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EMC issues for hysteretic converters in LED driving

Posted: 16 Oct 2008 ?? ?Print Version ?Bookmark and Share

Keywords:LED? lighting application? EMC standard?

By Adrian Wong and Silvestro Russo
Diodes Inc.

The use of LEDs in a wide range of lighting applications is rapidly increasing. In order to maximize the efficiency improvements that the LEDs offer they must be driven from switching regulators rather than power hungry linear circuits. One of the most versatile inductive switching topologies is the hysteretic converter. To ensure these converters are used effectively in a fully EMC compliant manner their limitations must be understood.

This article explores the EMC issues associated with hysteretic LED controllers. The topology is discussed, along with pulse width modulation (PWM) dimming and frequency shift considerations. The EMC standards to be met and two illumination examples are explained to show how to effectively design an EMC compliant solution for an MR16 replacement lamp and an automotive interior light.

Hysteretic converters HB LED control
The hysteretic converter is a switching topology that can be used in buck, boost or buck-boost configurations. Its inherent stability makes it particularly suitable for step-down LED driving applications.

The most important part of the topology, as highlighted in Figure 1, is a comparator with a predetermined hysteresis that defines the LED current control. The current in the LED is measured with a resistor and ramps up and down to the limits set by the comparator, with the limits being a trade-off between measurement accuracy/noise immunity and efficiency. Hysteretic converters are however always running in continuous mode, which means the inductor never saturates or is completely drained of current. This inherent stability means hysteretic controllers operate over a wide input and output voltage range. They do not require compensation with external components, and are not limited to duty cycle ranges as with many PWM topologies.

The key parameters in a self-oscillating hysteretic converter are the frequency of oscillation, the duty cycle and the propagation delay between sensing and switching phase. These affect the accuracy, the power management, and the EMI performances of the system. The EMI performance of a hysteretic converter, is particularly dependant on the frequency of oscillation, that is influenced by many factors, inductor choice being the most important. Since hysteretic converters are self-oscillating, this means the frequency will vary due to input voltage, LED current and the number of LEDs being driven.

Figure 1: Hysteretic converter, basic step-down configuration

Improving EMC performance
The size, stability and frequency of oscillation of hysteretic converters have a significant impact on the performance of the illumination system. Understanding this fact helps in the selection and usage of the best technique to improve system EMC performance.

The hysteretic converter topology allows a significant reduction in component count compared to other topologies, making system design more compact and shortening tracks between switching element, inductor and load. Shorter PCB tracks mean smaller parasitic inductance and capacitance, resulting in reduced electromagnetic emission and improved immunity. Moreover, the hysteretic converter is inherently stable, which means that it does not require external compensation which makes it robust and RF immune. Finally, radiated and conducted emissions are very dependent on the fundamental switching frequency. The peak EMI energy is concentrated at the switching frequency. As the hysteretic converter is a variable frequency technique it is possible to avoid the problems of fixed frequency topologies such as PWM controllers by avoiding persistent energy pulses.

The best techniques to cope with EMI issues in hysteretic converters are the usual improvements in layout design, filtering and shielding. This topic will be covered in the application examples provided later that use two specific examples that have quite different requirements: general LED lighting and automotive LED lighting.

Among the methods used to reduce EMI impact, the dithering technique has emerged as one of the most promising. The dithering technique consists of sequentially varying and shifting the frequency of the switcher from 10 percent to 20 percent of the nominal value, to spread the energy among a wider bandwidth, reducing the likelihood of having a persistent peak of energy at a given frequency (and hence failing to meet the specific EMC requirements). The use of dithering has to be done carefully though to ensure that it does not impact on optical performance.

Hysteretic converters do not allow an exact definition of operating frequency since this depends on a set of parameters rather than an individual one; this suggests that the frequency shift technique is not easily implemented on a hysteretic topology. Nonetheless, some techniques are emerging to limit the spread of the oscillation frequency in order to simplify the filter design. Moreover, Hysteretic converters show an inherent advantage when an AC input supply is used, as in the case of MR16 lighting, because the 100Hz input voltage ripple spreads the switching frequency up to 90 percent of the nominal value, simplifying the EMI filter design, as will be shown in the application example. However, care must be taken if low frequency PWM (typically 100Hz to 300Hz) is being used to control brightness. Spreading the frequency among several drivers to reduce EMI could interfere with the PWM dimming signal accuracy and possibly create beating effects (leading eventually to stroboscopic effects in extreme cases or even audible noise).

Standards and requirements
EMC requirements are set to specific international standards, and they can be categorized in terms of conducted emissions, ESD susceptibility, radiated emissions and immunity. Generally speaking, the applicable EMC standards for CE marking of general lighting equipment in domestic and residential environments are shown in Table 1.

Note that for the USA market, the applicable requirements for general lighting are stipulated in FCC part 15 and part 18. For automotive lighting applications, the EMC standards become vehicle manufacturer specific with various test limits and test frequency ranges. Table 2 presents a �Generic Automotive Tier 1 Supplier EMC Test Standard? summary that is essentially derived from consideration of most of the larger vehicle manufacturer EMC standards. The tests cover the supply of electrical products to a vehicle manufacturer only and do not extend to whole vehicle testing, which remains exclusively the domain of the vehicle manufacturer.

Table 1: General lighting EMC standards

Table 2: Automotive test standards

LED-based MR16 EMC solution
In MR16 lamp applications, the hysteretic converter is preferable to the fixed frequency PWM converter because of the lower component count. Moreover, its lower switching frequency minimizes the radiated emissions. Furthermore, an EMI filter along with frequency dithering is adequate to meet EMC emission requirements. The simplicity and ruggedness improves immunity.

The basic circuit with the proposed modifications is shown in Figure 2.

The LED driver is simply implemented using only two semiconductor devices. Full wave rectification is achieved by U2 (ZXSBMR16PT8) which contains a Schottky bridge. U1 (ZXLD1350) is a hysteretic current controller which uses a freewheeling Schottky diode from the combination part U2. The Zetex ZXLD1350 controller has an internal switch that greatly reduces both the PCB size and the component count. It also has a proprietary current monitor that significantly simplifies the current sensing circuit for the hysteretic control. Two input energy storage SMD tantalum capacitors, C1 and C2, of total value 300?F have been optimized as the minimum capacitance required to meet efficiency and LED current accuracy. Three 0805 type SMD common-mode chokes in series, L2, L3 and L4, are verified to be the minimum requirement to pass the EMC conducted EMI test (CISPR-25). A screened inductor L1 is selected to minimize the radiated EMI from the switching operation.

Figure 2: System diagram of ZXLD1350 MR16 lamp solution with EMI filter

Due to input ripple voltage, the switching frequency range in relation to the input variation of 14.5V2.5V can be found to shift from around 24kHz to 420kHz, as highlighted in Figure 3. In this way, the ZXLD1350 offers a 90 percent deviation of its nominal switching frequency from a nominal 230kHz thereby facilitating the frequency dithering.

In addition, an EMI filter is composed of two capacitors, C5and C6, and three common-mode chokes in series, L2, L3and L4. The 3dB cutoff frequency fc is estimated as 200kHz. Hence, the filter can provide a supplementary attenuation of -40dB/decade from 200kHz, which is just below the nominal switching frequency. In other words, it is effective to suppress the higher harmonics.

Figure 3: Switching frequency variation with 12VAC input voltage

There are also many critical EMC considerations in the PCB layout as detailed below:

  • A star ground connection is employed to avoid the common impedance effect.
  • A ground ring is used to protect the ADJ pin against any kind of electromagnetic coupling.
  • The sense tracks connecting R1 to ZXLD1350 are as short as possible.
  • The decoupling capacitor C3 is placed as close as possible to the VIN pin.
  • The freewheeling current path is as short as possible to ensure system precision and efficiency.
  • Power and ground tracks have been maximized around critical areas on both sides to create the intrinsic capacitors for high frequency filtering.
  • The EMC test results in accordance with the general lighting standard EN 55015: 2006 are presented in Figure 4. Most importantly, the stand-alone MR16 lamp is operated from a 12Vac source by means of a typical linear step down magnetic transformer directly connected to AC mains throughout the compliance verification. However, if an electronic transformer is intended to be used, the system EMC compliance level is unknown at this moment. In which case, the EMI filter may need to be re-designed for optimization. Also, the ESD test was passed according to IEC 61000-4-2 Level B which requires 4kV contact discharge and 8kV air discharge.

    Figure 4: Conducted EMI scan

    Automotive EMC
    Automotive EMC standards are mainly manufacturer specific. For example, a typical requirement for an automotive application is that the LED driver should operate over a wide input range with a transient capability to support both cold cranking and load dump conditions respectively. Furthermore, ESD tests in automotive LED applications require an air discharge voltage up to 25kV because automobiles are treated as isolated systems.

    A 350mA LED driver circuit for car interior lighting that meets with the essential automotive requirements is shown in Figure 5. For this lighting solution a 60V Zetex ZXLD1362 based buck converter employing hysteretic current control was used. For load dump protection a transient suppressor diode D3 is added. The EMI filter consists of an inductor L2 and two capacitors C1 and C5 to form a simple filter which attenuates the conducted EMI. A capacitor C3 of 10nF is connected from ADJ pin to ground to filter noise pickup which may create flickering during the immunity test. An optional RC snubber (R2-C6) could be connected across the diode D1 to control both the spike�s transition rate and shape. The capacitor controls the rise time and the resistor the peak voltage. In fact this was not required to meet the EMC test. Cores of both the switching inductor L1 and the filter inductor L2 are shielded, ferrite-based and closed magnetic field type, in order to provide suppression of radiated emissions as well as immunity to external fields.

    Figure 5: Circuit diagram of 350mA LED driver for car interior lighting

    The 4th-order low-pass filter, which is formed by C5, L2 and C1, offers more than 60dB attenuation at 300kHz according to the transfer function analysis. In this circuit, the switching frequency at VIN=12V is about 300kHz. So the filter has been optimized to provide enough attenuation at the fundamental frequency as well as its harmonic frequencies in order to meet the conducted EMI requirement.

    Again, there are many critical EMC considerations in the PCB layout as detailed below:

  • The capacitor C3 connected from ADJ pin to ground is as short as possible.
  • The high di/dt loop (LX-D1-VIN-C4) with a fast switching current is made as small as possible to minimise the loop inductance, and thereby the differential mode noise related L�di/dt effect.
  • A simple filter (C5-L2-C1) is placed as close as possible to the input terminals to ensure optimal conducted EMI attenuation.
  • The perpendicular configuration of the EMI filter components lowers the capacitive coupling between the inductor and capacitors.
  • A V-connection to the filter capacitors C1 and C5 helps prevent self-resonance of these capacitors and a degredation of EMI performance.
  • Careful component placement to avoid mutual coupling.
  • The EMI filter as a noise bypass was placed in close proximity to the radiation source (the switching regulator).
  • The EMC test results are shown in Figures 6, 7 and 8 respectively in accordance with the following automotive standards with limits identified by a car manufacturer.

  • CISPR-25: Conducted and Radiated Emissions (European and Worldwide Standards)
  • ISO11452: Radiated Immunity (North American and Worldwide Standards)
  • 95/54/EC: Radiated Emission (European Standards)
  • It should be noted that the radiated immunity test is correlated from the stripline measurement inside a GTEM cell, while the radiated emission test is correlated from absorber chamber verification using an active loop antenna at a 1m range.

    Figure 6: Conducted EMI scan

    Figure 7: Operating emission using GTEM

    Figure 8: Operating emission with loop antenna in 1m range

    Conclusion
    Hysteretic converters can be used to efficiently drive LEDs in an EMC friendly manner, by following good EMC practices associated with all switching regulator design and by being aware of the variable frequency nature of the hysteretic topology. The variability of frequency with input voltage can be taken advantage of in some AC applications, such as the MR16 lamp, to create a wide dynamic spread spectrum response that is beyond the expectations of some fixed frequency topologies. Even in the tough automotive EMC environment, successful LED lighting solutions can be created which take full advantage of the converter�s simple and stable topology.

    About the authors
    Adrian Wong
    is a systems engineer and Silvestro Russo is a systems application engineer at Diodes Inc.





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