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

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

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-lineAC Power Line

Series ResistanceESR

DC-DC ConverterLoad

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

Designing

Simulation

E1

D1D2

D3 D4

RS

Ce

VM1.V [V] Vload.V [V] RS.R [Ohm]

t [s]

0.25k

-25

50

0.1k

0.15k

0.2k

0 50m5m 10m15m20m25m30m35m40m

+

V

VM1

Simplified Capacitive Input Filter w/ Single-PhaseFull/Wave Bridge Rectifer and Lumped Total EffectiveSource Resistance.

Converter loadRL

R := 425

C := 375uAMPL := 0.240416305603426k

FREQ := 50

70,7%

RMS

VinRMS

VinRMS.VAL

t [s]

0.2k

-20

25

50

75

0.1k

0.13k

0.15k

0.18k

0 10.2 0.4 0.6 0.8

VinRMS.VAL 0.169913k

R := .42

MEAN VALUE

Vout_DC

Vout_DC.VAL

t [s]

0.25k

-25

50

0.1k

0.15k

0.2k

0 10.2 0.4 0.6 0.8

Vout_DC.VAL

0.232103k

70,7%

RMS

Icap_rms

Icap_rms.VAL

2.07848

Icap_rms.VAL Ce.I [A]

t [s]

30

-5

0

5

10

15

20

25

0 50m5m 10m 15m 20m 25m 30m 35m 40m

70,7%

RMS

Ie

Ie.VAL 2.16092 A

Ipeak

W

+

WM1

Simulation

Ipeak.I [A]

t [s]

30

-5

0

5

10

15

20

25

5m 50m10m 15m 20m 25m 30m 35m 40m 45m

WM1.P [W]

t [s]

4k

-0.5k

0

0.5k

1k

1.5k

2k

2.5k

3k

3.5k

5m 50m10m 15m 20m 25m 30m 35m 40m 45m

Simulation

1.260

1.280

1.300

1.320

1.340

1.360

1.380

1 10 100 1000

Power

Vo

(dc)/

Vin

(rm

s)

152.8

152.9

152.8

153

152.9

152.9

152.7

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. 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 η.• Pin = Pout/η = 357 W @ 100% 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. 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:• (2πf)*(Ce)*(RL) > 50• So the load resistance RL >= 425 Ω. I used RL at its minimum value.• From Fig. 1.6.7 w/ Pin = 357 W and Rs = 0.42 Ω, the ratio Vo(dc)/Vin(rms) = 1.34. 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