I wish to understand more on power parameter needed to design the power supply of audio amplifier.

Which power parameter should we use when designing the SMPS? Is it peak power or rated power?

Lets take an example as below:

SMPS topology:

  • Flyback quasi-resonant type

Assume Class-D audio amplifier requirement as below:

  1. Peak power : 250W (in short period, less than 5s)
  2. Rated power : 50W
  3. Speaker : 8 ohm

This is example I found on internet

Based on above example, the BTL output is 50W. This 50W is based on RMS value. Based on that, the minimum power supply voltage is 33.4V.

But if we use peak BTL output 250W, this will give minimum power supply voltage around ~70V.

Based on above calculation, I conclude as below:

  1. If use rated power, the minimum voltage power supply is ~33.4V
  2. If use peak power, the minimum voltage power supply is ~70.0V

Based on that, which voltage should I use for my power supply output voltage? Is it 33.4V or 70.0V?

  • \$\begingroup\$ Peak power tells you about the voltage rails you'll need to build and use. Rated power tells you about the average dissipation capability, without causing adversity. Both, plus a little more info, may also help size capacitors here and there. \$\endgroup\$
    – jonk
    Commented Jan 7, 2020 at 2:26
  • \$\begingroup\$ I understand your comment but based on my calculation, different power will lead to different power supply output voltage. \$\endgroup\$
    – user218385
    Commented Jan 7, 2020 at 3:17
  • \$\begingroup\$ (I'm not following that link you gave. Firefox for some reason is warning me.) The peak power and your supported loads tell you the voltage rail requirements. It's a simple formula. The average power rating tells you the average dissipation into the load that can be supported continuously (unless they say otherwise.) Or just go through the process of doing a design. By the end of just one attempt, if you are careful and thoughtful as you proceed, you'll pretty much know exactly which end is up and your question will be answered. It's not complex but I also don't want to argue. \$\endgroup\$
    – jonk
    Commented Jan 7, 2020 at 3:49
  • \$\begingroup\$ Different output voltage will lead to different number of turns on my transformer. Therefore, it is necessary for me to fully understand this power parameter before proceed to transformer design. However, thank you for your comment. \$\endgroup\$
    – user218385
    Commented Jan 7, 2020 at 4:01
  • \$\begingroup\$ Yes, you do need to understand before you can design. That's true enough. Just curious. Have you ever seen this expression: \$V_{_\text{RAIL}} = \pm\sqrt{2\:P_{_\text{PEAK}}\:R_{_\text{LOAD}}}\$? That does NOT apply to BTL. How would you change that for BTL? \$\endgroup\$
    – jonk
    Commented Jan 7, 2020 at 4:08

2 Answers 2


You are running class D, so to hit 250W BTL you will want somewhere in the region of about 65-80V or so assuming a single rail design and an 8 ohm load, dual rail would be +-35V to +-40V or so.

Now the 50W number tells you how to size heatsinks, but 5 seconds is forever in semiconductor junction terms so there is not a lot to be saved in the sand, it still needs to handle worst case voltage and current.

There is a bit of a trap, with a reactive load you can wind up with the amp supplying power to the supply during part of the cycle, make sure nothing exciting happens under these conditions.

On the subject of current, again reactive loads rear their heads, you can wind up with peak current demands very much greater then steady state on real program material, I would design the supply to run at least twice your calculated peak power for 100ms without tripping for overcurrent.


Peak power in a SMPS means peak Voltage and Peak current .Your semiconductors must be rated for the job .Your ferrite inductors must not saturate at these high peak currents .Your input and output caps must not give too much ripple at these high peak currents .So the design is much like a 250 watt SMPS but heatsinking needs are much less due to the average requirement .A simple 50Hertz transformer bridge rectifier and large electrolytic cap will handle these peaks well and could be cheaper to build .

  • \$\begingroup\$ I update my thread information. Is it peak power in SMPS is same as peak power in amplifier? Based on my understanding, peak power is the maximum power that the power supply can sustain for a short time. \$\endgroup\$
    – user218385
    Commented Jan 7, 2020 at 3:16
  • \$\begingroup\$ Due to the reactive nature of speaker loads you can get short term current peaks very much larger then implied by V/(nominal)R, so actual SMPSU peak power generally needs to be at least twice amplifier rating for all that it is very short term. \$\endgroup\$
    – Dan Mills
    Commented Jan 8, 2020 at 21:28

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