Active inrush current limiting (BP
A unique feature of BP Series BEAR Power Supplies
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
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
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
FIGURE 1: Typical inrush current for a 15 W BP Series BEAR power supply
trace1 = 240VAC 60Hz line voltage. trace2 = input current to 15W
BP Series BEAR power converter