U.S. patent application number 09/800337 was filed with the patent office on 2001-09-06 for efficient, dual-source, wide-input range, isolated dc-dc converter with effective current limit.
This patent application is currently assigned to Mitel Knowledge Corporation. Invention is credited to Burton, Scott Richard.
Application Number | 20010019492 09/800337 |
Document ID | / |
Family ID | 9887064 |
Filed Date | 2001-09-06 |
United States Patent
Application |
20010019492 |
Kind Code |
A1 |
Burton, Scott Richard |
September 6, 2001 |
Efficient, dual-source, wide-input range, isolated DC-DC converter
with effective current limit
Abstract
The present invention is a switching power supply having
efficient light load regulation at high input voltage. The power
supply comprises a pulse-width modulator integrate circuit along
with two separate voltage buses. A transformer is connected to the
first voltage bus via a first primary winding and is connected to
the second voltage bus via a second primary winding both primary
windings have a predetermined number of turns proportional to
voltage supplied by the two voltage buses. The power supply further
comprises a pair of switches connected to the two primary windings
and driven by the integrated circuit.
Inventors: |
Burton, Scott Richard;
(Ottawa, CA) |
Correspondence
Address: |
James W. McKee
Fay, Sharpe, Beall, Fagan,
Minnich & McKee, LLP
1100 Superior Avenue, 7th Floor
Cleveland
OH
44114-2518
US
|
Assignee: |
Mitel Knowledge Corporation
|
Family ID: |
9887064 |
Appl. No.: |
09/800337 |
Filed: |
March 6, 2001 |
Current U.S.
Class: |
363/21.12 |
Current CPC
Class: |
H02M 3/33569 20130101;
H02M 1/10 20130101 |
Class at
Publication: |
363/21.12 |
International
Class: |
H02M 003/335 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 6, 2000 |
GB |
0005375.1 |
Claims
What is claimed is:
1. A power supply having an output voltage, comprising: a first
voltage source supplying a first DC voltage that is switched on and
off by a first switch; a second voltage source supplying a second
DC voltage that is switched on and off by a second switch; a
transformer comprising a first primary winding connected to the
first voltage source, a second primary winding connected to the
second voltage source, and a secondary winding, where the secondary
winding has an output for supplying the output voltage and where
the first primary winding to the second primary winding has a turns
ratio that is proportional to a voltage ratio of the first voltage
source to the second voltage source; and a pulse-width modulator
for switching the first switch and the second switch on and off at
a duty cycle to control the output voltage.
2. The power supply of claim 1, wherein the first DC voltage varies
over a first voltage range and the second DC voltage varies over a
second voltage range.
3. The power supply of claim 2, wherein the output voltage is
supplied from whichever of the first voltage source and the second
voltage source is proportionally higher in voltage.
4. The power supply of claims 2 or 3, wherein the first voltage
range and the second voltage range do not overlap.
5. The power supply of any of claims 1 to 4, further comprising a
current limiting system to limit a first current of the first
voltage source and a second current of the second voltage
source.
6. The power supply of claim 5, wherein the current limiting system
comprises a resistor to generate a voltage drop for sensing a
combination of the first current and the second current by the
pulse-width modulator, where the pulse-width modulator varies the
duty cycle to limit the first current and the second current.
7. The power supply of any of claims 2 to 4, wherein the second
voltage range has a second peak voltage which is higher than a
first peak voltage of the first voltage range, and further
comprising a current limiting system to limit a first current of
the first voltage source and a second current of the second voltage
source that comprises a first resistor and a second resistor where
the first current passes through the first resistor and the second
current passes through the first resistor and the second resistor
such that the pulse-width modulator senses a voltage drop across
the first resistor and the second resistor accordingly changes the
duty cycle to limit current.
8. The power supply of claim 7, wherein the first resistor and the
second resistor are scaled such that the voltage drop sensed by the
pulse-width modulator is higher for a given current from the second
voltage source versus the first voltage source.
9. The power supply of any of claims 1 to 8, further comprising a
capacitor connected in parallel with the secondary winding via a
diode for rectifying and filtering the output of the secondary
winding, and wherein the turns ratio is set for a reduced reverse
voltage on the diode.
10. The power supply of claim 9, wherein the diode is a Schottky
diode.
11. A power supply having an output voltage, comprising: at least
three voltage sources, each voltage source supplying a DC voltage
and a current that is switched on and off by a switch; a
transformer comprising a primary winding connected to each of the
voltage sources, and a secondary winding, where the secondary
winding has an output for supplying the output voltage and where
the primary windings have turns ratios that are proportional to
voltage ratios of the voltage sources; and a pulse-width modulator
for switching the switch of each of the voltage sources on and off
at a duty cycle to control the output voltage.
12. The power supply of claim 11, wherein each of the DC voltages
varies over a voltage range.
13. The power supply of claim 12, wherein the output voltage is
substantially supplied from the voltage source that is
proportionally higher in voltage than the other voltage
sources.
14. The power supply of claims 12 or 13, wherein the voltage ranges
do not overlap.
15. The power supply of any of claims 11 to 14, further comprising
a current limiting system to limit the current of each of the
voltage sources.
16. The power supply of claim 15, wherein the current limiting
system comprises a resistor to generate a voltage drop for sensing
a combination of the currents of the voltage sources by the
pulse-width modulator, where the pulse-width modulator varies the
duty cycle to limit the currents.
17. The power supply of any of claims 12 to 14, wherein each of the
voltage ranges have a peak voltage and which peak voltages form a
peak range, and further comprising a current limiting system to
limit the currents of the voltage sources that comprises at least
three resistors where the currents of the voltage sources having
greater peak voltages passes through more of the resistors such
that the pulse-width modulator senses a voltage drop across the
resistors and accordingly changes the duty cycle to limit
current.
18. The power supply of claim 17, wherein the resistors are scaled
such that the voltage drop sensed by the pulse-width modulator is
higher for a given current from the voltage sources with a higher
peak voltage.
19. The power supply of any of claims 11 to 18, further comprising
a capacitor connected in parallel with the secondary winding via a
diode for rectifying and filtering the output of the secondary
winding, and wherein the turns ratios are set for a reduced reverse
voltage on the diode.
20. The power supply of claim 19, wherein the diode is a Schottky
diode.
Description
FIELD OF THE INVENTION
[0001] The present invention relates in general to power supplies
and more specifically to a switching power supply.
[0002] In the field IP telephony equipment, an IP phone can be
powered by a fixed-frequency, switching power supply employing an
industry-standard discontinuous mode flyback topology. With this
topology, the power supply output voltage is regulated by
controlling the pulse width of the switching waveform in response
to changes in both a source voltage and a power supply load. The
pulse width control method is commonly known as pulse-width
modulation (PWM). Inherent with this method is a potential for very
narrow pulse widths at a maximum input voltage and a minimum load
when the input operating voltage range is wide. However, the
problem with narrow pulse widths is that integrated power supply
controllers have a certain propagation delay from their control
inputs to their power switch control outputs and the power switch
also suffers from its own delay. These delays can become
significant at narrow pulse widths, particularly when the switching
frequency is high and, as a result, the period of each cycle is
relatively short.
[0003] There are other problems which arise from propagation delays
on narrow pulse widths. Firstly, output voltage may be difficult to
regulate at high input voltage and light load. Also, a peak primary
current limit threshold that is suitable for the rated load at the
minimum input voltage results in an excessive output load current
at the maximum input voltage.
[0004] There are also problems that result from a wide input
operating voltage range. One problem is that the blocking voltage
rating of power supply output diodes may need to be high which
preclude the use of Schottky diodes. Schottky diodes are beneficial
for use at low output voltages due to their forward voltage drop,
which is lower than standard fast-recovery diodes. This lower
voltage drop results in a more efficient power supply, however, it
also results in a lower reverse voltage rating. Standard Schottky
diodes send to have a maximum reverse voltage rating of 40 V.
Another problem is that the switching device must have both a high
voltage rating, for operation at the high end of an input voltage
range, and a high current rating, to conduct the larger currents
associated with operation at a low end of the input voltage range.
For efficient power supply operation this combination of high
voltage and high current rating in a single device may necessitate
the use of a physically larger and more expensive component than
would otherwise be required if the input voltage range were
narrower.
[0005] Presently, with respect to IP phone applications; the power
supply is required to operate from two independent voltage sources,
VSL and VSH, which have distinctly different voltage ranges. VSL
provides a voltage range from SVDC to 22 VDC while VSH provides a
voltage range from 22 VDC to 56 VDC. These voltage ranges result in
operation of the power supply over a source voltage range from
about 8 VDC to 56 VDC. For a given power supply load, this 7:1
range in input voltage results in a 7:1 range of PWM pulse width.
The 7:1 PWM pulse width ratio, in turn, results in the problems
associated with power supply operation using narrow pulse widths
and wide input operating voltage range described above.
[0006] Prior art techniques have combined the two voltage sources
together through coupling diodes to form a single voltage bus
having an operating range spanning that of the two sources
combined, which in this case would be 8 VDC to 56 VDC. A wide input
voltage range power converter converts the bus voltage to the
voltage required by the load. This has been implemented with
standard power converter topologies such as the buck converter and
the flyback converter. However, this technique still suffers from
the problems described above.
SUMMARY OF THE INVENTION
[0007] The present invention is a switching power supply that
improves light load regulation at a high input voltage by doubling
the pulse width, thereby overcoming, the problems of in the prior
art. The present invention also improves the current limit
performance at a high input voltage, provides lower reverse
voltages for an output diode and optimizes the use of switching
devices.
[0008] This apparatus of the present invention comprises two
distinct voltage buses and an isolating transformer having two
primary windings, each with its own associated switching device.
Each winding is fed from its own voltage bus with the number of
turns on each winding chosen to be proportional to the magnitude of
its particular voltage bus. In tis way, at any given time, power is
supplied from whichever of the two buses is proportionately higher
in voltage. The power supply operates from either voltage source
alone, or with both sources present simultaneously, with
transitions between sources being transparent to the output.
[0009] By keeping the two voltage buses separate, the duty cycle
range for a given load in this phone application is reduced to
2.75:1 for the low voltage bus and an even lower 2.55:1 for the
high voltage bus. This results in the minimum pulse width being
over twice as wide as that associated with the 7:1 duty cycle
range. This doubling of the pulse width significantly reduces the
impact of controller and switching device delays and thus
significantly improves light load regulation at high input
voltage.
[0010] Also, the reduction of the impact of controller and
switching device delays improves the current limit performance at
high input voltage.
[0011] Another advantage of the present invention is that
utilization of two, narrow, input voltage ranges allow the
transformer turns ratios to be adjusted such that the output diode
is subjected to a lower reverse voltage. This allows the use of
Schottky diodes fur output voltage rails of up to 5 V. The benefit
of using a Schottky diode is lower power dissipation and reduced
component stress.
[0012] Finally, having two switching devices allows each device to
be chosen so that its parameters are optimized for its particular
operating conditions. For example, the low voltage bus device may
conduct a high current without having to withstand a high voltage.
The opposite is true for the high voltage bus device. This aligns
well with switching device technology where the most easily
fabricated, and therefore less expensive, devices optimize one
parameter, either voltage or current, at the expense of the other.
Thus to smaller devices, each optimized for their particular
operating conditions, can replace one physically larger, more
expensive, device.
[0013] According to an aspect of the present invention, there is
provided a power supply having an output voltage, comprising: a
first voltage source supplying a first DC voltage that is switched
on and off by a first switch; a second, voltage source supplying a
second DC voltage that is switched on and off by a second switch; a
transformer comprising a first primary winding connected to the
first voltage source, a second primary winding connected to the
second voltage source, and a secondary winding, where the secondary
winding has an output for supplying the output voltage and where
the first primary winding to the second primary winding has a turns
ratio that is proportional to a voltage ratio of the first voltage
source to the second voltage source; and a pulse-width modulator
for switching the first switch and the second switch on and off at
a duty cycle to control the output voltage.
[0014] According to another aspect of the present invention, there
is provided A power supply having an output voltage, comprising: at
least three voltage sources, each voltage source supplying a DC
voltage and a current that is switched on and off by a switch; a
transformer comprising a primary winding connected to each of the
voltage sources, and a secondary winding, where the secondary
winding has an output for supplying the output voltage and where
the primary windings have turns ratios that are proportional to
voltage ratios of the voltage sources; and a pulse-width modulator
for switching the switch of each of the voltage sources on and off
at a duty cycle to control the output voltage.
GENERAL DESCRIPTION OF THE DETAILED DRAWINGS
[0015] Embodiments of the present invention are described below
with reference to the accompanying drawings, in which:
[0016] FIG. 1 is a schematic diagram of a first embodiment of a
switching power supply of the present invention; and
[0017] FIG. 2 is a schematic diagram of a second embodiment of a
switching power supply.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0018] A schematic diagram of the switching power supply is shown
in FIG. 1. The flyback switching power supply 10 comprises a pulse
width modulator (PWM) IC 12 and a transformer 16 having two primary
windings, W.sub.Low and W.sub.High, and a secondary winding
(W.sub.Out) 18. The primary winding W.sub.Low and W.sub.High are
connected to two separate voltage buses 20 and 22. Each of the
primary windings W.sub.Low and W.sub.High is connected to
associated switches Q.sub.Low and Q.sub.High. The switches
Q.sub.Low and Q.sub.High are driven by the PWM IC 12, which also
senses current via a resistor 24 to control the current. The
secondary winding 18 is connected in parallel with a capacitor 28
via an output diode 26. The capacitor 28 and the diode 26 function
to rectify and filter an output voltage (V.sub.out). As will be
understood by one skilled in the art, the black dots within the
windings W.sub.Low. W.sub.High and 18 represent the positive
terminal. PWM IC 12 further senses the V.sub.out (not shown) to
accordingly control such as will be understood by one skilled in
the art.
[0019] During operation, the signal output from IC 12 drives the
two switching devices Q.sub.Low and Q.sub.High at a switching
frequency such that both are either on or off at the same time.
When the switching devices Q.sub.Low and Q.sub.High are on, the
proportion of a primary current, I.sub.p, that each conducts is a
function of the voltage supplied by voltage buses 20 and 22 and the
number of winding turns, N. For example, if
N.sub.Whigh:N.sub.Wlow=2.8:1, Q.sub.High conducts significant
current only when the voltage supplied by voltage bus 22 is more
than 2.8 times higher than the voltage supplied by voltage bus 20.
Similarly, when, voltage bus 22 provides a voltage that is
substantially lower than 2.8 times the voltage voltage bus 20,
Q.sub.Low conducts the current. There is also a transition range
slightly above and below the 2.8 times voltage source multiplier
when both devices and their respective windings share the primary
current, I.sub.p.
[0020] When the switching devices Q.sub.Low and Q.sub.High are off,
both must withstand a flyback voltage. The flyback voltage is equal
to the output voltage multiplied by the primary to secondary turns
ratio pluse the input bus voltage, or
V.sub.Flyback=V.sub.Wout.times.N.sub.primary/N.sub.Wout+V.sub.S
[0021] where N.sub.primary =N.sub.Whigh and N.sub.Wflow for
Q.sub.High and Q.sub.Low respectively; and
[0022] V.sub.Wout-V.sub.out/V.sub.Dout; and
[0023] VS=V.sub.SHigh+V.sub.SLow (the voltages supplied by voltage
buses 22 and 20 respectively).
[0024] Therefore, with V.sub.out=5 V, V.sub.Dout=0.3 V,
N.sub.WHigh:N.sub.Wout=2.15:1, and N.sub.WLow:N.sub.Wout=0.77:1,
the switching devices Q.sub.Low and Q.sub.High are subjected to
maximum voltages of 26.1 V and 67.4 V respectively, at a ratio of
2.58;1. With respect to current, assuming comparable efficiency,
the ratio between the maximum currents conducted by Q.sub.Low and
Q.sub.High is inversely proportional to their respective minimum
input bus voltages supplied by voltage buses 20 and 22. For
example, with voltage bus 20 supplying a voltage of 8 V and voltage
bus 22 supplying a voltage of 22 V, the maximum current conducted
by Q.sub.Low is therefore 22/8=2.75 times the current conducted by
Q.sub.High. Thus, in comparing the two switching devices, Q.sub.Low
must carry 2.75 times the current, but Q.sub.High must withstand
2.58 times the voltage. Therefore, each switching device Q.sub.Low
or Q.sub.High can be chosen accordingly to optimize the power
supply design.
[0025] When Q.sub.Low and Q.sub.High are on, the output diode 26 is
off and is required to withstand a reverse voltage equal to the
input bus voltage multiplied by the primary to secondary turns
ratio plus the output voltage. Therefore, if the voltage on voltage
bus 22 is 56 V, output diode 26 is required to block a reverse
voltage equal to 56/2.15+5=31 V, and if the voltage on voltage bus
20 is 22 V, the output diode 26 is required to block a reverse
voltage of 22/0.77+5=33.6 V. As will be understood, since both
voltages are below 40 V, a standard Schottky diode can be employed
as the output diode 26.
[0026] In another embodiment, a two-resistor current sense network
is implemented to tailor current limit to the voltage bug that is
predominant at any given time. With reference to FIG. 2, the switch
Q.sub.High current is sensed by the PWM IC 12 via a pair of
resistors 32 and 34. The Q.sub.Low switching device current is
sensed by the PWM IC 12 via only the second resistor 34. When
switch Q.sub.Low is conducting, a current I.sub.Low flow through
the second resistor 34 and generates a voltage (V.sub.sense) that
is sensed by the PWM IC 12. The PWM IC 12 adjusts this voltage, as
necessary, to control the pulse width, and also to fix a maximum
value for the voltage that establishes a primary current limit
threshold. When switch Q.sub.High is conducting, a current
I.sub.High flows through a higher resistance value formed by the
sum of the pair of resistors 32 and 34. Since the resistance value
is higher but the maximum voltage value (V.sub.semsemax) remains
unchanged, a lower peak current limit value for the high voltage
source range threshold is achieved. This lower peak current limit
counteracts the tendency for the output current limit value to
increase with input voltage and reduces component stress under a
high input voltage overload of the output.
[0027] It will be appreciated that, although only two embodiments
of the invention have been described and illustrated in detail,
various changes and modification may be made. For example,
additional input sources can be accommodated by adding one primary
winding and one switching device for each source. This can be used
wherever a low-power, multi-source power supply is required and
would be suitable for both isolated as well as non-isolated
applications. All such changes and modifications may be made
without departing from the sphere and scope of the invention as
defined by the claims appended herein.
* * * * *