A New Series of High Frequency Core Loss
Measurement Methods
Mingkai Mu, Fred C. Lee
Center for Power Electronics Systems
Virginia Tech
February 8, 2012
February 8 , 2012
Orlando, FL
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Increasing Frequency for High Power
Density
Frequency
10MHz
Enpirion (9A)
TI(1A)
Maxim (3.6A)
TI
(1A)
Maxim (10A)
1MHz
Linear
(8A)
Power-one ( 15A)
Delta (15A)
Year
100kHz
1998
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2000
2002
2004
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2006
2008
2010
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Core Loss Map for Magnetic Materials
10
4
Core Loss Density (kW/m3)
Bm=20mT
10
10
3
2
1
10 2
10
Iron Powder
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10
Frequency (kHz)
Ferrite
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Flake
10
4
Alloy Thin Film
3
Outlines
• Overview
• Existing core loss measurement methods
• Proposed core loss measurement methods
– For sinusoidal excitations
– For general excitations
• Conclusion
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Core Loss Measurement Method I
P=
c p ⋅ m ⋅ ∆T
dt
Pros
Arbitrary excitation
Block Diagram of Closed Type Calorimeter
Cons
•Setup is complex and time
consuming.
•Not good for low power
measurement
•If use winding to excite the
core, winding resistance can
be hardly separated.
*Chucheng Xiao, Gang Chen and .G.Odendaal, “Overview of Power Loss Measurement Techniques in Power
Electronics Systems”, Industry Applications Conference, 2002
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Core Loss Measurement Method II
ω Li
i
i
ωC
Choose C to resonant with L at the measured frequency,
so voltage on L and C cancel each other.
Vout − pk
Vin − pk
Rcore ≈
1
1
jωC
=
≈
Rcore + Rcu + Rc ωC ( Rcore + Rcu + Rc )
Rc +
Pros.
Minimize phase error.
Cons.
•Hard to exclude winding loss
•C value is critical
•Sinusoidal waveform only
1 Vin − pk
− Rcu − Rc
ωC Vout − pk
*Yehui Han, Grace Cheung, An Li, Charles R. Sullivan† and David J. Perreault, “Evaluation of Magnetic
Materials for Very High Frequency Power Applications”, Power Electronics Specialists Conference, 2008.
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Core Loss Measurement Method III
Equivalent Circuit model
Calculation
1
B=
v2 (t )dt
∫
N 2 Ae T
iR
Lm
Pcore =
N1 f
N 2 Rref
∫ v (t )v
2
R
(t )dt
T
*Thottuvelil, V.J.; Wilson, T.G.; Owen, H.A., Jr.; , "High-frequency measurement techniques for magnetic
cores," Power Electronics, IEEE Transactions on , vol.5, no.1, pp.41-53, Jan 1990
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Limitation: Sensitivity to Phase Discrepancy
vR iR
1° discrepancy
For a film resistor of 2 ohm, if the ESL is 0.5nH,
it will cause 0.9° discrepancy at 5MHz.
% Power Error
v2 vR
iR
Lm
1° phase discrepancy
Phase angle between
V2 and IR
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∆ = tan(ϕ v −i ) ⋅ ∆ϕ
Phase angle between V2 and IR
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Sources of Phase Discrepancy
Phase Discrepancy
Sample rate limitation
Mismatch between
probes
Pros
•Exclude winding loss.
•Arbitrary waveform.
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Cons
•Sensitive to phase error. So not suitable
for high frequency core loss measurement
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Outlines
• Overview
• Existing core loss measurement methods
• Proposed core loss measurement methods
– For sinusoidal excitations
– For general excitations
• Conclusion
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Proposed New Method I
-- capacitive cancellation
IR
V3
1
V3 = ( Rcore + jω Lm − j
)IR
ω Cr
1 T
Pcore = ∫ V3 I R dt
T 0
Cr is used to cancel the observed reactive voltage seen by the probe.
* Mingkai Mu, Qiang Li, David Gilham, Fred C. Lee, Khai D.T. Ngo, “New core loss measurement method for high frequency magnetic materials”,
Energy Conversion Congress and Exposition, Sept. 2010, pp. 4384 – 4389.
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Proposed New Method I (cont’)
-- capacitive cancellation
Pcore
1
=
TRref
T
∫ v3vR dt
0
VR
V3
% Power Error
Tune the capacitor value
1° phase discrepancy
Phase angle
Phase
Phase angle between V2 and IR
angle between V3 and IR
Given 1 ° phase discrepancy, 30° phase angle is enough to reduce phase error below 1%
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Measurement Setup
Core material: NiZn ferrite (4F1)
Number of Turns: 14:14
Core Cross section area: A=5.98*10-6m2
Average flux path length: l=7.19 cm
Osc-scope: Tek TDS7054
Hot plate
core under test
immersed in
thermal oil
Oscilloscope
Power Input
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Working Waveform
V2
V2 (secondary side)
VR
VR (current sensing)
V3
V3 (cancelled voltage)
Lm
Cr
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Measurement Result @ 10MHz
4F1 (NiZn Ferrite) @100 °C
4
Core Loss Density Pv (kW/m3)
10
3
10
2
10
1
10
1
2
3
4
5 6 7 8 910
20
30
Peak Flux Density Bm (mT)
+
。
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Datasheet Result@100°C
Without Cap (conventional method) @100°C
With Cap (new method) @ 100°C
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Factors May Influence Accuracy
• Phase Error
– This error can be minimized when Cr resonates at
excitation frequency.
• Resonant Cap ESR
– An additional resistor is in series with the Rc, Lm, C branch.
• Parasitic Capacitor
– These parasitic capacitors (inter- and intra-winding
capacitor and probe loading effect) will introduce a small
current on the secondary side.
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Improved Circuit to Reduce
the Probe Loading Effect
Ll1
i1
Ll 2
Lm
Cr
C probe
Measure the voltage on resonant cap, instead of the voltage
of transformer’s secondary. So it merges the parasitic cap
into the resonant cap, and has less loading effect by probe
input cap.
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Test Setup with DC Bias Field
Choke
Choke
+
_
AC voltage
Add Winding for DC Pre-magnetization
Two Choke inductance should be large enough to block AC current
2Lchoke >> Lm
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Improved Scheme
+
_
AC voltage
_
AC voltage
+
The AC voltage seen by the DC current source is minimized.
Yongtao Han, Yan-Fei Liu, "A Practical Transformer Core Loss Measurement Scheme for High-Frequency Power Converter", IEEE
Transactions on Industrial Electronics, 2008, pp. 941-948.
Mingkai Mu, Yipeng Su, Qiang Li, Fred C. Lee, "Magnetic Characterization of Low Temperature Co-fired Ceramic (LTCC) Ferrite
Materials for High Frequency Power Converters", Energy Conversion Congress and Exposition, Sept. 2011, pp. 2133 - 2138.
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Limited for Sinusoidal Excitation
Ll1
Ll 2
*
Lm
1
V3 = ( Rcore + jω Lm − j
)IR
ω Cr
The capacitor can only cancel the reactive voltage at a
single frequency.
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Proposed New Method II
-- Inductive Cancellation
Ll1
Ll 2
Lm
*
V3 = (− Rcore − jω Lm + jω L) I R
Phase angle
V2
VR
V3
Pcore
N1 f
=
N 2 Rref
∫ v v dt
3 R
T
Using inductor, we can still cancel the observed reactive voltage.
Mingkai Mu, Fred C. Lee, Qiang Li, David Gilham, Khai D.T. Ngo, "A high frequency core loss measurement method for arbitrary
excitations", Applied Power Electronics Conference, 2011, pg.157-162.
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Proposed Method II (cont’)
-- Compatible with Arbitrary Waveform
Ll1
Ll 2
Lm
*
V3 = (− Rcore − jω Lm + jω L) I R
When L = Lm, the reactive voltage is cancelled out, leaving only the
voltage on resistor.
For this method, a high Q inductor is needed in order to reduce the error
introduced by the loss of the inductor.
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Using Air Core or Low-loss Core
Transformer’s Magnetizing Inductor
Ll1
Core under test
Ll 2
Air core transformer or Low
core loss transformer
Using the magnetizing inductance of air core or low loss core to eliminate
the error caused by winding resistance.
From another perspective, the principle is to compare the core loss of DUT
with the reference core.
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Reference Core
Choice 1: Air core
Advantage:
no core loss
Disadvantage: larger parasitic capacitance
and inductance
Choice 2: Low loss core
Advantage:
smaller parasitics
Disadvantage: hard to find if measure low
loss material
not convenient to adjust
inductance value.
Choice 3: Low loss core with adjustable air gap
Advantage:
smaller parasitics
easy to adjust inductance value
Disadvantage: hard to find if measure low
loss material
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Excitation
The circuit is excited by the function generator signal amplified by power
amplifier.
The power amplifier has 50Ω internal impedance, so an impedance
transformer is needed to reduce the voltage drop on the internal impedance.
Programmable function generator is used to generate desired waveforms.
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Test Example
Core-under-test and Reference Core
Core Loss @ 1.5MHz Sinusoidal Excitation*
4
10
3
10
2
10
LTCC 40011 (ESL®) 4F1 (Ferroxcube®)
Core sample
Reference
Toroid LTCC 40011
n1:n2=10:10
Ae=3.82mm2
le=64.5mm
ve=246.4mm3
Toroid 4F1
N1:N2=9:9
Ae=11.88mm2
le=24.5mm
ve=291mm3
1
10 0
10
BLTCC
B4 F 1
1
10
2
10
1
vLTCC dt
∫
nA
= 1 eLTCC
= 2.8
1
v4 F 1dt
∫
N1 Ae 4 F 1
Core loss in 4F1 core is much smaller than LTCC 40011.
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*The core loss data is measured using the capacitive cancellation.
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Measurement Setup
4F1
Test circuit
LTCC 40011
in oil bath
Hot plate
Impedance
transformer
Test LTCC 40011 using 4F1
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Measurement Waveforms
Sinusoidal Voltage
V2 (secondary side)
VR (current sensing)
V3 (cancelled voltage)
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Core Loss of Sinusoidal Excitation
Measured core loss with capacitive and inductive cancellation
4
10
3
10
2
10
Method with Capacitor
Method with Transformer
1
10
3
4
5
6
7
8
9 10
20
30
Peak Flux Density B (mT)
*1.5MHz, 100°C
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Measurement Waveforms
Rectangular Voltage (50% Duty Cycle)
V2 (secondary side)
VR (current sensing)
V3 (cancelled voltage)
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Measurement Waveforms
Rectangular Voltage (25% Duty Cycle)
V2 (secondary side)
VR (current sensing)
V3 (cancelled voltage)
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Core Loss of LTCC 40011
Rectangular Voltage Waveforms of Different Duty Cycles
1.5
1.4
1.3
1.2
1.1
1
0.9
0.8
0
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0.6
0.8
1
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Conclusion
• A new series of high frequency core loss measurement
methods are proposed, which excludes winding loss and
reduce phase discrepancy sensitivity.
• The principle is using capacitor or inductor to cancel the
reactive voltage seen by the probe to reduce the sensitivity to
phase error. The capacitor version is good for sinusoidal
excitation; the inductor version is broad-band, so it can
measure the core loss of general waveforms.
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Conclusion (cont’)
• For capacitor version, high Q capacitor is needed to reduce the
ESR induced error. For inductor version, the reference core is
important for this measurement. Air core, low loss core or air
gapped core should be selected according to the core under
test. Adjustable capacitor or transformer will make the
measurement more easier.
• With this new series of methods, high frequency core loss can
be accurately measured at different excitations. The core loss
measurement problem can be addressed for power electronics
applications and material science research.
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Q&A
Thank you!
mmk@vt.edu
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