LNK454/456-458/460
output capacitance and the value of the current sense resistor.
If the peak of the ripple voltage exceeds 550 mV, the device will
enter cycle skipping mode which will reduce PFC performance
(lower PF and increase THD).
Figure 8 shows the line voltage and current at the input of a
leading edge TRIAC dimmer. In this example, the TRIAC
conducts at 90 degrees.
Figure 9 shows the desired rectified bus voltage and current.
Transformer Considerations for use with
Leading Edge TRIAC Dimmers
PI-5984-060810
350
300
250
200
150
100
50
0.35
Audible noise can be created in the transformer due to the
abrupt change in flux when the TRIAC turns on. This can be
minimized by selecting cores with higher mechanical resonant
frequencies. Cores with long narrow legs should be avoided
(e.g. EEL types). RM and other pot core types are good choices
and produce less audible noise than EE cores for the same flux
density. Reducing the core flux density (BM) also reduces
audible noise generation. A value below 1500 Gauss usually
eliminates any noise generation but reduces the power
capability of a given core size.
Voltage
Current
0.3
0.25
0.2
0.15
0.1
0.05
0
0
Working with TRIAC Dimmers
50
100
0
150
200
250
300
350
400
The requirement to provide output dimming with low cost,
TRIAC based, leading edge phase dimmers introduces a
number of trade-offs in the design.
Conduction Angle (°)
Figure 9. Resultant Waveforms Following Rectification of Ideal TRIAC
Dimmer Output.
For correct operation incandescent phase angle dimmers
typically have a specified minimum load, typically ~40 W for a
230 VAC rated unit. This is to ensure that the current through
the internal TRIAC stays above its specified holding current
threshold.
Figure 10 shows undesired rectified bus voltage and current
with the TRIAC turning off prematurely and restarting. On the
first half-cycle this is due to the input current ringing below the
holding current of the TRIAC, excited by the initial inrush current.
The second half-cycle also shows the TRIAC turning off due to
the current falling below the holding current towards the end of
the conduction angle. This difference in behavior on alternate
half-cycles is often seen due to a difference in the holding
current of the TRIAC between the two operating quadrants.
Due to the much lower power consumed by LED lighting the
input current drawn by the lamp is below the holding current of
the TRIAC within the dimmer. The input capacitance of the
driver allows large inrush currents to flow when the TRIAC fires.
This then generates input current ringing with the input stage
and line inductance which may cause the current to fall below
the TRIAC holding current. Both of these mechanisms cause
undesirable behavior such as limited dimming range and/or
flickering.
PI-5985-102810
350
300
250
200
150
100
50
0.35
0.3
Voltage
Current
0.25
0.2
To overcome these issues two circuit blocks, damper and
bleeder, are incorporated in dimming applications. The
drawback of these circuits is increased dissipation and
therefore reduced efficiency of the supply.
0.15
0.1
0.05
PI-5983-060810
350
250
150
50
0.35
0.25
0.15
0.05
-0.05
-0.15
-0.25
0
0
Voltage
Current
50
100
0
150
Conduction Angle (°)
Figure 10. Example of Phase Angle Dimmer Showing Erratic Firing.
200
250
300
350
400
If the TRIAC is turning off before the end of the half-cycle or
rapidly turning on and off then a bleeder and damper circuit are
required.
0.5
50
100
150
200
250
300
350
400
-50
-150
-250
In general as power dissipated in the bleeder and damper circuits
increases, so does dimmer compatibility.
-350
-0.35
Initially install a bleeder network across the rectified power bus
(R10, R11 and C6 in Figure 7) with initial values of 0.1 µF and a
total resistance of 1 kW and power rating of 2 W.
Conduction Angle (°)
Figure 8.
Ideal Input Voltage and Current Waveforms for a Leading Edge
TRIAC Dimmer at 90° Conduction Angle.
8
Rev. C 10/11
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