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I'm working on a full bridge DC-DV converter, following the instructions on power supply cookbook. It has a 350 V output, 2 kW total power, and a 30 kHz switching frequency. I chose a 5% inductor current ripple.

The author says 30 mV output voltage ripple is OK for forward-mode converters, but in my situation that means a 2200 μF output capacitor and it frightens me because it's so big and forces me to use aluminium capacitors. I'm so confused here, should I listen to this 30 mV rule in the book or choose another ripple value?

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  • \$\begingroup\$ What are your design priorities , Specs? \$\endgroup\$
    – Tony Stewart EE75
    May 22, 2020 at 0:05
  • \$\begingroup\$ The output ripple is ultimately chosen by your application. If it can tolerate 100mV, then you can use less capacitance. We once bought an AC regulator that had a ripple of 5V! That was too much so we put some filters on it to get it down to about 100mV. \$\endgroup\$
    – Aaron
    May 22, 2020 at 0:05
  • \$\begingroup\$ You could also allow 1V ripple with 352V and use a compartor to regulate a FET in linear mode to get 1mV ripple out @ 6A with 12W max dissipation \$\endgroup\$
    – Tony Stewart EE75
    May 22, 2020 at 0:07
  • \$\begingroup\$ @TonyStewartSunnyskyguyEE75 It's a 48V-350V full bridge dc-dc converter operating in 30 kHz with 2kW max power it's going to feed an inverter, I forgot to add this to post. \$\endgroup\$
    – Das D.
    May 22, 2020 at 10:34
  • \$\begingroup\$ @Aaron Converter is going to supply a inverter. I couldn't find an answer for how much ripple can be on an inverter's input \$\endgroup\$
    – Das D.
    May 22, 2020 at 10:36

2 Answers 2

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The output ripple is an INPUT to the design process. Usually you as the engineer are aware of how much Voltage ripple the device that is consuming the power can tolerate. 30mV seems somewhat reasonable for general use, but would entirely too much for a small 1.8v supply that is power sensitive digital electronics. Only you can say how much output voltage ripple is enough.

As far as 2200uF goes, don't forget you can split your capacitance between several physical capacitors and connect them up in parallel.

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  • \$\begingroup\$ it's going to supply an inverter sir, I forgot to addd this detail to the post \$\endgroup\$
    – Das D.
    May 22, 2020 at 10:37
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3kW @ 350V is equivalent to 40.8 Ohms

350 mVpp ripple is 60 dB down from 350V or 41 mOhms 0, and 35 mV is -80 dB needs 4.1 mOhms.

2200 uF is around 2.4 mOhm @ 33kHz and ESR depends on quality and $ of Cap.

You would want DF to be low @ 30kHz but are always rated at 120Hz or 250 higher frequency.

It depends on the spectrum of your load for load ripple voltage and current and Pd temp rise in components and thus reliability stress factors.

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