DCDC Technical White Paper from Astec Power ____________________________________________________________________________________ A NEW APPROACH TO PFC INRUSH PROTECTION Linus Liu February 2003 ABSTRACT There are currently two common approaches to PFC inrush current protection. Such protection caters only for the first instance of power application at the switching on of the AC power. Faced with higher customer demand and expectations towards higher stability in power supply systems, a new concept is conceived that might fulfil the demand of an “available at all times” protection for the newer and improved generation of PFC units which themselves will be designed to be glitch-tolerant. INTRODUCTION The PFC circuit serves to generate a regulated, high voltage DC output while regulating the power factor of the power drawn from the input such that the current will be proportional to the input voltage at any particular instant. However, due to the fact that there is a large “bulk” capacitor across the output, high peak currents are drawn at the point of application of power to the input. This is known as “inrush” current. In order to prevent damage to the circuitry, a PFC converter usually requires an inrush protection circuit. In general, current protection methods only function at initial power application. Subsequent protection is not provided since most PFC converters self-protect by latching off or by entering into a nonoperation mode (protection mode) during a power irregularity. However, current market requirements increasingly demand power supply devices to be up and running at all times and to be able to recover as fast possible after a power interruption, rather than sitting in a “latched” mode which requires user intervention to reset the system.. REVIEW OF CURRENT PRACTISES The two common methods of current limiting are the resistive method and the SCR (TRIAC) method. 1. The resistive method uses a resistor of an appropriate rating connected in series with the main power connection to the PFC circuit. This resistor is removed (i.e. shorted with a relay contact) after the output capacitor is charged. Usually the relay drive circuit requires a low power supply from the PFC circuit itself to close the relay contact after the initial inrush has passed and the PFC converter has commenced operation. The PFC at this instance commences its own current limiting functions such as a “soft-start” or “Peak-Current detection”. This is an approach that causes power dissipation for a short duration. 1 Copyright © 2006 Astec International Limited For more information on the Astec product line go to www.astecpower.com or call 1-800-41-ASTEC 2. The SCR or TRIAC approach limits the inrush current by progressively varying the phase of the AC line voltage at which the SCR/TRIAC is switched on during start up. The instantaneous line voltage at which the SCR is activated is incrementally higher at each subsequent cycle ensuring that the difference between the line voltage and the output bulk capacitor voltage is small enough to result in negligible inrush current . This kind of circuit is considered to be non-power dissipating. However, the triggering circuit itself consumes power and the SCR has to be fed with a continuous pulse to prevent it switching off due to line glitches The power needed for this circuit is therefore not insignificant. These methods are “one-shot” which means that (unless the input power is cycled on/off or additional, complex circuitry is added) they cannot limit inrush due to subsequent disturbances on the power supply line after initial power on. NEW CIRCUIT DESCRIPTION Consider a PFC circuit consisting of a diode bridge feeding into a bulk capacitor through a boost choke and a boost diode, with the associated filtering and switching electronics. At the instant the power is applied, the circuit appears to the power line to be just a rectifying diode bridge feeding into a bulk capacitor. Since such a circuit has a very low reactance, the current may momentarily rise to a very high magnitude. FIG. 1 INRUSH-CURRENT LIMITING CIRCUIT CONNECTED TO A PFC Fig. 1 shows the INRUSH circuit diagram connected to a PFC unit. During normal operation, Q3 is off, and the FET Q1 conducts fully. When a high current level is detected (higher than the threshold, determined by the low-resistance connected to the source of the main FET) Q3 conducts and turns off 2 Copyright © 2006 Astec International Limited For more information on the Astec product line go to www.astecpower.com or call 1-800-41-ASTEC Q1. As Q1 turns off, a voltage is built up across R6 and D6 providing a bias to Q3 which remains on for the rest of the cycle or until the difference between Vin and V-PFC falls below the Q3 turn-on voltage. The bias to Q1 is maintained through R11, C5 and C19. The waveform during protection activation is shown in Fig. 2. FIG. 2 INRUSH CURRENT VERSUS INPUT VOLTAGE (SIMULATION) FIG. 3 INPUT CURRENT WITH PROTECTION ACTIVATED 3 Copyright © 2006 Astec International Limited For more information on the Astec product line go to www.astecpower.com or call 1-800-41-ASTEC DESIGN CONSIDERATIONS This circuit is simple and has the advantage of a low component count with the only penalty being a small dissipation in the rectifying diodes and the FET. A rectifying bridge is required but, as a bridge is always present for the PFC converter, this is not a problem. The maximum permitted inrush current is determined by appropriate selection of R1 and R3 and high voltage transients are filtered through the delay provide by R8 and C6. Care has to be taken in the translation from an RMS or average load power when calculating the appropriate values for these components. During activation, this protection circuit will disconnect the power for the current half-cycle which creates the possibility that the PFC may not recover from a power interrupt. This can be avoided by setting the current protection magnitude to a level below the current peak-limit of the PFC boost circuit. CONCLUSION A simple circuit of very few commonly available parts may fulfil the protection needs of a new generation of PFC models. It has clear advantages over existing solutions as it offers continuous inrush protection as opposed to at initial power up only. This is a useful contribution towards meeting the everincreasing customer demands and expectations for highly stable power sources. 4 Copyright © 2006 Astec International Limited For more information on the Astec product line go to www.astecpower.com or call 1-800-41-ASTEC
