IRPLHALO1E revB
International Rectifier
•
233 Kansas Street, El Segundo, CA 90245
USA
100VA dimmable electronic convertor for low voltage lighting
By Peter Green
TOPICS COVERED
Introduction
Functional Description
Supplying Vcc to the IR2161
Soft Start Operation
Run Mode
Shut Down Operation
Calculating RCS
Output Transformer Selection
Short Circuit Current
Adaptive Dead Time
Dimming
EMC Issues
Layout Issues
Component Selection
Bill of Materials
Introduction
Until now, almost all electronic convertors (often called electronic transformers) for Low Voltage
filament lamp applications have up to now been based around self-resonating bipolar transistor
half-bridge circuits. A new approach, using power MOSFETs driven by a control IC incorporating
additional functionality tailored to this application, is realized in the IR2161 and demonstrated in a
typical application by the IRPLHALO1E reference design kit.
The IR2161 is a dedicated intelligent half bridge driver IC for a Halogen convertor (1) or “Electronic
Transformer” targeted at medium and high end performance low voltage lighting applications.
The goal has been to incorporate as much functionality as possible within a low cost 8 pin DIP or
SOIC package to allow reliability and performance advantages over existing circuit designs with a
lower component count.
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IRPLHALO1E
Electronic transformers are commonly used in place of wire wound step down transformers in order to
provide the correct supply for low voltage (generally 12V) filament lamps such as Dichroic Halogen that
are widely used.
The advantages are:
1) Small size and weight
2) Fault protection circuitry
3) Safety because of low output voltage
(1) Throughout this data sheet “convertor” is spelled in accordance with standard IEC 61047 “DC or
AC supplied convertors for filament lamps – Performance requirements”.
Electronic transformers have become popular for low voltage lighting applications. The range of
products available ranges from very small 50W units, capable of driving only a single 50W lamp to
300W units capable of driving up to 6 X 50W lamps. In many applications the lamps are attached to
a track system in which the supply rails are exposed. Since the voltage is only approximately 12V
this does not present any safety problem.
The electronic transformer is generally smaller and lighter than a wire wound equivalent and may
be equipped with short circuit and thermal protection, which are generally not included when a wire
wound transformer is used. Generally a well-designed electronic transformer may be dimmed with
a triac based leading edge phase cut dimmer (as can a wire wound transformer) or by a trailing
edge transistor dimmer.
Functional description of the IR2161 based Halogen convertor
RD
RS
CD
DS
D1
LF
CVCC1
220VAC
INPUT
D2
CVCC
2
DCP2
DCP1
DBOOT
Q1
VCC
COM
CSD
DZ
CS
R1
C1
1
8
VB
HO
VS
LO
RB1
CBOOT
RB2
CSNUB
IR2161
2
3
4
7
6
5
CLF
Q2
C3
C4
D3
D4
RF
CSD
CF
RCS
12VAC
OUTPUT
R2
C2
Fig. 1 IRPLHALO1E Circuit Schematic
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IRPLHALO1E
The IR2161 provides low and high side output drives for the half-bridge MOSFETs or IGBTs. The
output from the half bridge is connected to a high frequency stepdown transformer, which supplies
approximately 12Vrms at the output to drive the lamps. The IR2161 also incorporates all of the
protection features needed in the system without the need for very many external components.
At switch on the frequency sweeps from a high frequency around 125kHz down to the normal
operating frequency 30 to 40kHz over a period of approximately 1 second. Because of the leakage
inductance in the transformer this causes the output voltage at the lamp to start at a reduced value
and gradually increase to the 12V nominal level. This reduces the inrush current at switch on. When
the lamp is cold the filament resistance is lower which tends to cause high inrush currents that can
cause false tripping of the shutdown circuit.
The IC includes a short circuit protection that operates if a high current is detected for approximately
50mS causing the outputs to switch off. Similarly if an >50% overload is detected for more than
approximately 500mS then the outputs will switch off. It should be noted that the shutdown time
under an overload condition will be reduced depending on the magnitude of the overload.
This dual mode shutdown circuit will protect the circuit from all output fault conditions and will
function when the convertor is dimmed, unlike the self-oscillating solution and it will also protect the
system from a short circuit at the end of the output leads.
An electronic transformer is normally required to provide a reasonably consistent output voltage
over a range of loads and so the IC senses the load through the current sense resistor and in-
creases the frequency as the load is reduced thus providing compensation for the output trans-
former load regulation. There is also some modulation of the frequency through the line voltage
half-cycle to spread the harmonics and reduce the size and cost of the EMC filtering components
required.
The IR2161 includes all necessary protection features and also allows the convertor to be dimmed
externally with a standard phase cut dimmer for leading or trailing edge. This IC provides the advan-
tage of longer lamp life due to soft start and output voltage shift (load regulation) compensation. It
causes the convertor to run with optimized harmonic behavior (i.e. almost unity power factor) also
at higher loads (solution with bipolar devices can have harmonics problems for high loads due to
the fact that the oscillator start-stop every half cycle, causing cross over distortion).
The IC also includes adaptive dead time to realize soft-switching and allow cool running MOSFETs
(and improves the EMI behavior due to frequency modulation during the line voltage half cycle).
Some points to consider when comparing a Halogen convertor circuit design with an electronic ballast
for fluorescent lamps:
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IRPLHALO1E
• Filament Lamp is a
Resistive
Load
• No Preheat / Ignition is required
• DC Bus is full wave rectified line without smoothing
• Close to
Unity Power Factor
is inherent in system
• Can be dimmed with
Triac
(standard domestic type) dimmer
• Dimming is achieved by
PHASE CUTTING
of the AC line
• Output is isolated
Low Voltage
• Protection required against output short circuit or overload not open circuit.
• Shutdown must be
Auto-Resetting
The protection circuitry is auto-resetting so that if the output is short circuited the system will
periodically try to restart and then shut down again. When the short-circuit is removed the lamp will
be able to come back on again without the need for cycling the AC supply off and back on again to
reset. The external capacitor at the CSD pin is used in several different operating modes allowing
the IC to be limited to 8 pins. The CSD pin is connected to different circuit blocks during different
modes, controlled by internal control logic.
The current sense (CS) input is connected to the voltage compensation and shutdown circuit blocks.
The CSD capacitor is switched between different circuits through internal transmission gates. The
oscillator is voltage controlled and its input is connected to the CSD capacitor during normal opera-
tion. In case of overtemperature or external shutdown, the IC will go into a fault mode. In this mode
the IC is in latched shutdown and will restart only after resetting the mains. In case of overvoltage or
overload, the IC will go into a auto-resetting fault.
The different modes of operation of the IC and the system implications are described here:
(See also the IR2161 data sheet)
Supplying VCC to the IR2161
The under-voltage lockout mode (UVLO) is defined as the state the IC is in when VCC is below the
turn-on threshold of the IC. To identify the different modes of the IC, refer to the State Diagram
shown in the IR2161 data sheet. The IR2161 under voltage lock-out is designed to maintain an ultra
low supply current under 300uA and to guarantee the IC is fully functional before the high and low
side output drivers are activated.
The capacitor (CVCC) is charged by current through supply resistor (RS) minus the start-up current
drawn by the IC. This resistor charges CVCC to the UVLO+ threshold, at which point the IR2161
starts to operate and the LO and HO outputs become active. In a Halogen convertor it is important
to consider that the DC bus is completely unsmoothed and has a full wave rectified shape. CVCC
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 CSNUB con-
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IRPLHALO1E
nected to two diodes, DCP1 and DCP2, feeds current back from the half bridge (VS) to VCC and
sustains the supply to the IC after it has been initially started. This arrangement avoids the neces-
sity for RS to supply all the current to VCC, which would require RS to dissipate around 2W in a
220VAC supplied system. This is undesirable as 2W power dissipated as heat within the convertor
adds significantly to the temperature rise. When selecting the value of RS, the trade-off is between
power dissipation and the ability of the convertor to start up when connected to a dimmer that is set
to a very low level. When a dimmer is set close to minimum and the supply to the convertor is
switched on, only a small voltage will appear on the DC bus and so RS must be small enough for
this to provide sufficient starting current for VCC. However the smaller the value of RS is, the more
power it will dissipate. It should be noted that the above considerations are important in a 220VAC
supplied system, but in a 120VAC supplied system a charge pump would not be necessary and RS
could supply VCC without dissipating significant power. Other considerations such as the high
primary current become more problematic in a 120VAC system.
An external 16V zener diode DZ has been added to avoid the need for the internal zener to dissi-
pate 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 rapidly charge CVCC to the maximum VCC voltage. In this way each line half cycle, the
system will receive a fast pull up on VCC when the traic in the dimmer is fired.
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. 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 Operation
The soft start mode is defined as the state the IC is in at switch on of the system when the lamp
filament is cold. As with any type of filament lamp the Dichroic Halogen lamp has a positive tem-
perature 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 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 125kHz). 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 allowing less
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