Comments for Engineering Connections http://engineeringconnections.com/blog Fairchild Semiconductor's blog Sat, 31 Jul 2010 11:23:29 +0000 http://wordpress.org/?v=2.7 hourly 1 Comment on Lifetime of an LED by Kerim Fahme http://engineeringconnections.com/blog/2009/07/02/lifetime-of-an-led/comment-page-1/#comment-364 Kerim Fahme Thu, 03 Sep 2009 12:12:50 +0000 http://engineeringconnections.com/blog/?p=64#comment-364 A pixel, in economical monocolor LED moving info panels, is usually formed by more than one LED. So based on what Mr Adithyan has pointed out earlier and for the static drive mode, driving LEDs of a pixel in series as possible decreases appreciably the track heat around the LED pins (for the same width). Less heat gives a more chance for a longer life. But this trend will likely need the addition of a voltage regulator to supply the driver ICs seperately. For instance, connecting LEDs in series is no more risky due the uniform high grade quality that most new LEDs are produced. A pixel, in economical monocolor LED moving info panels, is usually formed by more than one LED.
So based on what Mr Adithyan has pointed out earlier and for the static drive mode, driving LEDs of a pixel in series as possible decreases appreciably the track heat around the LED pins (for the same width). Less heat gives a more chance for a longer life. But this trend will likely need the addition of a voltage regulator to supply the driver ICs seperately.
For instance, connecting LEDs in series is no more risky due the uniform high grade quality that most new LEDs are produced.

]]> Comment on Lifetime of an LED by adithyan http://engineeringconnections.com/blog/2009/07/02/lifetime-of-an-led/comment-page-1/#comment-304 adithyan Sat, 29 Aug 2009 01:48:10 +0000 http://engineeringconnections.com/blog/?p=64#comment-304 Sir, I find in many LED display boards, the track is very narrow which means heating of the track which also generates heat and if enclosed in a tight environment, lack of ventilation can cause degradation leading to ultimate failure which will be baffling. As a person in the LED lighting side working on 5mm mostly, I keep the tracks as 50 mills which is considered sufficient for a load of 500 ma without heating. Providing a transparent acrylic cover affects light (light deviates from lighter to denser medium and back to lighter medium)as well as creating suffocating heat for the LEDs, I keep ventilation without any acrylic front cover. Also, the LEDs must be operated around 18 ma only and in case of LEDs with lesser mcd value, no doubt, emitted light will be less. Moral, adopt healthy practices for the longevity of leds Sir,

I find in many LED display boards, the track is very narrow which means heating of the track which also
generates heat and if enclosed in a tight environment, lack of ventilation can cause degradation leading to ultimate failure which will be baffling.

As a person in the LED lighting side working on 5mm mostly, I keep the tracks as 50 mills which is considered sufficient for a load of 500 ma without heating. Providing a transparent acrylic cover affects light (light deviates from lighter to denser medium and back to lighter medium)as well as creating suffocating heat for the LEDs, I keep ventilation without any acrylic front cover.
Also, the LEDs must be operated around 18 ma only and in case of LEDs with lesser mcd value, no doubt, emitted light will be less. Moral, adopt healthy practices for the longevity of leds

]]> Comment on Lifetime of an LED by Kerim Fahme http://engineeringconnections.com/blog/2009/07/02/lifetime-of-an-led/comment-page-1/#comment-303 Kerim Fahme Sat, 29 Aug 2009 00:20:59 +0000 http://engineeringconnections.com/blog/?p=64#comment-303 Assuming an LED is driven to get the same brightness by using a pulsed and constant current (as in scan and constant drive modes), its lifetime seems to be longer to a good extent in case of the constant current. Assuming an LED is driven to get the same brightness by using a pulsed and constant current (as in scan and constant drive modes), its lifetime seems to be longer to a good extent in case of the constant current.

]]> Comment on Why did my FET fail? by Craig Ranta http://engineeringconnections.com/blog/2009/08/25/why-did-my-fet-fail/comment-page-1/#comment-299 Craig Ranta Thu, 27 Aug 2009 20:51:04 +0000 http://engineeringconnections.com/blog/?p=186#comment-299 Dr. Engleberry left out another cause that I have experienced...ESD wounded parts. Althout this is not as common today as it was in the old days; a good ESD zap can seem to have no effect on the operation of a FET but can eventually lead to premature infarction of the gate insulator. Also, regarding slowing down the gate charging rate to reduce EMC, this is of course a second choice to reducing the drain-source circuit current loop area. But, if you are like most engineers in large companies, you are often commanded to fix the problem without changing anything, so sometimes a band-aid is called for to get that good review score and move to another group as quickly as possible. Dr. Engleberry left out another cause that I have experienced…ESD wounded parts. Althout this is not as common today as it was in the old days; a good ESD zap can seem to have no effect on the operation of a FET but can eventually lead to premature infarction of the gate insulator. Also, regarding slowing down the gate charging rate to reduce EMC, this is of course a second choice to reducing the drain-source circuit current loop area. But, if you are like most engineers in large companies, you are often commanded to fix the problem without changing anything, so sometimes a band-aid is called for to get that good review score and move to another group as quickly as possible.

]]> Comment on Why did my FET fail? by Ken Coffman http://engineeringconnections.com/blog/2009/08/25/why-did-my-fet-fail/comment-page-1/#comment-294 Ken Coffman Wed, 26 Aug 2009 20:51:22 +0000 http://engineeringconnections.com/blog/?p=186#comment-294 Hello Glen. Often a small resistor is used to slow down the turn-on and turn-off times and thus limit excitation of the parasitic resonances and reduce ringing (EMI), but 1000 ohms would be way too large. I've seen as much as 100 ohms used, but I think that's too large. I'm glad Dr. Engleberry is sleeping in right now because he'd give you a differential equation to solve just to punish you. The best solution is a great layout to minimize parasitics, then a snubber. Of course, as FET manufacturers, we'll always recommend something that makes the FET run as cool as possible. Slowing down the turn-on and turn-off makes the FET run in the linear region longer and increases switching losses. Hello Glen. Often a small resistor is used to slow down the turn-on and turn-off times and thus limit excitation of the parasitic resonances and reduce ringing (EMI), but 1000 ohms would be way too large. I’ve seen as much as 100 ohms used, but I think that’s too large. I’m glad Dr. Engleberry is sleeping in right now because he’d give you a differential equation to solve just to punish you. The best solution is a great layout to minimize parasitics, then a snubber. Of course, as FET manufacturers, we’ll always recommend something that makes the FET run as cool as possible. Slowing down the turn-on and turn-off makes the FET run in the linear region longer and increases switching losses.

]]> Comment on Why did my FET fail? by Glen Tennison http://engineeringconnections.com/blog/2009/08/25/why-did-my-fet-fail/comment-page-1/#comment-291 Glen Tennison Wed, 26 Aug 2009 16:36:58 +0000 http://engineeringconnections.com/blog/?p=186#comment-291 Dear Dr. FAE, In my experience the gate drive resistor is sometimes raised to a higher value to help pass compliance tests. The rise and fall times I am sure are now much faster but this can also lead to much higher radiated emsisons from hitting the FET with a sharper slew rate. Is there a trade off or a better way to control this problem? Thanks! Dear Dr. FAE,
In my experience the gate drive resistor is sometimes raised to a higher value to help pass compliance tests. The rise and fall times I am sure are now much faster but this can also lead to much higher radiated emsisons from hitting the FET with a sharper slew rate.
Is there a trade off or a better way to control this problem?
Thanks!

]]> Comment on Lifetime of an LED by dave lau http://engineeringconnections.com/blog/2009/07/02/lifetime-of-an-led/comment-page-1/#comment-266 dave lau Wed, 19 Aug 2009 08:48:23 +0000 http://engineeringconnections.com/blog/?p=64#comment-266 I heard that capacitor is a key component to affect on the lifetime of LED products. I think both CFL, LFL & LED also have light output degradation. I heard that capacitor is a key component to affect on the lifetime of LED products. I think both CFL, LFL & LED also have light output degradation.

]]> Comment on EMI: Back to the Drawing Board - Again! by Kevin Lindsay http://engineeringconnections.com/blog/2009/05/29/emi-back-to-the-drawing-board-again/comment-page-1/#comment-9 Kevin Lindsay Mon, 08 Jun 2009 22:19:09 +0000 http://engineeringconnections.com/blog/?p=34#comment-9 My experience with EMI and class-d audio circuits has left me with some memorable encounters. An EMI complaint from a customer designing a 100W subwoofer using a switching amp for the first time prompted a trip to taiwan. When I saw that the design used a wooden box for shielding I knew I had my work cut out for me. After explaining the concept of a Faraday shield and port decoupling to this engineer I left him to spin his design. This solution was the brute force method and would have worked. One year later I returned to see that the customer was still trying to pass FCC with a wooden box......... Years later EMI is now a concern in every consumer application. Well described application notes and reference designs with layout guidelines go a long way if the customer is willing to listen to your suggestions. When I hear auto-route I think wooden box. My experience with EMI and class-d audio circuits has left me with some memorable encounters. An EMI complaint from a customer designing a 100W subwoofer using a switching amp for the first time prompted a trip to taiwan. When I saw that the design used a wooden box for shielding I knew I had my work cut out for me. After explaining the concept of a Faraday shield and port decoupling to this engineer I left him to spin his design. This solution was the brute force method and would have worked. One year later I returned to see that the customer was still trying to pass FCC with a wooden box………

Years later EMI is now a concern in every consumer application. Well described application notes and reference designs with layout guidelines go a long way if the customer is willing to listen to your suggestions.

When I hear auto-route I think wooden box.

]]> Comment on EMI: Back to the Drawing Board - Again! by Bill Llewellyn http://engineeringconnections.com/blog/2009/05/29/emi-back-to-the-drawing-board-again/comment-page-1/#comment-8 Bill Llewellyn Mon, 08 Jun 2009 18:55:44 +0000 http://engineeringconnections.com/blog/?p=34#comment-8 Any onlookers who have experience in edge rate control for EMI mitigation? I'm currently on a class-D amplifier project where output edge rate control is a required option to be competitive. Interesting design challenge. Any onlookers who have experience in edge rate control for EMI mitigation? I’m currently on a class-D amplifier project where output edge rate control is a required option to be competitive. Interesting design challenge.

]]> Comment on EMI: Back to the Drawing Board - Again! by Ken Coffman http://engineeringconnections.com/blog/2009/05/29/emi-back-to-the-drawing-board-again/comment-page-1/#comment-7 Ken Coffman Wed, 03 Jun 2009 16:07:07 +0000 http://engineeringconnections.com/blog/?p=34#comment-7 EMI fixes are generally easy and free to fix during the initial circuit andf PWB design (keep loop areas small...keep high speed digital signals away from sensitive analog circuits...don't run that clock over a slot in the ground plane, ha!) but maddening and expensive after the PWB and mechanical design is locked down. It's an area where the Art of Electronics is in play. If you intuitively and instinctively "enjoy" minimizing current loops and keeping loud and noisy signals away from the areas that should be quiet...you're way ahead of the game. EMI fixes are generally easy and free to fix during the initial circuit andf PWB design (keep loop areas small…keep high speed digital signals away from sensitive analog circuits…don’t run that clock over a slot in the ground plane, ha!) but maddening and expensive after the PWB and mechanical design is locked down. It’s an area where the Art of Electronics is in play. If you intuitively and instinctively “enjoy” minimizing current loops and keeping loud and noisy signals away from the areas that should be quiet…you’re way ahead of the game.

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