What is best for LED dimming - constant voltage or constant current?
CV, CL, CC methods of LED dimming
ATX LED introduces: CL dimming
Background:
CV is a dimmable voltage drop intolerant design in our vocabulary- typically resistors. Because people have been using resistors in CV fixtures for a long time. The result is Voltage Drop intolerance, for example, fan speed changes significantly in a 24v CV fan, as a function of wire length. Strip ends are less bright than the beginning. CV @ 24v waste about 20% of the energy as heat.
CC is current controlled in our vocabulary. No drivers - bare LEDs. no loss.
CL is a device, that delivers a limited, and substantially the same, current to every fixture down to a voltage that is the expected remote worst case voltage in our projects. Budgeting 4% voltage drop. CL has high efficiency - we target 8% heat loss.
CL can have a BJT / FET design, or be a buck switcher - depending on wattage totals.
CL are also voltage dimmable, or PWM dimmable, without current spiking. The voltage dimming performance in CL ( like CV ) is inconsistent - lights will to be noticeably different brightness as the dimming goes below 10%. PWM dimming is to be avoided - because of the wire length.
CLS is a variation of CL for individual light fixtures with more than 10 watts - these are voltage dimmable and highest efficiency.
DCDC ( the worst )
There is another technology - that needs to be added to the vocabulary - these are DCDC fixtures - for example - Amazon 24v AC/DC E26 bulbs. These have an inverse VI curve, high inrush, and can put a power supply into flashing mode on startup. A 9w 24v DCDC bulb, requires 375mA at 24v, but over 1 amp at 10v, causing many many problems. They are undimmable, waste a lot of energy, and fail quickly.
MEPT
Many people are familiar with MPPT as used in Solar PV installations - it aims to get the ideal point of load vs capacity in the VI curve of a Solar panel. Well proven and vital technology
ATX LED applies this concept to LED lighting loads
An LED fixture has a VI curve. When I get time, I will create 5 curves
DCDC, CV, CC, CL, CLS
MEPT applies to CL fixtures - and allows extremely low cost fixtures to hit the performance of CC and CLS designs, at up to 100 watt Class 2 levels.
the details of MEPT will be captured in a patent. MEPT allows the highest number of fixtures to have the greatest lifetime sharing the fewest number of power supplies with tolerance for voltage drop.
Topics
- Bare LED, LEDs with resistors,
- LEDs with DC drivers inside ( 12v to 24v operating range) are not dimmable
Note - DC LEDs with an operating range of 12 to 24v cannot be dimmed by either CCR or PWM - if the operating range is greater than 1 to 1.5, the LED includes a DC-DC converter which makes dimming extremely unreliable. - efficiency
- Safety
- Turn on Delay
- Surge current at Power ON
Dimming LEDs via PWM and CCR Application Note #360 Revision C November 2016
For constant-current LED drivers, there are two mechanisms for dimming: pulse-width modulation (PWM) and constant current reduction (CCR). This application note will explain the difference between the two methods, allowing you to select the proper approach for the application. Note: Constant-voltage LED drivers are prim arily PWM dimming drivers and will not be addressed in this document.
Every LED has a rated current, which is the amount of current that needs to flow to get the maximum light output.
Comment: Lutron misrepresents how LEDs function;
A more correct statement is: Rated current is the point at which the maximum lifetime of the LED is expected, at an acceptable operating temperature. It is neither max light output nor max efficiency - it is a tradeoff of lifetime and usable light.
In a PWM driver, the current is switched at a high frequency between 0 and the rated output current. This means the LED load is either off or running at its rated current. The ratio of on time to off time determines the LED brightness. See the following diagram for an example of an LED that is dimmed to approximately 25% using PWM. The 25% level is a result of the current flowing for 25% of the time, then being turned off for the remaining 75% of the time.
- Constant Current Reduction (CCR)
In a CCR supply, the current flows continuously at a set amount for a given light level. Since the amount of light output is proportional to the current flowing through the LED, the current is reduced to reduce the brightness of the LED. See the following diagram for an example of an LED that is reduced to approximately 25% using CCR. Sometimes this type of dimming is referred to as “Analog Dimming”.
Use of both CCR and PWM based drivers will have the desired effect of dimming the lights. However, other factors come into play when each method is used.
Typically, LED specification sheets provide a particular characteristic, such as color or efficiency (in lumens per watt), at a particular forward current. These characteristics will vary based on the forward current. For example, the color temperature may be 3000 K at 700 mA of current, but only 2700 K at 350 mA.
ATX Comment: This statement is not supported by the data sheets of Cree, Bridgelux and other LED manufacturers, indeed, this article points out that the color shift is in the opposite direction ( the light becomes cooler at low current) by a factor of about 1%.
A PWM driver will only run the LEDs at the rated current level or zero, preventing these characteristics from changing as the load is dimmed. Practically, this means the same color temperature is maintained throughout the dimming range.
ATX Comment: This statement is not supported by material science.
PWM based drivers can provide a very precise output level, since the LEDs are always on at the same current level. Changing the operating current linearly, as done with CCR, may not result in a linear change in light output.
ATX Comment: the color shift is both insignificant,and barely perceptable according to published studies.
For devices that need to be rated as UL Class 2 for dry or damp locations, there is a lower output voltage limit for Class 2 devices that use PWM (24.8 V- for PWM frequencies between 10 Hz and 200 Hz) than those that use CCR (60 V-). Therefore, a Class 2 power supply that uses PWM will not be able to provide as much voltage as one that uses CCR.
ATX Comment: this is a very significant advantage - it means that we can use wire with 1/6 the amount of copper for the same delivered power - loss is related to the current squared so 60v vs 24v means 6.25 less loss allowing wire with 6x less mass to deliver the same energy
By definition, PWM power supplies have to run at frequencies high enough to be imperceptible to the human eye. The higher the frequency, the less likely someone is to observe flicker. Frequencies below 200 Hz may be observable in peripheral vision, and even higher frequencies are required to eliminate stroboscopic effects in fast motion environments. However, higher-frequency power supplies are generally more complex and expensive to manufacture, especially when low light levels are desired. Other programs, such as Energy Star, may place minimum requirements on PWM frequency.
Because of the fast rising and falling edges of a PWM driver (faster edges allow for higher frequencies and lower light levels), electromagnetic interference (EMI) can be generated. This EMI may not be suitable for certain applications.
ATX Comment: Transmitting PWM at any frequency turns the wire into a very long antenna - a room with 50 watts of LED at PWM rates will transmit an unacceptable amount of EMI in any home and do so over a very wide range of frequencies.
PWM drivers, due to their fast edges, may experience performance issues if mounted remotely from the light source. This is because the electrical characteristics (capacitance and inductance) of the long wire runs interfere with the fast rise and fall times required for precise light levels.
ATX Comment: Exactly for this reason - PWM is not acceptable
- Applications Suitable for PWM
Fixtures that must be dimmed lower than 40% and still maintain consistent color
ATX Comment: - not true
Color mixing applications, because of their need for precise levels of each color
ATX Comment: - not supported by material science
- Applications Suitable for CCR
Fixtures requiring a UL Class 2 rated output with an output voltage higher than the UL Class 2 PWM voltage level
Applications where long wire runs may exist between the driver and the light engines and high performance dimming is required
Applications that may have strict EMI requirements, such as medical suites
ATX Comment: - or people's homes
Applications with high motion activity or rotating machinery
ATX Comment: - or anyone taking a birthday video. You can see PWM artifacts in smartphone videos easily
Both PWM and CCR drivers can be used to adequately dim LED loads. However, each method has advantages that may make it more suitable for a particular application.
Lutron drivers can be ordered as either PWM or CCR modes for the ultimate in LED dimming flexibility and performance. Note: Lutron PWM drivers do not operate within the range of 10 Hz to 200 Hz and can go up to 60 V- while maintaining the Class 2 markings.
Safety
All CCR is operated at the voltage of the LED. Using series connected LEDs of up to 42 volts total for the series string, keeps the peak voltage low, the currents are less and overall safety is improved - especially compared to 120 VAC
Using CCR and perfect flicker free dimming - means that more lights in the house will be used at dimming levels below max - this saves a lot of energy compared to houses with 120vac wiring that offer no dimming when the house is built. Also - operating at the highest DC voltage allowed by code, allows thinner wire to be used saving copper.
LED with built in resistorsto be added, with photo.
Turn On Delay
to be added,
Surge Current
undimmable LED
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