[time-nuts] Parallel voltage regulators

[Bruce Griffiths]

J.D. Bakker wrote:
>> Poul-Henning Kamp wrote:
>>     
>>>  Considering the efficiency and easy availability of switchmode
>>>  supplies these days, I would never bother with a linear regulator
>>>  in a new design.
>>>
>>>  For instance national has a series of switch mode regulators (LM25xx)
>>>  which just requires a coil and a diode more than the usual LM78xx
>>>  types.
>>>
>>>  If you are worried about noise from the swichmode, you can add more
>>>  filtering to the output.
>>>
>>>  
>>>       
>> That is a recipe for disaster if one wants a really low noise oscillator.
>> This is particularly true if one is a beginner.
>> Reducing the output noise of a switching regulator to 100uV rms or less
>> is neither easy nor simple.
>>     
>
> http://www.linear.com/pc/downloadDocument.do?id=4159
>
> In this document, Linear Technology's Application Note 70, a 
> switchmode power supply design is presented with less than 100uV 
> output noise, and little RFI. This is achieved by using a switch 
> controller which has controlled output switch slewing, allowing a 
> trade-off between EMI/RFI and efficiency. Methods to measure and 
> control PSU noise are also discussed (IMHO, the EMI sniffer probe in 
> Section J is particularly interesting).
>
> BTW: contrary to popular belief, linear PSUs are NOT inherently 
> noise-free. The cap charging current spikes through the rectifier 
> diodes can produce plenty of EMI/RFI.
>
> JDB
> [not claiming it's easy, just pointing out one route that could be taken]
>   
However  this isnt a recommended approach for a beginner and you can
easily do much better with a well designed linear regulator.

The EMI due to diode reverse recovery and charging the reservoir
capacitor is exacerbated by the modern tendency to leave parts out of
designs, particularly if one doesn't understand their purpose.
A shunt capacitor across the input of a bridge rectifier or a capacitor
in parallel with each of the rectifier diodes can help by reducing the
area of the radiating EMI loop. However some damping may also be required.

Another problem with using switching regulators can arise when using an
input filter with the regulator.
Since the input power is to a first approximation independent of the
input voltage, for frequencies from dc to an upper limit determined by
the switching frequency the regulator has a negative resistance input
characteristic. This means that undesired oscillations are possible if
the input filter design doesn't take this into account. This situation
can easily arise when the load is another switching regulator (eg when
powering a Z3815 from a switching regulator usually Ok when powered by a
switchmode power supply, but if one decides to add a filter between the
switchmode power supply and the Z3815 one may just create an interesting
power oscillator instead of a quieter input supply).

Bruce