The Characteristics of LED Drivers

Power LEDs are becoming increasingly popular. After the brief introduction of how power LEDs work and how they are used in the lamp design, the article examines the main factors that designers usually take into account when designing a LED driver.

 

The World of LEDs

The LEDs market is evolving every day and high power LEDs are becoming increasingly popular due to rapid improvements in lighting efficiency, longer life, higher reliability, and overall cost-effectiveness. Typical LED applications include lighting for streets, interiors, shops, consumer lighting, decorative lighting, outdoor lighting.

The component behind all the above-mentioned applications is a quite simple two-terminal component: the LED:

 

Figure 1. LED symbol and physical appearance (from Cree® XLamp® XP-E LEDs datasheet).

 

But how do LEDs really work? The basic concept is that the luminous flux (lumen) depends on the LED current: the higher the current, the higher the luminous flux.
Each datasheet includes typical graphs where the luminous flux can be determined for each driving current.

 

Figure 2. Typical datasheet graph of luminous flux vs current (from Cree® XLamp® XP-E LEDs datasheet).

 

Inside a lamp the LEDs are typically connected as paralleled N strings, where each string is composed by M LEDs connected in series:

 

Figure 3. Typical LED connections

 

The result of this kind of connection is that the load can be considered, as a first approximation, as an equivalent LED where its current is the sum of the currents of each string and the voltage is the sum of the voltages across each diode.

 

Figure 4. The voltage is the sum of the voltages across each diode.

 

LED Drivers

Keeping the above basic concepts in mind, it is obvious that designing a led driver means to design a current source circuit to drive the equivalent LED load, obtaining the total required luminous flux.

Since the most common lighting applications are in the range 1W-100W, LED drivers are designed as switching mode circuits in order to reduce the power losses that generate heat inside the lamp.

While typical SMPS (Switching Mode Power supply) circuits have constant voltage output, a LED driver is a switching mode circuit with constant current output.

There are two main categories of LED drivers: DC LED Drivers and AC LED Drivers. 

 

Figure 5a. DC LED driver 

Figure 5b. AC LED driver 

 

• DC LED drivers – The input voltage is a DC source, typically a battery in the range 1V – 40V. An example of a DC LED driver is the one inside a smartphone that drives the torch LED, powered from the 3.7V Li-Ion battery.

• AC LED drivers – The input voltage is an AC source, typically the 115-230VAC public distribution network. The AC voltage comes out from a bridge as full-wave rectified and is able to power the LED driver. An example of an AC LED driver is the one inside a common LED bulb.

 

In the design of a LED driver, the main differences between DC and AC drivers are:

• Level of input voltage – The input voltage of an AC driver can be considered a factor 10x compared to the input voltage of a DC driver.

• Waveform of input voltage – The DC driver works with a constant level of input voltage, or at least with a voltage between a minimum and a maximum, while the AC driver works with a full-wave rectified voltage that in power each cycle goes from 0V to the maximum.

• Waveform of output current – The DC driver provides a constant output current, while the AC driver provides to the LED load a current that is full-wave rectified like the input voltage and whose average value is the one required to obtain the total luminous flux. 

 

Due to the differences indicated above, for the design of LED drivers, there are different circuit topologies with advantages and downsides depending on key factors that must be considered in the design. Here is a list of the main factors that are commonly considered by designers of LED drivers:

• Voltages (input voltage range and fixed output LED string voltage)

• Circuit size

• Component count

• Insulation

• Power factor correction (PFC – only for AC/DC drivers)

• Efficiency

• Design simplicity

• Cost

 

Each topology can be preferred for some of the mentioned factors, but not for others. The designer must choose for each project the topology that best suits the project specifications. Often, if not always, the selected topology is the best trade-off between the considered factors.

More than 90% of lighting applications are based on the following topologies:

• Buck and Reverse-buck

• Boost

• Buck-boost (inverter) or Reverse-buck-boost

• Flyback

In the next article, we will analyze each LED driver topology, including the buck and reverse buck topology, the boost topology, the buck-boost topology, the reverse buck-boost topology, and the flyback topology.