Problems caused by stepping up the power supply and then stepping it down to connect it to the power supply

jyz520

Problems caused by stepping up the power supply and then stepping it down to connect it to the power supply [Copy link]

Due to product requirements, difficult problems have arisen in circuit design. For example: a 12v battery is connected to a DC-DC boost circuit, and the voltage rises to 18v. The output is then connected to a DC-DC buck circuit, which drops it to 12v, and the output is then connected to the battery. Since the voltage cannot be exactly equal to the ideal value, the voltage after the buck is slightly higher than 12v. At this time, the buck output is connected to the positive pole of the battery. There is very little resistance between the positive pole of the battery at the connection point, and the slight voltage difference will cause a current to flow between the connection point and the positive pole of the battery. Where will this current flow?

maychang

"Where will this current flow?"

Flows into the boost circuit, offsetting part of the battery current.

秦天qintian0303

What is the purpose of such a design? Waste of money

秦天qintian0303

Just to stabilize the output?

maychang

Qintianqintian0303 posted on 2022-9-19 11:41 What is the purpose of such a design? Waste of time

The connection made by the OP is just an experiment. The actual circuit will not be connected in this way.

maychang

Qintianqintian0303 posted on 2022-9-19 11:41 What is the purpose of such a design? Waste of time

However, in some special cases, this connection is the normal way to use it.

Assume that the large box on the left in the first post is the power supply under test, and the large box on the right is an electronic load, which is used to test the characteristics of the power supply under test. Then the energy obtained by the large box on the left from the 12V power supply will be completely converted into heat dissipation by the electronic load. If the electronic load is made into a special switching power supply, whose input has constant resistance or constant power or other characteristics, and whose output can be fed back to the 12V power supply, then the energy converted into heat in the circuit will be very small.

chunyang

The 18.005V in the picture in the original post should be 12.005V.

At this time, if the buck DCDC has output current, it will merge with the battery output current and flow into the boost power supply. Further imagine that if the buck DCDC alone has a load output voltage of 13V within the rated output parameters, and then it is connected as the original poster said, what will happen?

jyz520

Supplementary F: DC-DC conversion is based on the input voltage. The boost circuit uses a 18/12 ratio to boost to 18v, and the buck circuit uses a 12/18 ratio to buck to 12v. Due to actual manufacturing errors, the buck output is slightly higher than 12v, 12.005v. Note: 18.005 in the figure is wrong, it should be 12.005v. When the buck output with a voltage of 12.005v is connected to the positive pole of the battery, how does the voltage change at each point? How does the current flow and size change at the positive end of the battery?

P0WER

The input and output are of the same voltage, so there is no need to increase first and then decrease. Moreover, it is a battery.

maychang

jyz520 posted on 2022-9-21 21:10 Supplementary F: DC-DC conversion is based on the input voltage. The boost circuit uses a 18/12 ratio to boost to 18v, and the buck circuit uses a 12/18 ratio to buck to 12v. Because...

This description is not clear enough. "When the buck output terminal with a voltage of 12.005v is connected to the positive terminal of the battery, how do the voltages at each point change?" It does not say what the buck output terminal is in before it is connected to the positive terminal of the battery. Is it floating? Is it connected to a load?

吾妻思萌

I'd like to make a wild guess, maybe a Boost-Buck circuit? If it's just voltage conversion, there's no need to step up first and then down, it's good to do it in one step. If you want to drive other loads or something, you can step up first and then down.