What Makes Switching Power Supplies Hiss?

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I attended a technical briefing two years in the past given by certainly one of our Product Managers on TDK-Lambda’s 400W CFE sequence, and was proven the assorted methods the product might probably scale back internal power losses - significantly at very mild masses – utilizing features out there from the power provide’s digital management circuits.  He did say that there have been a few small unintended effects if all of the features had been enabled.
Lately certainly one of our salespeople emailed me stating that in an off-load situation, his buyer might hear a 10 kHz hissing sound coming from certainly one of our 60W output power provides and was curious if the unit was defective.  Usually I might have requested about capacitive loading situations or if the product was being grounded appropriately.  Remembering the briefing I checked the product specification for no-load power draw.  Seeing that it was a low 200mW, I talked to certainly one of our Design Engineers.
Earlier than Power Star laws got here into drive, power provides would typically draw 5, 10 and even 20 Watts when in a no-load situation.  Apparently this was accounting for about 4 % of the entire electrical energy used within the US.  Now most small power provides will draw lower than 300mW.  Though the laws is geared toward client merchandise like phone chargers, many industrial clients are actually requesting related power saving merchandise.
I used to be knowledgeable that lots of the new power provide management chips make use of superior power saving features, and what my buyer was listening to was in all probability the converter going into “burst-mode”.
AC-DC power provides above 70W will typically have two converters in them; one is the increase PFC (power issue correction) converter which ensures that the enter present is just not drawn in huge “gulps”, however extra sinusoidal just like the utilized AC voltage.  An interleaved PFC converter proven under is used for this function.
 

The opposite is the primary switching converter which chops up the high voltage DC from the PFC part and offers isolation, voltage step down, regulation and filtering.  A generally used “half bridge” circuit is proven under.
 
Again to the topic of hissing!  With out power saving modes, each of those converters run repeatedly, whatever the utilized load.  Every time the FETs swap, power is utilized by the gate drive circuits and misplaced in parasitic inductance and capacitance.
To keep away from this, designers of management ICs have applied features the place the power provide engineer can have the PFC converter, the primary converter, or each converters flip off for brief intervals of time, aka “burst-mode” or pulse skipping, throughout mild loading.
Because of this as a substitute of the FETs being turned on each 10us for a power provide working at 100 kHz, the FETs can be switched on for a number of cycles say each 100us.  That is sufficient time to replenish the internal capacitors with power and preserve the output voltage in regulation.
 
The upside is way much less power is dissipated within the power provide, however the draw back is that as a substitute of the product working at 100 kHz (effectively above the human listening to vary) a faint hissing may be audible because the burst mode operation operates at 10 kHz.
If the power saving feature of the management IC is absolutely maximized, whereby a for much longer interval between bursts is enabled, a rise in output ripple could also be additionally noticed.  Usually that is acceptable to the top person offered that the output voltage stays regulated.
My suggestion to the salesperson was to supply our buyer an alternate power provide that had a barely larger off load power draw, however no burst-mode operation.  That suggestion was accepted.
Power Man