Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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SWITCHING POWER SUPPLY
BACKGROUND OF THE INVENTION
This invention relates to switching power supply
apparatus, and, more particularly, to a low C09t power
supply capable o~ accommodating a wide range of input
voltage and frequency for use in remotely located
programmable load control panels.
Description of the Prior Art
A system for remotely controlling electrical loads
distributed over a wide area, such as a large oEfice
building or factory, from a microprocessor-based central
controller is disclosed in U.S. Patent 4,367,414,
issued January 4, 1983 to Miller et al. The Miller
4,367,414 patent states in columns 29~30 that
a 20-40 volt switchleg power supply is required and
that a combination of resistors is used to provide
the 3.5-5 volts power required for the logic circuits.
The present invention provides a power supply system
to provide a plurality of d.c. vol.tayes for logic and
switchleg circuits. Power distribution systems in
various locations throughout the world deliver power with
a voltage ranging from 100 to 347 volts and with a fre-
quency ranging from 50 to lQ0 hertz. A power supply system
capable of operating successfully with any of the avail-
able voltage and frequency sources can avoid the need toprovide a multiplicity of products in order to accommodate
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each of the available power systems.
A power supply circuit for providing operating
potential to a load switching controller is disclosed
in U.S. Patent 4,333,138, issued June 1l 1982 to Huber,
and assigned to the present assignee. The aforementioned
U.S. Patent 4,333,138 discloses a power supply circuit in
which an output capacitor is charged during a single polarity
half-cycle of the source until a predetermined voltage
level is reached, at which point the charging is cut off.
This power supply circuit is capable of delivering a
constant voltage output from an a.c. power source.
A system for controlling distributed loads including
an arrangement for sensing remote binary inputs as
disclosed in Canadian Application Serial No. 481,260,
Beatty et al, filed May 10, 1985, a remote load control
relay processor as disclosed in Canadian Application
Serial No. 481,849, ~eatty et al, filed May 17, 1985 and
employinc3 a method of queued access of a common communica-
tions link as disclosed in Canadian Application Serial
No. 472,834, Miller et al, filed January 25, 1985 and
assigned to the present assignee requires a power supply
system able to accommodate a wide range of available a.c.
power systems and able to provide more than one d.c.
output voltage level. The present invention provides
a single power supply having the capability to accommodate
the range of power system voltages and frequencies used
commonly in various locations throughout the world and to
provide a plurality of constant d.c. outputs.
The prior art in power supplies includes many types
of systems with a wide variety of performance character-
istics. One prior art approach of accepting a wide range
of power inputs is to use a transformer having a multiplicity
(greater than 2) of primary and/or secondary taps. A
multi-tap transformer is quite costly and its complexity
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requires that users be trained to recognize ~he requirements
of proper use. Another approach is to use a dedicated
transformer for each voltage and frequency combination to
be served. This requires manufacturing and stocking a
large variety of products in order to meet a world market.
A single power supply of the present invention capable of
accommodating the full range of power supply input voltages
and frequencies can provide significant economies over
either supplying a separate power supply for each voltage
and frequency input or using a multi-tap transformer.
Prior art switching power supplies are generally fixed
within 10-20% of a given power distribution, and are ca~able
of delivering in excess of 200 watts. Prior art switching
power supplies are used primarily to reduce power dissipation
and size and are not directed to handling a plurality of
irlput voltag~s and frequencies. The prior art switching
power supplies operate on one of three modes: fixed on-time,
fixed off-time or fixed frequency. The switching action of
the switching element within the switching power supply is
employed to provide the on-time, off-time or fixed frequency
required. This typically requires an oscillator and timing
circuit to be included in the switching power supply. Fur-
thermore, the prior art switching power supplies on the
market require minimum loads with minimum switching frequency
and minimum duty cycle to insure safe operating conditions.
This minimum load is often a significant fraction of the
full load rating of the power supply varying typically
between 10% and 50%, thereby limiting design flexibility.
SUM~RY OF THE INVENTION
Accordingly, an object of the present invention is to
provide a power supply capable of accommodating a wide range
of input voltages and frequencies. A further object oE the
present invention is to provide a switching power supply for
providing a plurality of predetermined d.c. voltage outputs
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from a wide range of a.c. inputs. A still further object
of the present invention is to control the turn-on and
turn-off times of a peak voltage controlled switching
element in con~unction with a transformer impedance charac-
teristic to allow the con~ersion of a wide input voltageand frequency to a fixed ripple d.c. output.
Accordingly, the present invention includes an input
transformer for connecting any one of a plurality of input
power systems having a wide rd~nge of voltage and frequency
characteristics to the switching power supply, two power
switching stages having separate outputs, a halfwave
rectified, isolated, filtered power supply stage having a
distinct output and one monitoring stage with a power supply
status indicator.
BRIEF DESCRIPTION OF THE DRAWINGS
Further objects and advantages oE the present invention
together with its organization, method of operation and best
mode contemplated may best be understood by reference to the
following description taken in conjunction with the accom-
panying drawings, in which like reference characters refer
to like elements throughout, and in which the single figure
is a schematic circuit diagram illustrating the switching
power supply of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMæNTS
In the single figure, switching power supply 10 is
illustrated schematically. Input power is supplied by lines
12 and 14 on connector board 16 and jumper 18 is connected
either to terminal ~0 or terminal 22 to supply power to the
primary winding 24 of input transformer 26. ~ach terminal
can accept an input voltage range in which the maximum
voltage is at least twice the minimum voltage. For example,
in a particularly preferred embodiment terminal 20 is avail-
able for 70-140 volts a.c. inputs, and terminal 22 is
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available for 200-416 volts a.c. inputs with a frequency
range of 45 to 400 hertz. Secondary winding 28 is
connected to terminals 30 and 32 of bridge circuit 34
comprising diodes 36, 38, 40 and 42 and capacitor 44.
Also connected to terminals 30 and 32 is the primary
winding 46 of transformer 48. The secondary winding 50
of transformer 48 is connected to diode 52 and filter
capacitor 54 to provide a d.c. output at -terminals 56 and 58
of from 5 to 11.5 volts for an isolated d.c. power supply.
The negative output terminal 60 of the bridge 34 is
connected to the junction 62 and ground connection 64 which
are connected to respective output terminals 66 and 68.
The output from the bridge terminal 70 is brought to
transistor switch 72 (shown as a Darlington transistor
arrangement) which is cut off by resistor 74 across -the
base emitter junction. Current flows through the resistor
76, a soft start resistor, -to power the comparators 78,
80, 82 and 84, the 5-volt reference 86, for example, a
78L05 three terminal voltage regulator as sold by
National Semiconductor and the transistors 88 and 90.
The soft start resistor 76 protects transistor switch
72 by limiting the current drawn during start-up
needed, in particular, to charge capacitor 92 used
as a regulator charge storage device. Furthermore,
capacitors 106 and 144 aid in limiting current at power-
up to a safe level to protect the transistors 88 and 90.
The voltage on the line through resistor 76 is kept to
sa~e operating levels by current drawn from comparator 80,
reference 86, transistor 88 through resistor 94, and
transistor 90 through resistor 96. Clamping zener
diode 98 limits the absolute maximum voltage on
the line to less than 15 volts through the 9-volt
line load. The soft start is thereby achieved by
the controlled charging of capacitor 106 which controls
the base drive of transistor 90 through the RC combination
of resistor 76 and capacitor 106 which in turn controls
transistor 72 and its charging of capacitor 92.
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The power supplied by resistor 76 flows through
resistors 108 and 94 turning on transistor 88. This turns
on the pass transistor 110 thxough resistor 112. Current
is delivered through inductor 114 to capacitor 116 until
the voltage on capacitor 116 exceeds a~proximately 8-3/4
volts. At this point, the voitage, as seen by the com-
parator 80 on its inverting input pin 118, is the voltage
~to the 8-1/2 volt line divided by the combination~ of
resistors 120 and 122. When this voltage e*aee~ 5 volts
which is the reference voltage on pin 124, the output of
comparator 80 goes low ~below the turn-on threshold of
transistor 88) and sinks base drive away from transistor
88 shutting transistor 88 off. This in turn shuts off
transistor 110 very rapidly through the resistor 126. In-
ductor 114 then bucks the resulting change in current, the
DI/DT, and the voltage on the side 128 of inductor 114
adjacent the transistor 110 attempts to go very much nega-
tive in an attempt to keep current conducting. Diode 130
then turns on and continues to charge capacitor 116 through
inductor 114. Thus this section of the switching power
supply uses the inductor as a charge storage device.
Hysteresis and, therefore, the ripple voltage in this
section are controlled by diode 132 and resistors 134 and
136. That is, the voltage which must be applied to change
the state of the comparator is dependent upon ~he state of
the comparator at the time a change occurs. This arrange-
ment provides a 0.4 volt hysteresis at the output pin 138
of comparator 80 providing a 0.8 volt maximum ripple on the
8~1/2 volt output line 140. Without hysteresis switching
of the comparator output would always occur at the same
voltage causing an undesirable oscillation of output. 2ener
diode 98 acts in two modes. If the current supplied by
resistor 76 is less ~han 8-1/2 volts after the supply starts,
diode 98 conducts in the forward mode bringing ~he voltage
up to approximately 8 volts. This ensures that a high
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enough voltage is applied at the input 85 of ~-volt refer-
ence 86 for a stable region of operation. This minimum
voltage condition would occur, for example~ when a low
voltage i~ applied to the transformer 26 giving a minimum
secondary voltage. When a high voltage is applied to the
transformer 26 giving a high secondary voltage, resis~or 76
will attempt to supply more than 13 volts to the comparator
80. When this occurs, ~ener diode 98 turns on clamping the
voltage at approximat~ly 13-1/2 volts. This insures that
the comparator 78 will be able to sink ~he curren~ supplied
by resistor 142. Capacitors 106 and 144 serve as the
decoupling capacitors for the 5-volt reference 86, and
te~minal 87 i.s connected to syst~m common output 66.
I'herefore, the 8-1/2 volt outpuk supply utilizes a
switching transistor 110 with inductive charge storage to
achieve high power efficiency and low ripple, and allows a
second stage of regulation to be applied to derive a 5-volt
logic power supply. The switching regulator circuit operates
in a fixed on-time mode at maximum loading of 300 milliamps
at 8.1 volts d.c. The on-time, approximately 50 to 100
milliseconds, determines the amount of charge transferred
through inductor 114 to the load terminal 140. Exceeding
the fixed on-time could saturate the inductor 114 causing
transistor 110 to dissipate the excess power and possibly
damaging the transistor 110. Operation below ~ull load will
vary frequency and on-time according to load requirements.
In general, the on-time is controlle~ due to loading and
input power; the frequency of switching transistor 110 is
not controlled and will self-adjust to the loading require-
ments. It will be noted that no oscillator circuit is re-
quired, because the switching action is totally controlled
by the output voltage, thereby not requiring a minimum load.
In a particularly preferred embodiment of the present
invention, a maximum load of 300 milliamps, 0.5 volt maxi-
m~ ripple voltage, were selected for the low voltage output,
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and capacitor 116 was 220 microfarads with a discharge
period of 367 microseconds. The total charge transfer
in this time period is 110 x 10 6 coulombs. If this
circuit is operated in a fixed on-time mode with an
on-time of 50 microseconds, the value of inductor 114 can be
calculated as
I
where I = dQ/dT = 110 x 10 6 _ 2.2 amps and
V is the voltage differential between capacitor 116 and
capacitor 92 of about 20 volts. In this specific example,
inductor 114 must be 455 x 10 6 henries and be capable
of handling 1.5 amps peak.
In order to provide a high voltage stage which is
required for the load controller as described above,
power from resistor 76 is delivered to base 91 of
transistor 90 through the combination of resistors 142 and
96, in which resistor 142 acts as a pull-up sourcing
current resistor, and resistor 96 acts as a curren-t-
limiting resis-tor. The resis-tances of resis-tors 142
and 96 are chosen to insure proper drive to transistor
90 and the hysteresis of comparator 80 described above.
The current supplied to base 91 turns transistor 90 on,
which in turn turns transistor switch 72 on in a full
saturation mode through resistor 146. Current is thus
delivered to capacitor 92 building up the charge and
voltage on the capacitor 92. The voltage on capacitor
92 is divided by the resistor combination 148, 152 and
is sensed on negative input pin 150 of comparator 78.
The positive input pin 154 of comparator 78 is connected
to the reference voltage source output 196 through the
resistor 156. The output from comparator 78 allows
transistor switch 72 to be left on in a saturated
mode until it has delivered enough charge to
capacitor 92 that the voltage on capacitor 92
exceeds 36-40 volts. The voltage sensed on pin 150 is
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about 5 volts after being divided by the resistors 148, 152.
When ~he voltage exceeds 36-40 volts, the comparator 78
shuts off, that is, its output on pin 158 goes low (below
the turn-on threshold of transistor 90), turning off tran-
sistor 90 by shunting its base drive to ground. When tran-
sistor 90 turns off, it turns off transistor 72 through
resistor 74. Resistor 168 ensures that the voltage on base
91 goes to 0.3 volt and therefore that transistor 90 turns
off. Diode 160 and resistor 162, which are connected to
the output 158 of the comparator 78 and to the posit:ive
input 154 of comparator 78, form a hysteresis networ]c to
prevent oscillation in the switching supply as described
above.
In a particularly preferred embodiment when the output
on pin 158 of comparator 78 is logic zero, transistors 90
and 72 are off, and the voltage at comparator reference
input pin 154 is 4.645 volts. When the output on pin 158
is a logic 1, transistors 90 and 72 are on, and the reference
input on pin 154 is 5 volts providing a 0.355 volt hysteresis
at the comparator 80. It also defines the amount of rip21e
that will be present on the high voltage output line 170.
In the present case, the maximum ripple allowed is approxi~
mately 2-1/2 volts for a 36-40 volt output. When the voltage
on capacitor 92 is discharged, through some load, below
approximately 34 volts, the voltage on the inverting input
150 of comparator 78 goes below that of the positive reference
input 154, causing the co~parator to switch to the high
impedance state. This turns on transistor 90, turning on
the transistor switch 72 and chaxging up the capacitor 92
again. This cycle repeats indefinitely, and this switching
5upply therefore operates in a fixed ripple voltage mode.
It should be noted that when transistor switch 72 shuts
off, it will shut off rather abruptly causing an inductive
spike from the transformer 26. Capacitor 44 helps to atten-
uate this spike. The turn-off time of transistor switch 72
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determines the amplitude of the spike. To reduce the spike
amplitude, the turn-off time must be lengthened. The turn-
off time of transistor switch 72 is determined primarily by
~he combination of resistor 146 and capacitor 164 whose
S values are selected to provide a longer turn~o~f time than
the transistor switch intrinsic turn-off time. This pro-
~ides a lower dl/dt and a lower voltage transient from
transformer 26 and therefore a lower voltage spike upon
transistor switch 72. It is important to note that this
transient protection prevents the breakdown of transistor
switch 72 after it is turned off, thereby limiting the
unnecessary power dissipation by the transistor switch.
Capacitors 44 and 164 also minimize the electromagnetic
interference which is coupled back to the power line through
transformer 26. It should be noted that the transformer
impedance, that is, its inductance and d.c. resistance, is
used as voltage dropping element limiting the curxent
~hrough the transistor switch 720 The peak current handling
characteristic (transformex saturation current) also limits
the peak current through switch 72 to safe operational
limits. The transformer 26 operates in a saturation mode
while the secondary voltage is high. As the input voltage
decreases, the transformer 26 begins to operate in a linear
mode. Therefore, the transformer characteristics help limit
the extra power which switch 72 would otherwise have to
handle alone in accommodating the wide range of input voltages.
In a particularly preferred embodiment of the present
invention, capacitors 92 and 164 are 220 microfarads and
470 picofarads, respectively. Resistors 74 and 146 are
11~ ohms and 3000 ohms, respectively. Resistors 148 and 152
in the voltage divider -~upplying one input to comparator 78
are 22K ohms and 3.32K ohms, respectively.
The status circuit operates LED 102 and indicates
whether ~he two primary output voltages on lines 170 and 140
are within a tolerance range. It is meant primarily as a
gross indicator that the supply is operating properly.
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Undervoltage for the 8-1/2 volt supply at output 140 is
defined as 7 volts t and undervoltage for the 36-40 volt
output at line 170 is defined as 33 volts. Overvoltage is
defined as 10 volts for the 8-1/2 volt supply, and 39 volts
for the 36-40 volt supply. The circuit consisting of the
resistors 172, 174 and 176 is a voltage divider and is used
to set the over~olt detection threshold. The voltage
divider 178 feeds inverting input pin 180 of o~ervoltage
detecting comparator 82, and positive input pin 182 is
connected to the 5-volt reference 85. To detect an over-
voltage condition, the resistor equation for two simul-
taneous equations must be solved. That is, an overvoltage
condition should be detected if the 8-1/2 volt line is at
10 volts, and the 36-40 volt line 170 is at its nominal
voltage; or if the 8-1/2 volt line is at its nominal voltage
and the 36-4~ volt line goes to 39 volts. After substract-
ing the S-volt refexence voltage from this, the equations
read as follows: S x R2 + 31 x Rl = S volts;
3.5 x R2 + 34 x Rl = 5 volts. Taking the difference
between these two equations yields 1.5 x R2 - 3 x Rl = O.
Therefore, resistor 172 must be twice the resistance value
of the resistor 174. If resistor 174 is arbitrarily set
at lOOK ohms, then resistor 172 must be 50K ohms. In order
to meet the S-volt necessity for comparison, resistor 176
must be 12.2K ohms. The same voltage divider feeds pin 184,
the positive input of comparator 84, the undervoltage
detector, whose inverting input 186 is driven by a voltage
divider consisting of resistors 188 and 190. The total
xesistance oE resistors 188 and 190 must be kept low enough
so that the 5-volt reference 86 is loaded to at least one
milliamp to stabilize ~he reference. Normally, the over-
voltage input pin 180 is below the 5-volt reference, making
the output pin 192 of the comparator 82 a high impedance
which is connected in parallel with output pin ~ 94 of the
comparator 84. This is the output of ~he undervoltage
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detector. Normally r the undervolt positive inpu~ 184 which
is fed by the voltage sense resistor divider 178 is higher
than the divided down 5-volt reference from output 1~6 of
reference 86, making the pin 194 a high impedance output
allowing resistor 198 to drive txansistor 100 on, and the
LED 102 is on through resistor 104 in the 8-1/2 volt output.
However, if either an overvol~age is de~ected, that is,
pin 180 goes higher than the 5-~olt reference 86, the output
of comparator 82 will switch on, i.e., go low, or if an
undervoltage is detected, pin 194 will go low diverting the
base drive for transistor 100 to ground. Transistor 100
then turns off the LED 102 and gives a fault indication to
the user.
As will be appreciated by those skilled in the art, the
present invention provides a power supply syste~ capable
of outputting a plurality of d.c. voltages from a single
power input of any one of a variety of power sources of
widely varying voltage and frequency.
