Adaptive pulse width control power conversation method and device thereof

The present invention relates to a power conversion device, and in particular to an adaptive pulse width control power conversion method and a device thereof.

In the current electronic and/or computer system that is getting complicated and sophisticated, a power conversion device plays a very important role. Referring to FIG. 1 of the attached drawings, a control circuit diagram of a conventional power conversion device is shown. The conventional power conversion device, generally designated at 100 , comprising a control logic circuit 1 that generates a series of pulse control signals Pout 1 to control a switching circuit 2 to generate a series of switching circuit output signals LX 1 by conducting on/off semiconductor devices (such as metal oxide semiconductor field effect transistors (MOSFETs) of the switching circuit 2 and then a filter circuit 21 is employed to convert an input voltage source Vin into an output voltage Vout 1 for a load 3 .

The control logic circuit 1 operates for output control with pulse width modulation (PWM). The PWM control features synchronization of a switching frequency of the switching circuit 2 with a system clock signal CLK and power conversion is realized through modulation of the pulse width of the pulse control signal Pout 1 . The modulation of the pulse width is based on feedback status signals of inductor current iL 1 and output voltage Vout 1 . For example, an inductor current detection circuit 41 , an error amplifier circuit 42 , and a comparator 43 (that provides a slope compensation signal Vs) shown in the drawing constitute a feedback circuit of the output voltage Vout 1 . The inductor current detection circuit 41 functions to detect the inductor current iL 1 . The error amplifier circuit has an input terminal that is supplied with a reference voltage Vref and another input terminal that receives an input of the output voltage Vout 1 supplied to the load 3 . The comparator 43 compares outputs from the error amplifier circuit 42 and the inductor current detection circuit 41 and generates a feedback signal Fb that is fed back to the control logic circuit 1 .

Besides PWM control, power conversion devices also adopt pulse frequency modulation (PFM) for power conversion. The PFM control features controlling switching frequency of a switching circuit and the switching frequency of the switching circuit is adjusted according to the difference of voltage conversion ratio or load. In case of high voltage conversion ratio and large load, the switching frequency of the switching circuit is increased, and in low voltage conversion ratio and small load, the switching frequency of the switching circuit is decreased. Compared to the PWM control, the PFM control maintains a fixed pulse width of the pulse control signal regardless of the voltage conversion ratio, the load, and variations of external devices, such as an inductor L and a capacitor C.

Generally speaking, for a power conversion device operated with PFM control, the efficiency of power conversion is high for light loading. When the lose of power for switching of the switching device is kept fixed, the ratio of the power lose of switching of the switching device with respect to output power is inversely proportional to the load. Thus, in light loading, a major portion of power lose occurs for the switching operation of the switching circuit. The PFM control reduces the switching frequency in light loading so that the power lose occurring for switching operation is reduced. Relatively speaking, the PFM control is of better efficiency in light loading than the PWM control, and the PWM control gets lowered conversion efficiency for light loading. Thus, the state-of-the-art power conversion device adopts an operation mode of combining PWM and PFM and switches from PWM control to PFM control in a light loading condition. FIGS. 2 and 3 respectively show waveforms of inductor current iL 11 , output voltage Vout 11 , and switching circuit output signal LX 11 in a PWM mode and those of inductor current iL 12 , output voltage Vout 12 , and switching circuit output signal LX 12 in a PFM mode.

However, for a conventional power conversion device that switches from a PWM mode to a PFM mode in a light loading condition, since the pulse width of the pulse control signal is fixed regardless of voltage conversion ratio, load, and external devices (such as inductor and capacitor), the ripple of output voltage in the PFM mode (see FIG. 3 ) is greater than the output voltage ripples of the PWM mode (see FIG. 2 ) and the output voltage ripple increases with the decreasing voltage conversion ratio. This is a major problem of the power conversion devices of this type.

Thus, an objective of the present invention is to provide an adaptive pulse width control power conversion method and a device thereof, which lower ripple of output voltage in a PFM mode and provide better stability of output.

The solution adopted in the present invention to overcome the problems of the conventional techniques comprises generating a series of PFM control signals in accordance with a predetermined pulse width upon detecting a mode switching event signal, comparing pulse widths of a PFM control signal and a PWM control signal and supplying a larger one of the pulse widths as an output of a pulse width control signal based on which a switching circuit is controlled, and a filter circuit then supplying an output voltage to a load. Pulse width of the PFM control signal is sequentially adjusted from a low value to a high value according to a predetermined pulse width increment, and based on detection of a feedback status signal of the load, the pulse width of the PFM control signal is set to an optimum pulse width for supplying an output voltage to the load in the PFM mode.

A circuit in accordance with the present invention comprises a pulse width adjustable PFM control circuit that generates a series of PFM control signals, a PWM control circuit that generates a series of PWM control signals, a PWM/PFM switching unit that is connected between the pulse width adjustable PFM control circuit and the PWM control circuit, a switching circuit that is connected to an input voltage source and the pulse width adjustable PFM control circuit, and a load status detection circuit that detects a feedback status signal.

In accordance with the technique solutions of the present invention, when the power conversion device is to be switched from a PWM mode to a PFM mode, based on different voltage conversion ratio and external devices, and according to feedback status signals of inductor current and output voltage of a load, the pulse width of a PFM control signal is adjusted to an optimum pulse width, whereby when the power conversion device works in the PFM mode, the ripple of the output voltage thereof is improved and increase of the ripple of the output voltage due to decreased voltage conversion ratio is eliminated so as to reduce power lose in switching and maintain excellent stability of output.

The present invention will be apparent to those skilled in the art by reading the following description of preferred embodiment of the present invention and the best mode for carrying out the present invention, with reference to the attached drawings, in which:

FIG. 1 shows a control circuit diagram of a conventional power conversion device;

FIG. 2 shows waveforms of signals from various circuits of the conventional power conversion device in a pulse width modulation (PWM) mode;

FIG. 3 shows waveforms of signals from various circuits of the conventional power conversion device in a pulse frequency modulation (PFM) mode;

FIG. 4 shows a control circuit diagram of a preferred embodiment in accordance with the present invention;

FIG. 5 shows a control circuit diagram of a pulse width adjustable PFM control circuit of the preferred embodiment in accordance with the present invention;

FIG. 6 shows a waveform diagram of signals associated with circuits of the present invention;

FIG. 7 shows a flowchart of an operation process of the best mode in accordance with the present invention;

FIG. 8 shows waveforms of signals from various circuits in a PWM mode in accordance with the present invention; and

FIG. 9 shows waveforms of signals from various circuits in a PFM mode in accordance with the present invention.

With reference to the drawings and in particular to FIGS. 4 and 5 , an adaptive pulse width control power conversion device 100 a in accordance with the present invention comprises a control circuit that comprises a pulse width adjustable pulse-frequency modulation (PFM) control circuit 11 , a pulse-width modulation (PWM) control circuit 12 , a PWM/PFM switching unit 13 , a switching circuit 2 , and a load status detection circuit 5 .

The pulse width adjustable PFM control circuit 11 is connected to the PWM control circuit 12 by the PWM/PFM switching unit 13 . As shown in FIG. 5 , the pulse width adjustable PFM control circuit 11 comprises a control logic unit 111 , which is connected to a memory unit 112 . The memory unit 112 stores therein a starting value of pulse width W, a pulse width increment dW, and a correction multiplication parameter N, based on which the control logic unit 111 controls, sequentially through a counter 113 and a digital-to-analog converter 114 , a pulse generator 15 to supply a series of PFM control signals PWMF to a pulse width comparator 116 . It is noted that for those having ordinary skills in the art, using a counter 113 to generate PFM control signals PWMF is only an example that is currently considered the best mode in carrying out the present invention and other ways may be equally used.

The PWM control circuit 12 generates a series of PWM control signals PWM 0 ′, which are applied through the PWM/PFM switching unit 13 to the pulse width comparator 116 of the pulse width adjustable PFM control circuit 11 . The pulse width comparator 116 compares the PFM control signal PWMF and the PWM control signal PWM 0 to identify and supply one of the PFM control signal PWMF and the PWM control signal PWM 0 that has a larger pulse width as an output of a pulse width control signal Pout 2 .

The switching circuit 2 is connected to an input voltage source Vin and the pulse width adjustable PFM control circuit 11 . The switching circuit 2 is controlled by the pulse width control signal Pout 2 to generate a series of switching circuit output signals LX 2 , so as to supply, through a filter circuit 21 , an output voltage Vout 2 to a load 3 .

The load status detection circuit 5 detects voltage and current of the load 3 and generates a feedback status signal Ecout to the PWM control circuit 12 and the PWM/PFM switching unit 13 . In the instant embodiment, the load status detection circuit 5 comprises an inductor current detection circuit 51 , a feedback circuit 52 , an error amplifier circuit 53 , a phase compensation circuit 54 , and a difference comparison circuit 55 .

The inductor current detection circuit 51 is connected to a node between the switching circuit 2 and the filter circuit 21 to detect an inductor current iL 2 supplied to the load 3 . The feedback circuit 52 is connected to a node between the filter circuit 21 and the load 3 to detect the output voltage Vout 2 supplied to the load 3 . The error amplifier circuit 53 has an input terminal that is supplied with a reference voltage Vref and another input terminal that is connected to the feedback circuit 52 and an output terminal connected through the phase compensation circuit 54 to an input terminal of the difference comparison circuit 55 . Another input terminal of the difference comparison circuit 55 is connected to the inductor current detection circuit 51 , and an output terminal of the difference comparison circuit 55 supplies an output, which is applied to the PWM control circuit 12 and is also applied, through the PWM/PFM switching unit 13 , to the pulse width adjustable PFM control circuit 11 .

Also referring to FIG. 6 , a waveform diagram of signals associated with each circuit of the present invention is shown, illustrating variation of waveforms of the PWM control signal PWM 0 , the PFM control signal PWMF, the pulse width control signal Pout 2 , a clock signal CLK, a mode switching event signal Enable, a PFM mode switching event signal PFM_Enable, a discontinuous current mode signal DCM, and the inductor current iL 2 when the device switches from a PWM mode M 1 into a PFM mode M 2 .

FIG. 7 shows a flowchart of an operation process of the best mode in accordance with the present invention. Also referring to FIGS. 4-6 , an explanation of the operation process of the best mode of the present invention will be given. The adaptive pulse width control power conversion device 100 a in accordance with the present invention is switchable between the PWM mode M 1 and the PFM mode M 2 and when the adaptive pulse width control power conversion device 100 a is switched from the PWM mode M 1 to the PFM mode M 2 , a transition phase M 3 is present to adjust a pulse width of the PFM control signal PWMF to an optimum value.

In a practical operation, firstly, the pulse width adjustable PFM control circuit 11 determines if a mode switching event signal Enable is detected (Step 101 ). The mode switching event signal Enable is generated when the load 3 is set with a light loading. Those having ordinary skills are fully aware that light loading can be identified by for example inspecting current, voltage, and pulse width of a pulse width control signal. The mode switching event signal Enable can be generated externally or internally. In a normal loading, such as an intermediate loading or a heavy loading, the adaptive pulse width control power conversion device 100 a is set in PWM mode M 1 (Step 102 ) and the PWM control circuit 12 , based on the input signal Ecout and CLK, generates a series of PWM control signals PWM 0 , serving as an output of a pulse width control signal Pout 2 .

When a mode switching event signal Enable is detected, meaning the loading is changed to a light one, the adaptive pulse width control power conversion device 100 a is switched from the PWM mode M 1 to the transition phase M 3 , wherein the pulse width adjustable PFM control circuit 11 sets a starting value W of pulse width and an increment dW of pulse width (Step 103 ). In the instant embodiment, the starting pulse width W and the pulse width increment dW are pre-loaded in the memory unit 112 . However, it is apparent that they can be supplied from an external device through other means.

Further, the PWM control circuit 12 continuously applies the PWM control signals PWM 0 generated thereby to the pulse width adjustable PFM control circuit 11 . The control logic unit 111 of the pulse width adjustable PFM control circuit 11 , based on the starting pulse width W, generates a series of PFM control signals PWMF (Step 104 ), wherein the PFM control signal PWMF is triggered by a leading edge of the PWM control signal PWM 0 so as to be made synchronous with the PWM control signal PWM 0 and both are applied to the pulse width comparator 116 .

The PFM control signal PWMF and the PWM control signal PWM 0 , after being fed to the pulse width comparator 116 , are compared by the pulse width comparator 116 in respect of the pulse widths thereof (Step 105 ), whereby the one of the PFM control signal PWMF and the PWM control signal PWM 0 that has a large pulse width is outputted as the pulse width control signal Pout 2 (Step 106 ). In particular, the step (Step 106 ) is performed by first determining if the PFM control signal PWMF has a pulse width greater than a pulse width of the PWM control signal PWM 0 (Step 106 a ). When the pulse width of the PFM control signal PWMF is larger, the PFM control signal PWMF is outputted as the pulse width control signal Pout 2 (Step 106 b ); when the pulse width of the PWM control signal PWM 0 is larger, the PWM control signal PWM 0 is outputted as the pulse width control signal Pout 2 (Step 106 c ). Although in the best mode of the present invention, the above steps are taken to provide the pulse width control signal Pout 2 , it is apparent that the PWM control signal PWM 0 can be taken as a reference, which is then successively incremented until a similar result of output of the pulse width control signal Pout 2 is done.

The pulse width adjustable PFM control circuit 11 controls the switching circuit 2 with the pulse width control signal Pout 2 and the filter circuit 21 then supplies an output voltage Vout 2 to the load 3 (Step 107 ). The load status detection circuit 5 may then detect a feedback status signal Ecout of the load 3 (Step 108 ), which is fed back to the PWM/PFM switching unit circuit 13 . In the embodiment illustrated, the feedback status signal Ecout comprises the inductor current iL 2 and the output voltage Vout 2 supplied to the load 3 .

The feedback status signal Ecout fed back from the load status detection circuit 5 functions to adjust the PFM control signal PWMF and the PWM control signal PWM 0 . As shown in FIG. 6 , based on the feedback status signal Ecout, the PWM control circuit 12 gradually reduces the pulse width of the PWM control signal PWM 0 when the loading is changed to a light loading. The pulse width adjustable PFM control circuit 11 , based on the pre-set pulse width increment dW, sequentially adjusts the pulse width of the PFM control signal PWMF from a low value to a high value and sets an optimum value in accordance with the feedback status signal Ecout (Step 109 ). In particular, in the step (Step 109 ), it is first determined if the pulse width of PFM control signal PWMF reaches the optimum value or not (Step 109 a ) and if the pulse width of the PFM control signal PWMF has not yet reached the optimum value, then the control logic unit 111 sequentially adjusts the pulse width of the PFM control signal PWMF from a low value to a high value in accordance with pulse width increment dW (Step 109 b ), whereby the pulse width of the PFM control signal PWMF successively incremented with the clock signal CLK and through repeated cycles of output, detection, and adjustment, the optimum value can be reached.

Identification of the pulse width of the PFM control signal PWMF reaching the optimum pulse width can be done with detection of the inductor current iL 2 , the output voltage Vout 2 ripple, and the variation of the load 3 , and the following processes can be used: (1) detecting if the inductor current supplied to the load changes from a discontinuous current mode (DCM) to a continuous current mode (CCM), detection time point being the end of a cycle of the clock signal CLK or 1/10 to 1 cycle of the clock signal CLK, detection current being from zero to two times of the current of the DCM/CCM boundary; (2) detecting if ripple of the output voltage supplied to the load is greater than ripple of the output voltage at the DCM/CCM boundary; and (3) detecting if an average of the inductor current supplied to the load is greater than an average of the inductor current when working at the DCM/CCM boundary. In this way, the optimum pulse width can be determined respectively for different conditions in respect of various voltage conversion ratios and variants of external devices.

After the optimum pulse width is obtained, it may be feasible to further carry out modification of the optimum value through a pre-set correction multiplication parameter N (Step 110 ) so as to increase or decrease the optimum value in consideration of different loads and circuits. In case that the optimum pulse width is increased, the ripple of output voltage is increased and the efficiency is improved. If the optimum pulse width is decreased, then the ripple of output voltage is decreased and the efficiency is lowered. Once the optimum pulse width is set, the set value of the optimum pulse width is locked and the pulse width adjustable PFM control circuit 11 applies the PFM mode switching event signal PFM_Enable to the PWM/PFM switching unit 13 to switch to the PFM mode M 2 . Subsequently, the output voltage Vout 2 is supplied in the PFM mode M 2 to the load 3 according to the pulse-width-adjusted optimum pulse width (Step 111 ) and the feedback status signal Ecout serves for adjusting the pulse frequency of the PFM control signal PWMF.

Referring to FIGS. 8 and 9 , FIG. 8 shows waveforms of the inductor current iL 21 , the output voltage Vout 21 , and the switching circuit output signal LX 21 in the PWM mode in accordance with the present invention and FIG. 9 shows waveforms of the inductor current iL 22 , the output voltage Vout 22 , and the switching circuit output signal LX 22 in the PFM mode in accordance with the present invention. Compared to the known techniques (see FIGS. 2 and 3 ), the present invention additionally carries out pulse width adjustment of the PFM control signal PWMF before entering the PFM mode and this makes it possible to obtain the optimum pulse width. Thus, in the PFM mode of the present invention, the ripple of the output voltage Vout 22 is not greater than the ripple of the output voltage Vout 21 of the PWM mode, so that the drawbacks of the known techniques can be improved.

Although the present invention has been described with reference to the preferred embodiment thereof and the best mode for carrying the invention, it is apparent to those skilled in the art that a variety of modifications and changes may be made without departing from the scope of the present invention which is intended to be defined by the appended claims.