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Copyright © 2009 Luger Research & LED-professional. All rights reserved.
Driver
Off-Line LED Control Circuit
> Tom Ribarich, Director of Lighting Systems and Applications,
International Rectifier
Resonant mode topologies offer many benefits over traditional buck,
boost and flyback solutions. These include soft-switching, higher
operating frequencies, higher power density and higher efficiency.
Electronic ballast designs for fluorescent lighting applications have
already been taking advantage of these benefits for many decades.
Much can be learned from electronic ballast circuits and applied to LED
driver circuits. This article compares the load requirements for
fluorescent lamps and LEDs, explains the functionality of a new dimming
electronic ballast control IC, and describes a new resonant mode control
circuit for LEDs that uses the new IC. Experimental results of the new
circuit are also presented and summarized to show final performance.
Fluorescent vs. LEDs
Fluorescence is the conversion of UV light to visible light. Electrons flow
through the fluorescent lamp and collide with mercury atoms causing
photons of UV light to be released. The UV light is then converted into
visible light as it passes through the phosphor coating on the inside of
the glass tube wall. This two-stage conversion process results in about
25% of the total energy consumed by the lamp being used to generate
light. A typical fluorescent lamp also has a low lamp running temperature
(40degC) and a lifetime of about 10,000 hours. To control a fluorescent
lamp, the lamp requires a voltage or current to preheat the filaments, a
high-voltage for ignition, and a high-frequency AC current during
running.
LEDs work on a completely different principle than fluorescent lamps.
Individual electrons jump across a p-n junction (from the n-type region
to the p-type) of a semiconductor material. The ‘band-gap’ in certain
semiconductors such as gallium is very wide and requires appreciable
energy to make electrons jump across the junction. When each electron
recombines with an atom, it emits a particle of light known as a photon.
Because all of the light is being produced in a very small space down at
the junction, the resulting light source is a point source and requires
many LEDs to light a large area. Also, the heat inside an LED cannot be
thermally dissipated by the LED itself resulting in high LED working
temperatures and therefore requires heatsinking.
LEDs are much simpler to control but still have their own set of
requirements and challenges. They do not need to be ignited or
preheated but the current should be constant and matched in each LED.
Also, depending on the application, the electrical connection to the
LEDs may or may not need to be galvanically isolated. The circuit
requirements for fluorescent and LED have been summarized for
comparison (Table 1).
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Load Requirement
Fluorescent
LED
Filament Preheating
Current or Voltage
Control
1kV
No
Ignition Voltage
Running Control
No
High-Frequency AC
Current
Current Crest Factor 1.7
(peak/r.m.s.)
Dimming Control
Current Control
PWM On/Off
Isolation
Yes/No
No
Constant Current
1.42*
Table 1: Load requirement summary for fluorescent and LED
IRS2530D “DIM8TM” Control IC
Existing ballast non-dimming circuits include (Figure 1) an input filter
for blocking ballast generated noise, a rectifier and smoothing capacitor
for converting the AC line input into a DC bus voltage, a control IC and
half-bridge for producing a high-frequency square-wave voltage, and a
resonant output stage for preheating, igniting and running the
fluorescent lamp. The additional circuitry needed for dimming includes
(Figure 1) an isolated 0-to-10VDC dimming interface, a current-sensing
circuit to measure the lamp current, and a closed-loop feedback circuit
to keep the lamp current regulated to the user setting by continuously
adjusting the output frequency. A closed-loop system is needed to
regulate the lamp current due to the non-linear electrical characteristics
of the fluorescent lamp.
Figure 1: Fluorescent dimming ballast block diagram
The IRS2530D (Figure 2) is a 600V, 8-pin fluorescent dimming control
IC that provides the high- and low-side gate drive for the half-bridge,
includes all of the dimming ballast functions, and protects the circuit
against line and load fault conditions. The IC already uses 6 pins for very
basic but necessary functions: IC supply and ground (VCC, COM), and,
half-bridge high- and low-side gate drive (VB, HO, VS, LO). The challenge
is then to realize the other functions -- preheat, ignition and dimming
- with only two remaining pins (VCO, DIM).
Annotation:
*Philips LemiLeds 700mA Luxeon V Emitter, green/cyan/blue/royal blue, Absolute Maximum Ratings, (peak
pulsed forward current)/(average forward current).
IRS2530D or reference design kits: www.irf.com.
LED professional Review | Jan/Feb 2009 | page
21
Copyright © 2009 Luger Research & LED-professional. All rights reserved.
Figure 2: IRS2530D pin assignments and functions
When a voltage is first applied to VCC (14V, typical) the IC exits UVLO
mode and enters Preheat/Ignition mode. The half-bridge begins
oscillating at the maximum frequency and the internal current source
at the VCO pin begins charging up an external capacitor (CVCO)
linearly from COM (Figure 3). The output frequency decreases as the
VCO voltage increases and the lamp filaments are preheated by
secondary windings from the resonant tank inductor. As the VCO
voltage charges up, the frequency decreases towards the resonance
frequency of the resonant tank circuit and the output voltage across
the lamp increases. The lamp ignites when the output voltage exceeds
the lamp ignition threshold voltage, lamp current begins to flow, and
the IC enters Dim mode.
Figure 3: Preheat, ignition and dimming timing diagram
During Dim mode, a current sense resistor (RCS) is used to measure the
AC lamp current. This AC measurement is then coupled to the DC
reference at the DIM pin through a feedback capacitor (C2). The AC +
DC signal at the DIM pin is then compared to COM internally to the IC
and the frequency is controlled such that the valleys of the AC
component are held at COM continuously (Figure 4). As the DC reference
is increased or decreased while the AC valleys are held at COM, the AC
lamp current amplitude will then increase or decrease as well. By
combining the DC reference with the AC lamp current, a single pin can
then be used for both reference and feedback functions to achieve
closed-loop dimming control.
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Figure 4: AC + DC dimming control method
New LED Control Circuit
Typical LED control circuits are designed around a buck, boost or flyback
topology, and they are used to generate a constant DC current through
a string of a given number of LEDs. Each of these topologies has
advantages and disadvantages depending on the input voltage range,
the number of LEDs being driven in series, the number of parallel LED
strings, the LED output current, if isolation is required, if dimming is
required, efficiency, size and cost. For this reason, many circuit variations
exist to satisfy the many different LED applications. The new circuit is a
resonant mode circuit that has been slightly modified from dimming
fluorescent applications. It is for non-isolated, off-line applications, and
can drive one or many LEDs in series, can be easily scaled for different
LED current levels, and utilizes soft-switching for good efficiency. The
new circuit (Figure 5) is designed around the existing IRS2530D Dimming
Control IC, and the output stage has been modified to drive LEDs instead
of a fluorescent lamp. It is no longer necessary to preheat and ignite the
load so the resonant tank has been changed to a series L-C-LED type
(instead of a series L, parallel R-C for fluorescent). Since the output
current is AC, a full-wave bridge rectifier has been added to the output
so that current is always flowing through the LEDs during each highfrequency switching cycle.
The AC current sensing is still performed using a resistor (RCS) that is
placed in between the bottom of the rectifier and COM, and gives a
direct AC measurement of the full-wave rectified LED current amplitude.
This AC measurement is then coupled onto the DIM pin through resistor
RFB and capacitor CFB. The dimming control loop of the IRS2530D then
keeps the amplitude of the LED current regulated by continuously
adjusting the frequency of the half-bridge switching circuit such that
the nominal r.m.s. LED current is maintained within the manufacturer’s
specifications. If the LED current decreases, then the loop decreases the
frequency. This will increase the gain of the resonant tank circuit and
increase the LED current. If the LED current increases, then the loop
increases the frequency. This will decrease the gain of the resonant tank
circuit and decrease the LED current. The dimming control loop keeps
the LED current constant over line, load and temperature variations, and
will work for a single LED or many LEDs in series.
LED professional Review | Jan/Feb 2009 | page
22
Copyright © 2009 Luger Research & LED-professional. All rights reserved.
Figure 5: New IRS2530D off-line LED control circuit
Experimental Results
The experimental results show the waveforms during normal start-up
and running conditions (Figures 6 and 7). When the AC line voltage is
first applied, VCC charges up and the IC turns on. The output frequency
starts at the maximum frequency of the IC and sweeps down towards
the resonant frequency of the series L-C-LED resonant circuit. The
frequency sweep is performed by the capacitor CVCO at the VCO pin.
The LED current (sensed through resistor RCS) increases as the frequency
decreases. This causes the amplitude of the AC signal at the DIM pin to
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also increase until the valley of the AC signal reaches COM (Figure 6).
The IC then enters Dim mode and enables the dimming loop. The
dimming loop continuously adjusts the output frequency to keep valley
of the AC signal at the DIM pin maintained at COM and therefore
maintains a constant LED current amplitude. The LED current (Figure 7)
is full-wave rectified and operates at twice the frequency of the halfbridge switching node (VS pin). The shape of the LED current waveform
is sinusoidal due to the resonant behavior of the circuit. This helps keep
the current crest factor low so that the nominal LED r.m.s. current is
achieved without excessive peak currents.
LED professional Review | Jan/Feb 2009 | page
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Copyright © 2009 Luger Research & LED-professional. All rights reserved.
The IRS2530D also includes additional circuitry for protection against
all line and load fault conditions. These include AC mains brown-out,
open circuit (no load or LED failure) and short circuit fault conditions.
Figure 7: LED current (upper, 1A/div), D4:A – D3:A bridge rectifier voltage (middle, 25V/div), and VS pin voltage
(lower, 100V/div) during running. Time scale = 5usec/div
Conclusion
Figure 6: LED current (upper, 1A/div), DIM pin voltage (middle, 1V/div), and VCO pin voltage (lower, 2V/div) during start-up. Time scale = 50msec/div
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The new off-line LED control circuit is simple and provides good constant
current regulation for the LEDs. It is easily scalable for different input
voltage ranges and LED current levels, and is flexible to the number of
LEDs connected at the output. The IRS2530D successfully drives circuit
for both fluorescent and LED applications. The IC integrates the complete
control in a low-cost, 8-pin solution, and the control loop delivers good
constant current performance over all line and load conditions, and the
IC detects all fault conditions and deactivates the circuit safely.
Additional circuit improvements to be considered include PWM on/off
dimming of the LEDs.
LED professional Review | Jan/Feb 2009 | page
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