Offline SMPS for Class D Audio amplifier

Power Integrations has been used for class D audio in the past. The usual parts recommendations apply.

One useful thing to remember is that audio has a very low average to peak power requirement. This will prevent over-specing and over-designing the PSU. The average power rating dictates the thermal design of the PSU, the peak power as described by the average power over a 1~5 ms period will dictate the peak power rating of the PSU. Peak power over a shorter duration than 1 ms can be delivered by using a relatively large output capacitors (i.e. 1,000 ~ 4,700 uF).

In some cases, tt is useful to build the PSU with a CV-CC feeback loop characteristic. That is, when the max load current of the PSU is reached, the output voltage will drop. This will allow overloading of the PSU without shut down and re-start, and has the benefit of (when the voltage droops), increasing output distortion (clipping) during overload, which tends to make the user turn the volume down.

For the 30-100 W range you can use TOP-JX, for 100-440W, use HiperLCS (LLC).

I have personally run amplifier THD+N tests, comparing a TOPSwitch power supply vs. a traditional 60 Hz iron transformer + capacitor PSU. The amplifier when powered by TOPSwitch PSU showed lower distortion at 120 Hz and its multiples, at high power levels. This is due to the fact that an SMPS will have very little 120 Hz ripple at heavy load, as compared to a 60 Hz PSU. The power supply ripple in the latter was getting through the amplifier's PSRR and was measurable on our Audio Precision 1.

Another tip is to keep EMI low with a test setup with a grounded output. This is to ensure low common mode EMI from the PSU outputs, going to the amplifier board.

ok thanks but why do you suggest TOP-JX for up to 100W?.....your RDR203 suggests peak switch PKS607Y with on/off control....surely PWM control of topswitch does not have good enough transient response for audio purpose?
Also, HiperLCS LLC is surely no good for audio as LLC's are in voltage mode control and have much slower transient response than a flyback.....so surely LLC topology is not useful for audio amplifier supply?

You are correct, I missed recommending the RDR203 and the PKS family. However the transient response required in audio applications is nothing special. Consider that the old 60 Hz power supplies, without a feedback loop, worked for audio amplifiers.

Yes but those were class A or class A/B amplifiers....the modern class D ampplifiers need superb transient reponse from the power supply...surely you agree?

Ah yes, depending on the class D topology. Some of them have good PSRR.


HiperLCS which is suitable for >100W, operates at a recommended 250 kHz and as such has very good transient response. You can typically achieve > 7 kHz gain crossover frequency. Additionally, the LLC by its nature, has a very low output impedance when it is designed to run with its series resonant tank frequency close to the operating frequency.

You should not do LLC at 250KHz for offline smps, as the switch-OFF switching losses will be too high…….and LLC is of little use when split output rails are required, because each half will not see the same degree of coupling into the core

...oh sorry also, i forgot to say, you cannot get 7KHz feedback loop bandwidth with TL431/Opto feedback, so do you now agree that LLC is not worthwhile?

LLC converters by their nature have low switching losses which is why they are favored in high power, high efficiency applications e.g. Subwoofer amps, LCD TVs, PCs (Silver and Gold levels), LED street lights.

Typical opto response is in the 20-30 kHz range so 7 kHz bandwidth is achievable.

Cheers PI-Chekov