LED Driver Topologies
There are multiple topologies to consider when selecting an LED driver. This video provides an overview, as well as the advantages and disadvantages that come with the boost, buck, floating buck boost, SEPIC, and Cuk topologies.
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Hi, my name is Kelly. I am a product marketing engineer focused on TI's LED driver portfolio. Today I will be going over a basic overview of the advantages and disadvantages of common LED driver typologies.
Topology selection is always application dependent. The first question to ask when choosing a topology is whether your input voltage is higher or lower than the output voltage. If the regulated output voltage is always lower than the maximum input, then a buck converter is the ideal choice. A linear regulator, buck converter, or a buck controller could all be used. If the regulated output voltage is always higher than the maximum input, then a boost topology converter or controller is the ideal choice.
If the regulated output is sometimes higher or lower than the maximum input due to voltage spikes or other variations, then you have a couple of options. A single buck-boost regulator could be used to provide the output, regardless of the input voltage. There are several variations of the buck-boost topology, including a floating buck-boost, SEPIC, or a Cuk, which all have their own advantages and disadvantages. Another option would be using a two power stage solution where a boost regulator steps up the input voltage to a steady voltage which is subsequently stepped down by a buck regulator to the required voltage.
Most of the time when we discuss converters and controllers they are being used as voltage regulators. However, since we are focusing on driving LEDs, we actually are using them as a current regulator to maintain the current flowing through the LEDs.
Now we will go into a more detailed overview of each of these typologies.
A linear regulator is a regulating device that acts like a variable resistor in order to maintain a constant output. The transistor is operated in a linear region in a closed loop feedback system with a sense resistor used to measure the current. The voltage across the sense resistor is compared to a voltage reference in the feedback loop to produce the control signal to the transistor to adjust the resistance, if needed, to maintain the output current.
The advantages of a linear regulator include a high efficiency when V in is close to V out. Linear regulators also do not have switching components, so there is no output ripple. And they have a lower noise compared to a switching converter. They are also smaller and less complex with fewer components to design the switching regulators. In particular, they do not have magnetic components which can be bulky and expensive. However, there is a threshold of power output where the power losses of a linear regulator are so high, it makes sense to move to a higher efficiency switching converter instead. Linear regulators also can only step down voltage, unlike a switching regulator which has more topology choices.
The rest of the video will focus on switching typologies, starting with buck.
A buck topology converter or controller stuffs down the input voltage to a programmed output voltage. The conversion ratio, or V out over V in, is equal to the duty cycle. When the switch is turned on and the diode turns off, the inductor current ramps up and stores energy as current flows from the input through the inductor and into the load. The voltage drop across the inductor reduces the output voltage to the program value. When the switch is turned off and the diode turns on, the voltage source is disconnected, but stored energy in the inductor releases, charges the output capacitor, and provides current to the load, which keeps the output current continuous, even though the input current is discontinuous.
When using a switching regulator as a current source, a sense resistor in series with the LEDs is added to measure the current. The sense resistor can be placed on the high or low side of the LEDs, depending upon the LED driver selected. The current information is then fed back through the LED driver control network to set the duty cycle needed to produce the output voltage that corresponds to the target output current.
The specifics of how that control circuitry works will vary from part to part. This does apply to all switching typologies we will cover today when used as a current source.
The advantages of using a buck topology converter include continuous output current, which allows for the best dynamic control of LED current. This also means that the capacitor is not necessary to provide energy for the output and is only used to reduce the output ripple. This means the capacitance can be reduced or even eliminated. Bucks tend to have the simplest control architecture. Typically a buck regulator uses either hysteretic or current mode control. And finally, it handles short to input and short to ground conditions well.
The disadvantages of the buck converter include the discontinuous input current from the switching turning on and off. The transition from zero to peak current at the input causes conducted electromagnetic emissions which will require additional EMI filtering to mitigate. A buck device also cannot increase the output voltage, so the number of LED strings that can be supported is limited by the input voltage.
A boost converter steps up the input voltage to a higher output voltage. The conversion ratio, V out over V in, is equal to 1 over 1 minus the duty cycle. When the switch is on and the diode off, the inductor current ramps up and stores energy. Because the load is disconnected from the inductor current during an on cycle, the output current is discontinuous. This means an output capacitor is required to release energy stored in the previous off cycle to maintain the output current and voltage.
When the switch is off and the diode is on, the inductor releases energy and maintains the output current on the load. The polarity of the inductor reverses so the left side of the inductor is negative and the right is positive, which puts two sources in series, which means the output voltage is always higher than the input. The output capacitor will also be charged to this combined voltage.
Again, when used as a current source the addition of a sense resistor detects the current and feeds that information back through the control loop. Unlike the buck converter, the boost converter has continuous input current which reduces the amount of input EMI filtering necessary compared to the buck. This switches ground reference which also makes it simple to drive the FET. And finally, it handles short to input conditions well.
However, boost converters have discontinuous output current, which means that an output capacitor is required to maintain the current. The output cap also helps minimize EMI and the LED switching ripple which may be needed to make that peak to peak ripple requirements of the LED, but can make dynamic switching more difficult. Discontinuous output current also creates more differential mode noise on the output, which turns into common mode noise causing higher radiated emissions. Another potential disadvantage is that a boost can only step up the input voltage. And finally, a boost converter does not handle short to ground conditions well.
Our next topology is a buck-boost converter which can provide a steady output, regardless of if the input voltage is higher or lower than the output. In this example, we are looking at a variation of buck-boost converters called a floating buck-boost, where the load in the output capacitor is referenced to the input, instead of to ground, which means the output is not inverted like in a traditional buck-boost converter.
The conversion ratio of a floating buck-boost, V out over V in, is equal to the duty cycle over 1 minus the duty cycle. When the switch is on and the diode is off, the inductor current ramps up and stores energy and the inductor current flows through the closed switch. Just like the boost converter because the diode is off, the load is disconnected from the inductor current during the on cycle, so the output current is discontinuous and an output capacitor is required to release stored energy to maintain the output current.
When the switch is off and the diode is on, the inductor releases stored energy which flows through the closed diode, provides the output current, and charges the output capacitor. During the off cycle, the input current is discontinuous. Again, when used as a current source, the addition of a sense resistor detects the current and feeds that information back through the control loop to set the output voltage and current.
Buck-boost converters are advantageous because they can provide an output higher or lower than the input voltage. This means the second power stage can be eliminated. The floating buck-boost variation is a simpler buck-boost implementation than the SEPIC or the Cuk topologies we will cover next, and is also preferred over the traditional buck-boost because it provides a non-inverted output. The buck-boost also handles short to input conditions well.
The main disadvantage is the discontinuous input and output current, which means this topology has the poor input EMI performance of buck and the poor output EMI performance of the boost. A higher peak current also contributes to higher EMI. Additionally, just like the boost converter, the discontinuous output means an output capacitor is required to maintain the current. A floating buck-boost also does not handle short to ground conditions well.
The SEPIC topology is another variation of buck-boost topology with the same conversion ratio, V out over V in equals the duty cycle over 1 minus the duty cycle. This applies for a coupled of inductor with a 1 to 1 ratio or uncoupled inductor.
The SEPIC is characterized by the use of two inductors, one at the input and one going to ground, connected by a coupling capacitor. This means the SEPIC is more complex to design than the floating buck-boost and requires a larger board footprint due to the two inductors. Using a coupled conductor can mitigate the footprint size. But there are a number of cup-- there are a limited number of coupled conductors available off the shelf, so this may require a custom device.
When the switch is on and the diode is off, energy is stored in the inductor while the output capacitor releases energy to support the load. The input voltage is applied across the primary winding. There is a winding LED of 1 to 1, so the secondary winding also has a voltage equal to the input voltage.
But due to the polarity of the windings, the anode of the diode is pulled negative and the diode turns off. Because the diode is off, the coupling capacitor is charged the input voltage and the load is disconnected from the inductor current, so the output current is discontinuous. Again, like a boost converter, this means an output capacitor is required to release energy stored in the previous off cycle to maintain current to the LED load.
When the switch is off and the diode is on, the voltage on the windings reverse polarity to maintain current flow. The secondary voltage is now clamped to the output voltage and energy is transferred through the coupling capacitor and the diode into the output. The input current is continuous due to the constant L1 inductor current.
The advantages of the SEPIC is that it can provide any output higher or lower than the input and provides a non-inverted output. The continuous input, like a boost converter, minimizes the input EMI filtering needed. And it handles short to input and short to ground conditions well.
The disadvantages is the SEPIC has difficult control loop dynamics making it more difficult to design than the floating buck-boost. The discontinuous output current also means the regulator will have worse EMI on the output. And an output capacitor is required to maintain the current. As explained previously, an output capacitor can make dynamic switching more difficult.
The switching topology we will cover is the Cuk converter, which is another variation of a buck-boost converter. However, unlike the previous two examples it provides an inverted output. So the conversion ratio, V out over V in, is equal to the negative duty cycle over 1 minus the duty cycle.
As you can see from the schematic drawing the Cuk is essentially a boost converter followed by a buck converter with a coupling capacitor in between. Just like the SEPIC, the Cuk has two inductors, so similarly, will have a larger footprint and a more complex design.
When the switch is on and the diode is off, inductor L1 current rises and flows through the switch. The stored energy in the coupling capacitor also releases and flows through inductor L2 and the load. When the switch off and the diode is on, both conductor currents flow through the diode. L1 current charges the coupling capacitor while L2 current flows through the load.
The main advantages of the Cuk is that it combines the continuous output current of a buck and the continuous input current of a boost due to inductors on both the input and the output. This gives the Cuk the best EMI performance and allows you to reduce the capacitance needed. It also handles both short to input and short to ground conditions well.
The disadvantages are the inverted output voltage and more complex control dynamics, which make the Cuk more difficult to design.
To summarize, in order to select the correct topology for your application, you first need to determine whether or not the input voltage is always higher than the output, always lower than the output, or can be higher or lower than the output. If the voltage is always higher, then a linear regulator or buck converter or controller is the ideal choice to step down the voltage. If the voltage is always lower, a boost converter is the ideal choice to step up the voltage. If the input voltage varies and can be higher or lower than the output, then there are multiple types of typologies to choose from, including a floating buck-boost, SEPIC, and Cuk.
That's all I have for you today. Please check out the rest of our training portal for additional LED driver trainings. Thank you for watching.
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LED driver basics
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