Electricity: power supply or regulation systems – Output level responsive – Using a three or more terminal semiconductive device as the...
Reexamination Certificate
2000-03-16
2001-03-20
Berhane, Adolf Deneke (Department: 2838)
Electricity: power supply or regulation systems
Output level responsive
Using a three or more terminal semiconductive device as the...
C323S285000
Reexamination Certificate
active
06204649
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention generally relates to pulse-width modulated (PWM) power supplies; and in particular, the present invention relates to PWM power supplies with reduced electromagnetic interference (EMI).
2. Background of the Invention
Pulse-width modulated (PWM) power supplies encompass a wide variety of switching regulators having a constant switching frequency with variable “on” times (pulse widths). Adjusting the pulse widths based on a feedback voltage is used to generate a regulated dc voltage.
Exemplary circuit schematics for switching regulators can be found in Horowitz and Hill, “The Art of Electronics”, 2
nd
ed., Cambridge University Press, 1989, pages 355-367, which is incorporated herein by reference in its entirety. Some other switching regulator topologies can be found in U.S. Pat. No. 4,842,532 to Freedman, U.S. Pat. No. 4,823,070 to Nelson, U.S. Pat. No. 5,170,333 to Niwayama, and U.S. Pat. No. 5,028,861 to Pace et al., which are incorporated herein by reference in their entireties.
In brief, conventional PWM regulators include a PWM controller, one or more switching transistors, and a filter for providing the regulated voltage output. An oscillator in the PWM controller controls the switching frequency of the switching transistors.
It is well known that the switching action of a conventional switching regulator generates a substantial amount of “noise,” including conducted and radiated electromagnetic interference (EMI). Conducted EMI may be caused by output voltage or current ripple at the switching frequency. Radiated EMI is noise generated by the switching of the transistor(s) radiated into the environment at the switching frequency and its associated harmonics. Radiated EMI causes interference with radio transmissions in the vicinity of the regulator.
Because EMI noise generated by a switching regulator, either conducted or radiated, can interfere with the operation of nearby electronic equipment, design consideration of a switching regulator or its application must include careful management of spurious EMI emissions. Additionally, the Federal Communications Commission (FCC) imposes limits on the amount of conducted and radiated emission generated by electronic devices acting as unintentional radiators. (See, FCC regulations on Radio Frequency Devices, 47 C.F.R. §15.109.)
The substantial amount of spurious EMI emissions generated by conventional switching regulators sometimes limit their applications in electronic devices. Therefore, it is desirable to provide a method for reducing the EMI noise generated by switching regulators. It is further desirable to provide a switching regulator where unwanted EMI emissions are reduced to a level which meets the FCC regulations so that the switching regulator can be applied to a wide-range of electronic applications such as cellular telephones and hand-held instruments.
SUMMARY
A switching regulator is described having reduced electromagnetic interference (EMI). The regulator includes a PWM controller using a varying frequency oscillator for controlling the operating frequency of the switching regulator. The varying frequency oscillator provides an oscillating signal having a continuously varying frequency. The oscillating signal of the varying frequency oscillator varies about the center switching frequency of the switching regulator, causing the frequency components of the EMI noise to spread out, reducing the peak energy of the EMI emissions.
In accordance with the method and apparatus of the present invention, both the conducted and radiated EMI of the switching regulator are reduced.
In one embodiment of the present invention, the frequency of the varying frequency oscillator varies at a rate greater than about 20 kHz (above audio limits). The frequency of the oscillating signal varies within a frequency range of greater than ±10% of the center switching frequency. The oscillating signal can be one of a ramp signal, a sawtooth signal, a sine wave, or a square wave.
In one embodiment of the present invention, the varying frequency oscillator is implemented as a voltage controlled oscillator. The voltage controlled oscillator receives a voltage control input signal which is the sum of two voltage signals. The first voltage signal defines the center frequency of the voltage controlled oscillator. The second voltage signal defines the frequency variation rate and frequency variation range of the output signal of the voltage controlled oscillator.
In another embodiment, the varying frequency oscillator is implemented as a low frequency voltage controlled oscillator coupled to a waveform generator for generating an oscillating signal having a continously varying frequency.
The varying frequency oscillator may be external to a PWM controller integrated circuit (IC), such as connected to a sync pin or clock pin of the IC, or the oscillator may be formed on the PWM controller IC itself.
The present invention is better understood upon consideration of the detailed description below and the accompanying drawings.
REFERENCES:
patent: 4460951 (1984-07-01), Fenter et al.
patent: 4823070 (1989-04-01), Nelson
patent: 4843532 (1989-06-01), Freedman
patent: 5028861 (1991-07-01), Pace et al.
patent: 5170333 (1992-12-01), Niwayama
patent: 5235504 (1993-08-01), Sood
patent: 5479090 (1995-12-01), Schultz
patent: 5498995 (1996-03-01), Szepesi et al.
Arbetter, et al, “DC-DC Converter Design for Battery-Operated Systems,” 26thAnnual IEEE Power Electronics Specialists Conference, vol. 1, 103-109. 1970 “no month”.
Horowitz, et al, “The Art of Electronics”, 2nded., Cambridge University Press, 1989, pp. 355-367.
Wang, et al., “Programmed pulsewidth modulator waveforms for Electromagnetic Interference,” IEEE Trans. On Power Electronics, vol. 8, No. 4, Oct. 93, pp. 596-605.
Williams, “A Monolithic Switching Regulator with 100&mgr;V Output Noise,” Linear Technology Application Note 70, Oct. 1997. 71 pages.
Code of Federal Regulations, Title 47, Part 15, Section 15.109 entitled “Radiated emission limits.”, Oct., 1998 3 pages.
Micrel MIC2178 2.5A Synchronous Buck Regulator Data Sheet. Jun. 1998, pp. 1-16.
Berhane Adolf Deneke
Cook Carmen C.
Micrel Incorporated
Skjerven Morrill & MacPherson LLP
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