IR2161STRPBF Datasheet, PDF (15/20 Pages) International Rectifier – HALOGEN CONVERTOR CONTROL IC

DC BUS VOLTAGE
LAMP VOLTAGE
IR2161(S) & (PbF)
DC BUS VOLTAGE
LAMP VOLTAGE
Figure 13, Trailing Edge Dimming
Trailing edge dimmers are less common however than leading
edge dimmers. This is because they are more expensive to
make and need to incorporate a pair of MOSFETs or IGBTs
whereas a leading edge dimmer is based around a single
triac.
Conversely many Halogen convertors are not able to oper-
ate with leading edge dimmers because of the fact that
they are based around a triac. It is possible, however, to
design a Halogen convertor that will work effectively with
a triac based dimmer by designing the input filter compo-
nents correctly ensuring that at the firing point of the triac
the oscillator can start up rapidly. In the IR2161 based sys-
tem this is easy to achieve through the addition of RD and
CD, which conduct a large current to VCC due to the high
dv/dt that occurs when the triac fires. At the same time, the
bus voltage rises rapidly from zero to the AC line voltage. If
the VCC voltage falls below V_CCUV- during the time when
the triac in the dimmer is off, the soft start will not be initi-
ated because the soft start circuit is not reset until VCC
drops approxmately 2V below V_CCUV-. This takes some
time as the VCC capacitor discharges very slowly during
UVLO micro-power operation. The intermediate period is
referred to as Standby mode.
During dimming the voltage compensation circuit will cause
a frequency shift upward at angles above 90° because the
peak voltage at CS will be reduced (see figure 14). This will
result in a reduction of voltage at CSD and thus an increase
in frequency. However this will not have a noticeable effect
on the light output.
The problem associated with operation of Halogen conver-
tors with triac dimmers is due to the fact that after a triac
has been fired it will conduct until the current falls below its
Figure 14, Leading Edge Dimming
holding current. If the load is purely resistive (as in a fila-
ment lamp directly connected to the dimmer) this will natu-
rally happen at the end of the line voltage half cycle as the
current has to fall to zero. In a Halogen convertor it is nec-
essary to place a capacitor and inductor at the AC input to
comply with regulations regarding EMI conducted emis-
sions. This means that when the line voltage falls to zero
there could still be some current flowing that is enough to
keep the triac switched on and so the next cycle will follow
through and not be phase cut as required. This can happen
intermittently resulting in flickering of the lamps. The way to
avoid the problem is to ensure that the product has the
smallest possible filter capacitor CCS and to state a mini-
mum load for the convertor. This would be typically one
third of the maximum load to avoid problems of this kind.
Figure 15, Half Bridge voltage and current during dimming
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