Application Note

VISHAY DALE
www.vishay.com
Magnetics
Application Note
Selecting IHLP Composite Inductors for Non-Isolated
Converters Utilizing Vishay’s Application Sheet
INTRODUCTION
This application note will provide information to assist in the
specification of IHLP composite inductors based on given
operating conditions utilizing Vishay's IHLP application
sheets. It is assumed that the designer has a basic
understanding of non-isolated dc-to-dc converters as this is
not a design exercise in that family of converters. That being
said, tools will be introduced to allow the designer to select
an IHLP inductor and estimate its performance in their
applications. These tools will work for buck, boost and
buck/boost topologies. A set of application sheets will be
provided containing all the relevant constants and equations
needed for the selection process. This process is an
estimation only and all parts should be verified in their
application. It is also understood that the application sheets
are a work in progress and some of the data is estimated
through calculation. The estimated data will be highlighted
on the application sheets.
BACKGROUND
The IHLP inductor is constructed using an “open” or “air
coil” inductance coil. The two ends of the coil are connected
to a lead frame that acts as a means of transport through the
manufacturing operation at Vishay, and as the final
termination pads when the part is singulated from the lead
frame. A powdered iron core is pressed around the inductor
coil after the inductor coil is welded to the lead frame. The
characteristics of the powdered iron enhance the magnetic
properties of the inductor and also give the inductor its final
shape or footprint.
The only consistent item in a series of inductors will be the
performance of the iron powder from value to value;
therefore, the core loss constants for the material will remain
the same. There are, however, different iron powders used
in different product lines to cover a wider range of operating
conditions. Within these IHLP product lines the same
Revision: 08-Nov-11
Composite inductors are frequently used in non-isolated
dc-to-dc converters. This is not an issue, however the
waveforms associated with them are not in line with
conventional thinking. Core loss characterization and the
resulting data are often determined using sinusoidal
excitation. Dc-to-dc converters on the other hand do not
operate with sine waves, instead they use a pulsed DC
waveform. This means that the current waveform in the
inductor determining core loss will be a triangular wave, not
a sine wave. This difference will need to be compensated for
in the core loss calculations.
Increasingly, dc-to-dc converters are being asked to
operate at higher ambient temperatures. This in turn
requires the inductor to operate at the higher temperature in
addition to its own temperature rise incurred due to power
losses. It is known that iron powder exhibits the effects of
aging at higher temperatures in the form of increased core
losses. These losses must be accounted for during the
design process in order for a composite inductor to be used
at temperatures in excess of 125 °C. The effects of thermal
aging can be minimized by simply limiting the maximum
inductor temperature to 125 °C or less.
SELECTION TOOLS
Criteria
Start the inductor selection process by establishing the
selection criteria for the part. Composite inductors have
a recommended maximum component temperature of
125 °C. Subtracting the ambient temperature will give us
the maximum allowed temperature rise for the part. If this
number should exceed 40 °C it is recommended that 40 °C
be used for the allowed temperature rise. Core losses
should be limited to  1/3 of the total losses to mitigate any
aging effects associated with the powdered iron in the core
at elevated temperatures. Data sheets list a heat rated
current (IHEAT) as a parameter, which represents the current
needed to produce a certain temperature rise indicated on
the data sheet. This temperature rise is typically measured
using DC current and is due to copper losses only and does
not take into account core loss. However, this information is
Document Number: 34250
1
For technical questions, contact: [email protected]
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
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APPLICATION NOTE
The composite inductor is essentially built backwards from
a conventional inductor. In a conventional inductor the
magnet wire is wound either directly on the core as in a
toroid, or wound on a bobbin with the core halves inserted
into it as in “E” style cores. Since each IHLP size and value
has a unique coil dimension varying in outside and inside
diameter and height, each inductor has different geometric
parameters. This means that core constants must be
calculated for each inductor size and value.
inductance values do not use the same air coil, which means
constants will be required not only for geometry but for
material as well. What it comes down to is that each inductor
has its own unique parameters even within the same family
size.
Application Note
www.vishay.com
Vishay Dale
Selecting IHLP Composite Inductors for Non-Isolated Converters
Utilizing Vishay’s Application Sheet
useful since it can be used to determine maximum power
losses allowed in the inductor by multiplying the
temperature corrected resistance of the inductor by the
heating current squared. This will be the power loss (PHEAT)
to produce the temperature rise associated with the IHEAT
parameter.
In designing with the IHLP style of inductors core losses are
characterized using the Modified Steinmetz Equation (MSE)
[1] yielding Pv (core loss) in mW/cm3,
Pv = K0 x fe
Kf – 1
Kb
x B pk x fo
1
This equation uses an “effective frequency” of the
non-sinusoidal waveform along with the operational
frequency of the circuit while making use of the Steinmetz
parameters which are determined through curve fitting from
lab testing data.
Core Constants
The core loss equation shown previously has several
constants associated with it that must be determined. They
are the core constant K0, the frequency constant Kf, and the
flux density constant Kb. These constants are determined by
curve fitting of experimental core loss data established from
lab testing at Vishay. These constants are unique to the
IHLP material they are established for and do not change
from inductance value to inductance value. They will, of
course, change from material to material and are tabularized
on the application sheets.
Effective Frequency
Effective frequency (e) is a frequency other then the repeat
frequency of the circuit used to correct for non-sinusoidal
waveforms. The effective frequency for the waveforms
experienced in dc-to-dc converter inductors can be
characterized as follows [1]:
T
2
2
APPLICATION NOTE
 d---------B2- dt
0 dt
f e = -------------------------T
2  dB
------- dt
dt
0
This method equates the frequency to the sum of the
changes in the slope of the flux density divided by the
changes in amplitude divided by 2.
Method for Calculating Core Loss
To make use of the core loss equation, the effective
frequency (e) and maximum flux density (Bpk) need to be
established. These items are then plugged into the MSE to
determine core loss per unit volume. Since the V-μs product
of the circuit is known, or can be calculated, we will make
use of it to determine Bpk. Composite inductors can be
characterized in terms of their ability to handle a certain V-μs
product corresponding to 100 G. This parameter is referred
to as the ET100 constant. This constant will be different for
each inductor size and value and can be used to determine
Bpk as follows:
ET ckt
B pk = --------------- x 100
ET 100
3
where ETckt is the V-μs product of the circuit and the units
are in G. The V-μs product of the circuit can be taken from
the IC manufacturer's data sheets and application notes or
calculated from VOUT, frequency and duty cycle.
It can be shown that the effective frequency depends on the
duty cycle () of the circuit and the operational frequency
(0),
f0
f e = --------------------------------2
2 x   –  
4
Changes in the flux density in the magnetic circuit are
proportional to changes in the inductor current, assuming
that the inductance remains constant. Core losses are
caused by this ripple current according to the flux density
levels. The inductor is subjected to the input voltage during
the switch on time and a voltage of the opposite polarity
during the switch off time. The resulting plot of the flux
density is composed of two straight-line portions that
correspond to the integration of these two voltages over
time.
Once the effective frequency and the maximum flux density
are known, core losses can be estimated using the MSE.
The result from this equation will have the units mW/cm3 but
to simplify calculations the core volume is rolled into the K0
constant giving us the final version of the MSE for core loss
(Pcore) in W,
P core = K 0 x f e
Kf – 1
Kb
x B pk
x f0 x 10
– 14  5 
The core loss constants and associated parameters for the
IHLP composite inductor product line are contained in
Table 1, separated by inductance value and material type.
Revision: 08-Nov-11
Document Number: 34250
2
For technical questions, contact: [email protected]
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
Application Note
www.vishay.com
Vishay Dale
Selecting IHLP Composite Inductors for Non-Isolated Converters
Utilizing Vishay’s Application Sheet
SELECTION PROCESS
Method
Derating
To determine core losses a designer can calculate the peak
flux density, using V-μs product of the voltage waveform
across the inductor during operation. Utilizing this core loss
estimate, the designer can balance the combination of core
and copper losses to keep the total losses and associated
temperature rise less than the maximum 125 °C operating
temperature of the composite inductor. Care must be taken
to insure accurate copper losses by accounting for the
resistance increase of copper due the change in
temperature from room temperature (where DCR is
specified) to the maximum operating temperature and
losses due to AC components such as skin effect. Lastly,
check that IPEAK is  ISAT (found on the data sheet). Due to
the soft saturation feature of IHLP composite inductors,
IPEAK can exceed ISAT without a serious reduction in
inductance as observed in ferrite style inductors.
The heating current of IHLP composite inductors is
specified based on a 40 °C temperature rise due to the
current only; therefore we will need to derate the DC current
to allow for the heat rise due to core losses. We want to
select an inductor with a higher heating current than the
20 A the circuit demands, while minimizing size. Reviewing
the data sheets, we find the IHLP-4040DZ-01 series should
be a good fit.
Process
Data and application sheet specifications for the
IHLP-4040DZ-01 0.56 μH inductor are summarized in
Table 1.
IHLP-4040DZ-01
Inductance
0.56 μH
0.0017 
DCRMAX.
DESIGN EXAMPLE
IHEAT
Inputs
A designer is tasked with designing an inductor for the
following “buck” circuit, Vin = 5.0 V, Vout = 1.8 V, Iout = 20 A,
Frequency = 300 kHz, ambient temperature of 50 °C. To
strike a compromise between output capacitor size and cost
and inductor size we will design for a ripple ratio of
0.3  r  0.5. Ripple ratio is defined as the ratio of AC to DC
current components in the inductor. Allowing for a diode
drop of 0.5 V and estimating the switching voltage drop by,
27.5 A
ISAT
49.0 A
RTH
26.96 °C/W
PHEAT
1.48 W
ET100
0.88
K0
18.31
K1
0.00340
Kf
1.188
Kb
2.118
Table 1
V SW = I SW x R DSon
6
where Isw is equal to,
APPLICATION NOTE
I SW = I o x 
7
The above conditions produce design requirements of: Duty
cycle = 0.46, V-μs product of 4.14 and a required inductance
of 0.54 μH. Since we wish to keep the ripple ratio around 0.4
and since ripple ratio varies with inductance choose the
closest standard inductor value 0.56 μH. The circuit design
equations are summarized in Appendix A.
Start by determining Bpk, to do this use (3). Plugging in the
circuit ET product and the ET100 parameter of the inductor
gives us a Bpk of 470.5 G. The effective frequency from (4)
will be 192 216 kHz. The resulting core losses using (5) are:
P core = 18.31 x 192 216
300 000 x 10
1.188 – 1
– 14
x 470.5
2.118
To continue with the selection process we need to
determine total losses which must include copper losses. To
calculate copper losses we must determine operational
resistance (ROPER), circuit ripple current and the inductor DC
current. ROPER is the temperature corrected resistance of
the inductor and can be found, assuming a 40 °C rise in
temperature with
234.5 + T AMB + T RISE
R OPER = R MAX . x --------------------------------------------------------259.5
Revision: 08-Nov-11
x
= 0.248 W
8
Document Number: 34250
3
For technical questions, contact: [email protected]
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
Application Note
www.vishay.com
Vishay Dale
Selecting IHLP Composite Inductors for Non-Isolated Converters
Utilizing Vishay’s Application Sheet
Delta I (I), or ripple current, for a buck inductor is related to
the output voltage, inductance in μH, frequency in Hz and
duty cycle () according to,
V out + V DIODE
6
I = ------------------------------------- x  1 –   x 10
L x f0
9
The resulting I for this circuit is 7.39 A, the power loss
asoociated with AC effects can be determined using:
2
P ac = K 1 x I x
f OP x ROPER
 10 
Multiplying the square of the DC current in the inductor
(equal to the output current in a buck inductor) by the
temperature adjusted operational resistance from (9) results
in DC copper losses of 0.852 W. The power losses
associated with AC effects, from (10), are 0.217 W. Adding
these results to the core losses produces total losses of
1.317 W. The temperature rise can be found by multiplying
the total losses by the thermal resistance (RTH) found in the
inductor specification sheet. The temperature rise for the
inductor will then be,
T 0.56 = 1.917 x 26.96 = 35.51 C
It can be seen that by adding the temperature rise to
the ambient temperature the inductor will not exceed
the maximum recommended component temperature of
125 °C.
APPLICATION NOTE
Based on this it appears the IHLP-4040DZ-01 0.56 μH is
good choice for this circuit. The last item to check is to see
if the peak inductor current (IPEAK) is less then the saturation
current of the inductor. The peak current can be determined
by,
I  11 
I PEAK = I DC + ----2
IPEAK for this inductor is 23.70 A. Checking the data sheet
information the 0.56 μH part has a saturation current rating
of 49 A, therefore we can safely use this inductor for this
application.
References
[1] Jieli Li, T. Abdallah, and C.R. Sullivan, “Improved
calculation of core loss with non-sinusoidal waveforms”,
IEEE Industry Application Society annual meeting, 2001, pp.
2203-2210
Revision: 08-Nov-11
Document Number: 34250
4
For technical questions, contact: [email protected]
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
Application Note
www.vishay.com
Vishay Dale
Selecting IHLP Composite Inductors for Non-Isolated Converters
Utilizing Vishay’s Application Sheet
APPENDIX A
Buck Circuit Equations
Definitions
Vo + Vd
 = ------------------------------------V in – V SW + V d
Vo + Vd
6
- x  1 –   x 10
ET = ------------------f
Vo + Vd
6
x  1 –   x 10
L REQ = ----------------------Io x r x f
Vo + Vd
6
- x  1 –   x 10
I = ------------------L xf
I DC = I o

= Circuit duty cycle
Vo
= Circuit output voltage in V
Vd
= Diode voltage drop in V
Vin
= Circuit input voltage in V
VSW
= Switch (MOSFET) voltage drop in V
ET
= Circuit V-μs product
f
= Circuit switching frequency in Hz
LREQ
= Required inductance in μH
Io
= Circuit output current in A
r
= Ripple ratio
I
= AC current component in the inductor in A
IDC
= DC component in the inductor in A
ISW
= Current in the MOSFET switch in A
RDSon = The “on resistence” of the MOSFET switch in W
I SW = I o x 
APPLICATION NOTE
V SW = I SW x R DSon
Revision: 08-Nov-11
Document Number: 34250
5
For technical questions, contact: [email protected]
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
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