Abstract:
The disclosed embodiments relate to a power-supply circuit, an electronic device that includes the power-supply circuit, and a method for generating high-voltage DC power from AC line power using the power-supply circuit. This power-supply circuit includes a voltage multiplier and a low dropout (LDO) regulator, and does not include a step-up transformer. Conventional power supplies often use a custom step-up transformer, which is expensive unless the power supplies are manufactured in high quantities. In contrast, one embodiment of the present disclosure provides a solid-state implementation of a 700 V regulated power supply that can take up to a 1020 V input from an 6× voltage multiplier powered from the AC mains. Hence, the disclosed power-supply circuit eliminates the need for large, heavy and expensive step-up transformers and chokes that are used in conventional high-voltage DC power supplies.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No. 61/436,891, entitled “High Voltage DC Power Supply Using Voltage Multiplier with Feedback Regulation,” by Eric Smith, P. Jeffrey Ungar and Heather Sullens, filed on Jan. 27, 2011, having attorney docket number APL-P10764USP1, the contents of which are herein incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    The disclosed embodiments generally relate to power supplies. More specifically, the disclosed embodiments relate to the design of a high-voltage DC power supply, which is powered by an AC line, and which operates without using a step-up transformer. 
         [0004]    2. Related Art 
         [0005]    High-voltage DC power supplies that provide moderate power output are commonly used in a number of applications, such as: lasers, photographic flash units, displays and audio tube amplifiers. These high-voltage power supplies typically step up the AC line voltage by using a step-up transformer as the first stage before rectification and filtering to generate a DC output. However, in order to operate effectively at the line frequency, the step-up transformer generally needs to be large and heavy. Furthermore, the step-up transformer is likely to be an expensive custom component, because such step-up transformers are not widely used. 
         [0006]    Hence, what is needed is a high-voltage DC power supply that does not require such a step-up transformer. 
       SUMMARY 
       [0007]    The disclosed embodiments relate to a method for generating high-voltage DC power from AC line power using a power-supply circuit. This power-supply circuit includes a voltage multiplier and a low dropout (LDO) regulator, and does not include a step-up transformer. Conventional power supplies often use a custom step-up transformer, which is expensive unless the power supplies are manufactured in high quantities. In contrast, one embodiment of the present disclosure provides a solid-state implementation of a 700 V regulated power supply that can take up to a 1020 V input from a six-times voltage multiplier powered from the AC mains. Hence, the disclosed power-supply circuit eliminates the need for large, heavy and expensive step-up transformers and chokes that are used in conventional high-voltage DC power supplies. 
         [0008]    Note that, designs for stand-alone voltage multipliers and LDO regulators are fairly well known, it can be quite challenging to regulate the output voltage of a voltage multiplier. Moreover, existing LDO regulator designs are not configured to operate at high voltages and power levels. 
         [0009]    The power-supply circuit includes several features that facilitate operation under these conditions, including diodes having opposite polarities that clamp a differential voltage across the gates of a differential amplifier in a first stage of the LDO regulator during turn on. Moreover, a level-shifting circuit with a set of series-coupled diodes in a second stage of the LDO regulator shifts an output voltage from the differential amplifier to a larger intermediate voltage, thereby setting a gain of a power transistor in a third stage of the LDO regulator without exceeding a breakdown voltage of a transistor in the second stage. The third stage also includes a divider network with passive components that specify the DC output voltage, and which is electrically coupled to the differential amplifier in the first stage, thereby providing negative feedback to the differential amplifier. Furthermore, the output impedance of the divider network allows the power transistor to provide an approximately constant output power into a load over a range of audible frequencies (such as up to 10-20 kHz). Note that the power-supply circuit may include an input transformer that electrically isolates an input of the voltage multiplier from the AC line. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0010]      FIG. 1  is a block diagram illustrating a voltage multiplier in a power-supply circuit in accordance with an embodiment of the present disclosure. 
           [0011]      FIG. 2  is a block diagram illustrating a low dropout (LDO) regulator in the power-supply circuit of  FIG. 1  in accordance with an embodiment of the present disclosure. 
           [0012]      FIG. 3  is a block diagram illustrating an electronic device that includes the power-supply circuit of  FIGS. 1 and 2  in accordance with an embodiment of the present disclosure. 
           [0013]      FIG. 4  is a flow diagram illustrating a method for providing a DC output voltage using the power-supply circuit of  FIGS. 1 and 2  in accordance with an embodiment of the present disclosure. 
       
    
    
       [0014]    Note that like reference numerals refer to corresponding parts throughout the drawings. Moreover, multiple instances of the same part are designated by a common prefix separated from an instance number by a dash. 
       DETAILED DESCRIPTION 
       [0015]    Instead of using a custom step-up transformer to increase the voltage, the power-supply circuit in the present disclosure feeds AC power through a three-stage, half-wave Cockcroft-Walton multiplier which bumps the line voltage up above 1 kV (for example, to 1020 V). This is shown in  FIG. 1 , which presents a block diagram illustrating a voltage multiplier  110  (in this example, a Cockcroft-Walton multiplier) in power-supply circuit  100 . 
         [0016]    The Cockcroft-Walton multiplier uses only capacitors and diodes to produce a rectified and filtered output voltage Vcc  114 , which is a multiple of the peak AC input voltage  112 . In particular, when the voltages on capacitors C 1 , C 3  and C 5  on the top of voltage multiplier  110  (which are charged by the AC line voltage) exceed the voltage thresholds of the diodes (which act as unidirectional switches), charge flows into (and is held by) C 2 , C 4  and C 6  on the bottom of voltage multiplier  110 . Thus, C 1 , C 3  and C 5  dump charge into C 2 , C 4  and C 6 , and C 2 , C 4  and C 6  provide a voltage offset to the next stage in voltage multiplier  110  (each stage in voltage multiplier  110  provides an additional two-times multiplication). As the AC line current alternates, C 2 , C 4  and C 6  lift or pump up the voltage in both directions (because p can go below n), thereby providing a rectified output voltage. Note that the high-voltage diodes and high-voltage capacitors in  FIG. 1  are readily available. 
         [0017]    One problem with using a Cockcroft-Walton multiplier at the AC line frequency is that under load the output of this voltage multiplier can dip substantially and has significant ripple (much more than a conventional step-up transformer with rectified and filtered output). Moreover, when used in conjunction with an AC line voltage, the ripple is at 60 Hz, which is audible. Additional filtering can be added to reduce the ripple under load to acceptable levels. However, because of the output impedance of voltage multiplier  110 , the output-voltage level will remain sensitive to the AC line voltage level and the load. 
         [0018]    What is needed is an additional stage of active regulation that incorporates a reference so that a stable, low-ripple DC output voltage can be maintained for changing AC line voltage and changing loads. To achieve this end, power-supply circuit  100  includes a feedback regulator with the voltage referenced to the output of voltage multiplier  110 . 
         [0019]    This is shown in  FIG. 2 , which presents a block diagram illustrating an LDO regulator  210  in power-supply circuit  100 . LDO regulator  210  is a feedback regulator that includes a high-voltage first stage  212  that includes a differential amplifier  214 , which feeds through to a second stage  216  with a level-shifting circuit  218 . The output of level-shifting circuit  218  specifies a gain of a power transistor Q 3    226  in a third stage  224 . This third stage also includes a high power-divider network  228  (such as a passive voltage divider) that provides a filtered and amplified DC output voltage B+  230  relative to AC input voltage  112  in  FIG. 1  (for example, 700 V), as well as a voltage FB  232  corresponding to DC output voltage B+  230 . This voltage is compared to a reference voltage (which is provided by resistor R 1  and Zener diode Z 1 ) by differential amplifier  214 , thereby providing negative feedback. 
         [0020]    LDO regulator  210  includes a number of features to solve problems that occur at high voltages, including: (1) turn-on problems; and (2) a low output impedance so that power transistor Q 3    226  can provide an approximately constant output power into a load over audible frequencies, such as up to 10-20 kHz. (Thus, the resistances in the design need to be small enough so that third stage  224  has sufficient bandwidth to provide high power and to regulate over audio frequencies.) 
         [0021]    In particular, R 2  protects differential amplifier  214  even if Vcc  114  is very large. (Thus, differential amplifier  214  floats relative to the resistors so power-supply circuit  100  can operate safely at high voltages.) Because of the negative feedback, during normal operation the gates of the transistors in differential amplifier  214  are at similar voltages. However, when power-supply circuit  100  is first turned on, there is no voltage on DC output voltage B+  230 , while the voltage on Z 1  is approximately 200 V, which will destroy the transistors in differential amplifier  214 . To prevent this, the gates of these transistors are electrically coupled by Z 2  and Z 3  with opposite polarities, which clamp a differential voltage across the gates during turn on of power-supply circuit  100 . 
         [0022]    In second stage  216 , level-shifting circuit  218  includes an npn transistor M 1    220  having: a gate coupled to an output of differential amplifier  214 ; an emitter coupled to ground; and a collector coupled to set of series-coupled diodes  222 , which can include at least one diode. This set of series-coupled diodes shifts an output voltage from differential amplifier  214  to a larger intermediate voltage. This intermediate voltage specifies a gain V GX  of power transistor Q 3    226  with a differential voltage across the collector and the emitter that is less than a V CE  breakdown voltage of this transistor. 
         [0023]    Level-shifting circuit  218  works as follows. First, note that the resistances R 6  and R 8  are the same, and the mirror current in differential amplifier  214  and on npn transistor M 1    220  are similar (for a reasonable value of β for this transistor, they are almost the same). R 4  and R 5  are selected so that R 5  and R 8  are at the same voltage (or similar voltages). Then, if differential amplifier  214  is balanced because of negative feedback, the voltage on R 3  is similar to the voltages on R 4  and R 5 . Because of level-shifting circuit  218 , the voltages on R 6  and R 8  are similar, so the nominal gain V GX  for power transistor Q 3    226  can be specified. Moreover, even though R 8  is the ground reference, by using level-shifting circuit  218  it is shifted to a level that can control the gate voltage of power transistor Q 3    226 . Note that R 6  is referenced to high voltage Vcc  114 . 
         [0024]    LDO regulator  210  has reasonable gain at audio frequencies (up to 10-20 kHz). But large power transistors have large gate capacitance, so R 6  cannot be large. However, if R 6  is small there is a small associated voltage drop. In this case, the maximum V CE  of npn transistor M 1    220  would need to be very large. To avoid this, Zener diode(s) in set of series-coupled diodes  222  are used to shift the voltage level up. These diodes provide an AC short for currents and transients in one direction, and can hold voltage in the other direction. This set of series-coupled diodes can include multiple Zener diodes, such as Z 4  (each of which has a 200 V drop). This feature allows power-supply circuit  100  to maintain its high-frequency response. 
         [0025]    Note that power transistor Q 3    226  is where the voltage drops as the load current changes. To maintain DC output voltage B+  230  when this occurs, power transistor Q 3    226  dynamically changes its effective impedance. 
         [0026]    Furthermore, DC output voltage B+  230  equals a voltage reference times power-divider network  228 , so different DC output voltages can be selected. This power-divider network  228  also includes features that facilitate operation at high voltages. C 7  and C 8  (which each may have breakdown voltages of 300-400 V) are stacked so that they do not blow up. However, these capacitors cannot take the full voltage drop if the circuit assumptions are wrong (such as during turn off). Consequently, R 9  and R 10  are used to maintain half of the voltage drop on each of these capacitors during turn off. 
         [0027]    Additionally, voltage FB  232  is the feedback point. It provides this voltage to differential amplifier  214 . 
         [0028]    As described previously, power-divider network  228  may have an output impedance less than a predefined value, thereby allowing power transistor Q 3    226  to provide an approximately constant output power into a load over audible frequencies. 
         [0029]    Note that LDO regulator  210  uses readily available components yet successfully operates in the 1 kV range. For example, using the AC line as the input, DC output voltage B+  230  may be at least five times larger than a root-mean-square value of AC input voltage  112  ( FIG. 1 ). 
         [0030]    In an exemplary embodiment, voltage multiplier  110  ( FIG. 1 ) uses diode model STTH512FP (provided by STMicroelectronics, N.V., of Geneva, Switzerland). Moreover, LDO regulator  210  may use Zener diode model 1N5388B (provided by Freescale Semiconductor, of Austin, Tex.), transistors Q t  and Q 2  in differential amplifier  214  and power transistor Q 3    226  may be model MTB2P50E (provided by Freescale Semiconductor, of Austin, Tex.), and transistor M 1  may be model BUH50 (provided by Freescale Semiconductor, of Austin, Tex.). In addition, power transistor Q 3   226  may require a heat sink. 
         [0031]    Hence, power-supply circuit  100  in the present disclosure combines voltage multiplier  110  in  FIG. 1  (which provides a rectified, somewhat-filtered DC voltage Vcc  114 ) plus an LDO regulator  210  (which regulates Vcc  114  to provide DC output voltage B+  230 ) that produces moderate power output and operates using AC input voltage  112  ( FIG. 1 ). This voltage multiplier may be similar in size to a more conventional voltage multiplier that includes a custom transformer plus rectifier and filtration components, but is less expensive and, certainly, is much less massive. Power-supply circuit  100  has very good power-supply noise rejection without using an L-C network (which typically has a large output impedance). Thus, power-supply circuit  100  provides good filtering and a low output impedance. 
         [0032]      FIG. 3  presents a block diagram illustrating an electronic device  300  that includes power-supply circuit  100 . In this electronic device, power-supply circuit  100  drives a vacuum-tube load. In particular, the DC output voltage B+  230  is electrically coupled to the tube collector. 
         [0033]    Although power-supply circuit  100  ( FIGS. 1 and 2 ) and electronic device  300  are illustrated as having a number of discrete items, these embodiments are intended to be a functional description of the various features that may be present rather than a structural schematic of the embodiments described herein. In some embodiments, some or all of the functionality of power-supply circuit  100  ( FIGS. 1 and 2 ) and/or electronic device  300  may be implemented in one or more mixed signal integrated circuits. 
         [0034]    More generally, power-supply circuit  100  may be used in a wide variety of applications, including: lasers, photographic flash units, displays and audio tube amplifiers. Furthermore, electronic device  300  may include a computing device, such as: a personal computer, a laptop computer, a tablet computer, a cellular phone, a personal digital assistant, a server and/or a client computer (in a client-server architecture). 
         [0035]    Power-supply circuit  100  ( FIGS. 1 and 2 ) and/or electronic device  300  may include fewer components or additional components. For example, LDO regulator  210  ( FIG. 2 ) can also be applied to the filtered, rectified output of a step-up transformer. Alternatively or additionally, for specific applications the above-described voltage multiplier may be fed from the output of an optional isolation transformer  116  ( FIG. 1 ) that isolates an input of voltage multiplier  110  ( FIG. 1 ) from a source of AC input voltage  112  ( FIG. 1 ), but such isolation transformers are small and widely available. 
         [0036]    The Cockcroft-Walton multiplier could also be a full-wave version using an isolation transformer with a center-tapped secondary. This full-wave version can provide less ripple. Moreover, the full-wave version may include twice as many diodes and 1.5 times the number of capacitors as the half-wave version shown in  FIG. 1 , but these components may have smaller values. 
         [0037]    While the aforementioned embodiments illustrated power-supply circuit  100  ( FIGS. 1 and 2 ) using a particular configuration of a Cockcroft-Walton multiplier and LDO regulator  210  ( FIG. 2 ), a variety of configurations may be used, including a Cockcroft-Walton multiplier with fewer or additional multiplication stages and/or additional diodes in set of series-coupled diodes  222  ( FIG. 2 ) in level-shifting circuit  218  ( FIG. 2 ). Furthermore, other voltage multipliers and/or regulators may be used in power-supply circuit  100  ( FIGS. 1 and 2 ). 
         [0038]    In power-supply circuit  100  ( FIGS. 1 and 2 ) and/or electronic device  300 , two or more components may be combined into a single component, and/or a position of one or more components may be changed. Note that the functionality of power-supply circuit  100  ( FIGS. 1 and 2 ) and/or electronic device  300  may be implemented using n-type, p-type, CMOS, bipolar, vacuum tube, discrete and/or integrated components, as is known in the art. 
         [0039]    While some components are shown directly connected to one another in the preceding embodiments, others are shown connected via intermediate components. Nonetheless, electrical coupling may be accomplished using a number of circuit configurations, as is known in the art. For example, these embodiments can support AC and DC coupling between components. 
         [0040]      FIG. 4  presents a flow diagram illustrating a method  400  for providing a DC output voltage using power-supply circuit  100  ( FIGS. 1 and 2 ). During this method, the voltage multiplier in the power-supply circuit receives the AC input voltage (operation  410 ) and provides the output voltage to the LDO regulator in the power-supply circuit (operation  412 ). Then, the LDO regulator filters and amplifies the output voltage (operation  414 ). Moreover, the LDO regulator provides the DC output voltage (operation  416 ), where the power-supply circuit excludes a step-up transformer. 
         [0041]    In some embodiments of method  400  there may be additional or fewer operations. Moreover, the order of the operations may be changed, and/or two or more operations may be combined into a single operation. 
         [0042]    The foregoing description is intended to enable any person skilled in the art to make and use the disclosure, and is provided in the context of a particular application and its requirements. Moreover, the foregoing descriptions of embodiments of the present disclosure have been presented for purposes of illustration and description only. They are not intended to be exhaustive or to limit the present disclosure to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present disclosure. Additionally, the discussion of the preceding embodiments is not intended to limit the present disclosure. Thus, the present disclosure is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein.