Pulse width modulation frequency dithering in a switch mode power supply
View Patent ↗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.
1. 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.
2. The switch mode power supply according to claim 1 , 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.
3. The switch mode power supply according to claim 2 , 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.
4. The switch mode power supply according to claim 2 , wherein the roll count value changes randomly.
5. 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.
6. The PWM frequency dithering apparatus 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 PWM frequency dithering apparatus according to claim 5 , wherein the roll count value changes randomly.
8. 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.
9. The method according to claim 8 , 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.
10. The switch mode power supply according to claim 8 , wherein the step of changing the roll count value comprises the step of randomly changing the roll count value.