Lewispaul at 01:24, 9 December 2012

2012-12-09T01:24:23Z

Lewispaul: Created page with "For SMPS applications I have successfully used LTspice to design EMI filters, but I always start out by running an ac simulation rather than the much more time consuming time ..."

2012-12-09T01:23:00Z

Created page with "For SMPS applications I have successfully used LTspice to design EMI filters, but I always start out by running an ac simulation rather than the much more time consuming time ..."

New page

For SMPS applications I have successfully used LTspice to design EMI
filters, but I always start out by running an ac simulation rather
than the much more time consuming time domain simulation followed by
an fft. This is done by substituting frequency dependent B-source(s)
for the transistors/ diodes at the place in your circuit where the
rectangular switching waveforms would be during a transient run. The
B-source can be made to approximate the peaks of sin(x)/x sort of
spectrum you would see in an fft of a repetitive pulse with limited
transition times (your switching waveform minus the ringing).

Here is the netlist snippet for the EMI current injected into the
first capacitor of the input filter in a buck type design:

V1 1V 0 AC 1
E1 LIM 0 FREQ {V(1V)*1} ( 500k, 66, 0) (501k, 60, 0)
B1 EMI 0 I=Gain/1uV*V(1V) laplace=1/((1+s/{P1})*(1+s/{P2}))
.param Vdc=300 Idc=1.3 f=100kHz tr=60n d=.4
.param Gain={2*Idc*d} P1={2*f/d} P2={2/tr}

V1 sets up a unity (0dB) reference both for generating the limit
line (in E1) and the EMI shaped by the Laplace source, B1. The poles
for the Laplace statement depend on the amplitude, frequency, duty
cycle and rise time of the switching waveform (as defined in the
parameter statements). In real life your circuit's EMI source will
almost always have additional ringing in the time domain that will
generate several resonant peaks beyond the simple pulse frequency
domain envelope approximation given above (but it is a good start).

You can check the validity of this approximation by comparing it to
an fft of a pulse source set up matching the parameters. Bear in mind
that any SMPS switching cell invariably injects pulses of current
into the capacitor on its one end and pulses of voltage into the
inductor on its other side. If you poke around on the net you might
find one of the several papers justifying and explaining this
technique in detail.

Differential mode EMI results directly from the switching action
and can be well simulated by replacing the switching cell with their
equivalent sources as described above. However, the source of common
mode EMI is not so straightforward and will probably require several
guesses and/or measurements of various key stray capacitances between
EMI "hot" nodes and chassis (PE). The most common critical points are
switching transistor-to-heatsink mounting capacitance and transformer
primary-to-secondary interwinding capacitance.

It is very easy to make good models of your LISNs in LTspice and
you will have to model the line cord as loosely coupled inductors.
Depending on the specified test setup (i.e, whether your test unit
is bonded directly to the testing ground plane or sitting a specified
distance above) you may need to add into your simulation a few
chassis-to-ground plane capacitors (usually tens of pFs). These
can be responsible for some nasty common resonances in the upper
MHz range.

One last caveat: be sure to make use of LTspice's unique ability to
accept realistic parasitics (ESR, ESL, etc.) into the definition of
the capacitors and inductors in your EMI simulation circuit model.
Fill them all in even if you have to guess. EMI suppression film
capacitors typically have ESLs of 10nH to 50nH and ESRs of 3m to
30m ohms.

Good luck to all -- analogspiceman

@@ Line 12: / Line 12: @@
 first capacitor of the input filter in a buck type design:
-V1 1V 0 AC 1
+V1 1V 0 AC 1<br>
-E1 LIM 0 FREQ {V(1V)*1} ( 500k, 66, 0) (501k, 60, 0)
+E1 LIM 0 FREQ {V(1V)*1} ( 500k, 66, 0) (501k, 60, 0)<br>
-B1 EMI 0 I=Gain/1uV*V(1V) laplace=1/((1+s/{P1})*(1+s/{P2}))
+B1 EMI 0 I=Gain/1uV*V(1V) laplace=1/((1+s/{P1})*(1+s/{P2}))<br>
-.param Vdc=300 Idc=1.3 f=100kHz tr=60n d=.4
+.param Vdc=300 Idc=1.3 f=100kHz tr=60n d=.4<br>
 .param Gain={2*Idc*d} P1={2*f/d} P2={2/tr}

EMI Modelling using LTspice Hints - Revision history

Lewispaul at 01:24, 9 December 2012

Lewispaul: Created page with "For SMPS applications I have successfully used LTspice to design EMI filters, but I always start out by running an ac simulation rather than the much more time consuming time ..."