Regulated voltage reducing high-voltage isolated DC/DC converter system

[0002] This application claims the benefit of U.S. Provisional Patent Application No. 60/275,201 filed on Mar. 12, 2001.

[0003] 1. Field of the Invention

[0004] The present invention relates generally to DC-DC electrical power oonverter circuits, and more specifically to DC-DC power converter circuits adapted for converting electrical energy received from a high-voltage, poorly regulated electrical power source to a well regulated lower voltage.

[0005] 2. Description of the Prior Art

[0006] U.S. Pat. No. 5,999,417 entitled “High Efficiency Power Converter,” that issued Dec. 7, 1999, on a patent application filed by Martin F. Schlecht (“the '417 patent”), describes a DC-DC converter circuit adapted for converting electrical power received from a 48 volt direct current (“VDC”) power source to a 5 VDC output voltage for energizing the operation of computer digital logic circuits. As illustrated in FIG. 1 of the '417 patent, the DC-DC converter disclosed there includes a regulation stage, an isolation stage, and control circuit that is coupled both to the regulation stage and to the isolation stage.

[0007] FIG. 2 of the '417 patent depicts as the regulation stage a conventional voltage step-down converter circuit, a/k/a/ a buck converter circuit, which receives the 48 VDC battery voltage. Within the buck converter circuit, the 48 VDC battery voltage is applied across a capacitor, C_IN, and from a first terminal of the 48 VDC battery to a switching transistor, Q_R. When the switching transistor, Q_R, turns-on, electrical current flows from the 48 VDC battery source through the switching transistor, Q_R, and through a series connected inductor L into the isolation stage of the DC-DC converter. During normal operation of the buck converter circuit when the switching transistor, Q_R, turns-off to block current from flowing from the 48 VDC battery source through the inductor L, electrical current continues to flow through the inductor L via a free-wheeling diode, D_R, that connects between a second terminal of the 48 VDC battery source and a junction between the inductor L and the switching transistor, Q_R.

[0008] A version of the isolation stage of the DC-DC converter, illustrated in FIG. 2 of the '417 patent, includes two, separate transformers T1 and T2. Each of the transformers T1 and T2 includes three windings: a primary winding T1_PRIand T2_PRI; a secondary winding T1_SECand T2_SEC; and a tertiary winding T1_TERand T2_TER. The primary windings T1_PRIand T2_PRIof the transformers T1 and T2 are coupled to the inductor L of the regulator stage to receive electrical current therefrom, and are coupled respectively through MOSFETs Q1 and Q2 to the second terminal of the 48 VDC battery source. Connected in this way, while either of the transistors Q1 or Q2 are turned-on, the primary windings T1_PRIand T2_PRIof the transformers T1 and T2 are “current fed” from the inductor L of the regulation stage. By this it is meant that the electrical current flowing into the primary windings T1_PRIand T2_PRIof the isolation stage transformers T1 and T2 is held relatively constant throughout a switching cycle of the DC-DC converter. It also means that voltage across the primary windings T1_PRIand T2_PRIof the isolation stage transformers T1 and T2 is free to have large, high frequency components.

[0009] During normal operation of the DC-DC converter, approximately one half of the switching cycle, transistor Q1 is turned-on and transistor Q2 is turned-off. While the transistor Q1 is turned-on, electrical current flows through the series connected inductor L and primary winding T1_PRIof transformer T1. During a second half of the switching cycle, transistor Q2 is turned-on, and transistor Q1 is turned-off. While the transistor Q2 is turned-on, electrical current flows through the inductor L and through the primary winding T2_PRIof the transformer T2 in the same manner as described above for transformer T1,

[0010] While the transistor Q1 is turned-on, a positive voltage is imposed across the primary winding T1_PRI, and a magnetizing current flowing through the primary winding T1_PRIincreases. The voltage applied across the primary winding T1_PRIand the current flowing therethrough induce a corresponding flow of electrical current (transformed by the turns ratio between the primary winding T1_PRIand the secondary winding T1_SEC) through the secondary winding T1_SECof the transformer T1, and through a diode D1 connected in series with the secondary winding T1_SECboth to an output filter capacitor C_OUTand to a load that is coupled to the isolation stage. When the transistor Q1 turns-off thereby blocking an electrical current from flowing through the primary winding T1_PRI, the voltages across the windings T1_PRI, T1_SECand T1_TERreverse thereby causing electrical current to flow through the tertiary winding T1_TERof the transformer T1 and a diode D3 connected in series with the tertiary winding T1_TERto the output filter capacitor C_OUTand the load. Electrical current flowing through the tertiary winding T1_TERof the transformer T1 provides a means to reset the core of the transformer T1, and to recover most of the magnetizing inductance energy stored in the core while the transistor Q1 is turned-on. Since as described above the transistors Q1 and Q2 operate out of phase, the transformer T2 operates similar to but out of phase with the transformer T1 for supplying electrical currents respectively through the secondary winding T2_SECand a diode D2, and the tertiary winding T2^TERand a diode D4 to the output filter capacitor C_OUTand the load.

[0011] The control circuit illustrated in FIG. 1 of the '417 patent provides drive signals to control terminals of the transistors Q_R, Q1 and Q2 illustrated in FIG. 2. The '417 patent explains that the separate regulation stage, which in the illustration of FIG. 1 is on the primary side of the converter's isolation stage, regulates operation of the DC-DC converter. In this particular configuration, regulation is effected by controlling the duty cycle of the transistor Q_Rin response to one or more parameters sensed in the control circuit, which may be sensed on the primary side of the converter's isolation stage.

[0012] A significant fraction of the energy dissipated in a DC-DC converter such as that depicted in FIG. 2 of the '417 patent occurs in the diodes D1, D2, D3 and D4, particularly if the load and/or source voltages are low, e.g. 3.3, 5, or 12 volts. To reduce this rectification conduction power loss, the diodes D1, D2, D3 and D4 may be replaced with transistors which have an on-state voltage that is much less than the conduction voltage drop of the diodes D1, D2, D3 and D4. Transistors used in this way are frequently called synchronous rectifiers, and are typically power MOSFETs for DC-DC converters switching in the 100 kHz and higher range.

[0013] FIGS. 3, 5, 6A, 6B and 7-9 of the '417 patent illustrates an isolation stage for the DC-DC converter in which a pair of N-channel MOSFET synchronous rectifiers Q3 and Q4 replace the diodes D1, D2, D3 and D4. The positions of these synchronous rectifiers Q3 and Q4 in the circuit differs slightly from the positions of the diodes D1, D2, D3 and D4 in FIG. 2. The synchronous rectifiers Q3 and Q4 still connect in series with the respective secondary winding T1_SECand T2_SEC, but drains of the N-channel MOSFET synchronous rectifiers Q3 and Q4 connect to the negative output terminal of the respective secondary windings T1_SECand T2_SECrather than to the positive output terminal. The synchronous rectifiers Q3 and Q4 connect in this way to the respective secondary winding T1_SECand T2_SECso source terminals of both N-channel MOSFET synchronous rectifiers Q3 and Q4 connect to a single, common DC node, i.e. circuit ground.

[0014] If instead of N-channel MOSFETS, P-channel MOSFETs were used for the synchronous rectifiers Q3 and Q4, their respective drain terminals would connect to the positive output terminals of the respective secondary winding T1_SECand T2_SECas shown in the partial schematic of FIG. 4 in the '417 patent. The configuration for the P-channel MOSFETS synchronous rectifiers Q3 and Q4 shown in FIG. 4 permit connecting the source terminals of the synchronous rectifiers Q3 and Q4 to a single, common DC node.

[0015] As shown in FIGS. 3, 4, 5, 6A, 6B and 7-9, the gates of the MOSFET synchronous rectifiers Q3 and Q4, which drains are connected respectively to the secondary winding T1_SECand T2_SEC, are cross-coupled to the secondary winding T2_SECand T1_SECof the opposite transformers T2 and T1. Coupled in this way, the voltage across one transformer determines the gate voltage for the opposite MOSFET synchronous rectifier, and therefore the conduction state (on or off) of the MOSFET synchronous rectifier connected to the other transformer. This configuration for the MOSFET synchronous rectifiers inherently applies properly timed driving signals to the gates of the MOSFET synchronous rectifiers without requiring any special control circuitry on the secondary side of the transformers T1 and T2.

[0016] Frequently, operation of telecommunication systems is energized by relatively high-voltage battery power supplies, e.g. 48 VDC that at times may exhibit a poorly regulated output voltage. During re-charging of these high-voltage batteries, the voltage of this power source may increase to 75 VDC for extended intervals of time, with intermittent voltage spikes reaching 100 VDC. However, the equipment energized by a DC-DC converter such as that disclosed in the '417 patent must operate continuously and reliably while the high-voltage batteries are being recharged. Thus, there exists a need for a cost-effective DC-DC converter, capable of being energized by electrical power drawn from a poorly regulated power supply, that is also capable of supplying well-regulated electrical power to equipment at a much lower voltage, e.g. 1.0-3.0 VDC, at relatively high currents, e.g. up to 60 amperes (“AMPs”).

[0017] While use of high-voltage integrated circuit technology permits building a DC-DC converter having characteristics such as those outlined above, such an approach possesses several disadvantages. First, building high-voltage integrated circuits requires specialized integrated circuit manufacturing technology. A significant disadvantage of high-voltage integrated circuits made using such specialized manufacturing technology is that the integrated circuits switch slowly which increases power loss within the DC-DC converter. Moreover, high-voltage integrated circuits occupy a larger area of silicon than low-voltage integrated circuits which further increases the integrated circuits' cost.

[0018] An object of the present invention is to provide DC-DC converter that can be energized by a poorly regulated power supply and that can supply well-regulated electrical power.

[0019] Another object of the present invention is to provide DC-DC converter that can be energized by a poorly regulated, comparatively high-voltage power supply and that can supply well-regulated electrical power at a much lower voltage, and at a high current.

[0020] Another object of the present invention is to provide a cost-effective DC-DC converter that can be energized by a poorly regulated power supply and that can supply well-regulated electrical power.

[0021] Another object of the present invention is to provide DC-DC converter using only low-voltage integrated circuit technology that can be energized by a poorly regulated, comparatively high-voltage power supply and that can supply well-regulated electrical power at a much lower voltage, and at a high current.

[0022] Briefly, the present invention is a DC-DC converter adapted for converting direct current (“DC”) electrical power received from first and second output terminals of a high-voltage DC power supply. The DC-DC converter is preferably adapted for supplies DC electrical power to a load at a well-regulated output voltage that is significantly lower than the voltage which the DC-DC converter receives from the high-voltage DC power supply.

[0023] The DC-DC converter includes a regulated voltage-reduction stage which receives DC electrical power from the output terminals of the high-voltage DC power supply, and supplies DC electrical power from an output at a voltage which is lower than that received from the high-voltage DC power supply. The voltage-reduction stage includes a voltage-reduction electronic switch for alternatively:

[0024] 1. electrically coupling the first output terminal of the high-voltage DC power supply to the output of the voltage-reduction stage; and

[0025] 2. electrically de-coupling the first output terminal of the high-voltage DC power supply from the output of the voltage-reduction stage.

[0026] The voltage-reduction stage also includes a low-voltage, voltage-reduction integrated circuit (“IC”) that is energized by DC electrical power received from the output terminals of the high-voltage DC power supply. The voltage-reduction IC supplying an electrical signal to the voltage-reduction electronic switch which controls alternative electrical coupling and de-coupling effected by the voltage-reduction electronic switch. The voltage-reduction stage also includes a voltage-reduction current source that is coupled to the voltage-reduction IC to effect a controlled flow of electrical current between the output terminals of the high-voltage DC power supply through the voltage-reduction IC and the voltage-reduction current source.

[0027] The DC-DC converter also includes a separately regulated isolation stage adapted for supplying DC electrical power to the load at the output voltage that is significantly lower than the voltage which the DC-DC converter receives from the high-voltage DC power supply. The regulated isolation stage includes an isolation transformer having a primary winding that receives DC electrical power from the output of the voltage-reduction stage. The isolation transformer also has a secondary winding that is magnetically coupled to the primary winding. The regulated isolation stage includes also includes at least one transformer electronic switch connected to the primary winding of the isolation transformer. The transformer electronic switch alternatively:

[0028] 1. permits electrical current to flow between the output of the voltage-reduction stage through the primary winding of the isolation transformer and the transformer electronic switch to the second output terminal of the high-voltage DC power supply; and

[0029] 2. blocks the flow of electrical current between the output of the voltage-reduction stage through the primary winding of the isolation transformer and the transformer electronic switch to the second output terminal of the high-voltage DC power supply.

[0030] This operation of the transformer electronic switch induces an alternating current (“AC”) in the secondary winding of the isolation transformer.

[0031] The regulated isolation stage also includes a rectifier circuit coupled to the secondary winding of the isolation transformer. The rectifier circuit rectifies the AC received from the secondary winding to produce therefrom DC electrical power which the DC-DC converter is adapted for supplying to the load. An output-voltage sensor, included in the regulated isolation stage, produces an output signal which is responsive to the output voltage supplied to the load.

[0032] A low-voltage, isolation-stage IC, included in the regulated isolation stage, is energized by DC electrical power received from the output terminals of the high-voltage DC power supply. The isolation-stage IC receives the output signal produced by the output-voltage sensor and supplies an electrical signal to the transformer electronic switch for controlling the alternative electrical coupling and de-coupling effected by the transformer electronic switch. The isolation-stage IC produces this electrical signal responsive to the output signal received from the output-voltage sensor. An isolation-stage current source included in the regulated isolation stage, is coupled to the isolation-stage IC to effect a controlled flow of electrical current between the output terminals of the high-voltage DC power supply through the voltage-reduction IC and the voltage-reduction current source.

[0033] A reduction-stage feedback circuit couples an output signal produced by the isolation-stage IC to the voltage-reduction IC to control the electrical signal which the voltage-reduction IC supplies to the voltage-reduction electronic switch.

[0034] These and other features, objects and advantages will be understood or apparent to those of ordinary skill in the art from the following detailed description of the preferred embodiment as illustrated in the various drawing figures.

[0035] FIG. 1 is a block diagram depicting a DC-DC converter in accordance with the present invention.

[0036] FIG. 1 illustrates a DC-DC converter in accordance with the present invention identified by the general reference character 10. The DC-DC converter 10 is adapted for:

[0037] 1. receiving DC electrical power from a first output terminal 12 and a second output terminal 14 of a high-voltage DC battery power supply 16; and

[0038] 2. supplying DC electrical power to a load 18 at a well-regulated output voltage, e.g. 1.0-3.0 VDC, that is significantly lower than a supply voltage, e.g. 30 to 75 VDC, at which the DC-DC converter 10 receives electrical power from the battery power supply 16.

[0039] The DC-DC converter 10 includes a regulated voltage-reduction stage 22, enclosed within a dashed line in FIG. 1, that receives DC electrical power from the output terminals 12, 14 of the battery power supply 16. The voltage-reduction stage 22 includes a P-type MOSFET voltage-reduction electronic switch 24 having a source terminal 24swhich connects directly to the first output terminal 12 of the battery power supply 16. A drain terminal 24dof the electronic switch 24 connects to a junction between a cathode of a free-wheeling Schottky diode 26 and a first terminal of an inductor 28. An anode of the diode 26 connects to a circuit ground 32 for the voltage-reduction stage 22, which is the second output terminal 14 of the battery power supply 16. A second terminal of the inductor 28, which provides an output 34 for the voltage-reduction stage 22, connects to first terminals respectively of a filter capacitor 36 and a pair of series-connected output-voltage sensing resistors 38aand 38b.A second terminal of the capacitor 36 and a second terminal of the series-connected sensing resistors 38aand 38bboth connect to circuit ground 32. Configured in this way, the electronic switch 24, inductor 28 and capacitor 36 constitute a conventional voltage reduction buck converter circuit similar to that illustrated in FIG. 2 of the '419 patent.

[0040] The voltage-reduction stage 22 also includes a low-voltage, voltage-reduction IC 42 that is energized by DC electrical power received from the battery power supply 16. Accordingly, a first power terminal of the voltage-reduction IC 42 connects directly to the first output terminal 12 of the battery power supply 16, while a second power terminal of the voltage-reduction IC 42 is coupled through a current source 44 to the second output terminal 14 of the battery power supply 16. The current source 44 effects a controlled flow of electrical current between the output terminals 12, 14 of the battery power supply 16 through the voltage-reduction IC 42. In this way, the current source 44 establishes a voltage across the low-voltage voltage-reduction IC 42 that is compatible with electrical characteristics thereof. An output terminal of the voltage-reduction IC 42 connects to a gate terminal 24gof the electronic switch 24 for supplying a signal that causes the electronic switch 24 to alternatively:

[0041] 1. electrically couple the first output terminal 12 of the battery power supply 16 to the output 34 of the voltage-reduction stage 22; and

[0042] 2. electrically de-couple the first output terminal 12 of the battery power supply 16 from the output 34 of the voltage-reduction stage 22.

[0043] The DC-DC converter 10 also includes a regulated isolation stage 52, enclosed within a dashed line in FIG. 1, adapted for supplying DC electrical power from an output 54 thereof to the load 18 at the output voltage that is significantly lower than the voltage which the DC-DC converter receives from the battery power supply 16. The isolation stage 52 includes an isolation transformer 56 that preferably includes a center-tapped primary winding 56p.The primary winding 56preceives DC electrical power from the output 34 of the voltage-reduction stage 22. The isolation transformer 56 also includes a secondary winding 56sthat is coupled magnetically by the isolation transformer 56 to the primary winding 56p.

[0044] The isolation stage 52 also preferably includes a pair of N-type MOSFET electronic switches 62aand 62b.Each of the electronic switches 62aand 62bincludes a drain terminal 62adand 62bdthat connect respectively to one side of the center-tapped primary winding 56pof the isolation transformer 56. Each of the electronic switches 62aand 62balso includes a source terminal 62asand 62bsthat connect in parallel to the circuit ground 32, i.e. to the second output terminal 14 of the battery power supply 16.

[0045] The voltage-reduction stage 22 also includes a low-voltage, isolation-stage IC 66 that is energized by DC electrical power received from the battery power supply 16. Accordingly, a first power terminal of the isolation-stage IC 66 connects directly to the second output terminal 14 of the battery power supply 16, while a second power terminal of the isolation-stage IC 66 is coupled through a current source 68 to the first output terminal 12 of the battery power supply 16. The current source 68 effects a controlled flow of electrical current between the output terminals 12, 14 of the battery power supply 16 through the current source 68. In this way the isolation-stage IC 66 establishes a voltage across the low-voltage isolation-stage IC 66 that is compatible with electrical characteristics thereof. Output terminals of the isolation-stage IC 66 connect respectively to gate terminals 62agand 62bgof the isolation-stage IC 66. The isolation-stage IC 66 supplies signals to the gate terminals 62agand 62bgthat cause the electronic switches 62aand 62bto alternatively:

[0046] 1. electrically couple the second output terminal 14 of the battery power supply 16 to the output 34 of the voltage-reduction stage 22; and

[0047] 2. electrically de-couple the second output terminal 14 of the battery power supply 16 from the output 34 of the voltage-reduction stage 22.

[0048] The signals which the isolation-stage IC 66 supplies to the gate terminals 62agand 62bgcause the electronic coupling and decoupling of the electronic switches 62aand 62bto occur out of phase. In this way, at any instant in time signals from the isolation-stage IC 66 alternatively:

[0049] 1. permit a flow of electrical current between the output 34 of the voltage-reduction stage 22 through one-half of the primary winding 56pof the isolation transformer 56 and one or the other of the electronic switches 62aor 62bto the second output terminal 14 of the battery power supply 16; and

[0050] 2. block the flow of electrical current between the output 34 of the voltage-reduction stage 22 and the second output terminal 14 of the battery power supply 16 through the other half of the primary winding 56pof the isolation transformer 56 and one or the other of the electronic switches 62bor 62a.Arranged as described above, the DC-DC converter 10 receives DC electrical power from the output terminals 12, 14 of the battery power supply 16 through the source terminal 24sof the electronic switch 24 and the source terminal 62asand 62bsof the electronic switches 62aand 62b,and out of phase switching of the electronic switches 62aand 62bthereby inducing AC in the secondary winding 56sof the isolation transformer 56.

[0051] The voltage-reduction stage 22 also includes a rectifier circuit 72, enclosed within a dashed line in FIG. 1, that is coupled to the secondary winding 56sof the isolation transformer 56. Similar to the MOSFET synchronous rectifiers depicted in FIG. 6A and 6B of the '419 patent, the rectifier circuit 72 includes a pair of N-type MOSFETs synchronous rectifiers 74aand 74b.A gate terminal 74agand 74bgof each of the synchronous rectifiers 74aand 74bconnect respectively to opposite sides of the secondary winding 56sof the isolation transformer 56. Opposite sides of the secondary winding 56sconnect with cross-coupled drain terminals 74bdand 74adof the synchronous rectifiers 74aand 74b.An electrical ground 76 for the isolation stage 52, which is electrically insulated from the circuit ground 32 of the voltage-reduction stage 22 and the battery power supply 16, connects to both source terminals 74asand 74bsof the synchronous rectifiers 74aand 74b.A pair of Schottky diodes 78aand 78bconnect respectively in parallel with the source terminals 74asand 74bsand the drain terminals 74bdand 74adof the synchronous rectifiers 74aand 74b.

[0052] In addition to the rectifier circuit 72, first terminals of pair of inductors 82aand 82bconnect respectively to opposite sides of the secondary winding 56s,and second terminals thereof connect to the output 54 of the DC-DC converter 10. A filter capacitor 84 connects between the output 54 and the electrical ground 76 of the isolation stage 52. Configured in this way, the rectifier circuit 72 rectifies AC electrical power received from the secondary winding 56sto produce therefrom DC electrical power which the inductors 82aand 82band the filter capacitor 84 filter before being supplied by the DC-DC converter 10 to the load 18.

[0053] The voltage-reduction stage 22 also has an output voltage sensor that includes a pair of voltage divider resistors 92aand 92bconnected in series between the output 54 and the electrical ground 76 of the isolation stage 52. An inverting input of a sense amplifier 94 connects to a junction between the resistors 92aand 92b.A reference voltage, V Ref, is applied to a non-inverting input of the sense amplifier 94. An output of the sense amplifier 94 connects through a photo-diode of an opto-coupler 96 to electrical ground 76 of the isolation stage 52. An output of the opto-coupler 96 connects to circuit ground 32 of the voltage-reduction stage 22 and the battery power supply 16, and to an input of the isolation-stage IC 66. Connected in this way the opto-coupler 96 supplies an output signal to the isolation-stage IC 66 which responds to the voltage of DC electrical power which the DC-DC converter 10 supplies to the load 18. A junction between the resistors 92aand 92bin the voltage-reduction stage 22 also connects to an input of the isolation-stage IC 66. In this way, the isolation-stage IC 66 also receives directly a signal which is proportional to the voltage present at the output 34 of the voltage-reduction stage 22.

[0054] The DC-DC converter 10 also includes a N-type MOSFET 102 which feeds a control signal from the isolation-stage IC 66 back to the voltage-reduction IC 42 for controlling the operation of the electronic switch 24. The source terminal 102sof the MOSFET 102 in this voltage-reduction-stage feedback circuit connects to circuit ground 32 of the voltage-reduction stage 22 and the battery power supply 16, while the gate terminal 102gconnects to the isolation-stage IC 66. A divider formed by resistors 104aand 104bcouples a drain terminal 102dof the MOSFET 102 to the voltage-reduction IC 42 to provide the feedback signal thereto.

[0055] When the voltage-reduction stage initially receives DC electrical power from the output terminals 12, 14 of the battery power supply 16, the voltage-reduction IC 42 generates a pulse width modulated (“PWM”) electrical signal in a free wheeling mode, i.e. not fixed frequency. The voltage-reduction IC 42 supplies this PWM signal to the electronic switch 24 to effect alternative electrical coupling and de-coupling by the electronic switch 24 at a first frequency, e.g. 1.0 MHz. After a short interval of time controlled by a so called “soft start” function, the voltage at the output 34 of the electronic switch 24 becomes substantially equal to a pre-established value, e.g. 24 VDC. Upon reaching this pre-established value, the isolation-stage IC 66 and the MOSFET 102, responsive to the voltage at the output 34 of the electronic switch 24, cause the voltage-reduction IC 42 to alter the frequency of the PWM signal supplied to the gate terminal 24gof the electronic switch 24 to a different and fixed frequency. This second frequency, e.g. 300 KHz, differs markedly from the first frequency. The characteristics of the PWM signal supplied at the second frequency can vary slightly to maintain the voltage at the output 34 at the pre-established value.

[0056] The isolation-stage IC 66 also supplies PWM signals to the gate terminals 62agand 62bgof both electronic switches 62aand 62b.Responsive to the signal received from the opto-coupler 96, the characteristics of the PWM signals supplied to the gate terminals 62agand 62bgcan also vary slightly to maintain a substantially constant voltage across the load 18.

[0057] Although the present invention has been described in terms of the presently preferred embodiment, it is to be understood that such disclosure is purely illustrative and is not to be interpreted as limiting. For example, the isolation transformer 56 also preferably includes a center-tapped auxiliary secondary winding 56athat provides an un-regulated, low-power source for Vcc that energizes operation the sense amplifier 94. This un-regulated, low-power source also includes a pair of diodes 112aand 112bhaving anodes that connect respectively to opposite ends of the center-tapped auxiliary secondary winding 56a.A junction formed by cathodes of the diodes 112aand 112bconnects to a filter capacitor 114 and to the Vcc terminal of the sense amplifier 94.

[0058] Consequently, without departing from the spirit and scope of the invention, various alterations, modifications, and/or alternative applications of the invention will, no doubt, be suggested to those skilled in the art after having read the preceding disclosure. Accordingly, it is intended that the following claims be interpreted as encompassing all alterations, modifications, or alternative applications as fall within the true spirit and scope of the invention.