www.powerint.com

A

10/03

DI-67

Power Integrations

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San Jose, California 95138

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Power Integrations reserves the right to make changes to its products at any time to improve reliability or manufacturability. Power Integrations does not assume any

liability arising from the use of any device or circuit described herein, nor does it convey any license under its patent rights or the rights of others. The products and

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For the latest updates, visit www.powerint.com

025

50

Time (ms)

0.5 A/Div.

100 V/Div.

0.5 A/Div.

0 A

0 V

0 A

Table 1. Transformer Construction Information.

Core

Bobbin

Winding Details

Winding Order

(pin numbers)

Primary Inductance

Primary Resonant

Frequency

Leakage Inductance

TRANSFORMER PARAMETERS

EF-20 or Equivalent

Miles-Platts EF0700 EF20

8 Pin Horizontal

Primary: 70T, 2 layers, 29 AWG

Secondary: 13T, 2 x 32 AWG

Triple Insulated Wire

Bias: 9T, 2 x 23 AWG

Primary: 2-5, tape

Secondary: 8-7, tape

Primary: 5-1, tape

Bias: 3-4, tape 3 layers

350 µH ±10%

2.0 MHz (Min)

10 µH (Max)

Figure 2. PFC LED Driver Waveforms.

Top:

Input Current at 120 VAC 60 Hz.

Middle: C3 Capacitor Voltage.

Bottom: Output Current (with 20 V LED Load).

Figure 3. PFC Driver Current vs. Temperature.

Key Design Points

• To have high power factor, a constant duty factor must be

maintained over the 8.33 ms half cycle period. Therefore,

the bias supply voltage and the U1 CONTROL pin current

must remain extremely constant. The values of C6 and C8

must be chosen accordingly.

• Decreasing the value of C8 will reduce the turn-on delay

time, but will also degrade the power factor.

• Because low-cost is the goal of this low-loop-gain design,

the tolerance of the output current depends on the CTR of

the opto-coupler and the (unregulated) value of the bias

voltage. The restricted AC input voltage range allows

using the forward (not flyback) configuration for the bias

winding. If the AC input voltage range is extended, voltage

regulation of the bias supply circuit will be required.

-40

0

-20

20

40

60

80

Ambient Temperature (°C)

700

680

660

720

740

LED Current must be maintained

within manufacturer's specified

temperature range.

+5% Upper Specification Limit

-5% Lower Specification Limit