TW201128916A - Switching mode power supply with a spectrum shaping circuit - Google Patents

Abstract

In accordance with the teachings described herein, systems and methods are provided for a switching mode power supply. In one example, the switching mode power supply may include a transformer, a switching circuit and a switching control circuit. The transformer receives a DC input voltage on a primary winding and generates a DC output voltage on a secondary winding. The switching circuit, which may include a MOSFET switch, is coupled to the transformer and is configured to switch the transformer on and off. The switching circuit in order to regulate the DC output voltage of the transformer. The switching control circuit is configured to generate the switching control signal as a function of a timing signal having a varying frequency, wherein the varying frequency of the timing signal causes a switching frequency of the switching circuit to vary over a period of time in order to reduce electromagnetic interference caused by the switching circuit.

Description

201128916 VI. Description of the invention: [Technology leading to the invention] [(10) π The technology described in this patent application relates generally to a switched mode power supply. This application claims the priority and benefit of the Chinese Patent Application No. 200910059429. filed on May 26, 2009, the entire content of which is incorporated herein by reference. [Prior Art] 闺 - 典 典 换 换 换 换 换 换 换 换 换 换 换 换 换 换 换 换 换 换 换 换 换 换 换 换 换 换 换 换 换 换 换 换 换 换 换 换 换 换 换 换 换 换 换 换In particular, it is a problem. Therefore, it is preferable to provide a switching mode power supply that can operate at a high switching frequency and has a reducing effect. [Abstract]

A system and method for switching mode power supply is proposed in accordance with the teachings of the present invention. In the - real money, the Xianqi power cord can include a voltage regulator, a switching circuit and a switching control circuit. The transformer receives a -DC wheeling voltage on the primary winding and produces an output voltage on a secondary winding. The switching circuit (which may include a (10) switch) is coupled to the transformer and configured to turn the switch on and off transformer. The switching control circuit-switching control money control switching circuit to adjust the variable voltage H complex output voltage 1 switching control circuit is configured to generate the switching control signal as a function of the frequency coefficient of the variable frequency The varying frequency of the timing signal causes the switching frequency of the switching power to fluctuate over a period of time. In another real money, the power supply of the power supply is included - rectifier 099123789 Form No. A0101 Page 4 of 33 〇9933467〇〇-〇 201128916 Ο A transformer, a switching circuit, and a switching control circuit. The rectifier receives an AC input voltage and converts the AC input voltage to a DC input voltage. The transformer is configured to convert one of the DC input voltages on its primary winding to one of the DC output voltages on its secondary winding. The switching circuit is coupled to the primary winding of the transformer and is configured to control current through the primary winding of the transformer based on a switching control signal. The switching control circuit generates the switching control signal as a function of a timing signal having a varying frequency, wherein the varying frequency of the timing signal causes a switching frequency of the switching circuit to fluctuate over a period of time to reduce the switching circuit Electromagnetic interference. A method of adjusting an output voltage of a switched mode power supply can include the steps of: generating a timing signal having a varying frequency; monitoring one or more operational characteristics of the transformer that produces the output voltage to generate a feedback signal; Generating a switching control signal as a function of the timing signal and the feedback signal; and using the switching control signal to turn the transformer on and off to adjust the output voltage, wherein the varying frequency of the timing signal causes one of the transformers The switching frequency varies over a period of time to reduce electromagnetic interference. [Embodiment] FIG. 1 is a diagram showing an example switching mode power supply 100 having a spectrum shaping circuit 102. The switched mode power supply 100 includes a rectifier bridge 104, a transformer 106, a switching circuit 108, and a switching control circuit 110. In operation, rectifier bridge 104 receives an AC input voltage (V.) that translates the AC input voltage into a DC input voltage that is received by the primary winding in group of transformer 106. Transformer 106 is controlled by switching circuit 108 to produce a DC output voltage (V+) on the secondary winding with 099123789. May include an out form number A0101 page 5 / page 33 0993346700-0 201128916 mosfet (as shown) or some other suitable electronic switching device switching circuit 108 controls the current through the primary winding of transformer 1〇6 Effectively turn the transformer 1 () 6 on and off. Figure i also illustrates a storage input power (4) input capacitor (cin), an output electric valley for storing the output voltage of the Xia Lu (C〇ut), and a diode for preventing current from flowing back into the winding of the secondary transformer. (D). The switching circuit 108 is controlled by a switching control signal lu which is generated by the switching control circuit 110 as a function of a timing signal having a varying frequency. In the example shown in the figure, the timing signal is generated by J using the spectrum shaping circuit 1 〇 2 to change the frequency of the one-time turn signal (^κ), as described below. This causes the switching frequency of the switching circuit 1 to 8 to vary over a period of time, changing the shape of the switching spectrum to include a wider frequency band. By increasing the bandwidth of the switching frequency, the harmonic gain is reduced to cause EMI to decrease. To aid in exemplifying the resulting EMI reduction, Figure 2 provides a comparison of the harmonic gain of a typical switched mode power supply with a constant switching frequency, and the harmonic gain of Vds of the first example. In the diagram shown in Figure 2, three vertical solid lines illustrate the main ds of the v in a typical switched mode power supply. The dotted line represents the reduced harmonics that may occur in the switching mode power supply of Figure 1, where the switching frequency varies. As shown, by changing the switching frequency, the harmonics of vds are spread over a wider spectrum and the peak harmonics are reduced. This leads to a reduction in system EMI. Referring again to Figure 1, the switching circuit 1 1 〇 in the illustrated example includes an RS flip-flop 112, a feedback circuit (114, 116, 118), and a timing circuit (10 2, 12 0 ). The feedback circuit uses a current sensing circuit 1丨4, a voltage feedback circuit 116 and a comparison circuit 118 to generate a reset input (r). 099123789 Form No. A0101 Page 6 of 33 0993346700-0 201128916

Give the RS flip-flop Π 2. The timing circuit generates a clock signal (CLK) as a set input (S) to the RS flip-flop 112 by using the spectrum shaping circuit 102 and a oscillator. The rs flip-flop U2 causes the reset signal (r) to have a higher priority than the set signal (S). Therefore, when the reset input (R) is in a logic state, a logic low output (ie, the switching control signal 111) is generated on the Q output of the flip-flop, which causes the MOSFET switch 108 to be turned off and stopped. The current of the primary winding of transformer 1〇6. Set input (S) to one of the high-logic states of the flip-flop (where R is low) will be in Q

Generating a logic high output causes MOSFET switch 108 to return to turn on, allowing current to pass through the primary winding in the feedback circuit, and current sense circuit 114 produces a current sense signal proportional to the current through the primary winding of transformer 106. (丨) sense and the voltage feedback circuit 116 generates a feedback signal (FB) that provides a threshold to regulate the voltage output (V) of the transformer 106 out to a desired value. The current sense signal is compared by the comparison circuit 118 (! sense

And the feedback signal (FB) to generate a reset input (R) to the flip-flop U2, in the illustrated example, a single comparator. The comparison circuit 118 thus produces a logic high pulse at the reset input (R) when the current sense signal (Isense) reaches the threshold set by the feedback signal (FB). In the sequential circuit, a clock signal (CLK) is generated by the fixed frequency oscillator 12, and the frequency of the clock signal (CLK) is changed by the spectrum shaping circuit 1〇2 to be sent as a set input (S) to the rs positive and negative Timer 112 timing signal. In this way, since the current sense signal (!) causes the sense J MOSFET switch 1 〇 8 to disconnect the transformer 1 〇 6 and the clock pulse of the sequential circuit returns the transformer 106 to be turned on, the transformer 106 continues to cycle through the disconnection. And the on cycle. By changing the frequency of the clock pulse, the switch 099123789 0993346700-0 Form number A0101 Page 7 of 33 201128916

As shown in FIG. 5, this reduces the turn-on point of the system EM I 108 to become variable, and FIG. 3 is a simplified diagram of the example sequential circuit for the switching mode power supply of FIG. 1 The spectrum shaping circuit 1() 2 and a vibrating unit 120 are included. In the oscillating device 120, the voltage across the s electric grid (ct) is varied by a combination of a fixed current source (1) and a spectral shaping signal (ss) received from the spectral shaping circuit 102. A comparator 122 is provided. A clock pulse (CLK) is generated when the voltage across the charging capacitor (ct) reaches - threshold power M (,). The clock signal (CLK) also controls - switches to discharge the thermal capacitor at each clock pulse. In operation, the oscillator 120 generates a clock signal (CLK) having a varying frequency 'because of a varying current provided by the spectral shaping apostrophe (ss). The spectral shaping circuit 102 utilizes a shift register 126 to generate varying spectral shaping. A signal (ss) that controls the output of a series of switching current sources (13_1 〇) in a digital-to-analog ratio (D/A) converter 128. The shift register 126 includes a series of d flip-flops (q0i_q7) that are driven by a clock signal (CLK) from the oscillator 120 and configured to temporarily buffer each green pulse through the shift. One of the logic, " displacement. The output of the shift register (Q〇_Q7) is input to a series of logic gates (OR2-ORO) to generate a four-bit digital control word which is input to the D/A converter 128. In the D/A converter 128, the four-bit control word is converted into an analog spectrum shaping by changing the current of the SS by switching a series of switches (S0-S3) to switch the current source on and off. )signal. An example operation of the spectrum shaping circuit 102 undergoing one cycle of the SS signal is illustrated in FIG. As shown in Figure 4, each of the shift registers—clocks (099123789 Form No. A0101, Page 8 of 33, 0993346700-0, 201128916, Q7-Q0) causes the switch series (S3-S0) to increase the current of ss. The particular 'switched SO is used to supply 0.251, the switch S1 is used to supply 〇 5ι, the switch S2 is used to supply 0.75 Ϊ, and the switch S3 is used to supply I. In the illustrated example, spectrum shaping circuit 102 is configured to increase SS from 0 to a peak (I) and then back to 〇 in a repeating pattern. In this way,

Changing the current of the SS signal of the capacitor (Ct) in the oscillator 120 with each clock pulse (CLK) increment causes the frequency of the clock signal (clk) to change according to a function of SS. That is, the clock frequency increases as the % current increases. Fig. 5 shows an example of ds'' of the voltage (V) across the MQSFET of Fig. 1, in which case the switch is off. Figure 5 illustrates how the turn-on point 2 0 0 of the MOSFET switch 1 〇 8 varies due to a change in the clock signal :: (CLK...).::: (in the hatch area 200). This change in switch-on point 2 造成 results in a variable switching frequency that disperses the harmonics of vds across a wider spectrum when turned on, resulting in a reduction in EMI as previously described.

Figure 6 is a simplified diagram of a male-in-one switching mode power supply 300 having a spectral shaping circuit 3〇2 that varies the switching frequency of the power supply. In this example, the timing signal 304 that causes the switching frequency to vary is generated using a quasi-resonant converter that includes a valley detection circuit 306 to cause the switching frequency to be in the switching waveform (v) One of the troughs is aligned. This quasi-resonant switching technique reduces the power loss in the transformer and further increases system efficiency. Figure 6 also illustrates an example circuit implementation for current sensing circuit 308 and feedback circuit 31, which may also be used in the example of Figure 1. The switching mode power supply shown in Fig. 6 comprises a rectifier bridge 312, a 099123789 transformer 314 having a primary winding and two secondary windings, a switching form number A0101, a total of 33 S 0993346700-0 201128916 circuit 316, and a switching control circuit having an RS flip-flop 317, a feedback circuit, and a sequential circuit. In this example, the timing circuit includes a spectrum shaping circuit 302, a reference circuit 318, a valley detecting circuit 306, and a comparison circuit 320. The feedback circuit includes a current sensing circuit 308, a feedback circuit 310, a comparison circuit 322, a logic gate 324, and a delay circuit 326. A resistor (R) is included in the current sensing circuit 308 to provide a pass

S

The amount of current through the primary winding of transformer 314 is proportional to the voltage (I sense ). In feedback circuit 310, a voltage regulator 328 (e.g., TL431) is configured to maintain a DC voltage output by controlling the amount of time the transformer remains on during each cycle of the sequential circuit (V+

One of the desired voltage levels provides a reference voltage (FB). The desired DC voltage output (V + ) can be set by changing the resistor value out in the feedback circuit 310 to adjust the level of the threshold voltage (FB), which is compared by the comparison circuit 322 and the current sensing signal. (I) Compare. The output of the sense comparison circuit 322 is coupled through an OR logic gate 324 to the reset input (R) of the flip-flop 31 7 . As shown in Figure 7, this causes the MOSFET switch 316 to open sense (generating a voltage across V.) when the current sense signal (I) reaches the reference voltage (FB). Ds again referring to Fig. 6, comparing a periodic threshold voltage (ν + μ) and a valley detection signal (VD) to generate a timing signal 304 having a varying frequency, which is sent to the positive input by the set input (S) Counter 317. The valley detection circuit 306 monitors the voltage across the third winding of the transformer 314, which samples the voltage across the MOSFET switch 308 (Vds). The valley detection circuit 306 includes an RC network configured to generate 099123789 on its output (VD) when Vds falls below a predetermined threshold voltage. Form No. A0101 Page 10 of 33 0993346700-0 201128916 A pulse, the preset threshold voltage is determined by the resistance and capacitance values in the network. The periodic threshold voltage (vth) is generated by the shaping circuit 3^ reference circuit 318. Several possible examples of the periodic threshold voltage ('n) are illustrated in FIG. 10, and two examples of the spectrum shaping circuit 3〇2 are implemented. It will be explained below with reference to Fig. 11 through Fig. 13. Referring again to Fig. 6, the spectrum shaping circuit 302 generates a frequency-reading shaped signal (ss) to control the frequency variable. The frequency-shaping shaped signal (SS) is added by the reference circuit 318 to The reference voltage is used to generate a periodic threshold voltage (vth). The 0 DC reference voltage applied by the reference circuit 318 can be, for example, a sinusoidal reference voltage or a positive DC voltage, such as 50 millivolts. . . . . . . . . . 丨+ .:: Figure 7 is a timing diagram showing an example operation of the switching mode power supply 3〇〇 shown in Fig. 6. The upper part of Fig. 7 provides the switching circuit (ν&) and the switching control circuit. Timing diagram during the second burst of the timing signal (S). The lower part of Figure 7 illustrates an example of the periodic threshold voltage (Vth) and the 'sequence signal (s) over a long period of time. As shown, the timing signal The frequency of (S) varies according to a function of Vth. The upper part of Figure 7 is illustrated. The sequence signal ◎ (S) is pulsed out in the lower part of the second diagram. The timing diagram of Figure 7 begins at % 'At this point the MOSFET switch 316 is turned on causing current to flow through the primary winding of the transformer 312. The current through the primary winding increases as C is charged, as exemplified by the slope 111 sense slope in the current sense signal (I). Once the current sense signal (I) reaches the set in the t feedback power sense 1 path 310 The threshold (FB), reset input (R) causes MOSFET switch 316 to open. Referring again to Figure 6, delay circuit 326 causes MOSFET switch 316 to remain off for a predetermined delay time (9 in this example) Microseconds, which is illustrated in Figure 7 by the reset time limit of the reset signal (R). 099123789 Form No. A0101 Page 11 of 33 0993346700-0 201128916 As shown in Figure 7, the waveform is switched ( Vds) includes a valley 400 during the disconnection caused by leakage due to resonance with the parasitic capacitance of the MOSFET switch 316. When the valley 4 of the switching waveform (Vds) is below the threshold 402, the valley detector The measuring circuit 3〇6 causes an initial pulse 4〇4 to be set in the input The sound is emitted on (S). However, since this initial pulse 4〇4 occurs during the preset delay of the delay circuit 326, the initial pulse 4〇4 does not cause the 〇 output to change state (R has priority over S) The MOSFET switch 316 is then set to turn on (S) again at a leading edge of the timing signal, which occurs in Figure 7. The timing reference __3 in Figure 7 is illustrated in the timing signal, respectively. (S) The highest frequency and the lowest frequency of the turn-on point. As indicated by _, the frequency of the timing signal () is changed by the delay time limit set by the delay circuit 32 6 and

The distance between the switch-on points (, and %). The variation of the transformer connection point is also illustrated in the switching waveform shown in Fig. 8. By increasing the bandwidth of the switching frequency, as shown in Fig. 8, the Vds (four) wave gain reduction results in a reduction in system EMI. The obtained Vds spectrum wave is beneficial to the small - the actual pain is illustrated in Figure 9

The figure in Figure 9 contains an illustration - a solid line of the harmonics in a typical switched mode power supply and a line of dots that may occur as an example in the example of Figure 6. As shown, the sharp bee harmonic reduction results in a reduction in overall EM I. Figure U is - for the 6th (10) mode change; green red - the example frequency is discussed in the circuit 3〇2. The spectrum shaping circuit includes a -vibrator 500, a shift register 5G2, and a -D/A converter 5()4. The actuator 5 is a voltage controlled oscillator 099123789 which produces a clock output (clk) having a fixed frequency of a threshold voltage th). The clock output ^U) is used to drive the shift register 502 and is the RS flip-flop 317. Form number Α0101 a 10 0993346700 12 pages/total 33 pages 201128916 Supply clock (not shown). The shift register 502 includes a series of D flip-flops (q〇_q7) that are clocked by the clock output (CLK) of the oscillator 500 and configured to pass the parent clock. One of the displacement registers is logic, displacement. The output of the shift register (Q〇_Q7) is input to a series of logic gates (OR2-ORO) to generate a four-bit digital control word, which is converted by the D/A converter 504 into an analog signal (ss ). The example of a spectrum shaping circuit that undergoes a cycle of ss is illustrated in Figure 4. As shown in Figure 4, each clock of the shift register (Q7_Q〇) causes the switch series (S3-S0) in the D/A converter to increase the current of SS to generate a step waveform. This stepped SS waveform is input to the reference circuit 318 as shown in FIG. 6 to generate a one-cycle threshold voltage (V, th / Fig. 12 is another example of the source mode supply for the switching mode of Fig. 6. A schematic diagram of the spectrum shaping circuit 302. The spectrum shaping circuit 3〇2 includes a voltage controlled oscillator 600, a shift register 6〇2, and a D/A converter 6〇3. The voltage controlled oscillator 600 A fixed frequency clock signal 乂 "(1) is generated by the threshold voltage and drives the shift register 602. In this example, the shift register 602 includes a series of d flip-flops, and the D flip-flops are constructed The shape is used to generate a serial rounding signal (G). As shown in Fig. 13, the shift register 602 generates a single pulse on the serial output (g) every eight clock cycles. Referring again to Fig. 12, The serial output signal (G) from the shift register 6〇2 is used to control the SS output from a D/A converter 603. In particular, the 'D/A converter 603 includes a fixed current source (I). And its charging c 11 is stored by a charging capacitor (Cu). The serial output signal (g) from the shift register 602 controls the d/a converter 6 One of the switches in 03 099123789 Form No. A0101 Page 13 of 33 0993346700-0 201128916 (su) to discharge the charging capacitor every eight clock cycles)

The SS produces a saw wave output as shown in Figure 13. U The above description discloses the invention by way of example, and includes the best mode and the skilled in the art can make and use the invention. The patentable scope of the invention may encompass other examples that are apparent to those skilled in the art. BRIEF DESCRIPTION OF THE DRAWINGS [0005] Fig. 1 is a simplified diagram of an example switching mode power supply with a spectrum shaping circuit.

Fig. 2 is a diagram showing the result of the chopper gain reduction obtained by changing the switching frequency in the power supply mode of the first diagram switching mode. Figure 3 is a simplified diagram of an example sequential circuit for the switched mode power supply of Figure 1. Figure 4 illustrates an example phase of the spectrum shaping circuit of Figure 1. Figure 5 is a diagram showing the switching waveform of the switching mode power supply of Fig. 1 (v)

An example of ds J. Figure 6 is a simplified diagram of an alternative power supply with an instance-switched mode. Figs. 7A and 7B are timing diagrams showing an example operation of the switching mode power supply of Fig. 6. Fig. 8 is a view showing an example of the ds of the switching waveform (v) of the switching mode power supply of Fig. 6. Fig. 9 is a diagram showing the result of the harmonic gain reduction obtained by changing the switching frequency in the switching mode power supply of Fig. 6. Figure 10 illustrates an example periodic waveform that is not available for the sequential circuit of Figure 6. Figure 11 is a simplified diagram of an example spectrum shaping circuit for a switching mode power supply of Figure 6. 099123789 Form No. A0101 Page 14 of 33 0993346700-0 201128916 Figure 1 2 is a simplified diagram of another example spectrum shaping circuit for the switching mode power supply of Figure 6. Fig. 13 is a timing chart showing an example of the operation of the spectrum shaping circuit of Fig. 12. [Main component symbol description] [0006] CLK fixed frequency clock output Ct Charging capacitor FB threshold G serial output signal Ο

10, 11 ' 12 ' 13 Switching current source let Fixed current source I SENSE Current sensing signal Q0-Q7 D Positive and negative ORO, 0R1, 0R2 Logic gate S0, SI, S2, S3 Switch St Switch SS analog signal, spectrum shaping Rf, ':: Vds voltage Vth threshold voltage 100 example switching mode power supply 102 spectrum shaping circuit, sequential circuit 104 rectifier bridge 106 transformer 108 switching circuit 110 switching control circuit 112 RS flip-flop 114, 116, 118 feedback circuit 099123789 form No. A0101 Page 15 of 33 0993346700-0 201128916 120 Oscillator 122 Comparator 126 Displacement Register 128 Converter 200 Switch-on Point 300 Switching Mode Power Supply 302 Spectrum Shaping Circuit 304 Timing Signal 306 Valley Detection Circuit 308 Current Sensing circuit 310 feedback circuit 312, 314 transformer 316 M0SFET switch 317 RS flip-flop 318 reference circuit 320, 322 comparison circuit 324 logic gate 326 delay circuit 328 voltage regulator 400 valley 402 threshold 404 initial pulse 500 oscillator 502 displacement temporary storage 504 D/A converter 600 Controlled oscillator Form Number A0101 099123789 Page 16/33 Total 201 128 916 602 0993346700-0 shift register 603 D / A converter

❹ 099123789 Form No. A0101 Page 17 of 33 0993346700-0

Claims (1)

201128916 VII. Patent application scope: 1. A switching mode power supply, comprising: a rectifier; a transformer having a primary winding and a primary winding coupled to the rectifier; a switching circuit coupled to the transformer The primary winding controls the current through the primary winding of the transformer based on a switching control signal; and a switching control circuit for generating the switching control signal as a function of a timing signal having a varying frequency, wherein the timing signal The frequency of change f ··: causes the switching frequency of one of the switching circuits to vary over a period of time. 2. The switching mode power supply of claim 1, wherein the switching control circuit comprises: a flip flop that outputs the switching control signal as a function of a set input and a reset input; a feedback circuit Generating the reset input to the flip-flop, the feedback circuit configured to monitor current through the primary winding of the transformer and to generate a state change on the reset input when current through the primary winding reaches a threshold And a timing circuit that generates the timing signal as the set input to the flip-flop, the timing circuit including a spectrum shaping circuit that causes a varying frequency of the timing signal. 3. The switched mode power supply of claim 2, wherein the sequential circuit includes an oscillator coupled to the spectral shaping circuit, and wherein the spectral shaping circuit changes a frequency of a clock signal generated by the oscillator To generate the timing signal. 099123789 Form No. A0101 Page 18 of 33 0993346700-0 201128916 4. The switching mode power supply of claim 2, wherein the timing circuit comprises: a valley detecting circuit that generates a valley detecting signal a function of switching a voltage across one of the switching circuits, wherein the valley detection signal indicates when the switching voltage falls below a threshold voltage; a spectral shaping circuit configured to generate a spectrally shaped signal; a reference circuit, The method is configured to generate a periodic threshold voltage as a function of the spectral shaping signal, wherein the frequency of the periodic threshold voltage is set by the frequency 0 spectral shaping signal; and a comparison circuit that compares the valley detection The signal and the periodic threshold voltage are used to generate the timing signal as the set input input to the flip-flop, wherein the frequency of the timing signal is caused by the periodic threshold voltage. 5. The switching mode power supply of claim 4, wherein the feedback circuit comprises a delay circuit coupled between the switching control signal and the reset input to the flip-flop in a feedback loop The delay circuit causes the switching circuit to remain in an off state for a predetermined period of time after the state of the reset input is changed because the current through the primary winding reaches the threshold. 6. The switched mode power supply of claim 1, wherein the switching circuit comprises a MOSFET switch. 7. The switched mode power supply of claim 4, wherein the transformer includes an additional secondary winding and wherein the valley detecting circuit samples the switching voltage by monitoring a voltage across the additional secondary winding. 8. The switched mode power supply of claim 2, wherein the feedback circuit includes a voltage regulator configured to generate the threshold for comparison with current through the primary winding, wherein The voltage is 099123789 Form No. A0101 Page 19 of 33 0993346700-0 The threshold generated by the 201128916 section is set to generate a desired DC output voltage. 9. The switched mode power supply of claim 3, wherein the sequential circuit comprises: the oscillator that generates the clock signal; a shift register that is clocked by the clock signal and configured to generate a a digital control signal; and a digital-to-analog converter configured to convert the digital control signal into an analog spectrally shaped signal; wherein the analog spectrally shaped signal is fed back to the oscillator to change the frequency of the clock signal This timing signal is generated. 10. The switched mode power supply of claim 4, wherein the spectral shaping circuit comprises: an oscillator that generates a clock signal; a displacement register that is clocked by the clock signal and configured to Generating a digital control signal; and a digital-to-analog converter configured to convert the digital control signal into the spectrally shaped signal. 11. A method of regulating an output voltage in a switched mode power supply, comprising: generating a timing signal having a varying frequency; monitoring one or more operational characteristics of a transformer that produces the output voltage to generate a feedback signal Generating a switching control signal as a function of the timing signal and the feedback signal: and using the switching control signal to turn the transformer on and off to adjust the output voltage; 099123789 Form No. A0101 Page 20 of 33 0993346700- 0 201128916 wherein the varying frequency of the timing signal causes one of the switching frequencies of the transformer to fluctuate over a period of time to reduce electromagnetic interference. 12. The method of claim 11, further comprising: detecting one of a switching voltage associated with turning on and off the transformer; and causing the timing signal and the valley in the switching voltage alignment. ❹ ❹ 099123789 Form No. A0101 Page 21 of 33 0993346700-0