Reducing Emissions in DC-DC Switched Mode Power Supplies
advertisement
Reducing Emissions in DC-DC Switched Mode Power Supplies Scott Mee – Johnson Controls Jim Teune – Gentex 1 Outline Overview of SMPS designs and basic emissions issues Root Causes of Emissions Design Strategies for Reducing Emissions Schematic Design Component Selection Layout Considerations Trade-offs between EMI and other requirements Hardware Demonstration 2 Power Supplies Linear vs. switching Linear supplies Typically used when the input and output voltage levels are similar Large voltage drops and high current output cause low efficiency Low efficiency = higher heat Quiet from RF emissions point of view Switching supplies Preferred for applications where efficiency is important Buck Æ step down Æ i.e. 12Volts to 5Volts logic level Boost Æ step up Æ i.e. 12Volts to 40Volts LED lighting level Sudden changes in voltage & current cause EMC problems Circuit uses a switch, inductor and diode to transfer energy from input to output 3 Buck SMPS Vout = VIN x D, where D = tON/(tON + tOFF) 4 Buck SMPS Charge phase 5 Buck SMPS Discharge phase 6 Buck Circuit Voltages and Currents Switch State Iind Imax Imin Voltage Vin Vout Vind 0 volts 7 Boost SMPS Vout = VIN / (1 – D), where D = tON/(tON + tOFF) 8 Boost SMPS Charge phase 9 Boost SMPS Discharge phase 10 Boost Circuit Switching Voltages and Currents Switch State Iind Idiode Imax Imin Vout Voltage Level Vin Vind 0 volts Timing 11 Trapezoidal Periodic Signals Fourier Series ∞ x(t ) = c0 + ∑ 2 c n cos(nω 0 t + ∠c n ) n =1 Fourier Coefficients ⎛1 ⎞ ⎛1 ⎞ sin ⎜ nω 0τ ⎟ sin ⎜ nω 0τ r ⎟ τ ⎝2 ⎠ ⎝2 ⎠ e − jnω0 (τ +τ r ) 2 , cn = A 1 1 T nω 0τ nω 0τ r 2 2 τ c0 = A , τ r = τ f T 12 τ f =τr Bandwidth of Periodic Waveforms 1 Bounds on frequency spectrum Above the 2nd break point, πτ r the harmonics drop off at a rate of 40dB/decade. To be conservative we might choose a point, 3 times this second breakpoint this is approximately 3 Bandwidth of a periodic signal 13 1 πτ r BW = 1 τr τr Hz Noise Sources in SMPS Switching characteristics dv/dt & di/dt Fundamental frequency Harmonic series Resonances Step response to the RLC network Æ Ringing Secondary effects Power surges at input Ripple on power bus Ripple on system wiring Output ripple Magnetic fields 14 BUCK Supply Emissions Investigation 15 Emissions Investigation BUCK SMPS Circuit 16 Emissions Investigation BUCK Voltage Measurements 1 Voltage at Input to SMPS Switch Output Voltage 3 17 Emissions Investigation BUCK Voltage Measurements – Zoom Voltage at Input to SMPS Switch Output Voltage 18 Emissions Investigation Narrow Band vs. Broadband Frequency domain Time domain 19 Improvement came from - Front-end filtering (L/C filter) - Slew rate controls - Layout improvements Emissions Investigation Success Stories – 70kHz SMPS Conducted Emissions (150kHz – 2MHz) Before Techniques Applied After Techniques Applied 20 Emissions Investigation Success Stories – 150kHz SMPS (Low band) Improvement came from - Front-end filtering (L/C filter) - Layout improvements Conducted Emissions (150kHz – 2MHz) Before Techniques Applied After Techniques Applied 21 Emissions Investigation Success Stories – 150kHz SMPS (High band) Improvement came from - Diode snubber - Diode switching changes - Layout improvements CISPR 25 – Radiated Emissions (25MHz – 200MHz) Before Techniques Applied After Techniques Applied 22 BOOST Supply Emissions Investigation 23 Emissions Investigation Boost Supply Case Study 12volt input & 34volt output +34 V +12 V 24 Emissions Investigation Boost Supply – current loop when switch is closed Red = current flow to load, Blue = return current +34 V +12 V 25 Emissions Investigation Boost Supply – current loop when switch is open Red = current flow to load, Blue = return current +34 V +12 V 26 Emissions Investigation Boost Supply Æ Radiated Emissions 50MHz – 180MHz BL ON 123MHz 27 161MHz Emissions Investigation Boost Supply Æ Radiated Emissions 50MHz – 180MHz BL OFF 123MHz 28 161MHz Emissions Investigation Boost Supply Æ Measurement points i1 +12 V V1 29 +34 V Emissions Investigation Boost Supply Æ Measurement Setup Bench measurement setup overview LeCroy 6GHz 40GS/s Voltage probe 500MHz 1.8pf Current probe Langer HF magnetic field probe 30 Emissions Investigation Boost Supply Æ Measurement Setup Bench measurement setup overview Voltage probe used to show when switch is open/closed Current probe used to see shape of current flowing through the diode 31 Emissions Investigation Boost Supply Æ Voltage Measurement Results Overview of switching waveforms V1 i1 32 Emissions Investigation Boost Supply Æ Voltage Measurement Results Switch turns from off to on V1 i1 33 Emissions Investigation Boost Supply Æ Voltage Measurement Results Switch turns from on to off V1 i1 34 Emissions Investigation Boost Supply Æ Radiated Emissions 50MHz – 180MHz BL ON V1 123MHz 161MHz i1 35 Emissions Investigation Boost Supply Æ Diode Current Æ No changes / Baseline 123MHz ringing corresponds to 123MHz emissions 123MHz 36 Johnson Controls Emissions Investigation Boost Supply Æ Diode Current Æ 1nf cap across diode CR6401 77 MHz ringing corresponds to 77MHz emissions 77MHz 37 Johnson Controls Schematic Design 38 Schematic Design Buck topology 12V input Slew rate control Snubber Output filter +5 V 5V output +12 V Snubber Input filtering Soft-start capacitor Spread spectrum 39 Schematic Design Boost topology Front end Pi filter Slew rate control 40 Johnson Controls Snubber Output cap Schematic Design Snubber Calculations FRinging := 126MHz CSnubber := 1500pF FTuned := 40MHz CParasitic = 168.114pF −9 LParasitic = 9.491 × 10 H The optimum resistor to damp the overshoot is twice the inductive impedance at the new resonant frequency. This is calculated by the following equation: ( RSnubber := 2⋅ 2π⋅ FTuned ⋅ LParasitic RSnubber = 4.771Ω −9 CSnubber = 1.5 × 10 F ) Schematic Design Combination Selection Component Selection 43 Component Behavior Capacitors, Resistors, Inductances, Ferrites All passive components have resistance, capacitance and inductance Component behavior is different at low and high frequencies 44 Component Behavior Ceramic Capacitors ^ Z -20 dB/decade ESL - Equivalent Series Inductance (L) 20 dB/decade Capacitor has low impedance for a narrow range of frequencies Capacitive Inductive (ESL) f f res = 2π 1 L lead C 45 Component Behavior Electrolytic Capacitors – Example Æ 150uf 10V Power supply output filter BUCK 5V 150uf What does this mean??? 46 Component Behavior Electrolytic Capacitors – Example Æ 150uf 10V Capacitance measurement over frequency HP4284A Precision LCR Meter & 16047D adapter used 47 Component Behavior Electrolytic Capacitors – Example Æ 150uf 10V EPN_1214353_SUNCON 150uF 20% 10V 180 160 140 Capacitance (uF) 120 100 80 Capacitance (uF) 60 No real capacitance after a few kHz 40 Long Wave Band 20 Medium Wave Band 0 10 100 1000 10000 ‐20 Frequency (Hz) 48 100000 1000000 Component Behavior Inductors ^ Z 0 dB/decade -20 dB/decade 20 dB/decade Rpar Inductance resonates with parasitic capacitance between windings of the inductor Resistive Rpar 2π L Saturation can happen 1 fres = 2 π LC par 49 Inductive Capacitive 1 2π LCpar f Component Behavior Inductors Resonant Frequency Saturation Curves 10uH goes resonant at 30MHz 10uH has only ~180ohms impedance at 300kHz Our typical use cases are borderline saturation 50 Component Behavior Inductors Low profile is very important! LQH44 Murata 1.1mm Vishay IHLP-2020BZ 2 mm Vishay IHLP-4040DZ 4 mm EPCOS B82472P6 EPCOS B82477P4 4.5mm 8.5mm Too tall!! Shorter package Æ flux lines stay closer to the board Æ lower emissions 51 Component Behavior Resistor ^ Z 0 dB/decade R -20 dB/decade 20 dB/decade Resistors are not purely resistive as frequency increases Resistive Capacitive 1 2 π RCpar f res = 2π 1 L lead C par 52 Inductive 1 2 π Llead C par f Component Behavior Diodes Schottkey Soft start Slow start Fast start Efficiency vs. heat vs. di/dt for emissions I-V graph of a real diode 53 Component Behavior Ferrite bead Do not trust the curves you see in the datasheet!!! Be sure to understand the circuit where the ferrite will be used Impedance over frequency graphs change with DC bias 54 Component Behavior Ferrite bead Take care when choosing a ferrite by it’s rating Ratings are typically done at 100MHz 3 Devices Having 1000ohms @ 100MHz 55 Layout Design 56 Buck Power Supply Layout – Component Placement INPUT Controller Diode Snubber OUTPUT INDUCTOR 57 Buck Power Supply Layout – Copper Definitions GND INPUT PWR Controller Diode Snubber OUTPUT INDUCTOR 58 Buck Power Supply Layout – Switch Closed Start by drawing the path of the current Ensure area of loop formed by current is kept small Keep high di/dt components on same side of PCB Allow common ground between input cap, regulator, diode, snubber and output cap Keep snubber next to diode Inductor GND: to fill or not to fill? (efficiency vs. EMC) 59 Buck Power Supply Layout – Switch Open Don’t forget there are two switch states! 60 Boost Power Supply Schematic Boost Power Supply Component Placement Same as BUCK supply with these additional items: Keep switch node away from surrounding copper areas Make switch node as small as possible Inductor orientation/wiring makes a difference (node connected to winding on inside or outside) Boost Power Supply PCB Layout TOP Noisy switch node Ground node All other copper Keep switch node small Maintain spacing to surrounding copper areas BOTTOM 1.72pF Boost Power Supply PCB Layout Noisy loop Ground loop Avoid SMPS loop within a GND loop Æ provide continuous ground fill • L1 = 50nH • L2 = 270nH • K = 0.45 (represents poor coupling between loops; where 1 = perfect coupling) • Lm = 52nH Æ mutual inductance between loops dI/dt 2 loops Design Trade-Offs 65 Recommendations for a Balanced Design Thermal constraints prefer faster switching, larger copper areas and spacing EMC constraints prefer slow switching, smaller copper and spacing 0 spacing ∞ spacing Thermal Constraints Æ EMC Constraints Æ Common Solution Æ 66 Recommended Reading Power Electronics Technology trade magazine (www.powerelectronics.com) http://www.ridleyengineering.com/ National Semiconductor Application Note 1149, “Layout Guidelines for Switching Power Supplies”. Texas Instruments Application Report SLPA005, “Reducing Ringing Through PCB Layout Techniques” Demystifying Switching Power Supplies by Raymond A. Mack 67 Thank you for your attention Questions? 68 Hardware Demonstration 69 Hardware Demonstration Hardware Demonstration – Buck SMPS Base unit (no EMC components) Fully populated PCB
