Differences between isolated and non-isolated power supplies

Introduction:

Power supply hardware circuits can be designed as isolated or non-isolated, the core difference lies in the existence of electrical isolation in the circuit, there is an isolated circuit design of the power supply can be isolated from high-voltage inputs and outputs, to avoid electrocution and other hazardous events; conversely, non-isolated power supplies do not have isolation components and circuits, and are directly connected through diodes, capacitors, inductors and other components.

The main difference:

Circuit Principle Differences:

Schematic diagram with additional isolation components

Looking at the circuit design principles and analyzing their different electrical connections, grounding requirements, etc., we can get the following table of comparative results.

We will not discuss specific design options for isolated power supplies here, but only evalsuate the characteristics and differences between isolated and non-isolated power supplies.

Isolated power supplies must pass VDE, GB, UL and other certification levels ofDielectric withstand voltage,partial dischargeetc. tests, which are not required for non-isolated power supplies, so cost-wise isolated power supplies add extra testing and certification costs.

Isolation transformers are lossy and inefficient because of the transformer installed.

The advantages of non-isolated power supplies are that they do not require voltage withstand certification testing and are smaller and more efficient.

The main disadvantage of non-isolated power supplies is that they must be well grounded or they can lead to equipment damage or electrocution, whereas isolated power supplies do not have this problem present.

Usage Scenario Differences:

Which electronic products are suitable to be designed with isolated power supplies?

• Household appliances (washing machines, refrigerators, etc.)
• Medical equipment (ECG, ultrasound, etc.)
• Hand-held test instruments (spectrum analyzers)

The above three types of electronic products is just an example, many need to human contact with the electrical equipment need to consider the use of isolated power supply design, to prevent users from using leakage and electric shock problems, which is also required by the safety test standards.

In addition to household appliances, there is our daily use of step-down switching power supply, step-down power supply and other products, they are from 220V high voltage to low voltage, also need to consider doing electrical isolation, in addition to isolation components can be added to isolate the common-mode electrical interference, to ensure that the signal transmission is stable.

Which electronic products are suitable to be designed with non-isolated power supplies?

For some low-power electronic products, such as LED light strips, rechargeable treasure, cell phones charger light products, as well as single-chip computer systems, sensor power supply, as well as the product volume of the strict requirements of the occasion, we can consider the use of non-isolated power supply to design them, as well as the life of a common router, TV set-top boxes and other products, because the human body does not need to be directly in contact with the high-voltage part of the use of its products, the entire product Closed design, such occasions will also consider the use of non-isolated design.

Summary of differences:

Through the above introduction to the difference between isolated power supply and non-isolated power supply, we know the choice of which circuit design power supply is the core of the normal operation of this product is whether the human body needs to be contacted and whether it is susceptible to electrical interference.

• Isolated power supply design is preferred when human safety, high voltage/low voltage conversion, and the presence of strong external interference are considered.
• When the electronic products belong to low power, no human contact closed design, control of R & D costs, size constraints, give priority to non-isolated power supply design.

Case Notes:

Since we belong to the EMC industry, we use our familiar EMC test instruments to illustrate how to select isolated and non-isolated power supplies.

In the EMC EMC testing industry, we often usePreamplifier, Spectrum Analyzer,EMI ReceiverEMC equipment to measure the electromagnetic compatibility testing of electronic products. These devices are high-frequency and high-precision measuring instruments, the main requirements of high precision measurement, strong anti-interference ability, stable operation and safe operation, combined with the above description of the difference between isolated / non-isolated power supply, we can say with certainty that all of these devices should be used in the isolation of the power supply mode, the reasons are as follows:

Suppression of common mode interference and crosstalk

The RF front-end, IF processing module of a spectrum analyzer or EMI receiver is extremely sensitive to electromagnetic interference (EMI).

Spectrum Analyzer or EMI Receiver Operating Schematic

If non-isolated power supplies are used, their inputs/outputs share a common ground, and harmonics, surges, and stray interference in the 220V mains AC grid will be directly conducted to the core measurement circuit, resulting in elevated baseline noise, signal distortion, and increased measurement error in the spectrum analyzer.

If the isolated power supply is used, through the electrical isolation of the transformer / optocoupler, the common mode interference on the grid side can be completely blocked, and at the same time isolate the crosstalk between the internal modules (such as power supply module and RF module), to provide a clean power supply environment for high-precision spectrum analysis.

Safeguard EMC testers and equipment

As shown below, EUTTEST sells thespectrum analyzerMostly desktop or handheld equipment, EMC testers need to operate buttons or knobs directly on the instrument panel and are often in contact with other test equipment such assignal source,oscillograph) interconnection, which meets the gas that is described in the need for frequent human contact scenarioses. At this point, the choice of isolated power supply can avoid the utility high voltage strung into the equipment shell or operating interface, to prevent the risk of electric shock; and isolation design can avoid the ground loop interference (current interference caused by the difference in ground potential of different devices) when multiple devices are interconnected, to prevent damage to the equipment or measurement data anomalies.

Handheld Spectrum Analyzers and Benchtop Spectrum Analyzers

Internal multi-module independent power supply

Spectrum analyzer belongs to precision equipment, requires a large number of RF components and electronic modules to work together to work properly, such as built-in preamplifier, digital signal processing, digital-to-analog conversion, display, other control circuits and other modules, each module requires a different voltage, such as the RF module needs ± 12V, DSP needs 5V/3.3V, the choice of isolated power supply design can be achieved! Multi-channel independent isolated power output, no direct electrical connection between each module, effectively preventing power supply noise crosstalk between modules.

Some special modules:

Of course, the spectrum analyzer also has some internal cooling fans, indicators and other internal packaging of low-voltage load modules, these modules and the core of the measurement circuitry is not effectively connected, so you can consider the use of non-isolated power supply design on these modules, but when it is found that such a design causes abnormalities of the whole machine, it is still necessary to prioritize the modification of the power supply to the design of the isolated power supply method.

Summary:

Isolated power supply or non-isolated power supply design, in addition to considering the above reasons, of course, will also be affected by the company's leadership decision-making, supply chain and other aspects of the environment, so this article is only for reference, we only need to understand the difference between the two power supply design principles, and then in the hardware design process to flexibly utilize and put forward different programs, preparedness, from the comfort and ease of use.