Steve Sheard, On Semiconductor, Tempe, AZ; Edited by Martin Rowe and Fran Granville -August 11, 2011
LEDs are more efficient than
incandescent lights and can last
100 times longer, but they require specialized
electronic-drive circuits to avoid
overstress conditions. The main operating
parameter is relatively simple: Keep
the current through the LEDs constant
and under the specified maximum.
Traditional power supplies have
accurate voltage outputs with variations
in current. A resistor in series with an
LED string controls the current. Such a
design assumes a known voltage across
the LEDs that does not vary with changes
in LED temperature. Unfortunately,
LEDs’ forward voltage does change
with temperature. LED manufacturers
generally bin their devices by forward
voltage, allowing a lighting manufacturer
to build a lighting fixture to match
this forward voltage at a fixed temperature.
A circuit using unbinned LEDs
saves the LED manufacturer time and
results in less expensive LEDs. LEDs
also have a negative forward-voltage-to-temperature
coefficient that can cause
the drive circuit to go into thermal runaway,
requiring the designer to build
safeguards into the design.
The ideal approach for driving LEDs
is one in which the circuit monitors the
current and keeps it constant. LEDs’
forward voltage does not affect this
type of circuit, eliminating the need
for binning and the effect of the LEDs’
coefficient. These circuits can be
complex switching regulators or simple
linear regulators with feedback loops.
Complex switching regulators are ideal
for high-light-output applications, such
Simple, economical, and robust
hybrid circuits find use in architectural- and
interior-lighting fixtures. These circuits’
design may be less efficient than
that of a complex switching regulator,
but their low cost and simplicity make
them attractive. These circuits operate
over the full universal voltage specification
of 85 to 265V ac at 50 or 60 Hz.
The circuit in Figure 1 comprises a
bridge, a chopper, and a current regulator.
The full-wave bridge comprising
diodes D1, D2, D3, and D4, feeds into the
chopper circuit. MOSFET Q2 immediately
turns on, and capacitor C1 begins
form a voltage
divider. When the voltage on the cathode
reaches 43.5V, the zener diode
conducts and turns on Q1
, which pulls
the gate of Q2
low, causing it to turn off.
The voltage across C1
stays at 80 to
90V. The charge on C1
feeds the CCR
(constant-current regulator) and the
LED string. This circuit example has
22 LEDs. The CCR maintains
the current at 20 mA
through the LED string.
The circuit includes resistor
, in series with the LEDs,
for measuring the current
through the LED string.
Figure 2 shows the voltages
at different parts of the
cycle with an input voltage of
150V ac. Trace 1 is the output
of the bridge-rectifier circuit.
Trace 2 is the voltage across
C1, the output of the chopper
circuit. Trace 3 is the voltage
across the current-sense resistor.
The traces clearly show
that, when the voltage from the bridge increases to more than 80V,
the chopper circuit switches and limits
the voltage applied to the regulator circuit.
Figure 3 shows the voltages with an
input voltage of 85V ac.
The oscilloscope traces show that
there is still sufficient design head room,
staying on for a longer period,
during which C1
fully charges. The input
voltage drops to 54V ac before the current
through the LEDs begins to drop.
Figure 4 shows the circuit operation
at an input voltage of 265V ac. Trace
1 shows that, because of its high input
voltage, Q1 is on for a short time. Trace
2, however, shows that sufficient energy
still remains to charge Q1 and maintain
the current through the LEDs during
the off cycle.
You can scale this circuit to operate
with different LED arrays. CCRs are
available with current ratings as high
as 160 mA. For higher currents, you can
place the CCRs in parallel. The values
, and R2
match the type and
number of LEDs.