In a typical ac-dc power supply, diode bridges do not control inrush current due to the output capacitor sudden charge at line plug-in. A high inrush current can cause several issues for the system like component failure (fuse, bridge diode, etc.), but it also represents an excessive current stress for the power grid. If nothing is implemented to limit the inrush current, the start-up current can easily be 10 to 20 times higher than the steady-state current. The power grids components have then to be rated to deliver such a high current for a short time. Moreover, the sudden current variation will lead to line voltage dips. These fluctuations will decrease the power delivered to other loads. Lamps or displays connected on the line could then present a brightness variation called the flicker phenomenon. To avoid such unwanted phenomenon, the IEC 61000-3-3 Electromagnetic standard defines the maximum allowed voltage fluctuation, and therefore the maximum allowable inrush current.
To meet the IEC 61000-3-3 standard the capacitor inrush-current is usually limited by a resistor (RLIM in Fig. 1a, which can be either a fixed-value resistor or a temperature-variable resistor. Temperature-variable resistors usually present a negative thermal coefficient (NTC) so that the resistor will have a high impedance value at cold-state, i.e. at start-up, and a low impedance value at steady-state. The low-value resistor is required at steady-state to limit the power losses dissipated by this resistor. A better solution consists of turning a switch on in parallel to bypass its impedance.
Mechanical relays are usually used for the bypass switch (S2 in Fig. 1a. The drawback of this solution is that the RLIM resistor is always connected to the line and thus supplies the diode bridge even if the application is in stand-by mode. As the DC capacitor (C) is still charged, standby losses will then occur. To suppress these losses, a switch has to be added in series with the line voltage (S1 in Fig. 1a, which is opened to disconnect the diode bridge in stand-by mode.
This solution allows the limiting of inrush current by slowly charging the output capacitor with a progressive Silicon Controlled Rectifier (SCR) soft-start. The SCR is triggered at the end of each line half-cycle to apply a reduced voltage to the capacitor. The SCR turn-on delay is then progressively reduced to enlarge the SCR conduction time and increase the energy applied to the DC capacitor.
This solution works if an inductor is present in series with the line (L in Fig. 1b This inductor comes for free as most applications, using a DC bridge, feature an SMPS or a motor inverter that requires a high-frequency switching filter. And most EMI filters feature a common mode inductor that presents a spurious differential-mode inductance.
An auxiliary supply is required in this solution to supply the MCU before the DC output capacitor is charged so that the MCU can ensure the SCR’s soft-start.
The entire solution for an inrush current limiter (ICL) function and stand-by losses suppression is then implemented by two SCRs, instead of one ICL resistor and two mechanical relays. The solid-state technology could then be cheaper than the mechanical technology solution and will suppress following drawbacks of mechanical relays:
· Higher control current consumption due to the coil
· Risk of switch opening in case of machine vibrations
· Acoustic noise due to the mechanical contact
· Risk of explosion in flammable environment (due to switching spark)
· Poor reliability (especially is the relay is switched on or off when a high DC voltage or a high current is present).
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