Download Parallel LEDs evaluation

Evaluation of paralleled LEDs.
Date: Oct 2013
Intro.
This report lists concerns for certain non-robust and overly expensive LED product
designs which involve parallel LEDs.
The problem centres around the use of LEDs in parallel connection, and with no
circuitry present to equalise or limit the currents in each of the paralleled LEDs,
and little attempt being made to thermally couple the paralleled LEDs.
The problem that this can cause is that of “Thermal Runaway” (and subsequent
overheating) of individual LEDs which end up drawing more current than others. This
can lead to product failure in the field (i.e. with the customer) after only a short time
period.
Some problems due to “Thermal Runaway” in paralelled LEDs (without LED current
limiting resistors) can actually be avoided by using overly expensive LEDs, and
overly expensive, oversized heatsinking. –However, using such over-expensive
design is not a good idea.
Lastly, there are in fact specific situations where paralleled LEDs (even without
current equalisation/limiting circuitry) are used ‘successfully’. However, these
situations have their particulars, and in general, “paralleled LEDs without current
equalisation/limiting” would not be suitable for most applications. -It’d be too
expensive.
Reason for writing this report.
People aren’t likely to be happy with uncompetitive, overly expensive, or failing
products.
This report points out that there is a simple solution.
1
CONTENTS:
Page
Contents
3, 4
Methods of connecting LEDs:
5, 6
7
8, 9, 10
11, 12
13……
14
15, 16
17
18
A…LEDs in series
B…LEDs in parallel with current equalisation resistors
C…LEDs in parallel without current equalisation
Thermal Runaway
Thermal Runaway is a “positive feedback” process:
Matched LEDs
Matched & Batched LEDs for parallel operation.
Counterfeit LEDs infiltrating the market
LEDs with added series resistance.
Thermal testing of paralleled LED products
Problems due to sun’s direction
Long series strings of LEDs placed in parallel.
19
20,21
22
23
Examples of parallel LEDs in the market
Why would you want to put LEDs in parallel?
Being lucky with parallel LEDs
Optical effects
24
25
26
27, 28
LED manufacturers
LED Driver Manufacturers
ABBREVIATIONS
SUMMARY
2
Methods of connecting LEDs
A…LEDs in Series:
B….LEDs in Parallel with current equalisation resistors:
C…..LEDs in Parallel with no current equalisation:
3
Methods of connecting LEDs (continued)
A….LEDs in series. (see diagram on previous page)
All in all, this is the simplest and cheapest way to connect LEDs. It is the most robust
way.
B….LEDs in Parallel with current equalisation/limiting resistors:
This method is only recommended for low power designs, where the resistors can be
made small.
The sizing of the resistors is not straight-forward. The size of the resistors depends on
the type of LEDs used and the degree of thermal coupling between them.
However, B is generally preferable to C, since in B, the resistors act to ‘equalise’ the
currents between the LEDs.
C…..LEDs in Parallel with no current equalisation:
This method is not recommended.
The danger is that of “thermal runaway” , and that the majority of the current can end
up flowing through just one of the LEDs, causing overheating, and early product life
failure.
As you know, Diagram C can be made ‘workable’ (to an extent) by simply rating all
the LEDs, and the heatsinking , to the level that would be required if only one of the
LEDs were present…however, this is overly expensive.
The risk involved with diagram C’s method can be reduced by a lot of extra work and
expense, though this would make the product less competitive.
Its not to be forgotten that non-current-sharing paralleled LEDs can also look less
aesthetically pleasing, since some LEDs will be brighter than others.
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Thermal Runaway
For paralleled LEDs without current equalisation or current limiting circuitry, thermal
runaway is a danger.
Even with “Vf binned” LEDs from the same bin, there is a tolerance on Vf ,
typically about 100mV to 200mV, but often up to 300mV.
In a paralleled LED bank without current equalisation circuitry, the LED with the
lowest Vf will draw the most current. Due to this, it will heat up more than the other
LEDs. ..As it heats up more, so its Forward Voltage slightly reduces, meaning that it
draws even more current, and thus heats up even more, and thus suffers further slight
reduction in Vf, and subsequently draws even more current still, and so on, and so
on…..until the LED in question is drawing around 80% or more of the total current,
and running much hotter than it should….subsequently failing due to overheating.
This failure may well not be seen in any extended soak testing in the factory, though
early product failure in the field (with the customer) is the danger, since overheating
LEDs have reduced lifetimes…..and once one LED in a parallel circuit that’s supplied
by a current source fails, then the remaining LEDs then have to carry more current ,
and so they are then more likely to fail.
If there is no current equalisation circuitry for the LEDs, or limiting resistors, then the
only way to fight this is by:
1….Using expensive “Vf Matched” LEDs.
2….Tight thermal coupling of the LEDs. (thermal coupling = making sure that the
LEDs all ‘share’ their heat with each other)
Generally speaking, the methods of assuring increased LED thermal coupling are
hassle-some and expensive …..therefore, its best to just put the LEDs in series, or to
drive them with circuitry which ensures that the individual LED currents are forced
to be equal.
There are many ways to closely thermally couple LEDs…..
MCPCB:
The most effective way is to mount the LEDs on MCPCB (Metal Core PCB).
This is an expensive method, and thus generally not favourable for many products.
Mounting LEDs close to each other on thin FR4 PCB.
This method involves mounting the LEDs physically near to each other, and relying
on the interspacing PCB copper coating to transfer heat amongst the LEDs. This isn’t
as good as MCPCB, and it gives you the disadvantage that you are forced to mount
the LEDs physically close to each other, no more than about half an inch apart…and
preferably closer still than this.
The degree of thermal coupling can be increased by using a PCB with thick, double
sided copper coating, and using lots of thermal vias to thermally connect the top and
bottom copper layers…..the bottom copper layer can then be glued (with thermal
adhesive) to a metal heatsink.
…However, if you do this, you are left with the problem of how do you know that the
Assembly Staff properly & uniformly glued the LED PCB to the heatsink?
5
Thermal Runaway (continued)
Can you be sure that the Assembly Staff glued the LED PCB to the heatsink in a
manner which would ensure best thermal coupling between the LEDs?…
…This is one of the problems of putting LEDs in parallel, ..your assembly staff are
being more heavily relied upon to assemble the product in such a way that it doesn’t
suffer overheating due to insufficient thermal coupling between the LEDs.
If the assembly staff fail to evenly apply glue onto the PCB behind all the LEDs, then
the LEDs will not be so well thermally coupled, and your product will be less robust
against failure.
It’s difficult for Quality Staff to check for degree of thermal coupling between LEDs.
-In order to check for even glue application behind the PCB area that’s behind all of
the LEDs, the quality staff would have to prize the LED PCB away from the heatsink
and try and ascertain that none of the bottom copper area (behind the paralleled
LEDs) had been left without glue coating.
Supposing the assembly staff failed to glue behind some of the LEDs, -then those
LEDs would have a small airgap between their bottom copper and the heatsink….as
you know, air gaps are not good for thermal transfer.
In order to get round the above problem, you could use a thermal pad instead of glue,
and size the thermal pad the same size as the LED PCB.
The quality staff could inspect the product, and if they saw the edges of the thermal
pad all round the PCB edge, then they could be fairly certain that the thermal pad was
properly covering the back of the PCB behind all the LEDs.
This sounds OK, but are the quality staff going to be able to inspect every product in
this way?
If they do inspect every product like this , isn’t that overly expensive?
The point from all this is, that if LEDs are put in parallel without current equalisation
circuitry, then you saddle yourself with a number of problems about how you closely
thermally couple the LEDs, and how you check that your assembly staff are properly
carrying out the measures prescribed for thermal coupling.
To avoid these problems, it’s best to simply avoid putting LEDs in parallel without
current equalisation circuitry.
-Instead, put the LEDs in series. –This way, the LEDs don’t need to be thermally
coupled. It saves you all the hassle and expense.
Of course, as you know, LEDs in series still need to be thermally managed, but the
thermal management effort is less than when using “LEDs in parallel without current
equalisation circuitry”.
Some supporting articles:
http://ledsmagazine.com/features/6/2/2
6
Thermal Runaway is a “positive feedback” process:
In electronics, its normal for components and circuits to have tolerances. Often its
nothing to worry about. With ‘Vf matched’ LEDs, there’s actually a slight tolerance
on Vf.
‘Vf matched’ LEDs have a tolerance on Vf which is about 200mV or so. It would be
easy to think that that’s a small voltage and nothing to worry about. However, small
LED voltage changes can cause relatively large current differences in LEDs. –This is
especially so when the LED is operating at its rated current level, where the “i(led) vs
V(LED)” curve is steep.
Not only that, but the problem with very small voltage differences between LEDs
that are to be paralleled is that the one with the lowest Vf, will tend to draw the most
current….and the act of actually drawing more current means that it tends to then
draw even more current still, and so on, and so on…..this is “thermal runaway”. As
you can see, it’s a ‘positive feedback’ process, and its what we’re up against if we
wish to connect LEDs in parallel without current equalisation circuitry or current
limiting resistors.
As discussed above, small LED voltage changes can cause relatively large current
changes in LEDs, –This is especially so when the LED is operating at its rated current
level, where the “i(led) vs V(LED)” curve is steep.
This is a disadvantage when putting LEDs in parallel without current
limiting/equalisation circuitry.
It might be thought that a way round this is to simply operate all of the paralleled
LEDs near or below the “knee” of the “I(LED) vs V(LED)” curve. –Because then,
small changes in LED forward voltage don’t result in such large changes in LED
current.
-One problem of doing this is that you’re obviously going to be using the LEDs at
well below their rated current level,so therefore these LEDs will be rather more
expensive than they really need to be.
Another problem of operating LEDs at current levels which are well below their rated
current levels, is that the light quality from these LEDs (including brightness) will
not likely be spec’d for operation at this lowly current level. Its possible that some
batches of LEDs when operated at such low current levels could be particularly dim. –
It might well represent a very inefficient way to drive the LEDs, because you may
well not be getting much Lumens per Watt when the LEDs are driven at such low
current levels.
7
Matched LEDs
As you know, if one is going to utilise “parallel LEDs without equalised LED
currents” , then the LEDs chosen must be LEDs which are from the same “Vf bin”.
-This means that the LEDs must be sorted at the LED foundry , such that every LED
reel contains only those LEDs which have a similar forward voltage.
Such sorting inevitably means that these “Vf binned” LEDs are significantly more
expensive than standard LEDs.
-So this is already a disadvantage of using “parallel LEDs without equalised LED
currents”…….i.e. the fact that the LEDs that you will have to use will be significantly
more expensive, and since the LEDs are typically amongst the most expensive
(component) items in a LED product, such extra expense is not welcome.
Such “Vf binned “ LEDs are actually not perfectly Vf matched. There is usually a
100mV to 200mV difference in the Vf’s. –This can definitely be a problem since
such small voltage difference can mean a big difference in LED current.
So even though we can buy LEDs which are “Vf binned”, the binning is not that
accurate, and due to the nature of LEDs, even slight differences in forward Voltage
can result in large differences in LED current. (observing the LED’s “I vs Vf”
graph can show this)
(Of course, whether the actual Vf’s of these LEDs end up being the same in the final
product, depends on the degree of thermal coupling in the product design.)
In the UK, due to the absence of LED foundrys, there can be difficulty in sourcing
multiple LED reels all from the same Vf bin.
If the ordering of the LED reels for a LED product is left to a PCB assembly factory,
then you are trusting them to reliably ensure that only LEDs from the correct Vf bin
are used in your product. The problem is that if they don’t remember to select only
LEDs from the same Vf bin, then your product is much more likely to fail early in its
life with the customer.
It is quite confusing for the PCB assembly factory because they will have to
distinguish between same-part-number LEDs which differ only in the Vf code.
-There is a good chance that they can make a mistake here.
-There is also some chance that they may not be able to order the required number of
LED reels from the same Vf bin……there could then be a temptation to make up
numbers with same-part-number LEDs from a different Vf bin…..after all, when the
LEDs have been taken off the reels, and mounted on the PCB’s, there is no way of
knowing which Vf bin they came from.
-If you experience high product failure rates, then you will have no way of knowing
that the PCB assembly factory used LEDs from more than one “Vf bin”….the LEDs
themselves are not marked with the Vf code group.
8
Matched LEDs (continued)
Another problem is that of what happens every time the PCB assembly house
reaches the end of a reel whilst assembling your LED PCB’s…..
When using “Paralleled LEDs without current equalisation”, its usually adviseable to
use LEDs from the same reel in any given product, and not just from the same Vf bin.
..But what happens when they are down to the last 19 LEDs on a particular reel, but
the product being manufactured needs 24 LEDs?……
..Does the PCB assembly factory conscientiously put the 19 LEDs on the depleted
reel to one side, and start making further products from a fresh, full reel of LEDs?
In the ideal world, this would happen, but in reality, its likely that the PCB assemblers
will just use up the 19 LEDs remaining on the one reel, and then make up the number
to 24 LEDs by using LEDs from a different reel.
-After all, the individual LEDs are not Vf code marked, (-and are certainly not
marked with the reel number or batch number) and if there is a field failure of the
product, no one will ever know that LEDs from different reels/bins were used.
Problems in procuring Vf binned LEDs:
The problem for procuring several reels of LEDs from the same voltage bin is demonstrated
with the “LS E63B” Power TOPLED from Osram-os.com…
“LS E63B” LED datasheet:
http://catalog.osramos.com/catalogue/catalogue.do?favOid=000000000000076000020023&act=s
howBookmark
Quoting from page 2 of this datasheet:“In a similar manner for LED, where forward voltage groups are measured and binned, single forward
voltage groups will be shipped on any one reel. E.g. LA E63B-CBEA-24-1 means that only 1 forward
voltage group -3A, -3B, -4A or -4B will be shippable. In order to ensure availability, single forward
voltage groups will not be orderable (see page 5 for explanation).”
…..this quotation explains how its possible to order a reel of LEDs which contains
LEDs from a single voltage bin. However, it goes on to explain how it is not possible
to actually choose to order a particular voltage group.
This is bad news for people who wish to put LEDs in parallel without current
equalisation, since you cannot even order the voltage group that you need to order.
You would have to get very friendly with one of the big component distributors,
and hope that they can source you the LEDs that you need, when you need them, at a
decent price.
If sourcing the LEDs is being left to your PCB assembly factory, then how can you
tell that they are going to bother to source the correctly voltage binned LEDs?
The individual LEDs are not marked with the voltage bin…you will have no way of
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Matched LEDs (continued)
knowing whether or not they have gone the “extra mile” in order to ensure your LED
product gets built with LEDs all from the same voltage bin. The PCB assembly
factory may just procure the cheapest LEDs they can find which are the correct part
number , but the wrong voltage bin code.
This can definitely be a problem, and you’ll have no idea that it’s happening.
If your product fails on the field, you will have no way of knowing whether or not the
LEDs in the product were from the same voltage bin.
Putting the LEDs in series takes this entire problem away from you.
There are some (more expensive) brands of LEDs that can be ordered in specific
voltage bin groups. However, these are more expensive.
Cost of Matched LEDs:
If you are using “Vf matched” LEDs, then you will be paying significantly more
money for your LEDs than if you were using “non-Vf-Matched” LEDs.
Thus the type of LED connection involving “LEDs in parallel with no current
equalisation” will involve using more expensive LEDs.
With some products using 24 or more LEDs, its getting too expensive if you are
using “Vf Matched” LEDs.
Its better to use LEDs in series. This way you are free to use cheap, ‘non-VfMatched’ LEDs, and if their price increases, you have a much wider market of
LEDs to choose from as an alternative for your product. You won’t be limited to a
small number of expensive brands, which can be difficult to source.
Your PCB assembly factory may tell you that the correctly ‘Vf binned’ LEDs are easy
for them to source, ..but then, you will have no way of knowing that they actually
bothered to source LEDs from the same voltage bin. The individual LEDs, when
mounted on the PCB, are not marked with their voltage bin code.
10
“Matched & Batched” LEDs for parallel operation.
As discussed, if using paralleled LEDs without current equalisation or current limiting
circuitry in each paralleled LED string, then the LEDs used must be from the same
“Vf bin”.
However, this generally is not considered to be sufficient as a protective measure
against Thermal Runaway. This is because even “Vf binned” LEDs can have forward
voltages that vary by up to 250mV.
Therefore, even seasoned “Parallel LED enthusiasts” usually insist that the LEDs
used in the production of their parallel_LED lamps all come from the same
production batch. That is, the paralleled LEDs used are not only from the same “Vf
bin”, but are from the same ‘production batch’ from the LED foundry.
Very often, parallel LED lamp schematics indicate that all the LEDs used should be
“from the same reel”. By requesting LEDs “from the same reel”, the designer
believes that the LEDs will be from the same production batch.
However, this is not always the case. Often, LED distributors will buy back partial
reels from lighting companies, or from PCB assembly houses. The distributors then
may re-reel these LEDs, so as to make up full reels. There is obviously scope for
mistakes here, such as leds from different batchs or bins getting mixed up.
Also, supposing you request 10,000 LEDs from a LED distributor. –Suppose that you
also specify that all of these LEDs should be from the same production batch. What
is to stop the LED distributor from simply telling you that all of the LEDs are from
the same production batch even if they are not?
After all, the individual LEDs are not marked with the production batch number, so if
your customers experience LED product failure in the field, you will have no way of
knowing that LEDs from a mixture of batchs got used.
Realistically, the only way to provide some level of “guarantee” that the LEDs used
for manufacturing a particular production run of lamps are all from the same batch, is
for the LED foundrys themselves to make the LED lamps at the LED foundry…..ie
for the LED foundrys to take the LEDs off the LED production line, and transfer them
straight across to the LED lamp production line…or at least to transfer the LEDs to
the LED PCBs at the LED foundry.
Otherwise, how will you really know that all the LEDs came from the same
production batch?….i mean, whilst making the LEDs, the LED machine may
malfunction or something, and different wafers, different conditions, whatever, could
get used when the LED machine is started back up again, etc etc, resulting in wider
tolerancing of Vf..
11
Blank Page
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Counterfeit LEDs infiltrating the market
Fake electronics components of all descriptions have been reported in the market.
If this happens with “Vf binned” LEDs then that has obvious problems for
manufacturers of paralleled LED products, because the voltage matching of such
counterfeit products is liable to be very suspect.
13
LEDs with added series resistance.
Some of the cheap , Far Eastern LED lamp products found in supermarkets contain
LEDs in parallel.
One such 1.7W product contains 72 white LEDs in parallel….Each LED carries 8mA
of current.
(-its actually three groups of 24 LEDs in parallel, with each of the three groups having
a single resistor of 10R in series with it..so there’s three 10R resistors in total for the
whole product)
The forward voltage of the LEDs was seen to be 3V.
This is a very high forward voltage for a white LED carrying just 8mA.
This is indicative that the LEDs are from a LED foundry production run whereby
higher series resistance is placed into the bulk semiconductor region of each of the
LEDs. This in turn makes the LEDs more favourable for use in parallel operation
without extra current equalisation/limiting circuitry being used.
It is easier to make such “series resistance enhanced” LEDs in say China, because all
industries are basically owned by the government. Therefore, the ‘government’ can
confidently order a huge volume production run of “series resistance enhanced”
LEDs, and simply order various companies to use them to make lamps comprising
paralleled LEDs.
In the West, individual companies act more alone and independently, and none can
afford to get huge volume production runs of “series resistance enhanced” LEDs
made for themselves.
14
Thermal testing of paralleled LED products
Many LED light products are low power, eg just around 6 Watts.
However, some LED lights are housed in a transparent enclosure, which obviously
traps the suns rays, (-if for outdoor use) and can cause high levels of heating to the
LEDs and components.
To make matters worse, the internal heatsink in some LED lights does not emerge
externally from the enclosure (for cost reasons).
The internal aluminium heatsink is often totally enclosed in the poorly-thermallyconductive plastic enclosure.
Under such conditions, it’s difficult to transport heat away from the LEDs. (since the
convection currents from the heatsink are trapped inside the plastic enclosure)
Its worth remembering that heatsinks like in some LED lights can’t work well unless
they have somewhere to transfer their heat to. The method of cooling is called
“convection” cooling, -however, in some LED lights, as mentioned, there isn’t
anywhere cool for the internal heatsink to transfer convection currents away
to…because the convection currents are trapped inside the poor thermally conducting
plastic enclosure.
Therefore, Temperature testing is necessary for the LED lights.
[This should be done with the product in the thermal chamber, fitted inside its
enclosure, and not just the bear PCB being placed in the chamber, (since most thermal
chambers work by fans blowing heated air around the chamber, and such circulating
air currents can remove localised heated air from around a bear PCB)]
With “paralleled LEDs with no current equalisation circuitry”, which are only loosely
thermally coupled, effective thermal testing of the LEDs is going to be very time
consuming and expensive…
First of all, you have to assess which of the paralleled LEDs is carrying the most
current, so that you know where to mount the thermocouple.
Even if you do spend the time needed to find out which LED is carrying the most
current in a given product, you have no way of knowing whether that is typical of the
whole production batch of the same product.
So Thermal testing of “paralleled LEDs with no current equalisation circuitry”
(especially when the thermal coupling between the LEDs is not tight) becomes overly
time consuming and un-productive.
-its better to just put the LEDs in series, and feed the series string of LEDs with a
controlled current source.
This way (LEDs in series) , you can be certain that all of the LEDs carry the same
current as each other, regardless of the degree of thermal coupling between them. You
will be able to get meaningful thermal test results, done in thermal tests which are
repeatable. They’ll be repeatable because if you do the test again on the same product,
you’ll know exactly how much current is flowing in each LED.
Effective thermal testing of products containing “paralleled LEDs with no current
equalisation circuitry”, when the thermal coupling is not tight, is overly expensive,
and too time-consuming.
15
Putting the LEDs in series, and feeding them with a controlled current source, makes
thermal testing far more straightforward.
16
Problems due to sun’s direction
As you know, if using LED light products outdoors containing “paralleled LEDs
with no current equalisation circuitry”, then often the inside of the light comprises a
vertical, square-section mounting bar, with LED PCBs mounted on each of the four
faces.
Its possible that in use (with the customer), the beacon could spend a lot of its time
with the sun shining persistently on one of the four LED PCBs. ..And at least one of
the LED PCB’s is likely to be ‘shaded’ from the sun’s direct rays.
This obviously means that thermal coupling of the “paralleled LEDs with no current
equalisation circuitry” is poor.
-As you know, excellent thermal coupling of “paralleled LEDs with no current
equalisation circuitry” is essential.
Thus you would expect unfortunate overheating of the LEDs that were exposed to the
sun’s direct rays.
These LEDs would run hotter, and this in turn makes their Vf lower, which makes
them draw more current , making them hotter still, etc etc …and so on and so on..
This is not such a big problem with LEDs in series, since even though the LEDs that
are directly exposed to the sun’s rays will be hotter, they will not be able to undergo
thermal runaway, as thermal unaway does not happen in series LEDs.
Another problem for LED lights containing “paralleled LEDs with no current
equalisation circuitry”, is that the LEDs on the PCBs are sometimes mounted
vertically, so therefore the LEDs that are mounted higher up in the lamp will run
slightly hotter than the LEDs that are mounted lower down in the product, due to the
fact that heat rises.
-Once again, this doesn’t help thermal coupling of the paralleled LEDs at all…..and
excellent thermal coupling is essential for “paralleled LEDs with no current
equalisation circuitry”.
17
Long series strings of LEDs placed in parallel.
Some LED lamps comprise say three strings of eight or more LEDs in parallel. Such
longer LED strings provide an improvement for parallel connection, because having
such a larger number of LEDs tends to mean that a statistical spreading of the
forward voltages occurs, such that the overall forward voltage of the LED strings are
more likely to be more similar.
Even though the use of such long series/parallel LED banks is not as bad in terms of
paralleled LEDs, …the discussed facts about the significant extra expense of “Vf
matched” LEDs, the extra expense of the required thermal coupling, the difficulties of
ensuring that production staff are properly assembling the product so as to provide
the required thermal coupling, the problem of ensuring that the PCB assembly factory
are genuinely procuring the required “Vf matched & batched” LEDs, etc etc, all still
apply, and it is usually cheaper to do a series LED solution.
The following product by Forge Europa shows three series strings of eleven LEDs
being placed in parallel. –There are no current limiting resistors in each of the series
LED strings.
“200mm Round LED Light Engine”:
http://www.forge-europa.co.uk/solutions/200mm_round_led_light_engine
18
Examples of parallel LEDs in the market
If enough financial investment is made, then parallel LED light engines without
current equalisation can be produced which may work successfully.
In the Far East, often the availability of large numbers of lowly waged test engineers
means that they can afford extended soak testing whereby the testers measure the
currents in each parallel led branch over long periods of time, and make sure they are
equal.
The alternative (putting LEDs in series), works out simpler, cheaper and easier and
more failure proof.
Companies such as Forge Europa in Ulverston, UK, do however, (at the current time)
sell parallel led products comprising series/parallel strings of eleven leds in series. –
Some of these have no current limiting resistors in each paralleled LED string…
http://www.forge-europa.co.uk/solutions/200mm_round_led_light_engine
19
Why would you want to put LEDs in parallel?
Low voltage systems:
Some systems , such as low voltage, constant current systems, have a maximum
output voltage of 40V. They deliver a fixed constant current. If you want a luminaire
on this system with many LEDs (>12 white LEDs) in it to create an attractive lightspreading-appearance, then you have no option other than to put the LEDs in parallel.
Obviously, as discussed in this report, if LEDs are put in parallel with no current
equalisation, then doing that effectively is an expensive way of doing it.
Simple Switch mode converters:
LEDs can be driven by dissipative, inefficient “linear” power supplies, or efficient
“switch-mode” power supplies.
The simplest type of switch mode LED driver for driving LEDs, is the “hysteretic
Buck” type of LED driver. This is simple because pretty accurate LED current
regulation can be achieved without having to use “frequency compensation” in the
feedback loop. For example, the ZXLD1360 LED driver operates in this manner…..
ZXLD1360 LED driver IC datacheet:
http://www.diodes.com/datasheets/ZXLD1360.pdf
If your input voltage is low (e.g. 13V, as in an automobile), and you want to drive
lots of LEDs (say more than 4 white LEDs) using the “hysteretic Buck” type of
converter, then unless you use a separate driver for each series LED string, you will
have to put the LEDs in parallel.
However, even though “hysteretic Buck” type LED drivers are simple, it’s a fact that
all non-isolated LED drivers, even the ones utilising frequency compensated feedback
loops, are very easy to design, so its not really that helpful to always choose the
“Hysteretic Buck” type of LED drivers.
(Having said switch mode LED drivers are all easy to design, …Its worth noting that
since for cost reasons off-the-shelf inductors are used, and these generally don’t have
“delta B” type data on their datasheets for calculating “delta B” losses, its still always
necessary to carry out thermal testing on all LED driver prototypes.
Also Junction to Ambient thermal resistance data on FETs etc always refers to use on
certain size PCB copper lands….inevitably, in your design, you won’t be able to get
exactly those copper land sizes, (especially in a tiny products) so again, thermal
testing is always required on LED driver prototypes. Thermal testing needs to be
done with the PCB inside the intended enclosure, with the intended heatsink and
thermal adhesive etc etc.)
Multiple LED systems:
Maybe you want lots of LEDs in the lamp and would need multiple LED drivers to
drive them all. –So as an alternative, you may elect to put the LEDs in series/parallel
banks and just use one LED driver for all the LEDs. –This initially sounds like a
cheap way to do it, but the extra expense invoked by the measures taken for tight
thermal coupling between the LEDs etc, and all the other discussed problems, mean
that using multiple LED drivers is likely to be cheaper….specially since some LED
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drivers, like hysteretic bucks, are very cheap, and often have integrated FETs in the
controllers.
Tight layout of the power_switch and rectifier current loops will reduce the noise
problems significantly, inspite of having multiple switch mode LED drivers. Good
decoupling with capacitors also will help.
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Being lucky with parallel LEDs
There are often LED lamps that have parallel LEDs without current sharing cicuitry
that manage to pass through thermal testing.
However, the problem with “parallel LEDs without current sharing circuitry” is that
the failure mode can often take longer than the time the product spends in the thermal
chamber. The LED overheating that occurs due to “parallel LEDs without current
sharing circuitry” can make a LED beacon last for some weeks , before it fails.
However, there are some LED lamps containing “parallel LEDs with no current
sharing” which last for years in the field….even lamps where the LEDs are poorly
thermally coupled.
This longevity could be due to many reasons….
The most likely reason is that the LEDs and Heatsinks have been overspecified, and
are more expensive than they need to be.
Other reasons could be that the particular batch of LEDs that the lamp was made from
contained an excellent statistical match between the individual forward voltages of the
LEDs…….of course, a later production batch of LED lamps may contain LEDs from
a not so tightly toleranced batch, and then problems would be more likely to occur.
Another reason that failures may not occur with parallel LEDs, could be that the
customer is using the product in cool ambient temperatures…..put the product in a
situation where the ambient temperature was higher and then the LED failure problem
may then arise.
Another reason that failures may not occur with parallel LEDs, could be that the
customer is operating the product with a particularly low flash duty cycle, (ie if it a
flash lamp) and maybe the customer never uses the product on full, steady burn, or a
higher duty cycle flash pattern….this would mean that the LEDs would run cooler,
and would be less likely to fail.
Another reason that failures may not occur with parallel LEDs, could be that the
customer is actually not turning the product ON for long periods at a time. Maybe the
customer hardly ever uses the product.
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Optical Effects
Its accepted that a disadvantage of parallel LEDs is the problem of thermal runaway
and subsequent LED failure.
Another more obvious effect is the non uniformity of light emission that may well
occur in paralleled LED banks. (because those LEDs carrying more current than other
LEDs will emit more light than the LEDs carrying less current.)
-This makes the visual appearance of the product worse.
This problem is completely solved by using LEDs in series.
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LED manufacturers:
If you wish to know more about putting LEDs in parallel without using current
equalisation circuitry, then it could be worth talking to some of the LED
manufacturers…..
Philips:
http://www.philipslumileds.com/
Osram:
http://www.osram.com/osram_com/index.jsp
Cree:
http://www.cree.com/
Kingbright:
http://www.kingbrightusa.com/default.asp
Avago:
http://www.avagotech.com/pages/en/leds/avago_led_through_hole_lamps/
No LED Manufacturer Gaurantee for parallel LED operation:
To date, no single LED manufacturer produces literature stating that their LEDs can
be operated in parallel without measures being taken to equalise the currents, or
measures being taken to limit the current in each paralleled LED string.
This is extremely significant, and predisposes us to believe that putting LEDs in
parallel without taking measures to avoid thermal runaway, is a bad idea.
After all, if LEDs could be simply paralleled, then only one driver would be needed
for huge LED banks, and so LED manufacturers could make lots of money out of
selling all of the paralleled LEDs used…however, in spite of this, LED manufacturers
do not have official literature stating that parallel operation is possible, when no LED
current limitation circuitry is present.
For one thing, LED manufacturers want their LEDs to each be operated at a certain
rated current level, because at this particular level (usually stated in the LED
datasheet), the light from the LED has the spec’d amount of brightness and
chromaticity etc. If the LEDs are paralleled without current equalisation circuitry,
then there is no guarantee that the current in all the LEDs will be at the rated level. –
This could affect the aesthetic appearance of the LEDs, and such aesthetics are of
high importance to LED manufacturers.
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LED Driver Manufacturers.
Most LED luminaries/beacons are supplied by a LED driver, utilising a LED driver
Integrated Circuit (IC)
The manufacturers of LED driver IC’s will be able to advise you on the contents of
this report. (as well as the LED manufacturers).
ti.com:
http://www.ti.com/ww/en/lighting/index.htm
[email protected]
[email protected]
[email protected] (nsc.com is now ti.com)
“Driving high-power LEDs in series parallel arrays”, by Chris Richardson, National
semiconductor, November 27, 2008:
http://www.edn.com/design/led/4325905/Driving-high-power-LEDs-in-seriesparallel-arrays
supertex.com:
http://www.supertex.com/
[email protected]
diodes.com:
http://www.diodes.com/products/catalog/browse.php?parent-id=90
linear.com
http://www.linear.com/products/led_driver_ics
infineon.com:
http://www.infineon.com/cms/en/product/led-drivers-and-lightingics/channel.html?channel=db3a304319c6f18c011a154f7fb62712
[email protected]
nxp.com
http://www.nxp.com/products/power_management/lighting_driver_and_controller_ics
/led_drivers_for_automotive/
Distributors:
[email protected]
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ABBREVIATIONS:
Buck
I.C.
LED
MCPCB:
Refers to a step-down switch-mode power supply, where
the input voltage is greater than the output voltage.
This refers to the losses in the inductors of switch mode
LED drivers which are brought about due to the fact that
they have a constantly changing magnetic field in them.
Changing at high frequency, and with a certain amplitude.
The voltage across a LED that’s conducting current in the
forward biased direction (i.e. the voltage across a LED
that’s emitting light.
Also called “Vf”
Integrated circuit
Light Emitting Diode
Metal Core Printed Circuit Board.
PCB:
This is a printed circuit board which basically is made of
metal. (aluminium). Components mounted on it will share
their heat with each other relatively well, since the
thickness of the metal is far thicker than the very thin
coating of metal seen on FR4 based PCB’s. MCPCB is
significantly more expensive than FR4 based PCB’s
Printed Circuit Board
Delta B losses
Forward Voltage (Vf)
(also known as “Printed Wire Board”)
Thermal Coupling:
Making sure that the LEDs all share their heat with each
other.
Vf :
“forward Voltage” (refers to the voltage of the LED when
conducting current in forward bias, usually at a certain
temperature, and with a certain current in it)
Refers to a collection of LEDs that all have similar
Forward Voltage
Vf Bin:
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SUMMARY
Putting LEDs in parallel with no current equalisation_circuitry/limiting_resistors can
be made ‘workable’, to an extent, but is overly expensive.
If not done properly, then “LEDs in parallel with no current
equalisation_circuitry/limiting_resistors” can lead to large quantities of LED products
failing prematurely in the field (with the customer)
For many applications, its cheaper to put the LEDs in series and feed them with a
controlled current source.
“LEDs in parallel with no current equalisation circuitry” involves the following
additional costs……
Significant Extra cost of the “Vf Matched” LEDs which need to be used.
Cost of MCPCB or more thickly copper coated FR4 PCB which is needed to
ensure tight thermal coupling between paralleled LEDs.
Cost of any additional production testing procedures which you put in place
to examine current sharing in paralleled LED products where thermal coupling
between LEDs is more loose in order to save costs….plus any extended soak
testing which is more necessary with paralleled LED products.
Connecting LEDs in parallel also gives you a headache in terms of how do you check
that your PCB assembler company is actually using LEDs with the right forward
voltage bin code.
(once any LED is mounted on the PCB, there’s no way of knowing which voltage bin
it was from, or whether it was from the same batch as the other LEDs)
There can be supply chain difficulties in sourcing large numbers of LEDs all with the
same voltage bin code, or all from the same production batch.
Parallel connected LEDs also require significant effort in order to ensure that they are
very tightly thermally coupled. If you are using very expensive MCPCB on which to
mount the LEDs, then you will get very tight thermal coupling…..if not, and you are
glueing LED PCB’s to metal heatsinks, then the degree of thermal coupling that you
achieve will depend on the conscientiousness of your assembly staff. Once the
product is assembled, its very difficult for you to check that thermal adhesives or pads
have been correctly applied. –This is a much bigger problem for parallel connected
LEDs, since with series connected LEDs, the degree of thermal coupling between
LEDs is not important.
Thermal testing of paralleled LED products is also generally a more uncertain
process than with series LEDs. (especially if the thermal coupling between LEDs is
not very tight) Also, thermal testing of paralleled LEDs would be more timeconsuming, more uncertain, and more expensive.
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If you put LEDs in parallel with no current equalisation, and with poor thermal
coupling between the LEDs, then you potentially run the risk of suffering huge field
failure rates.
If a parallel LED product’s production line has an extra step added to check that over
time, all the paralleled LEDs do indeed share the current, then this represents extra
production process expense. –and such a checking process will be useless if in the
installation there is uneven heating to the fixture (eg by one side of the luminaire
being near a heat pipe etc)
For many applications, its overall cheaper & simpler to put the LEDs in series and
feed them with a controlled current source.
Driving LEDs in series , and designing LED drivers to drive LEDs in series, is very
simple. –And since its saves money, hassles and time, its worth doing.
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