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I'm having a 10A DC-DC Step Down Buck Converter, suppose I want to get X volts and Y amps at output, so do I need to have input DC of rating more than X and Y ? or how to decide input rating? Now let me explain by real voltage and current, if I want to have a 20V and 8A output from my buck converter, so for this scenario, what rating of SMPS(or a DC power supply) I should use at input of the buck converter.

Buck converter I'm using

Buck Converter Specifications

Rishabh Sharma
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  • You could get a 20V 8A DC power supply and then you won't need it... – user253751 Sep 10 '21 at 09:46
  • Does this answer your question? [Choosing power supply, how to get the voltage and current ratings?](https://electronics.stackexchange.com/questions/34745/choosing-power-supply-how-to-get-the-voltage-and-current-ratings) –  Sep 10 '21 at 12:35
  • This MUST be a duplicate... –  Sep 10 '21 at 12:35

2 Answers2

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$$P_{In} = P_{out} + P_{loss} = P_{in} \times Efficiency$$ and $$P = V \times I$$

In theory any combination or voltage and current can provide your required power. In practice best not to have the input and output voltages to be wildly different since it gets tricky. And of course, you have to stay within the input voltage rating of your converter.

You can usually assume an efficiency of 70-80% for switching converters if you don't know. Sometimes better.

DKNguyen
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You need to remember a couple of things about a buck converter in order to choose an SMPS properly.

Number 1: A buck converter is a step-down converter. Therefore, it needs a higher voltage at its input than its output.

Number 2: Because it is a step-down converter, the input current is lesser than the output current (why?)

Keeping these two things in mind, you can determine what kind of rating your SMPS needs.

Let's start with the voltage rating. You want 20V at the output, so your input needs to be higher than the output. 24V is a standard SPMS output voltage and is higher than the buck converter's output voltage, so choose an SMPS that outputs 24V.

Next, what should the SMPS's current rating be? You need 8A at the output at 20V. That's a power of 20V*8A=160W. Let's assume your converter has an efficiency of 90%. This means that only 90% of the power you feed to the buck converter makes it to the output. Therefore the minimum current drawn from the SMPS is

           24V * xA * 0.9 = 20V * 8A

Solving for x, you get 7.4A.

So look for a 24V SMPS that can output at least 7.4A.

Prathik Prashanth
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