PDF《balancing for an》

之后,误差乘以增益K,倍增输出用于相应调节相 位2的占空比.例如,如果相位1具有比相位2更高的电 流,则误差为正.倍增器输出(同样为正)增加相位2 的占空比及其电流.图2显示了这种配置. 交叉式升压PFC数字电流平衡 作者:Bosheng Sun,德州仪器 (TI) 系统工程师 图1 双相交叉式PFC Texas Instruments Incorporated

20 Analog Applications Journal High-Performance Analog Products www.ti.com/aaj 2Q

2013 Power Management output, which is also positive, increases the duty ratio of phase

2 and thus its current. This configuration is shown in Figure 2. Properly sampling the CT currents is critical for this approach. Since the CT outputs are saw waves, both currents need to be sampled at the same point for a fair comparison. An example would be to sample both at the middle of the switch'

s ON time, as shown in Figure 3. Here the unbalanced current causes different magnitudes of CT output. IIN PWM2 VBUS Q1 PWM1 D1 D2 L2 L1 PI PID + + C C VREF IREF A B C + + KM PWM1 PWM1 PWM2 VBUS VIN IIN ADC I_CT1 I_CT2 Digital Controller Duty-Ratio Adjustment K I_CT1 I_CT2 EMI Filter and Inrush Relay Q2 PWM2 + C +

1 VRMS

2 ADC ADC ADC ADC VS VIN RL CB RS1 Figure 2. Cycle-by-cycle duty-ratio adjustment I_CT1 Sample I_CT2 Sample I_CT I_CT2 I_CT1

0 Figure 3. Sampling CT signal 正确对CT电流采样是这种方法的关键所在.由于 CT输出为锯齿波,因此为公平比较,需在相同点 对两个电流采样.例如,在开关 导通 时间的 中间位置进行采样,如图3所示.此处,失衡电流 引起不同的CT输出大小. 正确CT电流采样以后,逐周期方法便可获得良好的电 流平衡.图4显示了360W、数字控制交叉式PFC的测 试结果.正如我们看到的那样,电感电流之间存在巨 大的差异,但在平衡以后它们几乎重叠在一起. 图2 逐周期占空比调节 图3 CT信号采样 Texas Instruments Incorporated

21 Analog Applications Journal 2Q

2013 www.ti.com/aaj High-Performance Analog Products Power Management With proper sampling of the CT currents, the cycle-by- cycle approach gives good current balancing. Figure

4 shows test results from a 360-W, digitally controlled inter- leaved PFC. As can be seen, there is a big difference between the inductor currents, but they almost overlap after being balanced. Because the second-phase duty ratio is adjusted on each switching cycle, and the adjustment may be different for each cycle since the current difference may vary between cycles, this method inevitably brings high-frequency noise to the AC input current. Figure 5a shows that the wave- form of the AC input current before current balancing is smooth and clean. Once current balancing is introduced, high-frequency noise appears (Figure 5b). Method 2: Half-AC-cycle duty-ratio adjustment Since adjusting the duty ratio on each switching cycle brings high-frequency noise to the total input current, it seems reasonable to try adjusting the duty ratio only once in each half AC cycle. Either average or peak inductor current in each half AC cycle can be used for current balancing. An example is to force the peak inductor currents to be equal in each half AC cycle by using a con- figuration similar to that in Figure 2. I_CT1 and I_CT2 are still sampled in each switching cycle, and the firmware finds out the peak value of I_CT1 and I_CT2 in each half AC cycle. These peak values are then compared, and the error is used to adjust the duty ratio. Since the current difference is calculated only once in each half AC cycle,