POWER SWITCHING SEMICONDUCTORS How to choose the right power switching device

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Today’s digital transformation depends on semiconductor technology – and semiconductors need efficient power management. This article compares the different types of power switching semiconductors currently available.

We’re all aware that our modern lifestyle, with its comfort, convenience, and rapid access to information, is based on a plethora of electronics-driven technologies including 5G, IoT, electric vehicles, renewable energy systems, and, more recently, the sudden growth in generative AI. It’s not so widely appreciated, however, that all these depend on power electronics - the application of semiconductor electronics to the control and conversion of electric power .

Power electronics is also essential to modern industrial processes, such as large-scale aluminium production, and smart grid management. Therefore, it’s not surprising that, for some time, it has processed 70% of electrical energy , and this will increase in the coming decades.

Power switching semiconductors of various types are the fundamental components of power electronics. Metal-oxide-semiconductor field-effect transistors (MOSFETs), bipolar junction transistors (BJTs), insulated-gate bipolar transistors (IGBTs), and thyristors can all be used for switching functions – but which type should you choose for any given application?

Below, we summarise the relative strengths and weaknesses of each device type for any given application, then we look in more detail at these devices’ characteristics. We also provide interesting manufacturers’ examples for each type.

BASIC KNOWLEDGE - MOSFET VS. BJT

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Power switching devices – relative merits

Each of these semiconductor power switching devices has both advantages and disadvantages, which you can weigh up to identify the best device for your particular application.

BJTs (Bipolar Junction Transistors) exhibit superior, more linear gain characteristics compared to MOSFETs, allowing higher voltage gain in some applications. They are also robust and can handle high current levels. They have a low forward voltage drop when fully saturated.

Disadvantages include slower switching speed than MOSFETs and IGBTs, and a requirement for a significant base current for proper operation.

IGBTs (Insulated Gate Bipolar Transistors) combine the physics of BJTs and power MOSFETs, offering the advantages of both device types. They can handle high voltage and current levels, and, like BJTs, have a low on-state power loss.

However, although their switching speed is faster than BJTs’, IGBTs are slower than MOSFETs. Also, their gate drive circuitry is more complex than that of MOSFETs.

MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) are fast-switching, and operate with simple, low-power gate drive circuitry. They also offer high input impedance.

However, MOSFETs are limited in their voltage handling capability, and have a higher on-state resistance than BJTs and IGBTs.

Thyristors can handle high voltages and currents, and have a straightforward structure. Once triggered, they remain on until the anode current drops below a certain level.

Power switching semiconductor details

BJT (Bipolar Junction Transistor) operation and key parameters

Although BJTs are commonly used as amplifiers, they can function as switches if they are biased into their saturation or cutoff region. In this mode, they play a crucial role in applications like power supplies and motor controls.

They are also used in inverters, buck converters, boost converters, and other DC-DC, DC-AC, AC-DC, and AC-AC converters.

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Key parameters include:

Current Gain (β or hFE): This parameter represents the ratio of output current (collector current, IC) to input current (base current, IB). For power transistors, the current gain is typically smaller than that of small-signal transistors, ranging from 20 to 100. It can also be influenced by collector current and temperature.

Maximum Rated Collector Current (I_Cmax): This value indicates the maximum current that the wires connecting the semiconductor to external terminals can handle. It might be related to the current at which the gain falls below a specified minimum or the current leading to maximum power dissipation when the transistor is biased in saturation.

Maximum Rated Voltage (V_CE): Associated with avalanche breakdown in the reverse-biased base-collector junction, this parameter defines the maximum voltage the BJT can handle. In the common-emitter configuration, the breakdown mechanism also involves the transistor gain and PN junction breakdown.

Junction Capacitance and Cutoff Frequency: Power transistors have a higher junction capacitance due to their large area. Consequently, their cutoff frequency is lower compared to small-signal transistors.

Product example: Diodes Incorporated’s DXTN07 and DXTP07 families of bipolar junction transistors can be used for high-current load-switching and power supply circuitry in industrial, consumer, and automotive applications. The DXTN07 series consists of four NPN devices, while the DXTP07 series has four PNP devices .

They tolerate collector-emitter voltages ranging from 25 V to 100 V, and they have a continuous-collector-current rating of either 2 A or 3 A. They could possibly be used in applications normally fulfilled by MOSFETs.

IGBTs (Insulated Gate Bipolar Transistors)

As higher voltage and current applications demand ever-higher efficiency power switching devices, power levels climb beyond MOSFETs’ ability to optimize efficiency, so alternative technologies become essential.

One solution is the insulated-gate bipolar transistor (IGBT), which is a combination of a power MOSFET control gate with a power NPN bipolar junction transistor. High power IGBTs have gained popularity as switching components in medium-to-high power converter designs such as motor control, power conversion, energy storage and industrial applications .

Critical parameters include:

Maximum Operating Temperature (Tc): IGBTs have specified parameters at different temperatures (e.g., 25°C, 100°C, 125°C, or 150°C). It’s essential to consider the temperature range within which the device operates effectively.

Voltage and Current Ratings:

• Maximum Collector-Emitter Voltage (V_ces): The maximum voltage that can appear across the IGBT collector to emitter connections without causing damage or degradation, whether during switching or steady state.

• The maximum current that the IGBT can switch is specified by I_c, whether operating in steady state (continuously), or during a pulse, when the maximum pulsed current is denoted by I_cp

Gate Drive Conditions:

• Gate Threshold Voltage (V_ge(th)): The voltage required to turn the IGBT on.

• Gate-Source Voltage (V_gs): The voltage applied to the gate.

• Gate Charge (Q_g): The total charge needed to switch the IGBT. (= I_g (gate current) multiplied by t (time))

Switching Characteristics:

• Turn-On and Turn-Off Times (t_on, t_off): The time taken for the IGBT to transition between on and off states.

• Switching Losses: Power consumption during the switching times is the major component of efficiency loss. Minimizing these losses enhances efficiency.

Thermal resistance (R_th): The thermal resistance expresses the difficulty of moving the heat generated in the semiconductor junction to the mounting surface of the IGBT package (the case)

Device Architecture: Punch-Through (PT) vs. Non-Punch-Through (NPT):

PT IGBTs are historically used, but are less common now. They have higher voltage ratings. NPT IGBTs: are symmetric, with equal forward and reverse breakdown voltages. They are commonly used due to improved performance.

Product example: The Bourns® IGBT discrete BID series combines technology from a MOS gate and a bipolar transistor, creating the right component for high voltage and high current applications. This device uses advanced Trench-Gate Field-Stop technology providing greater control of the dynamic characteristics while resulting in a lower Collector-Emitter Saturation Voltage (VCE(sat)) and fewer switching losses. In addition, this structure increases the robustness of the device and gives a lower RTH. The Bourns® IGBT solution is suitable for SMPS, UPS and PFC applications.

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Thyristors

A thyristor is a solid-state semiconductor device with four layers of alternating P- and N-type materials used for high-power applications.  It acts as a bistable switch (or a latch), increasingly in roles previously fulfilled by electromechanical devices.

Thyristors are mainly found where high currents and voltages are involved, and are often used to control alternating currents, where current polarity changing causes the device to switch off automatically. They can also be used as the control elements for phase-angle triggered controllers, also known as phase-fired controllers .

The most widely used members of this family are silicon controlled rectifiers (SCRs), Triacs, SIDACs, and DIACs.

An SCR is a four-layer NPN stack with a gate, anode, and cathode. Once triggered “On” by a gate signal, the device continues to conduct until it is turned “Off” by reducing the anode-to-cathode current below a critical threshold.

A Triac’s primary role is to control power bilaterally in an AC circuit. Operation of a Triac can be likened to two SCRs connected in parallel in opposite directions. A Triac has a single gate and can be triggered by either polarity.

A SIDAC is a multi-layer silicon semiconductor bidirectional switch operated by voltage. SIDACs are mostly found in ignition circuits or inexpensive high voltage power supplies.

A DIAC is constructed similarly to an open base NPN transistor. Since the DIAC is a bidirectional device, it makes a good economical trigger for firing Triacs in phase control circuits such as light dimmers and motor speed controls.

Essential parameters to be aware of include:

Maximum Allowable Forward Current: This represents the maximum current an open thyristor can handle. For powerful devices, this value can reach hundreds of amperes.

Maximum Allowable Reverse Current: This parameter specifies the maximum reverse current that the thyristor can withstand.

Forward Voltage refers to the voltage drop across the thyristor when it is conducting at its maximum current.

Reverse Voltage: This is the maximum permissible voltage across the thyristor in the closed state. It indicates the voltage at which the thyristor can operate without affecting its performance.

Switch-On Voltage: The minimum voltage applied to the anode required for the thyristor to start conducting. It’s the threshold voltage for turning the thyristor on.

Minimum Control Electrode Current: This current is necessary to activate the thyristor and switch it on.

Maximum Allowable Control Current: The maximum current that can be applied to the control electrode without damaging the thyristor.

Maximum Permissible Power Dissipation: This parameter defines the maximum power the thyristor can dissipate without overheating.

Transition Time: The time it takes for the thyristor to transition from the closed state to the open state. In some circuits, this parameter is critical.

BASIC KNOWLEDGE - SILICON

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Product examples – Littelfuse Teccor® brand thyristors

Teccor® brand Triacs are bidirectional solid-state switches designed for full-wave AC control in numerous applications. They can be used for variable output (phase control) of AC loads, and are also an effective, high-reliability replacement for electromechanical relays. The Triac family is available in a wide variety of package types and multiple performance variations to serve specific application needs.

Teccor brand Silicon Controlled Rectifiers (SCRs) are unidirectional solid-state switches. They can be used to switch or modulate half-wave or full-wave (rectified) AC circuits. Another common use is pulse control in capacitive discharge applications.

Teccor brand SIDACs are voltage switches typically used in high voltage lightning protection and flame ignition applications.

MOSFET operation and key parameters

The fundamental difference between MOSFETs and a BJT/NPN transistor is that a MOSFET turns on based on an applied voltage instead of current. A MOSFET gate essentially acts as a capacitor, that when charged, allows the source and drain to conduct. This is in contrast to a BJT/NPN which needs a current flow to conduct. Since MOSFET based circuits only require an applied voltage, they tend to be easier to implement. They also don’t have a well-defined linear region like NPN/BJT transistors do. As a result, they are usually used for on/off switching applications where they are activated and deactivated quickly .

Important specifications to check:

I_{d(Package Limited)} – This is the maximum theoretical drain current for the package. New designers commonly misunderstand this specification: You absolutely can’t exceed this value, but it does not mean you can actually drive a load at that current. The MOSFET will almost always burn up from heat before this specification is reached. So, it must be taken with a pinch of salt.

V_gs – Maximum voltage applied to the gate with respect to the source.

V_dss – The maximum allowable voltage difference from the drain to the source.

R_ds(on) – The maximum expected resistance from drain to source, at a given gate voltage.

R_{thetaJA (Junction-to-Ambient)} – This is the thermal resistance from the die junction to the outside of the package to ambient air. This will be stated for a specific amount of copper on the PCB.

Q_g – This is the total charge required to inject into the gate to fully turn the MOSFET “on”. This allows for the gate-to-source charge and gate-to-drain-charge, as well as any other internal parasitics. This is the easiest spec to use to calculate the maximum “theoretical” switching speed of the MOSFET.

Product example: Infineon’s IAUS300N08S5N012T is an 80 V, 1.2 mΩ max MOSFET for automotive applications. It features OptiMOS 5 technology, which is designed to improve system efficiency and power density while reducing system costs. OptiMOS 5 devices feature lower R_DS(on) and improved figure of merit (R_DS(on) x Qg) compared to alternative devices.

The devices are AEC Q101 qualified, with a 175°C operating temperature, and 100 % avalanche tested. They are RoHS compliant green products.

Applications include EV traction inverters, high-voltage DC-DC converters for EVs, and onboard battery chargers for EVs.

Ongoing developments for power semiconductors

Power trends across 270 component manufacturers at a recent exhibition remain focused on improving efficiency and reducing system complexity while driving down cost and package sizes. More highly integrated devices are also on the upswing.

A big area of development continues to be wide-bandgap (WBG)semiconductors, such asgallium nitride (GaN) and silicon carbide (SiC) power devices for a range of automotive, consumer electronics, communications and industrial applications. Several GaN manufacturers unveiled technology advances that claim higher performance and integration.

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For example, GaN Systems has announced a new GaN-based 11 kW/800 V on-board charger (OBC) reference design that delivers 36 % higher power density and up to 15 % lower bill-of-materials (BOM) cost compared with SiC transistors, according to the company. The improved efficiency of the 800 V OBC reference design reduces power losses during electric-vehicle (EV) charging, resulting in higher efficiency and lower cost.

SiC device manufacturers also announced several improvements. For example, Qorvo Inc. introduced a new surface-mount TO-leadless (TOLL) package for its 5.4 mΩ 750 V SiC FETs. It is the first product in a family of 750 V SiC FETs that will be released in the TOLL package with R_DS(on) ranging from 5.4 mΩ to 60 mΩ. Applications include AC/DC power supplies ranging from several hundreds of watts to multiple kilowatts, as well as solid-state relays and circuit breakers up to 100 A.

References

• https://www.hitachienergy.com/news/perspectives/2021/08/power-electronics-revolutionizing-the-world-s-future-energy-systems

• https://stateofgreen.com/en/solutions/efficient-and-reliable-power-electronics/

• https://www.electrical4u.com/application-of-bipolar-junction-transistor-or-bjt-history-of-bjt/

• https://www.eeeguide.com/power-bjt-construction-operation-and-its-characteristics/

• https://www.allaboutcircuits.com/news/new-high-power-bjt-diodes-incorporated-thermal-bipolar-junction-transistor/

• https://www.bourns.com/docs/technical-documents/technical-library/igbt/bourns_understanding_igbt_data_sheet_parameters_white_paper.pdf

• https://www.bourns.com/products/igbt

• https://en.wikipedia.org/wiki/Thyristor

• https://m.littelfuse.com/~/media/electronics/application_notes/switching_thyristors/littelfuse_thyristor_fundamental_characteristics_of_thyristors_application_note.pdf.pdf

• https://www.tti.com/content/ttiinc/en/manufacturers/littelfuse/products/fp_littelfuse_thyristors.html

• https://www.microtype.io/designing-power-mosfet-circuits/

• https://www.infineon.com/cms/en/product/power/mosfet/automotive-mosfet/iaus300n08s5n012t/

• https://community.infineon.com/t5/Knowledge-Base-Articles/Where-is-the-main-advantage-of-OptiMOS-5-technology/ta-p/464134

• https://www.electronicproducts.com/apec-2023-underscores-advances-in-power-devices/

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