Author:
Michael Zimmermann, Littelfuse, Inc.
Date
10/22/2024
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Traditional solutions, like NTC thermistors and standalone MOVs, often need more response time and reliability. This article explores how the SIDACtor+MOV combination offers a superior, cost-effective solution for safeguarding EV on-board chargers, providing faster response, lower clamping voltage, and enhanced durability in the harsh automotive environment.
Today’s component manufacturers offer multiple devices for safeguarding electronic circuits. Due to the connection to the grid, on-board charger protection from voltage surges using unique components is essential.
Littelfuse solutions focus on advanced overcurrent and overvoltage protection technologies, including MOV (Metal Oxide Varistor), TVS (Transient Voltage Suppressor), GDT (Gas Discharge Tube), and SIDACtor® protection thyristors. The challenge for design engineers is how to optimize component selection and determine the best combination of technologies to achieve the best performance and price.
A unique solution combines a SIDACtor and a Varistor (SMD or THT), reaching a low clamping voltage under a high surge pulse. The SIDACtor+MOV combination enables automotive engineers to optimize the selection and, therefore, the cost of the power semiconductors in the design. These parts are needed to convert the AC voltage into the DC voltage to charge the vehicle's on-board battery.
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Figure 1: On-board charger block diagram
The On-Board Charger (OBC) is at risk during EV charging due to exposure to overvoltage events that may occur on the power grid. The design must protect the power semiconductors from overvoltage transients because voltages above their maximum limits can damage them. To extend the EV's reliability and lifetime, engineers must address increasing surge current requirements and lower maximum clamping voltage in their designs.
Figure 1 shows the circuits requiring protection components and blocks that can employ high-efficiency components. The table lists the recommended technologies.
The potential surge pulses for the OBC come from indirect lightning strikes, load switching, and failure in the system. Imagine the power of a direct lightning strike of 100kA; a high surge current requirement in the specification is understandable. Other possible root causes for a surge pulse are abrupt load switching and faults in the power system.
Example sources of transient voltage surges include the following:
The coupling of the surge pulses is capacitive on parallel cables, inductive on conductor loops, and emission in the near field. The transient surge occurs over cable (on power, data, or signal lines), and it can be symmetrical (line-to-line) or asymmetrical (line-to-ground). It is crucial to know the coupling and propagation source to solve the application problem.
The IEC-61000-4-5 is the relevant standard for surge immunity. Table 1 lists maximum surge voltages up to 4kV. The 2Ω generator resistance results in a 2kA surge pulse (1a). The IEEE C62.41.2-2002 standard specifies a 6 kV/3 kA surge rating (1b). Today, most power grid-related AC power circuits are designed to resist the IEEE surge requirement.
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Table 1: (1a) IEC 61000-4-5 peak voltage and peak current withstand ratings and (1b) IEEE C62.41.2-2002 Standard 1.2/50 µs-8/20 µs, expected voltages and current surges
According to the 6kV/3kA surge, many designers use 14mm MOVs in the AC primary side circuit.
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Figure 2: Recommended circuit for differential and common mode transient voltage circuit protection using MOVs and a GDT.
A 20mm MOV is preferred for better reliability and protection. The 20mm MOV handles 45 pulses of 6kV/3kA surge current, which is much more robust than the 14mm MOV. The 14mm disc can only handle around 14 surges over its lifetime.
Voltage transient protection performance comparison
Compare an MOV's transient voltage protection performance with a SIDACtor+MOV combination. Figure 3 shows the clamping performance of a 14mm MOV when struck with a 2kV and a 4kV surge. The MOV has a maximum operating voltage of 385VACRMS. The clamping voltages are more than 1000V, which puts a high stress level on the power semiconductors.
MOV transient voltage performance
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Figure 3. Clamping performance of the Littelfuse V14P385AUTO MOV under 2kV and 4kV surges. The clamping voltage exceeds 1000V.
MOV Selection Parameters
Example selection determination
Level 1 Charger—120VAC, single-phase circuit: The expected ambient temperature is 100°C.
Step 1: Determine the minimum voltage rating of the MOV. The rule of thumb is to add 25% to the nominal AC line voltage to account for an imperfect power service: 120VAC x 1.25 = 150VAC. This is the minimum suggested voltage rating. The maximum peak surge current must be above 3kA.
Step 2: Repetitive Surge Capability must meet the standard requirements. The peak surge current and the energy rating must be reduced based on the temperature derating chart. The high potential capacity depends on the coating selection. Using a GDT helps the protection configuration achieve the leakage requirements of the High Potential test, which an MOV cannot meet alone.
SIDACtor+MOV transient voltage performance
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Figure 4. SIDACtor+MOV protection from voltage transients between line and neutral
The SIDACtor+MOV approach has several advantages. The primary benefit isthat for a 6KV/3KA surge, the clamping voltage is under 1000V as indicated in Table 2.
Figure 5 illustrates the voltage versus time response of the MOV and SIDACtor+MOV combination, again showing that the SIDACtor+MOV combination has a lower clamping voltage.
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Table 2. Clamping voltage of a Littelfuse V14H385A MOV compared with P3800FNL SIDACtor and a V14H250A MOV under different surge voltage
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Figure 5. Response of the MOV and the SIDACtor+MOV combination to a 6kV surge
An MOV alone shows degeneration after multiple surges. The leakage current increases with the number of surges the MOV must absorb. Also, the breakdown voltage is expected to fall with an increasing number of surge strikes. The rising leakage and the clamping voltage change show the MOV parameters' drift. Designer should select a larger disc size to avoid this situation with an MOV. This approach impacts the cost and consumes critical PCB space. However, their performance is more stable with a SIDACtor+MOV combination, and the SIDACtor extends the MOV lifetime.
SIDACtor+MOV: The superior solution for transient surge protection
While a designer will consider an MOV for voltage transient protection of downstream circuitry, Littelfuse can offer the designer a superior solution with its SIDACtor protection thyristor placed in series with an MOV. The SIDACtor+MOV combination has a lower clamping voltage to reduce semiconductor stress. In addition, the combination has a much lower leakage current and a breakdown voltage that degrades much less with increasing transient strikes. Using a SIDACtor+MOV combination for transient surge protection will result in a more reliable, robust on-board charger.
To learn more about using SIDACtor Protection Thyristors in electric vehicles, download the How to Select the Optimum Transient Surge Protection for EV On-Board Chargers application note, courtesy of Littelfuse, Inc.
