Switching regulator with reduced conducted emissions

1. A switching regulator comprising:

a transformer having a primary winding and a secondary winding;

a fixed-frequency switching element connected in series with said primary winding;

a controller provided to control said switching element;

a rectifying circuit connected to said secondary winding; and

a noise source injecting a signal into said rectifying circuit to reduce emissions,

wherein said noise source injects a random noise signal into said rectifying circuit to create a ripple voltage in an output voltage of said switching regulator.

2. The switching regulator of claim 1 , wherein a duty cycle of said fixed-frequency switching element is modified by said random noise signal.

3. The switching regulator of claim 2 , wherein said random noise signal is between 50 and 75 mV.

4. The switching regulator of claim 1 , further comprising an optical isolator circuit to provide closed-loop control of the switching regulator.

5. A switching regulator comprising:

a transformer having a primary winding and a secondary winding;

a controller having a fixed-frequency switching element, a duty cycle of said fixed-frequency switching element being determined in accordance with a closed-loop feedback circuit;

a rectifying circuit connected to said secondary winding; and

a random noise signal injected into said rectifying circuit, said random noise signal creating a ripple voltage in an output voltage of said switching regulator.

6. The switching regulator of claim 5 , wherein the duty cycle of said fixed-frequency switching element is modified by said random noise signal.

7. The switching regulator of claim 6 , further comprising an optical isolator circuit to provide closed-loop control of the switching regulator.

8. The switching regulator of claim 7 , wherein said random noise signal is between 50 and 75 mV.

9. A method of controlling a fixed-frequency switching regulator, the regulator including a transformer having a primary winding and a secondary winding; a fixed-frequency switching element connected in series with said primary winding; a controller; and a rectifying circuit connected to said secondary winding, the method comprising:

injecting a noise source into said rectifying circuit;

detecting an output voltage of said switching regulator;

controlling a duty cycle of said fixed-frequency switching element in accordance with said output voltage;

generating a random noise signal as said noise source; and

creating a ripple voltage in said output voltage of said switching regulator.

10. The method of claim 9 , further comprising altering a duty cycle of said fixed-frequency switching element in response to said random noise signal.

11. The method of claim 9 , wherein said random noise signal is between 50 and 75 mV.

12. The method of claim 9 , wherein said fixed-frequency switching regulator generates broadband conducted frequency components.

13. A method of controlling a fixed-frequency switching regulator, the regulator including a transformer having a primary winding and a secondary winding; a fixed-frequency switching element connected in series with said primary winding; a controller; and a rectifying circuit connected to said secondary winding, the method comprising:

detecting an output voltage of said switching regulator;

randomly modifying an applied load receiving said output voltage, wherein randomly modifying said applied load receiving said output voltage comprises generating a random noise signal in a microcontroller, and creating a ripple voltage in said output voltage of said switching regulator by randomly pulling a pin on said microcontroller high and low; and

controlling a duty cycle of said fixed-frequency switching element in accordance with said output voltage.

14. The method of claim 13 , wherein said applied load varies approximately 30 mA.