Monday, July 11, 2011

Spread-spectrum power converter designed for low EMI

DC/DC point-of-load switch mode regulator Circuit with spread spectrum frequency modulation can be designed for low EMI, write Alan Chern, Doug La Porte and Afshin Odabaee

Switch mode DC/DC regulators are popular means of voltage regulation in densely populated system boards because of the regulators’ low heat dissipation.

However, rapid switching of current, poorly defined layout, placement and selection of components such as inductors make their circuits a prime candidate for EMI (electromagnetic interference) source. Moreover, the potential problem for interference and noise can be exasperated when multiple DC/DC switch mode regulators are paralleled for current sharing and higher output power.

If all are operating (switching) at a similar frequency, the combined energy generated by multiple regulators in a circuit is then concentrated at one frequency. Presence of this energy can become a concern especially if the rest of ICs on the PC board as well as other system boards are close to each other and susceptible to this radiated energy.

One solution is to spread this energy among a range of many frequencies, instead of being concentrated at one frequency, thus lowering its amplitude and strength.

This solution is to use a spread spectrum frequency modulation (SSFM) clock. The idea behind using a spread spectrum technique to reduce EMI is to keep the clock moving.

A steady clock is an easy target for neighbouring devices and conformity test equipment to lock onto, allowing them time to accumulate emanating signal energy at the fixed clock frequency or its harmonics.

Additionally, a special modular DC/DC switch mode regulator system can provide high power, low heat dissipation, as well as low EMI power supply solution for densely populated boards. The benefit in using modular and pre-assembled DC/DC switch mode regulator circuits on a substrate is to optimize the layout by proper grounding and minimizing current loops while operating within a wide range of switching frequencies and to allow phase-lock-loop capability.

For best results, such device should include and enclose all the required components such as inductor, DC/DC regulator, mosfets, and compensation circuitry in a small package.

A technology attainable with complete DC/DC switching regulator systems-in-a-package allows high current modular point-of-load regulator with low EMI and low output and input ripple current.

The board is an example of a system (4.5V to 20V input, 0.6V to 5V output) with four LTM4601 µModule switching regulators in parallel. Each unit current share a maximum of 12A is placed in parallel delivering up to 48A of output current.

A schematic design is shown in Figure 2. AN119A and AN119B offer a detailed background explanation of this system which can be helpful in understanding the setup since this discussion will only cover the spread spectrum frequency modulation feature.

The previous discussion in AN119A and AN119B concentrated on replacing a single switcher with four paralleled µModule regulators which will reduce peak switching current through multiphase synchronization.

The switchers are externally driven by a clock that is phase shifted. Each switcher’s turn-on time is divided amongst themselves yielding even current spread across the input.

Figure 3 gives a clear example of how the switchers are synchronised out of phase with respect to the clock; spreading out the switching time evenly. In this example, the phase shift is 90 degrees.

The voltage ripple is therefore reduced in this high powered system. The key benefit to synchronized parallel regulators is the reduction of input and output capacitor size due to the cancelling of ripple currents on the input and output.

This eliminates the need for large bulk capacitors. There is recent interest in electromagnetic interference (EMI) qualms pertaining to Linear Technology µModule regulator units and will address EMI reduction using the new LTC6909 spread spectrum feature.

A key difference between the previous LTC6902 oscillator and the new oscillator is the increased number of outputs on the LTC6909.

The LTC6902 has a maximum of 4 outputs verses the 8 output, 8-phase LTC6909 with superior spread spectrum frequency modulation (SSFM) which effectively improves electromagnetic compatibility performance. Perhaps a future article will explore 8-phase parallel µModule system. But for the purposes of this article; the application design will remain the same, using 4-phases on the LTC6909 and focus only on SSFM.

Electromagnetic characterisation

The same design setup will be used as a benchmark using the LTC6909; a 4-phase paralleled µModule system, running 12V input to 1.5V output at 40A, though capable of up to a maximum of 48A.

The multiphase synchronization provides some EMI reduction; although it may not be sufficient to satisfy stringent EMI regulations. A spectrum analyzer is used to examine the frequency harmonics of the current system.

The fundamental and harmonics are observed ranging between 150kHz to 30MHz output frequency spectrum. The resolution bandwidth is 9kHz.

Observations on individual harmonic frequencies indicate specific levels of output on the current system. Depending on the specific EMI requirement, these harmonic spikes can exceed regulations and void certification. Failing emissions can be a system designer’s worst nightmare.

Thus having a trump card can be essential to saving time and money. The spread spectrum frequency modulation on the LTC6909 is exactly that. By continuously varying the µModule’s clock frequency, EMI is improved by forcing emitted energy to move around, preventing it from staying in any receiver’s band.

The ripple emitted by the µModule switcher is the key perpetrator in supplying the unwanted spectral harmonics. The LTC6909 SSFM feature will lower the harmonics through a pseudorandom noise signal to spread the energy over a wide frequency band thus decreasing peak electromagnetic radiation.

Reducing EMI with spread spectrum frequency modulation

The LTC6909 easily sets the SSFM by controlling the MOD pin. To activate; the MOD pin is floated for a frequency (700kHz set by a resistor) divide by 32 modulation rate.

An impressive 10dB reduction is observed on the harmonic frequencies. Such amazing reduction certainly cannot be without caveats and should be met with scepticism.

Further analysis of the system shows that the output ripple is not affected drastically when using SSFM.

A point of contention is that the load transient response may suffer due to the constant change in frequency from the SSFM. A load transient test performed on the output at 20A load step reveals that the SSFM does not affect the response time at all. Both response wave time and peak to peak values are similar.

It is possible to reduce the EMI by 10dB when using spread spectrum frequency modulation on 4-phase µModule DC/DC switching regulator 12V input, 1.5V output, 40A design.

This design utilizes multiphase synchronisation and SSFM to produce clear benefits, such as reduced output capacitor voltage rating, lower ripple, and decreased EMI. Furthermore, SSFM is easy to implement on the LTC6909 for this type of operation. This design is highly useful as a quick method for reducing EMI.


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