[time-nuts] need power trans. for HP3325

Didier Juges didier at cox.net
Sun Mar 25 21:40:45 EDT 2007


Dr Bruce Griffiths wrote:
>
> Didier
>
> Try using a Murata BNX002 filter between the switching supply output and 
> the linear regulator input.
> These should eliminate the High frequency noise (0.5MHz - 1GHz) from the 
> switching regulator output leaving the linear regulator to deal with the 
> low frequency noise.
> However for these to be effective a low impedance ground plane is required.
>
> Worst case linear regulator design allowing for low mains input and some 
> margin for the ripple from the rectifier certainly increases the series 
> pass element dissipation considerably over that when the input to the 
> regulator can be held within 0.1V or so with low ripple.
>
> The traditional preregulator employs SCRs in a phse controlled rectifier 
> arrangement, the resulting low frequency output noise from such 
> preregulators is easily dealt with by the linear regulator.
>
> Bruce
>   
Bruce,

I work for a company that designs and builds switchmode power supplies, 
so I have access to lots of power supply and filter components, and for 
better or for worse, we are used to dealing with switcher noise.

I will check the Murata filters though, I am not familiar with those.

Most switching supplies operating up to 200kHz make significant noise up 
to 30 to 50 MHz, and much less above that. Of course, if you design a 
receiver, you may still be able to detect much lower level emissions up 
to 300-400 MHz.

Another pitfall of cheap switchers is line emissions, where noise is fed 
back into the mains and may affect other equipment (by conduction or 
radiation from the AC line itself). This is normally controlled by 
regulations, such as FCC regulations here in the US for commercial 
equipment, and via MIL-STD-461 for military equipment.

It has been a long time since I have seen an SCR preregulator. They have 
almost the same complexity as a switcher (from a parts count 
standpoint), without any of the advantages, other than slightly lower 
noise levels at higher frequencies. However, they are bad for line noise 
(conducted emissions) and power quality (power factor, harmonic 
distortion) and require oversized transformers, inductors and 
capacitors, even compared to a normal linear supply. These are now 
completely abandoned (except maybe at very high power levels, as used in 
industrial processes and I believe in rail transportation) because they 
can't meet the power quality requirements and they cost too much/are too 
big and heavy.

In the past, we have been using what we call linear-switchers (switcher 
followed by a linear regulator) in some cases when the customer had high 
expectations for cleanliness and efficiency, but nowadays, we can meet 
all our customers needs with high frequency switchers, using good design 
practices to minimize noise in the first place, and effective filters to 
take care of the rest..

Didier



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