On 18.02.2016 12:35, Cursitor Doom wrote:
On Thu, 18 Feb 2016 00:47:43 +0100, Dimitrij Klingbeil wrote:
[...]
Dimitrij, you've gone to an awful lot of trouble here for which I'm
extremely grateful. The main impediment at this time is that my
fancy, high-end Marconi sig gen is temporarily out of order. Perhaps
I could rig up something to do the same job from a 555 timer, but
that's another story. I followed everything you said - except for one
important thing: you say this board has to be under load at *all*
times when mains powered or it will be catastrophically destroyed (or
words to that effect). I'm a complete switcher novice, so there have
been *many* times when I've probed this board under power with no
load connected; I didn't know any better. Before I go any further I
need to know if I've therefore toasted the board. Which components
exactly catastrophically fail under no-load conditions? Thanks again
for all your help.
Hi
As for the board being loaded at all times...
In that rather long description I posted a method for slowly bringing up
the board "from zero up to where a lamp lights up". Please note that the
first step was to make the board's internal switching controller
inoperative (by disconnecting its start-up power supply and the power
transistor driver transformer) and using a signal generator for timing.
In this particular state of affairs the board (obviously) has no
automatic control, feedback or regulation whatsoever. The only feedback
"control" is through your eyesight (lamp brightness) and the reaction of
your hands ("Turn the duty cycle and the voltage down quick NOW!"). This
is not a particularly fast nor a particularly reliable method of SMPS
control,
so I put in some extra words about exercising caution. When
running the SMPS in this "mode", you need to understand that the power
flow in it is governed by two aspects.
The switcher at a fixed duty cycle (that was set on a signal generator
manually) and at a fixed voltage (also manual from a variac) will
basically be pumping out a fixed amount of power (a fixed amount of
energy per unit time). Normally the power would be controlled (and
automatically reduced very fast if needed) by the SMPS controller chip,
but in full manual mode that's obviously not possible.
Now this "flow" of power is balanced by the dissipation in the lamp -
and only there, because there is nothing else where to put the power
into and no immediate way to reduce it either. As long as the lamp is
still OK, whenever the SMPS power "flow" gets higher, the output
capacitors will charge up, the output voltages will therefore rise, the
lamp will light brighter at higher voltage, pulling more power. By its
increased dissipation, the lamp power will balance the SMPS power, and
restore a net zero balance, preventing the voltage from rising further.
But if the lamp breaks, the SMPS will suddenly find itself without a
load, and with the power having "nowhere to go". Without automatic
control the power won't get reduced automatically, so it will continue
flowing into the output capacitors and charging them up. But they don't
have a very big capacitance and in a fraction of a second, their
voltages would rise way outside their ratings, causing some spectacular
bang somewhere. This happens so fast that you won't have a realistic
chance to turn down the operating parameters (voltage and duty cycle)
manually in time before something breaks. Power supply controller ICs
can handle power control and shutdown on a sub-millisecond basis, but
human reaction times and hand movements - unlikely.
As you see, the "keep it loaded with no interruption" is a particular
precaution that applied only to a particular mode of operation - full
manual power setting with no automatic feedback control. Yes, it's
tricky, and when you have an "uncontrolled" (unstable) SMPS waiting to
do something stupid as soon as the power balance gets upset, that's
where caution is definitely needed.
In "normal" circumstances, with the controller IC working and performing
its natural protective functions a power supply is way more robust and
can work with no load just fine.
Regards
Dimitrij