PWM in a switching power supply

[BradBrigade]

Hi,

First of all, I'm trying to figure out how switching power supplies
work (the ones in PCs). I've found very basic info, but I want more
technical stuff. If anyone has some good links please let me know.
These are questions I have yet to find an answer for.

Anyway, here's my question. One thing I read was that the output
voltage of the supply is fed back to the PWM which changes it's duty
cycle accordingly to keep the output voltage constant. But I thought
that the input-to-output ratio of a transformer is fixed. If the PWM
is outputting 100V at 20KHz to a 10:1 transformer, you get out 10V at
20KHz, right? What does it matter what the duty cycle is? It's still
100V at 20KHz. What am I missing?

Second, why does a switching power supply break without a load?

Third, in all my years in electronics, I have never used a choke, now I
see them all over these power supplies. Can someone clue me in about
what they do, and why they are in these things?

I appreciate any info at all. Thanks a lot.

 

[BradBrigade]

John Popelish wrote:

If the transformer is a voltage output (produces some ratio of the
primary voltage when the switches are on) and zero the rest of the
time), then, yes, the peak output voltage is essentially independent
of the duty cycle. but those kind of transformers also require an
additional LC filter that outputs a voltage about equal to the average
input voltage, not the peak. Holding the peak voltage for a smaller
part of the cycle lowers the average voltage.

OK, I got it, the output is filtered to create a stable voltage that is
the average of the duty cycle. But now I'm wondering, what is the
purpose of the transformer? If you want to convert 100V to 10V, why
not filter the output straight from a PWM with a 10% duty cycle?
What's the difference?

Thanks a lot.

 

[PeteS]

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

 

[BradBrigade]

Awesome! That really answered my question. Thanks for all the help
guys, and the links!

 

[PeteS]

Regarding Joel's notes on filter sections, it is true that in many
instances a relatively simple filter calculation may be done for a
workable switcher, but that depends on a number of things.

For a voltage mode controller, a widely varying input makes life
difficult and is where a multipole (4 or even 5 filter sections) may be
necessary, with the same provision applicable to large load steps for a
current mode controller.

Where one has both (the usual situation in a lot of embedded systems
nowadays), then a thorough knowledge of filter theory is certainly an
advantage when one must do their own switcher, even though one may use
spice programs; the issue is to use the program effectively, a
knowledge of what one is doing helps

There are a number of reasons for designing one's own, including space,
efficiency, unusual output voltages (althoguh there are 'adjustable'
bricks out there) and Vin / Vout functions that have a particularly
wide range. As an example, I had to do one that had 10-14V in nominal,
1.2V out, at loads from ~0 to 45A, with load steps of <30A in
<2millisec, and efficiency was required to be >90% across the load and
input range. That was quite a challenge. I managed to achieve >94% with
the assistance of the vendors involved.

I deliberately did not even attempt to cover everything (that's a
subject in it's own right that many spend entire careers on, and I
thank them for their assistance , but merely try to point out that
the loop filter is a critical issue in the design of a switcher
(although one may use the 'suggested application' in many cases) that
requires some attention, and is critical to understanding failure
causes.

Cheers

PeteS