Cirrus Logic AN368 - Manual (Page 13)

AN368
AN368REV2 13
a. Set the Value for Boost Output Voltage
The value of the boost output voltage V
BST
must be greater than the maximum input AC line voltage peak. The
maximum V
BST
voltage V
BST(max)
should be kept as low as possible to help keep the FET breakdown
requirement within economical constraints.
V
BST
is determined by an internal parameter and changes slightly depending on the type of dimmer detected.
With sense resistors R13, R14, R17, and R18 set to 750k  each, the resulting V
BST
is approximately 200V
for a 120V system. V
BST
is regulated by charging the boost output capacitor to its nominal value each half
line-cycle. V
BST
droops to its lowest value towards the end of each half line-cycle until the boosting process
starts again in the next half line-cycle.
b. Select an Appropriate FET
Determine the FET Q5 breakdown voltage V
Breakdown
and reflected voltage V
Reflected
. The FET maximum
drain voltage V
Drain(max)
is calculated using Equation 1.
The ringing associated with the transformer leakage inductance usually does not have enough energy to
cause a destructive avalanche breakdown. Voltages closely approaching the FET breakdown voltage are
acceptable. Alternatively, V
Zener
should be much greater than V
Reflected
to rapidly discharge the energy stored
in the transformer leakage inductance.
The FET breakdown voltage is constrained by cost and performance. A compromise must be reached in
partitioning voltage between V
BST
, V
Zener
, and V
Margin
. A second compromise then determines how to divide
V
Zener
into V
Reflected
and a reasonable overshoot voltage portion, V
Overshoot
.
The losses caused by the leakage inductance are inversely proportional to V
Overshoot
, which is determined by
Equation 2.
V
Drain max  
V
BST max  
V
Zener max  
+ = [Eq. 1]
V
Overshoot
V
Zener
V
Reflecteed
– = [Eq. 2]
V
Margin
V
Zener
V
BST
V
Overshoot
V
Reflected
FET Breakdown
Voltage Rating
Clamp
Zener
Voltage
Boost Output
Voltage
Margin
Reflected
Voltage
Overshoot
Voltage
Overshoot is a brief condition above
V
Reflected
, required to quickly dissipate
the energy stored in the transformer
leakage inductance.
During this time, the primary current is
kept from transferring to the second-
ary, siphoning energy from the load to
the clamp zener (snubber).
Figure 3. FET Breakdown Voltage