Transient Voltage Suppression Diode (TVS Device) Application in AC Circuit

Filter 18.432M

When dealing with DC lines, a unidirectional transient voltage suppression (TVS) diode is typically used, while for AC lines, a bidirectional TVS diode is required. This is because AC power comes from the grid, where transient voltages can occur unpredictably—sometimes even due to lightning strikes, which induce sudden voltage spikes. As a result, it's challenging to estimate the exact pulse power (PPR) that may occur. However, selecting the correct reverse operating voltage (VRWM) is crucial for effective protection.

The general rule of thumb is to multiply the AC voltage by 1.4 to determine the maximum reverse operating voltage for a bidirectional TVS. For DC applications, the VRWM should be selected between 1.1 to 1.2 times the DC voltage level. This ensures the TVS diode can handle any unexpected surges without being damaged.

Figure 2-8 illustrates the schematic of a microcomputer power supply that uses TVS diodes for line protection. This design helps ensure stable and reliable power delivery, especially in environments where electrical noise or surges are common.

Here are the key steps in the protection scheme:

• Install a TVS diode on the 220V AC line to suppress peak interference from the grid.
• Add an interference filter to the transformer input line to reduce small voltage spikes.
• Include a TVS diode at the 10V DC output stage to protect sensitive components downstream.

Based on the calculations, the VRWM values for the TVS diodes would be as follows: D1 (bidirectional TVS) = 220V × 1.4 = 308V; D2 (bidirectional TVS) = 20V × 1.4 = 28V; D3 (unidirectional TVS) = 10V × 1.2 = 12V.

After implementing these four stages of protection, the power supply becomes "purified," significantly reducing the risk of electrical noise affecting the microcomputer’s CPU and memory. Additional measures such as shielding, grounding, or filtering can further enhance the system’s immunity against interference.

According to statistical data, out of 100 microcomputer failures, approximately 90 are caused by power supply issues, while only 10 are related to the microcomputer itself. This highlights the critical importance of a reliable power supply in ensuring the overall system’s stability and performance. To improve the reliability of the entire system, the power supply must be the first priority in design and implementation.