The IEC1000-3-2 specification, the Power Factor Correction (PFC) standard, has been approved with the goal of minimizing the total harmonic distortion (THD) of the current that is drawn from the mains supply. In practice, the standard requires the current to be nearly sinusoidal and in phase with the ac line voltage. This constraint affects all manufacturers of line-powered appliances who will have to comply (by the year 2001 for TV receivers and monitors, for example.) ON Semiconductor has developed an IC to meet this new requirement at low cost.

Why A Standard?

Most line-powered appliances use a bridge rectifier charging a large capacitor to derive raw dc voltage from the ac line. This technique results in high-harmonic content and in a poor power factor. In effect, the simple rectification technique draws power from the mains supply when the instantaneous line voltage exceeds the bulk capacitor voltage. In practice this occurs for a short period near the line’s peak voltage and results in high-charge current spikes. As a result poor power factors in the range of 0.6 result, which cause the following main drawbacks:

• All low-PF appliances absorb the high-current spikes at the peak voltage of the ac line, i.e. at exactly the same time. These spikes add to each other and, consequently, the distribution network faces huge peak currents,
• Neutral currents that are too high for the system may flow in three-phase distribution networks,
• The harmonic current may produce such severe voltage distortions in a distribution branch that appliances it supplies may not work properly,
• At any given power, the resulting rms current is almost double it would be with a unity power factor, and the available power that can be drawn is nearly halved. At the same time the power distribution network has to be built oversized to compensate.

Generally speaking low power factors degrade the quality and the power capability of the distribution network. The proliferation of line-powered electronic equipment has necessitated regulation of the harmonic content of the current drawn from the supply

Uncorrected Power Factor Circuit and Input Waveforms

Active solutions

Active solutions are the most popular means to meet the regulations. A PFC pre-regulator can be inserted between the input bridge and the bulk capacitor. This intermediary stage is, in fact, a switched-mode power supply (SMPS) that is designed to output a constant voltage while drawing a sinusoidal current from the line. In practice the step-up (boost) configuration is adopted as it is easier to implement, which is why the output regulation level is generally set to around 400 V in universal mains supplies.

Corrected Power Factor Circuit

PFC boost preconverters typically require a transformer, a diode and a power switch, and they also need a power factor correction controller that is a circuit specially designed to drive PFC pre-regulators.

ON Semiconductor has developed such drivers to facilitate PFC implementation. The MC34262 that is currently a benchmark, is now accompanied by a high-voltage version (MC33368) and by the recently-released MC33260 to lower the PFC preconverter cost in the BOM.

Benefits Of MC33260-driven PFC solutions

The MC33260 is a voltage-mode controller designed to drive discontinuous-mode operations, featuring a large variety of functions to optimize PFC design:

• Synchronization: The MC33260 offers a synchronization facility for EMI reduction. If the synchronization pin is grounded the system operates in free-running mode,
• High safety level: The MC33260 offers a high robustness to PFC solutions. In addition to the usual protections of over-current limitation and over-voltage protection, it also features under-voltage protection, a maximum on-time and an in-rush current detection. These features prevent the power switch from overstresses. In particular, the in-rush current detection guarantees that in any conditions (start-up, normal operation, or overload), the power MOSFET cannot turn on as long as some current flows through the PFC inductor,
• Compactness and system cost effectiveness: The MC33260 requires a minimum number of external components. In addition, it permits other savings:
• An in-rush resistor is generally placed in series with the rectifier bridge to limit the bulk capacitor charge-current when the power supply is powered. This resistor can be used to sense the current with the MC33260 eliminating a specific current-sense resistor,
• The MC33260 performs the zero-current detection by sensing the inductor current. No auxiliary winding is required and a simple coil can be used.

Consequently the MC33260 reduces costs without sacrificing either performance or safety.

Follower Boost Concept

Traditional PFC preconverters provide the load with a fixed and regulated voltage that typically equals 220 V or 400 V according to the line supply conditions (for US, European or a universal range.) This working mode is offered here but the part can also drive the "Follower Boost" (FB) mode.

In FB operation, the preconverter output is still regulated but the regulation level varies linearly versus the ac line amplitude, aiming to reduce the gap between the output and input voltages to optimize boost efficiency. The method is extremely effective and the effect on the size of both the inductor and the power switch is dramatic: In a 200-W wide-mains application the reduction ratio can easily exceed 2, making the preconverter extremely compact and halving costs with a smaller PCB area and cheaper power elements.

The FB mode also has the advantage that the PFC output voltage "follows" the input voltage peak value and its level is roughly what it would be if obtained by the traditional simple rectification technique of bridge plus bulk capacitor. Consequently, an uncorrected power factor SMPS can be directly associated to a FB PFC without difficulty -- a painless implementation existing systems.

If desired it is possible to adjust the output voltage range by deciding how much "follower" there is. The quantity of FB is tuned by the value of the oscillator capacitor (C_T)up to a value where classic PFC operation is reached.