Freescale Semiconductor M68HC08 manual 2.2PFC Control Theory, Introduction

PFC Control Theory

power, battery operated fluorescent tubes are driven with a single switch fly-back topology, but, the output transformer is coupled to the tube by a capacitive network and the current through the lamp is alternating current. However, the filaments (if any) cannot be automatically turned off by this simple configuration and the global efficiency is downgraded accordingly.

Dual switch circuits are divided into two main topologies:

•Half-bridge, series resonant

•Current fed push-pull converter

The half-bridge is, by far, the most widely used in Europe (100% of the so-called “energy saving” lamps and industrial applications are based on this topology), while the push-pull is the preferred solution in the USA with around 80% of the electronic lamp ballasts using this scheme today.

Both of these topologies have their advantages and drawbacks, the consequence for the associated power transistors being not at all negligible, as shown by Table 2-1. The half-bridge topology controlled by the dedicated MC68HC908LB8 MCU is implemented in Chapter 3. For more details about electronic lamp ballast theory see Reference [1.].

Table 2-1. Main Characteristics of the Dual Switch Topologies

Parameters	Half-bridge	Push-pull
V(BR)CER	700 V(1)	1100 - 1600 V(1)
Inrush Current	3 to 4 times I nom(2)	2 to 3 times I nom(2)
tsi window	2.6 – 3.6 µs	1.9 – 2.3 µs
Drive	High and Low side	Low side only
Intrinsic Galvanic Isolation	no	yes

NOTES:

1.These numbers are typical for operation on a 230 V supply.

2.I nom is the current into the transistors in steady state.

2.2PFC Control Theory

2.2.1 Introduction

The most practical electronic systems contain a conventional single-phase full-bridge rectifier and an input filter capacitor. It is well known that this type of circuit draws high current peaks from the power line and produces a high level of harmonics. High total harmonic distortion (THD) and low power factor therefore reduce the maximum power available from the mains and the efficiency of the electricity supply networks. The European Normative EN 61000-3-2 defines the limits of the harmonic content of the input current for mains supplied equipment.To meet the norms, new designs require an active PFC at the input.

Many specific integrated circuit devices (ICs) are available on the market to perform power factor correction. This approach requires additional electronic components, which increases the system cost and complexity. On the other hand, there is a way to implement PFC control using the MCU, in addition to the MCU’s main control tasks, such as motor control. Digital PFC allows missing out these specific ICs, thereby reducing the system cost. Another benefit of the software implementation is the potential for easy modifications without changing the hardware.

Two power factor correction approaches were implemented in this design, discontinuous conduction mode and hysteresis current control mode. Each of these topologies has advantages and drawbacks. Both topologies are described in the following.

Dimmable Light Ballast with Power Factor Correction, Rev. 1

Freescale Semiconductor 15