Power Electronics

Power Management 101: Power MOSFETs

What are Power MOSFETs ?

Power MOSFETs (Metal-Oxide Semiconductor Field Effect Transistors) are three-terminal silicon devices that function by applying a signal to the gate that controls current conduction between source and drain. Their current conduction capabilities are up to several tens of amperes, with breakdown voltage ratings (BVDSS) of 10V to 1000V.

What type of power MOSFET is used in integrated circuits?

MOSFETs used in integrated circuits are lateral devices with gate, source and drain all on the top of the device, with current flow taking place in a path parallel to the surface. The Vertical Double diffused MOSFET (VDMOS) uses the device substrate as the drain terminal. MOSFETs used in integrated circuits exhibit a higher on-resistance than those of discrete MOSFETs.

What package styles are used for power MOSFETs?

MOSFETs are available in Small Outline IC (SOIC) packages for applications where space is at a premium. Larger through-hole TO-220, TO-247 and the surface mountable D2PAK or SMD-220 are also available. Newer package styles include chip scale devices and also the DirectFET™ and PolarPak™ packages.

What fabrication processes are used for power MOSFETs ?

The fabrication processes used to manufacture power MOSFETs are the same as those used in today's VLSI circuits, although the device geometry, voltage and current levels are significantly different. Discrete monolithic MOSFETs have tens or hundreds of thousands of individual cells paralleled together in order to reduce their on-resistance.

Is there an SiC power MOSFET?

Cree is the first to come up with a viable MOSFET. The ability to make these parts rests on the gate structure, which requires a physics and chemistry solution. The company still has some "tweaking" to do with the process, but they appear to be well ahead of the other companies that have ventured into this technology.

The commercial production of 1200 V SiC power MOSFETs is now feasible because of recent advances in substrate quality, improvements in epitaxy, optimized device design, advances made in increasing channel mobility with nitridation annealing, and optimization of device fabrication processes. SiC is a better power semiconductor than silicon (Si) because SiC has a much higher electric field breakdown capability (almost 10x), higher thermal conductivity, and higher temperature operation capability (wide electronic band gap).

SiC excels over Si as a semiconductor material in 600V and higher rated breakdown voltage devices. SiC Schottky diodes at 600V and 1200V ratings are commercially available today and are already accepted as the best solution for efficiency improvement in boost converter topologies as well as in solar inverters by substituting them for the previously-used Si PiN free-wheeling diodes that have significant switching losses

The SiC MOSFET being discussed here is a 1200V, 20A device from Cree that has a 100mW RDS(on) at a +15V gate-source voltage. Besides the inherent reduction in on-resistance, SiC also offers a substantially reduced on-resistance variation over operating temperature. From 25°C to 150°C, SiC variations are in the range of 20% versus 200% to 300% for Si. The SiC MOSFET die is capable of operation at junction temperatures greater than 200°C but for this particular example it is limited by its TO-247 plastic package to 150°C.

How does a power MOSFET turn on?

The gate turns the MOSFET on when its gate-to-source voltage is above a specific threshold. Typical gate thresholds range from 1 to 4 V. When a positive bias greater than the gate-to-source threshold voltage (VGS(th) ) is applied to the gate, a current flows between source and drain. For gate voltages less than VGS(th) the device remains in the off-state.

What circuit type is used to turn the power MOSFET on?

When power semiconductor switches first found wide use, discrete transistors, pulse transformers, opto-couplers, among other components were used to drive the power MOSFET on and off. Now, specially designed gate driver ICs are used in many applications. Fig. 5-1 shows the equivalent circuit of a gate driver driving a power MOSFET. This minimizes the drive requirements from a low power circuit, such as a microprocessor, and also acts as a buffer between the controlling signal and the power semiconductor switch. The gate driver supplies enough drive to ensure that the power switch turns on properly. Some gate drivers also have protection circuits to prevent failure of the power semiconductor switch and also its load.

Are there other power MOSFET technologies in general use ?

The trench MOSFET has replaced the planar device in many applications because it extends the cell density limit. Trench technology allows a higher cell density but is more difficult to manufacture than the planar device. Process refinements have yielded devices with steadily increasing density and lower on-resistance. TrenchFET devices have achieved on-resistance less than 1mW for a 25mm2 silicon die, exclusive of lead resistance.

Trench MOSFETs employs the same schematic configuration of the older planar MOSFETs. And, new Trench MOSFETs offer significant advantages over the older generation Trench MOSFETs and also some improvements over the older planar MOSFET technology.

Are there other power MOSFET technologies?

Among the other technologies are MDMesh . STMicroelectronics said that the improvement in RDS(ON) achieved with MDmesh V will significantly reduce losses in line-voltage PFC circuits and power supplies, which will in turn enable new generations of electronic products offering better energy ratings and smaller dimensions. This new technology should help designers with high efficiency targets and also save power.

MDmesh V achieves its RDS(ON) per area performance by improving the transistor drain structure to lower the drain-source voltage drop. This reduces the device’s on-state losses while also maintaining low gate charge (Qg), enabling energy-efficient switching at high speeds and delivering a low RDS(ON) x Qg Figure of Merit (FOM). ST claims that the breakdown voltage of 650V is also higher than competing 600V devices, delivering a valuable safety margin for designers. A further advantage of ST’s MDmesh V MOSFETs is a cleaner turn-off waveform, enabling easier gate control and simpler filtering due to reduced EMI.

STMicroelectronics’ STripFET technology uses an optimized layout and updated manufacturing process to improve the gate charge, gate resistance and input capacitance characteristics. The low gate charge enables excellent switching behavior and the low gate resistance means fast transient response. The technology also offers an extremely low figure-of-merit, meaning reduced conduction and switching losses.

STMicroelectronics has introduced a new series of 30V surface-mount power transistors, achieving on-resistance as low as 2 mΩ (max) to increase the energy efficiency of products such as computers, telecom and networking equipment. The latest-generation STripFET VI DeepGATE family process has high equivalent cell density and said to be best RDS(ON) in relation to active chip size. This is around 20 per cent better than the previous generation and allows the use of small surface-mount power packages in switching regulators and DC-to-DC converters, the company said.

The technology also benefits from inherently low gate charge, which allows designers to use high switching frequencies and thereby specify smaller passive components such as inductors and capacitors.

Infineon has developed CoolMOS™ technology for high voltage Power MOSFETs that reduces the RDS(ON) area product by a factor of five for 600V transistors. It has redefined the dependence of RDS(ON) on the breakdown voltage. The more than square-law dependence in the case of a standard MOSFET has been broken and a linear voltage dependence achieved. It is said that this opens the way to new fields of application even without avalanche operation. System miniaturization, higher switching frequencies, lower circuit parasitics, higher efficiency, reduced system costs are pointing the way towards future developments. It has also set new benchmarks for device capacitances. Due to chip shrink and novel internal structure, the technology shows a very small input capacitance as well as a strongly nonlinear output capacitance. The drastically lower gate charge facilitates and reduces the cost of controllability, and the smaller feedback capacitance reduces dynamic losses. This technology, improves the minimum RDS(ON) values in the 600 to 1000 V operating range.

What package types are used with power MOSFETs?

Devices with breakdown voltage ratings of 55V-60V and gate-threshold voltages of 2 to 3V are used mainly in through-hole packages such as TO-220, TO-247 or the surface mounted D2PAK (SMD220). These through-hole packages have very low thermal resistance. Despite their higher thermal resistances, more surface-mount SOIC packages are finding their way into applications due to the continuous reduction in on-resistance of power MOSFETs. SOIC packages save space and simplify system assembly. The newest generation of power MOSFETs use chip scale and ball grid array packages for low voltage power MOSFETs.

The International Rectifier DirectFET power package is surface-mount power MOSFET packaging technology designed for efficient topside cooling in a SO-8 footprint. In combination with improved bottom-side cooling, the new package can be cooled on both sides to cut part count by up to 60%, and board space by as much as 50% compared to devices in standard or enhanced SO-8 packages. This effectively doubles current density (A/in2) at a lower total system cost. The DirectFET MOSFET family offerings match 20V and 30V synchronous buck converter MOSFET chipsets, followed by the addition at 30V targeted for high frequency operation. The DirectFET MOSFET family is also available in three different can sizes.

Vishay’s PolarPAK® is a thermally enhanced package that facilitates MOSFET heat removal from an exposed top metal lead-frame connected to a drain surface in addition to a source lead-frame connected to a PCB. PolarPAK was specifically designed for easy handling and mounting onto the PCB with high-speed assembly equipment and thus to enable high assembly yields in mass-volume production. PolarPAK power MOSFETs have the same footprint dimensions of the standard SO-8, dissipate 1 °C/W from their top surface and 1 °C/W from their bottom surface. This provides a dual heat dissipation path that gives the devices twice the current density of the standard SO-8. With its improved junction-to-ambient thermal impedance, a PolarPAK power MOSFET can either handle more power or operate with a lower junction temperature. A lower junction temperature means a lower RDS(ON), which in turn means higher efficiency. A reduction in junction temperature of just 20 °C can also result in a 2.5 times increase in lifetime reliability.

What is the DrMOS power IC?

Intel’s November 2004 DrMOS specification identified a multi-chip module consisting of a gate driver and power MOSFET. A major advantage of using this module (Fig 5-2) is that the individual MOSFET’s performance characteristics can be optimized, whereas monolithic MOSFETs produce higher on-resistance. Although the component cost of a multi-chip module may be higher than a monolithic part. The designer must view the cost from a system viewpoint. That is, space is saved, potential noise problems are minimized, and fewer components reduce production time and cost. Here, the multi-chip approach is superior to use of a monolithic part.

Unlike discrete solutions whose parasitic elements combined with board layout significantly reduce system efficiency, the DRMOS module is designed to both thermally and electrically minimize parasitic effects and improve overall system efficiency. In operation, the high-side MOSFET is optimized for fast switching while the low-side device is optimized for low RDS(on). This arrangement ideally accommodates the low-duty-cycle switching requirements needed to convert the 12V bus to supply the processor core with 1.0V to 1.2V at up to 30A.

What are the necessary characteristics for power MOSFETs used in synchronous rectifiers?

Fig. 5-3 shows a simplified synchronous rectifier circuit. Typical synchronous rectifiers consist of high-side and low-side MOSFETs, which require different characteristics for an optimum design. Generally, the best high side MOSFET is one with the lowest Qswitch × RDS(ON) figure-of-merit. Qswitch is defined as the post gate threshold portion of the gate-to-source charge plus the gate-to-drain charge (Qgs2 + Qgd). In contrast, the best high side MOSFET must exhibit very low RDS(ON) coupled with good Cdv/dt immunity.

Low voltage (<20V) p-channel power MOSFETs are used as power management switches in cell phones and PDAs. Increasingly, n-channel devices are being used as switching or synchronous FETs in step-up or step-down regulators in these applications. Minimization of (RDS(on) x Active area) for power management switches and (RDS(on) × Qgd) for the dc-dc converters are important considerations in new product developments for this market. Defining RDS(on) max. @ Vgs of 4.5V and Qgd typ @ Vds of 15V, gate charge optimized planar MOSFETs and third generation trench devices with figures of merit of below 100 mΩ×nC have been developed.

How do you compare MOSFETs and BJTs?

Power MOSFETs are capable of operating at very high frequencies compared with Bipolar Junction Transistors (BJTs) whose switching speed is much slower than for a power MOSFET of similar size and voltage rating. Typical rise and fall times of power MOSFETs are of the order of several nanoseconds which is two orders of magnitude faster than bipolar devices of similar voltage rating and active area. BJTs are limited to frequencies of less than 100kHz whereas power MOSFETs can operate up to 1MHz before switching losses become unacceptably high. Recent advances in the design and processing of MOSFETs are pushing this frequency limit higher.

Power MOSFETs are voltage controlled devices with simple drive circuitry requirements. Power BJTs on the other hand are current controlled devices requiring large base drive currents to keep the device in the ON state. Power MOSFETs have been replacing power BJTs in power application due to faster switching capability and ease of drive, despite the very advanced state of manufacturability and lower costs of BJTs.

BJTs suffer from thermal runaway. The forward voltage drop of a BJT decreases with increasing temperature potentially leading to destruction. This is of special significance when several devices are paralleled in order to reduce forward voltage drop. Power MOSFETs can be paralleled easily because the forward voltage increases with temperature, ensuring an even distribution of current among all components. They can withstand simultaneous application of high current and high voltage without undergoing destructive failure due to second breakdown. However, at high breakdown voltages (>~200V) the on-state voltage drop of the power MOSFET becomes higher than that of a similar size bipolar device with similar voltage rating, making it more attractive to use the bipolar power transistor at the expense of worse high-frequency performance.

Breakdown voltage (BVDSS) is the drain-to-source voltage at which a current of 250µA starts to flow between source and drain while the gate and the source are shorted together. With no bias on the gate, the drain voltage is entirely supported by the reverse-biased body-drain p-n junction. Breakdown voltage is primarily determined by the resistivity of the epitaxial layer.

All applications of power MOSFET switches require some guardbanding when specifying BVDSS rating. It is important to remember that there is a price to be paid for this in the form of either higher RDS(on) or larger die. There may be applications where a reduction of conservative guardbanding on BVDSS can be justified by an improved RDS(on) specification or lower cost without jeopardizing performance or reliability.

Bipolar transistors have ratings for maximum current under continuous and pulsed conditions. Exceeding these ratings usually result in device failure. Current ratings on MOSFET transistors have a different meaning because they behave as a resistor when they turn on. This means that the maximum voltage drop or heat generated determines the maximum current. Turning the current on and off at high speeds reduces the average power or heat generated, thereby increasing the maximum allowable current.

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