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VRM Spread Spectrum Frequency Modulated for Reduced EMI

Published Date
Author OrCAD PCB Solutions

Key Takeaways

  • Spread spectrum modulation is one technique to scatter noise by concentrating on a particular frequency. This is an attempt to lower the energy, amplitude, and strength of the EMI.  It also offsets the input current and output voltage ripples in the DC-DC converter. 

  • The clock keeps on moving in spread spectrum modulation. The EMI peak energy is comparatively reduced by using a non-fixed frequency clock, so the EMI energy gets distributed to other frequencies.

  • Other popular approaches in pseudo-random modulation include random pulse position modulation (RPPM), random pulse width modulation (RPWM), and random carrier-frequency modulation with a fixed duty cycle (RCFMFD), or with a variable duty cycle (RCFMVD).

Voltage regulator module

Voltage regulator module

Electric and magnetic fields in communication devices interfere with communication signals, causing Electromagnetic Interference (EMI) and degrading the performance of the system by injecting noise. In portable electronic devices, Voltage Regulated Modules (VRM) generates both conducted and radiated EMI. VRMs are usually switch-mode power supplies, utilizing modulation techniques to output required DC voltage. In VRM, spread spectrum frequency modulation can be employed as a technique to reduce EMI. By using this technique, the noise is spread over a wider bandwidth and shaves off the peak and average noise occurrences at particular frequencies. 

Voltage regulator module

VRM with a clock signal given to switching device

VRM Spread Spectrum Frequency Modulation

Switch-mode DC-DC regulators used as VRMs in communication system boards utilize the spread spectrum frequency modulation technique for reducing the EMI and noise. Generally, DC-DC converters employ fixed or constant switching frequency operation. The rapid switching in this modulation method produces noises at fundamental and harmonic frequencies, along with conducted and radiated EMI.

Poor design layouts and bad placement of capacitors and inductors can increase the impact of EMI and noises, and are often the main cause of issues in VRMs. In certain electronic devices containing multiple DC-DC regulators, EMI and noises concentrate at one particular frequency and start to jeopardize the normal circuit operation and neighboring systems. The spread spectrum modulation technique works to scatter noise by concentrating on any particular frequency. This is an attempt to lower the energy, amplitude, and strength of the EMI.  It also offsets the input current and output voltage ripples in the DC-DC converter. 

In spread spectrum frequency modulation, the switching converter clock frequency is non-fixed. The frequency of the clock in modulation keeps on changing, thus generating noises and harmonics of different frequencies each time. The clock keeps on moving in spread spectrum modulation. The EMI peak energy is comparatively reduced by using a non-fixed frequency clock and the EMI energy gets distributed to other frequencies. 

Spread Spectrum Frequency Modulation Techniques

Even though the periodic frequency modulation is effective in spreading the frequency spectrum of noises, we will focus our discussion on pseudo-random modulation. In this technique, the clock shifts from one frequency to another in a pseudo-random fashion. This provides sufficient attenuation of the fundamental frequency and wider spectral spreading. 

The popular approaches in pseudo-random modulation are random pulse position modulation (RPPM), random pulse width modulation (RPWM), and random carrier-frequency modulation with a fixed duty cycle (RCFMFD), or with a variable duty cycle (RCFMVD). In RPPM, the position of the clock pulse is randomized for each switching cycle. It is comparable to constant PWM switching, with a random start position of the clock cycle, not commencing at the start of the switching cycle. The average pulse width is maintained at a desired duty cycle in RPWM, but the pulse width is varied continuously. The characteristics of RCFMFD are fixed duty cycles with a randomized switching period. When the constant duty cycle in RCFMFD is replaced with a constant pulse width, it transforms into RCFMVD.  In RCFMVD, the duty cycle is randomized, however, the average duty cycle meets the desired value. 

If you are working on regulated power supply circuits handling the telecom frequency range, there is an endless need for modulating them through the spread spectrum technique. The VRM spread spectrum frequency-modulation reduces EMI, noises, and ripples effects by spreading their frequencies over a wider bandwidth. It also improves efficiency compared to constant switching frequency modulation techniques, with a tremendous decrease in total losses. 

The Cadence Design tools can be utilized for studying the impact of spread spectrum modulation on DC-DC converters. OrCAD is one of the best simulation tools available today to compare the pros and cons of constant switching frequency modulation with the spread spectrum technique. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.