Slide 1: Stability Analysis in Time Domain
Stability Analysis in Time Domain No Bode, No Problem! Ensure Stability of Automotive DC-DC Converters through Time-Based Analysis Legal Disclaimer
Slide 2: Why Load Transients Matter?
Why Load Transients Matter? • Load Transients are critically important in Automotive System applications like ADAS, Infotainment and powertrain control. • Load Transients occur due to real-time system demands. For example: Activation of ADAS modules, Turning on headlights, electric steering or infot
Slide 3: Parameters defining DC-DC converter Stability
Parameters defining DC-DC converter Stability • Control loop stability refers to the system ability to maintain a desired state despite external disturbances or changes in system parameters. • Key parameters that define stability in DC-DC converters: Bandwidth (Crossover Frequency), Phase Margin and
Slide 4: Loop Stability Check – Frequency Domain
Loop Stability Check – Frequency Domain DC-DC Converter Frequency Response Analyzer VOUT RLOAD RINJ AC VIN VIN Power Stage RBOT RTOP CH-B CH-A VFB Control Loop Injection signal needs to be small enough to avoid non-linearities in the system ❑ Clear visualization of frequency response ❑ Easy determin
Slide 5: Loop Stability Check – Frequency Domain
Loop Stability Check – Frequency Domain DC-DC Converter Oscilloscope VOUT RBOT RTOP RDYNAMIC RSTATIC VIN Power Stage CH-1 CH-2 VFB Control Loop ❑ Easy and quick measurement with oscilloscope ❑ Can show the various the converter response and behavior ❑ Applies to both linear and non-linear systems ❑
Slide 6: Zero-Delay PWM (ZDP TM)
Zero-Delay PWM (ZDPTM) L + CURRENT HS SENSE COUT LOAD 1. Sudden load current increase VIN CIN VOUT DRIVERS LS causes VOUT to drop - AC COUPLER SLOPE COMP PWM Comparator OFFSET CANCELATION 2. VOUT dropping + VIN*K directly and immediately S - RFB1 Legend Q causes duty cycle to R CLOCK increase PWM CO
Slide 7: ZDP TM | Reducing Output Capacitance
ZDPTM | Reducing Output Capacitance MPQ4385 (ZDPTM) COMPETETION (PCMC) ±3% 2x 47uF (1210) 2x 47uF (1210) 4x 1210 1210 1210 1210 1210 1210 16mm2 footprint saving Legal Disclaimer
Slide 8: MPQ4385 Evaluation Board
MPQ4385 Evaluation Board Components that influence the MPQ4385 control loop stability MPQ4385 • Lower L1, Higher Bandwidth 40V 25A Buck w/ ZDPTM VIN 3.3-36V 4,5,19,20,31 • Higher L1, Lower Bandwidth VIN 10 C1A C1B C1C C1D BST 10μF 10μF 0.1μF 0.1μF 50V 50V 50V R1 C3 0.1μF GND 50V 1210 1210 0603 0603
Slide 9: Phase Margin Analysis in Time Domain
Phase Margin Analysis in Time Domain What is Ringing and How does it relate to Phase Relationship between Ringing and Phase Margin (1) Margin Analysis? ❑ Ringing is observed on the Vout due to a load transient. ❑ In time domain, the load step response of the system indicates how fast the system resp
Slide 10: Practical Example - Phase Margin Analysis in Time Domain
Practical Example - Phase Margin Analysis in Time Domain Vin = 12.5V, Vout = 5V, Fsw = 2.2MHz, L = 220nH, Cout = 2x22uF (1206,10V), Load Step = 10A <-> 20A @ 10A/us One ring is observed • One ring Observed • Corresponds to 45° Phase Margin (Acceptable Stability) • Critically Damped Response Legal Di
Slide 11: What Else You Can Decipher In Time Domain?
What Else You Can Decipher In Time Domain? ❑ The bandwidth of the DC-DC converter can also be estimated from the load transient waveform. ❑ The converter bandwidth is closely related to how fast it responds to changes in time domain. ❑ Quick way to estimate is from the ringing frequency. ✓ Not very
Slide 12: Bandwidth Analysis in Time Domain
Bandwidth Analysis in Time Domain Response Time Method ❑ Estimating DC-DC converter bandwidth from response time is a practical method in time domain analysis. ❑ Response time Tr is approximate indication of control bandwidth. Defined as the time it takes to go from 10% to 90% of the final output va
Slide 13: Practical Example - Bandwidth Analysis in Time Domain
Practical Example - Bandwidth Analysis in Time Domain Response Time Method Vin = 12.5V, Vout = 5V, Fsw = 2.2MHz, L = 220nH, Cout = 2x22uF (1206,10V), Load Step = 10A <-> 20A @ 10A/us 90% of Vout 10% of Vout Tr = 1.057µs In this example: • Tr = 1.057µs • BW = 0.35/ 1.057µs = ~331kHz Legal Disclaimer
Slide 14: Bandwidth Analysis in Time Domain
Bandwidth Analysis in Time Domain Undershoot Based Energy Method ❑ Undershoot refers to temporary dip in the output voltage when a sudden increase in load current. Vout Cout Undershoot Discharging ❑ Undershoot is caused by a deficit of charge in the output capacitor. Converter Recovering ❑ The bandw
Slide 15: Practical Example - Bandwidth Analysis in Time Domain
Practical Example - Bandwidth Analysis in Time Domain Undershoot Based Energy Method Vin = 12.5V, Vout = 5V, Fsw = 2.2MHz, L = 220nH, Cout = 2x22uF (1206,10V), Load Step = 10A <-> 20A @ 10A/us -188mV ∆𝐼𝑜𝑢𝑡 𝑓0 = 2∗𝜋∗∆𝑉𝑜𝑢𝑡∗𝐶𝑜𝑢𝑡 10A In this example: The bandwidth estimated doesn’t •…
Slide 16: Practical Example - Bandwidth Analysis in Time Domain (Cont'd)
Practical Example - Bandwidth Analysis in Time Domain(Cont’d) Undershoot Based Energy Method – Output Capacitor Influence ❑ Actual example application we used two 22µF / 10V 1206 Murata GCM31CR71A226KE02 in parallel. Be aware of MLCC capacitor DC bias characteristics!!! Source: https://ds.murata.co.
Slide 17: Practical Example - Bandwidth Analysis in Time Domain (Cont'd)
Practical Example - Bandwidth Analysis in Time Domain(Cont’d) Undershoot Based Energy Method - Output Capacitor Influence Vin = 12.5V, Vout = 5V, Fsw = 2.2MHz, L = 220nH, Cout = 2x22uF (1206,10V), Load Step = 10A <-> 20A @ 10A/us Effective capacitance = 2*13.16µF ≈ 26.4µF -188mV ∆𝐼𝑜𝑢𝑡 𝑓0 =…
Slide 18: Summary – Bandwidth Analysis in Time Domain
Summary – Bandwidth Analysis in Time Domain ❑ There are several ways to estimate the converter bandwidth. ➢ Estimation from Ringing Frequency ✓ Quick visual feedback from oscilloscope ✓ Not very useful for critically damped or high phase margin systems ➢ Estimation from response time ✓ Quick visual
Slide 19: Practical Example Comparison - Frequency Domain vs Time Domain
Practical Example Comparison - Frequency Domain vs Time Domain Time Domain Frequency Domain 46.7º 306kHz Case Frequency Domain Time Domain Phase Bandwidth Phase Bandwidth Bandwidth Margin Margin (Response (Undershoot based Time) energy method) Cout = 2x22µF 46.7º 306kHz ~45º ~331kHz ~320kHz Legal Di
Slide 20: Comparison - Frequency Domain vs Time Domain
Comparison - Frequency Domain vs Time Domain Case Frequency Domain Time Domain Phase Bandwidth Phase Bandwidth Bandwidth Margin Margin (Response (Undershoot based Time) energy method) Cout = 3x22µF 60.3º 224kHz ≥60º ~215kHz ~216kHz Cout = 2x22µF 46.7º 306kHz ~45º ~331kHz ~320kHz Cout = 22µF 28º 515k
Slide 21: Transient Response optimization for DC-DC converters
Transient Response optimization for DC-DC converters ❑ Internally compensated converters have limited transient response. How to improve the transient response? ✓ Adding a feed-forward capacitor (CFF) in the feedback networks boosts converter bandwidth with acceptable phase margin. ✓ CFF introduces
Slide 22: Transient Response optimization for DC-DC converters
Transient Response optimization for DC-DC converters Practical Example - Design Procedure for CFF Converter Vout ❑ Identify feedback resistor values to set the desired output. ❑Referring to MPQ4385 schematic, R1 = 100kΩ and R2 = CFF = ?? R1 19.1kΩ. VFB ❑ Determine the converter bandwidth without the
Slide 23: Transient Response optimization for DC-DC converters
Transient Response optimization for DC-DC converters Practical Example with calculated CFF Converter Vout CFF = R1 33pF VFB R2 • Zero added between ~40kHz and 50kHz range • Provides phase boost • Bandwidth rolls off at higher frequency with acceptable phase margin • 2x faster transient response 55º
Slide 24: Summary
Summary Comparison of Time Domain vs Frequency Domain Frequency Domain Loop Bandwidth (kHz) Phase Margin (Degrees) Gain Margin (dB) Time Domain Magnitude of output Output voltage behavior Does not show up in the voltage undershoot (mV) after load step (whether it load transient response has damped o
Slide 25: Thank You
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Slide 26: Questions???
Questions??? Legal Disclaimer