reducing emissions in dc-dc switched mode power...
TRANSCRIPT
Reducing Emissions in DC-DC Switched Mode Power Supplies
Scott Mee – Johnson ControlsJim 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 SuppliesLinear 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 step down i.e. 12Volts to 5Volts logic level
Boost step up 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
Iind
Imax
Imin
Switch StateVo
ltage
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
Switch State
Boost Circuit Switching Voltages and Currents
Iind
Idiode
Imax
Imin
Volta
ge L
evel
Timing
Vout
Vin
Vind
0 volts
11
Trapezoidal Periodic Signals
( ) ( )∑∞
=
∠++=1
00 cos2n
nn ctncctx ω( )
rfjn
r
r
nre
n
n
n
n
TAc ττ
τω
τω
τω
τωτ ττω =
⎟⎠⎞
⎜⎝⎛
⎟⎠⎞
⎜⎝⎛
= +− ,
21
21sin
21
21sin
2
0
0
0
00
frTAc τττ
== ,0
Fourier CoefficientsFourier Series
12
To be conservative we might choose a point, 3 times this second breakpoint this is approximately
Bandwidth of Periodic Waveforms
Bounds on frequency spectrum Above the 2nd break point, the harmonics drop off at a rate of -40dB/decade.
rπτ3
rπτ1
HzBWrτ
1=Bandwidth of a periodic signal
rτ1
13
Noise Sources in SMPS
Switching characteristics
dv/dt & di/dt
Fundamental frequency
Harmonic series
Resonances
Step response to the RLC network Ringing
Secondary effects
Power surges at input
Ripple on power bus
Ripple on system wiring
Output ripple
Magnetic fields
14
15
BUCK SupplyEmissions
Investigation
Emissions InvestigationBUCK SMPS Circuit
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Emissions InvestigationBUCK Voltage Measurements
Switch Output Voltage
Voltage at Input to SMPS
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1
3
Emissions InvestigationBUCK Voltage Measurements – Zoom
Switch Output Voltage
Voltage at Input to SMPS
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Time domain
Frequency domain
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Emissions InvestigationNarrow Band vs. Broadband
Conducted Emissions (150kHz – 2MHz)
Before Techniques Applied After Techniques Applied
Emissions InvestigationSuccess Stories – 70kHz SMPS
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Improvement came from- Front-end filtering (L/C filter)- Slew rate controls- Layout improvements
Conducted Emissions (150kHz – 2MHz)
Before Techniques Applied After Techniques Applied
Emissions InvestigationSuccess Stories – 150kHz SMPS (Low band)
21
Improvement came from- Front-end filtering (L/C filter)- Layout improvements
CISPR 25 – Radiated Emissions (25MHz – 200MHz)
Before Techniques Applied After Techniques Applied
Emissions InvestigationSuccess Stories – 150kHz SMPS (High band)
22
Improvement came from- Diode snubber- Diode switching changes- Layout improvements
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BOOST SupplyEmissions
Investigation
Emissions InvestigationBoost Supply Case Study
12volt input & 34volt output
+12 V +34 V
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Emissions InvestigationBoost Supply – current loop when switch is closed
Red = current flow to load, Blue = return current
+12 V +34 V
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Emissions InvestigationBoost Supply – current loop when switch is open
Red = current flow to load, Blue = return current
+12 V +34 V
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Emissions InvestigationBoost Supply Radiated Emissions 50MHz – 180MHz BL ON
123MHz 161MHz
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Emissions InvestigationBoost Supply Radiated Emissions 50MHz – 180MHz BL OFF
123MHz 161MHz
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Emissions InvestigationBoost Supply Measurement points
V1
i1+12 V +34 V
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Emissions InvestigationBoost Supply Measurement Setup
Bench measurement setup overview LeCroy 6GHz 40GS/s
Voltage probe 500MHz 1.8pf
Current probe Langer HF magnetic field probe
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Emissions InvestigationBoost Supply 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
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Emissions InvestigationBoost Supply Voltage Measurement Results
Overview of switching waveforms
V1
i1
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Emissions InvestigationBoost Supply Voltage Measurement Results
Switch turns from off to on
V1
i1
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Emissions InvestigationBoost Supply Voltage Measurement Results
Switch turns from on to off
V1
i1
34
Emissions InvestigationBoost Supply Radiated Emissions 50MHz – 180MHz BL ON
V1
i1
123MHz 161MHz
35
Johnson Controls36
Emissions InvestigationBoost Supply Diode Current No changes / Baseline
123MHz ringing corresponds to 123MHz emissions
123MHz
Johnson Controls37
Emissions InvestigationBoost Supply Diode Current 1nf cap across diode CR6401
77 MHz ringing corresponds to 77MHz emissions
77MHz
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Schematic Design
Schematic DesignBuck topology
12V input
5V output
+12 V+5 V
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Input filtering Snubber
Soft-start capacitor
SnubberSlew rate control
Spread spectrum
Output filter
Schematic DesignBoost topology
Johnson Controls40
SnubberSlew rate control Output cap
Front end Pi filter
Schematic DesignSnubber Calculations
FRinging 126MHz:=
CSnubber 1500pF:=
FTuned 40MHz:=
CParasitic 168.114pF=
LParasitic 9.491 10 9−× 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=
CSnubber 1.5 10 9−× F=
Schematic DesignCombination Selection
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Component Selection
Component BehaviorCapacitors, Resistors, Inductances, Ferrites
All passive components have resistance, capacitance and inductance
Component behavior is different at low and high frequencies
44
Component BehaviorCeramic Capacitors
^Z
Capacitive Inductive (ESL)
f
-20 dB/decade
20 dB/decadeESL - Equivalent Series Inductance (L)
Capacitor has low impedance for a narrow range of frequencies
CLf
lead
res
π21
=
45
Component BehaviorElectrolytic Capacitors – Example 150uf 10V
Power supply output filter
BUCK 5V150uf
What does this mean???
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Component BehaviorElectrolytic Capacitors – Example 150uf 10V
Capacitance measurement over frequency
HP4284A Precision LCR Meter & 16047D adapter used
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‐20
0
20
40
60
80
100
120
140
160
180
10 100 1000 10000 100000 1000000
EPN_1214353_SUNCON 150uF 20% 10V
Capacitance (uF)Capacitance (u
F)
Frequency (Hz)
Medium Wave BandLong Wave Band
Component BehaviorElectrolytic Capacitors – Example 150uf 10V
No real capacitance after a few kHz
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Component BehaviorInductors
f
^Z
Resistive CapacitiveInductive
Rpar
0 dB/decade
-20 dB/decade
20 dB/decade
LRpar
π2 parLCπ21
Inductance resonates with parasitic capacitance between windings of the inductor
Saturation can happen
parLCfres
π21
=
49
Component BehaviorInductors
Resonant Frequency Saturation Curves
10uH goes resonant at 30MHz
10uH has only ~180ohms impedance at 300kHz Our typical
use cases are borderline saturation
50
Low profile is very important!
Shorter package flux lines stay closer to the board lower emissions
Component BehaviorInductors
LQH44 Murata
1.1mm
Vishay IHLP-4040DZ EPCOS B82472P6Vishay IHLP-2020BZ
2 mm 4 mm 4.5mm 8.5mm
EPCOS B82477P4
Too tall!!
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Component BehaviorResistor
^Z
Resistive Capacitive Inductive
f
R0 dB/decade
-20 dB/decade
20 dB/decade
parRCπ21
parleadCLπ21
Resistors are not purely resistive as frequency increases
parlead CLfres
π21
=
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Component BehaviorDiodes
I-V graph of a real diode
SchottkeySoft startSlow startFast startEfficiency vs. heat vs. di/dt for emissions
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Component BehaviorFerrite 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 BehaviorFerrite bead
Take care when choosing a ferrite by it’s rating
Ratings are typically done at 100MHz
3 Devices Having 1000ohms @ 100MHz
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Layout Design
Buck Power SupplyLayout – Component Placement
INDUCTOR
OUTPUT
Diode
Snubber
Controller
INPUT
57
Buck Power SupplyLayout – Copper Definitions
INDUCTOR
OUTPUT
Diode
Snubber
Controller
INPUT
58
GND
PWR
Buck Power SupplyLayout – Switch Closed
59
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)
Buck Power SupplyLayout – Switch Open
60
Don’t forget there are two switch states!
Boost Power SupplySchematic
Boost Power SupplyComponent 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 SupplyPCB Layout
TOP BOTTOM
Noisy switch nodeGround nodeAll other copper
1.72pF
Keep switch node small
Maintain spacing to surrounding copper areas
Boost Power SupplyPCB Layout
Noisy loopGround loop
dI/dt 2 loops
• L1 = 50nH• L2 = 270nH• K = 0.45 (represents poor coupling between loops; where 1 = perfect coupling)
• Lm = 52nH mutual inductance between loops
Avoid SMPS loop within a GND loop provide continuous ground fill
65
Design Trade-Offs
Recommendations for a Balanced Design
0 spacing ∞ spacing
Thermal Constraints
EMC Constraints
Common Solution
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Thermal constraints prefer faster switching, larger copper areas and spacing
EMC constraints prefer slow switching, smaller copper and spacing
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
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Thank you for your attention
Questions?
68
Hardware Demonstration
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Hardware Demonstration
Hardware Demonstration – Buck SMPS
Base unit (no EMC components) Fully populated PCB