Electronic – Buck Converters Decreasing Current Consumption

buck

I have designed 3 power supplies with 3.3 V, 2.5 V, and 1.2 V outputs.

I use BD9870FPS Buck converter to step down 8 V – 24 V to 3.3 V.

Then the 3.3 V is used as inputs for 2 buck converters ENPIRION EN5322QI to step down to 2.5 V and 1.2 V. These 2 power supplies are used for load.

The 3.3 V output is also used for load.

The question is: when I tested three of them, the outputs (voltage) are stable, good. However, the current draw is decreasing while the input voltage to the 3.3 V buck converter is increased. Is this normal?

I read that a buck converter steps down voltage by increasing current. But what I got was another way around.

So, can you help me to explain this?

Best Answer

I read that a buck converter steps down voltage by increasing current.

This means that the output current is higher than the input current.

What you are seeing is that the input current depends on the input voltage. This is true because of, as others have mentioned, conservation of energy. Because of conservation of energy, we can define the following rule for a buck (or other switching) converter:

$$V_{in}I_{in} = \frac{1}{\eta}V_{out}I_{out}$$.

Here, \$\eta\$ is an efficiency factor to account for power lost in the converter itself. It does vary slightly when the other parameters are changed, but it's typically between 0.85 and 0.95 for a well-designed buck converter.

So you can see that if the output voltage and current stay the same, but you increase \$V_{in}\$, \$I_{in}\$ will decrease.

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