Compromises and work-arounds undertaken in an effort to meet the requirements of electro-magnetic compatibility (EMC) have had the potential in the past to markedly impair the reliability and efficiency, and increase the total production cost, of the power circuit in an LED luminaire. New component technologies, however, can now dramatically reduce the scale of the power system designer’s EMC problem, and help manufacturers of LED luminaires achieve a better balance of EMC performance, materials cost and production cost. This article gives an introduction to the operation, benefits and drawbacks of these new components.
A typical example of a power circuit for an LED luminaire using conventional power components is shown in Fig.1. Excessive radiated emissions will normally lead the design team to shield the entire housing. In practice, however, this increases the parasitic capacity between the (now larger) conductive area – that is, the chassis plus its shield – and the reference ground of any EMC measurement equipment. Common-mode conducted interference then becomes a large enough phenomenon to require attention. While a low-cost EMI filter will eliminate this problem, this author has seen designs in which even these counter-measures are not sufficient, since higher-frequency emissions radiated by the mains cable persist, and must be blocked by a more expensive filter with higher attenuation.
Fig. 1: A typical AC/DC LED driver design. The H-field is the result of winding leakage, the primary loop area and the secondary loop area. The E-field is the result of high dV/dt on conductive surfaces and of high-frequency ripple in cables. [Source: "Understanding noise-spreading techniques and their effects in switch-mode power supplies", by John Rice, Dirk Gehrke and Mike Segal of Texas Instruments.]
The designer could be forgiven for thinking that this cure is no better than the disease: to address the EMC problem, the luminaire manufacturer has to increase bill-of-materials and production costs and also increase component count, and ends up with a larger board and end product design.
The root cause of the problem in LED luminaires is the power supply’s high-speed switching circuits, which create wide-spectrum current and/or voltage ripples. Shielding and filtering might mitigate the emission problem, but do not eradicate it. A better solution would be to avoid generating high emissions at particular frequencies in the first place – and this is now possible through the use of new power components that use soft switching to minimise ripple currents, or to spread the noise energy over a wide frequency band.
Fig. 2 shows that there are broadly five architectures used today, each suited to different power outputs. Each of these LED driver topologies enables the designer to comply with the strict requirements of today’s EMC regulations.
While Fig. 2 indicates the power range in which each topology is most commonly used, it should be noted that each can be adapted for use in a higher or lower power output range.
Fig. 2: the evolution of components for AC/DC LED drivers.
RC = resonant converter, QR = quasi-resonant converter, PFC = power factor controller, CC = constant current control, CV = constant voltage control, CCM = continuous-conduction mode, CRM = critical conduction mode.
The PFC is the most common block in modern AC/DC LED drivers. A boost converter is inserted between the bridge rectifier and the main input capacitors. This regulator can operate in three modes. In discontinuous-conduction mode (DCM), the energy stored in the inductor (L) during the conduction interval of the switch is equal only to the energy required by the load for one switching cycle (see Fig. 3). The energy in the inductor drops to zero before the end of each switching cycle, resulting in a period of no energy flow, or discontinuous operation.
In transition mode (TM) – also called boundary conduction mode (BCM) or critical conduction mode (CRM), the converter operates at the boundary between DCM and continuous conduction mode (CCM), reducing the idle time of DCM to close to zero.
In CCM, the inductor has continuous current during the operation of the converter. The extra energy stored in the inductor during the conduction time of the switch is equal to the energy discharged into the output during the non-conductive time of the switch; at the end of the discharge interval, residual energy remains in the inductor. During the next conduction interval of the switch, energy builds from that residual level to that required by the load for the next switching cycle.
Fig. 3: Peak and average current in the inductor (IL) in a) discontinuous conduction mode b) transition conduction mode and c) continuous conduction mode.
CCM has a lower peak-to-average current ratio; thus it has lower ripple currents, lower coil conduction and core losses, and lower electromagnetic emission levels. The drawbacks are that it requires a very fast boost diode (otherwise diode recovery current starts to dominate, resulting in increased power losses and additional electromagnetic emissions). Unfortunately, it also requires hard MOSFET switching, and this results in high switching losses, and these are the main source of electromagnetic emissions in a CCM system.
The biggest advantage of TM or DCM operation is the absence of reverse recovery in the boost diode, which means that the circuit can use a low-cost diode with a low forward voltage. On the other hand, the cost of filters to block the electromagnetic emissions generated at the high peak currents might – depending on the quality of the board layout and the size of the load – be excessive.
New components in the second DC/DC stage of the LED driver (see Fig. 4) also offer new ways to reduce electromagnetic emissions. They often contain “resonant” LC networks, of which the voltage and current waveforms vary sinusoidally. The turn-on or turn-off transitions of semiconductor devices can occur at zero crossings of the tank voltage or current waveforms, thereby reducing or eliminating some of the switching loss. This means that resonant converters can operate at higher switching frequencies than comparable PWM converters, leading to smaller inductor and capacitor values and costs when designing high-power converters. In addition, zero-voltage switching reduces converter-generated EMI as it has no current or voltage ripples during switch commutation.
Resonant converters do, though, have several disadvantages. Performance can be optimised at a single operating point, but not across a wide range of input-voltage and load-power variations. Also, the quasi-sinusoidal waveforms found in a resonant converter exhibit higher peak values than their equivalent rectangular waveforms. In addition, current can circulate through the tank elements even when the load is disconnected, leading to poor efficiency at light load.
Fig. 4: Resonant converter circuit operating in a) zero-voltage switching mode when fsw > fLC and a delay between Q1 and Q2 commutation is introduced. Otherwise, the circuit operates in (b) hard switching mode.
A similar switching technology is today often employed in low-power LED drivers in which a quasi-resonant or valley-switching topology is implemented. To start, current IQ1 ramps up until the desired energy level is charged in to coil L (see Fig. 5). Then switch Q1 is turned off. When the switching transient is complete and the coil current equals zero, the drain voltage starts to oscillate around the input voltage level VDC . The amplitude equals V0. Circuitry connected to the Q1 drain pin senses when the voltage on the drain of the switch has reached its lowest value. The next cycle is then started.
The effect of this topology is to reduce capacitive switching losses and electromagnetic emissions. On the other hand, a quasi-resonant converter has the same disadvantages as a resonant converter, as described above.
Fig. 5: Operation of a quasi-resonant, zero-voltage switching circuit.
This article, then, has described the operation of some power circuit topologies which produce relatively low levels of electro-magnetic emissions. For some topologies, however, such as power supplies using PFC in CCM mode, and simple flyback converters, the requirements of EMC tests can remain hard to meet. In these cases, this is not because of excessive total noise across the spectrum, but because of concentrated energy in a narrow frequency band. It follows, then, that EMC can be achieved by spreading electromagnetic noise from hard-switching operations over a wider bandwidth. This can be achieved through fixed-frequency modulation or random modulation.
EMC standards for conducted emissions generally set peak energy limits within the frequency band from 150 kHz to 30 MHz. Although carefully selected fixed-frequency modulation, as shown in Fig. 6, can be effective in spreading harmonic content, the disadvantage is that in some cases it might not provide sufficient attenuation of the fundamental.
New research1, however, suggests that modulating at a fixed frequency is not as effective in reducing the peak energy in the fundamental as modulating the carrier with a complex, random, or pseudo-random waveform (see Fig. 7).
Fig. 6: Circuit implementing fixed-frequency dithering.
Fig. 7: Circuit implementing frequency hopping for random modulation of electromagnetic emissions.
As this article has shown, there are many architectural and circuit-design choices available to the designer of LED luminaires, and it is impossible to recommend one as preferable to the others for every end product. Microcontroller manufacturers have started to publish reference designs for software implementations of topologies such as flyback converters, PFC and resonant converters. By using a low-cost microcontroller and several high-voltage MOSFETs and diodes, it is now possible to build a generic power supply design which can be scaled for different power requirements. For the experienced engineer, this might be the best approach.
But in general, design engineers must build an understanding of circuit operation, of the criteria to use when selecting a PFC mode, of the correct way to design the second DC-DC stage, and of how to implement spread spectrum techniques.
Perhaps the most important lesson to learn from experience is the importance of testing prototypes for EMC, before going to pre-production. At the pre-production stage, it is too late – or if not too late, at least extremely expensive – to alter metal casting and plastic moulding equipment, or to modify elements of the PCB design such as its outline, connector position or grounding points. Yet these are exactly the elements which can make the difference between a product passing or failing its EMC test.
In this author’s view, then, far more time should be spent on prototype testing than is normally the case today. By designing for low electro-magnetic emissions, testing early and modifying prototypes accordingly, good EMC performance can be achieved without the expensive and unsatisfactory work-arounds, such as shielding and filtering, which bedevil conventional LED driver designs.
Reference:
[1] DA Stone, B Chambers, and D Howe, “Easing EMC problems in switched mode power converters by random modulation of the PWM carrier frequency,” Department of Electronic and Electrical Engineering, University of Sheffield, UK.
Acknowledgement
Future Technology Magazine Issue 1304. “Power for LED lighting: tackling noise at source” by Arnoldas Bagdonas, FAE, Future Electronics Lithuania.
Contact Craig Hemmes, Future Electronics, 021 421 8292, craig.hemmes@futureelectronics.com
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