Line Rectification and Capacitor Input Filters For Direct-off-Line Switchmode Power Supplies

1
Line Rectification and Capacitor Input Filters
For Direct-off-Line Switchmode Power Supplies
2
Overview

• Switchmode Supplies vs. Linear Supplies
• Characteristics of typical Direct-off-line
  Circuit for Switchmode Supplies
• Characteristics of Switchmode Supplies
• Simulation
• Calculations
• Applications

3
What is Direct-off-Line?

• Power supply operates directly from AC line
• No transformer prior to rectification and
  filtering

4
Switchmode vs. Linear
Switchmode Linear
Near constant efficiency and power load Constant current load
Wide input voltage range Low cost
Low heat generation Low noise
High reliability Easy to use
Small size transformer Large bulky transformer
Frequency 100/300 kHz Frequency 50/60 Hz
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Types of Switchmode Supplies

• Flyback topology
• Forward topology
• Half Bridge topology
• Push-Pull topology
• Full-Bridge topology

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Typical Off-line Dual Voltage Rectifier, Filter
Circuit
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Off-line AC Power Line
Series Resistance ESR
DC-DC Converter Load
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Characteristics

• Voltage Operation (120Vac/240Vac)
• 120V operation for voltage doubler
• 240V operation for full-bridge rectifier
• Effective Series Resistance
• Source resistance reduce peak currents
  20mOhms 600mOhms.
• Inrush Limiting thermistor used for startup
• 1Ohm

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Characteristics

• Constant Power Load
• Output voltage fixed under steady state
  conditions
• Near Constant efficiency for load
• Rising Current
• Resistance Factor
• Parameter used for calculating current loads

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Designing

• Effective Input Current Ie
• RMS Input Current Iin(rms)
• Capacitor Current (Icap)
• Peak Input Current (Ipeak)
• Capacitor Size (Ce)
• DC Output voltage (Vo)

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Designing
12
Designing
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Simulation
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Simulation
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Simulation
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Calculations

• Given parameters Vin(rms) 170 V
• ? 0.7
• Rs 0.42 O
• Pout 250 W
• F 50 Hz
• To find the minimum capacitance size of Ce, from
  Fig. 1.6.7, assume capacitance/Watt is 1.5
  uF/W.
• Ce (1.5 uF)(250 W) 375 uF
• The input power Pin can be found from the
  efficiency ?.
• Pin Pout/? 357 W _at_ 100 of load
• Using Eq. 2, the effective input current Ie
  2.1 A
• Using Eq. 4, the resistance factor Rsf 150 OW
• From Fig. 1.6.4 w/ 100 load, Rsf ratio yields
  Iin(rms)/Ie 1.48. The input current is
• Iin(rms) (2.1 A)(1.48) 3.11 A
• From Fig. 1.6.5 w/ 100 load, Rsf ratio yields
  Icap(rms)/Ie 1.3. The capacitor current is
• Icap(rms) (2.1 A)(1.3) 2.73 A
• From Fig. 1.6.6, Rsf ratio yields Ipeak/Ie 4.6.
  The peak input current is
• Ipeak (2.1 A)(4.6) 9.66 A
• As discussed earlier in the DC Output Voltage and
  Regulation For Rectifier Capacitor Input Filters
  section, when using Fig. 1.6.7
• (2pf)(Ce)(RL) gt 50

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More Design Parameters

• RMS Ripple current rating
• Prevents temperature rise in capacitor
• Ripple Voltage
• Allows wide input voltage range for load
• Voltage Rating
• Component voltage limit
• Holdup time
• Time output remains once input is shut off

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Applications

• Typically used in Digital Electronics
• Cell Phones
• PDAs
• PCs
• Laptops
• CD/DVD Players

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Applications
 
                                                                        
3.6 W Cell Phone Charger from Power Integrations
TINYSWITCH