Steven Shackell, ON Semiconductor
In recent years, LED suppliers have been building large portfolios of high-voltage (HV) LEDs. This high voltage-low current LED provides the ability to optimize the power conversion from the AC power line to simultaneously increase the overall efficacy and simplify the drive circuitry. With HV-LEDs the conventional AC to DC driver can be used along with other more unconventional driver topologies. These different drivers have their own challenges, but each offer a valid approach to drive the LEDs.
In general to be considered a HV-LED the turn-on voltage should be greater than 20 V compared to the 2-4 V turn-on voltage of conventional LEDs. To create this high turn-on voltage there are two common methods that LED suppliers use. First, is the Chip-on-Board, COB, method that uses multiple conventional LED die placed in a package and wire bonded together to create a high voltage array. The other method is to create a die that has multiple junctions within it. Each of these junctions produces light and has a typical 3 V drop. Both of these methods apply the same idea of placing multiple LEDs in series with each other, but it depends on the manufacturing capabilities of the LED supplier.
Voltage vs. efficiency
Now that the LED has a higher voltage, how does this help with the efficacy of the LED? The higher voltage demands that a lower current be used to keep the same power. This lower current is the key to the HV-LED having better efficacy. LEDs operated at lower currents have a lower current density, and this lower current density results in less internal quantum losses. This improvement in internal efficiency leads to more light being created under the same power, thus an increase in efficacy.
These HV-LEDs also help improve the efficiency and simplify the drive circuitry all while reducing the cost of the overall system. To understand how the HV-LEDs accomplish this, the conventional AC to DC driver will be analyzed.
AC to DC drivers work best and are most efficient when the DC output voltage is as close to the AC input voltage as possible. This means that the total forward voltage of the LED string needs to be close to the input voltage. This is not feasible to accomplish with conventional LEDs since the cost would be too expensive and it would take up a large amount of space. For 120V mains the number of conventional LEDs would be around 40 units.
However, if HV-LEDs are used the number of LEDs would be greatly reduced down to about 3-8 LED packages. This makes board layout more feasible and the overall cost significantly reduced. Not only is the number of LED packages decreased reducing the system size, but the passive components used in the drive circuitry can also be reduced. This is the result of there being a smaller difference between the input voltage and the LED string voltage reducing the power rating needed for these components.
AC to DC drivers do provide the best efficiency, but there are some drawbacks that are making LED luminaire manufacturers explore other driver options. One of these drawbacks is the need for an electrolytic capacitor. The lifetime of electrolytic capacitors is usually much shorter than that of the LEDs, especially when the luminaire is at elevated temperatures.
This reduced lifetime and reliability of the electrolytic capacitor hurts one of the major benefits of LEDs themselves, their long lifetime. In addition to the poor lifetime of these capacitors, is the drawback of their large size. This size makes it difficult to use AC to DC drivers in space-constrained applications such as flush mount downlights. The cost of the AC to DC driver is another disadvantage. As the price of a 60W equivalent LED bulb pushes past $10 USD manufacturers are looking for other cheaper alternatives for driving the LEDs.
Direct AC drivers
Most of these cheaper alternatives can be grouped together as Direct AC drivers. There are three major topologies of Direct AC drivers: Straight, Iterative VF, and Parallel-to-Series (P2S). The straight topology is the simplest and least expensive, while the iterative VF and P2S topologies both utilize some sort of switching that adds some complexity and cost.
A common hitch among all of these topologies is the negative phase of the AC voltage. During this time the LEDs would be off. To simply solve this, a full wave bridge rectifier is used to converter this negative voltage to a positive voltage. All three of the topologies use a full wave bridge rectifier (see Figures 1 & 2).
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Figure 1: LED Current with AC Input Voltage
Figure 2: LED Current with Rectified AC Input Voltage
The straight topology offers the simplest and least expensive solution, but it does so at the cost of performance. It is so called “straight” because it employs a linear driver in series with the LEDs. There are no other current paths, but just straight through the linear driver and LEDs. An example of this type of linear driver is ON Semiconductor’s NSI family of constant current regulator (CCR). These drivers are simple 2 terminal devices (3 terminals for adjustable versions) that provide a constant current to the LEDs (see Figures 3 & 4).
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Figure 3: Schematic of Straight Topology
Figure 4: LED Current in Straight Topology
For the straight topology the total LED voltage of the LED string is very important when understanding the performance. For best efficiency it is desired to have an LED string voltage as close to the max input voltage to reduce the amount of voltage that will be dropped across the linear driver. This will help reduce the amount of wasted power.
There are drawbacks to having a very high LED string voltage. First, the conduction time of the LEDs is reduced as the LED string voltage is increased. This will start to decrease the efficacy of the system since the LEDs light output will be decreased. Secondly, the total harmonic distortion (THD) will start to increase to undesired levels. Lastly, the power factor (PF) will decrease with a larger LED string voltage.
Understanding these tradeoffs is key for manufacturers to effectively choose the correct LED string voltage. For some applications where cost is the major consideration the high THD and lower PF is acceptable. In the cases where better PF and THD is needed the iterative VF and P2S topologies are adopted (see Figure 5)
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Figure 5: Efficiency, THD and PF versus LED string voltage
The iterative VF topology offers a solution that has improved THD and PF over the straight topology, and does so with improved efficiency. This solution essentially adds more LEDs to the LED string as the input voltage increases. The idea behind this solution is to turn on the LEDs at a low input voltage while also reducing the amount of voltage dropped across the driver as the input voltage is increased. The longer conduction time and better efficiency will result in a much better efficacy when compared to the straight topology. In addition to just switching in more LEDs to the LED string each stage, most implementations of the iterative VF topology will add a current source also. This current stepping provides a current waveform that follows the input voltage waveform resulting in improved THD and PF (see Figure 6).
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Figure 6: LED stages of Iterative VF
The downside of the iterative VF solution is the utilization of the LEDs. During the low input voltages the later stages of LEDs are sitting idle and are only used at the later stages. This is potential light output that is being wasted and the LEDs are using valuable PCB real-estate.
The parallel-to-series topology looks to solve this issue of under utilization of the LEDs. Instead of adding additional LEDs to the LED string, the P2S solution changes the configuration of the LEDs to increase the LED string voltage. As depicted in the Figure 7, the first stage has the 4 LEDs all in parallel to each other, the second stage has 2 strings of 2 LEDs, and the final stage has the 4 LEDs all in series with each other. If a 36 V HV-LED was used the voltage steps would be 36 V, 72 V, and 144 V. This allows for the same idea of the LED string voltage following the input voltage waveform to be implemented creating a high efficiency solution with good THD and PF. To further increase the THD and PF performance newer designs are switching in an additional current source at the last stage. This creates an input current waveform that matches the input voltage waveform very closely (see Figure 7).
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Figure 7: LED stages of Parallel-to-Series
A major complaint about Direct AC solutions is the flicker. Since most new designs use the full wave bridge rectifier there will be a zero volt point every 100 or 120 Hz depending on the region. This results in zero light output and possibly visual flicker. To fight this flickering, lighting manufactures are starting to implement valley fill capacitors. However, this takes away a major advantage that the Direct AC solution has over the AC to DC solution, the use of electrolytic capacitors.
In addition to 100/120Hz flicker, another common concern is dimmer compatibility. Consumers expect that their new LED bulb dims in the same manner as their conventional bulb. This is a difficult task for all LED bulbs regardless of the drive topology they are using. Dimmers have complex circuitry that usually requires a holding current. This means that the load attached to the dimmer needs to be always drawing current.
Since LEDs only conduct current above their forward voltage there is no holding current at the lower voltages causing the dimmer to incorrectly operate. To solve this holding current issue manufacturers are using bleeder circuits. These bleeder circuits draw a current during the off periods of the LEDs. This hurts the overall efficiency of the system, but allows for good dimming performance. Topologies that use some sort of switching will sometimes encounter jumps in the light output or issues at the switching points. Manufacturers are trying to solve this issue by implementing hysteresis and carefully selecting the switching points.
The development of HV-LEDs has opened the door to new driving techniques that are giving lighting manufacturers more options when designing new luminaires. When high efficiency solutions are needed an AC to DC topology can be implemented. When a low cost solution is needed a Direct AC straight topology can be implemented. And when an efficient mid cost solution is needed a Direct AC iterative VF or P2S topology can be implemented. Each of these topologies has their strengths and weaknesses, but HV-LEDs have allowed them all to be better performing and easier to implement.