Rev. D 08/11

10

LNK403-409/413-419

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Start by adding a bleeder circuit. Add a 0.44 mF capacitor and

510 W 1 W resistor (components in series) across the rectified

bus (C11 and R18 in Figure 7). If this results in satisfactory

operation reduce the capacitor value to the smallest that results

in acceptable performance to reduce losses and increase

efficiency.

If the bleeder circuit does not maintain conduction in the TRIAC,

then add an active damper as shown in Figure 7. This consists

of components R9, R10, R11, R12, D1, Q1, C6, VR2, Q2 in

conjunction with R13. This circuit limits the inrush current that

flows to charge C2 when the TRIAC turns on by placing R13 in

series for the first 1 ms of the TRIAC conduction. After

approximately 1 ms, Q2 turns on and shorts R13. This keeps

the power dissipation on R13 low and allows a larger value to

be used during current limiting. Increasing the delay before Q2

turns on by increasing the values of resistors R9 and R10 will

improve dimmer compatibility but cause more power to be

dissipated across R13. Monitor the AC line current and voltage

at the input of the power supply as you make the adjustments.

Increase the delay until the TRIAC operates properly but keep

the delay as short as possible for efficiency.

As a general rule the greater the power dissipated in the bleeder

and damper circuits, the more dimmer types will work with the

driver.

Trailing Edge Phase Controlled Dimmers

Figure 11 shows the line voltage and current at the input of the

power supply with a trailing edge dimmer. In this example, the

dimmer conducts at 90 degrees. Many of these dimmers use

back-to-back connected power FETs rather than a TRIAC to

control the load. This eliminates the holding current issue of

TRIACs and since the conduction begins at the zero crossing,

high current surges and line ringing are minimized. Typically

these types of dimmers do not require damping and bleeder

circuits.

Audible Noise Considerations for Use With

Leading Edge Dimmers

Noise created when dimming is typically created by the input

capacitors, EMI filter inductors and the transformer. The input

capacitors and inductors experience high di/dt and dv/dt every

AC half-cycle as the TRIAC fires and an inrush current flows to

charge the input capacitance. Noise can be minimized by

selecting film vs ceramic capacitors, minimizing the capacitor

value and selecting inductors that are physically short and wide.

The transformer may also create noise which can be minimized

by avoiding cores with long narrow legs (high mechanical

resonant frequency). For example, RM cores produce less

audible noise than EE cores for the same flux density. Reducing

the core flux density will also reduce the noise. Reducing the

maximum flux density (BM) to 1500 Gauss usually eliminates

any audible noise but must be balanced with the increased core

size needed for a given output power.

Thermal and Lifetime Considerations

Lighting applications present thermal challenges to the driver.

In many cases the LED load dissipation determines the working

ambient temperature experienced by the drive so thermal

evaluation should be performed with the driver inside the final

enclosure. Temperature has a direct impact on driver and LED

lifetime. For every 10 °C rise in temperature, component life is

reduced by a factor of 2. Therefore it is important to properly

heat sink and verify the operating temperatures of all devices.

0

50

100

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400

350

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Conduction Angle (°)

350

300

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150

100

50

0

0.35

0.3

0.25

0.2

0.15

0.1

0.05

0

PI-5985-102810

Voltage

Current

Figure 10. Example of Phase Angle Dimmer Showing Erratic Firing.

50

100

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350

Conduction Angle (°)

350

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-50

-150

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-350

0.35

0.25

0.15

0.05

-0.05

-0.15

-0.25

-0.35

PI-5986-060810

Voltage

Current

0

Figure 11. Ideal Dimmer Output Voltage and Current Waveforms for a Trailing Edge

Dimmer at 90° Conduction Angle.