TWI474595B - Switching mode power supply with a spectrum shaping circuit - Google Patents

Description

Switching mode power supply with spectral shaping circuit

The techniques described in this patent application relate generally to switched mode power supplies. The priority and benefit of Chinese Patent Application No. 200910059429.7, filed on May 26, 2009, is hereby incorporated by reference.

In a typical switched mode power supply, the harmonics caused by the switching frequency typically add to the overall system's electromagnetic interference (EMI). This is especially problematic for power supplies with a large amount of harmonic gain caused by a high switching frequency. Therefore, it is preferable to provide a switched mode power supply that can operate at a high switching frequency and has a reduced effect on EMI.

In accordance with the teachings of the present invention, systems and methods for switching mode power supplies are presented.

In an example, the switched mode power supply can include a transformer, a switching circuit, and a switching control circuit. The transformer receives a DC input voltage on a primary winding and a DC output voltage on a secondary winding. The switching circuit (which may include a MOSFET switch) is coupled to the transformer and configured to turn the transformer on and off. The switching control circuit generates a switching control signal for controlling the switching circuit to adjust 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 one of the switching circuits to switch The frequency changes over time.

In another example, the switched mode power supply includes a rectifier, 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 a DC input voltage on its primary winding to a DC output voltage on its secondary winding. The switching circuit is coupled to the primary winding of the transformer and 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 produce the output voltage to generate a reverse body 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 a varying frequency of the timing signal causes one of the transformers to switch The frequency changes over a period of time to reduce electromagnetic interference.

CLK‧‧‧ fixed frequency clock output

Ct‧‧‧Charging capacitor

FB‧‧‧ threshold

G‧‧‧Serial output signal

I0, I1, Í2, I3‧‧‧ switching current source

Ict‧‧‧fixed current source

ISENSE‧‧‧ current sensing signal

Q0-Q7‧‧‧D positive and negative

OR0, OR1, OR2‧‧‧ logic gate

S0, S1, S2, S3‧‧‧ switch

St‧‧ switch

SS‧‧‧ analog signal, spectrum shaping signal

Vds‧‧‧ voltage

Vth‧‧‧ threshold voltage

100‧‧‧Instance switching mode power supply

102‧‧‧Spectrum shaping circuit, sequential circuit

104‧‧‧Rectifier Bridge

106‧‧‧Transformers

108‧‧‧Switching circuit

110‧‧‧Switching control circuit

112‧‧‧RS forward and reverse

114, 116, 118‧‧‧ feedback circuit

120‧‧‧Oscillator

122‧‧‧ comparator

126‧‧‧Displacement register

128‧‧‧ converter

200‧‧‧Connected point

300‧‧‧Switch mode power supply

302‧‧‧Spectrum shaping circuit

304‧‧‧ timing signal

306‧‧‧ Valley Detection Circuit

308‧‧‧ Current sensing circuit

310‧‧‧ Feedback Circuit

312, 314‧‧‧ Transformers

316‧‧‧ MOSFET switch

317‧‧‧RS forward and reverse

318‧‧‧Reference circuit

320, 322‧‧‧ comparison circuit

324‧‧‧Logic gate

326‧‧‧delay circuit

328‧‧‧Voltage regulator

400‧‧‧ trough

402‧‧‧ threshold

404‧‧‧ initial pulse

500‧‧‧Oscillator

502‧‧‧Displacement register

504‧‧‧D/A converter

600‧‧‧Voltage controlled oscillator

602‧‧‧Displacement register

603‧‧‧D/A converter

Figure 1 is a simplified diagram of an example switching mode power supply with a spectral shaping circuit.

Fig. 2 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. 1.

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 operation of the spectrum shaping circuit of Figure 1.

Fig. 5 is a view showing an example of the switching waveform (V _ds ) of the switching mode power supply of Fig. 1.

Figure 6 is a simplified diagram of another example switching mode power supply with a spectrum shaping circuit.

Fig. 7A and Fig. 7B are timing charts showing an example of the operation of one example of the switching mode power supply of Fig. 6.

Fig. 8 shows an example of the switching waveform (V _ds ) 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 can be used by the sequential circuit of Figure 6.

Figure 11 is a simplified diagram of an example spectrum shaping circuit for a switched mode power supply of Figure 6.

Figure 12 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.

1 is a simplified diagram of an example switched mode power supply 100 having a spectral 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, bridge rectifier 104 receives an AC input voltage (V _in), the AC input voltage is converted to a primary winding of the transformer 106 receives the DC input voltage. Transformer 106 by the switching circuit 108 controls to generate a DC output voltage (V _out) on the secondary winding. A switching circuit 108, which may include a MOSFET (as shown) or some other suitable electronic switching device, controls the current through the primary winding of transformer 106 to effectively turn the transformer 106 on and off. Figure 1 also illustrates an input capacitor (C _in ) that stores the DC input voltage, an output capacitor (C _out ) that stores the DC output voltage, and a diode that prevents current from flowing back into the winding of the secondary transformer (D) ).

The switching circuit 108 is controlled by a switching control signal 111 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 Figure 1, the timing signal is generated by varying the frequency of a clock signal (CLK) using spectrum shaping circuit 102, as described below. This causes the switching frequency of the switching circuit 108 to fluctuate 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 of _Vds is reduced to cause EMI to decrease.

To help illustrate resultant EMI reduced degree, Fig. 2 includes a comparator providing a harmonic gain of V _ds harmonic typical gain constant switching frequency of the switching mode power supply V _ds to the first example of FIG.

In the diagram shown in Figure 2, three vertical solid lines illustrate the main harmonics of _Vds 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 FIG. 1, the switching circuit 110 of the illustrated example includes an RS flip-flop 112, a feedback circuit (114, 116, 118), and a timing circuit (102, 120). The feedback circuit utilizes a current sensing circuit 114, a voltage feedback circuit 116, and a comparison circuit 118 to generate a reset input (R) to the RS flip-flop 112. The timing circuit uses the spectrum shaping circuit 102 and an oscillator 120 to generate a clock signal (CLK) as a set input (S) to the RS flip-flop 112. The RS flip-flop 112 causes the reset signal (R) to have a higher priority than the set signal (S). Therefore, a logic low input is generated on the Q output of the flip-flop when the reset input (R) is in a high logic state. Out (ie, switching control signal 111), which causes MOSFET switch 108 to open, stopping current flow through the primary winding of transformer 106. Setting the input (S) to one of the high logic states of the flip flop (while R is low) will produce a logic high output at Q, causing the MOSFET switch 108 to return to turn on, allowing current to pass through the primary winding.

In the feedback circuit, the current sensing circuit 114 generates a current _sense signal (I _sense ) proportional to the current through the primary winding of the transformer 106, and the voltage feedback circuit 116 generates a feedback signal (FB), the feedback signal providing a threshold value to the DC voltage output of the transformer 106 (V _out) is adjusted to a desired value. The current sense signal (I _sense ) and the feedback signal (FB) are compared by the comparison circuit 118 to produce a reset input (R) to the flip flop 112, which in the illustrated example is a single comparator. The comparison circuit 118 thus produces a logic high pulse at the reset input (R) when the current sense signal (I _sense ) reaches the threshold set by the feedback signal (FB). In the sequential circuit, a clock signal (CLK) is generated by the fixed frequency oscillator 120, and the frequency of the clock signal (CLK) is changed by the spectrum shaping circuit 102 to be sent to the RS flip-flop 112 as a set input (S). Timing signal. In this manner, since the current sense signal (I _sense ) causes the MOSFET switch 108 to turn off the transformer 106 and the clock pulse of the sequential circuit returns the transformer 106 to turn on, the transformer 106 continues to cycle through the turn-off and turn-on cycles. By changing the frequency of the clock pulses, the turn-on point of MOSFET switch 108 becomes variable, as shown in Figure 5, which reduces system EMI.

Figure 3 is a simplified diagram of an example sequential circuit for a switched mode power supply of Figure 1. The timing circuit includes a spectrum shaping circuit 102 and an oscillator 120. In oscillator 120, the combination of a fixed current source (I _ct ) and a spectrally shaped signal (SS) received from spectral shaping circuit 102 varies the voltage across a charging capacitor (C _t ). A clock pulse (CLK) is generated by a comparator 122 when the voltage across the charging capacitor ( _Ct ) reaches a threshold voltage ( _Vth ). The clock signal (CLK) also controls a switch (S _t ) to discharge the charging capacitor (C _t ) at each clock pulse. In operation, oscillator 120 produces a clock signal (CLK) having a varying frequency due to a varying current provided by the spectral shaping signal (SS).

The spectrum shaping circuit 102 generates a varying spectrum shaping signal (SS) by using a shift register 126 that controls a series of switching current sources (13-10) in a digital-to-analog ratio (D/A) converter 128. Output. The shift register 126 includes a series of D flip-flops (Q0-Q7) that are driven by a clock signal (CLK) from the oscillator 120 and configured to pass each clock pulse. One of the displacement registers is logically 〝1〞 displaced. The output of the shift register (Q0-Q7) is input to a series of logic gates (OR2-OR0) 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 one by changing the current of the SS by controlling a series of switches (S0-S3) to switch the current source (I3-I0) on and off. Analog spectrum shaping (SS) 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 clock (Q7-Q0) of the shift register causes the switch series (S3-S0) to increase the current of SS. In particular, switch S0 is used to supply 0.25I, switch S1 is used to supply 0.5I, switch S2 is used to supply 0.75I, and switch S3 is used to supply I. In this illustrative example, spectrum shaping circuit 102 is configured to increase SS from 0 to a peak (I) and then back to zero in a repeating pattern. In this manner, 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 to say, the clock frequency increases as the SS current increases.

Figure 5 shows an example of the voltage (V _ds ) across the MOSFET switch 108 of Figure 1, where the switch is open. Figure 5 illustrates how the turn-on point 200 of the MOSFET switch 108 varies due to changes in the clock signal (CLK) (within the hatch region 200). This change in switch-on point 200 causes a variable switching frequency that, when turned on, disperses the harmonics of _Vds across a wider spectrum, resulting in a reduction in EMI as previously described.

Figure 6 is a simplified diagram of another example switched mode power supply 300 having a spectral shaping circuit 302 that varies the switching frequency of the power supply. In this example, the timing signal 304 for varying the switching frequency is generated by a quasi-resonant converter including a valley detecting circuit 306 to cause one of the switching frequency and the switching waveform (V _ds ). alignment. This quasi-resonant switching technique reduces power loss in the transformer and further increases system efficiency. FIG. 6 also illustrates an example circuit implementation for current sensing circuit 308 and feedback circuit 310, which may also be used in the example of FIG.

The switching mode power supply shown in FIG. 6 comprises a rectifier bridge 312, a transformer 314 having a primary winding and two secondary windings, a switching circuit 316, and an RS flip-flop 317, a feedback circuit and A switching control circuit for 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.

It comprises a resistor (R _s) in the current sensing circuit 308 to provide a voltage (I _sense) proportional to the amount of current through the primary winding of the transformer 314. In the feedback circuit 310, a voltage regulator 328 (e.g., the TL431) via configuration for the transformer by controlling the amount of time it turned a manner to maintain the DC voltage output (V _out) during each cycle of the timing circuit One of the desired voltage levels provides a reference voltage (FB). Be set to a desired DC output voltage (V _out) can be changed by the feedback circuit 310 of the resistor values to adjust the threshold voltage (FB) of the level of the embodiment, the threshold voltage level by the comparator circuit 322 and current sense signal ( I _sense ) to compare. The output of comparison circuit 322 is coupled through OR logic gate 324 to the reset input (R) of flip flop 317. As shown in Figure 7, this causes MOSFET switch 316 to turn off when the current sense signal (I _sense ) reaches the reference voltage (FB) (generating a voltage across _Vds ).

Referring again to FIG. 6, a periodic threshold voltage ( _Vth ) and a valley detection signal (VD) are compared to generate a timing signal 304 having a varying frequency, which is received by the set input (S) and sent to the flip-flop. 317. The valley detection circuit 306 monitors the voltage across the third winding of the transformer 314, which samples the shape of the voltage across the MOSFET switch 308 ( _Vds ). The valley detecting circuit 306 includes an RC network configured to generate a pulse on the output (VD) of the Vds when the Vds falls below a predetermined threshold voltage, the predetermined threshold voltage is determined by the The value of the resistor and capacitor in the RC network is determined. The periodic threshold voltage ( _Vth ) is generated by the spectral shaping circuit 302 and the reference circuit 318. Several possible examples of periodic threshold voltages ( _Vth ) are illustrated in Figure 10, and two example implementations of spectral shaping circuit 302 will be described below with reference to Figures 11 through 13.

Referring again to Figure 6, spectrum shaping circuit 302 produces a spectrally shaped signal (SS) to control the frequency variable. The spectral shaping signal (SS) is added by reference circuit 318 to a DC reference voltage to produce a periodic threshold voltage ( _Vth ). The DC reference voltage applied by reference circuit 318 can be, for example, a zero volt reference voltage or a positive DC voltage, such as 50 millivolts.

Fig. 7 is a timing chart showing an example of the operation of the switched mode power supply 300 shown in Fig. 6. The upper part of Fig. 7 provides a timing diagram of the switching circuit ( _Vds ) and the switching control circuit during the two pulses of the timing signal (S). The lower part of Fig. 7 illustrates an example of the periodic threshold voltage ( _Vth ) and the timing signal (S) over a long period of time. As shown, the frequency of the timing signal (S) _{varies as a} function of _Vth . The timing signal (S) exemplified in the upper part of Fig. 7 is circled in the lower part of Fig. 7.

The timing diagram of FIG. 7 begins at t ₀ , at which point MOSFET switch 316 is turned on causing current to flow through the primary winding of transformer 312. The current through the primary winding increases as _Cin is charged, as exemplified by the slope in the current sense signal (I _sense ). Once the current sense signal (I _sense ) reaches the threshold (FB) set by the feedback circuit 310 at t ₁ , resetting the input (R) causes the MOSFET switch 316 to open. Referring again to FIG. 6, delay circuit 326 causes MOSFET switch 316 to remain off for a predetermined delay time (9 microseconds in this example), which is broken by reset signal (R) in FIG. Time limit instantiation.

As shown in FIG. 7, the switching waveform ( _Vds ) includes a valley 400 during the off period, which is caused by leakage due to resonance with the parasitic capacitance of the MOSFET switch 316. When the valley 400 in the switching waveform ( _Vds ) passes below the threshold 402, the valley detecting circuit 306 causes an initial pulse 404 to sound on the set input (S). However, since this initial pulse 404 occurs during the preset delay of the delay circuit 326, the initial pulse 404 does not cause the Q output to change state (R has priority over S). MOSFET switch 316 is then set under a leading edge timing of the input signal (S) is switched on again, this system occurs at t ₂ of FIG. 7.

The timing reference points t ₂ and t _{3 in} Fig. 7 illustrate the turn-on points of the highest frequency and the lowest frequency of the timing signal (S), respectively. As shown, the frequency of the timing signal (S) changes the distance between the off time limit set by the delay circuit 326 and the turn-on point (t ₂ and t ₃ ). The variation of the transformer turn-on point is also exemplified in the switching waveform (V _ds ) shown in FIG. By increasing the bandwidth of the switching frequency, as shown in Figure 8, the reduction in harmonic gain of _Vds results in a reduction in system EMI. An example of the resulting V _ds harmonic gain reduction is illustrated in Figure 9. The diagram shown in Fig. 9 includes three solid lines exemplifying the main harmonics of _Vds in a typical switched mode power supply and three dotted lines exemplifying the harmonics that may occur in the example of Fig. 6. As shown, the peak harmonics are reduced, resulting in a reduction in overall EMI.

Figure 11 is a simplified diagram of an example spectral shaping circuit 302 for a switched mode power supply of Figure 6. The spectrum shaping circuit includes an oscillator 500, a shift register 502, and a D/A converter 504. The oscillator 500 in this example is a voltage controlled oscillator that produces a clock output (CLK) having a fixed frequency set by a threshold voltage ( _Vth ). The clock output (CLK) is used to drive the shift register 502 and supply a clock (not shown) to the RS flip-flop 317.

The shift register 502 includes a series of D flip-flops (Q0-Q7) that are clocked by the clock output (CLK) of the oscillator 500 and configured to pass each clock pulse through the One of the displacement registers is a logical 〝1〞 displacement. The output of the shift register (Q0-Q7) is input to a series of logic gates (OR2-OR0) to generate a four-bit digital control character that is converted by the D/A converter 504 into an analog signal (SS). . An example operation of a spectrum shaping circuit that undergoes one cycle of SS is illustrated in FIG. As shown in Fig. 4, each clock (Q7-Q0) of the shift register 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 reference circuit 318 as shown in FIG. 6 to generate a periodic threshold voltage ( _Vth ).

Figure 12 is a simplified diagram of another example spectrum shaping circuit 302 for the switched mode power supply of Figure 6. The spectrum shaping circuit 302 includes a voltage controlled oscillator 600, a shift register 602, and a D/A converter 603. The voltage controlled oscillator 600 generates a fixed frequency clock signal (CLK) that is set by the threshold voltage ( _Vth ) and drives the shift register 602. In this example, the shift register 602 includes a series of D flip-flops (Q0-Q7) that are configured to generate a serial output 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 602 is used to control the SS output from a D/A converter 603. In particular, D/A converter 603 includes a fixed current source (I _ct1 ) that charges the voltage stored by a charging capacitor (C _t1 ). The serial output signal (G) from the shift register 602 controls one of the switches (S _t1 ) in the D/A converter 603 to discharge the charging capacitor (C _t1 ) every eight clock cycles, and generates a saw wave in the SS. The output is shown in Figure 13.

The above description of the present invention is intended to be illustrative of the invention, and is intended to The patentable scope of the invention may encompass other examples that are apparent to those skilled in the art.

CLK‧‧‧ fixed frequency clock output

FB‧‧‧ threshold

ISENSE‧‧‧ current sensing signal

SS‧‧‧ analog signal, spectrum shaping signal

Vds‧‧‧ voltage

100‧‧‧Instance switching mode power supply

102‧‧‧Spectrum shaping circuit, sequential circuit

104‧‧‧Rectifier Bridge

106‧‧‧Transformers

108‧‧‧Switching circuit

110‧‧‧Switching control circuit

112‧‧‧RS forward and reverse

114, 116, 118‧‧‧ feedback circuit

120‧‧‧Oscillator

Claims (9)

A switching mode power supply, comprising: a rectifier; a transformer having a primary winding coupled to the rectifier and a primary winding; a switching circuit coupled to the primary winding of the transformer to switch based a control signal controls current through the primary winding of the transformer; and a switching control circuit for generating the switching control signal as a function of a timing signal having a varying frequency, wherein the varying frequency of the timing signal causes the switching The switching frequency of the circuit varies over a period of time, wherein the switching control circuit includes: a flip-flop that outputs the switching control signal as a function of a set input and a reset input; and a feedback circuit that generates the reset input Giving the flip-flop, the feedback circuit configured to monitor a current through the primary winding of the transformer and to generate a state change on the reset input when a 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 spectral shaping circuit that causes the varying frequency of the timing signal. The switching mode power supply of claim 1, wherein the timing circuit includes an oscillator coupled to the spectrum shaping circuit, and wherein the spectrum shaping circuit changes a frequency of a clock signal generated by the oscillator to This timing signal is generated. The switching mode power supply of claim 1, wherein the sequential circuit comprises: a valley detecting circuit that generates a valley detecting signal as a function of switching voltage across one of the switching circuits, wherein the valley The detection signal indicates when the switching voltage falls below a threshold voltage; the spectrum shaping circuit configured to generate a spectral shaping signal; a reference circuit configured to generate a periodic threshold voltage for the spectral shaping signal a function, wherein the frequency of the periodic threshold voltage is set by the spectral shaping signal; and a comparison circuit that compares the valley detection signal with the periodic threshold voltage to generate the timing signal as the setting input input to the positive The inverter, wherein the varying frequency of the timing signal is caused by the periodic threshold voltage. The switching mode power supply of claim 3, 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 change of the reset input due to the current reaching the threshold through the primary winding. A switching mode power supply as claimed in claim 1, wherein the switching circuit comprises a MOSFET switch. A switched mode power supply as claimed in claim 3, wherein the transformer includes an additional secondary winding and wherein the valley detecting circuit samples the switching voltage by monitoring a voltage across one of the additional secondary windings. The switching mode power supply of claim 1, wherein the feedback circuit includes a voltage regulator configured to generate the threshold value compared with a current through the primary winding, wherein The threshold generated by the voltage regulator is set to produce a desired DC output voltage. The switching mode power supply of claim 2, 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 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 vary the frequency of the clock signal to produce the timing signal. The switching mode power supply of claim 3, wherein the spectrum shaping circuit comprises: an oscillator that generates a clock signal; a displacement 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 the spectrally shaped signal.