Audio band conducted emissions suppression on power feeders

#Standards ETSI ETS 300 132-2 and ANSI T1.315 specify an allowable amount of audio band emissions (up to 20 kHz) that may be conducted from a telecom shelf onto DC power feeders supplying the shelf. Typically, the main source of these audio band conducted emissions is the forced air cooling system, which comprises fans or blowers drawing a current pulsed at less than 1 kHz. The invention suppresses the audio band conducted emissions resulting from the pulsed current sufficiently to comply with the stated standards.

#FIGS. 1 and 2 show two prior art approaches of limiting audio band emissions conducted onto DC power feeders supplying a telecom shelf. In the first approach, shown in FIG. 1, a large passive L-C filter 10 is placed between the power feeder connection point on the shelf and a high bandwidth DC-DC converter 11 that supplies the fans or blowers 12 of the shelf's cooling system. The passive filter limits the amount of audio band emissions that are conducted from the DC-DC converter 11 back onto the DC power feeders. In the second approach, shown in FIG. 2, a low bandwidth active filter 14 is placed between the high bandwidth DC-DC converter 13 and the fans or blowers 15. In this case the low bandwidth active filter limits the audio band emissions conducted through the DC-DC 13 converter onto the DC power feeders. In both approaches, the audio band emissions caused by the pulsed current used to drive the fans or blowers are sufficiently reduced before they are conducted back onto the DC power feeders, thereby enabling the telecom shelf to meet the aforementioned standards.

#Regarding the first prior art approach, shown in FIG. 1, the main problem with this approach is that the physical size of the components required for the passive filter 10 is large, and therefore they occupy more than a desirable amount of shelf space, which is usually at a premium.

#The above and other objects, advantages and features of the invention will become more apparent when considered with the following specification and accompanying drawings wherein:

#FIG. 1 is block diagram of a prior art power supply system for audio band conducted emission suppression,

#FIG. 2 is a further prior art approach to limiting audio band emissions onto DC power feeders supplying a telecom shelf,

#Referring to FIG. 3, according to the present solution the inherent filter, within the switch-mode power converter 16 that powers the fan or blower unit 17, is used to suppress the audio band noise, thereby eliminating any need for additional passive or active filters used in the prior art. Since this lower bandwidth power converter requires a similar (or same) number of components to implement as the wider bandwidth power converter used in the prior art approaches, it occupies substantially the same physical space. Consequently, not needing additional filtering, the invention uses fewer components than the prior art approaches, and therefore uses less shelf space and dissipates less heat.

#FIGS. 4 and 5 show the switch-mode DC-DC converter of FIG. 3 in more detail and the frequency response of its control loop, respectively. By modifying the feedback amplifier compensation network 20 such that the frequency control loop bandwidth of the power converter is substantially less than the AC load of the fans or blowers, the invention eliminates the need for additional filtering beyond that inherent in the power converter 16. This is because the frequency of the AC current drawn by the fans or blowers 17 is beyond the bandwidth of the power converter 16, and consequently the converter 16 will not react to changes in the AC current load. Since the converter 16 takes no action, it will continue to provide only the DC portion of the fan or blower current required, hence the current drawn from the shelf power feeders will be DC and the audio band AC noise will be eliminated.

#The fans/blowers units 17 commonly used in telecommunication systems and applicable to this invention are DC powered only. In other words, the fans/blowers 17 require a DC power source only. Internal to the fan/blower is a motor drive circuit (not shown) that derives from the DC source, the required AC voltage necessary to drive the fan/blower. The resulting load current signature from the fan/blower drawn from the DC source is an AC waveform with a DC offset as shown in FIG. 6.