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

Current through the load is given by:

Iload = (30 × Rpr) / (Rpr + 50)

ITVS = 21/50.7 = 0.414A

IMOV = 210/57 = 3.68A

The load resistance shown here is 50 Ω. During the current surge from 25ns to 35ns, the load being protected by the TVS Diode receives 10 times less current. The power (I2R) during the surge is determined by Iload × Iload × 50 Ω. Since the power is related to the square of the current (times the load resistance), is it easily recognized why current flow must be minimized.

CLAMPING VOLTAGE

Voltage peaks are limited at the load by the protection device. Laboratory test were performed by driving 300V pulses, 30ns width, through the various devices. Each device reacted quite quickly, however, the chart below indicates the relative voltage levels. The varistor granular structure does not allow for low clamping voltage. Areas of the array will force diodes in series, where their threshold voltages will sum. As seen in Figure 7, TVS diodes clamp at significantly lower voltages than their MOV counterparts, further reducing energy in low-voltage applications. As previously stated, these dynamic results are different than the Vclamp value that the datasheet would indicate. Where the current level of the input signal is known, Vclamp can be approximated as Vbreakdown + (Rdyn * Iknown) + L di/dt. For IEC61000-4-2 testing, after 10ns the L di/dt term approaches zero.

The requirement for dependable, repeatable, performance of the protection device will depend on the application. Leakage current is greatly impacted by device degradation, as shown in Figure 8. This parameter may be measured by repeating stress events and measuring the “normal” mode current. As expected, the varistor become more resistive after each over-voltage event. Carefully interpret the graph below, as the x-axis is logarithmic. TVS diodes do not degrade after each event. 10E-11A is approaching the limit of most laboratory equipment. When comparing two TVS devices, refer to the manufactures guaranteed specifications. Look for the lowest leakage current specification.

Figure 9 illustrates device degradation that also forces a shift in breakdown voltage (VBR). TVS diodes show no measurable shift in VBR over time. Some MOV devices show a clear reduction in VBR after each stress event. The failures of the MOV1 array (red) are creating a more conductive path through the varistor, seen by VBR heading towards zero. This structure (MOV1) will eventually become a short circuit. The failures of the MOV2 array (black) are creating a less conductive path through the varistor, seen by VBR heading towards infinity. Since MOV2 is less conductive initially (see the I-V curves), the ability to protect the load is further reduced with every stress event. Eventually, this structure (MOV2) will fail as an open circuit, providing no protection at all.

ENERGY LIMITING CAPABILITY

The best protection devices must limit voltage and current quickly. 15kV was used as the worst-case scenario. The common, simplified, 150pF and 330Ω , 15kV circuit was used to generate the input waveform. Peak current does occur earlier, but the same total power is contained in the input signal.

Figure 10 shows the power waveforms simulated at the load. The TVS diode combines low clamping voltage with low resistance and fast response time. Energy at the load is calculated by determining the area under the respective curves.

For this low-voltage application, the TVS diode allows 4.5µJ while the (red) MOV allows 18.0µJ at the load. Four time difference between these protection devices. This could easily be the difference between protection or failure, depending on the safe operating area (SOA) of the load. Select the protection device that offers the widest safety margin within the SOA of the load.

Some high-current, high-voltage applications will require either large MOV's or an array of TVS diodes. The designer needs to ensure system level protection against catastrophic failure. Stress beyond the specifications of most TVS diodes will result in sudden failure as a short circuit. This will result in the load not properly functioning, however, the system fails in a “safe” manner. The diode fails quickly and therefore does not have sufficient time to generate heat. Metal-oxide varistors fail in a different fashion. Their operating parameters shift with the number of stress events, even when used within specification. They become more conductive with use and thermal runaway occurs. Their ceramic construction can handle higher temperature than their silicon counterparts.

Some MOV devices will crack or explode if the temperature rises suddenly, possibly causing the device to fail as an open circuit. MOV devices that maintain their shape and form can reach temperatures above the combustion temperature of paper, which introduces the possibility of fire. Properly design your protection circuits to handle over-voltage and over-current conditions.

Many systems may have low-voltage supervisory microcontroller circuits or interface circuitry that is best protected by TVS diodes, while the AC mains or high-voltage DC stages may best be protected by MOV devices. Low-voltage signal paths are offered better protection from TVS diodes; however, some loads may operate within their SOA with either device.

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