Re: Gap, Creepage and Clearance for HF HV

On Sep 6, 10:23 am, D from BC <myrealaddr...@xxxxxxxxx> wrote:
On Thu, 06 Sep 2007 09:00:08 -0700, Paul Mathews <o...@xxxxxxxxxxx>
wrote:

[snip]



You can add some 'blanking' time by paralleling the current sense
resistor with a fairly large capacitance (needed because Rsense is
quite small) and/or adding an RC filter to the current sense input.
This will affect the minimum load for stability (increase it), but the
undesirable transient also has that effect.
It's easy to add some capacitance to your Spice model to generate the
switch-on transient that is the bane of current feedback designs.
Think about it.
Rectifier reverse recovery characteristics, leakage inductance and
layout inductance (loop area), and switch turn-on time are all
important determiners of this type of problem.
Paul Mathews

I think you're saying, assume the worst... :)

Yup...thought about RC filtering too..
But just the minimal amount to help out the following circuits I use
which also help reduce reaction to spikes.

I can easily make a blanking circuit but RC filtering is easier to add
to an existing prototype pcb.

Simply:

/\/\/\/\/\/\/
spice signal for current feedback

| | | | | |
/|\|/|\|/|\|/
| | | | | |
real world for current feedback (spikes and ringing)
D from BC

You need to add a 3rd picture, which is between the other two in
amplitude and relabel them as follows:

Spice (least amplitude)
As seen by control chip (medium amplitude)
As seen by oscilloscope (highest ampliltude) that you're calling 'real
world'

This is because it's difficult to setup a scope probe that doesn't add
to the spike amplitudes. One thing that helps quite a bit sometimes:
wind several turns of the scope coax through a large ferrite toroid or
clamp-on core to improve scope CMRR. Always check what you get when
you 'short' the probe tip using probe ground (whatever you're using).
Paul Mathews

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