Charged with efficiency

Although most domestic appliances and office equipment are plugged directly into wall outlets and powered from high voltage alternating current, nearly all of their internal circuitry requires a low voltage direct current. Accordingly, power supplies are required to convert ac voltage to low dc voltage. According to the research by Ecos Consulting, roughly 3billion ac/dc power supplies are currently used in the US and about 10billion globally. As ac/dc supplies become even more pervasive, their impact on the environment, in terms of power supply efficiencym has attracted growing attention in the international community. The California Energy Commission (CEC) has proposed mandatory efficiency standards for external power supplies and various other regions of the world, which currently depend on voluntary regulation, are considering moving to mandatory standards in order to encourage higher efficiency in power supplies. More than half of the external power supplies are used for portable electronic devices, such as laptops, mobile phones and MP3 players. This means they have output voltage and output current regulation capabilities for battery charging, as shown in Figure 1. When it comes to applications where precise output current regulation is required, there is also a need for current sensing and this results in additional losses. For power supply designers struggling in an environment of increasing regulatory pressures, the output current sensing always has been a daunting design challenge. Primary side regulation for power supplies can be an optimal solution for alleviating the burden of achieving CEC regulation in charger designs. The primary side regulation controls the output voltage and current precisely, with the information in the primary side of the power supply not only removing the output current sensing loss, but also eliminating all secondary feedback circuitry. This facilitates a higher efficiency power supply design without incurring tremendous costs. This article describes the basic operating principle behind primary side regulation and introduces a highly integrated primary side regulation PWM controller that offers distinct advantages over conventional secondary side regulation methods. Figure 2 shows the basic circuit diagram of primary side regulated flyback converter. Generally, discontinuous conduction mode (DCM) operation is preferred for primary side regulation due to better output regulation. The key to primary side regulation is how to obtain output voltage and current information without sensing them directly. Once these values are obtained, the control can be easily done by the conventional PI approach. During the mosfet on time (TON), the input voltage (VIN) is applied across the primary side inductor (Lm). In this period, mosfet current (Ids) increases linearly from zero to the peak value (Ipk) and energy is drawn from the input and stored in the inductor. When the mosfet is turned off, the energy stored in the inductor forces the rectifier diode (D) to be turned on. During the diode on time (TD), the output voltage (Vo) is applied across the secondary side inductor (Lm?Ns2/Np2) and the diode current (ID) decreases linearly from its peak value (Ipk ? Np/Ns) to zero. At the end of TD, all the energy stored in the inductor has been delivered to the output. During this period, the sum of output voltage and diode forward voltage drop is reflected to the auxiliary winding side as (Vo+VF).Na/Ns. Since the diode's forward voltage drop decreases as current decreases, the auxiliary winding voltage best reflects the output voltage at the end of diode conduction time, where the diode current diminishes to zero. Thus, by simply sampling the winding voltage at the end of the diode conduction time, the output voltage information can be obtained. The diode conduction time can be obtained by monitoring the auxiliary winding voltage. Meanwhile, the output current estimation requires some multiplying calculations. Assuming that output current is the same as the average of the diode current in steady state, the output current can be estimated as Io=Ipk.(Np /Ns).(TD /2Ts). The output current estimator picks up the peak value of the drain current with a peak detection circuit and calculates the output current using the diode conduction time (TD) One technology that addresses primary side regulated power supply designs specifically is the primary side regulation PWM controller, an example of which is Fairchild's FAN102. This technology significantly simplifies the challenge of meeting tighter efficiency requirements, while eliminating external components that add both to cost and reliability issues. Using the FAN102, power supply designers can meet existing and pending requirements such as those of Energy Star and the CEC. It also features 'green mode' standby operation and satisfies the International Energy Agency's 1 Watt Initiative, aimed at reducing standby power losses to less than 1W. The FAN102 has integrated output cable voltage drop compensation and external component temperature variation compensation circuitry, which allows high accuracy, even at the end of the output cable for charger applications. The internal oscillator is frequency hopped to reduce emi. Another important feature of FAN102 is its wide VDD operation range – from 5V to 28V. When the power supply operates in constant output current mode, the supply voltage for the control ic, Vdd, changes together with the output voltage. Thus, the Vdd operation range determines the constant current control range and FAN102 allows stable constant current regulation even with output voltage lower than a quarter of its nominal value. Conclusion Pressured by the present and emerging energy efficiency regulations and market needs, power supply designers are now looking for better efficiency, robustness and performance – and they want all of these improvements without any additional cost. Fairchild's primary side regulation controller promises to improve the overall cost and performance of future ac/dc and dc/dc power converter solutions. A proprietary combination of sampling and output estimation techniques provides a tight regulation and efficient implementation for ac adapters needed everywhere from mobile and cordless phones to MP3 players. These adapters can now be smaller, less expensive and more efficient, thanks to the use of efficient and cost effective semiconductor technologies.