U.S. patent application number 11/215622 was filed with the patent office on 2007-03-01 for pulse width modulation frequency dithering in a switch mode power supply.
This patent application is currently assigned to Microchip Technology Incorporated. Invention is credited to Bryan Kris.
Application Number | 20070047272 11/215622 |
Document ID | / |
Family ID | 37440918 |
Filed Date | 2007-03-01 |
United States Patent
Application |
20070047272 |
Kind Code |
A1 |
Kris; Bryan |
March 1, 2007 |
PULSE WIDTH MODULATION FREQUENCY DITHERING IN A SWITCH MODE POWER
SUPPLY
Abstract
A switch mode power supply has pulse width modulation (PWM)
frequency dithering of a PWM clock frequency. The PWM frequency
dithering circuit may change the frequency of the PWM clock based
upon each of a plurality of frequencies. A PWM time base circuit
may comprise a period register containing a PWM period value, a
comparator, and a PWM counter, wherein a PWM count value may be
incremented in the PWM counter by the variable frequency PWM clock,
the comparator may compare the PWM period value with the PWM count
value and when the PWM period value and the PWM count value are
substantially equal the PWM count value may be reset. The PWM
frequency dithering circuit may comprise a roll counter, wherein
the roll counter changes a roll count value each time the
comparator resets the PWM count value in the PWM counter; a
multiplexer having a plurality of inputs and an output, wherein the
output is coupled to each one of the plurality of inputs of the
multiplexer based upon the roll counter count value; and a
plurality of frequency registers, each one of the plurality of
frequency registers may be coupled to a respective one of the
plurality of inputs of the multiplexer; wherein the output of the
multiplexer may be coupled to a frequency control input of the
variable frequency PWM clock such that frequency values stored in
the plurality of frequency registers may be used in determining the
variable frequency PWM clock frequency.
Inventors: |
Kris; Bryan; (Phoenix,
AZ) |
Correspondence
Address: |
BAKER BOTTS, LLP
910 LOUISIANA
HOUSTON
TX
77002-4995
US
|
Assignee: |
Microchip Technology
Incorporated
|
Family ID: |
37440918 |
Appl. No.: |
11/215622 |
Filed: |
August 30, 2005 |
Current U.S.
Class: |
363/41 |
Current CPC
Class: |
H02M 1/44 20130101; H02M
1/12 20130101 |
Class at
Publication: |
363/041 |
International
Class: |
H02M 1/12 20060101
H02M001/12 |
Claims
1-3. (canceled)
4. A switch mode power supply having pulse width modulation (PWM)
frequency dithering comprising: a power charging circuit comprising
at least one power switching element; a PWM control circuit coupled
to and controlling the power charging circuit, wherein the PWM
control circuit has a PWM time base circuit comprising: a period
register containing a PWM period value; a comparator; and a PWM
counter, wherein a PWM count value is incremented in the PWM
counter by a variable frequency PWM clock, the comparator compares
the PWM period value with the PWM count value and when the PWM
period value and the PWM count value are substantially equal the
PWM count value is reset; and a PWM frequency dithering circuit,
wherein the PWM frequency dithering circuit changes the variable
frequency PWM clock frequency to each of a plurality of
frequencies.
5. The switch mode power supply according to claim 4, wherein the
PWM frequency dithering circuit comprises: a roll counter, wherein
the roll counter changes a roll count value each time the
comparator resets the PWM count value in the PWM counter; a
multiplexer having a plurality of inputs and an output, wherein the
output is coupled to each one of the plurality of inputs of the
multiplexer based upon the roll counter count value; and a
plurality of frequency registers, each one of the plurality of
frequency registers is coupled to a respective one of the plurality
of inputs of the multiplexer; wherein the output of the multiplexer
is coupled to a frequency control input of the variable frequency
PWM clock such that frequency values stored in the plurality of
frequency registers are used in determining the variable frequency
PWM clock frequency.
6. The switch mode power supply according to claim 5, wherein the
roll counter increases the roll count value sequentially until at a
maximum roll count value then the roll count value is reset to a
minimum roll count value.
7. The switch mode power supply according to claim 5, wherein the
roll count value changes randomly.
8. The switch mode power supply according to claim 1, wherein the
variable frequency PWM clock comprises a resistance-capacitance
(RC) circuit.
9. The switch mode power supply according to claim 1, wherein the
variable frequency PWM clock comprises a divide-by-N
phase-locked-loop circuit.
10. A pulse width modulation (PWM) frequency dithering apparatus
for controlling a plurality of PWM clock frequencies, said PWM
dithering apparatus comprising: a roll counter, wherein the roll
counter changes a roll count value each time a PWM count value is
substantially equal to a PWM count value; a multiplexer having a
plurality of inputs and an output, wherein the output is coupled to
each one of the plurality of inputs of the multiplexer based upon
the roll counter count value; and a plurality of frequency
registers, each one of the plurality of frequency registers is
coupled to a respective one of the plurality of inputs of the
multiplexer; wherein the output of the multiplexer is coupled to a
frequency control input of a variable frequency PWM clock such that
frequency values stored in the plurality of frequency registers are
used in determining the variable frequency PWM clock frequency.
11. The PWM frequency dithering apparatus according to claim 10,
wherein the roll counter increases the roll count value
sequentially until at a maximum roll count value then the roll
count value is reset to a minimum roll count value.
12. The PWM frequency dithering apparatus according to claim 10,
wherein the roll count value changes randomly.
13. A method for frequency dithering a pulse width modulation (PWM)
clock frequency of a switch mode power supply, said method
comprising: incrementing a PWM count value for each frequency cycle
of a PWM clock; comparing the PWM count value with a period value
until the PWM count value is substantially the same as the period
value then resetting the PWM count value; changing a roll count
value each time the PWM count value is reset; selecting from a
plurality of frequency values based upon the roll count value; and
adjusting the frequency of the PWM clock based upon the selected
one of the plurality of frequency values.
14. The method according to claim 13, wherein the step of changing
the roll count value comprises the step of sequentially
incrementing the roll counter value until at a maximum roll count
value then resetting the roll count value to a minimum roll count
value.
15. The switch mode power supply according to claim 13, wherein the
step of changing the roll count value comprises the step of
randomly changing the roll count value.
Description
TECHNICAL FIELD
[0001] The present disclosure, according to one embodiment, relates
to switching regulator power supplies, and more particularly, to
pulse width modulation frequency dithering in the switching
regulator power supply.
BACKGROUND
[0002] Power supply manufacturers must meet government (e.g.,
Federal Communications Commission) and customer electromagnetic
interference (EMI) emission requirements when selling switch mode
power supplies. A switch mode power supply may use pulse width
modulation (PWM) to control a switch, e.g., power transistor, power
field effect transistor, etc., that may charge an inductor to a
desired current. The longer the PWM signal is on, the longer the
inductor is charging through the control switch and the more
current that may be supplied from the switch mode power supply.
[0003] In the larger capacity switch mode power supplies, the power
transistors generate electromagnetic interference (EMI). These
larger capacity switch mode power supplies must be tested for and
pass stringent EMI tests that limit the amount of radio frequency
energy at a particular frequency for a certain length of time. If
the switch mode power supplies cannot meet these EMI test
requirements, they cannot be certified and/or type accepted by the
appropriate testing organization and/or government agency.
SUMMARY
[0004] What is needed in a larger capacity switch mode power supply
is some way to limit the amount of EMI energy generated at any
particular frequency for longer than a desired time period.
[0005] According to a specific example embodiment of the present
disclosure, a switch mode power supply having pulse width
modulation (PWM) frequency dithering may comprise a power charging
circuit having at least one power switching element, a PWM control
circuit coupled to and controlling the power charging circuit, and
a PWM frequency dithering circuit, wherein the PWM frequency
dithering circuit may change a variable frequency PWM clock
frequency to each of a plurality of frequencies. The PWM control
circuit may have a PWM time base circuit comprising a period
register containing a PWM period value, a comparator, and a PWM
counter, wherein a PWM count value may be incremented in the PWM
counter by the variable frequency PWM clock, the comparator may
compare the PWM period value with the PWM count value and when the
PWM period value and the PWM count value are substantially equal
the PWM count value may be reset. The PWM frequency dithering
circuit may comprise a roll counter, wherein the roll counter
changes a roll count value each time the comparator resets the PWM
count value in the PWM counter; a multiplexer having a plurality of
inputs and an output, wherein the output is coupled to each one of
the plurality of inputs of the multiplexer based upon the roll
counter count value; and a plurality of frequency registers, each
one of the plurality of frequency registers may be coupled to a
respective one of the plurality of inputs of the multiplexer;
wherein the output of the multiplexer may be coupled to a frequency
control input of the variable frequency PWM clock such that
frequency values stored in the plurality of frequency registers may
be used in determining the variable frequency PWM clock frequency.
The roll counter may increase the roll count value sequentially
until at a maximum roll count value then the roll count value may
reset to a minimum roll count value. The roll count value may also
change randomly.
[0006] According to another specific example embodiment of the
present disclosure, a pulse width modulation (PWM) frequency
dithering apparatus for controlling a plurality of PWM clock
frequencies, may comprise a roll counter, wherein the roll counter
changes a roll count value each time a PWM count value is
substantially equal to a PWM count value; a multiplexer having a
plurality of inputs and an output, wherein the output is coupled to
each one of the plurality of inputs of the multiplexer based upon
the roll counter count value; and a plurality of frequency
registers, each one of the plurality of frequency registers may be
coupled to a respective one of the plurality of inputs of the
multiplexer, wherein the output of the multiplexer may be coupled
to a frequency control input of a variable frequency PWM clock such
that frequency values stored in the plurality of frequency
registers may be used in determining the variable frequency PWM
clock frequency. The roll counter may increase the roll count value
sequentially until at a maximum roll count value then the roll
count value may be reset to a minimum roll count value. The roll
count value may also change randomly.
[0007] According to yet another specific example embodiment of the
present disclosure, a method for frequency dithering a pulse width
modulation (PWM) clock frequency of a switch mode power supply may
comprise incrementing a PWM count value for each frequency cycle of
a PWM clock; comparing the PWM count value with a period value
until the PWM count value is substantially the same as the period
value then resetting the PWM count value; changing a roll count
value each time the PWM count value is reset; selecting from a
plurality of frequency values based upon the roll count value; and
adjusting the frequency of the PWM clock based upon the selected
one of the plurality of frequency values.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] A more complete understanding of the present disclosure
thereof may be acquired by referring to the following description
taken in conjunction with the accompanying drawings wherein:
[0009] FIG. 1 is a schematic block diagram of an electronic system
powered by a switch mode power supply;
[0010] FIG. 2 is a schematic block diagram of switch mode power
supply having a PWM frequency dithering circuit; and
[0011] FIG. 3 is a more detailed schematic block diagram of the PWM
frequency dithering circuit of FIG. 2, according to a specific
example embodiment of the present disclosure.
[0012] While the present disclosure is susceptible to various
modifications and alternative forms, specific example embodiments
thereof have been shown in the drawings and are herein described in
detail. It should be understood, however, that the description
herein of specific example embodiments is not intended to limit the
disclosure to the particular forms disclosed herein, but on the
contrary, this disclosure is to cover all modifications and
equivalents as defined by the appended claims.
DETAILED DESCRIPTION
[0013] In switch mode power supplies, the power transistors may
generate electromagnetic interference (EMI). Therefore, it is
desirable to limit the EMI to meet customer needs and government
regulations. Dithering the pulse width modulation PWM period sweeps
the EMI over a range of frequencies thus lowering peak EMI
emissions at any one frequency.
[0014] Referring now to the drawings, the details of example
embodiments are schematically illustrated. Like elements in the
drawings will be represented by like numbers, and similar elements
will be represented by like numbers with a different lower case
letter suffix.
[0015] Referring to FIG. 1, depicted is a schematic block diagram
of an electronic system powered by a switch mode power supply. An
electronic system, generally represented by the numeral 100, may
comprise a power supply, e.g., switch mode power supply 102 that
may supply operating voltages and currents to electronic circuits
104 of the electronic system 100. The switch mode power supply 102
may convert a power source voltage 106, e.g., 120 VAC, 48 VDC,
etc., to all required operating voltages used by the electronic
circuits 104.
[0016] Referring to FIG. 2, depicted is a schematic block diagram
of switch mode power supply having a PWM frequency dithering
circuit. The switch mode power supply 102 may comprise a power
charging circuit 210, a PWM control circuit 208, and a PWM
frequency dithering circuit 212. The power charging circuit 210 may
be controlled by the PWM control circuit 208. The PWM frequency
dithering circuit 212 may vary a PWM clock frequency to the PWM
control circuit 208. The power charging circuit may comprise at
least one power switching element (not shown), e.g., power
transistor, power field effect transistor, etc.
[0017] Referring to FIG. 3, depicted is a schematic block diagram
of a PWM frequency dithering circuit 212, according to a specific
example embodiment of the present disclosure. The PWM frequency
dithering circuit 212 may comprise a PWM time base circuit 326
having a period register 320, a comparator 322, and a PWM counter
324; a roll counter 330, a variable frequency oscillator (VFO) 334,
a multiplexer 332 and a plurality of frequency registers 336 having
frequency values stored therein. These frequency values may be user
specified. Values at the period register 320 output 338 and the PWM
counter 324 output 342 may be compared in the comparator 322, and
when they are substantially equal the comparator 322 output 340 may
cause (at reset input 346) the PWM counter 324 to reset, and may
increment (at clock input 328) the roll counter 330. The comparator
322 compares the PWM time base counter 324 with a PWM period value
that may be stored in the period register 320. The PWM period value
may be user specified. When the PWM counter 324 reaches the
specified maximum count value stored in the period register 320,
the PWM period has been completed, and the PWM time base counter
324 is reset so the PWM cycle may start again.
[0018] The roll counter 330 may control which input of the
multiplexer 332 is coupled to its output 348 as determined at the
multiplexer select input 350. Depending upon the select value at
the multiplexer select input 350, the frequency value stored in the
selected one of the plurality of frequency registers 336 may be
applied to the frequency adjustment input 352 of the VFO 334. The
multiplexer 332 selects a desired frequency value from the
plurality of frequency registers 336 that contain frequency values
that may be specified by a user. The selected frequency value may
control a timing element in the VFO 334, e.g., current source of an
RC oscillator, divide-by-N phase-locked-loop (PLL), etc. The VFO
334 may then generate a PWM clock output 354 having a frequency
corresponding substantially to the frequency value from the
selected one of the plurality of frequency registers 336. The VFO
334 supplies this clock signal from the PWM clock output 354 to the
PWM time base counter 324 of the PWM time base 326. The PWM clock
output 354 may increment the PWM counter 324 through the clock
input 344 of the PWM counter 324.
[0019] The roll counter 330 may be clocked (incremented) every time
the PWM time base 326 reaches maximum (terminal) count. Therefore,
at the end of each PWM cycle, the frequency of the PWM clock (VFO
334 output 354) is changed to a next one of the frequency values
stored in the plurality of frequency registers 336. The roll
counter 330 may also comprise a pseudo random number generator
wherein the count value of the roll counter 330 may be randomly
generated numbers within the bit range (number of bits) of the
multiplexer select input 350 of the multiplexer 332.
[0020] As the frequency of the PWM clock (VFO 334 output 354) is
varied, the radiated EMI energy from the switching power
transistors (not shown) is spread over a range of frequencies. This
reduces the time average EMI energy at any specific frequency, thus
allowing EMI specifications to be more easily met.
[0021] While embodiments of this disclosure have been depicted,
described, and are defined by reference to example embodiments of
the disclosure, such references do not imply a limitation on the
disclosure, and no such limitation is to be inferred. The subject
matter disclosed is capable of considerable modification,
alteration, and equivalents in form and function, as will occur to
those ordinarily skilled in the pertinent art and having the
benefit of this disclosure. The depicted and described embodiments
of this disclosure are examples only, and are not exhaustive of the
scope of the disclosure.
* * * * *