PWM in a switching power supply

[Ban]

w_tom wrote:
That supply does not provide

sufficient galvanic isolation. All appliances must have
internal transient protection. Intel specs demand that
computers be even more robust. But that feedback circuit does
not provide thousands of voltages of isolation. That's right -
thousands of volts. An optoisolator for galvanic isolation
should have been located where those feedback resistors are
located.

Does your power supply for the 'who-dad device' require
galvanic isolation? Maybe. Maybe not. But galvanic
isolation is but another function in power supply design.
This noted because so many buy power supplies on price that
are then missing essential internal functions. Does your
clone computer power supply provide galvanic isolation? If
not, then internal transient protection has been compromised.

This posted just to demonstrate but another complication of
power supply design.

W_tom, you are wrong, the primary side is insulated by 3 transformers T2,T3
and the always on +5V supply T6. Depending on these transformers will be the
isolation, maybe even 1200-2000V for a short time. The outputs and the
internal circuits are not isolated from each other, which saves a lot of
optos and associated transmission problems. The same is true for the power
good line and the PS-ON line.
I wonder why there would not be galvanic isolation?

 

[Ban]

w_tom wrote:
That supply does not provide

sufficient galvanic isolation. All appliances must have
internal transient protection. Intel specs demand that
computers be even more robust. But that feedback circuit does
not provide thousands of voltages of isolation. That's right -
thousands of volts. An optoisolator for galvanic isolation
should have been located where those feedback resistors are
located.

Does your power supply for the 'who-dad device' require
galvanic isolation? Maybe. Maybe not. But galvanic
isolation is but another function in power supply design.
This noted because so many buy power supplies on price that
are then missing essential internal functions. Does your
clone computer power supply provide galvanic isolation? If
not, then internal transient protection has been compromised.

This posted just to demonstrate but another complication of
power supply design.

W_tom, you are wrong, the primary side is insulated by 3 transformers T2,T3
and the always on +5V supply T6. Depending on these transformers will be the
isolation, maybe even 1200-2000V for a short time. The outputs and the
internal circuits are not isolated from each other, which saves a lot of
optos and associated transmission problems. The same is true for the power
good line and the PS-ON line.
I wonder why there would not be galvanic isolation?

 

[w_tom]

I stand corrected. Transformers T2 and T6 do isolate the
R25 and R26 resistors from the mains. Those other
transformers do provide the necessary galvanic isolation even
to the PS-ON signal.

The point begin galvanic isolation is necessary when
circuits can be exposed to human contact and for other reasons
involving semiconductor protection. Not all power supplies
provide galvanic isolation nor need that function be
necessary. Just another of so many functions that must be
part of power supply designs.

 

[Joel Kolstad]

Pete,

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.

[Lots of good explanation deleted...]

Those are all good points, but keep in mind why chips such as National's
"Simple Switchers" are so popular: Many relatively inexperienced engineers are
given the job of designing relatively undemanding switchers, and they use
whatever they can to get the job down as best as possible. I think it's a
safe assumption that if someone can't tell you why a "Simple Switcher" IC
isn't an all-purpose solution, it's the kind of chip they should be using!

I am all for people learning traditional filter design theory. It's
unfortunately an area that's slowly dying due to more and more filters being
able to be replaced by DSP techniques.

---Joel Kolstad