EMBSY - D5
11/11/2010
Politecnico di Torino ICT School
Less. D5: Power supply system design
• Design from the following specifications
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Electronics for Embedded Systems
• Evaluate regulation Sv and output resistance Ro
• Add zener regulator
• Add Emitter follower
D5 - Design of power supply systems
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Input voltage range, frequency
Load nominal voltage
Load min and max current
Max ripple
Design from specifications
Evaluate Sv and Ro
Add zener regulator
Add Emitter follower
Modify into series regulator
– Evaluate new regulation Sv and output resistance Ro
• Modify into series regulator (Op Amp + Power BJT)
– Compare with commercial 3-pin regulator
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EMBSY - D5 - © 2010 DDC
EMBSY - D5 - © 2010 DDC
Linear PSU with input transformer
PSU design sequence
• Functional units:
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• Specifications
EMI Filter
Mains to low voltage AC (50 Hz transformer)
AC to DC, with ripple filter
Linear (or switching) voltage regulator
220V ≈
50 Hz
VOLTAGE
REG.
– Input voltage: 190-230Veff, 45-55 Hz (nominal 220 V, 50 Hz)
– Load: nominal Vo = 5 V ±10%, Io 0 – 1 A; Max ripple 0,1 Vpp
• Results:
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Vout =
• Problems
– Low efficiency
– Low PF
– Move to switching PSU with PFC
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Parameters of: fuse, transformer, diodes;
Value of filter capacitor
Evaluation of regulation Sv and output resistance Ro
Add zener regulator: new Sv and Ro
Add Emitter follower: new Sv and Ro
Add Op Amp + Power BJT (series reg.) new Sv and Ro
• Compare with commercial 3-pin regulator
– Redesign C to get same performance with 3-pin regulator
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EMBSY - D5 - © 2010 DDC
EMBSY - D5 - © 2010 DDC
Fuse and transformer
Ripple filter
• Fuse
• Actual waveform
– Types and parameters
– Value:
– Sine-segment charge
– Exponential discharge,
with long time constant
VRI
• Assumption
• Transformer
– Pulsed charge
– Constant I discharge
– Triangular ripple
– Parameters
» Ratio
» Power
» Efficiency
VRIPPLE 
– Values:
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VC
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EMBSY - D5 - © 2010 DDC
T /2
t
I
Q 1 T
 IL  L
C
C 2 2 fC
EMBSY - D5 - © 2010 DDC
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EMBSY - D5
11/11/2010
Design of capacitor
• Low ripple  Large capacitor
C
– Drawbacks
» Size and cost
» Low current circulation angle
Current in diodes
• Load current
IL
2f  VRIPPLE
– Pulsed
– Same total charge
– Various approximation
• Keep minimum required, considering torances
– More convenient to reduce ripple with regulator
Type
Capacitance value
Operting voltage
ESR
t
• Parameters
• Parameters of capacitors
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ID
– Almost DC
• Diode/transformer current
– Conduction angle defined
from sine/discharge intersection
– High capacitor
 low ripple
 low conduction angle
 high current peak
Scelta finale: 270μF
400VL
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EMBSY - D5 - © 2010 DDC
EMBSY - D5 - © 2010 DDC
Evaluating peak current
• Total charge to load
– Idc T/2
Selection of diodes
• Average current
• Peak current and inrush current
• Reverse voltage
ID
IPK
• Total charge from diodes
 V  VRIPPLE 

 C  arccos PK
VPK


– Id Tc
– Same total charge
• Different approximations
for diode current
T1
t
– Rectangle
– Triangle: ΔQ = IPK ·T1 / 2
– Parabolic: ΔQ = IPK ·T1 ·2 / 3
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iC  0
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EMBSY - D5 - © 2010 DDC
Final design result
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Fuse
Transformer
Diodes
Capacitor
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Output DC
Output ripple
Output line regulation
Output load regulation
EMBSY - D5 - © 2010 DDC
Choices for the regulator
• Shunt Zener regulator
• Zener + Emitter follower
• Op Amp + Power BJT or MOS
• Integrated regulator
• Different architecture: switching PSU
• Next: add active regulator
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EMBSY - D5 - © 2010 DDC
EMBSY - D5 - © 2010 DDC
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Shunt Zener regulator
Active series regulator
• Simple
• Series regulator: Good efficiency
• A feedback loop: Better line and load regulation
• Low output current capability
• High stability:
– Can increase with the Emitter Follower
Can use good reference
• Poor regulation
• Low efficiency
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EMBSY - D5 - © 2010 DDC
Op Amp + Power BJT or MOS
Integrated regulators
• The transistor operates as final power CC stage
• Can use BJT or MOS
• Can use darlington configuration
• 3-pin
– Fixed current limit
– No remote sense
– High current gain
– Requires more headroom
• More pins
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Additional commands
Remote V sense
External I sense
Add external power transistor
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EMBSY - D5 - © 2010 DDC
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