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Rev. A 09/06

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simple EMI filtering to comply with EN55022B. Resistor R2

and C4 form an RC snubber to reduce high-frequency EMI.

Outputovervoltageprotection(OVP)isprovidedontheprimary

sidebysensingthevoltageoftheauxiliarytransformerwinding.

Shouldthemainfeedbackloopopen duetofailureofU2,thenU1

willlatchoffoncethecurrentintotheBP/Mpin(viaVR3)exceeds

6.5 mA. Diode D6 and capacitor C7 rectify and smooth the

output of the auxiliary winding. The value of VR3 is selected

to trigger the OVP latch when the main output, and hence the

auxiliaryoutputvoltage,risesabovethenormalregulationrange.

Once triggered, cycling the AC power resets the OVP latch.

To reduce no-load input power and thus increase light load

efficiency, resistor R3 feeds the supply current for U1 from the

auxiliary winding on the transformer.

Key Design Points

• Verify maximum drain voltage is <650 V at high line,

maximum overload. Adjust values of R5, R6 and C5

accordingly. However, avoid making the clamp too

dissipative (low value of R5 and R6, and high value of

C5) as this will increase no-load consumption.

• To prevent an increase in no-load consumption or false

OVP triggering, VR3 should be selected to conduct

only when the output voltage is outside the normal

regulationrange.ResistorR4preventsexcessivecurrent

from flowing into the BP/M pin.

• Fast recovery glass-passivated diodes were selected for

D1 and D3 to reduce low frequency conducted EMI.

Fast diodes such as FR107 are also suitable.

Figure 2. Worst-Case Open Loop Output Overvoltage

(85 VAC, Full Load).

0

50

100

5

20

15

25

10

Time (ms)

30

35

0

-5

Table 1. Transformer Design Parameters.

T.I.W.: Triple Insulated Wire, NC: No Connection

TRANSFORMER PARAMETERS

Core Material

EF20, N67 or equivalent, gap

for A

2

Bobbin

5+5 pin horizontal

Winding Details

Shield: 24T, 28 AWG

Primary: 62T, 31 AWG

Bias: 5T, 4

× 28 AWG

12 V: 7T, 23 AWG T.I.W.

Winding Order

(Pin Numbers)

3 mm tape margin, shield

(1-NC), tape, primary (3-1),

tape, bias (5-4), tape, 12 V

(10-9), tape

Inductance

Primary: 790

µH ±7%

Leakage: 30

µH (max)

Primary Resonant

Frequency

650 kHz (min)