Re: PWM in a switching power supply

There are excellent resources for switching power supplies at all the
major manufacturers (TI, Linear Tech, Maxim and others).

One of my favourite design notes is from Linear Tech:
http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1142,C1114,P1134,D4162

(AN-73 [pdf] at http://www.linear.com/ should the link not work). This
shows the basic principle of the Switchmode power supply using a
specific device as an example, with the coil used as (as noted) an
energy storage device.

As to why some switching power supplies 'break' with no load, I would
agree it could be poor design, although to be fair to the designers
they may be designed for a specific load. Much depends on the specifics
of the type :
Topologies:
Buck (Step down)
Boost (Step up)
Buck-boost (inverting, usually)
SEPIC (step up and step down - for isntance, generate +5V from a
nominal 6V battery that may have an actual range of 4V to 7V)

Mode:
Current. Inductor (or switch) current is controlled directly
Voltage. Output voltage is controlled directly
Generally, current mode controllers are insensitive to *input voltage*
variations and voltage mode controllers are insensitive to *output
current* variations.

A switching power supply (actually, any regulated power supply) is a
closed loop system that has various (and numerous) filter elements in
the loop. To get regulation employs negative feedback (i.e. an output
variation causes a change at the input such as to [partially] negate
the output variation).

What makes negative feedback negative is the effective phase of the
feedback signal. The filters in the loop add their own phase
characteristics, and if not carefully considered cause sufficient phase
shift in the loop to make the negative feedback positive - giving an
oscillator if it happens at unity gain. This is one of the [many
possible] things that can happen at no load.

Feedback loops of this type have many analogies - the most basic
principles are found in servo theory.
For an excellent app note on loop compensation (the art of keeping
negative feedback negative) for a current mode controller, see
http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1042,C1143,C1083,P1735,D4165
(AN-76, once more from http://www.linear.com/ )
For the filters, the relevant equations

Capacitive filters:
Fx = 1/(2*pi*R*C) where R is the equivalent resistance of the ffilter
and Fx is the Frequency at which the difference between input and
output is 3dB, which is also the point at which the phase difference
between input and output is 45 degrees. The phase and relative
amplitudes may be either leading or lagging depending on the filter
configuration (a leading phase filter is known as a zero, a lagging
phase filter is known as a pole)

Forr voltage mode controllers, there is a 2-pole filter at the output,
given by 1/(2*pi* [sqr(LC)]) where L is the output inductor and C the
effective output capacitance. At this frequency, there is 180 degrees
of phase shift at the output.

Each pole (or zero) has a phase response of 45 degrees per decade, and
an amplitude response of 20dB per decade (alternatively, 6dB per
octave). (Note to others - I realise the filters may be -45 or +45 and
amplitude response could be rising or falling)

So there's a lot of terminolgy and a lot of fundamentals to learn to
understand these things.
I think there's plenty of reading noted here to be getting on with if
you want to understand the subject :)

Cheers

PeteS

.

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