U.S. patent application number 09/781501 was filed with the patent office on 2001-08-23 for apparatus for metering at least one type of electrical power over a predetermined range of service voltages.
Invention is credited to Hemminger, Rodney C., Munday, Mark L..
Application Number | 20010015640 09/781501 |
Document ID | / |
Family ID | 26947097 |
Filed Date | 2001-08-23 |
United States Patent
Application |
20010015640 |
Kind Code |
A1 |
Hemminger, Rodney C. ; et
al. |
August 23, 2001 |
Apparatus for metering at least one type of electrical power over a
predetermined range of service voltages
Abstract
Methods and apparatus for supplying power for use in metering
electrical energy over a wide range of voltages with a single meter
are disclosed. The wide ranging meter includes a processing unit
for processing divided input voltage and a current component in
order to determine electrical energy metering values. The
processing unit is operable in response to supply voltages. A power
supply, connected to receive the undivided voltage component,
generates the supply voltages over the wide dynamic range. It is
especially preferred for the power supply to include a transformer
having first, second and third windings, wherein the undivided
voltage component is provided to the first winding and wherein the
second winding defines the output of the power supply. A switching
member is connected to the first winding for permitting and
preventing the flow of current in response to a control signal. A
control member generates the control signal in response to the
output of the power supply. It is also preferred for the control
signal to disable the witch member. It is further preferred for the
power supply to include a voltage blocking clamp, connected to the
transformer for blocking the voltage applied to the transformer. It
is still further preferred for an oscillator to be used to generate
an oscillating signal for switching the switching member ON and OFF
so that the switching member is provided a substantially constant
OFF time.
Inventors: |
Hemminger, Rodney C.;
(Raleigh, NC) ; Munday, Mark L.; (Raleigh,
NC) |
Correspondence
Address: |
Woodcock Washburn Kurtz
Mackiewicz & Norris LLP
46th Floor
One Liberty Place
Philadelphia
PA
19103
US
|
Family ID: |
26947097 |
Appl. No.: |
09/781501 |
Filed: |
February 12, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09781501 |
Feb 12, 2001 |
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09047479 |
Mar 25, 1998 |
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6229295 |
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09047479 |
Mar 25, 1998 |
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08478605 |
Jun 7, 1995 |
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5903145 |
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08478605 |
Jun 7, 1995 |
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08384398 |
Feb 3, 1995 |
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5457621 |
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08384398 |
Feb 3, 1995 |
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08259116 |
Jun 10, 1994 |
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08259116 |
Jun 10, 1994 |
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07839967 |
Feb 21, 1992 |
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Current U.S.
Class: |
324/142 |
Current CPC
Class: |
H02M 3/33507 20130101;
G01R 21/133 20130101; G01R 21/1331 20130101; H02M 3/335
20130101 |
Class at
Publication: |
324/142 |
International
Class: |
G01R 007/00; G01R
011/32 |
Claims
What is claimed is:
1. A power supply for an apparatus for metering at least one type
of electrical power over a predetermined range of service voltages
supplied by electrical service providers, said apparatus comprising
a voltage input circuit connected to receive a voltage component
and a processing unit, said power supply comprising: a surge
protection circuit, connected to said voltage input circuit, which
receives an input voltage; a rectifier circuit, connected to said
surge protection circuit, which receives an alternating current
voltage from said surge protection circuit and outputs a rectified
direct current voltage; a transformer comprising first, second and
third windings, which receives said rectified direct current
voltage at said first winding so that current flows through said
first winding, wherein said second winding defines an unregulated
output voltage of said power supply, and wherein said third winding
is substantially similar to said second winding so that the voltage
across said third winding is similar to the voltage across said
second winding; a switching device, connected to said first
winding, for permitting and preventing the flow of current through
said first winding, wherein said switching device is operable in
response to a control signal; and a controller, connected to said
switching device, for generating said control signal based on the
voltage across said third winding, and wherein said control signal
operates to disable said switching member.
2. The apparatus as recited in claim 1, said controller comprising
a current-mode regulator, wherein a current reference signal is
generated by said current-mode regulator.
3. The apparatus as recited in claim 1, further comprising a linear
regulator, wherein said output of said power supply is input to
said linear regulator, wherein said linear regulator outputs a
regulated voltage.
4. The apparatus as recited in claim 3, wherein said regulated
voltage is less than said output voltage, and wherein said
regulated voltage is output to a precision voltage reference
generator.
5. The apparatus as recited in claim 3, wherein said unregulated
voltage is input to said apparatus to determine the presence of a
power fail condition.
6. The apparatus as recited in claim 3, further comprising a
non-volatile supply, wherein said regulated voltage is input to
said non-volatile supply, wherein said apparatus is switched to
said non-volatile supply when said regulated voltage is not
present.
Description
RELATED APPLICATION DATA
[0001] This application is a divisional application of U.S. patent
application Ser. No. 09/047,479, filed Mar. 25, 1998, which is a
continuation of U.S. patent application Ser. No. 08/478,605, filed
Jun. 7, 1995, now U.S. Pat. No. 5,903,145, which is a continuation
of U.S. patent application Ser. No. 08/384,398, filed Feb. 3, 1995,
now U.S. Pat. No. 5,457,621, which is a continuation of U.S. patent
application Ser. No. 08/259,116, filed Jun. 10, 1994, now
abandoned, which is a continuation of U.S. patent application Ser.
No. 07/839,967, filed Feb. 21, 1992, now abandoned.
FIELD OF INVENTION
[0002] The present invention relates generally to the field of
electric utility meters.
[0003] More particularly, the present invention relates to
electronic utility watthour meters or meters utilized to meter real
and reactive energy in both the forward and reverse directions.
BACKGROUND OF THE INVENTION
[0004] Electric utility companies and power consuming industries
have in the past employed a variety of approaches to metering
electrical energy. Typically, a metering system monitors power
lines through isolation and scaling components to derive polyphase
input representations of voltage and current. These basic inputs
are then selectively treated to determine the particular type of
electrical energy being metered. Because electrical uses can vary
significantly, electric utility companies have requirements for
meters configured to analyze several different nominal primary
voltages. The most common of these voltages are 120, 208, 240, 277
and 480 volts RMS. Presently, available meters have a different
style for each of these applications, both electro-mechanical and
electronic. This forces the electric utility companies to
inventory, test and maintain many different styles of meters.
Consequently, a need exists for reducing the number of meter types
a utility need inventory by providing a meter capable of operation
over a wide dynamic range.
[0005] The problem of wide amperage dynamic range was addressed in
U.S. Pat. No. 3,976,941--Milkovic. It was there recognized that
solid state electronic meters were becoming more desirable in
metering applications, however, such solid state meters had a
critical drawback in their amperage dynamic range. An effort was
described to improve the amperage dynamic range of solid state
meters so that such meters would be operationally equivalent to
prior electro-mechanical meters. The problem with such meters,
however, was their failure to address the multiple voltage
situation. Utility companies utilizing such meters would still be
forced to inventory, test and maintain many different styles of
meters in order to service the various voltages provided to
customers.
[0006] It has been recognized in various meter proposals that the
use of a microprocessor would make metering operations more
accurate. It will be understood, however, that the use of a
microprocessor requires the provision of one or more supply
voltages. Power supplies capable of generating a direct current
voltage from the line voltage have been used for this purpose.
Since electric utility companies have requirements for various
nominal primary voltages, it has been necessary to provide power
supplies having individualized components in order to generate the
microprocessor supply voltages from the nominal primary
voltage.
[0007] Consequently, a need exists for a single meter which is
capable of metering electrical energy associated with nominal
primary voltages in the range from 96 to 528 volts RMS. Applicants
resolve the above problems through the use of a switching power
supply and voltage dividers. It will be recognized that switching
power supplies are known. However, the use of such a power supply
in an electrical energy meter is new. Moreover, the manner of the
present invention, the particular power supply construction and its
use in an electrical energy meter is novel.
[0008] It will also be noted, in order to solve the inventory
problem, designing a wide voltage range meter in the past involved
the use of voltage transformers to sense line voltage. A
significant problem associated with the use of such transformers
was the change in phase shift and the introduction of
non-linearities that would occur over a wide voltage range. It was
not easy to remove such a widely changing phase shift or to
compensate for the non-linearities.
[0009] Consequently, a need still exists for a single meter which
is capable of metering electrical energy associated with nominal
primary voltages that also minimizes phase shift in the voltage
sensors over a wide voltage range.
SUMMARY OF THE INVENTION
[0010] The present invention is directed to a power supply for an
apparatus for metering at least one type of electrical power over a
predetermined range of service voltages supplied by electrical
service providers, where the apparatus comprises a voltage input
circuit connected to receive a voltage component, and a processing
unit. The power supply comprises a surge protection circuit which
receives an input voltage, a rectifier circuit which receives an
alternating current voltage from the surge protection circuit and
outputs a rectified direct current voltage, a transformer which
receives the rectified direct current voltage at a first winding so
that current flows through the first winding, and a second winding
defines an unregulated output voltage of the power supply, a
switching device for permitting and preventing the flow of current
through the first winding in response to a control signal, and a
controller for generating the control signal based on the voltage
across the third winding. The control signal output by the
controller operates to disable the switching member.
[0011] According to another feature of the present invention, the
output of the power supply is input to a linear regulator, which
outputs a regulated voltage. The regulated voltage is less than the
output voltage, and the regulated voltage is output to a precision
voltage reference generator. The unregulated voltage is input to
the apparatus to determine the presence of a power fail
condition.
[0012] According to yet another feature, the power supply comprises
a non-volatile supply, and the regulated voltage is input to the
non-volatile supply, such that the apparatus is switched to the
non-volatile supply when the regulated voltage is not present.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The present invention will be better understood, and its
numerous objects and advantages will become apparent to those
skilled in the art by reference to the following detailed
description of the invention when taken in conjunction with the
following drawings, in which:
[0014] FIG. 1 is a block diagram of an electronic meter constructed
in accordance with the present invention;
[0015] FIG. 2 is a schematic diagram of the resistive dividers
shown in FIG. 1;
[0016] FIG. 3 is a schematic diagram of the linear power supply
shown in FIG. 1;
[0017] FIG. 4 is a block diagram of the power supply shown in FIG.
1;
[0018] FIG. 5 is a schematic diagram of the control and switching
members shown in FIG. 4;
[0019] FIG. 6 is a schematic diagram of the startup/feedback shown
in FIG. 4; and
[0020] FIG. 7 is a schematic diagram of the voltage clamp shown in
FIG. 4.
DETAILED DESCRIPTION
[0021] A new and novel meter for metering electrical energy is
shown in FIG. 1 and generally designated 10. It is noted at the
outset that this meter is constructed so that the future
implementation of higher level metering functions can be
supported.
[0022] Meter 10 is shown to include three resistive voltage divider
networks 12A, 12B, 12C; a first processor--an ADC/DSP
(analog-to-digital converter/digital signal processor) chip 14; a
second processor--a microcontroller 16 which in the preferred
embodiment is a Mitsubishi Model 50428 microcontroller; three
current sensors 18A, 18B, 18C; a 12V switching power supply 20 that
is capable of receiving inputs in the range of 96-528V; a 5V linear
power supply 22; a non-volatile power supply 24 that switches to a
battery 26 when 5V supply 22 is inoperative; a 2.5V precision
voltage reference 28; a liquid crystal display (LCD) 30; a 32.768
kHz oscillator 32; a 6.2208 MHz oscillator 34 that provides timing
signals to chip 14 and whose signal is divided by 1.5 to provide a
4.1472 MHz clock signal to microcontroller 16; a 2 kbyte EEPROM 35;
a serial communications line 36; an option connector 38; and an
optical communications port 40 that may be used to read the meter.
The inter-relationship and specific details of each of these
components is set out more fully below.
[0023] It will be appreciated that electrical energy has both
voltage and current characteristics. In relation to meter 10
voltage signals are provided to resistive dividers 12A-12C and
current signals are induced in a current transformer (CT) and
shunted. The output of CT/shunt combinations 18A-18C is used to
determine electrical energy.
[0024] First processor 14 is connected to receive the voltage and
current signals provided by dividers 12A-12C and shunts 18A-18C. As
will be explained in greater detail below, processor 14 converts
the voltage and current signals to voltage and current digital
signals, determines electrical energy from the voltage and current
digital signals and generates an energy signal representative of
the electrical energy determination. Processor 14 will always
generate a watthour delivered (Whr Del) and, watthour received (Whr
Rec), depending on the type of energy being metered, will generate
either a volt amp reactive hour delivered (Varhr Del)/a volt amp
reactive hour received (Varhr Rec) signal or volt amp hour
delivered (Vahr Del)/volt amp hour received (Vahr Rec) signal. In
the preferred embodiment, each transition on conductors 42-48 (each
logic transition) is representative of the measurement of a unit of
energy. Second processor 16 is connected to first processor 14. As
will be explained in greater detail below, processor 16 receives
the energy signal(s) and generates an indication signal
representative of said energy signal.
[0025] It will be noted again that meter 10 is a wide range meter
capable of metering over a voltage range from 96-528V. The
components which enhance such a wide range meter include the
divider network 12A-12C, which as previously noted are connected to
receive the voltage component. The dividers generate a divided
voltage, wherein the divided voltage is substantially linear
voltage with minimal phase shift over the wide dynamic range, i.e.
96-528 Volts. A processing unit (processors 14 and 16) are
connected to receive the divided voltage and the current component.
The processing unit processes the divided voltages and the current
components to determine electrical energy metering values. It will
be appreciated from the following description that processors 14
and 16 require stable supply voltages to be operable. A power
supply, connected to receive the voltage component and connected to
processors 14 and 16, generate the necessary supply voltages from
the Phase A voltage component over the wide dynamic range. Power
supply 20 could also run off of phase B and phase C voltages or a
combination of the above. However, a combination embodiment would
require additional protection and rectifying components.
[0026] In relation to the preferred embodiment of meter 10,
currents and voltages are sensed using conventional current
transformers (CT's) and resistive voltage dividers, respectively.
The appropriate multiplication is accomplished in a new integrated
circuit, i.e. processor 14. Processor 14 is essentially a
programmable digital signal processor (DSP) with built in multiple
analog to digital (A/D) converters. The converters are capable of
sampling multiple input channels simultaneously at 2400 Hz each
with a resolution of 21 bits and then the integral DSP performs
various calculations on the results. For a more detailed
description of Processor 14, reference is made to U.S. Pat. No.
5,555,508, which is incorporated herein by reference and which is
owned by the same assignee as the present application.
[0027] Meter 10 can be operated as either a demand meter or as a
time-of-use (TOU) meter. It will be recognized that TOU meters are
becoming increasingly popular due to the greater differentiation by
which electrical energy is billed. For example, electrical energy
metered during peak hours will be billed differently than
electrical energy billed during non-peak hours. As will be
explained in greater detail below, first processor 14 determines
units of electrical energy while processor 16, in the TOU mode,
qualifies such energy units in relation to the time such units were
determined, i.e. the season as well as the time of day.
[0028] All indicators and test features are brought out through the
face of meter 10, either on LCD 30 or through optical
communications port 40. Power supply 20 for the electronics is a
switching power supply feeding low voltage linear supply 22. Such
an approach allows a wide operating voltage range for meter 10.
[0029] In the preferred embodiment of the present invention, the
so-called standard meter components and register electronics are
for the first time all located on a single printed circuit board
(not shown) defined as an electronics assembly. This electronics
assembly houses power supplies 20, 22, 24 and 28, resistive
dividers 12A-12C for all three phases, the shunt resistor portion
of 18A-18C, oscillator 34, processor 14, processor 16, reset
circuitry, EEPROM 35, oscillator 32, optical port components 40,
LCD 30, and an option board interface 38. When this assembly is
used for demand metering, the billing data is stored in EEPROM 35.
This same assembly is used for TOU metering applications by merely
utilizing battery 26 and reprogramming the configuration data in
EEPROM 35. The additional time-of use billing data is stored in the
internal RAM of processor 16, which RAM is backed by battery
26.
[0030] Consider now the various components of meter 10 in greater
detail. Primary current being metered may be sensed using
conventional current transformers. The shunt resistor portion of
devices 18A-18C are located on the electronics assembly.
[0031] The phase voltages are brought directly to the electronic
assembly where resistive dividers 12A-12C scale these inputs to
processor 14. In the preferred embodiment, the electronic
components are referenced to the vector sum of each line voltage
for three wire delta systems and to earth ground for all other
services. Resistive division is used to divide the input voltage so
that a very linear voltage with minimal phase shift over a wide
dynamic range can be obtained. This in combination with a switching
power supply allows the wide voltage operating range to be
implemented.
[0032] Referring briefly to FIG. 2, each resistive divider consists
of two 1 Meg, 1/2 watt resistors 50/52, 54/56 and 58/60,
respectively. Resistors 50-60 are used to drop the line voltage at
an acceptable watt loss. Each resistor pair feeds a resistor 62, 64
and 66, respectively. Resistors 62-66 are metal film resistors
having a minimal temperature coefficient. This combination is very
inexpensive compared to other voltage sensing techniques. Resistors
50-60 have an operating voltage rating of 300 Vrms each. These
resistors have been individually tested with the 6 kV IEEE 587
impulse waveforms to assure that the resistance is stable and that
the devices are not destroyed. Resistors 62-66 scales the input
voltage to be less than 1 Volt peak to peak to processor 14.
Resistors 62-66 should be in the range of from about 100 ohms to
about 1K ohms to assure this maximum voltage and maintain maximum
signal.
[0033] On grounded, three wire delta systems, those components of
the electronics assembly operating on logic voltage levels
(including the battery connector) can be at an elevated voltage. In
such situations, the two, 1 Meg resistor combinations (50/52,
54/56, 58/60) provide current limiting to the logic level
electronics. The worse case current occurs during testing of a 480
V, 3 wire delta meter with single phase excitation.
[0034] It will be appreciated that energy units are calculated in
processor 14 primarily from multiplication of voltage and current.
The preferred embodiment of processor 14, referenced above as being
described in U.S. Pat. No. 5,555,508, includes three analog to
digital converters. The necessity for three converters is primarily
due to the absense of voltage transformers, present in prior
meters.
[0035] The M37428 microcontroller 16 is a 6502 (a traditional 8 bit
microprocessor) derivative with an expanded instruction set for bit
test and manipulation. This microcontroller includes substantial
functionality including internal LCD drivers (128 quadraplexed
segments), 8 kbytes of ROM, 384 bytes of RAM, a full duplex
hardware UART, 5 timers, dual clock inputs (32.768 kHz and up to 8
MHz), and a low power operating mode.
[0036] During normal operation, processor 16 receives the 4.1472
MHz clock from processor 14 as described above. Such a clock signal
translates to a 1.0368 MHz cycle time. Upon power fail, processor
16 shifts to the 32.768 kHz crystal oscillator 32. This allows low
power operation with a cycle time of 16.384 kHz. During a power
failure, processor 16 keeps track of time by counting seconds and
rippling the time forward. Once processor 16 has rippled the time
forward, a WIT instruction is executed which places the unit in a
mode where only the 32.768 kHz oscillator and the timers are
operational. While in this mode a timer is setup to "wake up"
processor 16 every 32,768 cycles to count a second.
[0037] Consider now the particulars of the power supplies shown in
FIG. 1. As indicated previously, the off-line switching supply 20
is designed to operate over a 96-528 VAC input range. It connects
directly to the Phase A voltage alternating current (AC) line and
requires no line frequency transformer. A flyback converter serves
as the basis of the circuit. A flyback converter is a type of
switching power supply.
[0038] As used herein, the "AC cycle" refers to the 60 Hz or 50 Hz
input to power supply 20. The "switching cycle" refers to the 50
kHz to 140 kHz frequency at which the switching transformer of
power supply 20 operates. It will be noted that other switching
cycle frequences can be used.
[0039] Referring now to FIG. 4, power supply 20 for use in
electronic meters includes a transformer 300 having primary and
secondary windings. The input voltage (Phase A Voltage) is provided
to the primary winding so that current may flow therethrough. As
will be appreciated from FIG. 5, the secondary winding defines the
output of the power supply. Referring back to FIG. 4, a switching
member 302 is connected to the primary winding of transformer 300.
Switching member 302 permits and prevents the flow of current
through the primary winding. Switch member 302 is operable in
response to a control signal, which control signal is generated by
control circuit 304. Controller 304 generates the control signal in
response to a limit signal generated by the start/feedback circuit
306 in response to the output of power supply 20. Voltage clamp 308
serves to limit the voltage applied to transformer 300 and switch
302. Surge protection circuit 309 is provided at the input to
protect against surges appearing in the Phase A voltage.
[0040] Referring now to FIG. 5, transformer 300 and switch 302 are
shown in greater detail. It will be appreciated that switch 302 is
a transistor. At the beginning of each switching cycle, transistor
302 "turns on", i.e. becomes conductive, and magnetizes the core of
transformer 300 by applying voltage across the primary 310. At the
end of each cycle, transistor 302 turns off and allows the energy
stored in the core of transformer 300 to flow to the output of the
power supply, which "output" can be generally defined by secondary
312. Simultaneously, energy flows out of the bootstrap or tertiary
winding 314 to power the control circuitry 304.
[0041] Feedback circuit 306 and controller 304 control the output
of power supply 20 by varying the ON time of transistor 302.
Controller 304 will be described in greater detail in relation to
FIG. 5. Transistor 302 is connected through inverter 316 to receive
the output of an oscillator formed from inverters 318, 320 and 322.
It will be recognized that such inverters form a ring oscillator.
The oscillator has a free-run frequency of 50 KHz. The ON time of
transistor 302 may vary between 200 ns and 10 .mu.s. The OFF time
is always between 8 and 10 .mu.s. During operation, the bootstrap
winding 314 of transformer 300 (pins 10 and 11) powers controller
304, but this power is not available until the power supply has
started. The control circuit is a current-mode regulator.
[0042] At the beginning of a switching cycle, transistor 302 is
turned ON by the oscillator output. If left alone, transistor 302
would also be turned OFF by the oscillator output. Transistor 302
remains ON until the current in primary 310 of transformer 300
(pins 8 and 13) ramps up to the threshold current level I.sub.th
represented as a voltage V.sub.th. As will be explained below,
V.sub.th is generated by feedback circuit 306. When the primary
current of transformer 300, represented as a voltage V.sub.t and
sensed by resistor 326, ramps up to the threshold level V.sub.th,
pin 1 of comparator 324 terminates the ON period of the oscillator
by forcing the oscillator output HIGH, which output in turn is
inverted by inverter 316, shutting OFF transistor 302. Transistor
302 then turns OFF until the next switching cycle. Since the
V.sub.th indirectly controls the ON time of transistor 302,
controller 304 regulates the output voltage of the power supply by
comparing the sensed current in transformer 300 to this threshold
level.
[0043] Transistor 362 and pin 7 of comparator 326 can disable the
oscillator. Transistor 362, described in greater detail in FIG. 7,
disables the oscillator when the line voltage exceeds 400 volts.
Comparator 328 disables the oscillator when the controller 304 has
insufficient voltage to properly drive transistor 302. The voltage
in controller 304, V.sub.c, will be described in relation to FIG.
5.
[0044] Consider now feedback circuit 306, shown in FIG. 6. When
connected to the Phase A Voltage, resistor 330 slowly charges
capacitor 332. The high value of resistor 330 and the 400 volt
limit by voltage clamp 308 limit the power dissipation of resistor
330. After a few seconds, capacitor 332 charges above 13 volts.
Transistors 334 and 336 then provide positive feedback to each
other and snap ON. Controller 304 can run for tens of milliseconds
from the charge stored in capacitor 332. Normally, power supply 20
will successfully start and begin to power itself in this period.
If it fails to start, transistors 334 and 336 turn OFF when the
charge across capacitor 332 drops below 8.5 volts and capacitor 332
again charges through resistor 330. This cycle repeats until the
supply starts.
[0045] With high input voltages and without resistor 338 (FIG. 5),
the current sourced by resistor 330 can hold the control and
start-up circuits in a disabled state that does not recycle. When
Capacitor 332 drops below 8.5 volts, resistor 338 places a load on
the control circuit supply. This load insures that the start-up
circuit recycles properly with high input voltages.
[0046] As indicated above, when the primary current of transformer
300 sensed by resistor 326 ramps up to the threshold level
V.sub.th, pin 1 of comparator 324 can terminate the ON period of
the oscillator. When the voltage on capacitor 332 is less than 13
volts, zener diode 340 provides no voltage feedback. Under these
conditions, the base-emitter voltage of transistor 336 sets the
current threshold I.sub.th to about 650 mA. This maximum current
limit protects transistor 302, as well as those transistors in
voltage clamp 306, and prevents transformer 300 from
saturating.
[0047] As the voltage on capacitor 332, which is representative of
the output voltage of the supply, approaches the proper level,
zener diode 340 begins to conduct and effectively reduces the
current threshold, i.e. effectively reduces V.sub.th. Each
switching cycle will then transfers less power to the output, and
the supply begins to regulate its output.
[0048] When the regulating circuitry requires ON times of
transistor 302 less than about 400 ns, the current sense circuitry
does not have time to react to the primary current of transformer
300. In that case, the regulating circuit operates as a
voltage-mode pulse width modulator. Resistor 342 (FIG. 5) generates
a negative step at pin 3 of comparator 324 at the beginning of each
switching cycle. The regulator feedback voltage at pin 2 of
comparator 324, which contains little current information at the
beginning of each switching cycle, translates the step at pin 3
into various input overdrives of comparator 324, thereby driving
the output of comparator 324 to a logic HIGH level. The propagation
time of the comparator 324 decreases with increasing overdrive,
i.e. as the negative step increases, and the circuit acts as a
pulse width modulator. The negative step will increase due to the
changing level of V.sub.th.
[0049] Any leakage inductance between the bootstrap winding (pins
10 and 11 of transformer 300) and the output winding (pins 3 and 4
of transformer 300) causes inaccurate tracking between the voltage
on capacitor 332 and the output voltage of the supply. This leakage
inductance can cause poor load regulation of the supply. The
bootstrap and output windings are bifilar wound; they are tightly
coupled, have little leakage inductance, and provide acceptable
load regulation. Since the two windings are in direct contact, the
bootstrap winding requires Teflon insulation to meet the isolation
voltage specifications. A 100% hi-pot test during manufacture
insures the integrity of the insulation.
[0050] Consider now the details of voltage clamp 308, shown in FIG.
7. A 528 VAC input corresponds to 750 VDC after rectification.
Switching transistors that can directly handle these voltages are
extremely expensive. By using the voltage clamp of the present
invention, relatively inexpensive switching transistors can be
utilized.
[0051] In power supply 20, the switching member 302 is shut down
during parts of the AC cycle that exceed 400 volts. The switching
transistor, transistor 302, in conjunction with two other
transistors 344 and 346, can hold off 750 VDC. During surge
conditions, these three transistors can withstand over 1500 volts.
In the preferred embodiment, transistors 302, 344 and 346 are
600-volt MOSFETs.
[0052] Because high-voltage electrolytic capacitors are expensive
and large, this voltage clamp 308 has no bulk filter capacitor
after the bridge rectifier 348. Without a bulk filter capacitor,
this switching converter must shut down during parts of the AC
cycle. It intentionally shuts down during parts of the AC cycle
that exceed 400 volts, and no input power is available when the AC
cycle crosses zero. The 2200 .mu.F output capacitor 350 (FIG. 5),
provides output current during these periods.
[0053] As discussed above, transistors 344 and 346 act as a voltage
clamp and limit the voltage applied to switching member 302. At a
528 VAC line voltage, the input to the clamping circuit reaches 750
volts. During lightning-strike surges, this voltage may approach
1500 volts. When the voltage at the output of bridge rectifier 348
exceeds 400 volts, zener diodes 352 and 354 begin to conduct. These
diodes, along with the 33 K.OMEGA. resistors 356, 358 and 360,
create bias voltages for transistors 344 and 346. Transistors 344
and 346 act as source followers and maintain their source voltages
a few volts below their gate voltages.
[0054] If, for example, the output of bridge rectifier 348 is at
1000 volts, the gates of transistors 344 and 346 will be at
approximately 400 and 700 volts respectively. The source of
transistor 344 applies roughly 700 volts to the drain of 346; the
source of 346 feeds about 400 volts to switching member 302.
Transistors 344 and 346 each drop 300 volts under these conditions
and thereby share the drop from the 1000 volt input to the 400 volt
output, a level which the switching converter 302 can
withstand.
[0055] As zener diodes 352 and 354 begin to conduct and as
transistors 344 and 346 begin to clamp, transistor 362 turns ON and
shuts down the switching converter. Although transistors 344 and
346 limit the voltage fed to the converter to an acceptable level,
they would dissipate an excessive amount of heat if the switching
converter 302 consumed power during the clamping period.
[0056] When switching converter 302 shuts down, transistor 302 no
longer has to withstand the flyback voltage from transformer 300.
Resistor 364 takes advantage of this by allowing the output voltage
of the clamp to approach 500 volts (instead of 400 volts) as the
input to the clamp approaches 1500 volts. This removes some of the
burden from transistors 344 and 346.
[0057] Zener diodes 352 and 354 are off and the converter 302 runs
when the output of bridge rectifier 348 is below 400 volts. During
these parts of the AC cycle, the 33 K.OMEGA. resistors 356, 358 and
360 directly bias the gates of transistors 344 and 346. The voltage
drop across transistors 344 and 346 is then slightly more than the
threshold voltages of those transistors along with any voltage drop
generated by the channel resistance of those transistors.
[0058] During the off time of transistor 302, about 10 .mu.S, the
33 K.OMEGA. resistors can no longer bias the gates of transistors
344 and 346. Diode 366 prevents the gate capacitance of transistors
344 and 346 and the junction capacitance of zeners 368 and 370 from
discharging when transistor 302 is off. This keeps transistors 344
and 346 ON and ready to conduct when transistor 302 turns ON at the
next switching cycle. If the gates of transistors 344 and 346 had
discharged between switching cycles, they would create large
voltage drops and power losses during the time required to recharge
their gates through the 33 K.OMEGA. resistors.
[0059] In the preferred embodiment, two 33 K.OMEGA. resistors are
used in series to obtain the necessary voltage capability from 966
surface-mount packages.
[0060] This power supply must withstand an 8 KV, 1.2.times.50 .mu.S
short-branch test. Varistor 372, resistors 374, 376 and 378, and
capacitor 380 protect the power supply from lightning strike
surges.
[0061] A 550 VAC varistor 372 serves as the basis of the protection
circuit. It has the lowest standard voltage that can handle a 528
VAC input. The device has a maximum clamping voltage of 1500 volts
at 50 amps.
[0062] A varistor placed directly across an AC line is subject to
extremely high surge currents and may not protect the circuit
effectively. High surge currents can degrade the varistor and
ultimately lead to catastrophic failure of the device. Input
resistors 374 and 376 limit the surge currents to 35 amps. This
insures that the clamping voltage remains below 1500 volts and
extends the life of the varistor to tens of thousands of
strikes.
[0063] Resistor 378 and capacitor 380 act as an RC filter. The
filter limits the rate of voltage rise at the output of the bridge
rectifier. The voltage clamping circuit, transistors 344 and 346,
is able to track this reduced dv/dt. Current forced through diodes
382, 384 and capacitor 386 (FIG. 5) is also controlled by the
limited rate of voltage rise.
[0064] Resistors 374 and 376 are 1 watt carbon composition
resistors. These resistors can withstand the surge energies and
voltages. Resistor 378 is a flame-proof resistor that acts as a
fuse in the event of a failure in the remainder of the circuit.
[0065] The values of resistors 374, 376 and 378 are low enough so
that they do not interfere with the operation of the power supply
or dissipate excessive amounts of power.
[0066] Finally it is noted that resistors 388 and 390 act to
generate the power fail voltage PF.
[0067] By using the wide voltage ranging of the invention, a single
meter can be used in both a four wire wye application as well as in
a four wire delta application. It will be recognized that a four
wire delta application includes 96V sources as well as a 208V
source. In the past such an application required a unique meter in
order to accomodate the 208V source. Now all sources can be metered
using the same meter used in a four wire wye application.
[0068] While the invention has been described and illustrated with
reference to specific embodiments, those skilled in the art will
recognize that modification and variations may be made without
departing from the principles of the invention as described herein
above and set forth in the following claims.
* * * * *