Selecting Protection Devices: TVS Diodes vs. Metal-Oxide Varistors

Jun 1, 2010 12:00 PM
STEVEN J. GOLDMAN Field Applications Engineering Manager, Infineon Technologies, Durham, NC

An ideal protection device must limit the energy going to the load being defended, to a sufficient minimum level, such that the load is not damaged. The protection device must survive this energy burst to protect again another day. Energy at the load can be measured by Voltage × Current × Time (V × I × t). The best choice will combine low voltage clamping, low current passing, low dynamic resistance, and fast response time. Other factors such as longevity, repeatability, board space, cost, reliability, and a safe failure mechanism are also important. Laboratory tests and carefully correlated SPICE simulations were used to demonstrate and calculate the effect of 15kV transient events. In all cases, the standard 50 Ω resistance was used as the “load.” High-frequency (RF) response was not determined.

Designers should be aware of the differences between Device-Level and System-Level ESD Standards. Device-Level Standards such as Human Body Model (HBM), Machine Model (MM), and Charged Device Model (CDM) are used to define the handling conditions that the component can withstand. System-Level Standards, such as IEC61000-4-2 (Figure 1), address the conditions that the completed assembly must endure. Output current is very different for these tests, even at the same voltage.

For example, at 10kV the HBM waveform has a peak current of 6.67A, while IEC61000-4-2 waveform peaks at 37.5A. Likewise, the HBM peak occurs in 10 ns compared to 1ns for IEC61000-4-2. The dynamics are extremely different for this delta in di/dt. Understanding ESD device ratings is critical in selecting the best device. The rated specification from the manufacturer indicates the ESD level that the device can survive, without any correlation to system performance. Specifications indicated with the industry standard 8 µs/20 µs waveform do not correlate to 1ns/100ns performance. Additionally, the 1A industry standard rating is absolutely no indication of performance at 56.25A, which is the peak current level of the 15kV ESD strike. The best choice for each circuit application will provide the best protection for the load in question.

BASIC OPERATION

Protection devices operate in either “Normal” or “Protection” mode. During “Normal” mode (Figure 2), the system experiencing no unusual voltage or current surge events. Signaling is unaffected by the “ideal protection device.” No current flows in the branch containing the protection device. All of the current flows to the load, and no impact on signal integrity. Any current flowing through a practical protection device in “normal” mode is considered “leakage current.” This error term affects battery life in portable devices and signal integrity (when protecting communications lines, USB ports, HDMI lines, audio lines, etc.). While leakage currents can be quite small, these add up in battery-powered equipment and consume energy. Signal integrity is primarily influenced by the capacitance of the protection device. Compare these specifications carefully since not all manufacturers provide guaranteed rated maximum values.

Surges in voltage or current force the device into “Protection” mode (Figure 3). The ideal protection device then becomes a perfect short-circuit path to ground. Ideally, all of the energy flows through the protection device, defending the load from any damage. After the surge passes, the ideal device quickly returns to “Normal” mode, without any internal damage or changes in its performance.

MANUFACTURING DIFFERENCES

TVS semiconductor diodes are monolithic devices fabricated using standard semiconductor techniques. They can be easily implemented as arrays or incorporated into larger scale components, such as combination filter-protection devices. They feature very fast response time, low clamping voltage, and high reliability. When used within suggested design parameters, their specifications will not degrade over time or quantity of protection events. Charge carriers combine back and forth across the P-N junction during the different modes of operation. They are generally used to protect low voltage components.

MOV devices are ceramic masses composed of metal-oxide grains. Their structure is similar to that of a sugar cube. The boundary between grains forms a region with non-linear current and voltage performance, which behaves as a diode. These “diodes” arrange themselves in a random multitude of parallel and series combinations. This random structure leads to larger tolerances for specified parameters. MOV performance is affected by the volume of the device (Height × Length × Width). Large devices can handle very high voltage levels. For this reason, MOV devices are generally used to protect line powered circuitry.

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