Most switch-mode AC/DC power supplies tend to draw current from their power source in a non-sinusoidal fashion. Left uncorrected, this input current characteristic would result in current, and possibly also voltage, distortions on the supply line, degrading the quality of the supply and causing interference issues with other equipment on the same supply line. Furthermore, as the phase difference between the input voltage and current waveforms increases; i.e., as the power factor moves away from unity (1.00), higher input current is required.
To help prevent this, the International Electrotechnical Commission (IEC) established an international standard – IEC/EN 61000-3-2 – specifically for harmonic current emissions, as part of a suite of EMC standards. The standard became a mandatory requirement for applicable electronic products sold within the European Union in 2001, and nowadays is also accepted by most other industrialized countries in the world. EN 61000-3-2 applies to all electrical and electronic equipment with input power rated between 75W and 1kW with an input current of up to 16A per phase, designed for connection to the low-voltage AC public mains distribution network. The input stage of virtually all switch-mode AC/DC power supplies designed to operate from a mains supply consists of a bridge rectifier followed by a bulk capacitor and power stage as shown in Figure 1.
Click here for FIGURE 1 The bulk capacitor reduces the ripple on the voltage waveform into the DC/DC converter stage. The diode network and capacitor only draw current from the mains when the instantaneous mains voltage is greater than the capacitor voltage. Since the capacitor is chosen for a certain hold-up time, should the mains miss a number of cycles, its time constant is much greater than the frequency of the mains. This implies that the instantaneous mains voltage is greater than the capacitor voltage only for very short periods of time, tcharge. During these short periods, the capacitor must be charged fully, which means that large pulses of current of duration tcharge are drawn from the supply line over very short periods of time as shown in Figure 2. Click here for Figure 2
This is true for all rectified AC sinusoidal signals with capacitive filtering. They draw high amplitude current pulses from the mains supply, create significant harmonics and EMI, require over-sized components, and they are very inefficient. Power supply manufacturers consequently make every effort to incorporate some form of power factor correction (PFC) in their products.
There are basically two types of PFC: passive and active. Passive PFC circuits, as their name implies, use passive components – principally capacitors and inductors – to compensate for the inherent power factor of the circuit to be corrected. This approach is relatively simple to implement, and it is particularly cost-effective for low power applications. However, the line frequency components tend to be bulky, it cannot totally correct non-linear loads and it cannot easily accommodate dynamic load conditions. To overcome these limitations, several manufacturers have adopted a single-stage form of PFC for some of their AC/DC power supplies. This technique is cost-effective at low power levels, but it is not suitable for higher power supplies. Active PFC circuits utilize feedback with their own switch-mode stage, and they are designed to compensate for distortion as well as phase shifting of the input current waveform. Although significantly more complex than passive PFC, the availability of specialized control ICs has helped reduce implementation costs. Active PFC operates at frequencies much higher than the line frequency, enabling compensation of both distortion and phase shifting to occur within the time frame of each line frequency cycle, resulting in corrected power factors as high as 0.99. Conceptually, the popular basic converter topologies, including flyback and boost, are suitable for use in a high frequency PFC stage, though the most popular is the boost topology. Emerson Network Power, for example, uses a boost topology in its Astec NTS Series of 350W bulk AC/DC switch-mode power supplies (see photo). These 1U high power supplies accommodate an exceptionally wide mains supply range of 85V AC to 264V AC at any frequency from 47 Hz to 440 Hz (as well as DC inputs from 120 to 300V), and they use active PFC to minimize input harmonic distortion and maximize efficiency while maintaining a very compact form factor.
In order to achieve PFC over the entire range of input line voltages, the converter in the PFC stage must be designed so that the output voltage, Vout is greater than the peak of the input line voltage. Assuming a maximum line voltage of 264 Vrms and allowing for safety margin, this results in a nominal Vout of 380V DC. Vout is regulated via feedback to the operational amplifier U1 (see Figure 3). Click here for Figure 3
The sensed Vin will be in the form of a rectified sine wave which accurately reflects the instantaneous value of the AC input voltage. This signal, together with the Vout error voltage, is used as an input to the multiplier to formulate a voltage that is proportional to the desired current. This signal is then compared with the sensed actual converter current to form the error signal that drives the converter switch Q1. The result is that the input current waveform will track the AC input voltage waveform almost perfectly.
Tom Tillman is a Product Line Manager for Emerson Network Power with responsibility for the complete range of Astec standard low power AC/DC power supplies. He is based at the company’s facilities in Carlsbad, CA and can be contacted by email at tomtillman@astec.com. |
