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Digital control paves way for improved PFC design

Posted: 16 Jul 2014     Print Version  Bookmark and Share

Keywords:Digital control  power factor correction  THD  total harmonic injection  PFC 

This situation gets worse with PFC operating under a high-frequency AC environment, such as 400Hz, often used in airborne systems. The high input current quality required in these systems is difficult to achieve through traditional control methods. A new control method, called duty ratio feedforward (DFF) control, can effectively reduce input current distortion under high-line frequencies.

The basic idea of DFF control is to pre-calculate a duty ratio to alleviate the task of the feedback controller. For the boost topology operating in continuous conduction mode, the duty ratio dFF is given by:

This duty ratio pattern effectively produces a voltage across the switch whose average over a switching cycle is equal to the rectified input voltage. A regular current loop compensator changes the duty ratio around this calculated duty ratio pattern. Since the impedance of the boost inductor at the line frequency is very low, a small variation of the duty ratio produces enough voltage across the inductor to generate the required sinusoidal current waveform.

Figure 9: Duty ratio feedforward control for PFC.

The resulting control scheme is depicted in figure 9. The feedforward duty ratio dFF is calculated by Equation 1. It is then added to the traditional average current-mode control output dI. The final duty ration d is used to generate a PWM waveform to control PFC.

An enhanced version, which combines the regular duty ratio feedforward and the hardware digital filter of a digital controller, such as the UCD3138, has been developed. Compared to the regular duty ratio feedforward control, the enhanced version increases the control loop bandwidth. Thus, the current waveform and THD are greatly improved.

ZVS/Valley switching
When PFC operates in DCM-mode, once the MOSFET turns off, the boost inductor current starts to decrease. The current will not stop decreasing when it reaches zero. Rather, it will continue going to negative value and an oscillation between the inductor and the total parasitic capacitance at the switching node occurs. This oscillation results in a significant current distortion and seriously deteriorates the THD.

A new control method developed by TI can force the MOSFET to turn on at the point where the switch-node voltage is either zero or at its valley, depending on the instantaneous input voltage. This significantly reduces current distortion, and, because of zero voltage switching (ZVS) or valley switching, efficiency is also improved (figure 10).

Figure 10: ZVS/valley switching test result.

Summary
Digital control unveils a new epoch in PFC design. Lots of advanced control algorithms can be implemented to improve the performance. In addition to the above mentioned methods, other features such as over sampling, dynamic loop compensation, non-linear control, frequency dithering, housing keeping, primary/secondary communication, to name a few can be implemented in a single digital control chip, this not only significantly improved the PFC performance, but also reduced the total cost.

Reference
1. Bosheng Sun, Zhong Ye, PFC THD Reduction and Efficiency Improvement by ZVS or Valley Switching, Application Report (SLUA644), Texas Instruments, April 2012.
2. Bosheng Sun, A low-cost input power and RMS current measurement solution, Analog ('ue'?) Applications Journal (slyt545), Texas Instruments, 4Q 2013.
3. Bosheng Sun, Duty-Ratio Feedforward control of Digitally Controlled PFC, Power Systems Design, 2013.

About the author
Bosheng Sun is with Texas Instruments.

To download the PDF version of this article, click here.


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