Cirrus Logic AN379 - Manual (Page 18)

AN379
18 AN379REV2
Step 17) Boost Input Capacitor Selection
To sustain compatibility with a wide range of transformers paired with a leading-edge or trailing-edge dimmer,
the boost input capacitance should be minimized. Large input capacitance impacts the ability of the controller
to properly sustain the current required by a transformer paired with a dimmer and may cause oscillation.
Capacitors should not be connected to the AC line side of a bridge rectifier. Added AC line-side capacitance
alters the dimmer behavior in multi-lamp configurations and shifts the dimming curve. Excessive capacitance
C
rect
after the bridge generates current spikes that may introduce ringing. The ringing will cause a TRIAC to
prematurely open its switches.
A damping circuit is used in addition to capacitor C
rect
to attenuate ringing introduced by the presence of
leakage inductance of a magnetic or electronic transformer. This ringing can cause electronic transformers to
have potential problems in starting and sustaining switching. Therefore, an RC damping circuit is connected
in parallel to capacitor C
rect
to stabilize a transformer paired with a dimmer. Damping resistance R
damp
is
defined by setting damping factor d equal to 1 and using Equation 35:
Damping capacitance C
damp
is defined in Equation 36:
where
f
ring
= ringing frequency and equal to the cut-off frequency
Step 18) Clamp Circuit
The clamp circuit is managed by two functions in the Mode3 algorithm:
1. Keep the boost output voltage under control during startup; for example, the light is off or at minimum dim
and the phase angle suddenly jumps to full brightness.
2. Provide a mechanism for determining when the converter is producing enough power to turn the light back
on
The first function requires the clamp circuit to dissipate equal amounts of power being produced by the boost
stage; that is, the power dissipated in the clamp circuit, P
CLAMP
, is greater than or equal to target
power P
Target
. The true instantaneous input power is less than target power P
Target
; for example, due to current
ripple in the boost inductor. Clamp resistor R
CLAMP
must be designed to dissipate the power in the clamp circuit
and comply with the relation defined in Equation 37:
where
P
Target
= target power and designed to be approximately 3 to 3.5 times the output power P
OUT
V
CLAMP
= clamp voltage, which is defined using the clamp turn-on constant K
CLAMP(on)
The second function requires that the clamp duty cycle that triggers the second stage to turn on is sufficient to
dissipate as much power in the clamp circuit as the minimum load. If this criterion is not met, oscillation may
occur; for example, as the light turns on, the control loop is unable to maintain the boost output voltage, the
light turns off, and the boost output power is diverted into the clamp circuit with a sufficient duty cycle to turn
the light back on. The clamp duty cycle threshold is equal to 2.2% (that is, a 1.1ms pulse every 50ms). Clamp
R
damp
1
2d
-------
L
leak
C
rect
------------- 
1
2
---
L
leak
C
rect
-------------  = = [Eq. 35]
C
damp
1
2  R
damp
f
ring
 
-----------------------------------------------
2  L
leak
C
rect
 
2  R
damp

--------------------------------------------------
L
leak
C
rect

R
damp
------------------------------------- = = = [Eq. 36]
R
CLAMP
V
CLAMP
2
P
T et arg
---------------------
V
BST full  
K
CLAMP on  
  
2
P
T et arg
---------------------------------------------------------------------- =  [Eq. 37]