A new and ideal solution for EMI
28 February 2011
As the demand for energy efficiency increases, more electronics are being designed with switching regulators in place of linear regulators. Greg Zimmer, Senior Product Marketing Engineer at Linear Technology Corporation explores this subject.
Power systems with multiple switching regulators are becoming commonplace, and as the number of regulators increases, the impact of electromagnetic interference (EMI) also grows.
One of the simplest, most cost effective techniques for EMI reduction is the use of a multi-phase, spread-spectrum clock.
Multiphase synchronisation
The operating frequency of most switching regulators can be controlled with an external clock, which in turn sets the fundamental frequency of generated EMI.
This feature can be useful to set the EMI outside of a sensitive band and it is a particularly valuable feature when operating multiple switching regulators together.
Multiple independently running switching regulators have a potential to generate large peak EMI, as clock frequencies approach each other and create beat frequency conditions.
Similarly, if multiple regulators are operated on a single clock, the EMI will be synchronised and, therefore, very concentrated. One solution is for each regulator to be driven with the same clock frequency, but with different phases.
Multiphase synchronisation refers to the technique of externally driving multiple switchers with a single clock frequency that has a time shift placed between each regulator.
This reduces peak switching current by staggering the turn-on for each switcher such that there is input current drawn where previously there was a dead band.
As a result, multiple switching regulators synchronised out of phase, versus in phase, have a lower peak current and therefore, lower EMI.
Also, phase synchronisation increases the frequency of generated EMI. As the number of clock phase increases, the clock period effectively decreases, resulting in higher frequency EMI.
This enables simplified EMI since filtering is more effective at higher frequencies as using multiple parallel regulators in place of a single regulator, phase-synchronisation offers two additional benefits:
• The net cancelling of ripple current on both the input and output allows for a significant reduction in input and output capacitors
• A smaller equivalent inductance is required, providing a higher current slew rate
Spread Spectrum Frequency Modulation (SSFM)
In addition to multiphase synchronisation, EMI can be improved by continuously varying the frequency of the switching regulator clock. This technique, referred to as SSFM, improves EMI by not allowing emitted energy to stay in any receiver’s band for a significant length of time. There are four primary considerations for maximum SSFM effectiveness:
• The bandwidth of the impacted receiver
• The method for modulating the frequency
• The amount of frequency spreading
• The rate of modulation
Receiver
Whenever considering EMI, the designer should understand the bandwidth of the EMI affected receiver(s). These receivers could be real system devices, or they could be receivers used for regulatory conformance per CISPR 16-1.
Fortunately, regulatory agencies use receivers with bandwidths that reflect real-world devices. The receiver’s bandwidth determines two important characteristics; the range of frequencies for which the receiver will respond and the receiver’s response time when subjected to EMI.
Modulation method
Most switching regulators exhibit ripple that varies with frequency; more ripple at lower switching frequencies and less at higher switching frequencies.
As a result, a switcher’s ripple will exhibit an amplitude modulation if the switching clock is frequency modulated. If the clock’s modulating signal is periodic, such as a sine wave or triangle-wave, there will be a periodic ripple modulation and a distinct spectral component at the modulating frequency.
Since the modulating frequency is much lower than the switcher’s clock, it may be difficult to filter out. This could lead to problems, such as audible tones or visible display artefacts, due to supply noise coupling or limited power supply rejection in the downstream circuitry.
A pseudo-random frequency modulation can avoid this periodic ripple. With pseudo-random frequency modulation, the clock shifts from one frequency to another in a pseudo-random fashion.
Since the switcher’s output ripple is amplitude modulated by a noise-like signal, the output looks as if there is no modulation and the downstream system implications are negligible.
Modulation amount
As the range of SSFM frequencies increases, the percentage of in-band time is reduced. If the emitting signal enters the receiver’s band infrequently and for short periods, relative to its response time, significant EMI reduction occurs. For example, frequency modulation of ±10% will be much more effective at EMI reduction than frequency modulation of ±2%.
It should be noted that ±2% SSFM is common for microprocessors and data clocks, because they cannot tolerate large frequency variation. However, switching regulators have a limited range of frequencies for which they can tolerate. As a general rule, most switching regulators can easily tolerate frequency variation of ±10%.
Modulation rate
Similarly to the modulation amount, as the rate of frequency modulation increases, the time that EMI will be ‘in-band’ for a given receiver will decrease and EMI will be reduced. There is a limit, however, to the rate of frequency change (dF/dt) that a switcher can track.
The solution is to find the highest modulation rate that does not impact the switcher’s output regulation.
Silicon oscillators provide an ideal platform for multiphase, spread-spectrum switching regulator clocks. In addition to having an onboard clock generator, these solid state devices can combine spread spectrum modulation and multiphase outputs.
With this in mind, Linear Technology developed the LTC6909; a precision spread spectrum silicon oscillator with eight separate multiphase outputs. A single resistor selects the output frequency from 12.5 kHz to 6.67 MHz.
Three logic inputs set the output phase relationship in a range of 45° to 120°, allowing the LTC6909 to provide synchronisation for up to eight phases. A pseudo-random spread spectrum frequency modulation can be enabled with frequency spreading of ±10% of the centre frequency.
The user selects one of three modulation rates to ensure that the modulation rate does not exceed the regulator’s bandwidth.
In addition, the LTC6909 has an innovative filter that tracks the SSFM modulation rate and provides smoothing between frequency transitions.
Using multiple switching regulators in a single system can present a significant EMI concern. In addition to standard layout, filtering and shielding practices, the use of multiphase synchronisation and spreadspectrum frequency modulation can dramatically improve EMI performance.
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