A long time ago I promised I would do a topic to explain to the interested layman the difference between Linear, Buck, Boost and Direct Drive drivers. Well, here it is.
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Direct Drive
This is the simplest and cheapest design. As its name implies, it is a direct path to the LED, just like old incandescent flashlights. Sometimes there is a control chip and transistor to create modes (High, Low, Strobe, etc.) by rapidly switching ON and OFF the LED. That’s called PWM.
This is used by people who want to make extremely powerful flashlights that pull 6A or even more.
It is also used in cheap flashlights, but they add a resistor that will limit the current to a lower value. This is not good as the current will vary with the battery voltage. (Same as incandescent, but worst because of the LED)
In a direct drive flashlight, the battery voltage must be equal or higher than the LED voltage (An alkaline or NiMH battery can’t power a 3V LED using a direct drive driver).
Advantages:
Cheap
Disadvantages:
Current is varying greatly with battery voltage!
If a resistor is added to limit the current, the efficiency may be lower than a linear driver.
Voltage of the battery must be higher than the LED voltage (~3V) but not too much above. You can’t use this if you have multiple batteries in series.
Linear
This is the equivalent of a direct drive flashlight with a resistor to limit current… But smarter.
The only difference is that the resistor value is constantly adjusted to make a constant current. How is that possible? Well, that’s the job of the linear regulator. Most drivers use chips as linear regulators. Each chip will supply a constant 0.35A to the LED. A driver with four chips will supply a constant 1.4A to the LED.
This is also a very simple design. You can see the controller (that produces modes) surrounded by chips.
Don’t forget! Even though this is slightly smarter, it’s basically still a resistor to limit the current! This can never boost a lower voltage to give the LED the voltage it needs! That’s usually 3 - 3.3V.
This is the perfect driver for flashlights using single LED and single lithium battery. The battery has a voltage of 3 to 4.2V and the LED about 3 to 3.3V. So for most of the discharge the current will be constant.
Quick maths: What is the efficiency of a Linear driver?
Well, the linear driver has a variable resistance that burns off any excess power to reduce the voltage to 3V for the LED. That means that with a fully charged battery the efficiency will be lower than when the battery is discharged.
Efficiency=VLED/VBattery
Fully charged: Efficiency=3.3V/4.2V=78%
Half discharged: Efficiency=3.3V/3.7V=89%
Almost discharged: Efficiency=3.3V/3.3V =100% (Approximation, not taking into account all the parasitic resistances.)
When the battery voltage becomes too low, the linear driver will reduce the resistance to its minimum, to power the LED until the end (but dimmer).
“Why can’t I use a linear driver with two lithium batteries in series??”
Let’s do the maths:
Efficiency=3.3V/7.4V=45%!!! More power is wasted in the driver than fed to the LED! That will reduce battery life and the driver will overheat. On top of that the chip will fail above 6V…
Advantages:
Simple
Robust
Efficient if used in a single cell single LED configuration
Constant current for most of the discharge of the battery
Disadvantages:
Voltage of the battery must be higher than the LED voltage (~3V) but not too much above. You can’t use this if you have multiple Li-ion cells in series.
Buck
This is also called a step down driver. and is part of the SMPS (Switched Mode Power Supply) family.
It is easily recognizable thanks to the big inductor. Sometimes the inductor is black.
This is a more complicated design. I won’t go into details as it is well explained on Wikipedia .
In a nutshell, it uses an inductor and capacitor to step down a voltage. Compared to a linear () driver, the Buck driver will have a fairly constant efficiency, even with a battery voltage much higher the LED voltage. It can be used to power a single LED with 2 or more batteries in series.
What about the efficiency? Well, it is usually between 75% and 90%. That depends on the quality of the design.
For info, this is wildly used in many devices (PC, TV, Smartphone, Tablet, etc.) because it is efficient.
Advantages:
Can be used with batteries that have a voltage much higher than the LED voltage. For example three lithium batteries in series will produce about 11V. In that case you need a Buck driver to drive an LED that needs ~3V.
Good efficiency
If well designed it can produce a true PWM less low mode. That’s good for sea sickness and for the LED efficiency. (More on that below)
Disadvantages:
Slightly more expensive
Voltage of the battery must be higher than the LED voltage (~3V).
Bulky
Boost
This is also called a step up driver. and is part of the SMPS (Switched Mode Power Supply) family.
It is easily recognizable thanks to the big inductor. Sometimes the inductor is black.
This is very similar the the Buck driver, but as its name implies, it will increase the voltage.
This is typically used in flashlights using one or two AA/AAA batteries. Two AA batteries in series will have a voltage varying between 2 and 3V. The LED needs ~3.3V so the voltage needs to be stepped up. There is no other solution!
I measured the efficiency of a single AA flashlight and typically found:
At the battery: Vin=1.2V ; Iin=2.2A
At the LED: Vled=3.2V ; Iled=0.35A
Let’s do some maths:
Efficiency=(Vled*Iled)/(Vin*Iin)=(3.2*0.35)/(1.2*2.2)=42%!!
That’s really bad! Well yes, but it’s hard to step up a voltage as low as 1.2V… The efficiency is better at 2.4V (2*AA). That means that if you choose a 2*AA flashlight you’ll get more than twice the runtime for the same brightness! That’s definitely something to consider.
It can also be used to power a string of LEDs (for example 3 LEDs in series will need about 10V) with a single lithium battery.
Advantages:
Can be used with batteries that have a voltage lower than the LED voltage.
If well designed it can produce a true PWM less low mode. That’s good for sea sickness and for the LED efficiency. (More on that below)
Disadvantages:
Slightly more expensive
Doesn’t work if the battery voltage is higher than the LED voltage.
Bulky
What about PWM?
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PWM means Pulse Width Modulation. It’s a way to control the brightness of a flashlight by rapidly switching it on and off. If it doesn’t switch rapidly enough (PWM frequency too low) it can be unpleasant to the eye. The picture above was taken while rapidly moving the flashlight to show the effect.
Something that is often overlooked is the fact that a light using PWM to make a low mode will be less efficient than one using a constant current. Why is that?
Well, as you can see above, the lumen output is not linearly dependant to the current. Let’s take an example:
Driver 1 is driving the LED at mA at full mode. This driver also has a medium mode that is a duty cycle of 50%. That means that half of the time the LED is OFF and it’s half of time ON. We will get half the lumen of the full mode: about 500 lumen. On average it’ll consume mA.
Driver 2 is also capable to drive at mA. When in medium mode however, it uses a constant current of mA. The average consumption is the same as Driver 1. The lumen output however will be about 600 lumen! That’s 20% more lumen.
All this to say that a flashlight that uses constant current on all modes will be more efficient!
Conclusion
If you have a flashlight with a single LED single lithium battery then get a Linear driver.
If you have a flashlight with one or two NiMH/Alkaline batteries, then you need a Boost driver.
If your battery voltage is much higher than the LED voltage then get a Buck driver
Choosing the right LED driver is crucial for the efficiency and service life of LED lighting. There are two basic variants: external and integrated LED drivers. But what exactly is the difference between them and which variant is better suited to certain applications? In this blog post, we go into the most important differences in detail and help you to find the optimum solution for your lighting requirements.
An LED driver is an electronic circuit that supplies LEDs with the correct voltage and current. LEDs require a constant power source to ensure that they light up stably and efficiently. Without a suitable driver, LEDs would either not work or fail quickly due to overload.
LED drivers are necessary to compensate for voltage fluctuations and ensure uniform luminosity. They protect the LEDs from damage caused by voltage peaks and enable various brightness and dimming functions. Depending on the design, LED drivers can be integrated either as an external component or directly into the LED luminaire. Both variants have their own advantages and disadvantages, which are explained in more detail below.
External LED drivers are separate units that are installed outside the actual LED luminaire. This design offers several advantages, particularly in terms of flexibility and maintenance.
An important advantage of external LED drivers is their interchangeability. If the driver is defective or a different output is required, it can be easily replaced without having to replace the entire luminaire. This reduces maintenance costs and extends the overall service life of the lighting unit.
External drivers also offer greater flexibility when selecting the appropriate power. Depending on the number and type of LEDs connected, a special driver can be selected that provides the exact power supply required. This is particularly advantageous in professional lighting applications where customized solutions are required.
However, external LED drivers require additional space for installation. This can be a disadvantage in certain environments, especially where space is limited or a minimalist design is desired. Nevertheless, this variant is often used in industrial, commercial and outdoor lighting applications where longevity and ease of maintenance are paramount.
Integrated LED drivers are built directly into the LED luminaire. This design enables a particularly compact and space-saving construction, making them ideal for many modern lighting solutions.
A key advantage of integrated drivers is the ease of installation. As the driver is already in the luminaire, no additional components need to be connected. This makes installation much easier and reduces the amount of cabling required, which is particularly beneficial for home lighting and smaller commercial applications.
Another advantage is the uniform design. As no external components are visible, luminaires with integrated drivers look more aesthetically pleasing and are particularly suitable for modern interior designs.
However, the biggest disadvantage of this design is the limited interchangeability. If the integrated driver fails, the entire luminaire often has to be replaced, which can lead to higher maintenance costs. This is particularly problematic when complex or permanently installed lighting systems are involved.
The service life of an LED system depends largely on the quality of the driver. High-quality external LED drivers generally have a longer service life than integrated versions, as they can be cooled better and are not affected by the heat of the LED itself.
Integrated LED drivers are often more compact, which can result in them operating at higher temperatures. This can shorten the service life and increase the likelihood of premature failure.
External drivers can also be installed in environments with optimal cooling conditions, which further increases their durability. If an external driver does fail, it is easier to replace without having to replace the entire luminaire. Integrated LED drivers, on the other hand, are more difficult to maintain as they are permanently installed and can often only be replaced by replacing the entire luminaire.
In terms of energy efficiency, there are high-quality variants with high efficiency for both external and integrated LED drivers. However, external drivers often offer more options for power control and adaptation to specific requirements.
Many external LED drivers have options for dimming and controlling light intensity. This enables precise adaptation of the lighting to different situations and saves energy when less light is required.
Integrated LED drivers can also be energy efficient, but often offer less flexibility in terms of control. In many cases, they are designed for a fixed output power, which limits their customization options.
The choice between external and integrated LED drivers depends heavily on the area of application. In professional and industrial environments where high performance, long service life and ease of maintenance are crucial, external LED drivers are often preferred. They offer more flexibility, are easier to replace and are often more durable.
For living spaces, smaller offices or decorative lighting, integrated LED drivers are often the better choice. They allow for a compact design and easy installation, making them ideal for modern interior designs.
In outdoor areas where LED luminaires are exposed to extreme weather conditions, external LED drivers with special protective housings are often used to ensure a longer service life.
Whether an external or integrated LED driver is more suitable depends on the individual requirements of the lighting application. External LED drivers offer greater flexibility, longer service life and easier maintenance, but require more space for installation. Integrated LED drivers, on the other hand, are compact, aesthetically pleasing and easy to install, but are more difficult to replace in the event of a fault.
For long-lasting and powerful lighting solutions, external LED drivers are often the better choice, especially in commercial and industrial applications. In living spaces or for decorative lighting, on the other hand, integrated LED drivers score points thanks to their space-saving design and ease of use.
Ultimately, the decision should be based on the specific requirements, the planned use and the maintenance conditions. Both variants have their raison d'être and offer different advantages depending on the area of application.
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