On 28.02.2016 22:59, Dimitrij Klingbeil wrote:
On 28.02.2016 22:31, Cursitor Doom wrote:
On Sun, 28 Feb 2016 19:06:38 +0100, Dimitrij Klingbeil wrote:

Also, even with a dummy load connected, the stray capacitance of
an oscilloscope, when hanging off the loose end of a power
circuit with some 800 to 900 V worth of HF on it, would probably
cause so much undue capacitive loading that the power supply
circuitry would hardly handle it.

Isn't this just another example of the unsatisfactory nature of
this resonant converter design? If the thing is *that* fussy that a
little bit of stray capacitance can catastrophically destabilise
it, then AFAICS it's a fundamentally unreliable topology and it
would be better to have used one of the non-resonant forms of
converter. Unless there's some compelling reason I may be unaware
of not to for oscilloscope power supplies, of course.

That's definitely not "a little bit". By very far, not!

Muscling around a scope chassis (not the probe tip, but the probe
ground and the big scope chassis connected to it on the other end of
the cable) from zero to some 800 V in several dozen microseconds is
no small feat, much less doing that 20000 times a second
repetitively.

Not many power supplies will do that on an internal node without
running into major stability issues (unless you have a very small
battery-operated "pocket" scope, sitting on a wooden table far away
from any earthed metal, thereby being a "light" load).

The probe tip is not the issue, but the scope itself, hanging from
the probe ground, that is.

P.S.

There is a simple though unwritten rule about power supply testing:

"Never connect the ground (common, chassis etc.) of any test equipment
to the switching node (power transistor collector, drain or power IC
output pin and its associated signals) of a switching power supply!"

It is valid for all types, no matter if flyback, forward or resonant.

The reason for this rule is that a "switching node" usually drives a
square wave with high voltages (some 500 to 600 V in a flyback, may
happen to be as much as 800 or 1000 V in a resonant one), and that a
significant amperage is readily "available" at that node too, due to the
output transistor's low impedance. Neither is the supply designed to
safely drive that into "RF ground" nor is the test equipment made for
being "muscled around" at that sort of voltages and dV/dt rise times.

Grounding the test equipment would mean that the whole power supply
(plus any safety isolation transformer) is being swung around and
letting the test equipment "float" would mean to also swing around the
test equipment. Apart from the obvious safety hazard, this can also
damage the test equipment and even compromise the test equipment's
electrical safety by frying the "Y" capacitors between mains and
secondary or stressing the isolation barrier in the test equipment's
power supply and / or mains transformer, possibly beyond the level of
stress that it was rated for.

So, whenever you troubleshoot some switcher, take heed of this rule.

It's simple to remember, and it can save lives, test equipment,
and some power supplies under test too

Regards
Dimitrij