Electrical computers and digital processing systems: support – Clock – pulse – or timing signal generation or analysis – Multiple or variable intervals or frequencies
Reexamination Certificate
1998-11-16
2001-06-19
Butler, Dennis M. (Department: 2182)
Electrical computers and digital processing systems: support
Clock, pulse, or timing signal generation or analysis
Multiple or variable intervals or frequencies
C713S300000, C713S503000
Reexamination Certificate
active
06249876
ABSTRACT:
BACKGROUND
The present invention relates to an off-line switched mode control system with frequency jittering.
Many products rely on advanced electronic components to cost-effectively provide the product with the desired functionality. These electronic components require power regulation circuitry to supply them with a clean and steady source of power. The development of switched mode power supply technology has led to power supplies operating at high frequency to achieve small size and high efficiency. Each switched mode power supply typically relies on an oscillator switching at a fixed switching frequency or alternatively a variable frequency (such as in a ringing choke power supply).
Due to the high frequency operation relative to the frequency of an alternating current (AC) power line, switched mode power supplies can exacerbate problems associated with electromagnetic interference (EMI). EMI noise is generated when voltage and current are modulated by the switching power supply. This electrical noise can be transferred to the AC power line.
In addition to affecting the operation of other electronics within the vicinity of the power supply by conduction, EMI induced noise on a power line may radiate or leak from the power line and affect equipment which is not even connected to the power line. Both conducted and radiated electrical noise may adversely affect or interfere with the operation of the electronic equipment. For example, EMI noise generated by the switching power supply can cause problems for communication devices in the vicinity of the power supply. Radiated high frequency noise components may become a part of the AC mains signal and may be provided to other devices in the power grid. Further, power supply radiated EMI can interfere with radio and television transmissions.
To address EMI related interference, several specifications have been developed by government agencies in the United States and in the European Community. These agencies have established specifications that define the maximum amount of EMI that can be produced by various classes of electronic devices. Since power supplies generate a major component of the EMI for electronic devices, an important step in designing such supplies that conform to the specifications is to minimize EMI emission to the acceptable limits of the various specifications.
EMI may be reduced in a power supply by adding snubbers and input filters. These components reduce the noise transferred to the power line and by so doing, also reduce the electric and magnetic fields of noise generated by the power line. While these methods can reduce EMI, they usually complicate the design process as well as increase the production cost. In practice, noise filtering components are added in an ad hoc manner and on a trial-and-error basis during the final design process when EMI is found to exceed the compliance limits specified by the regulatory agencies. This inevitably adds unexpected costs to the products.
Further, extra components can undesirably increase the size and weight of the power supply and thus the resulting product.
SUMMARY OF THE INVENTION
EMI emission is reduced by jittering the switching frequency of a switched mode power supply. In one aspect, a frequency jittering circuit varies the switching frequency using an oscillator for generating a switching frequency signal, the oscillator having a control input for varying the switching frequency. A digital to analog converter is connected to the control input for varying the switching frequency, and a counter is connected to the output of the oscillator and to the digital to analog converter. The counter causes the digital to analog converter to adjust the control input and to vary the switching frequency.
Implementations of the invention include one or more of the following. The oscillator has a primary current source connected to the oscillator control input. A differential switch may be used with first and second transistors connected to the primary current source; a third transistor connected to the first transistor; and a fourth transistor connected to the second transistor at a junction. A capacitor and one or more comparators may be connected to the junction. The digital to analog converter has one or more current sources, with a transistor connected to each current source and to the counter. The primary current source may generate a current I and each of the current sources may generate a current lower than I. The current sources may generate binary weighted currents. The largest current source may generate a current which is less than about 0.1 of I.
In a second aspect, a method for generating a switching frequency in a power conversion system includes generating a primary current; cycling one or more secondary current sources to generate a secondary current which varies over time; and supplying the primary and secondary currents to a control input of an oscillator for generating a switching frequency which is varied over time.
Implementations of the invention include one or more of the following. A counter may be clocked with the output of the oscillator. The primary current may be generated by a current source. If the primary current is I, each of the secondary current sources may generate a supplemental current lower than I and which is passed to the oscillator control input. The supplemental current may be binary-weighted. The largest supplemental current may be less than approximately 0.1 of I.
In another aspect, a method for generating a switching frequency in a power conversion system includes generating a primary voltage; cycling one or more secondary voltage sources to generate a secondary voltage which varies over time; and supplying the primary and secondary voltages to a control input of a voltage-controlled oscillator for generating a switching frequency which is varied over time.
Implementations of the invention include one or more of the following. Where the primary voltage is V, each of the secondary voltage sources may generate a supplemental voltage lower than V which may be passed to the voltage-controlled oscillator. The supplemental voltage may be binary-weighted.
In another aspect, a frequency jittering circuit for varying a power supply switching frequency includes an oscillator for generating a switching frequency signal, the oscillator having a control input for varying the switching frequency; and means connected to the control input for varying the switching frequency.
Implementations of the invention include one or more of the following. The means for varying the frequency may include one or more current sources connected to the control input; and a counter connected to the output of the oscillator and to the one or more current sources. The oscillator may include a primary current source connected to the control input; and a differential switch connected to the primary current source. The differential switch may have first and second transistors connected to the primary current source; a third transistor connected to the first transistor; and a fourth transistor connected to the second transistor at a junction. A capacitor and a comparator may be connected to the junction. If the primary current source generates a current I, each of the current sources may generate a second current lower than the current I, further comprising a transistor connected to each current source connected to the counter. The means for varying the frequency may include one or more voltage sources connected to the control input; and a counter connected to the output of the oscillator and to the one or more voltage sources. The oscillator may include a primary voltage source connected to the control input; and a differential switch connected to the primary voltage source. The means for varying the frequency may include a capacitor; a current source adapted to charge the capacitor; and means for alternatingly charging and discharging the capacitor. One or more comparators may be connected to the capacitor and the means for alternatingly charging and discharging th
Balakrishnan Balu
Djenguerian Alex
Lund Leif
Blakely Sokoloff Taylor & Zafman LLP.
Butler Dennis M.
Power Integrations, Inc.
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