[BC] AC/DC & switching supplies

[WFIFeng@aol.com]

In a message dated 12/04/2006 11:17:52 AM Eastern Standard Time, 
DHultsman5 at aol.com writes:

>  Several companies are now using  switching 
>  techniques and using AC  direct with no transformers.

That's not entirely true. Granted, there is no tranny on the AC input side of 
these supplies, but there is, indeed, a transformer which isolates the DC 
output side from the AC input. 

Switching supplies take the incoming AC and rectify/filter it into high V DC. 
A PWM oscillator drives power transistors (in the dozens of Khz range) to 
drive relatively small, ferrite-based transformer(s) to step the voltage down. 
The PWM is used to regulate the output voltages. The frequency is usually not 
critical, and can be up/down by several percent or more and can vary with load 
current or supply voltage. (Without proper RFI filtering, these things are 
effective, wide-band jamming devices.)

The main reason for using switching supplies over analog is that they are 
much more efficient. (HF pulses vs 60Hz sinewaves.) Also, using high frequency 
pulses requires *much* smaller transformers than linear supplies running at 
60Hz. They also require much smaller filter caps for large currents. A few 
thousand uf will filter dozens of amps of low voltage output. You'd need hundreds of 
thousands of uf to filter the 120 hz ripple at those currents. (Hence, the 
large soup can sized 'lytics in old linear computer power supplies of the past.)

Willie...