Meet Green Standards in 24VAC and 12VAC Lighting Systems: Replace Halogen Bulbs with LEDs Driven by High Power Factor, High Efficiency Converter

Meet Green Standards in 24VAC and 12VAC Lighting
Systems: Replace Halogen Bulbs with LEDs Driven by
High Power Factor, High Efficiency Converter
Keith Szolusha
LEDs are increasingly used in 24VAC and 12VAC lighting systems as a robust,
energy efficient and high performance alternative to halogen lamps. Power
converters that drive the LEDs should have a high power factor (above 90% in order
to meet generally accepted green standards), should be efficient, use a minimal
number of components and should run cool. They do not need isolation.
One solution that meets these requirements combines a rectifier bridge and a
current-controlled synchronous step-up/
step-down converter. Specifically, a synchronous 4-switch buck-boost converter
can be paired with a 4-switch ideal diode
rectifier bridge for high power LEDs; lower
power solutions can use a standard diode
bridge. Both solutions are shown here.
The LT3791 60V 4-switch synchronous
buck-boost controller IC can drive constant current (either DC or pulsating) into
PVIN
24VRMS
PULSATING 120Hz
M5
RIN
0.003Ω
M6
CIN
1µF
50V
51Ω
0.1µF
TG2
24VAC
60Hz
TG1
IN1
LT4320
IN2
1µF
50V
100k
D3
OUTN
BG2
470nF
1M
OUTP
IVINP
CTRL
IVINN
VIN
INTVCC
TG1
22.6k
M1
SWI
BG1
INTVCC
200k
LT3791
M2
IVINMON
L1 7.8µH
M4
1M
M3
44.2k
RSENSE
0.008Ω
SHORTLED
OPENLED
0.1µF
OPENLED
SNSN
IVINMON
PGND
ISMON
ISMON
BG2
CLKOUT
CLKOUT
SW2
OVLO
TG2
SYNC
FB
SGND
ISP
0.1µF
SS
RT
VC
CSS
22nF
CC
22nF
45.3k
500kHz
D1, D2: NXP BAT46WJ
D3: SMAJ60A
L1: WÜRTH 744325780 7.8µH 8A
M1, M2: RENESAS RJK0651DPB 60VDS
M3, M4: RENESAS RJK0451DPB 40VDS
M5–M8: VISHAY Si7414DN 60VDS
Figure 1. 24V AC to 60W LED driver (600W halogen equivalent) features high power factor and high efficiency
28 | October 2013 : LT Journal of Analog Innovation
RLED
0.022Ω
PULSATING
LEDs
120Hz
15V–25V
0A–4.4A
ISN
PWMOUT
VREF
COUT
4.7µF
50V
×4
SNSP
200k
SHORTLED
CVCC
4.7µF
10V
0.1µF
BST1
EN/UVLO
M8
D1 D2
PWM
BST2
37.4k
BG1
M7
a string of high power LEDs. It features
an output current feedback loop used to
drive constant current through a string of
LEDs, and a CTRL dimming input pin that
can be tied to the 120Hz half-sine wave
design ideas
This eco-friendly 60W LED lighting solution is roughly
equivalent to 600W of halogen lighting without using
lead, mercury, argon, xenon or krypton gases.
IAC
2A/DIV
PVIN
10V/DIV
VLED
5V/DIV
IL1
2A/DIV
VAC
20V/DIV
ILED
2A/DIV
ILED
2A/DIV
5ms/DIV
5ms/DIV
5ms/DIV
Figure 2. 60Hz 24VAC input waveforms
Figure 3. 120Hz pulsating LED driver waveforms
Figure 4. 120Hz pulsating PVIN
output of a rectifier bridge to create a high
power factor pulsating LED current output.
output. When currents reach 5A and
higher, the diodes in a standard rectifier
bridge dissipate significant power and heat
up. The LT4320 helps high power AC applications run efficient and cool by driving
low resistance external N-channel FETs.
98.1% POWER FACTOR
The LT4320 is an ideal diode rectifier
bridge that drives four MOSFETs in place
of four typical rectifier diodes for highest efficiency conversion of the 60Hz
24VAC input to 24VRMS 120Hz pulsating
Figure 1 shows an LED driver that operates
with 98.1% power factor directly from
24VAC. It can drive up to 25V of LEDs with
120Hz pulsating power with LED current
peaking at 4.4A. At 120Hz, the pulsing of
the light is not detectable by the human
Figure 5. Components remain cool in the high efficiency LED driver shown in Figure 1. Note that the The LT4320 ideal driver remains cool at full LED current. The LT3791
high power buck-boost converter and supporting components rise less than 24°C while delivering 60W of LED power. The four ideal diode bridge MOSFETs on the back
of the board (inset) temperature rise less than 13°C (23°C ambient).
October 2013 : LT Journal of Analog Innovation | 29
PVIN
24VRMS
PULSATING 120Hz
D3
D4
D5
CIN
1µF
50V
51Ω
0.1µF
1µF
50V
D7
24VAC
60Hz
RIN
0.003Ω
470nF
1M
D6
100k
IVINP
CTRL
IVINN
VIN
INTVCC
D1 D2
PWM
BST2
37.4k
TG1
22.6k
M1
SWI
BG1
INTVCC
200k
LT3791
M2
IVINMON
L1 15µH
M4
1M
M3
44.2k
RSENSE
0.015Ω
SHORTLED
OPENLED
0.1µF
SNSP
200k
SHORTLED
COUT
4.7µF
50V
×4
0.1µF
BST1
EN/UVLO
CVCC
4.7µF
10V
OPENLED
SNSN
IVINMON
PGND
ISMON
ISMON
BG2
CLKOUT
CLKOUT
SW2
OVLO
TG2
SYNC
FB
SGND
ISP
RLED
0.05Ω
PULSATING
LEDs
120Hz
15V–25V
0A–2A
ISN
0.1µF
PWMOUT
VREF
SS
RT
VC
CSS
22nF
CC
22nF
45.3k
500kHz
D1, D2: NXP BAT46WJ
D3–D6: PDS360
D7: SMAJ60A
L1: WÜRTH 744071150 15µH
M1, M2: RENESAS RJK0651DPB 60VDS
M3, M4: RENESAS RJK0451DPB 40VDS
Figure 6. Alternate, 24W solution uses a standard diode rectifier for simplicity
eye and is seen as constant brightness.
The high power factor 24VAC input voltage and current waveforms are shown in
Figure 2. The 120Hz pulsating LED current waveforms are shown in Figure 3.
start-up is not harsh and inrush currents
do not affect the high power factor.
LED current foldback with the CTRL pin
voltage is used to achieve the high power
factor. The maximum LED current is set
by RLED at 4.5A, but the CTRL pin monitors
the post-rectifier 120Hz PVIN input voltage
(see Figure 4) and shapes the LED current waveform to match the input. When
the input drops below the shutdown pin
threshold, the IC goes into shutdown and
switching stops. The LED current trails off
as the output capacitors are discharged
and soon enough, the input rises above the
shutdown pin threshold and the LT3791
starts back up. With the CTRL pin folding back the LED current at low input,
The 24VAC pulsating LED driver converter in Figure 1 delivers approximately
60W of LED lighting at 94% efficiency.
This eco-friendly solution is roughly
equivalent to 600W of halogen lighting
replacement without using lead, mercury, argon, xenon or krypton gases. The
four synchronous switches of the LT3791
buck-boost converter and those of the
LT4320 ideal diode bridge are responsible
for the high efficiency. Figure 5 shows
the circuit components remaining cool
despite the 60W conversion. The components have less than 24°C temperature rise,
showing that there is plenty of room to
spare for even higher power applications.
30 | October 2013 : LT Journal of Analog Innovation
HIGH EFFICIENCY & HIGH POWER
FACTOR 60W PULSATING LED
DRIVER
A standard rectifier bridge would produce about a 50°C temperature rise and
run several efficiency points lower.
Total efficiency is calculated by measuring the input power, the power factor, and the delivered output power
separately. The values of 63.0W real input
power, 64.4W apparent input power
and 98.1% power factor are measured
with an HP 6812A AC power source.
Measurement of the output power is a
bit more complex. A current probe and
oscilloscope are used to capture the pulsing current and voltage waveforms at the
output of the converter. From these waveforms, the converter output RMS current
and voltage is calculated for the on-time
(tON) of the LED. The on-time output power
is POUT(ON) = VRMS(ON) • IRMS(ON). Output
power is zero during LED off-time, where
design ideas
The principals of the 24W circuit are the same as the 60W circuit
and the two operate in the same manner. Efficiency of the 24W
circuit is 90%, lower than the 94% achieved by the 60W circuit.
Nevertheless, this loss is acceptable due to the overall lower power.
Figure 7. Thermal performance of 24W solution
the current is zero. The output power of
60W is calculated via a simple duty cycle
equation: POUT = POUT(ON) • tON • 120Hz.
Overall efficiency = output power divided
by real input power.
HIGH EFFICIENCY & HIGH POWER
FACTOR 24W PULSATING
LED DRIVER
The circuit in Figure 6 is a high efficiency
and high power factor 24W pulsating
LED driver that operates from 24VAC input.
Because the power level here is less than
half of the 60W LED driver in Figure 1,
the rectifier bridge shown in Figure 8 is
made from four discrete Schottky diodes,
instead of ideal diodes. The trade-offs
for simplicity are slightly lower efficiency and additional heat dissipation.
The principals of the 24W circuit are the
same as the 60W circuit and the two operate in the same manner. Efficiency of the
24W circuit is 90%, lower than the 94%
achieved by the 60W circuit. Nevertheless,
this loss is acceptable due to the overall
lower power, making the temperature
rise in the discrete rectifier bridge components comparable between the two.
With the discrete diode rectifier bridge,
the components only heat up to 49°C as
shown in Figure 7, well within the requirements of most high power LED drivers.
CONCLUSION
The LT4320 and LT3791 synchronous
buck-boost pulsating LED driver combine to deliver 60W of LED power at
120Hz with 98.1% power factor and 94%
efficiency. This circuit can be used to
easily replace high power 24VAC halogen lighting with more robust and ecofriendly LEDs. At lower power levels,
the LT3791 can be used with a simple
discrete diode rectifier bridge—such as
in a 24W LED driver with 90% efficiency
and similarly high power factor. n
For higher efficiency, simply replace
the discrete rectifier with a LT4320based rectifier. In general, as power
levels and temperatures rise, the need
for synchronous rectification in both
the converter and rectifier goes up.
October 2013 : LT Journal of Analog Innovation | 31