# Peak Power Vs Rated Power

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?

• 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.
– jonk
Commented Jan 7, 2020 at 2:26
• I understand your comment but based on my calculation, different power will lead to different power supply output voltage. Commented Jan 7, 2020 at 3:17
• (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.
– jonk
Commented Jan 7, 2020 at 3:49
• 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. Commented Jan 7, 2020 at 4:01
• 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?
– jonk
Commented Jan 7, 2020 at 4:08