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Active inrush current limiting (BP Series)

A unique feature of BP Series BEAR Power Supplies

Download printable tech note (160Kb) with additional information about inrush current limiting techniques.

In any power converter, a large inrush current flows when the input capacitors are suddenly charged during the initial application of power.  If unrestricted, this current can easily exceed 50 Amps at high line. This high inrush current severely stresses the converter’s input rectifiers and capacitors.  It is also a common cause of nuisance fuse failures.

Typical power converters made by other manufactures require the user to add external inrush current limiting components such as an NTC thermistor. In contrast, all BEAR converters have active inrush current limiting circuitry built into the module.  This circuitry limits peak currents to less than 8 A, which is well below the 35A surge rating of the bridge diodes and eliminates stress on the internal fuses. The result is longer power supply life, less heat dissipation, and more reliable operation.

Figure 1 shows a typical inrush current for a 15 W BP Series BEAR Power Supply at 240 VAC.  The current limiting circuitry in the module senses the input voltage zero crossing, and then allows the converter’s capacitors to start charging. By starting the charging cycle at zero voltage, the internal active circuitry sufficiently limits the inrush current and eliminates the need to add external components to perform this function.

BEAR Power Supplies' active circuitry has two key advantages over conventional approaches, which use NTCs. 

  1. When power is applied to a circuit with an NTC, the NTC’s resistance starts off high: usually between 20 and 50 ohms for converters less than 15 W.  After the inrush current has passed and the converter is running at steady state, the NTC heats up and its resistance drops in a manner such that the NTC will dissipate about a 0.5 W for a 15 W converter.  That corresponds to a 3% loss in efficiency and added heat within the unit.  BEAR's active circuitry dissipates less than 0.25 W under the same conditions.

  2. NTCs limit the inrush current only when they are cold.  When power is turned off and quickly on again, the NTC does not have time to cool down and will not limit the inrush current.  The active circuitry in BEAR Power Supplies is effective even when an impatient user quickly cycles the power.

FIGURE 1: Typical inrush current for a 15 W BP Series BEAR power supply surge current graph

trace1 = 240VAC 60Hz line voltage. trace2 = input current to 15W BP Series BEAR power converter

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