Tuesday, December 23, 2014

HAIER LTF42K1 AND LTF47K1 POWER SUPPLY CIRCUIT WITH POWER FACTOR CORRECTION

  HAIER LTF42K1 AND LTF47K1

SOME NOTES ABOUT POWER FACTOR CORRECTION (PFC)

                        Every year, millions and millions of notebook computers, LCD monitors and LCD televisions are produced. With such a fast growing number of these and other electronic devices using more and more power, actions must to be taken to ensure the functionality of the nationwide power grid. In 2001, the European Union put EN61000-3-2 into effect to set the harmonic regulation standard on any power grid supplied application with power consumption over 75 watts. This essentially requires power factor correction (PFC). Additionally, a standby power dissipation limit is set to conserve power when a load is OFF. “80 PLUS” is an initiative funded by electric utilities to integrate more energy efficient Power Supply Units (PSUs) - especially for desktop computers and servers. 80 PLUS certifies to more than 80% energy efficiency at 20%, 50% and 100% of rated load. To meet the 80 PLUS certification, PSUs require a PFC of 0.9 or greater at 100% load. This means PSUs that waste 20% or less electric energy (as heat at the specified load levels) will lead to reduced electricity consumption and lower bills. Rebates are sometimes given to manufacturers who use 80 PLUS certified PSUs. Implementing power factor correction (PFC) into switch mode power supplies will maximize:

1 .The power handling capability of the power supply

2.Current handling capacities of power distribution networks Input power factor (PF) is defined as:

PFC = REAL POWER (WATTS) / APPARENT POWER (VA )

PF is expressed as decimal number between zero and one (0 and 1). A non-corrected power supply with a typical PF equal to 0.65 will draw approximately 1.5 times greater input current than a PFC supply (PF = 0.99) for the same output loading. The non-corrected supply requires additional AC current to be generated which is not consumed by the load, creating I 2R losses in the power distribution network. There are two types of PFCs:

1. Active

2. Passive

Passive PFC

The simplest form of PFC is passive (Passive PFC). A passive PFC uses a filter at the AC input to correct poor power factor. The passive PFC circuitry uses only passive components > an inductor and some capacitors

Active PFC
 
Active PFC offers better THD and is significantly smaller and lighter than a passive PFC circuit. To reduce the size and cost of passive filter elements, an active PFC operates at a higher switching frequency than the 50Hz/60Hz line frequency.
Active PFC functions include:
1. Active wave shaping of the input current
2. Filtering of the high frequency switching
3. Feedback sensing of the source current for waveform control
4. Feedback control to regulate output voltage