[time-nuts] The home time-lab

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Jul 24 23:25:31 EDT 2016


The PFC power supply architecture I had in mind uses a flyback inverter to regulate the input rail voltage for the output dc-dc converter stage. The flyback stage peak current is modulated by a sinewave. A feedback loop adjusts the effective amplitude of this sinusoidal modulation to regulate the rail voltage. With some high frequency input filtering the input current is sinusoidal with low power factor.
I've seen a lot of equipment that's either unhappy or has even been damaged by use of modified sinewave inverters. Some equipment even fails to work with modified sine input although it subsequently works with a sinewave input. Laptop/notebook supplies in particular can be something of a lottery in this respect.
Machines with capacitor start motors also require a near sine wave input

Bruce
 

    On Monday, 25 July 2016 2:17 PM, David <davidwhess at gmail.com> wrote:
 

 Are there some reference designs you can point to where this is a
problem?

Because all of the application notes and references designs I have
checked so far have no requirements for input wave shape other than
peak and to a lessor extent RMS voltage.  The input power factor
degrades with inputs which have higher frequency content but the
converter should not fail.

The input after the bridge rectifier is sampled by one control loop
which adjusts the duty cycle so that the input current is proportional
to input voltage (resistive) while a second slower control loop
adjusts the average input power to roughly regulate the output
voltage.  The output voltage ripple mimics the input voltage waveform
for practical reasons.

If PFC power supplies have an issue with modified sine inverters, then
that would be a pretty big problem given how common they are.

On Mon, 25 Jul 2016 00:51:36 +0000 (UTC), you wrote:

>The PFC stage needs a line frequency sinewave reference, if this is produced by attenuating the line input, then it may not function well with badly distorted line input waveforms (modified sine, square wave etc).Phase locking a sinewave to the line input may be a better approach.
>Bruce
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