移动通信领域包络线跟踪电源的研究
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摘要
为提高移动通信的数据传输率和有效利用日益拥挤的频带资源，现代移动通信系统广泛采用非恒定包络的数字调制方式。此时若仍然沿用传统的恒压供电方式为移动通信基站中的功率放大器(PowerAmplifier, PA)供电，则PA会产生较大的功率损耗，造成巨大的能源浪费。包络线跟踪(Envelope Tracking, ET)电源是指其输出电压跟踪射频输入信号包络线的变化而变化，使PA始终工作在高效率状态，进而提升整个系统的变换效率。本文主要探讨ET电源的系统架构及相应的控制方法。
为同时满足高效率和高跟踪线性度的要求，可以采用开关线性组合结构来构成ET电源，本文讨论了开关变换器和线性放大器的具体组合方式，指出电压型输出的开关线性并联结构和开关线性串联结构适于构成ET电源。通过分析开关变换器PWM工作时的边带效应和多频域控制模型，指出开关变换器的高带宽设计面临的主要问题是所需要的开关频率过高，而实际电路中开关管的开关速度往往难以满足要求。为此，探讨提高系统跟踪带宽的同时降低所需要的开关频率的控制方法和电路结构，提出采用串入阻断二极管的多电平开关切换结构来拟合输出电压的方案和多幅值电流源拟合输出电流的方案，它们分别适用于开关线性串联结构和开关线性并联结构ET电源中开关变换器的设计。在此基础上，为适应两种结构对线性放大器的不同要求，讨论了线性放大器的拓扑选择。
开关线性并联结构ET电源采用电压型输出的AB类线性放大器和电流型输出的同步整流Buck变换器相并联。其中，线性放大器控制ET电源的输出电压，Buck变换器提供绝大部分的负载电流。通过推导线性放大器的输出电流表达式可以发现，当跟踪信号频率处环路增益不足够大时，线性放大器的输出电流会受到输出电压的影响而包含较大幅值的基波量，这大大降低了系统效率。为此，本文提出一种输出电压全前馈方法，可以完全消除输出电压对线性放大器输出电流的影响，大幅减小线性放大器输出电流中的基波电流成分，提高ET电源的效率。设计制作了一台跟踪频率为100kHz，输出电压为5V~15V正弦波，峰值输出功率为75W的ET电源原理样机进行实验验证，实验结果证明了所提出方法的有效性。
开关线性串联结构ET电源采用多电平开关切换结构和A类线性放大器相串联。其中，多电平开关切换结构用阶梯波电压的形式拟合出输出电压的形状，线性放大器仅需提供阶梯波电压与输出电压之间的差值。由于阶梯波电压包含非常丰富的谐波，系统闭环无法有效抑制其对输出电压的影响，导致输出电压上出现高频电压尖峰。为此，本文提出一种阶梯波电压前馈方法，以消除阶梯波电压对ET电源输出电压的影响，仿真结果证明了其有效性。但在实际电路实现时，前馈电路中运算放大器的放大能力会受自身带宽的限制，影响了前馈方法的实现效果。为此，提出了运算放大器级联通路的频率补偿方法，以扩展其有效带宽；提出了输出电压高频尖峰抑制方法，进一步消除阶梯波电压中的高频分量对ET电源输出电压的影响。设计制作了一台跟踪频率为300kHz，输出电压为10V~32V正弦波，峰值输出功率为50W的ET电源原理样机进行实验验证，实验结果证明了所提出方法的有效性。
For high data transmission rate and efficient utilization of the increasingly crowded spectrum,non-constant-envelope modulation methods have been widely used in modern mobile communicationsystem. In order to avoid considerable power loss of power amplifier (PA) in mobile base stationpowered by a constant voltage, envelope tracking (ET) technique is adopted, which provides a variablepower supply voltage tracking the envelope of the radio frequency (RF) input signal, so that the PA canalways operate in its theoretical high efficiency region, and the system efficiency can be improvedaccordingly. This dissertation is dedicated to the ET power supply configuration, control andexperimental verifications.
The switch-linear hybrid configuration for constructing the ET power supply can achieve bothhigh efficiency and high linearity. This dissertation discusses the possible combinations of switchingconverter and linear amplifier, and it is pointed out that the series-connected and parallel-connectedswitch-linear hybrid schemes with voltage-controlled output are preferred to construct the ET powersupply. By analyzing the sideband frequencies of the pulse-width modulation (PWM) based switchingconverters and the multi-frequency small signal model, it is indicated that achieving a high bandwidthof the switching converter often requires too high switching frequency, which is very difficult to obtainsuitable power switches. The possible control methods and new configurations are discussed to reducethe required switching frequency while retaining a high tracking bandwidth. The multilevel converterwith block diodes scheme and the multi-phase current sources scheme are proposed forseries-connected and parallel-connected hybrid configuration, respectively. Additionally, the topologyselection of the linear amplifier is discussed for the two hybrid schemes.
A voltage-controlled Class AB linear amplifier in parallel with a current-controlled synchronousrectification buck converter is employed to construct the parallel-connected hybrid ET power supply,where the linear amplifier controls the output voltage while the buck converter provides the most partof the load power. The expression of the linear amplifier output current is derived, and from which, it isfound that when the loop gain magnitude at the tracking frequency is not sufficiently high, the linearamplifier is forced to output a fundamental-wave current by the influence of the output voltage, whichgreatly degrades the system efficiency. This dissertation proposes a full feed-forward of the outputvoltage scheme, which can substantially reduce the linear amplifier output current. A prototype for100kHz sine wave tracking, with5V~15V output voltage, and75W peak output power is fabricated and tested in the lab. The experimental results verify the effectiveness of the proposed full feed-forward ofthe output voltage scheme.
A multilevel converter in series with a Class A linear amplifier is employed to construct theseries-connected hybrid ET power supply, where the multilevel converter fits the output voltage with astep-wave, and the linear amplifier provides the voltage difference between the step-wave voltage andthe final output voltage. Due to the abundant harmonics, the influence from the step-wave voltage onthe output voltage cannot be suppressed effectively by the loop gain, which leads to high spikesappearing at the ET power supply output voltage. This dissertation proposes a feed-forward of thestep-wave voltage scheme to eliminate this influence, and it is verified effectively by the simulationresults. However, in practical implementation, the operational amplifiers suffer bandwidth limitation inrelatively high-frequency range, thus better output voltage linearity cannot be achieved as expected. Toimprove the tracking performance, the frequency compensation scheme is proposed to extend theeffective bandwidth of the operational amplifier cascade path, and the output voltage high-frequencyspike elimination scheme is proposed to further eliminate the influence from the high frequencyharmonics of the step-wave voltage on the final output voltage. A prototype for300-kHz sine wavetracking, with10V~32V output voltage, and50W peak output power is fabricated and tested in thelab. The experimental results validate the proposed schemes.
为同时满足高效率和高跟踪线性度的要求，可以采用开关线性组合结构来构成ET电源，本文讨论了开关变换器和线性放大器的具体组合方式，指出电压型输出的开关线性并联结构和开关线性串联结构适于构成ET电源。通过分析开关变换器PWM工作时的边带效应和多频域控制模型，指出开关变换器的高带宽设计面临的主要问题是所需要的开关频率过高，而实际电路中开关管的开关速度往往难以满足要求。为此，探讨提高系统跟踪带宽的同时降低所需要的开关频率的控制方法和电路结构，提出采用串入阻断二极管的多电平开关切换结构来拟合输出电压的方案和多幅值电流源拟合输出电流的方案，它们分别适用于开关线性串联结构和开关线性并联结构ET电源中开关变换器的设计。在此基础上，为适应两种结构对线性放大器的不同要求，讨论了线性放大器的拓扑选择。
开关线性并联结构ET电源采用电压型输出的AB类线性放大器和电流型输出的同步整流Buck变换器相并联。其中，线性放大器控制ET电源的输出电压，Buck变换器提供绝大部分的负载电流。通过推导线性放大器的输出电流表达式可以发现，当跟踪信号频率处环路增益不足够大时，线性放大器的输出电流会受到输出电压的影响而包含较大幅值的基波量，这大大降低了系统效率。为此，本文提出一种输出电压全前馈方法，可以完全消除输出电压对线性放大器输出电流的影响，大幅减小线性放大器输出电流中的基波电流成分，提高ET电源的效率。设计制作了一台跟踪频率为100kHz，输出电压为5V~15V正弦波，峰值输出功率为75W的ET电源原理样机进行实验验证，实验结果证明了所提出方法的有效性。
开关线性串联结构ET电源采用多电平开关切换结构和A类线性放大器相串联。其中，多电平开关切换结构用阶梯波电压的形式拟合出输出电压的形状，线性放大器仅需提供阶梯波电压与输出电压之间的差值。由于阶梯波电压包含非常丰富的谐波，系统闭环无法有效抑制其对输出电压的影响，导致输出电压上出现高频电压尖峰。为此，本文提出一种阶梯波电压前馈方法，以消除阶梯波电压对ET电源输出电压的影响，仿真结果证明了其有效性。但在实际电路实现时，前馈电路中运算放大器的放大能力会受自身带宽的限制，影响了前馈方法的实现效果。为此，提出了运算放大器级联通路的频率补偿方法，以扩展其有效带宽；提出了输出电压高频尖峰抑制方法，进一步消除阶梯波电压中的高频分量对ET电源输出电压的影响。设计制作了一台跟踪频率为300kHz，输出电压为10V~32V正弦波，峰值输出功率为50W的ET电源原理样机进行实验验证，实验结果证明了所提出方法的有效性。
For high data transmission rate and efficient utilization of the increasingly crowded spectrum,non-constant-envelope modulation methods have been widely used in modern mobile communicationsystem. In order to avoid considerable power loss of power amplifier (PA) in mobile base stationpowered by a constant voltage, envelope tracking (ET) technique is adopted, which provides a variablepower supply voltage tracking the envelope of the radio frequency (RF) input signal, so that the PA canalways operate in its theoretical high efficiency region, and the system efficiency can be improvedaccordingly. This dissertation is dedicated to the ET power supply configuration, control andexperimental verifications.
The switch-linear hybrid configuration for constructing the ET power supply can achieve bothhigh efficiency and high linearity. This dissertation discusses the possible combinations of switchingconverter and linear amplifier, and it is pointed out that the series-connected and parallel-connectedswitch-linear hybrid schemes with voltage-controlled output are preferred to construct the ET powersupply. By analyzing the sideband frequencies of the pulse-width modulation (PWM) based switchingconverters and the multi-frequency small signal model, it is indicated that achieving a high bandwidthof the switching converter often requires too high switching frequency, which is very difficult to obtainsuitable power switches. The possible control methods and new configurations are discussed to reducethe required switching frequency while retaining a high tracking bandwidth. The multilevel converterwith block diodes scheme and the multi-phase current sources scheme are proposed forseries-connected and parallel-connected hybrid configuration, respectively. Additionally, the topologyselection of the linear amplifier is discussed for the two hybrid schemes.
A voltage-controlled Class AB linear amplifier in parallel with a current-controlled synchronousrectification buck converter is employed to construct the parallel-connected hybrid ET power supply,where the linear amplifier controls the output voltage while the buck converter provides the most partof the load power. The expression of the linear amplifier output current is derived, and from which, it isfound that when the loop gain magnitude at the tracking frequency is not sufficiently high, the linearamplifier is forced to output a fundamental-wave current by the influence of the output voltage, whichgreatly degrades the system efficiency. This dissertation proposes a full feed-forward of the outputvoltage scheme, which can substantially reduce the linear amplifier output current. A prototype for100kHz sine wave tracking, with5V~15V output voltage, and75W peak output power is fabricated and tested in the lab. The experimental results verify the effectiveness of the proposed full feed-forward ofthe output voltage scheme.
A multilevel converter in series with a Class A linear amplifier is employed to construct theseries-connected hybrid ET power supply, where the multilevel converter fits the output voltage with astep-wave, and the linear amplifier provides the voltage difference between the step-wave voltage andthe final output voltage. Due to the abundant harmonics, the influence from the step-wave voltage onthe output voltage cannot be suppressed effectively by the loop gain, which leads to high spikesappearing at the ET power supply output voltage. This dissertation proposes a feed-forward of thestep-wave voltage scheme to eliminate this influence, and it is verified effectively by the simulationresults. However, in practical implementation, the operational amplifiers suffer bandwidth limitation inrelatively high-frequency range, thus better output voltage linearity cannot be achieved as expected. Toimprove the tracking performance, the frequency compensation scheme is proposed to extend theeffective bandwidth of the operational amplifier cascade path, and the output voltage high-frequencyspike elimination scheme is proposed to further eliminate the influence from the high frequencyharmonics of the step-wave voltage on the final output voltage. A prototype for300-kHz sine wavetracking, with10V~32V output voltage, and50W peak output power is fabricated and tested in thelab. The experimental results validate the proposed schemes.
引文
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