When designing power circuits for embedded systems or selecting finished power modules, one of the important issues to consider is whether to use isolated or non-isolated power solutions. Before proceeding with the discussion, let us first understand the concepts of isolation and non-isolation, and the main characteristics of the two.
1. The concept of power isolation and non-isolation
Isolation and non-isolation of power supplies are mainly for switching power supplies. The industry’s more common views are:
1. Isolated power supply: There is no direct electrical connection between the input circuit and the output circuit of the power supply, and the input and output are in an insulated high-impedance state, 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 share a common ground.
Take BuckBoost and its isolated version of the flyback circuit as an example. The schematic diagrams are shown in Figure 1 and Figure 2.
Two, the advantages and disadvantages of isolated power supply and non-isolated power supply
It can be seen from the above concepts that for commonly used power supply topologies, non-isolated power supplies mainly include: Buck, Boost, Buck-Boost, etc.; and isolated power supplies mainly include various flyback, forward, half-bridge, LLC, etc. with isolation transformers. Topology.
Combining commonly used isolated and non-isolated power supplies, we can intuitively draw some of their advantages and disadvantages, as shown in Table 1 and Table 2, the advantages and disadvantages of the two are almost the opposite.
Table 1 Advantages and disadvantages of non-isolated power supply
Table 2 Advantages and disadvantages of isolated power supply
Most of the above-mentioned advantages and disadvantages are well understood by us. Because of the damage to the load after the power supply is abnormal, whether the power supply is isolated or not, we simply analyze the Buck and its corresponding isolation circuit, namely the forward circuit, as shown in the following figure. Show.
It can be seen from Figures 3 and 4 that for Buck circuits, if the switch tube breaks down and short-circuits, because there is no isolation, the higher voltage at the input will directly act on the load through the inductance, and the load is likely to be burned out due to overvoltage.
It can be seen from Fig. 5 and Fig. 6 that for the forward circuit, the switch tube breaks down and short-circuits. For the load, it just loses the power supply and cuts off the power, and it will not cause other effects on the load itself.
3. Applications of isolated and non-isolated power supplies
By understanding the advantages and disadvantages of isolated and non-isolated power supplies, we can see that they have their own advantages. For some commonly used embedded power supply options, we can already make accurate judgments:
1. In order to improve the anti-interference performance and ensure the reliability of the power supply of the front stage of the system, an isolated power supply is generally used;
2. The IC or part of the circuit in the circuit board is powered. From the perspective of cost performance and volume, the non-isolated solution is preferred;
3. In the occasions where safety is required, if it is necessary to connect to the AC-DC of the city power, or the power supply for medical use, in order to ensure personal safety, an isolated power supply must be used, and in some cases, a reinforced isolated power supply must be used;
4. For the power supply of long-distance 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 power each communication node separately;
5. For occasions that use battery power supply and have stringent endurance requirements, use non-isolated power supply.