Planar Inductors For
PQC's proprietary planar inductors are used as differential
chokes in practically all power topologies in variety
of applications. Among those are single or multi-winding
filtering chokes in industrial and commercial off-line
power supplies, as well as in military DC-DC converters.
Other applications include EMI differential filter
stages, power inductors for nonisolated topologies
such as buck, boost, buck-boost and others.
PQC inductors utilize the same advanced geometry
cores, patent pending SMD packages, heavy copper
planar windings and flexible tab technology as our
transformers. Benefits of PQC planar inductors are
similar to those of planar transformers such as small
size, high quality, reliability, repeatability, and
superior thermal management. PQC inductors cover
a wide range of rated currents and inductance values
from low power SMD chokes with less than a postage
stamp footprint to up to 400 amperes capability and
yet smaller than a credit card footprint parts.
A typical inductance curve of a single-winding inductor as a function of bias current is shown in Fig. 2-1 below.
In region I inductance value drops slightly mostly as a linear function of bias current. PQC specifies
the end of region I when inductance drops 10% to Lnom from the
value at zero bias current. The current associated
with Lnom is identified as I max, which is temperature dependent.
PQC characterizes cores at 100°C,
which is a reasonable maximum steady state operating
temperature of a power inductor. When inductor runs
at lower temperature than 100°C, the I max value
increases, and the "knee" moves to the
right. An opposite
happens at higher operating temperatures. PQC recommends
that designers run all planar inductors in region I
where core losses are low and total losses are dominated
by winding losses. In region I inductance value is
defined which helps to keep output ripple within specification,
and feedback loop stabilization is more predictable.
If bias current is allowed to rise beyond I max,
inductance drops in nonlinear fashion along "the
identifies the end of region II when inductance drops
25% from the value at zero bias current. Core losses
increase significantly in region II and together
with winding losses, which are proportional to Ibias ,
total losses to
a threshold of thermal runaway. Sometimes, designers
allow inductor to enter region II for a short time
duty cycle during current limiting operation with
a soft restart.
If bias current is allowed to rise beyond the point
of 25% drop in inductance value, the part enters region
where the core looses permeability fast and saturates.
No inductor should operate in this region, because
runaway and permanent damage are likely. In case
of an unexpected power surge, which may take the
in this region for a very short period of time, once
the surge is removed, the inductor returns to normal
without any damage.
Planar Inductor Selection
Every ferrite core has a
well defined ampere-turns capacity for a given gap
at elevated working temperature.
Having just a few standard cores a designer can
come up with an infinite variety of inductors
gap/turns combination for inductance/current requirement.
To help customers select an appropriate core size
PQC presents a matrix of parts in Table 2. The
matrix places preselected single-winding planar
in cells where their I max value intersects corresponding
Lnom values. A limited quantity of cells is filled,
they give sufficient understanding of what is
achievable. To meet multi-winding requirements PQC
Prototype Request Form.
Planar Inductor cooling
Depending on current rating and core size planar
inductors may run without any air flow for cooling.
In this case
they must be attached to a base plate with controlled
temperature. Inductors operating close to I
max and at elevated
ambient temperature must be cooled by a combination
of conduction and air flow.