With the rapid development of the embedded industry, power requirements in various applications are becoming increasingly stringent. To ensure system stability, isolated power supplies have emerged. But what exactly is the isolation voltage, a key indicator in isolated power supplies? And how does it relate to creepage distance? This article will reveal the answers by explaining the principles of isolated power supplies.
With the rapid development of the microelectronics industry, the electromagnetic environment in which products are used is becoming increasingly complex, significantly impacting product stability. Embedded product manufacturers are incorporating various isolation devices or circuits into their products to reduce interference in the working environment and enhance equipment stability.
As the power supply component of embedded devices, the power supply is a prerequisite for stable product operation. Power isolation is particularly important, and the application of power isolation modules has become an essential part of embedded device design.
In industrial equipment, power isolation between two devices is required. A DC-DC converter with a transformer is used to separate the two power supplies, making them independent of each other, thereby reducing external interference.
Introduction to Isolation Withstand Voltage and Creepage Distance of Isolation Power Supplies 1. Introduction to Isolation Withstand Voltage of Isolation Power Supplies
Figure 1 Internal block diagram of the isolated power supply
Figure 1 shows the internal block diagram of the isolated DC-DC power supply module.
Isolation withstand voltage refers to the highest insulation voltage that two systems that are not directly electrically connected can withstand.
The selection of parameters varies depending on the application of power isolation. For example, AD-DC power isolation generally requires an isolation withstand voltage of 3000VAC to 4000VAC in industrial applications; DC-DC power isolation generally requires 1000VDC to 2000VDC in industrial applications, but may be higher in special industries, such as the medical industry, which requires 6000VDC.
First, let's distinguish the units of various voltage indicators. Common ones include ADC, VAC, and RMS, as shown below.
VAC/VDC refers to AC voltage and DC voltage, respectively. However, AC and DC cannot be simply converted in isolation withstand voltage. For example, the amplitude voltage of 3000VAC is 4242V, but in practical applications, the isolation withstand voltage of 3000VAC and 4242VDC are not equivalent.
The specific reasons include two points:
1. For isolation modules, there is an isolation capacitor between the input and output. For DC signals, the impedance of the capacitor is infinitely large, so the size of the isolation capacitor has little effect on DC signals, but it will have a significant impact on AC signals, which will manifest as increased leakage current or even exceeding the limit, triggering a system alarm.
2. Another difference between AC and DC is that frequency affects the dielectric constant of the insulating medium. Frequency will cause the dielectric constant of the insulating medium to decrease. Generally, the higher the dielectric constant, the stronger the insulation ability.
RMS stands for True Effective Value, which simply means that alternating current (AC) does the same amount of work as direct current (DC) per unit time. In other words, AC with a true effective value of 10V does the same amount of work as DC with a true effective value of 10V over the same load in the same amount of time. This unit is not typically used as a unit of measurement for isolation withstand voltage.
2. Introduction to creepage distance of isolated power supplies
To ensure the stability and safety of the isolation withstand voltage and prevent the isolation power module from being broken down, we need to calculate the creepage distance. Creepage distance is the shortest spatial distance measured along the surface of the insulating material between two conductive parts , or between a conductive part and equipment or easily accessible surfaces. The distance along the insulating surface where electricity is discharged is the leakage distance, also known as the creepage distance. Creepage distance = surface distance / system maximum voltage, and varies depending on the degree of contamination.
Figure 2. Schematic diagram of creepage distance
The IEC 60950 and GB 4943-2011 standards specify the minimum safety distances required for different voltage levels. These safety distances include both electrical clearance and creepage distance. For switching power supplies, the main areas where minimum safety distances need to be ensured are as follows:
1. Safe distance between the primary circuit and the casing (protective ground);
2. Safety distance between the primary circuit and the secondary circuit.
The advantages and disadvantages of isolated power supplies compared to non-isolated power supplies are shown in Figure 3 below.
Figure 3 Comparison of isolated and non-isolated power supplies
Applications of isolated power supplies
In embedded product applications, various types of interference can threaten the stability of the product. During the product design phase, selecting good isolated power supply modules and isolated communication modules can effectively shield the interference caused by the operating environment and ensure the stable operation of the product.
We often see news reports in the media about mobile phone explosions and electric shocks while charging. Isolated power modules can effectively ensure the stable operation of products, and more importantly, protect the lives and property of users.
There are currently two ways to use isolated power supplies: discrete component construction and the use of DC-DC power modules, as shown in the comparison below.
1. Option Selection
Once the product performance requirements are somewhat clear, the next step is to start design and development. The first thing to do is to select a circuit scheme. Below are some common "bad examples".
For example, when designing and developing a device that converts AC input to DC output, many people immediately think of using a power frequency converter circuit because it's relatively simple—just a power frequency transformer and a rectifier/filter, as shown in Figure 4. Products using this approach have very low efficiency and are extremely large, and they also produce annoying power frequency eddy current noise during application. In contrast, a modular approach uses a suitable transformer and employs multiple processes to ensure transformer consistency, guaranteeing the final product performance.
Figure 4 Comparison of Transformer Schemes
2. Material selection and PCB design
Once the circuit design is finalized, the next step is to design the product performance parameters. This involves parameter design, calculation, and material selection for the electronic components in the circuit design, requiring a balance of various factors.
1. Material selection. Professional modular power supply manufacturers can achieve both goals by discarding unnecessary material specifications and selecting the optimal ones based on different product specifications and application conditions.
2. PCB Design of Power Module. Because modular power supply products have specific PCB design specifications, which need to consider thermal design, EMC design, interference design, manufacturing process design, etc., involving a great deal of content, PCB design is treated as one of the most important aspects in the development of modular power supply products, as shown in Figure 5.
Figure 5 PCB Design Requirements
ZLG Zhiyuan Electronics' isolated DC-DC power supplies provide stable and reliable drive voltages for user systems and effectively solve power instability issues caused by static electricity and surges. They are an ideal power supply solution for onboard data acquisition, communication, and other subsystems. Isolation voltage levels cover 1000VDC-6000VDC, meeting the needs of all operating conditions.
Compared to traditional designs, ZLG Zhiyuan Electronics' constant voltage series isolated power modules adopt the industry's best solutions in terms of topology, chip design and components, and have higher integration and reliability, providing users with standard and reliable solutions for I/O and communication isolation applications.
