[time-nuts] One sure way to kill your FE-5680A or FE-5650A

David davidwhess at gmail.com
Wed Jun 8 13:42:40 EDT 2016

On Wed, 8 Jun 2016 08:20:45 -0700, you wrote:

>Interesting, we just had a similar issue on a circuit here at work.. 
>someone slowly brought the supply voltage up on a bunch of DC/DC 
>converters, and some didn't start. This was in initial checkout of a new 
>Switch it on with a bang, and it works just fine.
>So for some of these things there's apparently a minimum dv/dt.

This problem also occasionally shows up in integrated circuits where a
slow power ramp does not allow the bias circuits to start.  I think
Bob Pease related an instance where this was caused by a missing
connection and capacitive coupling to the substrate was enough to
start the bias circuits but nobody noticed the problem until after it
was in production.

>I've seen this before with DC/DC converters.. if the voltage drops too 
>low, they draw too much current - because they're basically constant 
>power devices- and the overcurrent trip shuts them down.  There's a 
>delicate interplay between the overcurrent and undervoltage trips,both 
>of which have some sort of time constant, and I suspect that for a lot 
>of circuits, the "slow ramp up of input voltage" isn't something they 
>are designed for.  Once it's up and running, when the supply sags, the 
>UV trip works just fine, tripping before the OC trip goes.

These problem seems to crop up more with newer designs.  In old
off-line switching power supply designs that I have studied, most have
a deliberately designed in hard start capability where when an
extended fault condition is detected, the regulator is completely
reset by momentarily shorting the bias supply.  If the fault
continues, then the power supply periodically ticks as it tries to
restart so there is a nice indication of the problem without self

The negative input resistance characteristic of switching power
supplies can have another bad result.  Some will continue to operate
at low input voltages drawing excessive current eventually damaging
themselves do to I^2R losses.

Power on reset circuits can have this problem in a different way.  If
power is removed and then reapplied within a short time, an RC circuit
may not discharge enough to retrigger.  Adding a diode to discharge
the capacitor when the supply falls is usually enough to fix this.

Some "universal input" off-line switching power supplies are marked to
run from 90 VAC to 270 VAC but actually cannot because they use an
automatic switching voltage doubler at their input.  If the input is
between the 120 VAC and 240 VAC ranges, they toggle back and forth
until they self destruct.  Luckily these are less common now with
active power factor correction inputs becoming ubiquitous.

>Linear regulators.. they may be not the most efficient thing in the 
>world, but they have a lot less "weird" behavior.  (although I've had 
>linear regulators go into thermally driven oscillation)

Linear regulators with foldback current limiting can have startup
problems with some loads.  Integrated regulators are usually designed
to put out full current as long as secondary breakdown limits are
observed and rely on their thermal protection which is itself designed
to have significant hysterisis to allow for hard starts under any
conditions.  But if the input to output voltage difference is high,
they can fail to start into some loads.

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