Cirrus Logic AN375 - Manual (Page 9)

AN375
AN375REV4 9
When operating in Dimmer Mode, the line current must be at least as high as dimmer hold current I
hold
. The
dimmer hold current is a system parameter. For compatibility with the widest range of dimmers, a value of
60mA is typical for 120V applications, and 40mA is typical for 230V applications. The constant current value
used in the design is determined using Equation 4:
When operating in No-dimmer Mode, the line current is nearly sinusoidal, and its peak current value will never
exceed current I
IN(CC)
, given by Equation 4.
Step 2) Select an Appropriate FET
A transformer snubber circuit can take the form of a zener diode in series with a fast rectifier diode or an RC
network in series with a fast rectifier diode. The snubber circuit across the flyback transformer primary must
absorb the energy stored in the primary leakage inductance. Without a snubber, the current in the uncoupled
leakage inductance cannot transfer to the secondary winding which causes the power FET drain voltage V
Drain
to rise above the combined voltage (V
rect
+ V
Reflected
), risking the chance of exceeding the FET breakdown
voltage. For certain designs it is acceptable to operate the power FET in avalanche mode, provided the time
duration and peak current through the power FET during avalanche are controllable and verified to be within
the power FET ratings.
The peak current I
PK
at the end of the FET ‘ON’ time T1 varies with the phase across the line voltage.
Therefore the energy E
leakage
produced by the leakage inductance L
leakage
varies cycle by cycle and must be
dissipated. See Equation 5:
The zener diode snubber circuit defines a constant zener clamping voltage. The power FET drain voltage is
controlled to a voltage level of (V
rect
+ V
zener
). A zener diode based snubber is straightforward to design,
resulting in a predictable circuit behavior against all the tolerances involved as long as the zener diode is
capable of handling the maximum dissipation.
When selecting a power FET using a zener diode snubber circuit, determine the breakdown voltage V
Breakdown
of the power FET and reflected voltage V
Reflected
. The maximum drain voltage V
Drain(max)
of the power FET is
calculated using Equation 6:
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.
I
IN CC  
I
hold
= if I
hold
I
fb

[Eq. 4] or
I
IN CC  
I
fb
= if I
hold
I
fb

E
leakage
L
leakage
I
PK
2 
2
--------------------------------------- = [Eq. 5]
V
Drain max  
V
INPK max  
V
Zener max  
+ = [Eq. 6]