[ARC5] Scope photos - Chinese DC-DC converters

[don davis]

These converters should definitely NOT be putting out any 170/300 Hz.  There
is nothing cornered that low in the loop for normal operation.  It might,
however be going into a shut-down mode due to high FET / input dc spikes.
In this mode, the usual response is for the FETs to open for some time
period and then re-trigger the start mode logic, or may drive it back on
without the start mode (soft start).  I assume the waveforms are with ac
coupling and probes?  The "hash" is the switching frequency.  In order to
properly diagnose these you need a dc offset amplifier so you can see dc on
top of the input dc without the common mode distortion, and a good dc
current probe Tek TM xxx.  Look at the FET drain current w.r.t. FET Vds and
look for cycle-by-cycle variation for shut down, LF oscillation, etc.

Other things:  What is the input supply?  Must not be current limited or
high Z output - can get stuck in start mode.  Inrush current for start may
be 10 times steady state!   Also no L in the input line - will oscillate at
LF.  What is rated Vin and Pout?  To get past obvious things have you tried
running in the middle of the spec range for both?  Some dc-dc converters are
just that - converters, and may not have proper input / output filters.
This is common in acft (Interpoint, for example needs outboard filters to
meet MIL-STD-461 CE01, CE02, CE06, and RE specs).

And, I did this for a living for 35 years in the aerospace business, where
we have a technical name for designs that behave this was from the design:
POFS

73 de don ad6pb

-----Original Message-----
From: ARC5 [mailto:arc5-bounces at mailman.qth.net] On Behalf Of Ben Hall
Sent: Saturday, October 17, 2015 2:34 PM
To: arc5 at mailman.qth.net
Subject: [ARC5] Scope photos - Chinese DC-DC converters

Afternoon all,

I bought two of the Chinese DC-DC converters, so I decided to hook up and
test the second one to see if it had the same "sine wave" like output.