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SNVS570L – JANUARY 2009 – REVISED MAY 2013
45° 90° 135°
V PEAK
V AC
t
Figure 30. AC Line with Firing Angles
Figure 31 show three triac dimmed waveforms. One can easily see that the peak voltage (V PEAK ) from 0° to 90°
will always be:
V AC-RMS-PK 2
Once the triac is firing at an angle greater than 90° the peak voltage will lower and equal to:
V AC-RMS-PK 2 x SIN( T )
(27)
(28)
The voltage at V BUCK with a valley fill stage of two will look similar to the waveforms of Figure 32 .
The purpose of the valley fill circuit is to allow the buck converter to pull power directly off of the AC line when
the line voltage is greater than its peak voltage divided by two (two stage valley fill circuit). During this time, the
capacitors within the valley fill circuit (C7 and C8) are charged up to the peak of the AC line voltage. Once the
line drops below its peak divided by two, the two capacitors are placed in parallel and deliver power to the buck
converter. One can now see that if the peak of the AC line voltage is lowered due to variations in the line voltage,
or if the triac is firing at an angle above 90°, the DC offset (V DC ) will lower. V DC is the lowest value that voltage
V BUCK will encounter.
V BUCK(MIN) =
V AC-RMS(MIN) 2 x SIN( T )
#stages
(29)
Example:
Line voltage = 90V AC to 135V AC
Valley-Fill = two stage
90 2 x SIN(135 )
V BUCK(MIN) =
2
o
= 45V
(30)
Depending on what type and value of capacitors are used, some derating should be used for voltage droop when
the capacitors are delivering power to the buck converter. When the triac is firing at 135° the current through the
LED string will be small. Therefore the droop should be small at this point and a 5% voltage droop should be a
sufficient derating. With this derating, the lowest voltage the buck converter will see is about 42.5V in this
example.
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