PDF《balancing for an》

19 Analog Applications Journal Texas Instruments Incorporated 2Q

2013 www.

ti.com/aaj High-Performance Analog Products Digital current balancing for an interleaved boost PFC Introduction A power-factor correction (PFC) converter lets the input current track the input voltage so that the load appears like a resistor to the voltage source that powers it. The most popular power topology used in active PFC is the non-isolated boost converter. For high power levels, two boost units can connect to the same bridge rectifier and operate at 180° out of phase (Figure 1). This is called two- phase interleaved PFC. By controlling two phases'

inductor currents 180° out of phase, both input- and output- current ripple can be reduced. As a result, a smaller electromagnetic-interference filter can be used, which reduces material costs. Due to discrepancies between the two sets of components used in the two boost circuits, the two inductor currents inevitably will be different. This situ- ation gets worse when PFC enters continuous-conduction mode (CCM). While the unbalanced current causes more thermal stress on one phase, it may also mistrigger over- current protection. Therefore, a current-balancing mecha- nism is necessary for the interleaved PFC design.1C4 This article discusses three different digital-control methods of balancing inductor currents. The first method senses the inductor current on each switching cycle, compares the current difference between the two phases, then adjusts the duty ratio of one phase cycle-by-cycle. The second method only adjusts the duty ratio in each half AC cycle. The third method uses two independent current loops to control each phase individually. Since these loops share the same current reference, the current is balanced automatically. Method 1: Cycle-by-cycle duty-ratio adjustment In this approach, a shunt is used to sense the total current. An average-current mode control is employed to force the input current to track input voltage. The pulse-width- modulation (PWM) controller generates two signals, each with the same duty ratio but shifted by 180° to drive the two boost stages. A current transformer (CT) is put right above the MOSFET in each phase to sense the switching current. The CT outputs are sampled and compared to each other;

then the error is multiplied by a gain K, and the multiplier output is used to adjust the duty ratio of phase

2 accordingly. For example, if phase

1 has higher current than phase 2, the error is positive. The multiplier Power Management By Bosheng Sun Systems Engineer VS RS1 RL Q1 CB VIN D1 D2 L2 L1 EMI Filter and Inrush Relay Q2 VBUS Figure 1. A two-phase interleaved PFC 引言 功率因数校正转换器让输入电流追随输入电压,这样,负载就好像是一个连接为其供电的电压源的电阻 器.有源PFC中使用的最为普遍的电源拓扑是非隔离 式升压转换器.就高功率级而言,两个升压单元可连 接同一个桥整流器,并工作在180°异相下(图1). 这被称作双相交叉式PFC.通过控制两个相位的电感 电流180°异相,可同时降低输入和输出电流纹波.结果,我们便可以使用更小的电磁干扰滤波器,从而降 低材料成本.由于两个升压电路中所使用的两套组件 之间的差异,两个电感电流必然不同.当PFC进入连 续导电模式(CCM)时,这种情况更甚.失衡电流在 一个相引起更多热应力,并且它还可能会误触发过电 流保护.因此,对于交叉式PFC设计而言,电流平衡 机制非常必要1-4. 本文为你介绍平衡电感电流的三种不同数字控制方 法.第一种方法检测每个开关周期的电感电流,比较 两个相位之间的电流差异,然后逐周期调节一个相位 的占空比.第二种方法仅对每半个AC周期的占空比进 行调节.第三种方法使用两个独立电流环路,单独控 制每个相位.由于这些环路共用相同的电流基准,因 此电流被自动平衡. 方法1:逐周期占空比调节 在这种方法中,使用一个分流器检测总电流.一个平 均电流模式控制用于强制输入电流追随输入电压.脉 宽调制(PWM)控制器产生两个信号,每个信号都有 相同的占空比但异相180°,用以驱动两个升压级. 在每个相位,电流变压器(CT)放置于MOSFET正 上方,以检测开关电流.对CT输出采样,然后相互比 较;