When it comes to SMPS applications, both transistors have their advantages, but which one’s right for you?
There are many types of switch-mode power supply (SMPS) transistors to choose from today. Two of the more popular versions are the metal-oxide semiconductor field effect transistor (MOSFET) and the insulated-gate bipolar transistor (IGBT). Historically speaking, low-voltage, low-current and high switching frequencies favor MOSFETs. High-voltage, high-current and low switching frequencies, on the other hand, favor IGBTs.
Some additional basics:
While everyone has an opinion on which device works best in an SMPS application, the truth is this: there’s no universal standard to determine which device offers better performance in a specific type of circuit. It varies from application to application, and a wide range of factors, such as speed, size, and cost, all play a role in determining the right choice.
So, rather than say that one is outright better than the other, here’s a basic overview on the differences between both transistors.
The MOSFET in a nutshell
The MOSFET is a three-terminal (gate, drain, and source) fully-controlled switch. The gate/control signal occurs between the gate and source, and its switch terminals are the drain and source. The gate itself is made of metal, separated from the source and drain using a metal oxide. This allows for less power consumption, and makes the transistor a great choice for use as an electronic switch or common-source amplifier.
In order to function properly, MOSFETs have to maintain a positive temperature coefficient. This means there’s little-to-no chance of thermal runaway. On-state losses are lower because the transistor’s on-state-resistance, theoretically speaking, has no limit. Also, because MOSFETs can operate at high frequencies, they can perform fast switching applications with little turn-off losses.
Power MOSFETs
There are many different types of MOSFETs, but the one most comparable to the IGBT is the power MOSFET. It’s specially designed to handle significant power levels. They’re only used in “on” or “off” states, which has resulted in their being the most widely used low-voltage switch. When compared to the IGBT, a power MOSFET has the advantages of higher commutation speed and greater efficiency during operation at low voltages. What’s more, it can sustain a high blocking voltage and maintain a high current. This is because most power MOSFETs structures are vertical (not planar). Its voltage rating is a direct function of the doping and thickness of the N-epitaxial layer, and its current rating is related to the channel’s width (the wider the channel, the higher the current). Due to its efficiency, power MOSFETs are used in power supplies, dc/dc converters, and low-voltage motor controllers.
The IGBT in a nutshell
The IGBT is also a three terminal (gate, collector, and emitter) full-controlled switch. Its gate/control signal takes place between the gate and emitter, and its switch terminals are the drain and emitter.
The IGBT combines the simple gate-drive characteristics found in the MOSFET with the high-current and low-saturation-voltage capability of a bipolar transistor. It does this by using an isolated gate field effect transistor for the control input, and a bipolar power transistor as a switch.
The IGBT is specially designed to turn on and off rapidly. In fact, its pulse repetition frequency actually gets into the ultrasonic range. This unique capability is why IGBTs are often used with amplifiers to synthesize complex waveforms with pulse width modulation and low-pass filters. They’re also used to generate large power pulses in areas like particle and plasma physics, and have established a role in modern appliances like electric cars, trains, variable-speed refrigerators, air conditioners, and more.
Comparing structures
The structures of both transistors are very similar. When it comes to electron current flow, an important difference is the addition of a p-substrate layer beneath the n-substrate layer in the IGBT. In this extra layer, holes are injected into the highly-resistive n-layer, creating a carrier overflow. This increase in conductivity within the n-layer helps reduce the total on-state voltage of the IGBT. Unfortunately, it also blocks reverse current flow. As a result, an additional diode (often referred to as a “freewheeling” diode) gets placed parallel with the IGBT to conduct the current in an opposite direction.
The absence of minority carrier transports allow MOSFETs to switch at higher frequencies. There are, however, two limitations: the transit time of electrons across the drift region and the time required to charge/discharge the input gate and “Miller” capacitances.
Switching power
A reduction in on-state voltage can cost the IGBT to experience slower switching speed at turn-off. The reason is that while electron flow can be abruptly halted simply by reducing the gate-emitter voltage below the gate threshold voltage (as is the case with the MOSFET), there’s still the matter of the holes that are left in the drift and body regions (there’s no terminal connection to remove them). The only way to get them out of there is by sweep-out, which is dependent upon voltage across the device and internal recombination. As a result, the device displays a tail current at turn-off until the recombination is complete. This has always been a big drawback for the IGBT.
Advancements
A lot of these facts cover the historical basics for both devices. Advancements and breakthroughs have led to fairly significant performance improvements throughout the years for both devices.
MOSFETs:
• Improved switching speeds.
• Improved dynamic performance that requires even less power from the driver.
• Lower gate-to-drain feedback capacitance
• Lower thermal impedance which, in turn, has enabled much better power dissipation
• Lower rise and fall times, which has allowed for operation at higher switching frequencies
IGBTs:
• Improved production techniques, which has resulted in a lower cost
• Improved durability to overloads
• Improved parallel current sharing
• Faster and smoother turn-on/-off waveforms
• Lower on-state and switching losses
• Lower thermal impedance
• Lower input capacitance
Conclusion
MOSFETs and IGBTs are fast replacing a large majority of older solid-state and mechanical devices. It’s a movement that doesn’t look like it’s going to slow down any time soon either, especially with the development of silicon carbide (SiC) material quality. SiC power devices are showing developers advantages like less loss, smaller size, and improved efficiency. Innovations like this will continue to push the limits of MOSFETs and IGBTs into higher-voltage and higher-power applications. As a result, tradeoffs and overlaps are likely to continue in many applications. With that being the case, careful analysis of the device itself is perhaps the most logical solution when faced with the task of selecting a transistor for your SMPS application. ■
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