LED Technology
Figure 1. Circuit diagram of a boost converter, a very common topology for LED drivers.
Selecting switching regulator ICs that are already designed for minimal emissions and optimal EMC behaviour. In this case, either minimal or no filtering is needed. Most LED drivers are boost (step-up) converters. Figure 1 shows a schematic circuit diagram of this type of converter. Boost converters usually have lower conducted emissions on the input side. The input currents are nonpulsating (blue current loop). On the output side, however, there are very high emissions because here pulsed currents flow through the flyback diode (red current loop). During the on-time−that is, when the switch connected to ground is on−the inductor is charged and there is no flow of current through the flyback diode. The total energy to supply the load in this time section comes from the output capacitor.
In Figure 1, the current flow during the on-time is shown in blue and the current flow during the off-time is shown in green. All paths in which the current flow changes over a very short time, or the switching transition time, are shown in red in Figure 1. These paths change their state from current flow to no current flow in just a few nanoseconds. They are the critical paths and must be designed to be as small and compact as possible in order to reduce the radiated emissions. Switching regulator ICs that generate much lower radiated emissions as a result of innovations have recently become available. The critical paths are laid out so symmetrically that the generated magnetic fields largely cancel each other out due to different directions of current flow. Figure 2 shows the symmetrical arrangement of this topology. The magnetic field generated in the top red loop is the same magnitude as the field in the bottom red loop, but points in the opposite direction. This yields the effect of field cancellation. At Analog Devices, this technology is marketed under the name Silent Switcher. In addition to this innovation, there is a strong reduction in the parasitic inductance in all critical line segments, resulting in a considerable reduction in the radiated fields. The Silent Switcher topology utilizes a proprietary layout of the power transistors to achieve this magnetic cancellation effect. The length of the path between the power transistors and
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the output capacitors for the boost converter (the hot loop) determines the inductance involved with this magnetic field. In Silent Switcher 2 technology, the length of this path is greatly reduced. This is accomplished through the so-called flip chip technology. Here, the silicon in the switching regulator IC is connected to the IC housing not with bond wires, but rather with copper pillars. These pillars have a much lower inductance. Thus, for the same current switching speed, there is a much lower voltage offset and, through this, a lower radiated emissions level. It is hence very possible to achieve a considerable reduction in EMI by using optimized LED driver ICs. In some cases, it is even possible to stay within certain EMI limits without using EMI filters.
A practical circuit with very low radiated emissions is shown in Figure 3. Here, the LT3922-1 (
https://www.analog.com/en/ products/
lt3922-1.html) is operated in a boost circuit. A chain of 10 LEDs with 333 mA is driven with an input voltage of 8 V to 27 V. For this constellation, switching is done at a switching frequency of 2 MHz and the generated emissions are minimal. In Figure 4, the average radiated emissions from the circuit in Figure 3 are shown. The red lines show the respective limits from the CISPR 25 specification. As can be seen, this
Figure 2. Silent Switcher concept applied to a boost converter with magnetic fields that cancel each other out.
Figure 3. Example circuit for a Silent Switcher LED driver optimized for minimal emissions and the best EMC behaviour.
specification is easily met (undershot). An LED driver such as the LT3922-1, which is designed for low emissions, frequently also offers the option of activating a spread spectrum frequency modulation (SSFM) function. This may not reduce the real emissions generated, but it spreads the emissions over a wider frequency range. Through this, better results can be obtained in the measurements for individual EMC standards. The LT3922-1 offers this function between the respectively set switching
Figure 4. Average radiated EMI (CISPR 25) from the LT3922-1 in Figure 3.
frequency and 125 per cent of this value. Spread spectrum can also have a very significant effect in the VHV and UHV bands, reducing the emission of any given frequency below the level that would affect radio communication.
As is the case for every switching regulator, for LED drivers the design of the board layout is very critical. Modern innovations such as the Silent Switcher and Silent Switcher 2 technologies help to dramatically improve EMC behaviour, but it is still important to avoid any mistakes with the printed circuit board layout. Proper placement of critical components that conduct rapidly switched currents is especially decisive for minimizing radiated emissions. As little parasitic inductance as possible should be included in these paths. The current loops should also be designed as compactly as possible. To aid in the successful consideration of these aspects, detailed documents such as the LT3922-1 data sheet offer valuable and clear information. Some of today’s modern LED drivers are specialized in minimizing electromagnetic emissions. For this, they use some key innovations in the field of switching regulators, including the Silent Switcher and Silent Switcher 2 technologies from Analog Devices. When designing with these ICs, the effort required to comply with EMI limits is relatively low.
www.analog.com Components in Electronics February 2025 43
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