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How to reduce EMI without adding flicker

Posted: 16 Jan 2014     Print Version  Bookmark and Share

Keywords:LED drivers  flicker-free  PWM dimming  SMPS  EMI 

LED drivers for automotives must be compact and efficient. They should also allow for flicker-free PWM dimming, and not produce significant conducted EMI at and around the AM radio band. Unfortunately, low EMI is not in the nature of high power switch mode power supplies. The constant switching frequency produces a significant EMI signature at a number of frequencies, including the power supply's fundamental operating frequency and its harmonics. Odds are good that something will fall into the AM band.

One way to minimise EMI peaks is to allow the switch mode power supply (SMPS) operating frequency to cover a range of values, namely spread spectrum switching. The desired effect of spread spectrum switching is to push down the EMI peaks that would occur at the SMPS fundamental operating frequency and harmonics, spreading the EMI energy over a range of frequencies instead.

LED driver SPMSs have an additional requirement: the frequency spreading should also be synchronised with the PWM dimming (brightness control) frequency to ensure that there is no resulting LED flicker.

Figure 1: 80V, 400mA automotive LED driver with internal spread spectrum for low EMI.

To this end, the LT3795 generates its own spread spectrum ramp signal and aligns it with the lower frequency PWM dimming input with a patent pending technique. This eliminates the chance that the spread spectrum frequency could combine with the PWM signal to produce visible flicker in the LEDs, even at the highest PWM dimming ratio.

Figure 2: Conducted peak EMI around the AM band is reduced by 3dBµV–6dBµV when the LT3795's spread spectrum frequency modulation is used. The CISPR25 Class 5 AM-band limit is provided for reference.

Figure 3: Spectrum analyser scan of the LT3795 150kHz–30MHz peak conducted EMI.

High power LED driver
The LT3795 is a high power LED driver that uses the same PWM dimming scheme as the LT3756/LT3796 family, but with the additional feature of the internal spread spectrum ramp for reduced EMI. It is a 4.5V-to-110V input to 0V-to-110V output single-switch controller IC that can be configured as a boost, SEPIC, buck-boost mode or buck mode LED driver. It features a 100kHz to 1MHz switching frequency range, open LED protection, short-circuit protection, and can also be operated as a constant voltage regulator with current limit or as a constant current SLA battery or supercapacitor charger. Figure 1 shows a 92% high efficiency 80V, 400mA, 300kHz- 450kHz automotive LED head-lamp driver with spread spectrum frequency modulation and short-circuit protection.

Internal spread spectrum
Unlike other high power LED drivers, the LT3795 generates its own spread spectrum ramp to produce 30% switching frequency modulation below the programmed switching frequency. This lowers its conducted EMI peaks, reducing the need for costly and bulky EMI input filter capacitors and inductors.

Using an external, or separate, spread spectrum clock to produce the switching frequency in an LED driver can produce visible flicker during PWM dimming since the spread spectrum frequency pattern is not synchronised with the PWM period. For this reason, in many high end LED driver applications, implementing spread spectrum is not trivial. Without spread spectrum, designers must rely upon bulky EMI filters, gate resistors that slow down switching edges (but reduce efficiency) and snubbers on the switch and catch diode.

Figure 2 shows a comparison of the conducted EMI measurements of the LT3795 LED driver around the AM band when spread spectrum is enabled and disabled. Normal (non-spread spectrum) operation yields high energy peaks at the switching frequency and its harmonics. These peaks can prevent the design from passing stringent EMI requirements in EMI sensi-tive applications such as automobiles. For reference, the CISPR 25 class 5 automotive con-ducted EMI limits are shown in figure 2. Figure 3 shows the effect of spread spectrum over a wider frequency band.

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