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 circuit and the output circuit 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.
Take BuckBoost and its isolated version of the flyback circuit as an example. The schematic diagram is shown in Figure 1 and Figure 2.
Figure 1 non-isolated power supply diagram
Figure 2 uses an isolated power supply for the transformer
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, as shown in Table 1 and Table 2, the advantages and disadvantages of both are almost the opposite.
Table 1 advantages and disadvantages of non-isolated power supply
Advantages Disadvantages
High conversion efficiency and poor anti-interference ability
The volume is relatively small, and the buck-boost conversion with the same input and output polarity is not good.
Low cost is difficult to achieve multiple outputs
Easy to design and less secure
- After the power supply is abnormal, the damage to the load is large.
-- It is difficult to achieve a wide input voltage range
Table 2 advantages and disadvantages of isolated power supply
Advantages Disadvantages
Strong anti-interference ability, low conversion efficiency
Easy to realize the buck-boost conversion volume is relatively large
Easy to achieve multiple output costs are more expensive
Higher security, more complex design
After the power supply is abnormal, the damage to the load is small --
Easy to achieve a wide input voltage range --
For the above advantages and disadvantages, most of us understand very well. Due to the abnormality of the power supply after the power supply is abnormal, the damage to the load is caused by Buck and its corresponding isolation circuit, ie, the forward circuit, as shown in the following figure. Show.
Figure 3 Buck circuit 
Figure 4: The electric energy trend after the breakdown of the Buck switch 
Figure 5 forward circuit 
Figure 6 shows the switching circuit of the forward circuit after the breakdown of the switch
As can be seen from FIG. 3 and FIG. 4, for the Buck circuit, if the switch tube is short-circuited, since there is no isolation, the higher voltage at the input end directly acts on the load end through the inductor, and the load is likely to be burnt due to over-voltage.
As can be seen from FIG. 5 and FIG. 6, for the forward circuit, the same switch tube is short-circuited, and for the load, only the power supply is lost and the power is turned off, and the load itself is not affected.
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. From the perspective 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, a power supply with enhanced isolation 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;
Non-isolated power supply for battery-powered applications where endurance requirements are critical
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