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Rev. A 02/07

Power Integrations

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signal. The circuit formed by R10, U3 and its associated

componentsprovidesthelowfrequency(LF)portionofthe

feedbacksignalanddeterminestheDCregulationsetpoint.

The HF circuit gives the supply good transient load

responsiveness.TheLFcircuitgivesthesupplygoodoutput

voltageaccuracy.Bothcircuitsworkbyforwardbiasingthe

LEDinU2whenevertheoutputvoltageexceedsthesetpoint

value.TheseriesvoltagedropsacrossVR2,theU2LEDand

R16determinethesetpointfortheHFcircuit.Thevoltage

divider(R12andR13)andU3determinethesetpointforthe

LFcircuit.CapacitorC17isphysicallylocatedneartheHF

circuit,andattenuatesnoisethatcouldfalselytriggeritsset

pointthreshold.ZenerdiodeVR3clampstheoutputvoltage

at16V,ifthefeedbackloopbecomesanopencircuit,until

auto-restartactivates(occursafternocycleshavebeenskipped

for30ms).

Key Design Points

• ThevaluesforR11andR16werechosensothatthevoltage

onthecathodeoftheLEDinU2isabout2.2V.

Table 2. Transformer Parameters.

TRANSFORMER PARAMETERS

Core

Material

EE25

NC-2H or Equivalent, gapped for A

Bobbin

EE25, 10 pin

Winding

Details

1/2 Primary: 26T, 27 AWG, 1 layer, tape

Bias: 8T X2, 29 AWG, tape

Secondary: 6T, X4, 23 AWG TIW, 2 layers

Shield: 7T X4, 29 AWG, tape

1/2 Primary: 26T, 27 AWG, 1 layer

Primary

Inductance 370 mH ±12%

Primary

Resonant

Frequency

180 kHz (minimum)

Leakage

Inductance 8 mH (maximum)

Figure 2. Efficiency vs. Load, at Standard Line Voltages.

Figure 3. Worst Case Conducted EMI (230 VAC).

0

0.5

1.5

1

2

Output Current (A)

82.5

82

81.5

81

80.5

80

79.5

79

Efficiency @ VAC = 230 V

Efficiency @ VAC = 115 V

• Thecoresizeandthewindingwirediametersizes

werechosenbasedontheaverageofthepeakandthe

continuousoutputpower.

• Thenumberofturnsintheprimaryandsecondary

windingsandtheprimaryinductancevalueswere

chosenbasedonthepeakoutputpower.

• DiodeD5mustbeanultrafastdiodewithareverse

recoverytimeof<75ns.

1.0

0.15

10.0

70.0

-20

-10

0

10

20

30

40

50

60

80

70

MHz

QP

AV

QP

AV