Spread Spectrum
For Power Supplies
Speaker: Christian Kueck
May 2019

Speaker Intro: Christian Kueck• Senior FAE supporting automotive Tier-1 customers
throughout Germany• Over two decades of experience managing EMI challenges• Deeply involved in the definition and compliance testing ofour leading AEC-Q100 power management solutions• 22 years at Linear Technology
o Strategic Marketing Manager for Europe – Productdefinition and product support for PSU and LED circuitso Field Application Engineer• Additional:o Design Engineer, Quality Assurance, Materials Engineer• Microelectronics. Dipl. Ing., Elektrotechnik University ofDortmund

Agenda
The Motivation

PSU Fixed Frequency Spectrum
Effect of Switching Frequency and Duty Cycle on EMI
What Can Spread Spectrum Accomplish
Effective Methods to Make Spread Spectrum

Frequently Asked Questions About Spread Spectrum
Open Q&A

Motivation
Often results are just a hair or two above the limits. So what can you do?

Fixed Frequency PSU Spectrum
The typical PSU switch excitation
source creates a flat periodic “fence”
of spurs in the frequency domain,
spaced by the switching frequency.

ffSW

2fSW

3fSW

4fSW

5fSW6fSW

Fixed Frequency PSU Spectrum
At a certain frequency, it willattenuate like a high order low pass.
For most modern hard switch
designs, there should not be muchenergy left above 500MHz.
PSUs have harmonics into the hightriple digits. Spread spectrum onhigh harmonics is seldom covered inliterature.

How Does Duty Cycle Affect the Lower Harmonics?

5dB/div[08]

How Does Duty Cycle Affect the Lower Harmonics?

At 50% duty cycle, all even harmonics disappear.
Lower harmonics distribution depends on duty cycle = VOUT / VIN for a buck topology

Harmonic energy goes up with input voltage

How Does Switching Frequency Affect the EMI <10MHz?

5dB/div[09]
Switching Frequency

How Does Switching Frequency Affect EMI 150kHz-300MHz?
500k
5dB/div

1MHz2MHz[10]

EMI Receiver According to CISPR 16-1-1
RBW – Resolution Bandwidth Filter

Pk, QP or AV Filter

http://www.schwarzbeck.de/appnotes/EMIRcvrCISPR16.pdf

EMI Receiver Spectral View
The RBW filter moves with constant velocity (frequency change) through the receiver span fromFreq start to Freq stop. This is a convolution operation in the frequency domain.

ffSW

2fSW

3fSW

4fSW

5fSW

6fSW

EMI Receiver Spectral View
Area where you can still hide spur energythat the EMI receiver does not seeffSW

2fSW

3fSW

4fSW

5fSW

6fSW

EMI Receiver Spectral View
Attenuation = 10 * log10

𝑅𝐵𝑊
𝑓𝑠𝑤

Maximum Achievable Spread Spectrum Attenuation
0.15MHz–30MHz
RBW [MHz]

30MHz–1GHz

>1GHz

0.009

0.12

1

0.1

-10.5

0.0

0.0

0.2

-13.5

-2.2

0.0

0.4

-16.5

-5.2

0.0

1

-20.5

-9.2

0.02

-23.5

-12.2

-3.03

-25.2

-14.0

-4.8

5

-27.4

-16.2

-7.0

10-30.5

-19.2

-10.0

100

-40.5

-29.2

-20.0fSW [MHz]ffSW2fSW 3fSW 4fSW 5fSW 6fSW

RBW Filter Time Domain View of Spur in the Frequency Domain
1If the carrier stays longer than the bandpass impulse response time
inside the passband,
𝑅𝐵𝑊
there will be no attenuation through the RBW filter, and you see the spurs’ original energy.

1= 111µ𝑠
9𝑘𝐻𝑧

1= 8.33µs
120𝑘𝐻𝑧RBW Filter Time Domain View of Spur in the Frequency Domain
To get attenuation through spectral movement, one needs to rip with spurs faster through theRBW filter than their time domain impulse response.
The speed at which the frequency changes needs to be faster than:
𝑅𝐵𝑊𝐼𝑚𝑝𝑢𝑙𝑠𝑒 𝑟𝑒𝑠𝑝𝑜𝑛𝑠𝑒 𝑡𝑖𝑚𝑒

=𝑅𝐵𝑊
1
𝑅𝐵𝑊= 𝑅𝐵𝑊 2 = a with be measured in𝐻𝑧𝑃𝑒𝑟𝑖𝑜𝑑𝑠
or in𝑠𝑠2

According to CISPR16 / 22 / 25, for the bands to get attenuation in PK mode at all
the frequency change (speed or sweep) needs to be faster than:

150kHz to 30MHz with RBW = 9kHz> 81MHz/s
30MHz to 1GHz with RBW = 120KHz > 14.4GHz/s>1GHzwith RBW = 1MHz > 1THz/sFM Modulation Frequency Domain View
Signal needs to be somewhat periodic (can’t move constant to infinity).
Constant energy for no additional output ripple => FM = frequency modulation

Modulation frequency (FM) – The repetitive frequency FM = 1/period
where the modulation waveform repeats.
Modulation depth (Δf) – fMAX - fMIN. The span or stroke of the modulated signal

ΔfModulation index – h = FMFM Modulation Attenuation
Modulation depth Δf rises linearly with the harmonic number of the spur = N * Δf
The same applies for the modulation index h =0Attenuation of FM in [dB]-2-4-6-8-10-12-14-16-18

h=Δffm

1
5913
17
21
25
29
33
37
41
45
49
53
57
61
65
69
73
77
81
85
89
93
97
101

Modulation index

N∗Δf
fm

High harmonic numbers easily reach alarge modulation index.
However, for fundamental and low
harmonic numbers, a low modulation
frequency is needed.
This violates the time domain view thatis needed to be much faster than 111µs
or 8.33µs out of the RBW window.

Low Modulation Index / High Modulation Frequency
5dB/div

Too high a modulation
frequency makes only afew spurs with littleattenuation. For the
9kHz RBW area to be<30MHz, a modulation
frequency of about 9kHz
gives best results.

[15]150kHz to 30MHz View Over Modulation Frequency
5dB/div

Too high a modulation
frequency makes only afew spurs with little
attenuation. For the
9kHz RBW area to be<30MHz, a modulation
frequency of about 9kHz
gives best results.

[17]150kHz to 30MHz View Over Modulation Span
5dB/div

A high modulation span
will give goodattenuation results, butis demanding on theregulation loop.

[18]

120kHz RBW Range >30MHz

5dB/div

For the 120kHz RBW
frequency range,>30MHz highmodulation frequency
would be best because
their spurs still “move”
fast enough against the>14.4GHz/s that the120kHz RBW filter
impulse responserequires.

[16]

Modulation Waveforms
We learned that for a given frequency span, the modulation frequency should not be too high,or too low. For typical modulation spans, modulation frequencies around the RBW seem to workbest. That is typically 9kHz for the <30MHz range, and 120kHz for the >30MHz region.
Usually, you choose for the 9kHz RBW range since you get the most attenuation.

Now, what is the best modulation waveform shape?
We know it should be a constant velocity in Hz/s. A high number would be good, but if it isperiodic, it must come back to its origin.

[16]

Modulation Waveforms – Triangular
The problem here is that in
the same time (t1) makes
only half of the frequencychange.

Δf/2

Δft1t1[16]

Modulation Waveforms – Sawtooth

The df/dt here is high or infinity,
which is good. However, theabrupt frequency change often
creates additional output ripple in
fixed frequency designs.
Δf

t1

t1

[16]

Modulation Waveforms – “Hershey kiss” and Stepped Triangular

Δf

Δf

=Δf

Δf

t1
t1

t1

t1

Constant df/dt[16]

Pseudo-Random Noise for Spread Spectrum Modulation

5dB/div

Fixed frequency carrier[19]
Industry-available, pseudo-random spread
spectrum generator.
No attenuation in PK nor in AV mode at 9kHz RBW.

Pseudo-Random 150kHz to 30MHz

5dB/div[20]
9kHz Triangle Modulation @ 20% Span

Industry-available, pseudo-random spreadspectrum generator. A few dB.

Spread Spectrum Implementation for Parts without Internal FSS
If they have a resistor program FREQ pin

Monopole 150kHz-30MHz Radiated Results for MPQ4430

Digital Implementation in the MPQ8875

Spread Spectrum Benefits
Spectral method spread spectrum works independently from other EMI mitigation methods likefiltering, layout, and shielding. Therefore, fixed frequency results provide additional attenuationfrom spread spectrum, regardless of whether one measures conducted or radiated.
A good engineering practice is to develop and test the switch mode PSU first with fixed frequency.
Using spread spectrum from the beginning may hide an excessive jitter or stability issue if
focused at the switch node.
General spread spectrum adds minimal additional output ripple to fixed frequency designs. On
non-fixed frequency loop topologies like COT, hysteretic, and fixed ripple, one might not see
anything additional at the output.
Transient response is not affected unless excessive frequency spans are used.

Conclusion
Spread spectrum is an excellent tool to reduce EMI with little additional cost
The optimum modulation frequency for each EMI RBW is around the RBW frequency
Optimize with spread spectrum for 9kHz RBW in the <30MHz frequency band
With spread spectrum for <30MHz, one saves the most with reduced filter and shielding requirements
Test PSU with a fixed frequency setting

Parts with a FREQ setting resistor can be easily made spread spectrum
Excessive frequency span only helps for the first few harmonics, not in the overlap region
The effect of 120kHz RBW or more (>30Mhz range) offers limited improved optimizing for 9kHz RBWFAQ: Question 1
How many dBs reduction can I get from the MPS spread spectrum feature?
See table onslide 14Maximum Achievable Spread Spectrum Attenuation
0.15MHz–30MHz
RBW [MHz]

30MHz–1GHz

>1GHz

0.009

0.12

1

0.1

-10.5

0.0

0.0

0.2

-13.5

-2.2

0.0

0.4

-16.5

-5.2

0.0

1

-20.5

-9.2

0.0

2

-23.5

-12.2

-3.0

3

-25.2

-14.0

-4.8

5-27.4

-16.2

-7.0

10-30.5

-19.2

-10.0

100

-40.5

-29.2

-20.0fSW [MHz]FAQ: Question 2
Q: Does spread spectrum affect the transient response of a DC/DC buck regulator?
A: It has almost no effect since the mean switching frequency stays the same.

FAQ: Question 3
Q: Which algorithm is used to generate spread spectrum inside MPS’s DC/DC converters?
A: Analog triangle or stepped triangle generators are used as well as digital implementations.
As long as the digital implementation changes the frequency value on each switch node cycle,
there is no difference in the resulting spectrum and attenuation.

FAQ: Question 4
Q: Which frequency modulation envelope is more effective: sawtooth, triangle, or sinusoid?
A: Sawtooth and stepped triangle are the most effective. However, sawtooth makes a jump over thecomplete range in one step, which can introduce loop perturbation for the switcher. Because of this, a more
continuous method like triangle or stepped triangle are often used.
Sinusoid or even rectangular modulation stay at the top and bottom signal too long around a small
frequency range (inside the RBW), which results in poor attenuation.

FAQ: Question 5
Q: What is the cost difference, if any, to be expected in a SS part vs. non-SS part? Will there be an overall
reduction is system cost (capacitors, inductor, filters)?
A: The additional audio frequency modulator adds very little area on the device, so the additional cost is low.
The biggest solution cost improvement is on the filter requirements for the lower frequency area fromswitching frequency to the first harmonics.

Often, spread spectrums save the complete solution, so ultimately only you can answer what your project isworth.

FAQ: Question 6
Q: Does spread spectrum really help the EMI? Or just cheat the test equipment?
A: The total emitted energy into the universe stays the same. You change the spectral distribution.

FAQ: Question 7
Q: Which kind of EMI does it help? CE? RE? <30MHz? >30MHz?

A: All are the same. Spread spectrum is a spectral method, and always gives additional dB attenuation toconventional methods like filtering, layout, and shielding.
Therefore, it helps the same for both conducted and radiated, and gives the same dB attenuation at any
frequency, whether radiated and conducted.
With the different RBW (resolution bandwidth) of 9kHz <30MHz, 120kHz >30MHz, and 1MHz >1GHz, insome OEM defined ranges spread spectrum attenuates the most for low RBW like 9kHz.
See slide 16 for carrier frequency change minimums.
150kHz to 30MHz with RBW = 9kHz
> 81MHz/s
30MHz to 1GHz with RBW = 120KHz > 14.4GHz/s>1GHz
with RBW = 1MHz> 1THz/sFAQ: Question 8
Q: Are there any secret military or patented spread spectrum methods to get more attenuation?
A: Spread spectrum works to the basic principle of quantum physics according to Heisenberg’s uncertainty
principle (frequency form) ∆𝑓 ∗ ∆𝑡 ≤ 1
The best method is to spread signal energy evenly over the spectrum. For a comb-style periodic spectrum.
only the area in between two comb teeth can be filled.
This reduces the possible attenuation to the formula from slide 14:

Attenuation = 10 * log10𝑅𝐵𝑊𝑓𝑠𝑤

FAQ: Question 9
Q: Why is random or pseudo-random modulation a bad choice?
A: PSU spread spectrum has to use a constant power modulation scheme, which is FM. FM gives aquadradic spectral energy between ±Carson frequency. Therefore, low modulation frequencies make more
spectral energy then high ones for the same frequency span.
That is the reason that audio FM modulation uses pre-emphasis (50µs in Europe 75µs in US). Otherwise,
the signal to noise ratio for high frequencies would be much lower than for low frequencies. Low carrier
movement staying inside the RBW does not give any attenuation at all.
White noise/random or pseudo-random yields a flat frequency response. One would have to differentiate thenoise signal (high frequencies amplified), pre-emphasize it, and cut out the low amplitude area, which would
result in the carrier staying inside the RBW. The high frequency range requires limiting the energy giving a
low modulation index. In total, one would have to bandpass, pre-emphasize, and omit low-amplitudechanges (inside RBW). In the end, the signal would not be much different from a stepped triangle.

Currently, white noise/pseudo-random generators give lower attenuation than otherwise possible for theabove reasons.

Q&A

New MPS EMI test center in Hangzhou, China
Additional labs to be built in Detroit, Germany & Shanghai

Attachment Optimum Modulation Frequency and SpreadMaximize

𝑁 ∗ 𝐹𝑚𝑜𝑑 ∗ Δ𝐹

RBW impulse response

Minimize𝑁 ∗ Δ𝐹

Overlap starts

Maximize

3𝑁∗Δ𝐹𝐹𝑚𝑜𝑑

Modulation index attenuation from FM modulation
math

Conflicting so there is anoptimum middle ground

𝑁 = Harmonic Number

𝐹𝑚𝑜𝑑 = Modulation Frequency
Δ𝐹 = Frequency Span

Attachment FM Modulation
In the frequency domain, FM modulated signals have a discrete spectrum spaced by the modulation
signal frequency, with magnitudes according to the Bessel function of the first kind. Measure themodulation frequency using low RBW (e.g. 100Hz) on the analyzer.
Excel supports the Bessel function of the first kind, so there is an opportunity for deeper research if
interested
J0

J−2

J−4

J−3

J−1

Modulation Index

J1 J2 J3J4

Attachment Overlap

The spread frequency span goes up with the harmonic number N, andstarts to overlap at a certain frequency.
Above the overlap frequency, EMI will not improve by increasing themodulation frequency span/spread.

0 𝑓0

𝑓1 𝑓2 𝑓3

𝑓4 …..

Attachment Switching Frequency Effect on EMI
Doubling the switching frequency doubles the number of switch transitions, so it is best to double
the EMI energy. That is 3dB more (power is 10 * log(2)).
But with double the frequency, only half the bins are available to locate this energy.
The spurs have 6dB more amplitude per doubling the frequency for fixed frequency, and at least
3dB more for a perfect working spread spectrum.

6dB0dBfAttachment Switching Frequency Effect on EMIWith fixed frequency forevery doubling, only half
the bins are available forthe energy.

If all stays the same except the switching frequency:

Result is 6dB/octave =20db/decade

EMI Increase in dB

Fixed Frequency

Perfect Spread Spectrum

EMI energy isproportional to theswitching frequency
3db/octave =10db/decadex

2x4xx

Increase in Switching Frequencyx