PULSE WIDTH MODULATION CONTROLLER AND RELEVANT CONTROL METHOD HAVING MINIMUM ON TIME IN RESPONSE TO VOLTAGE PEAK OF LINE VOLTAGE

Abstract

A PWM controller in a switching mode power supply provides to a power switch a PWM signal determining an ON time and an OFF time. A peak detector detects a voltage peak of a line voltage generated by rectifying an alternating-current input voltage. An OFF-time control unit controls the PWM signal and determines the OFF time in response to a compensation voltage, which is in response to an output voltage of the switching mode power supply. An ON-time control unit controls the PWM signal and determines the ON time in response to the compensation voltage and the voltage peak. The ON-time control unit is configured to make the ON time not less than a minimum ON time, and the minimum ON time is determined in response to the voltage peak.

Claims

1. A PWM controller for providing to a power switch a PWM signal determining an ON time and an OFF time of the power switch, the PWM controller comprising: a peak detector for detecting a voltage peak of a line voltage generated by rectifying an alternating-current input voltage; an OFF-time control unit for controlling the PWM signal and determining the OFF time in response to a compensation voltage, wherein the compensation voltage is in response to an output voltage of a power supply comprising the power switch and the PWM controller; and an ON-time control unit for controlling the PWM signal and determining the ON time in response to the compensation voltage and the voltage peak, wherein the ON-time control unit is configured to make the ON time not less than a minimum ON time, and the minimum ON time is determined in response to the voltage peak.

2. The PWM controller as claimed in claim 1, wherein the minimum ON time increases when the voltage peak decreases.

3. The PWM controller as claimed in claim 1, wherein the ON-time control unit comprises a ramp signal generator for providing a ramp signal compared with the compensation voltage to determine the ON time.

4. The PWM controller as claimed in claim 3, wherein the ramp signal generator provides a charging current in response to the voltage peak to charge a capacitor and generate the ramp signal.

5. The PWM controller as claimed in claim 4, wherein the charging current is capable of being expressed by a polynomial function using the voltage peak as an indeterminate, and the degree of the polynomial is more than one.

6. The PWM controller as claimed in claim 4, wherein the ramp signal generator comprises: a first circuit for providing a setting signal in response to the voltage peak byway of a linear transformation; and a divider for dividing the voltage peak by the setting signal to control the charging current.

7. The PWM controller as claimed in claim 6, wherein the divider is a translinear circuit.

8. The PWM controller as claimed in claim 3, wherein the ramp signal is limited to not less than a bottom voltage, and the bottom voltage is higher than a ground voltage.

9. The PWM controller as claimed in claim 8, wherein during the OFF time the ramp signal is reset to be the bottom voltage.

10. The PWM controller as claimed in claim 1, wherein when the compensation voltage is below a reference voltage the ON time is equal to the minimum ON time.

11. The PWM controller as claimed in claim 1, wherein the power switch is connected in series with an inductive device, and during the OFF-time control unit makes the power switch perform valley switching by detecting a cross voltage of the inductive device.

12. A control method suitable for use in a switching mode power supply, wherein the switching mode power supply is powered by a line voltage to output an output voltage, and comprises a power switch and an inductive device, the control method comprising: detecting a voltage peak of the line voltage; providing a compensation voltage in response to the output voltage; providing a PWM signal in response to the compensation signal and the voltage peak, wherein the PWM signal controls the power switch to define an ON time and an OFF time; limiting the ON time to not less than a minimum ON time; and controlling the minimum ON time in response to the voltage peak.

13. The control method as claimed in claim 12, comprising: generating a ramp signal in response to the voltage peak; and comparing the ramp signal and the compensation voltage to end the ON time.

14. The control method as claimed in claim 13, comprising: providing a charging current in response to the voltage peak to charge a capacitor and generate the ramp signal.

15. The control method as claimed in claim 14, wherein the charging current is capable of being expressed by a polynomial function using the voltage peak as an indeterminate, and the degree of the polynomial is more than one.

16. The control method as claimed in claim 14, comprising: providing a setting signal in response to the voltage peak by way of a linear transformation; and dividing the voltage peak by the setting signal to control the charging current.

17. The control method as claimed in claim 13, comprising: resetting the ramp signal to be a bottom voltage during the OFF time.

18. The control method as claimed in claim 12, comprising: detecting a cross voltage of the inductive device to provide the PWM signal and make the power switch perform valley switching.

19. The control method as claimed in claim 12, comprising: providing a reference voltage; and making the ON time equal to the minimum ON time if the compensation voltage is below the reference voltage.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following drawings. In the drawings, like reference numerals refer to like parts throughout the various figures unless otherwise specified. These drawings are not necessarily drawn to scale. Likewise, the relative sizes of elements illustrated by the drawings may differ from the relative sizes depicted.

[0006] The invention can be more fully understood by the subsequent detailed description and examples with references made to the accompanying drawings, wherein:

[0007] FIG. 1 demonstrates a switching mode power supply according to embodiments of the invention;

[0008] FIG. 2 exemplifies the PWM controller in FIG. 1;

[0009] FIG. 3 shows the relationship between switching frequency f.sub.SW and compensation voltage V.sub.COM;

[0010] FIG. 4 demonstrates line voltage V.sub.LINE, detected voltage V.sub.DET, voltage peak V.sub.LINE-PEAK, and voltage peak V.sub.DET-PEAK; and

[0011] FIG. 5 shows some signal waveforms of signals in the PWM controller of FIG. 2, including ramp signal V.sub.AP, PWM signal S.sub.DRV, and current sense signal V.sub.CS.

DETAILED DESCRIPTION

[0012] To achieve high power factor, a switching mode power supply normally rectifies the AC input voltage from a power grid into a direct-current (DC) line voltage, which is then converted into an output voltage to power a load.

[0013] In order to reduce switching loss and increase conversion efficiency, a switching mode power supply could enter burst mode operation during a light-load or no-load state. Burst mode operation generally refers to a condition that power conversion from an input voltage to an output voltage of a switching mode power supply continues for several consecutive switching cycles and then discontinues for a long period of time before the power conversion resumes. To make the power conversion of each switching cycle efficient, the ON time of a power switch that mandates the power conversion is limited to not less than a minimum ON time. The ON time of a power switch is usually the minimum ON time if the switching mode power supply with the power switch enters burst mode operation.

[0014] According to one embodiment of this invention, the minimum ON time is not a constant, and could vary in response to the change in a voltage peak of a line voltage. For instance, when the line voltage has a voltage peak of 110 volt, the minimum ON time of a power switch is set to be a first minimum ON time; and when the voltage peak changes into 240 volt, the minimum ON time becomes a second minimum ON time, which is less than the first one according to one embodiment of this invention. Power conversion of a switching mode power supply, if well designed to make the minimum ON time depend on a voltage peak of a line voltage, could be substantially independent to the change in the voltage peak.

[0015] FIG. 1 demonstrates a switching mode power supply 100 according to embodiments of the invention. Switching mode power supply 100 employs primary side control (PSC), which controls output voltage V.sub.OUT and current precisely with the information in the primary side SD.sub.PRM of the power supply only. But this invention is not limited to, however. Embodiments of the invention might use secondary side control (SSC), which relies on the information in the secondary side of a power supply to regulate the output voltage and current.

[0016] Bridge rectifier 102 provides full-wave rectification, converting AC input voltage V.sub.AC from a power grid into DC line voltage V.sub.LINE and primary-side ground GND.sub.LINE. Input voltage V.sub.AC could be 240 VAC or 100 VAC, for example. The transformer in FIG. 1 includes a primary winding PRM, a secondary winding SEC, and an auxiliary winding AUX, all inductively coupled to one another. Primary winding PRM and power switch 104 are connected in series between line voltage V.sub.LINE and primary-side ground GND.sub.LINE. PWM controller 106 provides at drive node DRV pulse width modulation signal S.sub.DRV to turn ON or OFF power switch 104. During an ON time when power switch 104 is turned ON to provide a short circuit, line voltage V.sub.LINE and primary-side ground GND.sub.LINE together energize primary winding PRM. In the opposite, during an OFF time when power switch 104 is turned OFF to provide an open circuit, the energy stored in the transformer releases via secondary winding SEC and/or auxiliary winding AUX to build up operation voltage V.sub.CC in the primary side SD.sub.PRM and output voltage V.sub.OUT in the secondary side SD.sub.SEC. Operation voltage V.sub.CC powers PWM controller 106, and output voltage V.sub.OUT load 108.

[0017] A voltage divider consisting of resistors 109 and 110 detects the across voltage of auxiliary winding AUX, and the joint between resistors 109 and 110 is connected to feedback node FB of PWM controller 106. The voltage divider could provide to PWM controller 106 information of line voltage V.sub.LINE or output voltage V.sub.OUT.

[0018] PWM controller 106 could perform constant ON-time control to make switching mode power supply 100 have an excellent power factor close to 1.

[0019] FIG. 2 exemplifies PWM controller 106 in FIG. 1. PWM controller 106 could be in form of a packaged integrated circuit on a monolithic silicon chip, including compensation circuit 130, peak detector 132, OFF-time control unit 136, ON-time control unit 134, and logic unit 138.

[0020] Logic unit 138 including SR flip flop 150 and driver 152 provides PWM signal S.sub.DRV at drive node DRV. PWM signal S.sub.DRV is capable of determining ON time T.sub.ON and OFF time T.sub.OFF of power switch 104. If SR flip flop 150 is set, driver 152 turns power switch 104 ON, so an OFF time T.sub.OFF ends and an ON time T.sub.ON starts. When SR flip flop 150 is reset, driver 152 turns power switch 104 OFF, so an ON time T.sub.ON ends and an OFF time T.sub.OFF starts. A switching cycle T.sub.CYC consists of one ON time T.sub.ON and one OFF time T.sub.OFF.

[0021] Sampler 140 inside compensation circuit 130 samples the voltage at the feedback node FB during de-energizing of the transformer in FIG. 1, to hold sample signal V.sub.SAM. According to inductive coupling, sample signal V.sub.SAM could be a representative of output voltage V.sub.OUT. Transconductor 142 compares sample signal V.sub.SAM and a target voltage V.sub.TAR, and accordingly provides compensation voltage V.sub.COM at compensation node COM. Compensation voltage V.sub.COM is therefore generated in response to output voltage V.sub.OUT. The higher output voltage V.sub.OUT the lower compensation voltage V.sub.COM. From another technology perspective, PWM controller 106 provides a close loop to make sample signal V.sub.SAM about target voltage V.sub.TAR, and the heavier load 108 the higher compensation voltage V.sub.COM.

[0022] Another embodiment using SSC has an error amplifier, TL431 for example, and a photo coupler in the secondary side SD.sub.SEC, to replace compensation circuit 130 in the primary side SD.sub.PRM of FIG. 2. The error amplifier monitors output voltage V.sub.OUT, and the photo coupler feeds the output of the error amplifier back to the primary side SD.sub.PRM by controlling compensation voltage V.sub.COM at compensation node COM.

[0023] In other words, compensation voltage V.sub.COM could be controlled by compensation circuit 130 inside PWM controller 106 in the primary side SD.sub.PRM as shown in FIG. 2, or by circuitry in the secondary side SD.sub.SEC that monitors output voltage V.sub.OUT.

[0024] OFF-time control unit 136 has feedback node FB and compensation node COM as inputs, capable of setting SR flip flop 150 to end and conclude an OFF time T.sub.OFF of power switch 104. For example, OFF-time control unit 136 has a valley selector 156 that is able to set SR flip flop 150 via AND gate 158 when feedback node FB is determined to be having a voltage valley, thereby achieving valley switching and reducing the switching loss of power switch 104. The voltage valley selected to conclude an OFF time T.sub.OFF might be the first voltage valley at feedback node FB during an OFF time T.sub.OFF, or anyone subsequent to the first voltage valley during the OFF time T.sub.OFF. Which voltage valley is selected to conclude an OFF time T.sub.OFF is for example determined by compensation voltage V.sub.COM.

[0025] Comparator 154 compares compensation voltage V.sub.COM with a burst-mode reference voltage V.sub.BST-REF When compensation voltage V.sub.COM is larger than burst-mode reference voltage V.sub.BST-REF, valley selector 156 is allowed to set SR flip flop 150. When compensation voltage V.sub.COM drops below burst-mode reference voltage V.sub.BST-REF, valley selector 156 cannot set SR flip flop 150 until the passage of a sleep time T.sub.SLEEP determined by sleep-time generator 155. In other words, when compensation voltage V.sub.COM happens to go downward and drop across burst-mode reference voltage V.sub.BST-REF, the present OFF time T.sub.OFF will be about sleep time T.sub.SLEEP, which could be as long as several microseconds.

[0026] FIG. 3 shows the relationship between switching frequency f.sub.SW of power switch 104 and compensation voltage V.sub.COM that OFF-time control unit 136 is possibly in control of, where switching frequency f.sub.SW is the reciprocal of switching cycle T.sub.CYC. If compensation voltage V.sub.COM is below burst-mode reference voltage V.sub.BST-REF switching frequency f.sub.SW is as low as about the reciprocal of sleep time T.sub.SLEEP because power switch 104 remains OFF until about the passage of sleep time T.sub.SLEEP. If compensation voltage V.sub.COM exceeds burst-mode reference voltage V.sub.BST-REF switching frequency f.sub.SW is limited to the range between the curves of the maximum switching frequency f.sub.SW-MAX and the minimum switching frequency f.sub.SW-MIN, which is shown in FIG. 3 by dotted region Z. As demonstrated by FIG. 3, each of the maximum switching frequency f.sub.SW-MAX and the minimum switching frequency f.sub.SW-MIN could correlate with compensation voltage V.sub.COM positively.

[0027] Peak detector 132 detects voltage peak V.sub.LINE-PEAK of line voltage V.sub.LINE Via high-voltage node HV of PWM controller 106, peak detector 132 is connected to line voltage V.sub.LINE The joint between resistors 144 and 146 provides detected voltage V.sub.DET in proportion to line voltage V.sub.LINE Peak holder 148 generates voltage peak V.sub.DET-PEAK in response to detected voltage V.sub.DET. FIG. 4 demonstrates line voltage V.sub.LINE, detected voltage V.sub.DET, voltage peak V.sub.LINE-PEAK of line voltage V.sub.LINE and voltage peak V.sub.DET-PEAK of detected voltage V.sub.DET, where detected voltage V.sub.DET and voltage peak V.sub.DET-PEAK are in proportion to line voltage V.sub.LINE and voltage peak V.sub.LINE-PEAK respectively. Voltage peak V.sub.DET-PEAK an internal signal in an integrated circuit, is seemingly equivalent to voltage peak V.sub.LINE-PEAK an external signal. Hereinafter, voltage peak V.sub.DET-PEAK might be used for detailing circuit operations, but it could be equivalently replaced by voltage peak V.sub.LINE-PEAK without any influence to the explanation.

[0028] On-time control unit 134 can reset SR flip flop 150 in response to compensation voltage V.sub.com, so as to turn power switch 104 OFF and to conclude an ON time T.sub.ON. On-time control unit 134 performs constant ON-time control, which, as named, makes the length of ON time T.sub.ON about constant. ON time T.sub.ON nevertheless increases if compensation voltage V.sub.COM increases, and the detail of dependence between them will be explained later. When compensation voltage V.sub.COM is equal to or below a reference voltage V.sub.MIN-ON-REF, ON time T.sub.ON, if started, is equal to a minimum ON time T.sub.ON-MIN, which is determined in response to voltage peak V.sub.DET-PEAK and will be detailed later. For example, the minimum ON time T.sub.ON-MIN lengthens if the voltage peak V.sub.DET-PEAK lessens.

[0029] On-time control unit 134 includes ramp signal generator 180 and comparator 182. Ramp signal generator generates periodic ramp signal V.sub.RAMP in response to voltage peak V.sub.DET-PEAK. Synchronized by the signal at node GT from the output of SR flip flop 150, ramp signal V.sub.RAMP starts ramping up at the beginning of an ON time T.sub.ON. Comparator 182 compares ramp signal V.sub.RAMP with the bigger one between compensation voltage V.sub.COM and reference voltage V.sub.MIN-ON-REF. When ramp signal V.sub.RAMP exceeds both compensation voltage V.sub.COM and reference voltage V.sub.MIN-ON-REF, comparator 182 resets SR flip flop 150, drive 152 in response turns power switch 104 OFF via drive node DRV, and an ON time T.sub.ON is concluded. The higher compensation voltage V.sub.COM, the longer ON time T.sub.ON, because it takes more time for the ramp signal V.sub.RAMP to exceed the higher compensation voltage V.sub.COM. ON time T.sub.ON is never less than minimum ON time T.sub.ON-MIN however. ON time T.sub.ON is equal to minimum ON time T.sub.ON-MIN only if compensation voltage V.sub.COM is equal to or below reference voltage V.sub.MIN-ON-REF. For some embodiments of the invention, reference voltage V.sub.MIN-ON-REF is the same with burst-mode reference voltage V.sub.BST-REF Other embodiments might have reference voltage V.sub.MIN-ON-REF different from burst-mode reference voltage V.sub.BST-REF.

[0030] Ramp signal generator 180 includes first circuit 184, divider 186, voltage-to-current converter 188, reset switch 190, and capacitor 192.

[0031] First circuit 184 provide setting signal V.sub.SET in response to voltage peak V.sub.DET-PEAK by way of a linear transformation. For instance, the relationship between setting signal V.sub.SET and voltage peak V.sub.DET-PEAK can be expressed by the following equation (1).

V.sub.SET=K.sub.1−K.sub.2V.sub.DET-PEAK  (1),

where K.sub.1 and K.sub.2 both are positive constants.

[0032] Divider 186 divides voltage peak V.sub.DET-PEAK by setting signal V.sub.SET, to generate signal V.sub.FF controlling charging current I.sub.CHG. Divider 186, in one embodiment, is implemented by a translinear circuit. For instance, charging current I.sub.CHG can be expressed by the following equation (2).

[00001]$\begin{array}{cc}{I}_{\mathrm{CHG}}=\frac{K_3.Math.V_{\mathrm{DET}.Math.\text{-}.Math.\mathrm{PEAK}}}{K_1-K_2.Math.V_{\mathrm{DET}.Math.\text{-}.Math.\mathrm{PEAK}}}.& \left(2\right)\end{array}$

By way of Taylor expansion, equation (2) could become

[00002]$\begin{array}{cc}\begin{array}{c}{I}_{\mathrm{CHG}}=.Math.K_3.Math.V_{\mathrm{DET}.Math.\text{-}.Math.\mathrm{PEAK}}\\ .Math.\left(K_{a.Math..Math.1}+K_{a.Math..Math.2}.Math.V_{\mathrm{DET}.Math.\text{-}.Math.\mathrm{PEAK}}+K_{a.Math..Math.3}.Math.V_{\mathrm{DET}.Math.\text{-}.Math.\mathrm{PEAK}}^2+K_{a.Math..Math.4}.Math.V_{\mathrm{DET}.Math.\text{-}.Math.\mathrm{PEAK}}^3+\mathrm{.Math.}\right)\\ =.Math.K_{b.Math..Math.1}.Math.V_{\mathrm{DET}.Math.\text{-}.Math.\mathrm{PEAK}}+K_{b.Math..Math.2}.Math.V_{\mathrm{DET}.Math.\text{-}.Math.\mathrm{PEAK}}^2+K_{b.Math..Math.3}.Math.V_{\mathrm{DET}.Math.\text{-}.Math.\mathrm{PEAK}}^3+\mathrm{.Math.}.Math.,\end{array}& \left(3\right)\end{array}$

where all K.sub.X are positive constants. It can be seen from equation (3) that charging current I.sub.CHG is capable of being expressed by a polynomial function using voltage peak V.sub.DET-PEAK as an indeterminate, and the degree of the polynomial is more than one.

[0033] Reset switch 190, during an OFF time T.sub.OFF when power switch 104 is turned OFF, is ON and resets ramp signal V.sub.AP, making it equal to bottom voltage V.sub.BTM, which is the minimum voltage that ramp signal V.sub.RAMP can be. In one embodiment, bottom voltage V.sub.BTM is less than burst-mode reference voltage V.sub.BST-REF. For example, bottom voltage V.sub.BTM could be 0V, the voltage of primary-side ground GND.sub.LINE. For another embodiment of the invention, bottom voltage V.sub.BTM could be 2V.

[0034] The relationship between ON time T.sub.ON and compensation voltage V.sub.COM can be expressed by the following equations.

[00003]$\begin{array}{cc}.Math.I_{\mathrm{CHG}}T_{\mathrm{on}}=C_{192}\left(V_{\mathrm{COM}}-V_{\mathrm{BTM}}\right);.Math..Math.T_{\mathrm{ON}}=\frac{C_{192}\left(V_{\mathrm{COM}}-V_{\mathrm{BTM}}\right)}{I_{\mathrm{CHG}}}=\frac{C_{192}\left(V_{\mathrm{COM}}-V_{\mathrm{BTM}}\right)}{K_{b.Math..Math.1}.Math.V_{\mathrm{DET}.Math.\text{-}.Math.\mathrm{PEAK}}+K_{b.Math..Math.2}.Math.V_{\mathrm{DET}.Math.\text{-}.Math.\mathrm{PEAK}}^2+K_{b.Math..Math.3}.Math.V_{\mathrm{DET}.Math.\text{-}.Math.\mathrm{PEAK}}^3+\mathrm{.Math.}},& \left(4\right)\end{array}$

where C.sub.192 is capacitance of capacitor 192.

[0035] Replacing compensation voltage V.sub.COM in the equation (4) with reference voltage V.sub.MIN-ON-REF, minimum ON time T.sub.ON-MIN can be found from the following equation (5).

[00004]$\begin{array}{cc}{T}_{\mathrm{ON}.Math.\text{-}.Math.\mathrm{MIN}}=\frac{C_{192}\left(V_{\mathrm{MIN}.Math.\text{-}.Math.\mathrm{ON}.Math.\text{-}.Math.\mathrm{REF}}-V_{\mathrm{BTM}}\right)}{K_{b.Math..Math.1}.Math.V_{\mathrm{DET}.Math.\text{-}.Math.\mathrm{PEAK}}+K_{b.Math..Math.2}.Math.V_{\mathrm{DET}.Math.\text{-}.Math.\mathrm{PEAK}}^2+K_{b.Math..Math.3}.Math.V_{\mathrm{DET}.Math.\text{-}.Math.\mathrm{PEAK}}^3+\mathrm{.Math.}}& \left(5\right)\end{array}$

[0036] FIG. 5 shows some signal waveforms of signals in PWM controller 106, including ramp signal V.sub.RAMP, PWM signal S.sub.DRV and current sense signal V.sub.CS. Current sense signal V.sub.CS, at current sense node CS in FIG. 1, could represent inductor current I.sub.PRM flowing through primary winding PRM. In light of convenient comparison, each signal in FIG. 5 is drawn to have two signal waveforms, the one with the broader line corresponding to the condition when voltage peak V.sub.LINE-PEAK is 240V, and the other with the narrower line the condition when voltage peak V.sub.LINE-PEAK is 100V.

[0037] It is supposed compensation voltage V.sub.COM is equal to or less than reference voltage V.sub.MIN-ON-REF in FIG. 5, so power switch 104 operates to have ON time T.sub.ON minimum. Minimum ON times T.sub.ON-MIN-240 and T.sub.ON-MIN-100 are two minimum ON times T.sub.ON corresponding to the conditions when voltage peaks V.sub.LINE-PEAK are 240V and 100V respectively. As shown in FIG. 5, Minimum ON time T.sub.ON-MIN-240 is less than Minimum ON time T.sub.ON-MIN-100 As derivable from equation (5), it is evident as well that minimum ON time T.sub.ON-MIN reduces when voltage peak V.sub.DET-PEAK or V.sub.LINE-PEAK increases.

[0038] Proper design to PWM controller 106 can set constants in equation (5) to make peak values V.sub.CS-PEAK-240 and V.sub.CS-PEAK-100 of current sense signal V.sub.CS, as denoted in FIG. 5, very close to each other. That is, peak value V.sub.CS-PEAK of current sense signal V.sub.CS could be about independent from voltage peak V.sub.LINE-PEAK Peak value V.sub.CS-PEAK represents a peak current of inductor current I.sub.PRM, and is capable of indicating the total energy converted from primary side SD.sub.PRM to secondary side SD.sub.SEC in one switching cycle T.sub.CYC. If the change in voltage peak V.sub.LINE-PEAK contributes substantially no influence to peak value V.sub.CS-PEAK it implies that energy converted in one switching cycle T.sub.CYC is about independent from voltage peak V.sub.LINE-PEAK of line voltage V.sub.LINE.

[0039] In one embodiment, reference voltage V.sub.MIN-ON-REF in the embodiment of FIG. 2 is the same with burst-mode reference voltage V.sub.BST-REF. When load 108 of switching mode power supply 100 becomes lighter and lighter, compensation voltage V.sub.COM drops and if it drops below burst-mode reference voltage V.sub.BST-REF switching mode power supply 100 enters burst mode operation. The quantity of load 108 for switching mode power supply 100 entering burst mode operation is hereinafter referred to as burst-mode load. In view of power supply system design, it is preferable the burst-mode load is about a constant independent from line voltage V.sub.LINE Switching mode power supply 100 according to embodiments of the invention could make this preference come true. If compensation voltage V.sub.COM is equal to burst-mode reference voltage V.sub.BST-REF the energy converted in one switching cycle T.sub.CYC, according to previous analysis, is about independent from voltage peak V.sub.LINE-PEAK of line voltage V.sub.LINE, implying the burst-mode load consuming the energy is independent from voltage peak V.sub.LINE-PEAK. Therefore, if input voltage V.sub.AC changes from 240 VAC to 100 VAC, or vice versa, the burst-mode load of switching mode power supply 100 could remain the same.

[0040] Furthermore, by properly setting the constants in equation (4), switching mode power supply 100 could trigger over-load protection when compensation voltage V.sub.COM exceeds an over-load reference value V.sub.OLP-REF, and this over-load reference value V.sub.OLP-REF corresponds to a specific load substantially not varying if the input voltage V.sub.AC changes from 240 VAC to 100 VAC.

[0041] While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.