[time-nuts] Thunderbolt - any negatives ?

Robert Atkinson robert8rpi at yahoo.co.uk
Mon Jun 1 15:46:00 EDT 2009


Most switch mode power supplies actually run a voltage doubler on the input when running on 110V. This puts over 300V across the transformer and switch. Also the regulation loop crosses the isolation barrier introducing more failure points that can result in overvoltage.

Robert G8RPI.

--- On Mon, 1/6/09, SAIDJACK at aol.com <SAIDJACK at aol.com> wrote:

> From: SAIDJACK at aol.com <SAIDJACK at aol.com>
> Subject: Re: [time-nuts] Thunderbolt - any negatives ?
> To: time-nuts at febo.com
> Date: Monday, 1 June, 2009, 6:17 PM
> Hi there,
>  
> A switcher has much more stresses on the components, since
> it  usually 
> switches the primary side rectified 110/220V high-voltage
> across a  transformer. 
> Thus the switching FET has to be very high voltage capable
> (about  ~170V DC 
> in the US), and the second  component under stress is
> the primary  high 
> voltage capacitor, because it sees a very fast AC switching
> current on it  
> (current draw is on when the FET is on, and off when the
> Fet is off).  Also 
> there has to be a fast snubber network to prevent the
> back-emf from  destroying 
> the primary Fet with over-voltage.
>  
> A linear supply has none of these fast current/voltage
> transients on it,  
> only a couple of diodes switching the 60Hz secondary onto a
> capacitor at low  
> voltage.
>  
> A secondary concern is thermally induced stress, switchers
> will usually be  
> packed into a very small enclosure with very high power
> capability/density. 
> This  is not possible for linear supplies, since the
> transformer size will 
> usually  determine overall sizing. Compare a Laptop
> power supply size 
> (usually these have  between 40W and 90W rating!) to a
> similar rated linear supply.
>  
> bye,
> Said
>  
>  
> In a message dated 6/1/2009 09:48:29 Pacific Daylight
> Time,  
> hmurray at megapathdsl.net
> writes:
> 
> Is there  something I don't understand in this
> area?  What makes a linear  
> supply more reliable than a switcher?
> 
> My first guess would be a  switcher would be more
> reliable because it would 
> run cooler.
> 
> That's  probably assuming the same amount of design
> effort which is 
> probably 
> not a  valid assumption if I'm comparing a brand-X
> linear with a brand-Z  
> switcher.  A quick glance at the general construction
> might give a  better 
> answer.
> 
> 
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