Touch current analysis on SMPS designed for energy efficiency regulations

Presented by: William Meng/孟昭德

Date: 2011/09/23

how many times have you heard that “it’s not the volts that’ll kill you, but the 

amps”?

nothing but ‘closed loop’

Basis of touch current: ac, iec 60479‐1

10

100

1000

10000

0.1 1 10 100 1000 10000

duratio

n of current flow

 (ms)

body current (mA)

perception possible, yet no startle

perception and involuntary muscular 

contractions likely, yet no harmful effect

cardiac arrest, breathing arrest,and burns or other cellular damage.

Steady state

Basis of touch current: HF effect

1

10

100

0.1 1 10 100

freq

. factor

freq. (kHz)

freq. factor, perception

freq. factor, reaction

almost linear

measurement network by iec 60990, for perception

1500Ω

500Ω

0.22 uF

10,000Ω

0.22 uF U2

Test terminals

Weighted (for perception/ reaction) touch current = U2/500 (peak value)

measurement network by iec 60990, for let‐go

1500Ω

500Ω

0.22 uF

10,000Ω

0.22 uF U3

Test terminals

Weighted (for let-go) touch current = U3/500 (peak value)

20,000Ω

0.0062 uF

0.0091 uF

0.5

1

1.5

2

2.5

10 100 1,000 10,000 100,000 1,000,000

U3/U2

Frequency (Hz)

Difference between Fig. 4 and Fig. 5

approx. 2.42 for freq. 

higher than 20,000 Hz

EMI and touch current

CSE

CPSACVDS

PWM Control and Feedback

CY

EMI and touch current

CSE

CPSACVDS

PWM Control and Feedback

CY

Touch current of an SMPS

PWM and Feedback I : Leakage current 

through filter Y‐cap

I : Leakage current through bridging Y‐cap

I : Leakage current through insulation

I : Leakage current through winding capacitance

Accessible conductive parts

Insulation

Set‐up of experiment

EUT(SMPS)

V+

V-

L

N

PE

Measuring network(U2/U3)

Electric load

264 V, 60 Hz

OscilloscopeTektronix: TDS3032B, DOP7104

Chroma: 63030Prodigit: 3311F

Samples: EPSOutput(W) Topology PFC Filter Y‐cap 

(PF)Bridging Y‐cap (PF)

Efficiency level

15 Flyback None None 680 x 1

90 Flyback active None 2200 x 1

120 Flyback active None 1000 x 2

180 Resonant active 2200 x 2 2200 x 2

*Loading conditions: 0%, 25%, 50%, 75%, 100% of max. nameplate current

V

Samples: PC powersOutput(W) Topology PFC Filter Y‐cap 

(PF)Bridging Y‐cap (PF)

Efficiency level

300 Dual‐switchforward + flyback

active 2200 x 21000 x 2 2200 x 1350

400300 Dual‐switch

forward + flyback

active 2200 x 21000 x 2 2200 x 1 none350

400Loading conditions: 0%, 25%, 50%, 75%, 100% of max. nameplate currentFlyback: transformer for standby power (+5Vsb)

Result: EPS

0%

20%

40%

60%

80%

100%

120%

0% 25% 50% 75% 100%

U2, pk U2, rms U3, pk U3, rms

0%

40%

80%

120%

160%

200%

0% 25% 50% 75% 100%

U2, pk U2, rms U3, pk U3, rms

0%20%40%60%80%100%120%

0% 25% 50% 75% 100%

U2, pk U2, rms U3, pk U3, rms

0%20%40%60%80%100%120%

0% 25% 50% 75% 100%

U2, pk U2, rms U3, pk U3, rms

Result: PC powers

0.6

0.8

1

1.2

0 0.2 0.5 1

300W, 80+, U2, pk 300W, 80+, U2, rms

300W, 80+, U3, pk 300W, 80+, U3, rms

0.6

0.8

1

1.2

0 0.2 0.5 1

300W, non‐80+, U2, pk 300W, non‐80+, U2, rms

300W, non‐80+, U3, pk 300W, non‐80+, U3, rms

0.6

0.8

1

1.2

0 0.2 0.5 1

350W, 80+, U2, pk 350W, 80+, U2, rms

350W, 80+, U3, pk 350W, 80+, U3, rms

0.6

0.8

1

1.2

0 0.2 0.5 1

350W, non‐80+, U2, pk 350W, non‐80+, U2, rms

350W, non‐80+, U3, pk 350W, non‐80+, U3, rms

0.6

0.8

1

1.2

0 0.2 0.5 1

400W, 80+, U2, pk 400W, 80+, U2, rms

400W, 80+, U3, pk 400W, 80+, U3, rms

0.6

0.8

1

1.2

0 0.2 0.5 1

400W, non‐80+, U2, pk 400W, non‐80+, U2, rms

400W, non‐80+, U3, pk 400W, non‐80+, U3, rms

Result: PC powers (cont.)

0.6

0.8

1

1.2

0 0.2 0.5 1

80/non‐80, 300W, U2, pk 80/non‐80, 300W, U2, rms

80/non‐80, 300W, U3, pk 80/non‐80, 300W, U3, rms

0.6

0.8

1

1.2

0 0.2 0.5 1

80/non‐80, 350W, U2, pk 80/non‐80, 350W, U2, rms

80/non‐80, 350W, U3, pk 80/non‐80, 350W, U3, rms

0.8

1

1.2

1.4

0 0.2 0.5 1

80/non‐80, 400W, U2, pk 80/non‐80, 400W, U2, rms

80/non‐80, 400W, U3, pk 80/non‐80, 400W, U3, rms

Observation: EPS, 90WInput current

Touchcurrent

Mains voltage

0%

25%

50%

75%

100%

Hypothesis: only bridging Y is influenced by loading

0

50

100

150

200

250

300

0% 25% 50% 75% 100%

TC (m

A)

U2, no CY

U2, 680P

U2, 1000P

U2, 1680P

U2, 2000P

U2, 2680P

0

50

100

150

200

250

300

0 500 1000 1500 2000 2500 3000

touch curren

t (mA)

Y‐Cap (PF)

U2, 0%

U2, 25%

U2, 50%

U2, 75%

U2, 100%

Recommendation

• Touch current shall be measured with the EuTloaded

• Peak measurement is more appropriate than rms measurement

• Keep the EuT completed!!