When driving inductive loads such as solenoids, motors, or even the parasitic inductance of a long cable harness, turning off the switch does not immediately stop the current. Physics demands that the current continues to flow. If this energy has nowhere to go, it manifests as a massive voltage spike that forces the MOSFET into breakdown. Whether the device survives this event depends on its Avalanche Ruggedness.
This guide explores the physics of Unclamped Inductive Switching (UIS), decodes the Single Pulse Avalanche Energy (EAS) rating in datasheets, and explains why “ruggedness” is a critical selection criterion for reliability. For a broader understanding of MOSFET structure and general selection strategies, please first review our foundational article: Power MOSFET Deep Dive: The Engineer’s Guide to Selection, Losses, and Thermal Design.
1. The Physics of the Voltage Spike: What is Avalanche?
To understand avalanche ratings, we must first understand the enemy: Inductive Kickback (or Flyback).
The Inductive Law
An inductor stores energy in its magnetic field. The fundamental law of inductance is that current cannot change instantaneously. When a MOSFET turns off an inductive load, the current path is broken. The inductor fights this change by reversing its polarity and generating a high voltage spike in an attempt to keep the current flowing.
The voltage generated is defined by:
V = -L × (di/dt)
If there is no external “freewheeling” diode to clamp this voltage, the voltage across the MOSFET (Drain-Source voltage, VDS) rises rapidly. It will rise until it exceeds the MOSFET’s breakdown voltage (BVDSS). At this point, the MOSFET’s internal PN junction breaks down and begins to conduct current in reverse bias, acting exactly like a Zener diode.
The “Avalanche” Mode
This state is called “Avalanche.” During this event, the MOSFET is conducting the full inductor current while sustaining a voltage higher than its rated breakdown voltage. This results in massive instantaneous power dissipation within the silicon die.
The Danger: The heat generated is not distributed evenly; it is often concentrated in small areas of the die. If the temperature in these hotspots exceeds the intrinsic limits of silicon, the device fails catastrophically. This is why Avalanche Ruggedness is a key specification for systems like Anti-Lock Braking Systems (ABS) or Fuel Injection, where inductive spikes are routine.
2. Decoding the EAS Datasheet Parameter
The primary metric engineers use to quantify a device’s robustness against these spikes is Single Pulse Avalanche Energy, denoted as EAS.
The Formula
EAS represents the maximum amount of energy (in Joules) the device can absorb during a single, non-repetitive event without failing. For a standard unclamped inductive switching (UIS) test, the energy is calculated as:
EAS = ½ × L × IAS2 × [ V(BR)DSS / (V(BR)DSS – VDD) ]
- L: The inductance of the load.
- IAS: The avalanche current (usually the peak current flowing through the inductor just before turn-off).
- V(BR)DSS: The breakdown voltage of the MOSFET.
- VDD: The supply voltage.
The Trap: Why You Cannot Trust the “Headline” Number
If you look at the first page of a MOSFET datasheet, you might see a prominent EAS rating, such as “500 mJ”. However, experienced automotive engineers know this number is often misleading if taken out of context.
The Crucial Dependency on Current (ID): The EAS rating is strongly dependent on the current magnitude. A MOSFET might handle 500 mJ if the current is very low (e.g., 10 A) because the discharge time is long, allowing heat to spread across the die. However, at high currents (e.g., 50 A), the discharge is extremely fast, creating intense localized hotspots. At high currents, the allowed EAS might drop to only 50 mJ.
Key Takeaway: Never rely solely on the table value. You must consult the “Maximum Avalanche Energy vs. Starting Junction Temperature” and “Avalanche Current vs. Time” graphs in the datasheet.
3. Failure Mechanisms: Thermal vs. Parasitic BJT
Why exactly does a MOSFET fail during avalanche? Understanding this helps in selecting robust devices for harsh environments like Electric Vehicle (EV) powertrains.
Old Tech: Parasitic BJT Latch-up
Inside every MOSFET structure, there is an intrinsic, parasitic Bipolar Junction Transistor (BJT). In older planar MOSFET technologies, high avalanche current could forward-bias this parasitic BJT. Once the BJT turns on, it “latches up,” creating a short circuit that cannot be turned off. This leads to immediate destruction.
Modern Tech: Thermal Limit
Modern Trench and Superjunction MOSFET technologies are designed to suppress this parasitic BJT. In robust, “Avalanche Rated” modern devices, the failure mode is purely thermal. The device fails only when the silicon temperature rises so high (often exceeding 300°C to 400°C locally) that the silicon loses its semiconductor properties (becomes intrinsic). This improvement makes modern devices significantly more reliable for inductive loads.
For a comparison of how different modern power technologies handle stress, including SiC and IGBTs in automotive contexts, refer to: SiC vs. IGBT: The Technology Showdown Powering the Future of EVs.
4. Single Pulse (EAS) vs. Repetitive Avalanche (EAR)
Datasheets distinguish between a one-time survival event and recurring stress. Understanding this difference is vital for motor drive designers.
Single Pulse (EAS)
This rating assumes a “one-shot” event, such as a battery disconnect, a fuse blowing, or an emergency stop. The key constraint is that the junction temperature (Tj) starts at a known value (e.g., 25°C or 150°C) and is allowed to cool down completely afterward.
Repetitive Pulse (EAR)
Repetitive avalanche occurs in every switching cycle. This is common in certain inefficient solenoid drive circuits or high-speed switching with poor layout (high parasitic inductance). Here, the failure mechanism is not just the peak temperature of one spike, but the accumulated average heat.
Modern Specification Trend: You will notice that many modern datasheets no longer provide a specific “EAR” value in Joules. Instead, they state that repetitive avalanche operation is allowed as long as the average junction temperature (Tj) does not exceed the absolute maximum rating (e.g., 175°C). This shifts the responsibility to the engineer to calculate the thermal budget.
When designing for repetitive inductive switching, such as in robotic servo drives, ensuring thermal stability is paramount. See our insights here: Powering Precision: The Role of Power Devices in Robotic Servo Drives.
5. Design Strategies: Avoiding vs. Enduring
While buying a “rugged” MOSFET is good, good circuit design is better. Relying on the MOSFET to act as a Zener clamp dissipates energy in the switch, which is inefficient and heats up the system.
Strategy A: The Freewheeling Diode (The Best Practice)
The most effective way to handle inductive energy is to provide a path for the current to recirculate. Placing a diode in anti-parallel with the inductive load allows the current to “freewheel” and decay naturally through the resistance of the load and diode.
- Benefit: Clamps the voltage spike to the supply rail (plus diode drop).
- Result: The MOSFET never enters avalanche breakdown. EAS becomes irrelevant for normal operation.
Strategy B: Active Clamping
In applications where fast decay is needed (like fuel injectors), a simple diode is too slow. Active clamping uses a Zener diode between the Drain and Gate of the MOSFET. When the drain voltage rises, the Zener forces the MOSFET to turn partially back ON (in linear mode). This dissipates the energy in the MOSFET but controls the voltage precisely, preventing over-voltage breakdown.
Strategy C: Snubber Circuits
RC (Resistor-Capacitor) snubbers placed across the MOSFET can absorb high-frequency ringing energy caused by parasitic inductance, protecting the device from voltage spikes that might otherwise trigger avalanche.
Analyzing failure modes in these protection circuits is critical. For a broader look at how overvoltage leads to failure, read: Failure Analysis: Preventing Overcurrent, Overvoltage, and Overtemperature.
6. Summary and Engineer’s Checklist
Avalanche capability is a safety net, not a standard operating mode for efficient power conversion. When evaluating EAS for your design:
- Verify the Current: Check the datasheet curve for EAS at your specific operating current (ID). Do not rely on the table header value.
- Check the Temperature: De-rate the energy capability if your device starts at a high junction temperature (e.g., 100°C).
- Distinguish Single vs. Repetitive: EAS is for emergency survival. If your circuit repetitively avalanches the MOSFET, you likely need a better clamping circuit or a different topology.
- Prioritize Layout: Minimize parasitic board inductance. The energy stored in stray inductance ($½ L I^2$) ends up as heat in your MOSFET during every switching cycle.
Further Resources and Authority Links
For deep technical validation of avalanche ratings and physical failure analysis, we recommend consulting the following manufacturer resources:
External Resource: Vishay provides an excellent technical note on the definitions and test conditions for avalanche ruggedness: Vishay – Power MOSFET Avalanche Design Guidelines.
External Resource: Infineon offers detailed insights into the “Unclamped Inductive Switching” (UIS) behavior of their OptiMOS series: Infineon – Avalanche Capability of Power MOSFETs.