Power isolation and non-isolation concepts and advantages and disadvantages

One of the important issues to consider when designing a power circuit for an embedded system or selecting a finished power module is to use an isolated or non-isolated power solution. Before we discuss it, let's first understand the concepts of isolation and non-isolation, and the main features of both.

First, the concept of power isolation and non-isolation

The isolation and non-isolation of the power supply is mainly for the switching power supply. The general view in the industry is:

1. Isolated power supply: There is no direct electrical connection between the input and output circuits of the power supply. There is a high impedance state between the input and output, and there is no current loop.

2. Non-isolated power supply: There is a direct current loop between the input and output. For example, the input and output are common ground.

The isolated power supply is shown in the figure.

Second, the advantages and disadvantages of isolated power supply and non-isolated power supply

According to the above concept, for the common power supply topology, the non-isolated power supply mainly includes: Buck, Boost, Buck-Boost, etc.; and the isolated power supply mainly has various flyback, forward, half bridge, LLC, etc. with isolation transformers. Topology.

Combined with the commonly used isolated and non-isolated power supplies, we can intuitively derive some of their advantages and disadvantages, the advantages and disadvantages of both are almost the opposite.

Using isolated or non-isolated power supplies, you need to know what the actual project needs for the power supply, but before that, you can understand the main differences between the isolated and non-isolated power supplies: 1. The isolation module has high reliability but high cost. The efficiency is almost the same. 2. The structure of the non-isolated module is simple, low cost, high efficiency and poor safety performance. Therefore, in the following occasions, it is recommended to use isolated power supply: 1. In the case of possible electric shock, such as taking power from the power grid and converting to low-voltage DC, it is necessary to use isolated AC-DC power supply; 2. Serial communication bus is adopted. Physical networks such as RS-232, RS-485, and Controller Area Network (CAN) transmit data. Each of these interconnected systems is equipped with its own power supply, and the systems are often separated from each other. Therefore, we usually need to isolate The power supply is electrically isolated to ensure the physical safety of the system, and the ground loop is cut off by isolation to protect the system from transient high voltage surges and reduce signal distortion. 3. External I/O ports, in order to ensure reliable operation of the system, Power isolation is also recommended for I/O ports.

Third, isolated and non-isolated power supply applications

By understanding the advantages and disadvantages of isolated and non-isolated power supplies, they have their own advantages. For some common embedded power supply options, we can make an accurate judgment:

1. The power supply of the front stage of the system is used to improve the anti-interference performance and ensure the reliability. Generally, the isolated power supply is used.

2. The IC or part of the circuit in the circuit board is powered. In terms of cost performance and volume, the non-isolated solution is preferred.

3. For occasions where safety is required, if AC-DC is required to be connected to the mains, or the power supply for medical use, in order to ensure personal safety, an isolated power supply must be used. In some occasions, an isolated power supply must be used.

4. For the power supply of remote industrial communication, in order to effectively reduce the influence of ground potential difference and wire coupling interference, an isolated power supply is generally used to separately supply power to each communication node.

5. For battery-powered applications, where non-isolated power is required for applications with severe endurance requirements.

Fourth, the electric strength

The isolated withstand voltage of the power supply is also called the electric strength in GB-4943. This GB-4943 standard is the safety standard of information equipment that we often say, and is the national standard to prevent people from being physically and electrically injured, including avoiding People are exposed to electric shock, physical damage, explosions, etc. The following figure shows the isolated power supply structure.

Isolated power supply structure

As an important indicator of the module power supply, the standard test method for isolation withstand voltage is also specified in the standard. For simple test, the equipotential bonding test is generally used. The connection diagram is as follows:

Isolation withstand voltage test diagram

Test Methods:

The voltage of the withstand voltage meter is set to a predetermined withstand voltage value, the current is set to a predetermined leakage current value, and the time is set to a predetermined test time value;

The operation pressure gauge starts the test and starts to pressurize. During the specified test time, the module should have no breakdown and no arcing.

Note that the welding power module should be selected at the appropriate temperature during testing to avoid repeated soldering and damage to the power module.

5. What are the advantages and disadvantages of isolated power and non-isolated power?

Advantages and disadvantages of isolated and non-isolated power supplies

By understanding the advantages and disadvantages of isolated and non-isolated power supplies, they have their own advantages. For some common embedded power supply options, we can observe the following conditions:

For safety requirements, if you need to connect AC-DC to the mains or the power supply for medical use, you must use an isolated power supply to ensure personal safety. In some cases, you must use a reinforced power supply.

In general, the power supply isolation voltage requirement for the module is not very high, but the higher isolation voltage can ensure that the module power supply has less leakage current, higher safety and reliability, and better EMC characteristics. The universal isolation voltage level is above 1500 VDC.