treez
Computer
- Jan 10, 2008
- 87
Hello,
I have been working on Domestic Mains White LED Lighting (for sale to the cheap-end, mass market) for a while.
I have been working at the 15W level as this is equivalent luminous output to a 60W incandescent.
However, there are multiple problems with LED lights here, and in fact fluorescents are far better when all is accounted for.
Fluorescent bulbs driven by a high-frequency electronic ballast have a significantly greater efficiency (110 Lumens per Watt) than LED Lamps (55 Lumens per Watt for affordable LEDs).
--------------------------------------------------------------------------------
The only significant disadvantages of Fluorescent Lamps, compared to LED Lamps is that…….
1) None of the fluorescent lamp components (ferrites, connectors, enclosure, ) can be re-cycled since the fluorescent lamp contains mercury.
2) Also, as stated, Fluorescent Lamps contain mercury and ploughing several hundred million used-up fluorescent bulbs into our landfill sites, year after year, may or may not cause a pollution problem.
-This shouldn’t be under-estimated as a potential fluorescent lamp show-stopper, since newly introduced RoHS regulations have banned mercury from any electronic devices, and here we are introducing billions of CFL’s, -each one containing ~4mg of Mercury, as a replacement for incandescent bulbs.
3) In theory, fluorescent bulbs have other problems, such as dim cold-start, poor repeated on/off ability etc, but these can be quite cheaply mitigated with cheap additional circuitry.
4) The extrusion of a fluorescent lamp’s glass bulb is a high energy process, and so production energy usage is higher for Fluorescents Lamps than LED Lamps.
–LED Lamps still use up significant energy in manufacture, since the metal LED heatsink needs to be made. –however, this component is re-cycle-able.
4) IN THEORY, fluorescent lamps don’t last as long as LED lamps.
-However, the fact is that in truth the life-time of cheap LED Lamps and Cheap Fluorescent Lamps is actually determined by the life-time of the electrolytic capacitors that such cheap lamps end up needing to contain.
In other words. Cheap Fluorescent Lamps and Cheap LED lamps have the SAME life-expectancy. –The life-expectancy of their electrolytics.
--------------------------------------------------------------------------------
It is point (4) that is the “deciding” factor that says that LED lamps just cannot compete with fluorescents…………
LED lamps have a greater need of electrolytics than fluorescent lamps.
Cheap LED lamps produce generally more ripple current in their electrolytics than cheap fluorescent lamps.
Take apart a 15W fluorescent CFL…..
You’ll see that it only needs one small electrolytic capacitor (A 3.3uF 400V Electrolytic with high ripple and temperature rating )
-This cap occurs after the mains diode bridge.
-It does NOT give good Voltage smoothing of the post diode bridge waveform, since this would just mean poorer power factor…..in fact, in UK, the DC Bus ripples from some 200 to 330V in a typical 15W CFL.
Now try and make a CHEAP mains LED Lamp with this small amount of electrolytic capacitance !
I guarantee that you will not manage.
It can be done, -but the solution becomes too expensive for the mass domestic lighting market. Even if you take away the (great) cost of the LEDs……..the electronics (SMPS) is more expensive for a cheap LED Lamp than a cheap fluorescent lamp.
--------------------------------------------------------------------------------
There are certain design constraints which must be imposed on a cheap 15W mains LED lamp…………..
a) Only 1 offline SMPS to be used (not two stages in cascade as its too expensive)
b) DC Bus voltage (post mains diode bridge) cannot be flat, it must have 100Hz voltage ripple on it of magnitude at least 100V (like in the CFL)
-This is because a flat DC Bus would mean too poor power factor, and even though 15W is below the statutory PFC level, there will be several 100 million of these lamps in each country, so an improved power factor is needed in reality.
c) Secondary must be isolated and must have a max voltage of no more than 42V. –Any more than 42V and it would be dangerous.
-Isolation on the secondary is essential, since the LEDs will have to be heatsink mounted, and parts of this heatsink will have to be exposed to free air on the outside of this cheap LED lamp, where human fingers can touch it.
…like the LED lamp heatsinks you can see here:-
d) LED current cannot be very rippley.
-If it is, then the average LED current will be down, and a further LED or two will have to be added to make up the power.
Since power LEDs are very expensive, adding another LED in to the lamp is just not an option.
The need to conform to the above constraints, as well as the fact that it must be cheap…………….points the designer to pretty much just ONE viable SMPS topology for a Cheap Mains LED Lamp…………….
That Topology is a Current-Mode, 1 Transistor Forward Converter with opto isolation.
Schematic of practical SMPS for cheap mains LED lamp:-
-The current regulation, has to be done by a current sense resistor in the secondary rectifier loop, -put it any other place and you loose bandwidth and your LED current gets too rippley.
The current sense resistor voltage has to be put into a compensated op-amp error amplifier, whose other input is a reference voltage.
The output of this error amplifier feeds the opto-coupler diode. The opto-coupler transistor acts on the PWM comparator to control the main primary FET.
The use of a forward converter means you need less secondary electrolytic capacitance because of the secondary inductor.
-Even so, a fairly significant amount of electrolytic capacitance is still needed because of the need to de-rate it so that it lasts a long time, whilst it hangs up there on the ceiling, -the hottest place in the house –also alongside those hot power LEDs.
Try to use just a 3.3uF, 400V Electrolytic following the mains diode bridge (like in the 15W CFL) and you run into instant problems.
–your primary ripple current is way too much in such a forward converter……….
-You can’t solve this problem by derating the input capacitance, since then you end up with a too_poor conduction angle on the mains diode bridge diodes, and a poor power factor.
-Your only option is then to use a passive “Valley Fill” Power Factor Corrector, which ups your component count but does allow you to sufficiently derate the input capacitance.
-Unfortunately, the “snap-off” (every 10ms) of the centre diode in the “Valley Filler” suddenly breaks the current in the inductance (ESL) of the “Valley Fill” electrolytics and causes a voltage spike and a worsening of Common-Mode-Noise……which you have to mitigate by keeping the capacitor charge current down with a resistor in series with the “Valley Fill ” electrolytics….this makes your efficiency worse.
-----------------------------------------
-Sounds bad, but Flybacks (CCM or DCM) are even more hopeless in this situation.
Flybacks need much more secondary electrolytic capacitance than forwards, which has to be well derated to keep the life-time of the LED lamp relatively high….which means using even more electrolytic capacitance………..and thus the size and expense of the SMPS becomes impractical.
Another unfortunate point about Power LEDs, is that even though they can be made with an illumination angle of 180 degrees…….-that is, sufficient angle of illumination to light-up an entire room………………the luminous intensity cannot be made to be equal over every ste-radian……………..and the LED’s illumination intensity falls to
about half the axial intensity at an angle of some 60 degrees from the axis.
-eg see page 18 of this white power LED datasheet…
This means that if your power LEDs point directly downwards from the ceiling, your room will not be evenly illuminated, and if you have just enough brightness in the axis, then it will be too dim on the periphery.
-You can get round this by having complicated, angled , mounting fixtures, but these will add significantly to the expense of the LED lamp.
-You can also get round this by having a diffuser, but then you need higher power LEDs to get the same luminous intensity, and your costs increase too much.
-----------------------------------------------
I believe the above shows that for the mass, cheap, domestic, white lighting market, Fluorescent Lamps are clear winners over LED lamps.
-----------------------------------------------
However, Professor Colin Humphreys, of Cambridge University, UK, claims to be able to provide us all with cheap White, GaN, Power LEDs of some 250+ Lumens Per Watt in about 2014.
-This would mean LED Lamps win over Fluorescents.
This kind of efficiency improvement would be too significant to ignore.
Until that hopeful day, Fluorescents come first…….At least for the Cheap, Mass Market, Domestic Mains White Lighting World.
I wonder if others agree?
I have been working on Domestic Mains White LED Lighting (for sale to the cheap-end, mass market) for a while.
I have been working at the 15W level as this is equivalent luminous output to a 60W incandescent.
However, there are multiple problems with LED lights here, and in fact fluorescents are far better when all is accounted for.
Fluorescent bulbs driven by a high-frequency electronic ballast have a significantly greater efficiency (110 Lumens per Watt) than LED Lamps (55 Lumens per Watt for affordable LEDs).
--------------------------------------------------------------------------------
The only significant disadvantages of Fluorescent Lamps, compared to LED Lamps is that…….
1) None of the fluorescent lamp components (ferrites, connectors, enclosure, ) can be re-cycled since the fluorescent lamp contains mercury.
2) Also, as stated, Fluorescent Lamps contain mercury and ploughing several hundred million used-up fluorescent bulbs into our landfill sites, year after year, may or may not cause a pollution problem.
-This shouldn’t be under-estimated as a potential fluorescent lamp show-stopper, since newly introduced RoHS regulations have banned mercury from any electronic devices, and here we are introducing billions of CFL’s, -each one containing ~4mg of Mercury, as a replacement for incandescent bulbs.
3) In theory, fluorescent bulbs have other problems, such as dim cold-start, poor repeated on/off ability etc, but these can be quite cheaply mitigated with cheap additional circuitry.
4) The extrusion of a fluorescent lamp’s glass bulb is a high energy process, and so production energy usage is higher for Fluorescents Lamps than LED Lamps.
–LED Lamps still use up significant energy in manufacture, since the metal LED heatsink needs to be made. –however, this component is re-cycle-able.
4) IN THEORY, fluorescent lamps don’t last as long as LED lamps.
-However, the fact is that in truth the life-time of cheap LED Lamps and Cheap Fluorescent Lamps is actually determined by the life-time of the electrolytic capacitors that such cheap lamps end up needing to contain.
In other words. Cheap Fluorescent Lamps and Cheap LED lamps have the SAME life-expectancy. –The life-expectancy of their electrolytics.
--------------------------------------------------------------------------------
It is point (4) that is the “deciding” factor that says that LED lamps just cannot compete with fluorescents…………
LED lamps have a greater need of electrolytics than fluorescent lamps.
Cheap LED lamps produce generally more ripple current in their electrolytics than cheap fluorescent lamps.
Take apart a 15W fluorescent CFL…..
You’ll see that it only needs one small electrolytic capacitor (A 3.3uF 400V Electrolytic with high ripple and temperature rating )
-This cap occurs after the mains diode bridge.
-It does NOT give good Voltage smoothing of the post diode bridge waveform, since this would just mean poorer power factor…..in fact, in UK, the DC Bus ripples from some 200 to 330V in a typical 15W CFL.
Now try and make a CHEAP mains LED Lamp with this small amount of electrolytic capacitance !
I guarantee that you will not manage.
It can be done, -but the solution becomes too expensive for the mass domestic lighting market. Even if you take away the (great) cost of the LEDs……..the electronics (SMPS) is more expensive for a cheap LED Lamp than a cheap fluorescent lamp.
--------------------------------------------------------------------------------
There are certain design constraints which must be imposed on a cheap 15W mains LED lamp…………..
a) Only 1 offline SMPS to be used (not two stages in cascade as its too expensive)
b) DC Bus voltage (post mains diode bridge) cannot be flat, it must have 100Hz voltage ripple on it of magnitude at least 100V (like in the CFL)
-This is because a flat DC Bus would mean too poor power factor, and even though 15W is below the statutory PFC level, there will be several 100 million of these lamps in each country, so an improved power factor is needed in reality.
c) Secondary must be isolated and must have a max voltage of no more than 42V. –Any more than 42V and it would be dangerous.
-Isolation on the secondary is essential, since the LEDs will have to be heatsink mounted, and parts of this heatsink will have to be exposed to free air on the outside of this cheap LED lamp, where human fingers can touch it.
…like the LED lamp heatsinks you can see here:-
d) LED current cannot be very rippley.
-If it is, then the average LED current will be down, and a further LED or two will have to be added to make up the power.
Since power LEDs are very expensive, adding another LED in to the lamp is just not an option.
The need to conform to the above constraints, as well as the fact that it must be cheap…………….points the designer to pretty much just ONE viable SMPS topology for a Cheap Mains LED Lamp…………….
That Topology is a Current-Mode, 1 Transistor Forward Converter with opto isolation.
Schematic of practical SMPS for cheap mains LED lamp:-
-The current regulation, has to be done by a current sense resistor in the secondary rectifier loop, -put it any other place and you loose bandwidth and your LED current gets too rippley.
The current sense resistor voltage has to be put into a compensated op-amp error amplifier, whose other input is a reference voltage.
The output of this error amplifier feeds the opto-coupler diode. The opto-coupler transistor acts on the PWM comparator to control the main primary FET.
The use of a forward converter means you need less secondary electrolytic capacitance because of the secondary inductor.
-Even so, a fairly significant amount of electrolytic capacitance is still needed because of the need to de-rate it so that it lasts a long time, whilst it hangs up there on the ceiling, -the hottest place in the house –also alongside those hot power LEDs.
Try to use just a 3.3uF, 400V Electrolytic following the mains diode bridge (like in the 15W CFL) and you run into instant problems.
–your primary ripple current is way too much in such a forward converter……….
-You can’t solve this problem by derating the input capacitance, since then you end up with a too_poor conduction angle on the mains diode bridge diodes, and a poor power factor.
-Your only option is then to use a passive “Valley Fill” Power Factor Corrector, which ups your component count but does allow you to sufficiently derate the input capacitance.
-Unfortunately, the “snap-off” (every 10ms) of the centre diode in the “Valley Filler” suddenly breaks the current in the inductance (ESL) of the “Valley Fill” electrolytics and causes a voltage spike and a worsening of Common-Mode-Noise……which you have to mitigate by keeping the capacitor charge current down with a resistor in series with the “Valley Fill ” electrolytics….this makes your efficiency worse.
-----------------------------------------
-Sounds bad, but Flybacks (CCM or DCM) are even more hopeless in this situation.
Flybacks need much more secondary electrolytic capacitance than forwards, which has to be well derated to keep the life-time of the LED lamp relatively high….which means using even more electrolytic capacitance………..and thus the size and expense of the SMPS becomes impractical.
Another unfortunate point about Power LEDs, is that even though they can be made with an illumination angle of 180 degrees…….-that is, sufficient angle of illumination to light-up an entire room………………the luminous intensity cannot be made to be equal over every ste-radian……………..and the LED’s illumination intensity falls to
about half the axial intensity at an angle of some 60 degrees from the axis.
-eg see page 18 of this white power LED datasheet…
This means that if your power LEDs point directly downwards from the ceiling, your room will not be evenly illuminated, and if you have just enough brightness in the axis, then it will be too dim on the periphery.
-You can get round this by having complicated, angled , mounting fixtures, but these will add significantly to the expense of the LED lamp.
-You can also get round this by having a diffuser, but then you need higher power LEDs to get the same luminous intensity, and your costs increase too much.
-----------------------------------------------
I believe the above shows that for the mass, cheap, domestic, white lighting market, Fluorescent Lamps are clear winners over LED lamps.
-----------------------------------------------
However, Professor Colin Humphreys, of Cambridge University, UK, claims to be able to provide us all with cheap White, GaN, Power LEDs of some 250+ Lumens Per Watt in about 2014.
-This would mean LED Lamps win over Fluorescents.
This kind of efficiency improvement would be too significant to ignore.
Until that hopeful day, Fluorescents come first…….At least for the Cheap, Mass Market, Domestic Mains White Lighting World.
I wonder if others agree?
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