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

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

What is Direct-off-Line? Power supply operates directly from AC line No transformer prior to rectification and filtering

Switchmode vs. Linear SwitchmodeLinear Near constant efficiency and power load Constant current load Wide input voltage rangeLow cost Low heat generationLow noise High reliabilityEasy to use Small size transformerLarge bulky transformer Frequency 100/300 kHzFrequency 50/60 Hz

Types of Switchmode Supplies Flyback topology Forward topology Half Bridge topology Push-Pull topology Full-Bridge topology

Typical Off-line Dual Voltage Rectifier, Filter Circuit

Off-line AC Power Line Series Resistance ESR DC-DC Converter Load

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

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

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

Designing

Simulation

Calculations Given parameters: Vin(rms) = 170 V η = 0.7 Rs = 0.42 Ω Pout = 250 W F = 50 Hz To find the minimum capacitance size of Ce, from Fig , 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 η. Pin = Pout/η = % of load Using Eq. #2, the effective input current Ie = 2.1 A Using Eq. #4, the resistance factor Rsf = 150 Ω∙W From Fig w/ 100% load, Rsf ratio yields Iin(rms)/Ie = The input current is: Iin(rms) = (2.1 A)*(1.48) = 3.11 A From Fig 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 , 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 : (2πf)*(Ce)*(RL) > 50 So the load resistance RL >= 425 Ω. I used RL at its minimum value. From Fig w/ Pin = 357 W and Rs = 0.42 Ω, the ratio Vo(dc)/Vin(rms) = The filter DC output voltage is: Vo(dc) = 228 V.

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

Applications Typically used in Digital Electronics –Cell Phones –PDA’s –PC’s –Laptops –CD/DVD Players

Applications 3.6 W Cell Phone Charger from Power Integration’s TINYSWITCH