Abstract:
A switched magamp post regulator in a power converter incorporating a switched set mode control circuit which minimizes the power loss associated with the control transistor of a set mode magamp post regulator is disclosed. Power loss in set mode is minimized by switching the control transistor on and off synchronously with the main transformer. The incorporation of set mode and switching allows the use of less expensive ferrite core materials with increased efficiency for operation at higher frequencies and higher temperatures.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to power converters having magnetic amplifier (magamp) post regulators and, more particularly, to circuitry used to reduce the power and efficiency loss in the control transistor of a set-mode magamp post regulator. 
     BACKGROUND OF THE INVENTION 
     The magamp post regulator is a popular power supply topology for regulating the outputs of a power converter in many applications. Modern electronic devices often require several voltage outputs; and need a low cost, energy efficient and well regulated way of providing these outputs. Magamps are typically used to provide an efficient and reliable way of providing precise voltage regulation of independent outputs of a multiple output power converter. A magamp post regulator provides improved regulation of power converter output voltage using a small control current. 
     The basis function of a magamp is to block a positive incoming voltage for a certain time (t block ) before allowing it to pass through an output filter. The duty cycle reduction occurs because the magamp delays the leading edge of the voltage waveform. The magamp acts to reduce the duty cycle to the rest of the circuit from the duty cycle of the incoming voltage so as to maintain the required average output voltage. 
     Conventional magamp post regulator circuits use a reset control to control the magamp using a control transistor operated in a linear mode. FIG. 1 illustrates a prior art example of a conventional reset-controlled magamp circuit  10 . FIG. 2 illustrates the hysteresis characteristic of the core element of the magamp of the circuit of FIG.  1 . The conventional magamp circuit includes a magamp  16 , a diode  12 , a reset transistor  20  and an error amplifier (error amp)  18 . In FIG. 1, when a power switch  34  is turned on, a secondary voltage V sec  is developed across a transformer  14  secondary winding. Magamp  16  is forced into saturation due to the action of the voltage V sec  forced upon it. The B-H hysteresis curve in FIG. 2 shows the saturation point, B saturation  at the top of the path. Since the magamp  16  is “in saturation”, forward biased and highly conductive, current flows through the magamp to a forward output rectifier diode  22  after which it is filtered by an L-C circuit, comprised of inductor  24  and capacitor  26 . The output voltage is coupled to a load, not shown, and is also divided by a voltage divider formed by series resistors  36  and  38  to generate a Voltage sense signal at node  35 . At the end of the switch “on” time, the magamp  16  remains forward biased and in saturation. 
     When the main power switch  34  turns off and the transformer  14  voltage reverses polarity to −V sec  the current through the magamp  16  is caused to ramp down. As a result, a vertical rectifier diode  30  must pick up the output current, causing the voltage at node  15  to drop. In this off state the magamp voltage V m  is not allowed to reach zero. Instead a reset control circuitry supplies a voltage that reversely biases the magamp  16 , such that the magnetic flux density is reset to a point below remanence (below the point B remanence  of the left side of dark shaded area in FIG.  2 .). Then the main switch is turned on and the transformer  14  secondary voltage becomes +V sec . Since Magamp  16  is well below the saturation point and not conductive, it acts as an open circuit and blocks the secondary voltage. Vertical diode  30  continues to provide a path for the output current so the voltage at node  15  remains at zero. The magamp voltage Vm then equals +V sec . In time, the voltage across magamp  16  causes it to reach saturation and become conductive. The current through the magamp rises to the output current level and remaining at this level till the end of the on time. 
     The flux excursion on the B-H curve of FIG. 2 depends on how much volt-time is applied across the magamp  16  during resetting. The amount of volt-seconds is controlled by the output of error amp  18 . The blocking time equation is given by            t   block     =       Δ                   B   ·   turns   ·     A   core         V       ;                          
     where A core  is the core area, ΔB is the change in flux density, turns is the number of turns for the core, and V is the voltage. It can be seen from this equation that the loop in FIG. 2 corresponding to ΔB 2  gives a longer blocking time that the loop of ΔB 1 . The cores required for this prior art method of reset control exhibit a relatively square B-H curve. To lower the output voltage and increase the blocking time, the loop followed is the lightly shaded part of the B-H curve as compared to the dark part. The control circuit forces the B-H loop larger by pushing the vertical, descending part of the locus. Thus, the minimum blocking voltage-time is the locus where it just touches the vertical axis. To maximize the difference between maximum and minimum volt-time blocking, the B-H loop of the core material must have a small difference between B saturation  and B remanence , where it intercepts the vertical axis. 
     Compared to square loop amorphous core magamps, ferrite magamps are lower cost, better for high frequencies and can run at higher temperature. However, a drawback associated with this conventional reset control approach is that lower cost non-square ferrite cores perform poorly under reset control because the power dissipation at high flux excursion is too large, especially for operation at high frequency. 
     A prior art example of a conventional circuit for magamp post regulator control without using reset control but instead using a “set” mode with a control circuit in a linear mode, is shown in FIG.  3 . This set control enables the use of lower cost ferrite cores for the magamp core, however, operation in linear mode leads to unacceptable losses in the circuit. The corresponding B-H hysteresis characteristic of the core member of the magamp of the circuit is shown in FIG.  4 . For the magamp post regulator  40  in FIG. 3, an error amp  48  feeds a control transistor  50  which is operated in linear mode. When the transformer  44  secondary voltage V sec  turns negative in response to power switch  64 , a diode  42  and a control transistor  50  “catch” the current through magamp  46 . Depending on the voltage output from error amp  48 , the current through the loop of diode  42 , control transistor  50  and magamp  46  is decreased, and the corresponding change in ΔH and ΔB is achieved (as shown in FIG. 4, the current is related to H by the equation H*L core =turns * I.) During the next positive cycle, the magamp  46  will block the secondary voltage V sec  The blocking time, T block , according to the equation described above,            t   block     =       Δ                   B   ·   turns   ·     A   core         V       ,                          
     is proportional to ΔB (turns, A core  and V are constant for the equation). As the curve in FIG. 4 illustrates, set control mode operates only at one quadrant of the B-H curve while the reset control, as shown in FIG. 2, can operate at all four quadrants. In this “set” mode circuit, the control circuit tries to prevent the core from resetting, i.e. tries to make a smaller loop. Since there is no requirement for the core to be square, non-square less costly ferrites can be used. 
     FIG. 5 shows another prior art version of set control for a magamp post regulator. For this magamp post regulator circuit  70 , in addition to the magamp  76  power winding, there is an extra magamp control winding  77 . A driver diode  72  and a control transistor  80  control the magamp control winding  77 , with the control elements isolated from the transformer  74  secondary power winding. The current through the diode  72  and control transistor  80  can be reduced depending on the turns ratio of the control winding and power winding. FIG. 6 shows a corresponding set of timing curves for the magamp set control circuit of FIG.  5 . The top curve  1 , is the secondary voltage and V p  is the transformer  74  primary voltage, curve  2  is the V error  voltage, curve  3  is the transistor  80  collector-emitter voltage, V cc , and curve  4  is the magamp voltage Vm. FIG. 7 shows a set of measured voltage curve traces for the magamp set control circuit of FIG.  5 . Curve  5  is the secondary voltage, curve  6  is the voltage at the anode of the horizontal diode  82  and the lower curve  7  is the magamp voltage V m . 
     The conventional set control circuits of FIGS. 3 and 5 allow the use of lower cost non-square ferrites. A drawback of these circuits, however, is that the circuits exhibit unacceptable power and efficiency loss. FIG. 8 illustrates the unacceptable energy loss. The stored energy in the core is the area bounded by the B-H curve and the B axis. When traversing the lower part of the B-H curve up to saturation, the energy stored is equal to the light shaded area A e  plus the dark shaded area A h ;with A h  representing the energy lost due to hysteresis. When traversing the curve from saturation to the area between saturation and remanence, a part of the area A e  is associated with the movement. Under set control, the energy is dissipated in the control transistor. 
     FIG. 9 is a set of measured trace curves illustrating the power dissipation drawback of the conventional set mode circuits. Curve  8  is the secondary voltage V sec , curve  9  is the current for the control transistor and  10  is the control transistor voltage. The voltage and current waveforms between the vertical cursors illustrate that the power is being dissipated in the control transistor that is operating in its linear region. At higher power levels, more power will be dissipated. Under set mode control, the energy has been found to be dissipated in the control transistor that is the driver element for the magamp post regulator. 
     To allow the use of any kind of loop material regardless of its residual flux and to use ferrites effectively at lower frequencies, a conventional “full control” method has also been used. For this full control method, both the reset and set control methods are selectively used; with either being applied to the same core. 
     A drawback associated with the “set” control and the “full control” methods, as described above, is that losses in the control transistor are quite high, resulting in unacceptable reductions in power and efficiency. Parasitic energy stored in the magamp during the power delivery is burned in the control transistor. Therefore, there is a need for circuitry to reduce this power and efficiency loss in the control transistor of a set mode magamp post regulator circuit. 
     SUMMARY OF THE INVENTION 
     The aforementioned drawbacks associated with losses in the control transistor in “set” mode magamp post regulators are substantially reduced or eliminated by the present invention. One aspect of the present invention is directed to a switched set mode magamp post regulator circuit operative to eliminate the power loss associated with operation of the control transistor in linear mode, by switching the control transistor on and off synchronously with the main transformer. The switched magamp post regulator circuit enables the parasitic energy stored in the magamp to be recycled to the output load. The switched magamp circuit also reduces cost over the more commonly used reset-mode magamp circuits by employing the set mode which enables the use of different materials for the magamp core. The magamp post regulator control circuit embodiments described below are for regulating one or more output voltages of a power converter. The embodiments are described where the power converter is a forward converter, however, the present invention is equally applicable to other topologies including push-pull, half-bridge, full bridge and flyback; especially when there is a periodic rectangular voltage source similar to the V sec  transformer secondary waveform. 
     One exemplary embodiment of the present invention, shown in FIG. 10, provides a set mode magamp post regulator control circuit for regulating the output voltage of a power converter. The magamp post regulator circuit comprises a magnetic amplifier, a control transistor, a set mode control circuit and an output circuit. In this embodiment the control transistor is preferably a MOSFET. A power switch signal is turned “on” a secondary voltage V sec  is developed across the transformer winding. A set mode control circuit switches the control transistor on and off synchronously with the main transformer. A winding on the magamp is allowed to “fly” then subsequently gets shorted out, during every cycle, during the off-time for the primary of the power converter, in order to get the desired B-H excursion curve of the magamp core. When the control transistor is off, the energy from the magamp is returned to the load. When the control transistor turns on later in the cycle, current will circulate in the control windings of the magamp. The magamp preferably includes multiple magamp windings and a low-cost ferrite core. 
     An advantage of this embodiment is that the use of a switching mode of operation improves the power and efficiency by reducing losses in the control transistor compared to conventional circuits in which the control transistor is operated in linear mode. The set mode also allows the efficient use of lower cost core materials including ferrites. 
     In another exemplary embodiment of the present invention shown in FIG. 13, the switched magamp post regulator control circuit uses set mode control with a feedback control using pulse width modulation (PWM). The magamp post regulator control circuit comprises a magnetic amplifier, a control transistor, a control circuit and an output circuit. The control transistor to be switched on and off is also preferably a MOSFET. The control circuit for switching the control transistor is comprised of a comparator, an error amp and a ramp generator circuit. When the control transistor is off, the energy from the magamp is returned to the load. When the control transistor turns on later in the cycle, current will circulate in the control windings of the magamp. 
     FIG. 15 shows the preferred embodiment of the magamp post regulator circuit of FIG.  13 . There are two main differences between FIGS. 13 and 15. One is that the ramp voltage waveform, V ramp , produced in the embodiment in FIG. 13 is triangular whereas the ramp voltage waveform produced in FIG. 15 is trapezoidal. Secondly, for the embodiment in FIG. 15, the voltage at the negative input of the comparator passes through a diode and is DC biased. The DC bias feature incorporated into FIG. 15 is essential to ensure that the MOSFET control transistor is off during the time when the secondary voltage V s  is positive even in cases wherein the output error voltage goes to its lowest possible voltage. Note that in FIG. 13 if the error amp  208  is saturated and the ramp transistor  211  is fully on, the output of the comparator  221  is unpredictable and would be dependent on which of the two voltages is larger. The trapezoidal waveform in the embodiment in FIG. 15 raises the effective error voltage needed to operate in the ramp&#39;s dynamic range. This makes the circuit more immune to false triggering. Note also that FIG. 15 easily allows the addition of another error amp circuit for constant current operation if needed. The embodiment may optionally include a drive circuit to drive the control transistor. The embodiment in FIG. 15 shows the error amp portion of the control circuit configured “for constant voltage” control. 
     FIG. 18 shows an alternative embodiment of FIG. 15 with an error amp circuit that provides both constant voltage and constant current control. Alternatively the constant current control, shown in FIG. 17, could be provided without the constant voltage control circuit. 
     An alternate embodiment of the present invention shown in FIG. 19, provides “full control” over the magamp post regulator control circuit for regulating the output voltage of a power converter. “Full control” refers to control over the full range of the hysteresis loop [from −B saturation  to +B saturation ]. Unlike the conventional full control circuits, this embodiment uses both a set mode (switched) and reset mode (conventional linear, non-switched) depending on the operating condition. A further advantage of the full control embodiment is that it reduces core size and at the same time reduces the required number of power turns. This embodiment also allows efficient use of any kind of loop material and allows the use of lower cost ferrites at lower frequencies. 
     An advantage of the present invention is that it improves the operating efficiency of the power converter by minimizing the power loss associated with the control transistor element. Another advantage of the present invention is that it allows the use of lower cost ferrite cores which are lower cost than conventional amorphous cores, run better at high frequencies and can run at higher temperatures. A feature of the present invention is that it is inexpensive to manufacture since magamps have lower parts count and are easier to design than conventional post regulators. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The aforementioned and related advantages and features of the present invention will become apparent upon review of the following detailed description of the invention, taken in conjunction with the following drawings, where like numerals represent like elements, in which: 
     FIG. 1 is a schematic diagram of a prior art magamp post regulator using reset control; 
     FIG. 2 is a graph illustrating a hysteresis characteristic of the core member of the magamp of the circuit of FIG. 1; 
     FIG. 3 is a schematic diagram of a prior art magamp post regulator using set control instead of reset control; 
     FIG. 4 is a graph illustrating a hysteresis characteristic of the core member of the magamp of the circuit of FIG. 3; 
     FIG. 5 is a schematic diagram of a prior art magamp post regulator circuit having an extra control winding and using set control with the control transistor in linear mode; 
     FIG. 6 is a set of voltage timing curves for the circuit of FIG. 5, 
     FIG. 7 is a set of measured voltage curve traces for the magamp set control circuit of FIG.  5 . 
     FIG. 8 is a graph illustrating the stored energy and energy loss in the core; 
     FIG. 9 is a set of curves illustrating the power dissipation drawback of the prior art; 
     FIG. 10 is a schematic diagram of an alternate embodiment of a magamp post regulator circuit of the present invention; 
     FIGS. 11-12 are a set of curves illustrating the operation of the circuit of FIG. 10; 
     FIG. 13 is a schematic diagram of an exemplary embodiment of a magamp post regulator circuit of the present invention using a comparator; 
     FIG. 14 is a set of curves illustrating the operation of the circuit of FIG. 13; 
     FIG. 15 is a schematic diagram of the preferred embodiment of the magamp post regulator circuit of FIG. 13 with constant voltage control; 
     FIG.  16 (A) is a set of voltage curves illustrating the operation of the circuit of FIG. 15; 
     FIG.  16 (B) is a set of voltage curves illustrating regulation timing and voltage differences between the set mode linear and the set mode switching operation; 
     FIG. 17 is a schematic diagram of an error amp circuit for constant current control that could be used instead of, or in addition to, the error amp constant voltage control circuit in the embodiment in FIG. 15; 
     FIG. 18 is a schematic diagram of an alternative embodiment of FIG. 15 with an error amp circuit that provides both constant voltage and constant current control. 
     FIG. 19 is a schematic diagram of an alternate embodiment of a magamp post regulator circuit of the present invention implementing full control. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The switched magamp post regulator circuits according to the embodiments of the present invention allow the use of lower cost ferrite cores, at higher frequencies, while also minimizing the power loss associated with the control transistor element. 
     The switched magamp post regulator of the present invention will now be described with reference to FIGS. 10-19. FIG. 10 shows one embodiment of a magamp post regulator circuit  100 . This embodiment comprises a magamp  101 , a control transistor  110 , a control circuit  140  and an output circuit  150 . The control transistor  110  is operated as an on/off switch to control the set mode; and is preferably a MOSFET. The magamp includes a main magamp winding  106 , with a magamp control winding  105  and an additional magamp winding  107  inductively coupled to the main magamp winding  106 , preferably also provided. During the positive pulse of the transformer  104  winding, diode  102  blocks the conductive MOSFET control transistor  110  from clamping the magamp secondary control winding  105 . 
     In order to get the desired B-H excursion curve of the magamp  101  core, the magamp control winding  105  is allowed to “fly” then subsequently gets shorted out every cycle, during the off-time of the transformer  104  primary. Power switch  124  connects in series with the transformer  104  and is coupled to an input power source (not shown). The power switch  124  alternately switches between an on period and an off period such than an ac voltage is generated across the secondary winding of transformer  104  in response. The present invention provides a control circuit  140  to accomplish the switched set mode. The control circuit  140  provides an error amp  108  and a ramp generator and drive circuitry to generate and drive a control signal to turn the control transistor  110  on and off. An error amp  108  produces an amplified error signal when a “voltage sense” from the output varies from a reference voltage (error amp input details not shown but well known in the art). The voltage sense is obtained from a node  127  tap from a voltage divider (formed by series resistors  126  and  128  across the output, and well known in the art) at the output in the output circuit  150 . This error signal feeds through resistor  122  coupled to the gate of control transistor  110  at node  119 . Ramp generator and drive circuitry is provided to present a ramped voltage signal at node  119 . The error amp controls the amplitude of this voltage. The time when the MOSFET will turn on depends on the slope of the ramped voltage signal. Timing resistors  122  and  113  and capacitors  115  and  117  of the ramp generator and drive circuitry determine the slope of the ramped voltage signal. The ramp generator and drive circuitry further includes a transistor  111  connected to node  119 , with a resistor  109  coupling the base of transistor  111  to the transformer  104  secondary. 
     The output circuit provided includes a forward rectifier horizontal diode  112  coupled to an LC output filter formed by an inductor  114  and a bulk capacitor  116 . The LC output filter provides a substantially constant dc component flowing to the output with the ac component of the inductor  114  current flowing through the bulk capacitor  116 ; which has the output voltage V out  across it. During the off state, the inductor  114  current flows through a side path provided by vertical rectifier diode  120  that prevents the forward rectifier horizontal diode  112  from becoming reverse biased during the off state. 
     In the conventional set mode circuit, the control transistor is operated in linear mode and not switched as for the present invention. One advantage of the present invention is overcoming the power loss and increased parasitic stored energy in the control transistor of the conventional set mode circuits. This power dissipation of the control transistor is reduced in the present invention, since when the MOSFET switch is off, the energy from the magamp is returned to the load; and when the magamp turns on later in the cycle, current will circulate in the control windings. 
     FIGS. 11 and 12 are sets of voltage curves illustrating the operation of the circuit of FIG.  10 . In FIG. 11, as shown, the curve A represents the voltage V sec  from transformer  104  secondary winding and curves B and C represents the gate voltage and the drain to source voltage, respectively, for MOSFET control transistor  110 ; with the output set to 3 volts. In FIG. 12, curve D represents V sec , curve E is the drain current and curve F is the drain to source voltage, with the output set to 3.3 volts. From these curves, it can be seen that when the secondary voltage turns negative, the MOSFET control transistor  110  is off and turns on only when the voltage at the gate reaches its threshold level. MOSFET control transistor  110 , is fully on during the remaining period of the cycle. An advantage of this embodiment compared to the conventional set mode circuit shown in FIG. 5, is the intersection of the drain to source voltage and the drain current was greatly reduced. 
     FIG. 13 shows the exemplary embodiment of the present invention. This embodiment shows magamp post regulator set mode circuitry for regulating the output voltage of a power converter. The magamp post regulator circuit  200  uses a Pulse Width Modulation (PWM) concept for controlling switching of the control transistor. The magamp post regulator circuit  200  comprises a magamp  201 , control transistor  210 , diode  202 , control circuit  240  and output circuit  260 . The magamp  201  preferably has a secondary magamp control winding  205  inductively coupled to the main magamp winding  206 . The control circuit  240  includes a ramp generator circuit  215 , an error amp  208 , a comparator  221 , and a drive circuit  250 . 
     For this embodiment, the ramp generator circuit  230  is comprised of resistors  209  and  219 , capacitor  217  and transistor  211 . The ramp generator circuit  215  is controlled to produce a ramped voltage signal during the off time of the transformer  204 . Power switch  203  connects in series with the transformer  204  and is coupled to an input power source (not shown). The power switch  203  alternately switches between an on period and an off period such than an ac voltage is generated across the secondary winding of transformer  204  in response. An error amp  208  produces an amplified error signal  207  when the voltage sense (V sense ) tapped at node  227  from a voltage divider, formed by series resistors  226  and  228  across the output in output circuit  260 , varies from a reference voltage (details not shown but well known in the art). A comparator  221  compares the ramped voltage signal with the error signal  207  from the error amp  208 . The comparator  221  provides a signal whenever the error signal  207  is less than the magnitude of the ramped voltage signal from the ramp generator circuit  215 . The control transistor  210  is preferably a MOSFET. The signal from comparator  221  feeds a drive circuit  250 , formed by resistor  213 , transistor  223  and diode  222 , which drives the gate of the MOSFET control transistor  210 , switching the MOSFET control transistor  210  on to the conducting state. During the positive pulse of the transformer  204  winding, however, diode  202  blocks the conductive control transistor  210  from clamping the magamp secondary control winding  205 . 
     This embodiment has the advantage of further reducing the power dissipation (and the device temperature) for the control transistor  210 . The magamp  201  stored parasitic energy is returned instead to a bulk capacitor  216  until the desired volt-seconds part of the duty cycle is reached. At that point, the conductive control transistor  210  clamps the magamp secondary control winding  205  to set the magamp  201  core and keep the core at the desired point in the B-H loop. 
     FIG. 14 is a set of curves illustrating the operation of the circuit of FIG.  13 . Curve G represents the voltage V sec  from the transformer  204  secondary winding. Curves H and I represents the drain current and drain to source voltage, respectively, for the MOSFET control transistor  210 . From these curves, it can be seen that at the minimum blocking state, when the secondary voltage V sec  turns negative, the MOSFET control transistor  210  is off(no drain current in curve H). The drain current pulses in curve H occur during the interval, described above, when the ramped voltage signal rises above the error signal  207  threshold causing the comparator  221  to provides a signal that turns MOSFET control transistor  210  on to the conductive state. 
     FIG. 15 shows a schematic diagram of the preferred embodiment of the magamp post regulator circuit in FIG.  13 . As can be seen from the figure, the magamp post regulator circuit  400  in FIG. 15 shows additional circuit details and an optional different drive circuit (drive ckt), a different error amp circuit (for constant voltage) and a different ramp generator circuit (using a zener diode) than that shown in FIG.  13 . FIG.  16 (A) shows voltage and timing curves illustrating the operation of the circuit of FIG.  15 . FIG.  16 (B) is a set of voltage and timing curves to show the difference in regulation voltage and timing for the set mode using switching versus the linear operation. 
     FIG. 17 shows an alternative error amp circuit  510  “for constant current” control using an output current sense. FIG. 18 shows a magamp post regulator circuit  600  that is an alternative embodiment of FIG. 15 using an error amp circuit that provides both constant voltage control and constant current control circuitry. Alternatively the constant current control circuit as shown in FIG. 17 could be provided without the constant voltage control circuit. 
     FIG. 19 shows an alternate embodiment of the switched magamp post regulator of the present invention that implements a “full control” over the range of the hysteresis loop in regulating the output voltage of a power converter. In addition to the advantage of improved efficiency and substantial reduction in the power loss in the control transistor, magamp core size is reduced along with a reduction in the required number of power turns. This embodiment also allows efficient use of any kind of loop material and allows the use of lower cost ferrites at lower frequencies. This alternate embodiment of the switched magamp post regulator uses both set mode and reset mode control, depending on the operating conditions of the converter. The set mode part of the circuit uses the inventive switching aspect; while for the reset mode part, the control transistor is operated in the conventional non-switching linear because switching yields no advantage for the reset mode. 
     The switched magamp post regulator circuit  300  of FIG. 19 is comprised of a magamp  301 , a diode  302 , a set mode control circuit  240 , a reset mode control circuit  390 , a mode arbitrator circuit  380  and an output circuit  260 . The set mode control circuit  240  is as described for the set mode embodiment in FIG.  13 . The reset control circuit  390  and mode arbitrator circuit  380 , however, are unique to the “full control” embodiment of FIG. 19, and thus, will be described in more detail. 
     The reset control circuit  390  operates a reset mode control transistor  335  in a conventional linear (non-switched) mode. This circuit controls the amount of current through magamp  301  when the magamp  301  core is driven beyond remanence. Since the magamp  301  core can be driven beyond remanence, a higher flux is achieved. Applying the equation for blocking time that is found on FIG. 4,            t   block     =       Δ                   B   ·   turns   ·     A   core         V       ,                          
     indicates that even with a smaller number of turns and a smaller core area, A core , the necessary blocking time of the magamp  301  can still be achieved since the change in flux density, ΔB, can be made larger. 
     For this full control operation of this embodiment, however, the set and reset modes are never applied at the same time. Thus, set mode control transistor  310 , preferably a MOSFET, and reset mode control transistor  335  are never on simultaneously in this embodiment. The mode arbitrator circuit  380  which provides this control of the two modes includes a transistor  340 , coupled to the base of reset mode transistor  335  through a resistor  343 , additional resistors  341 ,  344  and  345 ; and a zener diode  342 , coupled to an connection between the error amp  208  and comparator  221  of the control circuit  240 . 
     The foregoing detailed description of the invention has been provided for the purposes of illustration and description. Although exemplary embodiments of the present invention have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to the precise embodiments disclosed, and that various changes and modifications to the present invention are possible in light of the above teaching. Accordingly, the scope of the present invention is to be defined by the claims appended hereto.