Abstract
A method and a circuit dynamically adjust a frequency of a clock signal that drives the operations of a power converter. The method includes (a) detecting a change from a predetermined value in an output voltage of the power converter; and (b) upon detecting the change, changing the frequency of the clock signal so as to restore the output voltage. The change, such as a load step-up, may be detected by comparing a feedback signal generated from the output voltage and a predetermined threshold voltage. In one implementation, changing the switching frequency is achieved in increasing (e.g., doubling) the frequency of the clock signal, as needed. The frequency of the clock signal need only be changed for a predetermined time period.
Claims
1. A method for dynamically adjusting a frequency of a clock signal that determines a switching frequency of one or more switches in a power converter, comprising: detecting a change from a predetermined steady state value in an output voltage of the power converter and indicating the change by an enable signal of a predetermined duration; and upon receiving the enable signal, doubling the frequency of the clock signal by combining in time the clock signal with its complement for the predetermined duration, so as to increase the switching frequency of the switches, thereby restoring the output voltage to the predetermined steady state value.
2. The method of claim 1, wherein detecting the change comprises comparing a feedback signal generated from the output voltage and a predetermined threshold voltage.
3. The method of claim 1, wherein the change comprises a load step-up.
4. The method of claim 1, wherein the change comprises a load step-down.
5. The method of claim 1, wherein the power converter is a multi-phase power converter.
6. A circuit for dynamically adjusting a frequency of a clock signal that determines a switching frequency of one or more switches in a power converter, comprising: detector circuit which indicates a change from a predetermined steady state value in an output voltage of the power converter by generating an enable signal of a predetermined duration; and frequency adjusting circuit including a frequency doubler circuit which, upon the frequency adjustment circuit receiving the enable signal, is enabled to double the frequency of the clock signal by combining in time the clock signal with its complement for the predetermined duration, so as to increase the switching frequency of the switches, thereby restoring the output voltage to the predetermined steady state value.
7. The circuit of claim 6, where the detector circuit compares a comparator that compares a feedback signal generated from the output voltage and a predetermined threshold voltage.
8. The circuit of claim 6, wherein the change comprises a load step-up.
9. The circuit of claim 6, further comprising a one-shot circuit which, upon detecting the change, provides the pulse of the predetermined duration during which to carry out doubling of the clock signal.
10. The circuit of claim 7, wherein the power converter is a multi-phase power converter.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) FIG. 1(a) is a schematic diagram showing single-phase circuit configuration 100 for one type of power converter.
(2) FIG. 1(b) shows the waveforms of output voltage (V.sub.O), the output current (I.sub.O), and the switching signal that controls the top-side switch of a power converter, in response to a step increase of 15 A load current.
(3) FIG. 2 illustrates a dynamic frequency adjustment scheme for improving transient response, according to one embodiment of the present invention.
(4) FIGS. 3(a) and 3(b) show the performances of a conventional system and the same system adapted for using a dynamic switching frequency adjustment scheme of the present invention, respectively.
(5) FIGS. 4(a) and 4(b) show the performances of a conventional system during a 10 A load current step-up and a 10 A load current step-down, respectively.
(6) FIGS. 5(a) and 5(b) show the operations of a system using a dynamic switching frequency adjustment scheme of the present invention that substantially meets the design specifications of the conventional system of FIGS. 4(a) and 4(b) under a 0 A-to-10 A step-up and under 10 A-to-0 A step-down in load current, respectively.
(7) FIG. 6 shows the voltage spike reductions for various threshold values, in accordance with one embodiment of the present invention.
(8) FIG. 7(a) shows clock circuit 700 which provides a clock signal for dynamically adjusting the switching frequency of a power converter for a load step-up, in accordance with one embodiment of the present invention.
(9) FIG. 7(b) shows selected signals of circuit 700 for implementing the dynamically adjusted switching frequency scheme.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
(10) According to one embodiment of the present invention, a dynamic switching frequency adjustment scheme improves transient response. FIG. 2 illustrates this dynamic frequency adjustment scheme, according to one embodiment of the present invention. FIG. 2 shows output voltage V.sub.O, feedback signal V.sub.FB, and the switching clock signals of the present invention. Feedback signal V.sub.FB may be derived from and may be made proportional to output voltage V.sub.O. The methods of the present invention detect a transient change in output voltage V.sub.O, such as a voltage undershoot condition. The voltage undershoot condition occurs, for example, when output voltage V.sub.O falls below a threshold voltage, such as during a load step-up (i.e., a sharp rise in load current). In the example of FIG. 2, feedback voltage V.sub.FB is 0.6V and the threshold voltage is set at 0.975 times V.sub.FB, or 585 mV. When the voltage undershoot condition is detected, a controller switches to a higher switching frequency, so as to reduce the switching cycle delay. In FIG. 2, the frequency is doubled. As shown in FIG. 2, at the higher switching frequency, the delay between detecting the voltage undershoot condition and the time the top-side switch is turned on (i.e., the switching cycle delay) is reduced from 2.31 s to 1.05 s. Consequently, the voltage undershoot is reduced from 86 mV (FIG. 1) to 46 mv, which is approximately a 46% reduction. The higher frequency operation may be maintained for 10 to 20 original switching cycles to ensure output voltage V.sub.O recovers smoothly. Thus, the voltage spike experience during the transient condition is significantly reduced, or equivalently, a smaller output capacitance is required to meet the same transient spike window. The methods of the present invention are equally applicable in multi-phase power converters as in single-phase power converters.
(11) FIGS. 3(a) and 3(b) show the performances of a conventional system and the same system adapted for using the dynamic switching frequency adjustment scheme of the present invention, respectively. The system of FIGS. 3(a) and 3(b) has the following design parameters: (a) a 12-volt input voltage (V.sub.in), (b) a 1-volt nominal output voltage (V.sub.O), (c) a 400 kHz switching frequency (f.sub.SW), (d) a 330 nH inductor (L), and (e) a 860 F output capacitance (C.sub.OUT), provided by two 330 F/9 m tantalum polymer capacitors, and two 100 F/2 m ceramic capacitors. As shown in FIGS. 3(a) and 3(b), the voltage undershoot is reduced from 133 mV to 89 mV by doubling the clock frequency for a load current step-up from 0 A to 20 A.
(12) As discussed above, the methods of the present invention allow the same design specification to be achieved with a lesser output capacitance requirement. For example, FIGS. 4(a) and 4(b) show the performances of a conventional system during a 10 A load current step-up and a 10 A load current step-down, respectively. This conventional system uses peak current mode control. The design specification for that conventional system is: (a) a 12-volt input voltage (V.sub.in), (b) a 1-volt nominal output voltage (V.sub.O), (c) a 400 kHz switching frequency (f.sub.SW), and (d) a 40 mV peak-to-peak voltage (V.sub.pp) limit for 10 A step-up and 10 A step-down in load currents. In the example of FIGS. 4(a) and 4(b), these specifications are substantially satisfied by a 330 nH inductor (L), and a 2220 F output capacitance (C.sub.OUT), which was provided by four 330 F/6 m tantalum polymer capacitors, and nine 100 F/2 m ceramic capacitors. As seen in FIGS. 4(a) and 4(b), a 22.25 mV negative voltage spike is experienced during a 0 to 10 A step-up in load current, and a 19.5 mV during a 10 A to 0 A step-down in load current, thus providing a total peak-to-peak voltage spike of 41.75 mA.
(13) The design specification of the conventional system of FIGS. 4(a) and 4(b) may be met using a dynamic switching frequency adjustment scheme of the present invention with a lesser requirement on the output capacitance. FIGS. 5(a) and 5(b) show the operations of such a system under a 0 A-to-10 A step-up and under 10 A-to-0 A step-down in load current, respectively. In the example of FIGS. 5(a) and 5(b), the switching frequency is doubled, when a voltage undershoot condition (i.e., load current step up) is detected, and halved, when a voltage overshoot condition is detected (i.e., load current step-down) is detected. In FIGS. 5(a) and 5(b), a 18.3 mV negative voltage spike is experienced during a 0 to 10 A step-up in load current, and a 23.75 mV during a 10 A to 0 A step-down in load current, thus providing a total peak-to-peak voltage spike of 42.05 mA. The specification is met by a 330 nH inductor (L), and a 1720 F output capacitance (C.sub.OUT), which was provided by four 330 F/6 m tantalum polymer capacitors, and four 100 F/2 m ceramic capacitors, which represents a reduction of output capacitance by 23%. Fewer ceramic capacitors also save significant cost. Further, as compared to the conventional nonlinear control method described above, a power converter using a dynamic switching frequency adjustment scheme of the present invention need only run in a linear control loop. Consequently, there is no concern related to interactions between a nonlinear control loop and a linear control loop, so that transient recovery can occur smoothly.
(14) An additional advantage of a system using a method of the present invention is its relative insensitivity to threshold setting. FIG. 6 shows the voltage spike reductions for threshold values that are set from 0.99 times of reference voltage V.sub.ref to 0.95 times reference voltage V.sub.ref. Reference voltage V.sub.ref may be set to, for example, 0.6V. As shown in FIG. 6, for a 10 A load current step-up, doubling the switching frequency provides the same performance improvement (i.e., a voltage spike reduction from 86 mV to 46 mV) over the range of threshold voltages between 0.96*V.sub.ref and 0.99*V.sub.ref.
(15) FIG. 7(a) shows clock circuit 700 which provides a clock signal for dynamically adjusting the switching frequency of a power converter for a load step-up, in accordance with one embodiment of the present invention. FIG. 7(b) shows selected signals of circuit 700 for implementing the dynamically adjusted switching frequency scheme. As shown in FIG. 7(a), circuit 700 receives (i) feedback signal V.sub.FB, representative of output voltage V.sub.O, (ii) threshold voltage V.sub.threshold, and (iii) clock signals CLK1 and CLK2 of the same frequency, but separated in phase by a 180. The waveforms of clock signals CLK1 and CLK2 are shown as waveform 751 and 752 in FIG. 7(b). When comparator 701 detects a load step-up condition, which occurs when V.sub.FB falls below threshold voltage V.sub.threshold, its output signal triggers one-shot timer 702 to provide a pulse in enable signal 703. The pulse in enable signal 703 has a duration spanning about 10 cycles of clock signal CLK1. Enable signal 703 is shown as waveform 753 in FIG. 7(b). Enable signal 703 causes clock signal CLK2 to be merged by AND gate 704 and OR gate 705 with clock signal CLK1 to provide output clock signal CLKx. The waveform of output clock signal CLKx is shown as waveform 754 in FIG. 7(b). As shown in FIG. 7(b), in waveform 754, the frequency of output clock signal CLKx is doubled during the duration of the pulse in enable signal 703.
(16) The above detailed description is provided above to illustrate specific embodiments of the present invention and is not intended to be limiting. Numerous modifications and variations within the scope of the present invention are possible. The present invention is set forth in the accompanying claims.
