[ARC5] comments on a DC - DC B+ converter -- capacitor ripple currents

[Brian Clarke]

There are books by Irving M Gottlieb covering all manner of regulated power
supplies, including SMPSUs, that you may find useful. I used them when
teaching PSUs at TAFE.

73 de Brian, VK2GCE

On Friday, 20 April 2018 11:33 AM, Les Smith said:


Hello David & others,

<snip>

I would be most interested to hear comment about capacitor rating and
ripple.    I thank Bob & Scott for their observations.    Scott gave typical
values (22uF/260mA) and I note that the output cap on this board is
10uF/400V.  0.5x capacitance suggests 130mA ripple rating.  I tested the
current capacity at 250V/4k load or (about) 60mA.  Seemed OK for 1/2 hour or
so.  Taking the load up to 85mA (about 20W) was OK, but the next step above
that (30W) tripped the over-current protection circuit in my Marconi
Instruments supply.   At any rate, observations of this type should be
presented in graphical form; I haven't done that work yet.  I might say that
the efficiency drops noticeably under load.

I noted the posting where ripple current was judged to be up to 3x the load
current.  That surprised me.  I made some calculations and would have put
that figure at 5x or 8x the load current - but this figure was for lower
voltage higher current supplies with (say) capacitance values of 5 to 10
thousand "muffs".  Also my figure ignored the resistance and inductance of
the transformer winding.  

I would be interested to read more comment on ripple current in the filter
caps and how to test the power supply more thoroughly.  A simple static test
might show the card working well for a month or a year (or longer).
Estimating the service life of the card is another matter, and testing the
same at full load (or discovering the real capacity of the card) may take
months, even if tested at +65C (or some elevated temperature).

I'm interested in any comment about how the life may be estimated and
measured.

Les Smith.
VK2BCU "at" OperaMail "dot" com.