Driver circuit lights architectural and interior LEDs
Hold the current constant and within tolerance.
Steve Sheard, On Semiconductor, Tempe, AZ; Edited by Martin Rowe and Fran Granville -- EDN, 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’
negative forward-voltage-to-temperature
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
as streetlights.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
to charge.
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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,
with Q1 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 of C1, R1, and R2 match the type and number of LEDs.
Talkback
-
1. Let assume the voltage across C1 is equal to 80V, an input AC voltage 85V, voltage across 22 LED is equal 66V, LED current is 20mA, so, without MOSFET and bridge rectifier losses it is easy to calculate that efficiency will be less than 76%.
2. With AC voltage increase to 230V, the efficiency will drop to 51%.
3. CCR and sense resistor R4 (not clear what is its function) dissipate almost 0.28W compare with LED 1.32W power dissipation. It is almost 21% additional losses compare with previously published design.
Vladimir Doubovis - 2011-7-9 15:26:54 PDT -
1.) Design is a very similar to previously published by TA Babu (EDN, April 21, 2011) with such difference as added current limited diode and missed a resistor between base and emitter of BJT transistor. This resistor shall be required to prevent open base conditions.
a. Yes, this is very similar to TA Badu’s “Offline supply drives LEDs” (EDN April 21, 2011). Once the 39V zener conducts, the 39K + 10K resistors dissipate more power than the 330K + 390K resistors, thus making this application dissipate less power. This design incorporates a CCR to control the current through the LED thus providing a higher level of protection.
b. A 10K Base-Emitter Resistor can be used to stabilize the operation of the BJT, a good safeguard.
2.) How reliable is this circuit? How long will the 22 MFD capacitor last, supplying the LED power like that? And how much power is wasted in the constant current regulator circuit?
The reason that most designers use a switching supply of some sort is efficiency, which switchers are more efficient. Presently the reliability problems come mostly from under-rated parts and moisture entry. So using a better quality of adequately rated parts will be much better in the long term.
a. The weak link in this circuit is the 22uF electrolytic capacitor. Electrolytic Capacitors usually last 10K Hours at 108C. This is very much an end product design requirement. Since none of the components in this circuit reach above 40C thermal management is not an issue.
b. The Constant current regulator is run at only a few volts over the minimum overhead voltage (1.8V) so the CCR’s power dissipation is pre-determined by the chop voltage. One of the largest issues with LED lighting is electrical over stress. LEDs, like a diode, have an exponential current-voltage relationship. So even a small voltage ripple will cause a large change in LED current reducing the lifetime and reliability of the LEDs. The CCR completely removes this ripple by maintaining the current constant.
c. This design is optimized for efficiency with lowest cost. ON Semiconductor has a full portfolio switching supplies designed specifically for very high efficiency.
3.) I like the simplicity, but how efficient is this circuit? Could you measure the voltage across the string and list the input current at 115V and 230V?
a. Thank you. This circuit can be upwards of 90% efficient. the Resistor divider will dissipate between 0.02 W and 0.195 W, R3 can dissipate between 0.25 W to 2.5 W but the value can be increased and the power can be decreased to below 0.2 W while the output is 1.7W. For this particular configuration the efficiency can be 88 %. Due to the Constant current regulator, it keeps the current through the LEDs the same and the other sources of current are almost negligible.
Steve Sheard - 2011-23-8 15:20:37 PDT -
Design is a very similar to previously published by TA Babu (EDN, April 21, 2011) with such difference as added current limited diode and missed a resistor between base and emitter of BJT transistor.
This resistor shall be required to prevent open base conditions.
Vladimir Doubovis - 2011-21-8 13:42:27 PDT -
How reliable is this circuit? How long will the 22 MFD capacitor last, supplying the LED power like that? And how much power is wasted in the constant current regulator circuit? The reason that most designers use a switching supply of some sort is efficiency, which switchers are more efficient. Presently the reliability problems come mostly from under-rated parts and moisture entry. So using a better quality of adequately rated parts will be much better in the long term.
William Ketel - 2011-16-8 12:16:46 PDT -
I like the simplicity, but how efficient is this circuit? Could you measure the voltage across the string and list the input current at 115V and 230V?
Thanks!
Brian German - 2011-16-8 12:15:38 PDT
