Current balance circuit and multiphase DC-DC converter and current balance method thereof

[0001] 1. Field of the Invention

[0002] The present invention relates to a current balance circuit and a multiphase DC-DC converter and a current balance method thereof, and more particularly, to a current balance circuit and a multiphase DC-DC converter and a current balance method thereof capable of shifting a time of starting to increase a voltage of a ramp signal to perform the current balance, so as to maintain a slope and amplitude of the ramp signal to be constant to prevent a loop gain of the current balance technique from varying with output and input voltages.

[0003] 2. Description of the Prior Art

[0004] With the electronic products developing rapidly, requirements for specification on power management chips of the electronic products become more rigorous, especially on those for the microprocessors with high performance. In addition to high efficiency, the power management chips are also required to have an extreme high current driving capability and low ripples on the output voltage. In comparison with the conventional single channel DC-DC converter, a multiphase DC-DC converter has multiple parallel channels, and can distribute currents equally to each channel to provide a larger current. Then the multiphase DC-DC converter subtracts the output voltage by a reference voltage to be amplified via an amplifier to generate a voltage error signal, which will be compared with ramp signals with multiphase to adjust the duty cycles of pulse width modulation signals to control the power transistors of each channel, so as to adjust the currents of each channel. As a result, the multi-channel can provide power with multiphase to a load simultaneously, and thus can reduce ripples on the output voltage by well controlling the power with multiphase. Therefore, the multiphase DC-DC converter is most suitable for the high quality power management chips.

[0005] However, since there may be mismatches between the power transistor, inductor, and controller in each channel and on the printed circuit board (PCB), such that the currents of each channel may be imbalance and unequal. In addition to efficiency reduction, when the imbalance is severe, most of the currents flow through the same channel, and hence heats cannot be dissipated and the channel will be burnt. Therefore, the conventional multiphase DC-DC converter still needs a current balance technique to make the currents equally distributed to each channel to avoid the above situation to occur.

[0006] In detail, the conventional current balance technique applied in the multiphase DC-DC converters detects magnitudes of the currents flowing through each channel and calculates the average current, and then subtracts the current flowing through each channel by the average current to obtain a difference between the current flowing through each channel and the average current, so as to generate a current error signal. In the conventional current balance technique, the above current error signal is added to a voltage error signal, and thus each channel is corresponding to a voltage error signal, which can be utilized for controlling the output voltage and correcting the imbalance of the current. However, since this method combines the output voltage regulating and the current balance control on a voltage error signal, the control is more complex.

[0007] On the other hand, regarding the conventional current balance technique of adjusting slopes (i.e. amplitudes) of ramp signals, please refer to FIG. 1A to FIG. 1C. FIG. 1A is a waveform diagram of not performing the current balance control, and FIG. 1B and FIG. 1C are waveform diagrams of increasing and reducing amplitudes of ramp signals to perform the current balance, respectively. As shown in FIG. 1A, in a channel of the multiphase DC-DC converter, when current of the channel and the average current of all channels are equal and the current balance control is not performed, a voltage error signal V_C is directly compared with a ramp signal V_Ramp to obtain a duty cycle D required in this channel for stabilizing the output voltage. On the other hand, as shown in FIG. 1B, when the current of the channel is greater than the average current, the current of the channel should be reduced to achieve the current balance, and thus the slope (i.e. amplitude) of the ramp signal V_Ramp is increased according to the current error signal first, and then the ramp signal V_Ramp is compared with the voltage error signal V_C to reduce the duty cycle D, so as to reduce the current of the channel. On the contrary, as shown in FIG. 1C, when the current of the channel is less than the average current, the current of the channel should be increased to achieve the current balance, and thus the slope (i.e. amplitude) of the ramp signal V_Ramp is reduced according to the current error signal first, and then the ramp signal V_Ramp is compared with the voltage error signal V_C to increase the duty cycle D, so as to increase the current of the channel. In such a situation, for the channels with different currents, the current balance technique can simply adjust the slopes (i.e. amplitudes) of the ramp signals according to the currents of the channels to achieve the current balance.

[0008] However, regarding the above current balance technique of adjusting the slopes (i.e. amplitudes) of the ramp signals, loop gain of the current balance may change with variation of the duty cycle D. In such a condition, when the input voltage is far greater than the output voltage, the loop gain of the current balance is low, and performance of the current balance is worse, e.g. when the voltage error signal V_C is lower, the duty cycle D has less variation with the same degree of variation in the slope of the ramp signal V_Ramp; when the input voltage is close to the output voltage, the loop gain of the current balance is high, and performance of the current balance is better, e.g. when the voltage error signal V_C is higher, the duty cycle D has greater variation with the same degree of variation in the slope of the ramp signal V_Ramp, but the system stability is worse. Thus, there is a need for improvement of the prior art.

[0009] It is therefore an objective of the present invention to provide a current balance circuit and a multiphase DC-DC converter and a current balance method thereof capable of shifting a time of starting to increase a voltage of a ramp signal to perform current balance, so as to maintain a slope and amplitude of the ramp signal to be constant to prevent a loop gain of the current balance technique from varying with output and input voltages.

[0010] The present invention discloses a current balance circuit for a multiphase DC-DC converter. The current balance circuit comprises a current error calculation circuit, for generating a plurality of current balance signals indicating imbalance levels of a plurality of inductor currents of a plurality of channels of the multiphase DC-DC converter according to a plurality of current sensing signals of the plurality of channels, a time shift circuit, for adjusting pulse widths of a plurality of clock signals according to the plurality of current balance signals, and a ramp generator, for deciding shift levels of a plurality of ramp signals according to the plurality of clock signals.

[0011] The present invention further discloses a multiphase DC-DC converter. The multiphase DC-DC converter comprises a plurality of channel, for outputting a plurality of inductor currents, a current sensing circuit, for detecting magnitudes of the plurality of inductor currents to generate a plurality of current sensing signals, and a current balance circuit, comprising a current error calculation circuit, for generating a plurality of current balance signals indicating imbalance levels of the plurality of inductor currents according to the plurality of current sensing signals, a time shift circuit, for adjusting pulse widths of a plurality of clock signals according to the plurality of current balance signals, and a ramp generator, for deciding shift levels of a plurality of ramp signals according to the plurality of clock signals.

[0012] The present invention further discloses a current balance method for a multiphase DC-DC converter. The current balance method comprises generating a plurality of current balance signals indicating imbalance levels of a plurality of inductor currents of a plurality of channels of the multiphase DC-DC converter according to a plurality of current sensing signals of the plurality of channels, adjusting pulse widths of a plurality of clock signals according to the plurality of current balance signals, and deciding shift levels of a plurality of ramp signals according to the plurality of clock signals.

[0013] These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.

[0025] Please refer to FIG. 2, which is a schematic diagram of a multiphase DC-DC converter 20 according to an embodiment of the present invention. As shown in FIG. 2, the multiphase DC-DC converter 20 has N channels outputting inductor currents IL₁˜IL_N to an output load LD. The multiphase DC-DC converter 20 includes power stages PS₁˜PS_N, an error amplifier 202, a current sensing circuit 204, a current balance circuit 206, comparators Comp₁˜Comp_N, and a logic circuit 208.

[0026] In short, in the voltage-controlled multiphase DC-DC converter 20, each power stage among the power stages PS₁˜PS_N includes a high-side power transistor and a low-side power transistor, respectively, which can be turned on and off by means of pulse width modulation (PWM) according to duty cycles D₁˜D_N, to charge a capacitor C_OUT with an input voltage V_IN through inductors L₁˜L_N and a resistor R_esr, so as to generate an output voltage V_OUT for the output load LD. The error amplifier 202 amplifies a difference between the output voltage V_OUT and a reference voltage V_REF to generate a voltage error signal V_C for the comparators Comp₁˜Comp_N. Then the comparators Comp₁˜Comp_N compare the voltage error signal V_C with N ramp signals V_Ramp1˜V_RampN with multiphase, i.e. voltage-controlled structure, such that the logic circuit 208 generates the duty cycles D₁˜D_N to control the high-side power transistor and the low-side power transistor by means of PWM. In such a situation, when the output voltage V_OUT is deviated from a desired voltage level to cause the voltage error signal V_C to vary, e.g. the output load LD varies, the duty cycles D₁˜D_N may vary accordingly to drive the output voltage V_OUT back to the desired voltage level, so as to stabilize the output voltage V_OUT. The operation related to voltage stabilization is similar to the operation of the single channel DC-DC converter.

[0027] On the other hand, in order to ensure the currents flowing through each channel are equal, the current sensing circuit 204 detects magnitudes of the inductor currents IL₁˜IL_N flowing through each channel to generate current sensing signals I_s1˜I_sN indicating inductor current information in each channel for the current balance circuit 206, and then the current balance circuit 206 adjusts a time of starting to increase voltages of the N ramp signals V_Ramp1˜V_RampN with multiphase. Then the comparators Comp₁˜Comp_N compare the N ramp signals V_Ramp1˜V_RampN with multiphase with the voltage error signal V_C, such that the logic circuit 208 generates the different duty cycles D₁˜D_N to control the N channels, respectively. As a result, the present invention can shift the time of starting to increase the voltages of the ramp signals V_Ramp1˜V_RampN to perform the current balance, so as to maintain the slope of the ramp signals to be constant to prevent the loop gain of the current balance technique from varying with the output and input voltages.

[0028] In detail, please refer to FIG. 3A, which is a schematic diagram of the current balance circuit 206 shown in FIG. 2. As shown in FIG. 3A, the current balance circuit 206 includes a current error calculation circuit 300, a time shift circuit 302, and a ramp generator 304. The current error calculation circuit 300 generates current balance signals V_CB1˜V_CBN indicating imbalance levels of the inductor currents IL₁˜IL_N according to the current sensing signals I_s1˜I_sN, the time shift circuit 302 adjusts pulse widths of clock signals CLK₁˜CLK_N according to the current balance signals V_CB1˜V_CBN and a predefined clock signal CLK, and the ramp generator 304 decides shift levels (i.e. starting times) of the ramp signals V_Ramp1˜V_RampN according to the clock signals CLK₁˜CLK_N, and then outputs the ramp signals V_Ramp1˜V_RampN to be compared with the voltage error signal V_C, so as to generate the duty cycles D₁˜D_N.

[0029] In such a situation, please refer to FIG. 3B to FIG. 3D. FIG. 3B is a waveform diagram of the current balance circuit 206 shown in FIG. 3A not performing the current balance control, and FIG. 3C and FIG. 3D are waveform diagrams of the current balance circuit 206 shown in FIG. 3A advancing and delaying the time of starting to increase the voltage of the ramp signal V_Rampa to perform the current balance, respectively. As shown in FIG. 3B, in a channel (i.e. any one of the N channels) of the multiphase DC-DC converter 20, when an inductor current IL_a of the channel is equal to an average current IL_avg of the inductor currents IL₁˜IL_N of all channels and the current balance control is not performed, the ramp signal V_Rampa is controlled by a clock signal CLK_a. That is, when the clock signal CLK_a is high (i.e. logic “1”), the ramp signal V_Rampa is maintained at a predefined voltage level; when the clock signal CLK_a is low (i.e. logic “0”), the voltage of the ramp signal V_Rampa increases with a fixed slope. As a result, the required duty cycle D_a can be obtained by comparing the voltage error signal V_C with the ramp signal V_Rampa, i.e. when the output voltage V_OUT varies to cause the voltage error signal V_C to vary, the duty cycle D_a can vary accordingly to stabilize the output voltage V_OUT.

[0030] On the other hand, as shown in FIG. 3C, when the inductor current IL_a of the channel is greater than the average current IL_avg, the current of this channel should be reduced to achieve the current balance. At this moment time, the time shift circuit 302 reduces the pulse width of the clock signal CLK_a, and the ramp generator 304 advances the time of starting to increase the voltage of the ramp signal V_Rampa with a fixed slope, wherein the original ramp signal V_Rampa is denoted by a dotted line, and the adjusted ramp signal V_Rampa is denoted by a solid line. In such a situation, the duty cycle D_a obtained by comparing the ramp signal V_Rampa with the same voltage error signal V_C may be reduced, and thus the current of this channel will be reduced accordingly.

[0031] On the contrary, as shown in FIG. 3D, when the inductor current IL_a of the channel is less than the average current IL_avg, the current of this channel should be increased to achieve the current balance. At this moment time, the time shift circuit 302 increases the pulse width of the clock signal CLK_a, and the ramp generator 304 delays the time of starting to increase the voltage of the ramp signal V_Rampa with a fixed slope, wherein the original ramp signal V_Rampa is denoted by a dotted line, and the adjusted ramp signal V_Rampa is denoted by a solid line. In such a situation, the duty cycle D_a obtained by comparing the ramp signal V_Rampa with the same voltage error signal V_C may be increased, and thus the current of this channel will be increased accordingly. As a result, the present invention can shift the time of starting to increase the voltages of the ramp signals V_Ramp1˜V_RampN to perform the current balance for the voltage-controlled multiphase DC-DC converter 20.

[0032] Noticeably, the spirit of the present invention is to shift the time of starting to increase the voltages of the ramp signals V_Ramp1˜V_RampN to perform the current balance. Therefore, the slope of the ramp signals can be maintained as a constant to prevent the loop gain of the current balance technique from varying with the output and input voltages. Those skilled in the art can make modifications and alterations accordingly. For example, in the above embodiment, the ramp signal V_Rampa is controlled by the clock signal CLK_a to maintain at a predefined voltage level when the clock signal CLK_a is high and to rise with a fixed slope when the clock signal CLK_a is low. However, in other embodiments, the ramp signal V_Rampa can also be controlled by means of other methods. Besides, the above multiphase DC-DC converter 20 is voltage-controlled, and hence the comparators Comp₁˜Comp_N compare the voltage error signal V_C with the ramp signals V_Ramp1˜V_RampN to generate the duty cycles D₁˜D_N. Although the voltage control is easier, the system bandwidth is smaller and the transient response is slower. Therefore, in other embodiments, the multiphase DC-DC converter 20 may also be current-controlled. Although the current control is more complex, the system bandwidth is larger and the transient response will be faster. Note that the elements should be modified and alternated accordingly.

[0033] In detail, please refer to FIG. 4A, which is a schematic diagram of a multiphase DC-DC converter 40 according to an embodiment of the present invention. The multiphase DC-DC converter 40 is similar to the multiphase DC-DC converter 20, and hence elements and signals with similar functions are denoted by the same symbols. The main difference between the multiphase DC-DC converter 40 and the multiphase DC-DC converter 20 is that the multiphase DC-DC converter 40 is current-controlled. Therefore, the duty cycles D₁˜D_N is not derived from the voltage error signal V_C and the ramp signals V_Ramp1˜V_RampN, but is derived from the voltage error signal V_C and peak values of the inductor currents IL₁˜IL_N. Since the current-controlled DC-DC converter 40 needs to perform slope compensation to avoid sub-harmonic oscillation, a current balance circuit 406 of the multiphase DC-DC converter 40 shifts the time of starting to increase the voltage of the ramp signals V_Ramp1˜V_RampN, and then adds the ramp signals V_Ramp1˜V_RampN with the current sensing signals I_s1˜I_sN to output summation signals V_SUM1˜V_SUMN to be compared with the voltage error signal V_C, so as to generate the duty cycles D₁˜D_N, i.e. the ramp signals V_Ramp1˜V_RampN have the functions of both the slope compensation and the current balance.

[0034] Therefore, please refer to FIG. 4B, which is a schematic diagram of the current balance circuit 406 shown in FIG. 4A. The current balance circuit 406 shown in FIG. 4B and the current balance circuit 206 shown in FIG. 3A are similar, and hence elements and signals with similar functions are denoted by the same symbols. The main difference between the current balance circuit 406 shown in FIG. 4B and the current balance circuit 206 shown in FIG. 3A is that a ramp generator 408 of the current balance circuit 406 adds the ramp signals V_Ramp1˜V_RampN with the current sensing signals I_s1˜I_sN to output the summation signals V_SUM1˜V_SUMN.

[0035] In such a situation, please refer to FIG. 4C to FIG. 4E. FIG. 4C is a waveform diagram of the current balance circuit 406 shown in FIG. 4B not performing the current balance control, and FIG. 4D and FIG. 4E are waveform diagrams of the current balance circuit 406 shown in FIG. 4B advancing and delaying the time of starting to increase the voltage of the ramp signal V_Rampa to perform the current balance, respectively. As shown in FIG. 4C, in a channel (i.e. any one of the N channels) of the multiphase DC-DC converter 40, when an inductor current IL_a in the channel is equal to an average current IL_avg of the inductor currents IL₁˜IL_N of all channels and the current balance control is not performed, the current balance circuit 406 adds the ramp signal V_Rampa with the current sensing signal I_sa to generate the summation signal V_SUMa, and then the voltage error signal V_C is compared with the summation signal V_SUMa, so as to obtain the required duty cycle D_a, i.e. when the output voltage V_OUT varies to cause the voltage error signal V_C to vary, the duty cycle D_a can vary accordingly to stabilize the output voltage V_OUT.

[0036] On the other hand, as shown in FIG. 4C, when the inductor current IL_a of the channel is greater than the average current IL_avg, the current of this channel should be reduced to achieve the current balance. At this moment time, the time shift circuit 302 reduces the pulse width of the clock signal CLK_a, and the ramp generator 304 advances the time of starting to increase the voltage of the ramp signal V_Rampa with a fixed slope, wherein the original ramp signal V_Rampa is denoted by a dotted line, and the adjusted ramp signal V_Rampa is denoted by a solid line. In such a situation, the duty cycle D_a obtained by adding the ramp signal V_Rampa with the current sensing signal I_sa to generate the summation signal V_SUMa to be compared with the same voltage error signal V_C may be reduced, and thus the current of this channel will be reduced accordingly.

[0037] On the contrary, as shown in FIG. 4D, when the inductor current IL_a of the channel is less than the average current IL_avg, the current of this channel should be increased to achieve the current balance. At this moment time, the time shift circuit 302 increases the pulse width of the clock signal CLK_a, and the ramp generator 304 delays the time of starting to increase the voltage of the ramp signal V_Rampa with a fixed slope, wherein the original ramp signal V_Rampa is denoted by a dotted line, and the adjusted ramp signal V_Rampa is denoted by a solid line. In such a situation, the duty cycle D_a obtained by adding the ramp signal V_Rampa with the current sensing signal I_sa to generate the summation signal V_SUMa to be compared with the same voltage error signal V_C may be increased, and thus the current of this channel will be increased accordingly. As a result, the present invention can shift the time of starting to increase the voltages of the ramp signals V_Ramp1˜V_RampN to perform the current balance for the current-controlled multiphase DC-DC converter 40.

[0038] The operation of the above current balance circuits 206 and 406 can be summarized into a current balance process 50. As shown in FIG. 5, the current balance process 50 includes the following Steps:

[0039] Step 500: Start.

[0040] Step 502: Generate the current balance signals V_CB1˜V_CBN indicating imbalance levels of the inductor currents IL₁˜IL_N of the N channels of the multiphase DC-DC converter according to the current sensing signals I_s1˜I_sN of the N channels.

[0041] Step 504: Adjust pulse widths of the clock signals CLK₁˜CLK_N according to the current balance signals V_CB1˜V_CBN.

[0042] Step 506: Decide shift levels of the ramp signals V_Ramp1˜V_RampN according to the clock signals CLK₁˜CLK_N.

[0043] Step 508: End.

[0044] The detailed operation of the current balance process 50 can be referred to the above illustration, and will not be narrated hereinafter.

[0045] In the prior art, the method of adding the current error signal to the voltage error signal is to combine stabilizing control of the output voltage and the current balance control on the voltage error signal, and the control is more complex. On the other hand, regarding the current balance technique of adjusting slopes (i.e. amplitudes) of the ramp signals, since loop gain of the current balance may change with variation of the duty cycle D, when the input voltage is far greater than the output voltage, the loop gain of the current balance is low and performance of the current balance is worse, and when the input voltage is close to the output voltage, the loop gain of the current balance is high and performance of the current balance is better, but the system stability is worse.

[0046] In comparison, the present invention is capable of shifting the time of starting to increase the voltage of the ramp signal to perform the current balance, so as to maintain the slope of the ramp signal to be constant to prevent the loop gain of the current balance technique from varying with the output and input voltages. Therefore, the loop gain can be higher, and the system stability will not be influenced by the duty cycle.

[0047] Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.