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High brightness white LEDs damaged by custom switcher

P

Paul E. Schoen

Jan 1, 1970
0
Some of you may recall my discussions about a 12 VDC powered switching
converter to drive strings of 7 or 13 high power white LEDs, in posts from
1/5/07 (transients), 2/22/07 (Inductor gaps), and 3/10/07 (TC1410 driver
problems). We built the circuit, found a few problems, then tested it and
it seemed OK. Basically this is a switching boost regulator using a 10 uH
inductor and a PIC with PWM at 100 kHz. I did a lot of LTSpice simulations
as well.

The circuit is somewhat unique in that the PIC is programmed to produce
alternate high and low brightness modes, at about 700 and 100 mA, when
power is applied and removed. The PWM is controlled by the voltage on a 1
ohm sense resistor, and it is designed to ramp up slowly and cut back
quickly. There are also undervoltage input and overvoltage output sensing
circuits to shut down the PWM until the next power on reset.

My friend had three different 7-LED clusters (made by Cree), of low,
medium, and high brightness. While testing the medium-brightness cluster,
he noticed a less than normal difference in brightness, and found that two
of the LEDs were not lit. They tested open, but apparently the voltage was
enough to bridge the gap and provide a current path for the remaining 5 in
series.

The manufacturer examined the LEDs and stated: "#1 appeared to have an
evidence of damaged chip which looked to be a burnt metal traces around the
bond pads, it also had a blown wire bond on the Anode side which had an
appearance of a fused wire. The lamp #4 had the blown off bond pads on the
chip side." They concluded it was a high current surge.

My recommendation to my friend was to perform a deliberate destructive test
on one or two lamps to see what level of current would cause such damage. I
think he had previously tried up to 1.5 or 2 amps for a few seconds without
sign of damage, although possibly not with these exact model LEDs.

Without going into more detail for the sake of brevity, I would like to
discuss possible scenarios, testing suggestions, and design improvements to
minimize the risk of more failures in the field. Terry Given was kind
enough to analyze my circuit and layout, with many suggestions for
improvement, and I plan to implement them in the next iteration of the
design. However, someone else was adamant that a PIC would never be as
reliable as a pure analog design, and suggested an off-the shelf Linear
Tech or other part, which would involve a complete redesign and loss of the
dual brightness feature.

Thanks,

Paul
 
D

D from BC

Jan 1, 1970
0
Some of you may recall my discussions about a 12 VDC powered switching
converter to drive strings of 7 or 13 high power white LEDs, in posts from
1/5/07 (transients), 2/22/07 (Inductor gaps), and 3/10/07 (TC1410 driver
problems). We built the circuit, found a few problems, then tested it and
it seemed OK. Basically this is a switching boost regulator using a 10 uH
inductor and a PIC with PWM at 100 kHz. I did a lot of LTSpice simulations
as well.

The circuit is somewhat unique in that the PIC is programmed to produce
alternate high and low brightness modes, at about 700 and 100 mA, when
power is applied and removed. The PWM is controlled by the voltage on a 1
ohm sense resistor, and it is designed to ramp up slowly and cut back
quickly. There are also undervoltage input and overvoltage output sensing
circuits to shut down the PWM until the next power on reset.

My friend had three different 7-LED clusters (made by Cree), of low,
medium, and high brightness. While testing the medium-brightness cluster,
he noticed a less than normal difference in brightness, and found that two
of the LEDs were not lit. They tested open, but apparently the voltage was
enough to bridge the gap and provide a current path for the remaining 5 in
series.

The manufacturer examined the LEDs and stated: "#1 appeared to have an
evidence of damaged chip which looked to be a burnt metal traces around the
bond pads, it also had a blown wire bond on the Anode side which had an
appearance of a fused wire. The lamp #4 had the blown off bond pads on the
chip side." They concluded it was a high current surge.

My recommendation to my friend was to perform a deliberate destructive test
on one or two lamps to see what level of current would cause such damage. I
think he had previously tried up to 1.5 or 2 amps for a few seconds without
sign of damage, although possibly not with these exact model LEDs.

Without going into more detail for the sake of brevity, I would like to
discuss possible scenarios, testing suggestions, and design improvements to
minimize the risk of more failures in the field. Terry Given was kind
enough to analyze my circuit and layout, with many suggestions for
improvement, and I plan to implement them in the next iteration of the
design. However, someone else was adamant that a PIC would never be as
reliable as a pure analog design, and suggested an off-the shelf Linear
Tech or other part, which would involve a complete redesign and loss of the
dual brightness feature.

Thanks,

Paul

How about asking Microchip Inc. if PIC's are just as reliable as
dedicated smps PWM controller IC's.
D from BC
 
P

Paul E. Schoen

Jan 1, 1970
0
D from BC said:
Some of you may recall my discussions about a 12 VDC powered switching
converter to drive strings of 7 or 13 high power white LEDs, in posts
from
1/5/07 (transients), 2/22/07 (Inductor gaps), and 3/10/07 (TC1410 driver
problems). We built the circuit, found a few problems, then tested it and
it seemed OK. Basically this is a switching boost regulator using a 10 uH
inductor and a PIC with PWM at 100 kHz. I did a lot of LTSpice
simulations
as well.
[snip]
However, someone else was adamant that a PIC would never be as
reliable as a pure analog design, and suggested an off-the shelf Linear
Tech or other part, which would involve a complete redesign and loss of
the
dual brightness feature.

Thanks,

Paul

How about asking Microchip Inc. if PIC's are just as reliable as
dedicated smps PWM controller IC's.
D from BC

I'm sure they would "stand by their chips", but I think the concern was
that the software could go awry and damage could be done before it could
react. That's why I am considering a separate fail-safe mechanism, such as
a fast acting fuse, or an NPN transistor that would turn on, when the
voltage on the 1 ohm sense resistor reached Vbe, and turn off the MOSFET.
Maybe an SCR would be better, requiring a complete power down to reset.

Paul
 
R

rebel

Jan 1, 1970
0
(snip history)
My friend had three different 7-LED clusters (made by Cree), of low,
medium, and high brightness. While testing the medium-brightness cluster,
he noticed a less than normal difference in brightness, and found that two
of the LEDs were not lit. They tested open, but apparently the voltage was
enough to bridge the gap and provide a current path for the remaining 5 in
series.

The manufacturer examined the LEDs and stated: "#1 appeared to have an
evidence of damaged chip which looked to be a burnt metal traces around the
bond pads, it also had a blown wire bond on the Anode side which had an
appearance of a fused wire. The lamp #4 had the blown off bond pads on the
chip side." They concluded it was a high current surge.

Did the manufacturer examine the entire string, and if so what were their
conclusions (if any) about the diodes that *hadn't* failed? (It is easy to say
that a "current surge" blew the shit out of the failed ones., just like most
house fires always seem to be caused by an electrical fault.)
 
D

D from BC

Jan 1, 1970
0
D from BC said:
Some of you may recall my discussions about a 12 VDC powered switching
converter to drive strings of 7 or 13 high power white LEDs, in posts
from
1/5/07 (transients), 2/22/07 (Inductor gaps), and 3/10/07 (TC1410 driver
problems). We built the circuit, found a few problems, then tested it and
it seemed OK. Basically this is a switching boost regulator using a 10 uH
inductor and a PIC with PWM at 100 kHz. I did a lot of LTSpice
simulations
as well.
[snip]
However, someone else was adamant that a PIC would never be as
reliable as a pure analog design, and suggested an off-the shelf Linear
Tech or other part, which would involve a complete redesign and loss of
the
dual brightness feature.

Thanks,

Paul

How about asking Microchip Inc. if PIC's are just as reliable as
dedicated smps PWM controller IC's.
D from BC

I'm sure they would "stand by their chips", but I think the concern was
that the software could go awry and damage could be done before it could
react. That's why I am considering a separate fail-safe mechanism, such as
a fast acting fuse, or an NPN transistor that would turn on, when the
voltage on the 1 ohm sense resistor reached Vbe, and turn off the MOSFET.
Maybe an SCR would be better, requiring a complete power down to reset.

Paul

Arrghh..I'm forgetting lots of my PIC stuff..
Then there's the PIC watchdog timer...
IIRC ..it's to help with "getting stuck in the hole" type glitches"...
The uP will recover however I can imagine that the recovery period
might still cause some smps external damage.

The method of OC protection will depend on worst case over current
rate of rise. MegaAmps/picosecond??

Alternatively, Polyswitch devices (PTC) are used for overcurrent
protection but I don't know if I'll protect LED"s in your app.
It's a datasheet exploration...
D from BC
 
T

Tony Williams

Jan 1, 1970
0
Paul E. Schoen said:
I'm sure they would "stand by their chips", but I think the
concern was that the software could go awry and damage could be
done before it could react. That's why I am considering a
separate fail-safe mechanism, such as a fast acting fuse, or an
NPN transistor that would turn on, when the voltage on the 1 ohm
sense resistor reached Vbe, and turn off the MOSFET.

Perhaps for development you could use a power NPN
that directly did a limiting clamp of the current
through the LED strings. Also a sensor that flags
when the NPN is conducting.

This would allow any fault to remain long enough
for investigation, without damaging the LEDs.
 
R

rebel

Jan 1, 1970
0
I'm sure they would "stand by their chips", but I think the concern was
that the software could go awry and damage could be done before it could
react. That's why I am considering a separate fail-safe mechanism, such as
a fast acting fuse, or an NPN transistor that would turn on, when the
voltage on the 1 ohm sense resistor reached Vbe, and turn off the MOSFET.
Maybe an SCR would be better, requiring a complete power down to reset.

Why not use something like an LM317 in the (series) feed to the LEDs as a
programmable current source. That would facilitate (a) avoiding possibly
damaging current surges (sorry, tongue in cheek), and (b) allowing the stepping
of the LED current through 0/100/700mA or whatever steps you want.

Pierre (who loves 34063's and 317's)
 
J

Joel Kolstad

Jan 1, 1970
0
Paul E. Schoen said:
However, someone else was adamant that a PIC would never be as reliable as a
pure analog design

That's a little naive. There are plenty of switching power supply out there
being run by microcontrollers these days, and on ones where it's preventing
catastrophic failure is important, you certailny to see various current
limiting/fail-safe devices... ***just as you do on analog designs, since
there's plenty to fail on them as well***.
and suggested an off-the shelf Linear Tech or other part, which would
involve a complete redesign and loss of the dual brightness feature.

You don't necessarily lose the dual-brightness feature -- somewhere an analog
design still has a reference voltage (or similar) that it's trying to match,
and you can usually find some means of changing that reference, PWMing the
output, etc. to get variable brightness.
 
P

Phil Hobbs

Jan 1, 1970
0
rebel said:
Why not use something like an LM317 in the (series) feed to the LEDs as a
programmable current source. That would facilitate (a) avoiding possibly
damaging current surges (sorry, tongue in cheek), and (b) allowing the stepping
of the LED current through 0/100/700mA or whatever steps you want.

Pierre (who loves 34063's and 317's)

I usually use two-transistor current limiters--you know, the classical
two-terminal ones--rather than ICs for protecting sensitive devices like
diode lasers. I'm much more confident that I understand their transient
response than the IC's, especially in areas like coming out of thermal
limiting.

Cheers,

Phil Hobbs
 
P

Paul E. Schoen

Jan 1, 1970
0
rebel said:
Why not use something like an LM317 in the (series) feed to the LEDs as a
programmable current source. That would facilitate (a) avoiding possibly
damaging current surges (sorry, tongue in cheek), and (b) allowing the
stepping
of the LED current through 0/100/700mA or whatever steps you want.

Pierre (who loves 34063's and 317's)

Thanks for the suggestion, but the power required for the clusters is at
least 21 watts for 7 and 39 watts for 13. Even if the device could handle
the voltage and power, it would be inefficient and unusable in the
application, which is a diving flashlight. The efficiency appears to be in
the order of 90%, and 4 watts is not excessive, especially when the water
acts as a huge heat sink. However, efficiency and battery life are
important.

I think the additional transistor or SCR will work. I can't add too many
parts, as the entire board is only 0.95" x 2". The transistor would
effectively form a linear current regulator, but would subject the MOSFET
to excessive power. If I can get a small SCR, or make one from two
transistors, it would latch the MOSFET off, but then the 12 volt gate drive
would be applied across the 20 ohm gate resistor.

Maybe I will use the TI UCC27321 MOSFET driver which has an enable line
with a Schmitt trigger. Luckily it is an active High with internal pull-up,
so the saturated NPN transistor would disable the drive. Adding a capacitor
should hold the output off for long enough to produce a "flashing" effect,
which would be very noticeable.

It needs a redesign anyway, and I'm a little leery of the Microchip
drivers. The TI products have a combination MOSFET and bipolar output
designed to switch more efficiently through the on threshold.

Additional suggestions are welcome!

Paul
 
W

Winfield Hill

Jan 1, 1970
0
Paul said:
power is applied and removed. The PWM is controlled by the voltage
on a 1 ohm sense resistor, and it is designed to ramp up slowly and
cut back quickly. [ snip, or should I say snap? ]

Hmm, are you trying to maximize thermal-gradient stresses?
 
P

Paul E. Schoen

Jan 1, 1970
0
Winfield Hill said:
Paul said:
power is applied and removed. The PWM is controlled by the voltage
on a 1 ohm sense resistor, and it is designed to ramp up slowly and
cut back quickly. [ snip, or should I say snap? ]

Hmm, are you trying to maximize thermal-gradient stresses?

I was trying to minimize the possibility of overshoot. In the simulation,
with 12 VDC input, an 80% duty cycle produces a peak of 2 amps at 55 volts
in 600 uSec. The desired 700 mA is reached in about 300 uSec, and the first
100 uSec is used to pump up the voltage of the 47 uF output capacitor. This
is with a simulated 13 LED cluster. The 7 LED cluster also takes about 600
uSec to reach a peak of 3.8 amps, and reaches 700 mA in about 250 uSec.

A 45% duty cycle, on the 7 LED cluster, reaches and holds 700 mA at 27
volts in about 210 uSec, and a 65% duty cycle PWM reaches and holds 700 mA
at 46 volts in about 1.2 mSec.

The circuit is certainly capable of generating enough current to damage the
LEDs, particularly the cluster of 7. However, I am reading the output
current at a 1 kHz rate, so I think I may have discovered the problem. The
PWM starts at 0, and can only increment by about 1/63 per mSec, so it
should take at least 30 mSec for the PWM to reach the 45 or 65% level
needed for the target output current. However, if somehow the PWM is set
too high, it will take less than 600 uSec for the current to shoot up to as
much as 4 amps, which would probably cause the damage seen.

I could easily increase the sampling rate, at least for the output current,
to 100 uSec, as the conversion time is 51 uSec. It seemed like the slow
rise of PWM would have been enough, but this is very likely the culprit.

Another simple thing I could do is generate an interrupt when my proposed
NPN overcurrent sensor indicates more than 700 mA, and then shut down the
PWM (as well as lowering the duty cycle) until it clears. I will probably
want to change the sense resistor to about 0.68 ohms so that it will only
trip over about 1 ampere, which is still safe. The same logic signal could
be tied to the enable line of the driver as well.

OK, back to the drawing (and coding) board...

Thanks,

Paul
 
G

Guest

Jan 1, 1970
0
is device to device parameter variations being accounted for?

most of these LED's are not anywhere near being matched pairs/triples or
other, unless you get the manufacturer to custom mount them from the same
die and then do a parametric test on each and then sort them and mark them,
(Very costly)

even clusters are assembled from random devices!

MIL specs may offer you some leeway, but even controlled groups will be bulk
tested only for basic params.

if spiked current is failing some devices, they may have beeen borderline to
begin with, as most will just heat up and dim before catastrophic failure
occurs. usually over time.

your blown bonds and similar symptoms tells me that you had EXTREME current.

rise time between units may force some to go into failure befor the adjacent
units absorb the available current also.

as with all design, the simplest is the best. trying to parlay your
education of PICs and neatsy circuits willl yield expensive and hard to
manufacture systems.

take some lessons from the chinese,tiawanese and others, cheap simple and
basic.

stay within some easy set standard limitations and you may have good
results.
 
F

Fred Bloggs

Jan 1, 1970
0
Paul said:
Some of you may recall my discussions about a 12 VDC powered switching
converter to drive strings of 7 or 13 high power white LEDs, in posts from
1/5/07 (transients), 2/22/07 (Inductor gaps), and 3/10/07 (TC1410 driver
problems). We built the circuit, found a few problems, then tested it and
it seemed OK. Basically this is a switching boost regulator using a 10 uH
inductor and a PIC with PWM at 100 kHz. I did a lot of LTSpice simulations
as well.

The circuit is somewhat unique in that the PIC is programmed to produce
alternate high and low brightness modes, at about 700 and 100 mA, when
power is applied and removed. The PWM is controlled by the voltage on a 1
ohm sense resistor, and it is designed to ramp up slowly and cut back
quickly. There are also undervoltage input and overvoltage output sensing
circuits to shut down the PWM until the next power on reset.

My friend had three different 7-LED clusters (made by Cree), of low,
medium, and high brightness. While testing the medium-brightness cluster,
he noticed a less than normal difference in brightness, and found that two
of the LEDs were not lit. They tested open, but apparently the voltage was
enough to bridge the gap and provide a current path for the remaining 5 in
series.

The manufacturer examined the LEDs and stated: "#1 appeared to have an
evidence of damaged chip which looked to be a burnt metal traces around the
bond pads, it also had a blown wire bond on the Anode side which had an
appearance of a fused wire. The lamp #4 had the blown off bond pads on the
chip side." They concluded it was a high current surge.

My recommendation to my friend was to perform a deliberate destructive test
on one or two lamps to see what level of current would cause such damage. I
think he had previously tried up to 1.5 or 2 amps for a few seconds without
sign of damage, although possibly not with these exact model LEDs.

Without going into more detail for the sake of brevity, I would like to
discuss possible scenarios, testing suggestions, and design improvements to
minimize the risk of more failures in the field. Terry Given was kind
enough to analyze my circuit and layout, with many suggestions for
improvement, and I plan to implement them in the next iteration of the
design. However, someone else was adamant that a PIC would never be as
reliable as a pure analog design, and suggested an off-the shelf Linear
Tech or other part, which would involve a complete redesign and loss of the
dual brightness feature.

Thanks,

Paul

I don't recall seeing the previous posts on this project so really don't
know what your circuit looks like. The most reasonable current limit
topology for almost any kind of switching power control is one which
truncates the power flow upon over current. In your case this would mean
a small sense resistor + comparator monitoring the source current of the
MOSFET and truncating the gate turn-on pulse as qualified by a fixed
threshold corresponding to a level somewhat below the peak current
capability of the LEDs. This will provide protection against an
instantaneous over-current while the PIC will handle the averages. It is
most likely that your design should include protection against the PIC
in addition to the usual analog protection measures. On the analog side
of things there could be several causes of the over-current if this is
in fact a failure mode. Some possible causes are: 1) the inductor
transiently saturates for some unanticipated reason having to do with
the PIC or possibly a bad rating selection, 2) the filter capacitor
displays too much ESL and ESR relative to the LEDs+ 1 ohm sense resistor
resulting in higher peak current transients being driven into the LEDs
than planned, 3) too high a gain of dIleds per unit percentile of duty
cycle corrupting the PIC control algorithm into a hunting condition or
resulting in a faulted steady state which is an alias of a fault
condition. This last may require that you employ some sort of dual
modulus PWM scheme for the duty cycle generation to obtain better
resolution on your control. After reading your other replies, it looks
like your sample rate is too low by a factor of ten. There is such a
thing as an effective Nyquist criteria that must be satisfied for
applications such as this.
 
D

default

Jan 1, 1970
0
Some of you may recall my discussions about a 12 VDC powered switching
converter to drive strings of 7 or 13 high power white LEDs, in posts from
1/5/07 (transients), 2/22/07 (Inductor gaps), and 3/10/07 (TC1410 driver
problems). We built the circuit, found a few problems, then tested it and
it seemed OK. Basically this is a switching boost regulator using a 10 uH
inductor and a PIC with PWM at 100 kHz. I did a lot of LTSpice simulations
as well.

The circuit is somewhat unique in that the PIC is programmed to produce
alternate high and low brightness modes, at about 700 and 100 mA, when
power is applied and removed. The PWM is controlled by the voltage on a 1
ohm sense resistor, and it is designed to ramp up slowly and cut back
quickly. There are also undervoltage input and overvoltage output sensing
circuits to shut down the PWM until the next power on reset.

My friend had three different 7-LED clusters (made by Cree), of low,
medium, and high brightness. While testing the medium-brightness cluster,
he noticed a less than normal difference in brightness, and found that two
of the LEDs were not lit. They tested open, but apparently the voltage was
enough to bridge the gap and provide a current path for the remaining 5 in
series.

The manufacturer examined the LEDs and stated: "#1 appeared to have an
evidence of damaged chip which looked to be a burnt metal traces around the
bond pads, it also had a blown wire bond on the Anode side which had an
appearance of a fused wire. The lamp #4 had the blown off bond pads on the
chip side." They concluded it was a high current surge.

My recommendation to my friend was to perform a deliberate destructive test
on one or two lamps to see what level of current would cause such damage. I
think he had previously tried up to 1.5 or 2 amps for a few seconds without
sign of damage, although possibly not with these exact model LEDs.

Without going into more detail for the sake of brevity, I would like to
discuss possible scenarios, testing suggestions, and design improvements to
minimize the risk of more failures in the field. Terry Given was kind
enough to analyze my circuit and layout, with many suggestions for
improvement, and I plan to implement them in the next iteration of the
design. However, someone else was adamant that a PIC would never be as
reliable as a pure analog design, and suggested an off-the shelf Linear
Tech or other part, which would involve a complete redesign and loss of the
dual brightness feature.

Thanks,

Paul
Hmm . . . saw something similar in my own experience. Bought Cree one
watt leds and connected them in series and blew one up - linear supply
with a dropping resistor.

Problem turned out to be the little aluminum heat spreader pad they
were mounted to and multi layer board. I used something like a #6
machine screws to fasten it to the heat sink and the screw threads
contacted an exposed bit of conductor on the white insulating board.
Apparently the copper multi layer board uses large traces to aid heat
removal and there's no provision in manufacture to prevent the edge of
the copper from contacting mounting screws. Once the screws were
tightened down the copper deformed and made a permanent short to the
heat sink - the problem was solvable in the others (that weren't
destroyed) by taking a drill bit and reaming the hole a little to
break the bond, then using smaller insulated screws.

Check for shorts to the heat sink?
 
A

Al

Jan 1, 1970
0
is device to device parameter variations being accounted for?

most of these LED's are not anywhere near being matched pairs/triples or
other, unless you get the manufacturer to custom mount them from the same
die and then do a parametric test on each and then sort them and mark them,
(Very costly)

even clusters are assembled from random devices!

MIL specs may offer you some leeway, but even controlled groups will be bulk
tested only for basic params.

if spiked current is failing some devices, they may have beeen borderline to
begin with, as most will just heat up and dim before catastrophic failure
occurs. usually over time.

your blown bonds and similar symptoms tells me that you had EXTREME current.

rise time between units may force some to go into failure befor the adjacent
units absorb the available current also.

as with all design, the simplest is the best. trying to parlay your
education of PICs and neatsy circuits willl yield expensive and hard to
manufacture systems.

take some lessons from the chinese,tiawanese and others, cheap simple and
basic.

stay within some easy set standard limitations and you may have good
results.

Amen!

Many years ago I did some analysis one a power switching circuit which
used two transistors in the primary of a center tapped transformer. The
transistors would switch alternately to produce a square wave on the
secondary which was then rectified to produce a higher voltage.

Modules which failed had one blown transistor on them. As it was blown,
it was not possible to read the parameters. Analysis of stock
transistors showed that their characteristics, although within spec,
varied. When the characteristics of the transistors were matched and
they were used in matched sets, the problem disappeared.

The problem disappeared until a couple of years later, when a new
manufacturing guru wondered why they were going through the expense of
checking all the transistors. He abolised the practice, and voila,
problem reappeared.

Al
 
J

Jim Thompson

Jan 1, 1970
0
[snip]
Many years ago I did some analysis one a power switching circuit which
used two transistors in the primary of a center tapped transformer. The
transistors would switch alternately to produce a square wave on the
secondary which was then rectified to produce a higher voltage.

Modules which failed had one blown transistor on them. As it was blown,
it was not possible to read the parameters. Analysis of stock
transistors showed that their characteristics, although within spec,
varied. When the characteristics of the transistors were matched and
they were used in matched sets, the problem disappeared.

The problem disappeared until a couple of years later, when a new
manufacturing guru wondered why they were going through the expense of
checking all the transistors. He abolised the practice, and voila,
problem reappeared.

Al

Of course there was no "engineer" there to ponder WHY? And fix the
frigging problem the right way. Selecting parts is the weenie's way
of "engineering :-(

...Jim Thompson
 
F

Fred Bartoli

Jan 1, 1970
0
Al a écrit :
Amen!

Many years ago I did some analysis one a power switching circuit which
used two transistors in the primary of a center tapped transformer. The
transistors would switch alternately to produce a square wave on the
secondary which was then rectified to produce a higher voltage.

Modules which failed had one blown transistor on them. As it was blown,
it was not possible to read the parameters. Analysis of stock
transistors showed that their characteristics, although within spec,
varied. When the characteristics of the transistors were matched and
they were used in matched sets, the problem disappeared.

The problem disappeared until a couple of years later, when a new
manufacturing guru wondered why they were going through the expense of
checking all the transistors. He abolised the practice, and voila,
problem reappeared.

Bad design to start with.
Bad answer to go on: for the cost of matching, you could easily pay for
the components cost to fix the problem.
 
F

Fred Bartoli

Jan 1, 1970
0
Paul E. Schoen a écrit :
Winfield Hill said:
Paul said:
power is applied and removed. The PWM is controlled by the voltage
on a 1 ohm sense resistor, and it is designed to ramp up slowly and
cut back quickly. [ snip, or should I say snap? ]
Hmm, are you trying to maximize thermal-gradient stresses?

I was trying to minimize the possibility of overshoot. In the simulation,
with 12 VDC input, an 80% duty cycle produces a peak of 2 amps at 55 volts
in 600 uSec. The desired 700 mA is reached in about 300 uSec, and the first
100 uSec is used to pump up the voltage of the 47 uF output capacitor. This
is with a simulated 13 LED cluster. The 7 LED cluster also takes about 600
uSec to reach a peak of 3.8 amps, and reaches 700 mA in about 250 uSec.

A 45% duty cycle, on the 7 LED cluster, reaches and holds 700 mA at 27
volts in about 210 uSec, and a 65% duty cycle PWM reaches and holds 700 mA
at 46 volts in about 1.2 mSec.

The circuit is certainly capable of generating enough current to damage the
LEDs, particularly the cluster of 7. However, I am reading the output
current at a 1 kHz rate, so I think I may have discovered the problem. The
PWM starts at 0, and can only increment by about 1/63 per mSec, so it
should take at least 30 mSec for the PWM to reach the 45 or 65% level
needed for the target output current. However, if somehow the PWM is set
too high, it will take less than 600 uSec for the current to shoot up to as
much as 4 amps, which would probably cause the damage seen.

I could easily increase the sampling rate, at least for the output current,
to 100 uSec, as the conversion time is 51 uSec. It seemed like the slow
rise of PWM would have been enough, but this is very likely the culprit.

Another simple thing I could do is generate an interrupt when my proposed
NPN overcurrent sensor indicates more than 700 mA, and then shut down the
PWM (as well as lowering the duty cycle) until it clears. I will probably
want to change the sense resistor to about 0.68 ohms so that it will only
trip over about 1 ampere, which is still safe. The same logic signal could
be tied to the enable line of the driver as well.

OK, back to the drawing (and coding) board...

What's your circuit? In details...

With 7/13 leds you're having about 28V/52V from your strings of white leds.
Your circuit is a boost, so you're reasonably safe from some POV, except
this:
at 100kHz, L=10uH, E=12V, and Io = 700mA you're in CC mode with:
@ Vout=28V : duty ratio= 0.57, Imin=1.3A, Imax=2A => Irms=1.1A
@ Vout=52V : duty ratio= 0.77, Imin=2.6A, Imax=3.5A => Irms=1.5A

depending on how you've filtered (which kind of bypass cap) your output
the led might see a big chunk of the pulsating current: the 47uF may not
be as good as you think and the dynamic impedance of your leds will be
very low at 700mA.
Thus the Leds bonding might really see an effective 1.1/1.5A rms, not
even speaking of any regulation problems involved by the low loop
crossover frequency (for ex. what will happens if you have a small +0.5V
step on your supply once the circuit is running full power?).

And why messing with a pic (everything implying a pic is a mess, anyway)
while you could easily do this with a cheap dedicated SMPS controller?
 
Some of you may recall my discussions about a 12 VDC powered switching
converter to drive strings of 7 or 13 high power white LEDs, in posts from
1/5/07 (transients), 2/22/07 (Inductor gaps), and 3/10/07 (TC1410 driver
problems). We built the circuit, found a few problems, then tested it and
it seemed OK. Basically this is a switching boost regulator using a 10 uH
inductor and a PIC with PWM at 100 kHz. I did a lot of LTSpice simulations
as well.

The circuit is somewhat unique in that the PIC is programmed to produce
alternate high and low brightness modes, at about 700 and 100 mA, when
power is applied and removed. The PWM is controlled by the voltage on a 1
ohm sense resistor, and it is designed to ramp up slowly and cut back
quickly. There are also undervoltage input and overvoltage output sensing
circuits to shut down the PWM until the next power on reset.

My friend had three different 7-LED clusters (made by Cree), of low,
medium, and high brightness. While testing the medium-brightness cluster,
he noticed a less than normal difference in brightness, and found that two
of the LEDs were not lit. They tested open, but apparently the voltage was
enough to bridge the gap and provide a current path for the remaining 5 in
series.

The manufacturer examined the LEDs and stated: "#1 appeared to have an
evidence of damaged chip which looked to be a burnt metal traces around the
bond pads, it also had a blown wire bond on the Anode side which had an
appearance of a fused wire. The lamp #4 had the blown off bond pads on the
chip side." They concluded it was a high current surge.

My recommendation to my friend was to perform a deliberate destructive test
on one or two lamps to see what level of current would cause such damage. I
think he had previously tried up to 1.5 or 2 amps for a few seconds without
sign of damage, although possibly not with these exact model LEDs.

Without going into more detail for the sake of brevity, I would like to
discuss possible scenarios, testing suggestions, and design improvements to
minimize the risk of more failures in the field. Terry Given was kind
enough to analyze my circuit and layout, with many suggestions for
improvement, and I plan to implement them in the next iteration of the
design. However, someone else was adamant that a PIC would never be as
reliable as a pure analog design, and suggested an off-the shelf Linear
Tech or other part, which would involve a complete redesign and loss of the
dual brightness feature.

Thanks,

Paul

Just so I'm reading this correctly, you are not paralleling strings of
leds. That is, you only drive one string of leds. If that is not the
case, I would expect problems.

When I think of the effort it takes to make a bullet-proof DC/DC chip,
I just shake my head at the idea of doing it in software. In a chip,
events take place simultaneously, while a uP is a step at a time using
polling.
 
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