Monolithic Power Systems, Inc. · NASDAQ:MPWR
Monolithic Power Systems — Technical Presentation (April 2022)
52 slides from Monolithic Power Systems's technical presentation, Apr 6, 2022
Slide 1: Spread Spectrum to Reduce EMI
Spread Spectrum to Reduce EMI Presenter: Christian Kueck April 6th , 2022
Slide 2: Presenter Intro: Christian Kueck
Presenter Intro: Christian Kueck • Senior FAE supporting automotive Tier-1 customers throughout Germany • About three decades of experience managing EMI challenges • Deeply involved in the definition and compliance testing of our leading AEC-Q100 power management solutions • 6 years at MPS as Power
Slide 3: Today's Agenda
Today’s 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
Slide 4: Motivation
Motivation Often results are just a hair or two above the limits. So what can you do?
Slide 5: Fixed Frequency PSU Spectrum
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. f fSW 2fSW 3fSW 4fSW 5fSW 6fSW
Slide 6: Fixed Frequency PSU Spectrum
Fixed Frequency PSU Spectrum At a certain frequency, it will attenuate like a high order low pass. For most modern hard switch designs, there should not be much energy left above 500MHz. PSUs have harmonics into the high triple digits. Spread spectrum on high harmonics is seldom covered in literatur
Slide 7: How Does Duty Cycle Affect the Lower Harmonics? Video
How Does Duty Cycle Affect the Lower Harmonics? Video 5dB/div Fsw Vsw Vin I in [08]
Slide 8: How Does Duty Cycle Affect the Lower Harmonics?
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 and can be derived by fourier transformation of the switch node waveform Harmonic energy goes up with input voltage
Slide 9: How Does Switching Frequency Affect the EMI <10MHz? Video
How Does Switching Frequency Affect the EMI <10MHz? Video 5dB/div Fsw Vsw Vin I in [09] Switching Frequency
Slide 10: How Does Switching Frequency Affect EMI 150kHz-300MHz? Video
How Does Switching Frequency Affect EMI 150kHz-300MHz? Video 500k 5dB/div Fsw 1MHz 2MHz Vin I in [10]
Slide 11: EMI Receiver According to CISPR 16-1-1
EMI Receiver According to CISPR 16-1-1 RBW – Resolution Bandwidth Filter 9 KHz <30MHz 120 KHz >30MHz Pk, QP or AV Filter http://www.schwarzbeck.de/appnotes/EMIRcvrCISPR16.pdf
Slide 12: EMI Receiver Spectral View
EMI Receiver Spectral View The RBW filter moves with constant velocity (frequency change) through the receiver span from Freq start to Freq stop. This is a convolution operation in the frequency domain. f fSW 2fSW 3fSW 4fSW 5fSW 6fSW
Slide 13: EMI Receiver Spectral View
EMI Receiver Spectral View Area where you can still hide spur energy that the EMI receiver does not see f fSW 2fSW 3fSW 4fSW 5fSW 6fSW
Slide 14: EMI Receiver Spectral View
EMI Receiver Spectral View Ideal would be a white noise like spectrum f fSW 2fSW 3fSW 4fSW 5fSW 6fSW
Slide 15: EMI Receiver Spectral View Optimal Attenuation
EMI Receiver Spectral View Optimal Attenuation 𝑅𝐵𝑊 Lower attenuation >30 MHz Attenuation = 10 * log10 𝑓𝑠𝑤 Maximum Achievable Spread Spectrum Attenuation 0.15MHz–30MHz 30MHz–1GHz >1GHz RBW [kHz] 9 120 1000 fSW [kHz] [dB] [dB] [dB] 100 -10.5 0.0 0.0 200 -13.5 -2.2 0.0 400 -16.5 -5.2 0.0 1000…
Slide 16: RBW Filter Time Domain View of Spur in the Frequency Domain
RBW Filter Time Domain View of Spur in the Frequency Domain 1 If 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
Slide 17: RBW Filter Time Domain View of Spur in the Frequency Domain
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 the RBW filter than their time domain impulse response. The speed at which the frequency changes needs to be faster than: 𝑅𝐵𝑊 𝑅𝐵𝑊 𝐻𝑧…
Slide 18: FM Modulation Frequency Domain View
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
Slide 19: Attenuation of FM in [dB]
FM Modulation Attenuation Modulation depth Δf rises linearly with the harmonic number of the spur = N * Δf N∗Δf The same applies for the modulation index h = fm Δf Modulation index h= fm 0 High harmonic numbers easily reach a 101 1 5 9 73 13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 77 81 85 89 93 9
Slide 20: Low Modulation Index / High Modulation Frequency Video
Low Modulation Index / High Modulation Frequency Video Too high modulation Modulation Signal time domain 50μs/div 5dB/div frequency = low modulation index makes only a few spurs with little attenuation. For the 9kHz RBW area <30MHz, a modulation frequency of about 9kHz gives best results. [15]
Slide 21: 150kHz to 30MHz View Over Modulation Frequency Video
150kHz to 30MHz View Over Modulation Frequency Video Too high a modulation Modulation Signal time domain 50μs/div 5dB/div frequency makes only a few spurs with little attenuation. For the 9kHz RBW area <30MHz, a modulation frequency of about 9kHz gives best results. Too low modulation frequencies gi
Slide 22: 150kHz to 30MHz View Over Modulation Span Video
150kHz to 30MHz View Over Modulation Span Video A high modulation span Modulation Signal time domain 50μs/div 5dB/div will give good attenuation results, but is demanding on the regulation loop. Modulation index goes up with modulation span Δf Δf h= fm [18]
Slide 23: 120kHz RBW Range >30MHz Video
120kHz RBW Range >30MHz Video For the 120kHz RBW Modulation Signal time domain 50μs/div frequency range, >30MHz high 5dB/div modulation frequency would be best because their spurs still “move” fast enough against the >14.4GHz/s that the 120kHz RBW filter impulse response requires. The absolute possi
Slide 24: Attenuation [dB]
Modulation Frequency RBW = receiver bandwidth Too low FM frequency for RBW filter impulse response Too low modulation index Spur stays too long inside RBW Too few spurs with too much individual energy Attenuation [dB] FM Modulation frequency [kHz] 9kHz for <30MHz range or 120kHz for >30Mhz range
Slide 25: Modulation Waveforms
Modulation Waveforms For a given RBW, the modulation frequency should not be too high, or too low. High modulation frequencies = low modulation index (bad). Low modulation frequencies = low RBW filter impulse response attenuation (bad). For typical modulation spans, modulation frequencies around the
Slide 26: Modulation Waveforms – Triangular
Modulation Waveforms – Triangular The problem here is that in Δf/2 the same time (t1) makes only half of the frequency change. Δf t1 t1 [16]
Slide 27: Modulation Waveforms – Sawtooth
Modulation Waveforms – Sawtooth The df/dt here is high or infinity, which is good. However, the abrupt frequency change often creates additional output ripple in fixed frequency control loop designs. Δf t1 t1 [16]
Slide 28: Modulation Waveforms – "Hershey kiss" and Stepped Triangular
Modulation Waveforms – “Hershey kiss” and Stepped Triangular Δf Δf Δf = Δf t1 t1 t1 t1 Constant df/dt [16]
Slide 29: Spread Spectrum Implementation for Parts without Internal FSS
Spread Spectrum Implementation for Parts without Internal FSS If they have a resistor program FREQ pin
Slide 30: Monopole 150kHz-30MHz Radiated Results for MPQ4430
Monopole 150kHz-30MHz Radiated Results for MPQ4430
Slide 31: Digital Implementation in the MPQ8875 Video
Digital Implementation in the MPQ8875 Video
Slide 32: Spread Spectrum Benefits
Spread Spectrum Benefits Spread spectrum works independently from other EMI mitigation methods like filtering, layout, and shielding. Therefore, fixed frequency design results get additional attenuation from spread spectrum, in conducted and radiated areas. A good engineering practice is to develop
Slide 33: What is the optimum switching Frequency
What is the optimum switching Frequency EMI goes general up with voltage on the switch node by 6dB for every 2x switch voltage So for 5V or 3.3V Vin switchers you get away with >2MHz switching frequency. Keep the inductors small and low profile. For higher Vsw switchers like 12V and more keep the sw
Slide 34: Frequency Synchronisation
Frequency Synchronisation Even fixed frequency switcher have some phase noise (FM) which will multiply by the harmonic number. So expect some natural FM attenuation on high harmonic numbers (i.e. >80MHz). On non fixed frequency schemes like COT those natural phase noise / FM is even more pronounced.
Slide 35: Conclusion
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 Spread spectrum optimized for <30MHz, saves the most by redu
Slide 36: FAQ: Question 1
FAQ: Question 1 How many dBs maximum reduction can I get from the MPS spread spectrum feature? See table on Maximum Achievable Spread Spectrum Attenuation slide 14 0.15MHz–30MHz 30MHz–1GHz >1GHz RBW [kHz] 9 120 1000 fSW [kHz] [dB] [dB] [dB] 100 -10.5 0.0 0.0 200 -13.5 -2.2 0.0 400 -16.5 -5.2 0.0 100
Slide 37: FAQ: Question 2
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 and control loop stays the same.
Slide 38: FAQ: Question 3
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 n
Slide 39: FAQ: Question 4
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 the complete range in one step, which can introduce loop perturbation for the switcher. Because of t
Slide 40: FAQ: Question 5
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 spread spectrum audio frequency modulator adds very little area on the device, so the additional c
Slide 41: FAQ: Question 6
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.
Slide 42: FAQ: Question 7
FAQ: Question 7 Q: Which kind of EMI does it help? CE? RE? <30MHz? >30MHz? A: All. Spread spectrum is a spectral method, and always gives additional dB attenuation which add to conventional methods like filtering, layout, and shielding. Therefore, it helps for conducted and radiated, and gives the s
Slide 43: FAQ: Question 8
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…
Slide 44: FAQ: Question 9
FAQ: Question 9 Q: Why random or pseudo-random modulation can be a bad choice? A: PSU spread spectrum has to use a constant power modulation scheme, which is FM. FM gives a quadradic spectral energy between ±Carson frequency. Therefore, low modulation frequencies make more spectral energy then high
Slide 45: MPS operates 3 EMC Labs worldwide open to customer use
MPS operates 3 EMC Labs worldwide open to customer use SAC3 – Chamber CISPR25 – Chamber Shielded – Chamber Pulses and Application Engineering
Slide 46: Attachment Optimum Modulation Frequency and Spread
Attachment Optimum Modulation Frequency and Spread Maximize 𝑁 ∗ 𝐹𝑚𝑜𝑑 ∗ Δ𝐹 RBW impulse response Minimize 𝑁 ∗ Δ𝐹 Overlap starts 3 𝑁∗Δ𝐹 Modulation index attenuation from FM modulation Maximize 𝐹𝑚𝑜𝑑 math Conflicting so there is an optimum middle ground 𝑁 = Harmonic Number 𝐹𝑚𝑜𝑑 =…
Slide 47: Attachment FM Modulation
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 the modulation frequency using low RBW (e.g. 100Hz) on the analyzer. Excel supports t
Slide 48: Attachment Overlap
Attachment Overlap The spread frequency span goes up with the harmonic number N, and starts to overlap at a certain frequency. Above the overlap frequency, EMI will not improve by increasing the modulation frequency span/spread. 0 𝑓0 𝑓1 𝑓2 𝑓3 𝑓4 …..
Slide 49: Attachment Switching Frequency Effect on EMI
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 ha
Slide 50: Attachment Switching Frequency Effect on EMI
Attachment Switching Frequency Effect on EMI With fixed frequency for every doubling, only half the bins are available for the energy. If all stays the same except the switching frequency: Result is 6dB/octave = 20db/decade Fixed Frequency EMI Increase in dB Perfect Spread Spectrum EMI energy is pro
Slide 51: Pseudo-Random Noise for Spread Spectrum Modulation
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.
Slide 52: Pseudo-Random 150kHz to 30MHz
Pseudo-Random 150kHz to 30MHz 5dB/div [20] 9kHz Triangle Modulation @ 20% Span Industry-available, pseudo-random spread spectrum generator. A few dB.
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