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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 

Figure 8 is the setup for input power and IRMS measurements. The setup shown is of a conventional PFC application. The input line and neutral voltage are both sensed through an attenuation network and subsequently sampled by two separate ADC inputs. Output voltage sensed by another ADC is used for voltage loop control. The current signal is sensed by a current shunt and is amplified and filtered by a signal-conditioning circuit. It is then connected to an error ADC (EADC) for current loop control, and to an ADC for input power and IRMS measurement. The measurements use the same conventional PFC setup. The traditional dedicated power metering chip and extra sensing circuit are eliminated. It greatly reduces the cost and design effort.

Figure 8: Input power and Irms measurement setup.

The current sense circuit needs to be calibrated. A simple 2 points calibration is developed: apply a fixed input voltage (either AC or DC), apply light load, then heavy load, record the actual input current and ADC measurements respectively, then the gain and offset of the current sense circuit can be derived.

For accurate power calculations, VIN and IIN need to be sampled at the same time. TIs UCD3138 provides a mechanism called dual sample and hold, which allows these two signals to be sampled simultaneously. The measurement error caused by VIN and IIN sampled at different time instants is eliminated and accurate measurement can be achieved.

All the mathematical calculations are carefully optimised, overhead on CPU usage is little, and there is no impact on normal PFC control.

This input power and RMS current measurement has been tested on 360W PFC. The results show this method gives excellent measurement accuracy.

Duty ratio feedforward control
Average current-mode control has been used in PFC for decades. Various analogue PFC control chips employing this control algorithm can be found in the commercial market. The performance of average current-mode control is often considered adequate for most commercial power applications with 50/60Hz AC line input. However, the traditional average current-mode control causes the inductor current to lead the input voltage, resulting in a non-unity fundamental displacement power factor and zero-crossing distortion.

Table 2: e-metering test results.

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