Advancements in power TVS diodes have made them an effective alternative to metal oxide varistors (MOVs) in protecting AC and DC power supplies from power line surges and indirect lightning strikes. These devices not only provide improved reliability and increased durability against repetitive surges when compared to MOVs in AC and DC power line applications, their use of surface mount packaging delivers an enhanced surge response due to lower lead inductance.
An important element of the circuit protection design must limit the peak surge voltage to an acceptable level and by not short-circuiting the line for an extended period of time. Advancements in power TVS diodes have made them optimal solution to meet the demands of these applications, and offer a more effective alternative to metal oxide varistors (MOVs). For example, the peak voltage of a MOV at its maximum rated current can be as much as three times its breakdown voltage at low current.
Power TVS diodes have become an effective alternative to metal oxide varistors (MOVs) in protecting AC and DC power supplies from power line surges and indirect lightning strikes, due to their improved reliability and increased durability against repetitive surges in AC and DC power line applications.
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Here, we will discuss the basics of power TVS diode design and how these devices provide improved reliability and increased durability against repetitive surges when compared to MOVs in AC and DC power line applications. It will also illustrate the benefits of surface mount packaging in delivering enhanced surge response, due to its lower lead inductance, for a more robust circuit protection design.
Power TVS Diode Design Basics
A Power Transient Voltage Suppressor (PTVS) diode is a circuit protection device designed to protect electronic circuits against high-level transients such as indirect lightning strikes. The silicon technology used in the design of PTVS diodes offers lower clamping voltage under surge compared to competing MOV technology. For example, after an initial short duration peak, the voltage across a PTVS diode folds back to a lower level and then remains relatively constant as the current through the device continues to increase. In contrast, the peak voltage across a MOV at its maximum rated current may be as high as three times its breakdown voltage at low current.
Fig. 1 compares a Bourns® Model PTVS3-380C PTVS diode with a typical breakdown voltage (VBR) of 420 V to a 10 mm MOV with a typical VBR of 430 V. Note the rising voltage of the MOV compared to the voltage foldback characteristic of the power diode. The diode voltage is approximately 700 V lower than the MOV at the peak current of 2500 Amps. Additionally, because the PTVS diode is a semiconductor device, it provides improved reliability and durability against repetitive surges when compared to the capabilities of a typical MOV.
One factor that can compromise the performance of a lower voltage PTVS diode at high currents is the lead inductance of the packaged device. Lower voltage PTVS diodes have longer lead lengths and therefore higher lead inductance. This additional inductance results in larger voltage drops across the leads of the device (V=Ldi/dt).
Fig. 2 compares the response time of a Bourns® Model PTVS10-058C to a Bourns® Model PTVS10-380C. After the initial rise and foldback response of the device, there is a negative slope in the voltage across each device. The lead inductance of each device is the primary cause. Because the lead inductance is higher and the breakdown voltage is much lower for the -058C device, the effect of the lead inductance on the response is considerably more pronounced. The use of a surface mount package is the best solution for this problem because the larger cross-sectional area and shorter length of the leads have significantly lower inductance. PTVS diodes in a surface mount (SM) package are now available and specific models feature standoff voltages of 58 V and 76 V. They deliver an improved surge response due to their low lead inductance. In addition, these devices also eliminate an extra assembly step in cases where they may have been the only through-hole device(s) used on the PCB assembly.
Fig. 3 compares the voltage waveforms of an SM (Bourns® Model PTVS15-076C-SH) with an axial lead (Bourns® Model PTVS15-076C-TH) 15 kA PTVS diode. Note that the voltage and peak current ratings are identical. When tested using the 1.2/50 µs, 8/20 µs combination wave with a peak current of 15 kA, the lower inductance of the SM package reduces the peak voltage by 35 V. This is a significant improvement in protection performance since limiting the peak surge voltage to an acceptable level is critical in most AC and DC power line applications.
The contribution of the lead inductance to the voltage response of the device is provided by the equation V = L di/dt and is important to note. Assuming the voltage contribution is directly proportional to the rate of change of the surge current (di/dt), the performance improvement obtained with an SM package will increase for waveforms with faster rise and decay times as well as at higher currents for the same waveform. The improvement will be less significant at lower peak surge currents for the same waveform and for slower waveforms, since the rate of change of the surge current (di/dt) will be reduced.
Enhanced Circuit Protection
PTVS diodes are offered in a wide voltage range with surge current ratings of 3, 6, 10 and 15 kA to deliver protection advantages compared to MOVs in typical AC and DC power line applications. By limiting the peak surge voltage to an acceptable level and by protecting against short-circuiting the line, PTVS diodes provide a more robust circuit protection solution for critical systems and exposed applications.