Apollo Charalambous
Xibo Yuan
apollo.charalambous@bristol.ac.uk
xibo.yuan@bristol.ac.uk
EMI Reduction with the Soft-Switched Auxiliary Commutated
Pole Inverter
Introduction
Boost currents and ACPI control
For given resonant components, the boost currents Iboost and
Iboca control the resonant intervals tres_on and tres_off, both in value
and in being fixed or variable during a fundamental cycle T.
PWM voltage-sourced inverters used in motor drives operate
with high dv/dt’s and di/dt’s, which are linked with EMI
generation. Interference with surrounding equipment and
problems within the motor drive then occur. Thus, bulky EMI
filters are often required. Soft switching techniques, specifically
Zero Voltage Switching (ZVS) inverters, cannot only reduce
switching losses but also dv/dt’s. EMI can then be addressed at
its root, and filter size and weight can therefore be reduced.
𝑉𝑑𝑐
2
2
𝑡𝑟𝑒𝑠_𝑜𝑛 =
tan−1
𝜔0
𝑍0 𝐼𝑏𝑜𝑜𝑠𝑡
𝑉𝑑𝑐
2
2
−1
𝑡𝑟𝑒𝑠_𝑜𝑓𝑓 =
tan
𝜔0
𝑍0 𝐼𝑏𝑜𝑐𝑎 + 𝐼𝑙𝑜𝑎𝑑
A fixed-Iboost/variable-Iboca control scheme has been devised,
referred to as standard fixed-timing control. It’s simple in
design, but tres_on and tres_off vary throughout T. An optimized
scheme could lead to fixed tres_on and tres_off values. Hence the
vpole ramps could be fixed in duration, and the vpole line
spectrum could become more predictable and controllable.
The ACPI is a widely-studied ZVS inverter topology thanks to
such merits as PWM capability, independent phase-leg control,
modularity, and high efficiency [1]. An auxiliary branch
comprised of an inductor and auxiliary devices is added to an
inverter phase-leg, as well as snubber capacitors across the leg’s
main devices. The inductor and the capacitors form the
resonant circuit. Whenever a transition between the main
devices is imminent, the auxiliary switches turn on and the
inductor current iLr starts rising. When enough energy is
accumulated, resonance takes place which shapes the output
voltage (vpole) ramps in a slow, smooth manner. Consequently,
both ZVS turn-on and turn-off of the main devices are achieved.
The boost currents Iboost and Iboca are crucial for completing
resonance and are important resonance control parameters [2].
iLr
O
+-
Cr1
iLr
D1
Sa1
Sa4
Da1
Da4
Vdc/2
A
Lr
Cr4
D4
N
tramp_off
tramp_off
t
tres_on
vpole
tramp_on
Iboca
ΔΑ = 29 dBμV
@ 10 MHz
120
-40 dB/dec
100
Vdc
The ACPI half-leg and the resonant
magnitudes iLr and vpole during a switching
cycle under positive load current.
fc2 ≈ 8 MHz
140
0
tramp_on
fc2 ≈ 300 kHz
Hard-switched vpole spectrum
ACPI vpole spectrum
160
Iboost
+
-
-20 dB/dec
Iload > 0
tres_off
+- vpole
S4
180
Iload
Iload
200
switch turn-off
Itrip
+-
S1
Vdc/2
A hard-switched phase-leg’s vpole spectrum is compared against
that of the ACPI under standard fixed-timing control. The
designed control results in a highly asymmetrical ACPI vpole
pulse-train. Nevertheless a corner frequency is seen at around
300 kHz, marked as fc2. The hard-switched fc2 appears at 8 MHz.
Consequently, the slower ramps generated by the ACPI lead to
fewer EMI emissions in the higher frequency range. The -60
dB/dec slope is missing though, due to the unpredictability in
vpole ramp generation introduced by the control scheme; most
ramps are linear instead of sinusoidal.
80
-40 dB/dec
ΔΑ = 37 dBμV
@ 150 MHz
60
PWM signal
40
20
0
t
100
Line spectra of trapezoidal
and sinusoidal pulses
vpole
Trapezoidal pulse
Sinusoidal ramp pulse
Vdc
Vdc
τ
10k
700
Ideally during a switching cycle, a hard-switched inverter
produces trapezoidal vpole pulses, and the ACPI sinusoidal ones.
The trapezoidal pulse includes the rise time tr; the sinusoidal
pulse includes both tr and the first derivative rise time tr(dv/dt) [3].
vpole
1k
600
500
tr
t
tr = 2tr(dv/dt)
400
300
200
100
0
tres_on = 918 ns
tres_off = 370 ns
tr = 2tr(dv/dt)
170
fc1 = 1/(πτ) = 31.8 kHz
fs = 50 kHz
150
0 dB/dec
-20 dB/dec
fc2_trap = 1/(πtr) = 637 kHz
Trapezoidal pulse spectrum
Sinusoidal pulse spectrum
fc2_sine = 1/[π(tr - tr(dv/dt)] = 1.27 MHz
130
ΔΑ = 11.9 dBμV
@ 10 MHz
110
90
ΔΑ = 31.9 dBμV
@ 100 MHz
70
-40 dB/dec
50
30
-60 dB/dec
10
-10
-30
10k
100k
1M
10M
Frequency (Hz)
100k
Trapezoidal and sinusoidal spectra with Vdc = 650 V, fs = 50 kHz, τ = 10 μs,
tr = 500 ns, and tr(dv/dt) = 250 ns.
1G
Only the
vpole ramps
close to
900 ns turn
out
sinusoidal.
vpole
iLr
-300
The trapezoidal’s spectral envelope has two corner frequencies;
fc1 marks a slope change from 0 to -20 dB/dec and is defined by
the pulse-width τ, whereas fc2 marks a change from -20 to
-40 dB/dec and is defined by tr. In the sinusoidal’s envelope, fc2
is influenced by tr(dv/dt) and the slope immediately changes from
-20 to -60 dB/dec, with more attenuation at higher frequencies.
190
1G
100M
linear ramp shaped
by switch turn-off
-200
tr
10M
sinusoidal ramp shaped
by switch turn-on
-100
τ
t
100k
1M
Frequency (Hz)
Output voltage spectra for Vdc = 650 V, fs = 50 kHz, tr_hard-sw. = 40 ns, 360 ns ≤ tr_ACPI ≤ 900 ns.
inductor current iLr (A)
output voltage vpole (V)
+-
switch turn-on
SABER simulation results
Amplitude (dBuV)
P
Amplitude (dBuV)
Electrical Energy Management Group
The Auxiliary Commutated
Pole Inverter (ACPI)
844u
844.5u
845u
845.5u
t (s)
846u
846.5u
847u
847.5u
Conclusions
• Active dv/dt control is possible with the Auxiliary Commutated
Pole Inverter. This way, EMI emissions can be pre-determined
and mitigated at the design stage, leading to savings in filter size
and weight.
• Standard fixed-timing control leads to a highly asymmetrical vpole
pulse-train, which impairs dv/dt controllability and predictability.
• Through proper boost current control, fixed and predictable vpole
ramps are possible and the -60 dB/dec slope can be introduced.
• An ACPI half-leg prototype is in the works, as well as
development of the optimised control scheme.
[1] R. De Doncker and J. Lyons, 'The auxiliary resonant commutated pole converter', Industry
Applications Society Annual Meeting, 1990., Conference Record of the 1990 IEEE, 7-12 Oct.
1990, vol 2, pp. 1228-1235.
[2] R. Teichmann, 'Control parameter selection in auxiliary resonant commutated pole
converters', Industrial Electronics Society, 2001. IECON '01. The 27th Annual Conference of
the IEEE, 29 Nov-02 Dec 2001, vol 2, pp. 862--869.
[3] N. Oswald, B. Stark, D. Holliday, C. Hargis and B. Drury, 'Analysis of shaped pulse
transitions in power electronic switching waveforms for reduced EMI generation', Industry
Applications, IEEE Transactions on, vol 47, iss 5, pp. 2154--2165, 2011.