Page 15 - Issue 2 2023

ELECTRONICS

Simple methods for limiting current

using an integrated MOSFET

By Frederik Dostal, field applications engineer

urrents often have to be but also for limiting the current, in which case the switch works like a linear regulator.
limited in electronic circuits, for Such a switch must, however, ensure that the MOSFET is always operated within
Cexample in a USB port, excessive its safe operating area (SOA). If this is not the case, the semi-conductor and thus
current flow must be prevented so that the circuit will be damaged. Unfortunately, it’s not always easy to select a suitable
the electrical circuit can be reliably MOSFET and operate it in just such a way that it never leaves the SOA. The operating
protected. Similarly, in a power bank, temperature, the voltage, the current, and especially the time are all factors that
battery discharge must be prevented. influence this. They all have to be right in order to ensure safe operation. Figure 2
Discharging with too high a current can shows an SOA diagram for a typical N-channel MOSFET. Operation of the MOSFET
lead to an impermissibly high voltage below the lines shown is permitted.
drop in the battery and an insufficient Figure 3 shows a dedicated current limiter IC, the MAX17523 from Analog Devices.
supply voltage to the downstream It has two MOSFETs that can limit current to a value between 150 mA and 1 A. If the
device. current flow reaches the limit, it is either cut off and resumed after a certain waiting
Thus, it is often necessary to limit period, or the current flow is interrupted continuously until the next switch-on, or the
a current flow to a specific value. Most current is limited through a reduction in voltage. The internal MOSFET is then operated
power converters have overcurrent in the ohmic region. This is then a type of linear regulator function. In each of these
limiters to protect themselves from
damage due to excess currents. In some
DC-to-DC converters, the threshold can
even be adjusted.
In Figure 1, a DC-to-DC boost
converter with a built-in, possibly even
adjustable, current limiter could also be Figure 1: For current limiting in a power bank with an output current of 1 A per port.
used. In this case, the additional current
limiter block could be omitted. However,
there are also many applications in
which no DC-to-DC converter is used
in the power path. One example is
when a voltage of 24 V is available in a
system and the current flow in this line
should be limited, but the load has to
be operated at exactly 24 V. In this case,
an additional current limiter block, as
shown in Figure 1 in blue, can be used.
A current limiter circuit provides
a solution to this problem. It comes
from the family of protection modules,
which includes, for example, hot swap
controllers, surge protectors, electronic
circuit protectors and ideal diodes.
Most of these ICs on the market
use external MOSFETs as switches for
switching the current flow on and off, Figure 2: A typical SOA of a MOSFET.

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