Monolithic Power Systems, Inc. · NASDAQ:MPWR
Monolithic Power Systems — Technical Presentation (May 2019)
48 slides from Monolithic Power Systems's technical presentation, May 2019
Slide 1: Spread Spectrum For Power Supplies
Spread Spectrum For Power Supplies Speaker: Christian Kueck May 2019
Slide 2: Speaker Intro: Christian Kueck
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 of our leading AEC-Q100 power management solutions • 22 years at Linear Technology
Slide 3: Agenda
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?
How Does Duty Cycle Affect the Lower Harmonics? 5dB/div [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 Harmonic energy goes up with input voltage
Slide 9: How Does Switching Frequency Affect the EMI <10MHz?
How Does Switching Frequency Affect the EMI <10MHz? 5dB/div [09] Switching Frequency
Slide 10: How Does Switching Frequency Affect EMI 150kHz-300MHz?
How Does Switching Frequency Affect EMI 150kHz-300MHz? 500k 5dB/div 1MHz 2MHz [10]
Slide 11: EMI Receiver According to CISPR 16-1-1
EMI Receiver According to CISPR 16-1-1 RBW – Resolution Bandwidth Filter 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 𝑅𝐵𝑊 Attenuation = 10 * log10 𝑓𝑠𝑤 Maximum Achievable Spread Spectrum Attenuation 0.15MHz–30MHz 30MHz–1GHz >1GHz RBW [MHz] 0.009 0.12 1 fSW [MHz] 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…
Slide 15: 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 16: 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 17: 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. Modulatio
Slide 18: FM Modulation Attenuation
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 1 5 9 101 13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 73 77 81 85 89 93 9
Slide 19: Low Modulation Index / High Modulation Frequency
Low Modulation Index / High Modulation Frequency Too high a modulation 5dB/div frequency makes only a few spurs with little attenuation. For the 9kHz RBW area to be <30MHz, a modulation frequency of about 9kHz gives best results. [15]
Slide 20: 150kHz to 30MHz View Over Modulation Frequency
150kHz to 30MHz View Over Modulation Frequency Too high a modulation 5dB/div frequency makes only a few spurs with little attenuation. For the 9kHz RBW area to be <30MHz, a modulation frequency of about 9kHz gives best results. [17]
Slide 21: 150kHz to 30MHz View Over Modulation Span
150kHz to 30MHz View Over Modulation Span A high modulation span 5dB/div will give good attenuation results, but is demanding on the regulation loop. [18]
Slide 22: 120kHz RBW Range >30MHz
120kHz RBW Range >30MHz For the 120kHz RBW 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. [16]
Slide 23: Modulation Waveforms
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 work best. That is typically 9kHz for the <30MHz range, and 120kHz for the >30MHz region. Usually,
Slide 24: 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 25: 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 designs. Δf t1 t1 [16]
Slide 26: 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 27: 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 28: 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.
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
Digital Implementation in the MPQ8875
Slide 32: Spread Spectrum Benefits
Spread Spectrum Benefits Spectral method spread spectrum works independently from other EMI mitigation methods like filtering, layout, and shielding. Therefore, fixed frequency results provide additional attenuation from spread spectrum, regardless of whether one measures conducted or radiated. A go
Slide 33: 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 With spread spectrum for <30MHz, one saves the most with red
Slide 34: FAQ: Question 1
FAQ: Question 1 How many dBs 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 [MHz] 0.009 0.12 1 fSW [MHz] 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
Slide 35: 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 stays the same.
Slide 36: 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 37: 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 38: 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 audio frequency modulator adds very little area on the device, so the additional cost is low. The
Slide 39: 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 40: FAQ: Question 7
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 to conventional methods like filtering, layout, and shielding. Therefore, it helps the same for both conducted and radiated
Slide 41: 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 42: FAQ: Question 9
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 a quadradic spectral energy between ±Carson frequency. Therefore, low modulation frequencies make more spectral energy then high ones
Slide 43: Q&A
Q&A New MPS EMI test center in Hangzhou, China Additional labs to be built in Detroit, Germany & Shanghai
Slide 44: 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 45: 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 46: 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 47: 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 48: EMI Increase in dB
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
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