AD power supply circuit design experience, worth collecting

This article is mainly to understand the use of LDO and switching power supply in actual circuits, and the specific details will be slowly added and corrected later.

Circuit power classification

In the circuit, the power supply is one of the most important parts to ensure the stable operation of the circuit. Only by ensuring the quality of the power supply output can the normal operation of the system be ensured, so this time we will first explain the power supply part of the circuit.

LDO power supply

LDO (linear regulated power supply) is a commonly used power supply in circuits. During work, we often encounter various situations with different voltages. Let's give a simple example:

Take the STM32 and 51 microcontrollers as examples. The power supply of the 51 microcontroller is 5V, while the power supply of the STM32 is 3.3V. Assuming that you are using an ordinary mobile phone charger, that is, a 5V output charging head, this can directly power the 51 microcontroller. If you need to power the STM32 microcontroller, you need to step down the external voltage to 3.3V. Otherwise, the chip will be directly burned out and the chip will not work properly.

In this case, we need to reduce the voltage in the circuit to the voltage we need, then we can choose the LDO chip. The commonly used chip is the often heard AMS1117-3.3 chip. The schematic is as follows:

Linear voltage regulator circuit

Through this chip, the 5V voltage can be converted into an output voltage of 3.3V, which can be used for STM32. This is what we often call a linear voltage regulator circuit.

Switching power supply

LDO power supply is often used in daily circuits, so you often find that assuming your existing input voltage is 12V or 24V, and you need a voltage of 3.3V, can it be used?

The answer is yes, as long as the voltage you input is within the input range, you can directly use the chip to get the voltage you need, but we generally do not do this. For linear voltage regulator chips, when you When the difference between the input voltage and your output voltage is too large, the efficiency of the power supply will be greatly affected.

The current you output is the same as the current you input (assuming your output current is 0.5A, and similarly your input current is also 0.5A), then in this case you will find that the actual useful power is 1.65W (assuming the input 12V, output 3.3V), the input power requires 6W, then the efficiency of the actual power supply is only about %25, and the other power is dissipated in the form of heat energy. Power utilization will be extremely low. At this time, you need to consider the switching power supply.

The most important thing about a switching power supply is high power utilization. The following will introduce several commonly used switching power supply classifications:

AC-DC power supply

AC-DC power supply: The most common one is the charger for mobile phones. It converts the 220V AC power in daily life into 5V DC power, which is what we call AC-DC power supply. Most of them have an efficiency of %60–%90. between.

AC-AC power supply

AC-AC power supply: This is an alternating current to alternating current power supply. I have relatively few contacts with this type of chip. My personal opinion is that it is somewhat similar to the nature of a transformer. It only changes the voltage, but does not change the frequency in the AC signal.

DC-DC power supply

DC-DC power supply is more common in actual circuits. Going back to the problem of power supply voltage regulation in LDO, suppose you only have a 12V power supply now, and now you need a 3.3V power signal, then you can use linear voltage regulation to solve this problem. , but as we discussed above, when the input and output pressure difference is large, the efficiency is low. Then you can consider using a DC-DC power supply at this time, which can convert the voltage 12V into a 3.3V output. At the same time, the efficiency can reach %70-%90, and the power utilization rate will be particularly high. Let’s analyze the circuit of a DC-DC chip (TPS62140) that I use myself:

DC-DC circuit

We can see in the picture that this is a circuit that converts 12V voltage to 5V voltage. The output result obtained is that the output efficiency we get will be relatively high (the efficiency of this chip is around %92), which is suitable for portable devices. is particularly convenient.

In addition, the LDO method cannot meet your voltage boosting needs. The linear voltage regulator chip can only convert the high voltage in the circuit into a low voltage (5V–3.3V), and cannot meet the voltage change of 3.3V–5V. Suppose you have a 4.2V lithium battery and you need a 5V voltage supply, then you can consider using DC-DC.

Because DC-DC can not only step down but also boost voltage, let’s use another DC-DC chip (TP8350) to analyze:

DC-DC boost circuit

This is a lithium battery that boosts the voltage to 5V through a DC-DC chip, thus ensuring the normal power supply of external devices.

DC-AC power supply

DC-AC seems to have never been used in practice. Friends who know it can leave a message and let me know (it seems that the chip has never been seen).