Info!! What Makes Switching Ability Supplies Hiss?

I attended a technical briefing ii years agone given yesteryear ane of our Product Managers on TDK-Lambda’s 400W CFE series, too was shown the diverse ways the production could potentially trim back internal position out energy losses - peculiarly at real calorie-free loads – using features available from the ability supply’s digital command circuits.  He did state that in that location were a span of modest side effects if all the features were enabled.

Recently ane of our salespeople emailed me stating that inward an off-load condition, his client could withdraw heed a 10 kHz hissing audio coming from ane of our 60W output ability supplies too was curious if the unit of measurement was faulty.  Normally I would stimulate got asked almost capacitive loading weather condition or if the production was beingness grounded correctly.  Remembering the briefing I checked the production specification for no-load ability draw.  Seeing that it was a depression 200mW, I talked to ane of our Design Engineers.

Before Energy Star legislation came into force, ability supplies would oftentimes describe 5, 10 or fifty-fifty xx Watts when inward a no-load condition.  Apparently this was accounting for almost 4 pct of the total electricity used inward the US.  Now most modest ability supplies volition describe less than 300mW.  Although the legislation is aimed at consumer products similar telephone chargers, many industrial customers are straight off requesting similar position out energy saving products.

I was informed that many of the novel ability provide command chips employ advanced position out energy saving features, too what my client was hearing was likely the converter going into “burst-mode”.

AC-DC ability supplies higher upwardly 70W volition oftentimes stimulate got ii converters inward them; ane is the boost PFC (power constituent correction) converter which ensures that the input electrical flow is non drawn inward big “gulps”, but to a greater extent than sinusoidal similar the applied air conditioning voltage.  An interleaved PFC converter shown below is used for this purpose.

 

The other is the primary switching converter which chops upwardly the high voltage DC from the PFC department too provides isolation, voltage measuring down, rule too filtering.  A usually used “half bridge” circuit is shown below.

 

Back to the dependent area of hissing!  Without position out energy saving modes, both of these converters run continuously, regardless of the applied load.  Each fourth dimension the FETs switch, position out energy is used yesteryear the gate campaign circuits too lost inward parasitic inductance too capacitance.

To avoid this, designers of command ICs stimulate got implemented features where the ability provide engineer tin stimulate got the PFC converter, the primary converter, or both converters plough off for brusk periods of time, aka “burst-mode” or pulse skipping, during calorie-free loading.

This agency that instead of the FETs beingness turned on every 10us for a ability provide operating at 100 kHz, the FETs volition hold upwardly switched on for a few cycles state every 100us.  This is plenty fourth dimension to replenish the internal capacitors amongst position out energy too choke on the output voltage inward regulation.

 

The upside is much less ability is dissipated inward the ability supply, but the downside is that instead of the production operating at 100 kHz (well higher upwardly the human hearing range) a faint hissing powerfulness hold upwardly audible equally the flare-up manner functioning operates at 10 kHz.

If the position out energy saving characteristic of the command IC is fully maximized, whereby a much longer interval betwixt bursts is enabled, an growth inward output ripple may hold upwardly besides observed.  Often this is acceptable to the halt user provided that the output voltage remains regulated.

My proposition to the salesperson was to offering our client an choice ability provide that had a slightly higher off charge ability draw, but no burst-mode operation.  That proposition was accepted.

Power Guy