PFC Circuit Halts Inrush Currents

Jun 1, 2008 12:00 PM
By John Bottrill, Senior Applications Engineer, Texas Instruments, Manchester, N.H.

The use of power-factor correction (PFC) is becoming more common in the design of power converters. Their use is now being mandated by some countries and will continue to expand as line power-carrying capabilities become more and more critical. Additionally, the new ENERGY STAR efficiency category of converters will push this topology. This will cause many engineers to try designing PFCs for the first time.

Powering up a converter that's plugged into a live ac line and uses a PFC front end brings with it design challenges. When power is first applied to a circuit, there is usually a large transient current surge that warrants careful considerations, particularly for first-time designers.

Most PFC circuits consist of a boost front end that takes the universal ac line voltage and transforms it into a higher dc voltage at the output of the PFC stage. From there it is down-converted to the voltages that are needed by the unit being powered.

The key power components of a boost converter before the activation of the PFC control are the series-connected PFC inductor L1, a diode and the PFC capacitor C1. The effects of the input filter can be ignored because they are insignificant as compared to L1 and C1.

In a 500-W system, L1 will be on the order of 1 mH, and C1 will be on the order of 400 µF. When power is first applied, the output capacitor is charged by current that flows through L1 into C1.

If the ac voltage being applied is at a zero crossing of the ac line voltage when the connection is made, the output capacitor will charge within a quarter cycle and not overshoot because the resonant frequency of the LC, defined as 250 Hz, is significantly higher than the input line voltage, typically 50 Hz or 60 Hz. Charging the capacitor with a rectified 60-Hz sinewave is much lower than the resonant frequency of the LC (Fig. 1).

The peak current into the capacitor is approximately 55 A. This can create problems since the worst-case operating peak current is about 8 A.

Applying power at the zero crossing of the ac line voltage, however, is not the worst-case condition. If the input power is applied at a point where the input ac voltage is approaching the peak voltage, the output voltage will charge in much less than a half-cycle. Because it will now be limited to the resonance of the LC circuit, the output voltage will overshoot to a voltage that is potentially twice the input voltage, and the peak charge current will increase significantly (Fig. 2). The only limiting factors are parasitic elements such as the resistance of the inductor winding, the pc-board traces and the source impedance; however, these would not be enough to save components from failure.

The traditional method of preventing the voltage overshoot is to put a diode from the positive terminal of the input bridge rectifier to the positive terminal of the output capacitor. This will prevent any voltage overshoot of the output capacitor at turn-on. However, it does not prevent the overcurrent. In fact, it will increase the current spike. Though it is not shown here and is not required for start-up, with the PFC circuit that will be presented later in the article it does help stabilize the control for input-voltage transients.

The overcurrent is normally handled with either a positive temperature coefficient resistor (PTC) or a negative temperature coefficient resistor (NTC). A PTC is put in series with the input and limits the current while the output capacitor is initially charged. Once charged, an internal housekeeping power supply starts and operates a relay that shorts the PTC, at which point the PFC boost converter can be started. If there is a failure, the PTC's temperature increases and results in an increase in resistance, resulting in less current through the line.

The NTC starts with a high resistance, and as its temperature increases, its resistance decreases. It is usually left in the circuit, so as long as it stays hot, the resistance stays low. Care must be taken to ensure that it gets hot enough to become insignificant even under light loads, but has enough impedance with a high ambient temperature to still perform the defined function. Furthermore, although the resistance is small, it will still result in losses during operation.

There are cases where the inrush current must be actively controlled. A circuit to accomplish this is presented in Fig. 3. This circuit limits the current at turn-on to a maximum of 10% more than the maximum low-line operation limit. The circuit also limits the power dissipation in the active element, FET Q3. Thermal junction calculations are shown to assist in determining the device needed to provide the current-limiting function.

This circuit safely limits the start-up inrush current to the maximum expected line current during operation. It prevents overshoot of the output upon power application.

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