based upon variations in a signal that is to be filtered. In implementations of the invention, the peak-to-peak variations in the signal to be filtered are measured, and the average voltage is set to be slightly higher than the peak-to-peak variations. 
   Other advantages and features of the invention will become apparent from the following description and from the claims. 

   
     DESCRIPTION 
       FIG. 1  shows a circuit board. 
       FIG. 2  shows parallel connection of components. 
       FIGS. 3 through 16  show protection components. 
       FIGS. 17 through 21  show filter components. 
   

   As shown in  FIG. 1 , a service or secondary function can be provided as an electronic component  14  that operates in conjunction with a primary circuit  10  mounted on a board  12 . In some cases, the secondary function is provided by an appropriate number of parallel-connected, small, low-priced electronic components  14  that include controlled elements  16 , that are governed by active control circuits  18 ,. 
   The primary circuit  10  may have a wide variety of purposes and the electronic components  14  may provide a broad range of secondary functions or services to the primary circuit. Two classes of secondary functions and services are circuit protection and filtering. 
   PROTECTION COMPONENTS 
   Short Circuit Protection 
   As shown in an idealized form in  FIG. 3 , one kind of electronic component  11  includes a controlled element  17  to provide a short circuit protection function for a primary circuit (not shown) that is connected to an output  13  and is powered from a DC voltage source (say, between 1.5 and 30 volts, not shown) connected to an input  15 . A third terminal  19  is grounded. 
   The short circuit protection function is provided by detecting when too large a current is being drawn at output  13  and either opening the controlled element  17  quickly enough to avoid damage to the protected circuit or linearly controlling the controlled element to limit the current flow to some value or range of values. 
   In one implementation, shown in  FIG. 4 , the protection component is formed as an integrated circuit  20  that includes a MOSFET  22  (and its body diode  24 ) and a control circuit  25  that controls the MOSFET  22  through a control terminal  26 . The control circuit is grounded at the terminal  16  and is also connected to the input  12  and the output  21  through the terminals  30  and  32 . 
   The control circuit  24  provides three primary functions using conventional approaches. 
   One function  40  monitors the amount of power being delivered through the output  21  to quickly detect when a fault in the primary circuit is causing more than a threshold amount of current to be drawn through the output  21 . One way to detect the current is by measuring the voltage across the resistance channel represented by the MOSFET. 
   A second function  42  monitors the temperature of the component  20  against a threshold temperature maximum, to protect component  20  against damage. 
   A third function  44  generates Vcc from the voltage at the input terminal using a conventional charge pump circuit. The voltage Vcc is used both to power the control circuit  25  and to drive the gate  50  of the MOSFET. The MOSFET is either switched open quickly when either the detected current being drawn or the detected temperature exceed the preset limits  46 ,  48 , or the MOSFET is controlled to limit the current or temperature in accordance with some continuous control strategy (e.g., the current may be controlled to be a constant current or “foldback current limiting” may be used). 
   If the “switched” protection approach is used (i.e., the MOSFET is switched open when a fault condition is detected) then several components  20  may be paralleled to provide protection at higher current and power levels. One way to parallel the components is simply to connect their inputs  12  and outputs  21  ( FIG. 4 ) together (as illustrated in FIG.  2 ). In this case, differences in current limit thresholds will result in one device opening first on occurrence of an overcurrent condition. This will then result in other devices carrying higher current, which will cause their overcurrent thresholds to be exceeded. Alternatively, a “parallel” pin may be provided on each component  20  (not shown in the figures) which would connect to a similar pin on all other units. Any unit which senses an overcurrent condition would open its MOSFET and deliver a signal to its parallel pin; the signal would be sensed at the parallel pin of all other units, causing them to open their MOSFETs. 
   In one example, the component  20  can handle any positive input voltage up to 30 volts and an output current up to a maximum of 10 Amps, and the switch is a 5 milliohm, 30 Volt MOSFET. 
   Another example would handle negative voltages instead. 
   Component  20  could be fabricated as a single integrated circuit or as an integrated circuit controller and a separate MOSFET. In either case the parts could be packaged using MLP techniques in a tiny surface mount component and made available for a cost as low as $1. 
   Smart OR&#39;ing Diode 
   Another type of electronic component that provides a different protective service function is shown in  FIG. 5. A  load  35  is powered by two (or more) redundant power sources  33 ,  34 . In the absence of the OR&#39;ing circuits  36 ,  38 , certain types of failures (such as a short-circuited output) in one or the other of the sources would cause the output voltage delivered to the load to drop and power delivery to the load to be interrupted. The goal of the OR&#39;ing circuits  36 ,  38  is to enable a single one of the sources  36 ,  38  to provide uninterrupted delivery of power to the load under such circumstances. 
   As shown in an idealized form in  FIG. 6 , the OR&#39;ing function may be achieved by a diode  71  in a device that has an input terminal  73  to connect to one of the sources and an output terminal  75  to connect to the load. Conventional bipolar or Schottky diodes are commonly used for this purpose. However, the voltage drops in such devices (e.g., 0.7 V or more for a bipolar and 0.4 V or more for a Schottky) may represent significant power loss when the voltage being delivered to the load is relatively low (e.g., 2.2 V or 5 V). 
   One implementation of a circuit  70  to provide the OR&#39;ing function, shown in  FIG. 7 , includes a MOSFET  77  (and its body diode  72 ) and a control circuit  79  that controls the gate of the MOSFET. The input source is connected to terminal  73  and the load is connected to terminal  75 . If the source that is connected to the input  73  fails, to prevent current from flowing backward through the MOSFET, the control circuit  79  detects the condition and turns off the MOSFET. The OR&#39;ing diodes of other sources that are connected to the same load do not shut off and their sources cooperate to continue to provide all of the power needed by the load. 
   For this purpose, the control circuit has two functional elements that use conventional approaches. As in  FIG. 4 , a Vcc generation element  74  accepts power from the load connection  75  and uses it to generate Vcc both to power the control circuit and to drive the gate of the FET (by connecting to the load, the control circuit, e.g., control circuit  36 , will continue to operate even if its input source, e.g., source  33 , becomes inoperative, provided that at least one operating source is able to provide power to the load via it own OR&#39;ing diode). A comparator  69  compares the voltages at terminals  73 ,  75 . When the voltage at terminal  75  is higher, a condition that would otherwise drive current backward through the circuit, the MOSFET is quickly shut off. Vcc continues to be generated to keep the control circuit powered. The polarity of the body diode  72  of the FET  77  enables power to be delivered from the input  73  to the output  75  should the control circuit be inoperative, e.g., during the time period after initial power-up when Vcc has not risen to its final value). 
   As in the other components described above and below, the smart OR&#39;ing diode component can be fabricated using discrete or integrated circuit techniques and packaged in a small, low cost MLP package for commercialization. 
   As shown in  FIG. 2  (which also applies generically to a wide range of different kinds of components  14  including others described above and below), multiple units of the smart OR&#39;ing diode (at any chosen level of granularity) can be connected in parallel between an input source and an output load to achieve higher current capacity. The current is shared naturally by the MOSFETs. 
   Short-Circuit Protection with OR&#39;ing Diode 
   As shown in  FIGS. 8 through 11 , the service functions of the OR&#39;ing diode component and the short-circuit protection circuit can be combined in a single commercial component. 
     FIGS. 8 and 10  show idealized components  80 ,  89  in which the diode and protection elements  84 ,  90  are respectively connected in different orders. Functionally the two approaches are essentially the same. But they provide different fabrication opportunities and burdens. 
     FIGS. 9 and 11  show implementations of the respective components, in both cases using two MOSFETs  83 ,  86  (and their body diodes  88 ,  90 ) connected in series. In both cases, the gates of the MOSFETs are managed by a control circuit  82  that includes the same kinds of functional elements shown earlier for the OR&#39;ing diode and the protection circuit. 
   In  FIGS. 8 and 9 , the protection function is on the source side of the OR&#39;ing diode function and the MOSFETs are connected in a common source configuration. The control circuitry is simpler than in  FIGS. 10 and 11  because (in  FIGS. 8 and 9 ) the two gates can be driven in a parallel mode rather than separately. But the fabrication of a fully-integrated (i.e., the FETs and control circuitry integrated onto a common semiconductor die) version of the component of  FIGS. 8 and 9  is more complex than that of  FIGS. 10 and 11  because the drains of the two MOSFETs must be isolated from each other. 
   Voltage Shock Absorber 
   Another protection component in the form of a voltage shock absorber  250  is illustrated as an ideal element in FIG.  12  and in an implementation in FIG.  13 . 
   The goal of component  250  is to “shock absorb” against changes in a voltage (either increases or decreases) between a point of load  252  and ground  254  to prevent damage to or failure of operation of a load  256 . The component  250  is responsive to transients which might cause the load voltage to go above or below pre-defined limits. 
   Component  250  has two sub-circuits  258 , 260  that include controlled elements  251 ,  253 , respectively. One sub-circuit  260  is arranged to quickly pull down the voltage at the point of load toward ground. The other side  258  is arranged to quickly pull up the point of load toward a voltage that is approximately double the nominal voltage V0 of the point of load. 
   The 2*V0 voltage is achieved by charge pumping in a conventional manner into a capacitor  262 . 
   As shown in  FIG. 13 , the controlled elements can be implemented as MOSFETs  261 ,  263  (with their body diodes  264 ,  266 ). Functional elements of the control circuit  268  include a voltage averaging function  270  that averages the load voltage over time as a way to calibrate the circuit for later control of the gates of the MOSFETs. A temperature limiter  272  forces a shutoff of the MOSFET if it gets too hot. A dv/dt detector  274  watches for sharp changes up or down in the point of load. Positive and negative linear regulators  276 ,  278  provide very rapid control of the FETs to achieve the shunting up or down of the point of load. 
   A conventional charge pump  279  accepts V0 as an input and charges capacitor  262  to approximately 2*V0. The charge pump voltage is the input to positive linear regulator  276  and to the drain of MOSFET  261 . The feedback loop comprising the positive linear regulator  276  and MOSFET  261  seeks to counteract any negative rate-of-change in the voltage V0. Transients having a negative rate-of-change with respect to the average value of V0 are detected by the dv/dt detector  274 , which causes the positive linear regulator  276  to drive the gate of MOSFET  261 , thereby delivering energy to the load from capacitor  262  as a means of counteracting the transient. A more rapid rate-of-change of V0 will result in a faster rate of delivery of energy to the load. 
   The feedback loop comprising the negative linear regulator  278  and MOSFET  263 , with input from dv/dt detector  274 , operates in a complementary manner with respect to dips in load voltage V0. Transients having a positive rate-of-change with respect to the average value of V0 are detected by the dv/dt detector  274 , which causes the negative linear regulator  278  to drive the gate of MOSFET  263 , thereby diverting energy away from the load to ground to counteract the transient. A more rapid rate-of-change of V0 will result in a faster rate of diversion of energy away from the load. 
   Multiple similar shock absorber components can be connected in parallel (with any desired level of granularity) between the point of load and ground to increase the total “shock absorbing” capacity of the system. For example, if the “pulldown” thresholds of three parallel shock absorbers are 5.1, 5.2, and 5.3 volts, respectively, and if the voltage at the point of load rises to 5.1 volts, the first shock absorber will start to draw current. If that unit alone can successfully absorb the shock, then 5.2 volts is never reached, and the other two shock absorbers are not triggered. Otherwise, the voltage will continue to rise and, eventually, the second shock absorber, and, if needed, the third one, will be triggered. 
   Efficient Shock Absorber 
   The efficient shock absorber  280  of  FIGS. 14 and 15  has the same goal as in  FIGS. 12 and 13 , but uses a more complex circuit to operate more efficiently. Instead of pulling up by connecting the point of load  282  to a 2*V0 source and pushing down by connecting to ground, the circuit includes a boost regulator  284  to pull up to V0+delta*V0 and a buck regulator  286  to pull down to V0−delta*V0. This reduces the difference in voltage between the point of load and the pullup and pulldown points, which reduces the amount of energy loss in the circuit. 
   In general, if the worst case current transient which must be absorbed is X Amps, then the MOSFET should be selected to have a minimum controllable value of drain-to-source resistance (the ON resistance) of delta*V0 divided by X. For example, if delta*V0 is 50 millivolts and the maximum current transient is 10 Amps, then the ON resistance of the MOSFET should be less than 5 milliohms. 
   In  FIG. 15 , the control circuit has other control elements that are similar to the ones shown in  FIG. 13 , including voltage averaging 290, dv/dt detection  292 , temperature limiter  294 , and positive and negative linear regulators  296 ,  298 . 
   Simple and Efficient Shock Absorber 
   The component  310  of  FIG. 16  achieves a compromise between the simplicity of the component of FIG.  13  and the efficiency of the component of FIG.  15 . 
   The component  310  is simpler than the one in  FIG. 15 , for it uses a single boost converter  312 . The other control circuit elements are similar to the ones in FIG.  15 . Note that the diode  314  is poled differently than the body diodes in  FIGS. 13 and 15 . This allows the circuit to start up when power is initially applied. Thereafter, the value of Vccu may be set to any value above V0, but the minimum value of Vccd will be limited to be above (V0−Vd), where Vd is the drop in diode  314  (which may be a discrete diode or the body diode of the MOSFET). T 
   The circuit of  FIG. 16  is more efficient than the circuit of  FIG. 13 , but is more complex and expensive, and is simpler and lower cost than the circuit of FIG.  15 . but is less efficient. 
   FILTER COMPONENTS 
   Another class of components that use controlled elements and control circuits provides filtering service functions to primary circuits. 
   Active Output Filter 
   As shown in  FIG. 17 , for example, a component  202  includes an idealized filtering element  204  connected between a unipolar, but non-ideal, input voltage source, Vin  207  , connected to input terminal  206 , and a unipolar output load  203  connected to output terminal  208 . A third terminal  210  is grounded. 
   The goal is to prevent ripple  209  ( FIG. 18 ) generated by the input voltage source  207  from appearing at the load by controlling the filtering element in a manner to offset the ripple of the input voltage source. 
   As shown in  FIG. 18 , an implementation of the active filter component  202  includes a MOSFET series pass element  212  (with its body diode  214 ) and a control circuit  218  that controls the conductivity of the MOSFET through a control terminal  220 . The control circuit is grounded at the terminal  210  and is connected to the input  208  and the output  208  through the terminals  222  and  224 . 
   The control circuit  218  provides five primary functions using conventional approaches:
     a. an error amplifier  234  for comparing the AC ripple component of Vout (as indicated by DC blocking capacitor  235 ) to an essentially zero voltage AC reference.   b. a circuit  238  for measuring the peak-to-peak ripple of the input source.   c. a headroom adjustment circuit  236 , explained below.   d. a gate control circuit for controlling the AC and DC voltage across the MOSFET, as explained below.   e. a Vcc circuit  230  (e.g., a charge pump) for powering the circuitry in control circuit  218 .   

   Using these functional elements, the gate is controlled in a two-layer control regime. 
   In one layer, called “headroom adjustment” or “adaptive headroom”, the gate of the MOSFET is controlled so that the DC voltage level across the MOSFET series pass element  212  is regulated to be as small as it can be while still spanning the peak-to-peak range  240  of the ripple of the input source. This reduces average power losses in the circuit while maintaining sufficient dynamic range to cancel the ripple. Headroom adjustment is accomplished by measuring the peak-to-peak ripple at the input (using peak-to-peak ripple measuring circuit  238 ), comparing the peak-to-peak ripple to the average voltage across the MOSFET (i.e., the difference between the average value of Vin and the average value of Vout) in error amplifier  236 , and closing a feedback loop, via gate control circuit  239 , to control the average voltage across the MOSFET  212  to be slightly above the measured peak-to-peak voltage. This process occurs continuously: the average value of MOSFET voltage, over a time span which is relatively large compared to the time scale over which the variations in the ripple take place, is adaptively adjusted as the envelope of the peak-to-peak ripple changes. 
   In a second layer of the control regime, error amplifier  234  compares the AC ripple component of Vout (as indicated by DC blocking capacitor  235 ) to an essentially zero voltage AC reference point and generates an error signal which controls the gate of the MOSFET, via gate control circuit  239 , so that the AC voltage across the MOSFET  212  exactly (in an ideal world) offsets the input source ripple variations. 
   Active Input Filter 
     FIG. 19  illustrates an idealized component, an active input filter  302 , which is intended to prevent ripple currents, IR, generated by a load  303  (e.g., the input of a DC—DC converter) from being reflected back into the input source  307 . In the figure, a component  302  includes an idealized filtering element  304  connected between a unipolar, but non-ideal, input voltage source, Vin  307 , connected to input terminal  306 , a unipolar output load  303  connected to output terminal  308 , and a bypass capacitor  305  across the load. A third terminal  310  is grounded. The active input filter may be thought of as operating at relatively high frequencies relative to the frequency content of the input source. For example, if the load is a DC—DC converter operating at a conversion frequency of 300 KHz, the active input filter might act on the first three or four harmonics of the conversion frequency. Low frequency variations in the input source, such as 100 or 120 Hz ripple deriving from rectification of an AC utility source, will be below the frequency range of operation of the active input filter. Thus, the source may be considered to be a DC source. 
   The goal is to prevent ripple generated by the load  309  from appearing at the input source  307  by controlling the filtering element  304  in a manner which forces the currents to flow in the bypass capacitors  305 , thereby (in an ideal world) preventing any reflected ripple current IF from flowing back into the input source. 
   As shown in  FIG. 20 , an implementation of the active input filter component  302  includes a MOSFET series pass element  312  (with its body diode  315 ) and a control circuit  318  that controls the conductivity of the MOSFET through a control terminal  320 . The control circuit is grounded at the terminal  310  and is connected to the input  308  and the output  308  through the terminals  322  and  324 . 
   As illustrated, the control circuit  318  provides the same primary functions described above for the output filter of  FIG. 18 , using conventional approaches:
     a. an error amplifier  334  for comparing the AC ripple component of Vin to an essentially zero voltage AC reference.   b. a circuit  338  for measuring the peak-to-peak ripple of the input source.   c. a headroom adjustment circuit  236 , explained below.   d. a gate control circuit for controlling the AC and DC voltage across the MOSFET, as explained below.   e. a Vcc circuit (e.g., a charge pump)  330  for powering the circuitry in control circuit  318 .   

   Using these functional elements, the gate is controlled in a two-layer control regime. 
   In one layer, called “headroom adjustment” or “adaptive headroom”, the gate of the MOSFET series pass element  312  is controlled so that the DC voltage level across the MOSFET is regulated to be as small as it can be while still spanning the peak-to-peak range  240  of the ripple across the bypass capacitors  305 . This reduces average power losses in the circuit while maintaining sufficient dynamic range to cancel the reflected input ripple current, IF. Headroom adjustment is accomplished by measuring the peak-to-peak ripple at the output (using peak-to-peak ripple measuring circuit  338 ), comparing the peak-to-peak ripple to the average voltage across the MOSFET (i.e., the difference between the average value of Vin and the average value of Vout) in error amplifier  336 , and closing a feedback loop, via gate control circuit  339 , to control the average voltage across the MOSFET  312  to be slightly above the measured peak-to-peak voltage. This process occurs continuously: the average value of MOSFET voltage, over a time span which is relatively large compared to the time scale over which the variations in the ripple take place, is adaptively adjusted as the envelope of the peak-to-peak ripple changes. 
   In one example of a second layer of the control regime, error amplifier  334  compares the AC ripple component of Vin (as indicated by DC blocking capacitor  335 ) to an essentially zero voltage AC reference point and generates an error signal which controls the gate of the MOSFET, via gate control circuit  239 , so that the AC voltage across the MOSFET  212  exactly (in an ideal world) offsets the output source ripple variations. By this means, IF is effectively eliminated. 
   In another example of the second layer of control, the AC error amplifier  334  of  FIG. 20  is replaced with a current sense amplifier  434  (shown in FIG.  21 ). The current sense amplifier compares the current IF to an essentially zero current reference point and generates an error signal which controls the gate of the MOSFET, via gate control circuit  239 , so that the conductivity of the MOSFET  212  exactly (in an ideal world) cancels the flow of AC current, IF. 
   Other implementations are within the scope of the following claims. For example, the components may comprise an integrated circuit for control functions and separate MOSFET devices or one or more MOSFETs may be integrated onto the same die as the control circuitry. The MOSFETs in the active filters may be placed in either the positive or the negative current path. All components may be implemented for use with positive or negative sources.