„Diotec design idea“
Design Idea
Cost Effective Driving of Standard LEDs from 10VDC up to 110/230VAC with Current Limiting Diodes
Cost Effective Driving of Standard LEDs from 10VDC
up to 110/230VAC with Current Limiting Diodes
Standard LEDs are widely used for all kind of lighting purposes such as optical
indicators and signal lights, marking lights, display backlights, interior and furniture
lighting and much more. New approaches even use standard LED arrays to replace
fluorescent lights (neon tubes), especially in areas
where no high-level lighting is required, e. g. floor
lightings, wash and lunch room luminaire, outdoor
lights etc. Standard LEDs are robust, long-living and
© Mike Deal aka ZoneDancer
available in high volumes at low costs; their power
consumption is on a very low level below 100mW. The driving current of such
devices is typically 20mA1, resulting in a forward voltage drop between 2 and
4V. The following describes a cost effective solution to drive such LEDs by
means of the Current Limiting Diodes CL20M45 and CL40M45 offered by
Diotec.
Current Limiting Diodes (CLD)
Like a Zener diode keeps a voltage constant over a
wide range of Zener current, the Current Limiting
Diode or CLD keeps a current constant over a wide
range of forward voltage. The left figure shows the
typical IF vs VF curve of such CLD device, including
the symbol and current/voltage definitions. When
applying a positive voltage from anode to the
cathode (indicated by cathode mark), the current
rises until it reaches a constant value IP. The start of
this current limiting area is defined by the limiting
voltage VL, above which IL = 80% of IP is reached. IP
remains constant, unless a maximum admissible
voltage VAK is reached, above which a breakdown
happens and the device can be destroyed. In
reverse direction, the voltage VR is quite quickly
reached. So operating voltage range is between VL
and VAK, where current is kept to a constant value IP;
the reverse direction is normally not used.
Taking the function of the CLD into account, such device
can be used to drive LEDs with a constant current IP from a
variable voltage source VIN, see right figure. The only
condition is to limit the voltage across the CLD to a value
less than VAK; that voltage is the difference between V IN
and the voltage drop at the LED, VF-LED.
The CL20M45 is designed for an IP of 20mA, which is the
typical driving current for Standard LEDs; the VAK is 45V. So
in the easiest case, this CLD can be used to drive a single
LED from a voltage source ranging from about 10VDC up
to 45VDC. For higher driving currents, the CL40M45 offers an IP of 40mA. CLDs can be even operated in
parallel; of course then the power dissipation / power losses will increase as well.
1
One should not mix up Standard LEDs with Power LEDs, where driving current is in the range of 350 to 700mA
(thereabout refer to the Application Note “Diotec Products for Power LED Drivers”)
© Diotec Semiconductor AG
http://www.diotec.com/
2011-05-04
1/3
Cost Effective Driving of Standard LEDs from 10VDC up to 110/230VAC with Current Limiting Diodes
However, in most cases there is an AC voltage source, so additionally a (bridge) rectifier device is required.
If the circuit is connected directly to the 110V/230VAC mains, VIN can reach quite high levels up to 350V peak.
So existing designs require a lot of additional devices, in order to reduce the incoming voltage to an
acceptable level; they further require electrolytic capacitors to keep that voltage constant. As a result, such
circuits are complex and expensive, and their lifetime is limited mainly by the electrolytic capacitors used.
The following describes how by means of just a bridge rectifier, one or two CLDs, either an array of LEDs or
an AC capacitor, a circuit for direct operation at 110VAC or 230VAC mains can be build2.
Solution up to now: requiring 5 components, having limited lifetime
The right picture shows
how up to now LEDs are
driven at AC mains. The
Zener diode, electrolytic
capacitor
and
power
resistor is required to keep
the forward current IF
through the LED constant.
The electrolytic capacitor
has got the shortest lifetime of all components in
the circuit and is the
limiting element. During
turn-on, a current peak can occur at the bridge, caused during initial charge of the AC and electrolytic
capacitor. Therefore here the S250 by Diotec is used, featuring a very high forward surge rating of 40A (at
50Hz).
New Solution, Example 1: only 3 components and improved lifetime
The circuit on the left simply needs 3
components to drive one (or more)
Standard LEDs at a wide input range. The
AC capacitor C has got a certain dynamic
impedance XC = 1/(2πfC). Depending on the
output power (and related to that, the input
or mains current), there is a voltage drop
across this impedance. C has to be chosen
such way that the voltage drop is big
enough to ensure that VAK of the CLD is not
exceeded.
2
2/3
This design idea describes an application proposal and shall not considered as assured and proven characteristic of a circuit. No
warranty or guarantee, expressed or implied is made regarding the capacity, performance or suitability of any circuit etc, neither
does it convey any license under its patent rights of others.
2011-02-07
http://www.diotec.com/
© Diotec Semiconductor AG
Design Idea
Cost Effective Driving of Standard LEDs from 10VDC up to 110/230VAC with Current Limiting Diodes
New Solution, Example 2: only 2 components for driving LED arrays at 110VAC
Here not only one, but an
array of Standard LEDs is
used. The resulting voltage
drop across the series of
LEDs is big enough to ensure
VAK is not exceeded. Thus, by
only one rectifier bridge and
one single CLD a complete
LED luminaire, operated at
110VAC, can be done. Since
there is no inrush current,
the lower cost MS series of
bridge rectifiers can be used.
LED array driven at 110VAC by a
bridge rectifier and a CLD
New Solution, Example 3: only 3 components for driving LED arrays at 230VAC
Here the same circuit, but
operated at 230VAC mains.
Due to the higher peak
voltage, more LEDs have to
be connected in series, and
also two CLD are used.
Anyway, a single rectifier
bridge is enough to complete
the whole circuit!
In the above circuits, no electrolytic capacitors are needed nor other complex devices 3. One may add a
simple inductor (coil) in series to the LEDs, in order to reduce the (small) ripple in driving current (occurring
during the zero-crossing of mains voltage). Furthermore, a mains fuse can provide circuit protection in case
of any unforeseen shorts.
3
This design idea describes an application proposal and shall not considered as assured and proven characteristic of a circuit. No
warranty or guarantee, expressed or implied is made regarding the capacity, performance or suitability of any circuit etc, neither
does it convey any license under its patent rights of others.
© Diotec Semiconductor AG
http://www.diotec.com/
2011-05-04
3/3
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