Dave Lag wrote:
> Are you saying PID can never work or you just can't tune it to do this
> well?

PID can be useful for many control problems, and power supply regulation can
be thought of as a control problem.  However, switching power supply
regulation also has other attributes such that often a simple pulse on
demand system produces good results.  Transient response is very good with
this scheme since it doesn't have to "ramp up".  It is not predictive, so a
clever and well tuned algorithm could theoretically do better in some cases.
In other words, pulse on demand doesn't include the logic "if the voltage is
already near the threshold and heading down, do a pulse a little sooner so
the bottom of the dip doesn't get so low".

The flip side is that prediction is by definition an assumption, and
assumptions can be incorrect.  If something stops drawing current suddenly,
then the predictive scheme could cause higher overshoot than a dumb pulse on
demand system.

Overall, due to the highly unpredictable and high frequency nature of load
current draw, and dumb but reliable system is often a better choice for
switching power supply control.  You make sure that the worst case
excursions above and below the set point as acceptable, and work with that.


******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist