A switch mode power supply

Abstract: A method and apparatus is disclosed in which a transformer (22) which has at least one DC output and one input, and a precision timer (38) which generates a control signal having a frequency within a predetermined range of an integer multiple of the carrier frequency spacing. A switch (36) which is connected to the transformer (22) controls the input of the transformer (22). A controller (34) which is electrically connected to said DC output, said precision timer (38) and the switch (36) pulse width modulates said switch (36) at an integer multiple of the carrier frequency spacing whereby the DC output of the transformer (22) is regulated.

Description: The invention relates generally to a switch mode power supply and more specifically to preventing the RF noise generated from a switch mode power supply from interfering with the reception of a radio signal in a nearby radio receiver. The operation of a switch mode power supply generates noise impulses at frequencies which extend into the frequencies of AM broadcast frequencies. When the noise signal is received by a radio receiver, the noise signal typically demodulates into an undesirable tone in the audible range of a human ear (20 Hz - 22 KHz). One method to reduce the received noise is to locate the radio receiver away from the noise source. However, in automotive applications and, more specifically, in an automobile instrument panel, the distance a radio receiver can be separated from a switch mode power supply utilised in the instrument panel is limited. In prior art applications, the operating frequency of a switch mode power supply is controlled by timing circuits such as RC circuits. The operating frequency of such circuits tend to drift over the temperature range typically encountered in automobiles. Thus, a time variable RF noise component may be picked up by an AM receiver. US Patent 4 672 527 describes apparatus for supplying power to a system, such as a digital audio system, for sampling an information signal at a sampling frequency f s. The power supply includes a switch mode power supply having a switching frequency which is a multiple of one or more times half of the sampling frequency f s and synchronised with the sampling frequency. Fixed interfering frequencies between the sampling frequency and the switching frequency are thereby avoided in the audible range. US Patent 4 606 076 describes a communication receiver system for receiving coded signals and which has at least one predetermined intermediate frequency (IF) signal. The receiver has a DC/DC converter to provide a power supply and operates at a frequency below the IF signal. A frequency trim circuit controls the operating frequency of the converter to avoid interference with the IF signal. It is desirable to provide a switch mode power supply capable of operating in close proximity to a radio receiver without producing substantial audible noise in the demodulated output of the radio receiver. An advantage of the present invention is that a switching power supply can be placed in close proximity to an AM radio without generating noise on the demodulated signal. The present invention provides a switch mode power supply for use near a radio receiver for receiving radio broadcasts transmitted at carrier frequencies spaced according to a predetermined carrier frequency spacing of 10 kHz or 9 kHz, the system mode power supply comprising: a transformer having at least one DC output and one input; precision timing means which generates a control signal having an operating frequency within a predetermined range of an integer multiple of said carrier frequency spacing; switch means electrically connected to said transformer for controlling said input of said transformer; and controller means electrically connected to said DC output, said precision timing means and said switch means; said controller means being adapted to pulse width modulate said switch means at the said operating frequency of said control signal to regulate the said DC output of said transformer, the said predetermined range being such that the pulse width modulation is within 20 Hz of said integer multiple of said carrier frequency spacing. The present invention further provides an instrument panel comprising; a radio receiver means for receiving radio frequencies having a carrier frequency assignment spacing of 10 kHz or 9 kHz ; a switch mode power supply having an operating frequency; and oscillator means which is connected to said power supply and which controls the operating frequency of said power supply within 20 Hz of an integer multiple of said carrier frequency assignment spacing. The present invention also provides a method of operating a switch mode power supply for concealing noise in a radio receiver, said method comprising the steps of: receiving radio frequencies with the radio receiver having a carrier frequency assignment spacing; generating a control signal having a frequency within 20 Hz of an integer multiple of said carrier frequency assignment spacing; and controlling the switching frequency of said power supply with said control signal. The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: FIG. 1 is an instrument panel of an automobile; FIG. 2 is a block diagram of a preferred embodiment of the present invention; FIG. 3A is a plot of a portion of carrier frequency spacings; FIG. 3B is a plot showing harmonic RF emission from a prior art RC-controlled power supply; and FIG. 3C is a plot showing harmonic RF emission from the crystal controlled power supply of the invention. Referring to FIG. 1, an instrument panel 10 in an automotive vehicle typically has several components including an instrument cluster 12 and radio receiver 14. A power supply 20, which is preferably a pulse width modulated switch mode power supply, powers instrument cluster 12. The switch mode power supply can be of the boost regulator type (e.g., flyback) or buck type. Radio receiver 14 has an AM band which has carrier frequency assignment spacings of 10 KHz in the North America and 9 KHz in Europe. Because of its switching characteristics, power supply 20 typically generates RF noise that potentially interferes with AM reception in radio receiver 14. Referring now to FIG. 2, a detailed block diagram of a switch mode power supply 20 is shown. Power supply 20 has a transformer 22 with a primary winding 24 and a secondary winding 26. A battery input 28 is connected to primary winding 24 through a series combination of an electromagnetic interference filter 30 and a reverse battery protection circuit 32, both of which are well known in the art of power supplies. Electromagnetic interference filter 30 filters electrical noise from travelling from the battery input 28 into primary winding 24 and vice versa. Reverse battery protection 32 prevents damage to the electrical system if the battery connections are connected incorrectly. The other side of primary winding 24 is connected to ground through a switching device 36, (for example, a transistor such as a JFET) and a resistor 37. A controller 34 is connected to the gate terminal of JFET 36 to pulse width modulate the current through primary winding 24 and thereby control the voltage across the output of secondary winding 26. Thus, controller 34 operates switch 36 at a predetermined operating frequency at a variable pulse width determined in response to feedback of the output voltage of secondary 26. The operating frequency of controller 34 is controlled by a precision timer 38 which is electrically connected to controller 34. Timer 38 preferably consisting of a crystal 40 and divider circuit 42 is used to generate a control signal. Crystal 40 can be any precision crystal having a desired operating frequency and accuracy. For example, a crystal having 3.84 MHz operating frequency has been successfully used. The frequency of control signal is an integer multiple of 9 KHz or 10 KHz depending on whether the circuit is going to be used in Europe or in North America, respectively. For example, a divider circuit 42 can be used to divide the frequency of the 3.84 MHz crystal by 128 to obtain an output frequency of 30 KHz. Secondary winding 26 has a series of DC outputs which are connected to rectification and filter circuitry 44 in order to provide a clean DC voltage. The output of secondary winding 26 provides feedback for controller 34 so that controller 34 maintains the desired output voltage. For example, if the feedback voltage drops below a predetermined level, the pulse width is increased by said controller 34. If said feedback rises above a predetermined level, then said controller 34 decreases said pulse width. Referring now to FIG. 3A, the AM radio frequency band spacing in North America is 10 KHz, a portion of which is shown with information signal 50. FIG. 3B shows a typical RC controlled power supply impulse which switches at 32 KHz. The 31st harmonic, 992 KHz, is within the AM band. FIG. 3C shows a power supply impulse provided from a power supply controlled by a precision timer of the present invention. In operation, switch 36 radiates an RF signal in the form of power impulses at the fundamental operating frequency and its associated harmonic frequencies which typically extend into the AM band and thus when demodulated possibly causing noise in the audible range of the human ear. When an AM signal is demodulated, an information signal within 20 Hz of the centre frequency will be demodulated into a range below the audible range of the human ear. In the prior art when an RC timing circuit is used, an extremely accurate switching time through a broad range of operating temperatures is not possible. The impulse as shown in FIG. 3B is 2000 Hz from the centre frequency and will demodulate into an audible tone. However, when a crystal 40 is used for controlling such circuit, the accuracy can be easily controlled within 20 Hz of the centre frequency at various operating temperatures experienced by automotive vehicles so that an RF noise generated by the power supply is demodulated to a frequency below the audible range.

Claims: A switch mode power supply (20) for use near a radio receiver (14) for receiving radio broadcasts transmitted at carrier frequencies spaced according to a predetermined carrier frequency spacing of 10 kHz or 9 kHz, the switch mode power supply (20) comprising: a transformer (22) having at least one DC output and one input; precision timing means (38) which generates a control signal having an operating frequency within a predetermined range of an integer multiple of said carrier frequency spacing; switch means (36) electrically connected to said transformer for controlling said input of said transformer; and controller means (34) electrically connected to said DC output, said precision timing means (38) and said switch means (36); said controller means (34) being adapted to pulse width modulate said switch means at the said operating frequency of said control signal to regulate the said DC output of said transformer (22), said predetermined range being such that the pulse width modulation is within 20 Hz of said integer multiple of said carrier frequency spacing. A switch mode power supply as claimed in claim 1, wherein said precision timing means (38) comprises a crystal (40) oscillating at a predetermined frequency. A switch mode power supply as claimed in claim 2, wherein said precision timing means (38) further comprises a divider circuit (42) for dividing said predetermined frequency to a lower predetermined frequency. An instrument panel comprising; a radio receiver means (14) for receiving radio frequencies having a carrier frequency assignment spacing of 10 kHz or 9 kHz ; a switch mode power supply (26) having an operating frequency; and oscillator means (38) which is connected to said power supply (38) and which controls the operating frequency of said power supply within 20 Hz of an integer multiple of said carrier frequency assignment spacing. An instrument panel as claimed in claim 4, wherein said oscillator means (38) comprises a crystal (40) generating a predetermined frequency and a divider circuit (42) for dividing said predetermined frequency to a lower predetermined frequency. An instrument panel as claimed in claim 4, wherein said oscillator means (38) comprises a crystal (40) generating a predetermined frequency and a frequency divider (42) to divide said frequency of said crystal into an integer multiple of said carrier frequency assignment spacing. A method of operating a switch mode power supply for concealing noise in a radio receiver, said method comprising the steps of: receiving radio frequencies with the radio receiver having a carrier frequency assignment spacing; generating a control signal having a frequency within 20 Hz of an integer multiple of said carrier frequency assignment spacing; and controlling the switching frequency of said power supply with said control signal. A method of operating a switch mode power supply as claimed in claim 7, wherein said spacing is 10 kHz or 9 kHz.

Inventors: HIPP KENNETH MICHAEL, HIPP, KENNETH MICHAEL
Applicants: FORD MOTOR CO, FORD MOTOR COMPANY
Application year: 1995
Main classification: H02M3
IPC codes: H02M1, H04B15, H02M3