Hi David,
> Say you have a simple, non-stabilised switcher; then, in a push pull or
> bridge design, a small guard band is needed between each side
> energising/de-energising transition to allow for storage effects, to prevent
> cross-conduction.
Now I see what you mean. Yes, of course. It was a naming issue: I know
this as "dead time"!
> This guard band is "filled in" with the output capacitance.
Yes, but in any case it's bad practice to use such a simple switcher
without any inductance. There should be some inductance to limit the
inrush pulse in each conduction cycle. Of course, this inductance can be
very small.
> This value of capacitance would be a lot smaller than needed
> in a stabilised design.
Not necessarily. In big switchers, it's more economical to do most of
the output filtering (and energy storage) in the coil, and not the
capacitor! High power switchers often have a large filter inductor,
followed by just a few polystyrene filter caps totalling 1uF or so.
I would not build such an unregulated switcher, though. It's very bad
economy! In such a thing, the output 100 (or 120) Hz ripple is
essentially the same as appears on the primary filter capacitor, so you
need a huge primary cap to get decent ripple. And large electrolytics
are expensive, often becoming by far the most expensive part of such a
power supply!
If instead we use regulation by pulse width modulation or by phase
shifting, then we can maintain the output constant, with essentially no
100 or 120Hz ripple, while using a much smaller and cheaper primary
filter cap running with 30 or even 40% ripple on it. The small parts
needed to implement the control of the power devices are dirt cheap, and
so the whole power supply with the smaller input cap ends up less
expensive, with less output ripple (essentially none), and there are two
added bonuses: Tightly regulated output voltage, and an improved input
power factor caused by the shallower and more stretched current pulses
taken from the power line! That's without adding any power factor
control circuitry, which of course can be done too, but increases
complexity and cost.
For the above reasons, the reduced cost and much better performance of a
regulated switcher compared to an unregulated one, I truly wonder why
the power supply commented in this thread is unregulated! I would like
to know if the designers had any good reason for it, or if it was done
simply due to lack of expertise.
It has been an extremely long time since I last came across any
unregulated switching power supply in commercial production! That last
time was back in a time where pulse width modulation was an esoteric
technique and had to be implemented using lots of discrete components!
Manfred.
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