IR2161(
S
) & (PbF)
8 www.irf.com
Halogen Convertor Controller
Functional Description
Under-voltage Lock-Out Mode (UVLO)
The under-voltage lockout mode (UVLO) is defined as the
state the IC is in when VCC is below the turn-on threshold.
To identify the different modes of operation, refer to the
State Diagram shown on page 7 of this data sheet. The
IR2161 under voltage lock-out is designed to maintain an
ultra low supply current of less than 300uA, and to guaran-
tee the IC is fully functional before the high and low side
output drivers are activated. Figure 1 shows a simple VCC
supply arrangement that will work effectively, also when
the convertor is being dimmed from a conventional triac
based wall dimmer
Figure 1, Halogen Convertor.
The start-up capacitor (C
VCC
) is charged by current through
supply resistor (RS) minus the start-up current drawn by
the IC. This resistor is chosen to provide sufficient current
to supply the IR2161 from the DC bus. In a Halogen conver-
tor it is important to consider that the DC bus is completely
unsmoothed and has a full wave rectified shape. C
VCC
should
be large enough to hold the voltage at Vcc above the UVLO
threshold for one half cycle of the line voltage as it will only
be charged at the peak. A charge pump consisting of two
diodes (DCP1 and DCP2) connected to CSNUB is recom-
mended to supply VCC as this allows RS to be a large value
since it is only needed at startup. IF RS is required to supply
the circuit without a charge pump it needs to be a relatively
low value and consequently dissipates 1 to 2W, which is
undesirable.
An external 16V zener diode DZ has been added to avoid
the need for the internal zener to dissipate power (it should
be rated at 1.3W). The resistor RD in series with CD is
necessary if the convertor is required to operate from a
triac based (leading edge) phase cut dimmer. When the
triac fires at a point during the mains half-cycle the high dv/
dt allows a large current to flow through this path to instantly
charge C
VCC
to the maximum Vcc voltage.
The external zener (DZ) will prevent possible damage to
the IC by shunting excess current to COM.
Once the capacitor voltage on VCC reaches the start-up
threshold the IC turns on and HO and LO begin to
oscillate.
The supply resistor (RS) and RD/CD must be selected such
that enough supply current is available over all ballast
operating conditions. A bootstrap diode (DB) and supply
capacitor (CB) comprise the supply voltage for the high
side driver circuitry. To guarantee that the high-side supply
is charged up before the first pulse on pin HO, the first
pulse from the output drivers comes from the LO pin. During
under voltage lock-out mode, the high and low-side driver
outputs HO and LO are both low.
Soft Start Mode
The soft start mode is defined as the state the IC is in at
system switch on when the lamp filament is cold. As with
any type of filament lamp, the Dichroic Halogen lamp has a
positive temperature coefficient of resistance such that the
cold resistance (at switch on when the lamp has been off
long enough to cool) is much lower than the hot resistance
when the lamp is running. This normally results in a high
inrush current occurring at switch on. Under worst-case
conditions this could potentially trigger the convertors shut
down circuit. To overcome this problem the IR2161
incorporates the soft start function.
When the IC starts oscillating the frequency is initially very
high (about 130kHz). This causes the output voltage of the
convertor to be lower since the HF transformer in the system
has a fixed primary leakage inductance that will present a
higher impedance at higher frequency and thus allowing
less AC voltage to appear across the primary. The reduced
output voltage will naturally result in a reduced current in
the lamp which eases the inrush current thus avoiding
tripping of the shutdown circuit and will ease the stress on
the lamp filament as well as reducing the current in the half
bridge MOSFETs (M1 and M2).
The frequency sweeps down gradually from 130kHz to the
OUTPUT
1
2
3
4
IR2161
VCC
COM
CS LO
VS
HO
VB
8
7
6
5
CSD
M1
M2
RS
RD
CD
BR
LF
DZ
DS
DB
CSD
RCS
CB
RL
CVCC
CF
CCS
