xDSL Pushes DC-DC Converters to Higher Frequencies

The throughput or user bit rate for xDSL is highly dependent on the receiver's signal-to-interference ratio (SIR). There are many sources of interference that affect channel bandwidth and, hence, throughput. Most of these are plant related, such as crosstalk from adjacent wire pairs within the binder group, RF EMI ingress from the drop, and impulse noise from appliances that couple in from the home's inside wiring. There is another source of noise — the noise generated by the switching power-supply circuitry within the modem itself — that has been the bane of DSL customer premises equipment (CPE) designers.

The switcher noise is generally common-mode noise that couples in from the ground plane and noise on the power-supply rails that enters the front-end, equalizer and demodulator sections. In the past, the problem was avoided by using linear regulators, which do not generate switching noise, throughout the modem. However, the poor efficiency of the linear regulators meant that a more expensive ac-dc converter was required, and the heat generated by the linear regulators led to reliability and safety issues. These safety and reliability issues, together with cost considerations, have driven DSL-modem OEMs to use switch-mode dc-dc converters.

One common approach to dealing with switcher noise is to use a second-stage L-C filter to reduce the ripple voltage at the immediate output of the supply. While this can greatly reduce the output switcher noise, the introduction of a second-stage filter limits the ability of the regulator to respond to large load transients. Poor transient performance can mean that the regulated output voltage may go outside the limits of the modem ICs and cause data corruption or the modem to hang up or reset. Further, the second-stage filter may not resolve the common-mode noise problem.

It is helpful to understand how switching noise affects the DSL modem performance. The downstream carrier band is divided into discrete multi-tone (DMT) subcarriers, spaced approximately 4 kHz apart. Bandwidth is achieved by aggregating these subcarriers. If noise generated by the switch-mode converter's fundamental switching frequency, and/or its harmonics, falls within the DSL downstream band, throughput will be severely degraded. Hence, switching frequencies that fall outside of these bands are highly desirable.

The downstream band for ADSL/ADSL2 falls between 0.138 MHz and 1.1 MHz, and the band for ADSL2+ falls between 0.138 MHz and 2.2 MHz. A switcher that operates above that band is guaranteed not to cause interference in the downstream band. The newer VDSL2 standard (G.993.2) is a bit more complicated having two or three downstream bands designated as D1, D2 and D3. To further complicate this, the band plans vary by region or operator as follows:

Plan 997: D1 = 0.138 MHz to 3 MHz; D2 = 5.1 MHz to 7.05 MHz
Plan 998: D1 = 0.138 MHz to 3.75 MHz; D2 = 5.2 MHz to 8.5 MHz
Annex C (Japan): D1 = 0.138 MHz to 3.75 MHz; D2 = 5.2 MHz to 8.5 MHz; D3 = 12.0 MHz to 18.1 MHz.

To ensure spectral compatibility with VDSL2, the switching frequency would need to fall outside of these downstream bands, ideally at or just under 5 MHz, to avoid harmonic content from falling into D2, or above 8.5 MHz.

Today, typical switch-mode dc-dc converters are fabricated in 1.2 micron to 0.5 micron geometry silicon. MOSFETs fabricated at these geometries tend to become very lossy at switching frequencies above 500 kHz to 1 MHz. Fortunately, a new class of converter silicon has been pioneered by companies such as Enpirion that enable low-loss operation at very high switching frequencies. This new class of converter silicon is fabricated in deep submicron geometries, supports frequencies today as high as 5 MHz, and can be extended to 10 MHz or even 20 MHz while maintaining favorable loss characteristics.

Future xDSL evolution will move the spectral requirements ever higher. To complicate the matter, the higher frequency downstream bands are more noise sensitive. The drive to push the xDSL downstream spectrum ever higher also will push manufacturers of power semiconductors to finer silicon process geometries and higher switching frequencies — that is, if they care about their customers.

Michael Laflin is the director of marketing at Enpirion. He has more than 20 years of experience in wireline and wireless telecommunications systems, data storage systems and data communications.