How MOSFETs and TVS Diodes Prevent Power Losses
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Electric Vehicles Are on the Rise: How MOSFETs and TVS Diodes Prevent Power Losses
Power and performance remain important objectives in the electric vehicle industry. Consumers already accustomed to the capabilities of vehicles using fossil fuels require electric vehicles to perform similarly (if not better) for mass adoption. At the heart of converting electricity to motion, semiconductors like MOSFETs are essential in power converter circuits. These circuits convert mains electricity into voltage levels appropriate to the vehicle’s battery, internal circuitry, and operating requirements.
Repeated power converter switching generates unwanted transients/spikes through the circuits. If left uncorrected, these transients can propagate along conducting lines and damage downstream components. Transient Voltage Suppressor (TVS) diodes protect against damage from these spikes in energy. These diodes allow designers to incorporate additional circuit protection without making extensive changes to the circuit layout because of their small package footprint.
MDE Semiconductor, Inc. manufactures MAX™ 40 Series TVS diodes, a high current TVS with a 40,000 W peak pulse power rating and excellent clamping capability, designed for automotive applications. We have a long and proven track record in manufacturing TVS diodes and semiconductor components for a wide range of markets. We are committed to facilitating more production for the advancing electric vehicle infrastructure and entire transportation industry.
Read further for a discussion on how MOSFETs and TVS diodes prevent power losses in electric vehicles.
The Growing Popularity of Electric Vehicles (EV) and the Challenges Still Ahead
In 2017, the global sales of battery-operated electric vehicles and plug-in hybrids exceeded one million units for the first time, moving the EV market share above one per cent of global car sales. Reasons for their growing popularity include their more accessible prices (although they’re still out of reach of the regular car owner), and policy shifts towards low to zero carbon emissions in the transportation sector.
In an ever-expanding list, major world cities like Copenhagen and Central London have announced plans to ban gasoline and diesel vehicles from their city centers by 2030. Reduced noise pollution from internal combustion engines is a welcome side effect.
Environmental and health benefits aside, future projections favor more EV purchases because of the desire for cost savings. With EVs, users and territories reduce their reliance on oil which is susceptible to price spikes and supply disruptions.
However, concerns and challenges still exist such as the lack of widespread availability of charging infrastructure, the time it takes to fully charge a vehicle, and achieving similar speeds and driving ranges per charge at the price of traditional cars.
Electric Vehicles and Power Semiconductors
Converting electric energy to its kinetic form is not 100% efficient. What’s more, there is no standard best practice in terms of a vehicle’s internal architecture to maximize performance while keeping costs at a minimum. Designers have multiple approaches to choose from.
However, lithium-ion batteries have stood out as a technological solution. Thanks to continued advancement over the years, battery-powered systems are now lighter in weight and more efficient compared to their predecessors. Our discussion will refer exclusively to battery-powered electric vehicles as opposed to hybrids or fuel-cell based automobiles.
There are three main power blocks involved in the process of converting electricity to motion, where MOSFETs are required. These include the on-board-charger, the DC/DC converter, and the traction inverter.
The onboard charger (OBC):
Converts AC from the grid (110VAC or 220-240VAC with output powers from 1.4 kW to 19.4 kW) to DC voltage to charge the car’s battery (400V-800V depending on the battery). The charging process is painfully long, taking 6 to 12 hours to fully charge, but comes with the added benefit of home charging. The driving range of a typical EV after recharge is between 70-100 miles, although some high-performance cars can exceed 200 miles.
A faster way to charge a vehicle termed DC fast charging uses charging stations called Electric Vehicle Supply Equipment, EVSE. This system bypasses the onboard charger and connects directly to the battery. Providing up to 400 kW of DC output power, it’s designed to take 30 minutes or less to charge depleted batteries to 80% full. What’s more, charging system developers are working on reducing that time to be as quick as the time it takes to fill up with gasoline.
An onboard DC/DC converter steps down the battery voltage (400V-800V) to power on-board equipment like interior lighting, doors, windows, heater, entertainment systems, and other subsystems which need to operate at 12V/48V. The converter isolates the battery, its circuitry and components from the mains circuit for safety purposes.
The traction inverter:
A DC/AC converter provides the power to propel the vehicle, by converting energy from the batteries to the electric motor (at up to 200kW) to drive the wheels and control the car’s speed.
Developments are underway that will allow vehicles in the future to be charged wirelessly by inductive power transmission.
Power MOSFETs and their application:
MOSFETs, short for Metal Oxide Semiconductor Field Effect Transistor, are used in the power converters discussed above and also in the charging station equipment because of their highly efficient switching capabilities at high frequencies, including:
- low ON-state resistance which reduces power losses
- low internal series gate resistance which determines the input impedance and switching speeds
- low thermal resistance (ambient/junction) so that self-generated heat is easily removed
The behavior of a MOSFET depends on the gate-to-source voltage applied. When acting as an open switch (OFF-state) in the cut-off region, no current flows through the drain and source terminals. The magnitude of the voltage applied between the gate and source is lower than the gate threshold voltage.
When the gate-to-source voltage approaches the threshold, a conducting channel between the output terminals is created which behaves like a constant resistance. This is the ohmic region, where the MOSFET is in the ON-state and the resistance is linearly proportional to drain-source voltage and the drain current.
In the active region, the gate-to-source voltage alone determines the current flow between the drain and source terminals. And the MOSFET acts as an amplifier. Increasing the applied control voltage increases the width of the conducting channel and allows more current to flow between the output terminals.
Protecting MOSFETs Using TVS Diodes
In power converters, designers must make a trade-off between the high switching frequency and the inductive spikes that arise as a result. These transients lasting in the region of 10 -100 microseconds cause overshoots of voltage and current in the circuit, ringing and parasitic turn-on. To deal with these issues, TVS diodes can be incorporated into the design to protect the MOSFET and downstream loads from destruction by absorbing the energy associated with the transients.
TVS diodes have exceptionally high energy absorption abilities. They act as clamping devices to absorb overvoltages greater than their breakdown voltage, and are characterized by very fast switch-on times – as fast as 50 ps. An added advantage is that their performance doesn’t degrade with the number of transients they experience in their lifetime.
To protect a MOSFET, the clamping voltage of the selected TVS diode should be less than the avalanche breakdown voltage of the MOSFET’s drain-to-source junction. However, the clamping voltage should be high enough so it allows normal operation and will not be triggered by it.
Beyond an EV’s performance, circuit protection to ensure the safety of the user is non-negotiable, considering the system’s operation involves hundreds of volts and amps. An accident or even damage to the battery could cause a thermal incident. Designers should account for these possibilities which require the appropriate placement of fuses and surge protection devices and other fail-safe measures. A few examples among many are:
- Battery management system overvoltage protection: High-voltage TVS diodes
- On-board charger, Traction inverter, DC/DC converter, EVSE, protection against transients: High-voltage TVS diodes
- Overall protection barrier: High-voltage/high-current fuses placed in series with the main switch
MDE Semiconductor, Inc. Manufactures TVS Diodes and Surge Protection Devices for Electrical Vehicles
Customer demands combined with regulations will have a huge impact on the future EV market. Safety, efficiency, and affordability will never go out of style, and intense R&D projects are taking place at car manufacturers in response. Any designs will need circuit protection elements since the performance of an EV depends completely on the reliable operation of its electrical systems. This is what drives us as at MDE Semiconductor, Inc.—keeping in step with the changing times.
MDE Semiconductor, Inc. specializes in the manufacture of a broad range of reliable and high-quality TVS diodes applicable in the protection of electric vehicle circuits including inverters, converters, controllers, sensors, and more. Our MAX™ 40 Series TVS diodes can prevent high energy transients from damaging MOSFETs and connected components. They have fast response times (under 1.0 ps), and a 40,000 W peak pulse power rating.
These diodes, because of their small package footprint, allow designers to incorporate protective measures to circuits without making drastic changes to the layout. However, designers should be aware that customized solutions, which we also provide, could be more applicable in specific designs.
Our team continues to push boundaries to lead the market with circuit protection innovations. Customers can expect our products to meet strict quality standards and most importantly, in time to market to ensure your market leadership.
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