Fast Switching Semiconductors: Selecting Power MOSFETs to Minimize Switch Losses

Power MOSFETs are small form factor and are used for power supply topologies that require a small footprint. This makes them ideal for use in the wide variety of compact consumer and industrial devices. Power MOSFETs became available when silicon manufacturers were able to build high-power transistors into small surface-mount packages.

As demand increased, silicon vendors evolved their materials to enable improved voltage and current ratings. Additionally, focus was given to providing low input capacitances to support required switching speeds. On-resistance (RDS(on)) and charge gate (Qg) continue to improve as well allowing for minimal power and switching losses.

Defining Characteristics of Power MOSFETs

When designing with a Power MOSFET there are some interesting features to consider. The advantage of a MOSFET rather than a BJT is the turn-on voltage at its gate. The FET requires at least a few volts to activate the drain current making it immune to noisy transients at the startup of the system. Its DC characteristics and knowledge of how to apply them that will help you bias your circuit for best effect. Make yourself familiar with the following:

Gate Charge, Qg: The amount of charge consumed by the capacitance of the gate.

On-state Resistance, RDS(on): Basically, is the resistance across the drain-source terminals of the MOSFET, that dissipates heat when current is flowing. When using in your circuit, consider other resistances in the surrounding materials as this may add to the on-resistance of the MOSFET. Consider accumulation in surrounding material, drift-region, substrate, bond wire, contact, and leadframe resistances.

Switching speeds: The turn-on and turn-off timing of the MOSFET.

Terminal Capacitances: These are capacitances across each terminal, the gate-drain, gate-source, and drain-source, as well as the input and output capacitances.

Input Capacitance, Ciss: Capacitance from gate to source.

Reverse-transfer capacitance, Crss: Capacitance from gate to drain.

Miller Capacitance, Cgd: Combination of Input and reverse-transfer capacitance.

Many consider on-resistance and gate capacitance when biasing their MOSFETs to minimize both power dissipation and fast transients when switching. To do this, both RDS(on) and gate capacitances must be studied for their ability to bias the FET with both the required currents to drive the gate and with the appropriate response timing to achieve the drain current at the right time.

Parameters to Consider When Selecting a Power MOSFET

If your application is using a Power transistor for switching operations, you want to consider the tradeoff between RDS(on) and gate charge (Qg). Qg determines capacitance values on your gate. Lowering Qg increases RDS(on) so you’ll need to determine if the Power transistor with lower Qg is able to handle increased power dissipation.

Part of your analysis should include a deeper look at the input impedance at the gate with respect to the overall design. To drive the gate, large currents are needed to supply the gate capacitances, both the input and the reverse-transfer capacities. As the gate voltage increases, the current across Crss changes and more current is demanded through Ciss. During this dynamic change of capacity, the voltage on the gate flattens to allow the input impedance to readjust. This is called the Miller effect and it produces the Miller plateau.

Dynamic changes in gate capacitance during startup cause the Miller plateau

You also want to consider how timing for bringup of your gate effects turn-on of your Power MOSFET drain current. Switching large currents too fast will have the detrimental effect of transients. Transients inject noise onto both your ground plane and voltage rails, so you want to avoid creating them. In addition, the transients provide an opportunity to capacitively couple onto adjacent wires or nodes causing noise, so these too should be avoided. It turns out that the gate response timing is slower than drain current activation. To avoid overdriving the output current, IDS, consider underdriving the gate. You want to aim for the lowest VDS to achieve your required drive current.

Here are a few more terms you may run into when using Power MOSFETs in your designs:

Active (or Synchronous) Rectification: A technique for improving the efficiency of rectification by replacing diodes with actively controlled switches such as transistors, usually power MOSFETs or power BJTs.

Figure of Merit: A method for evaluating MOSFETs by comparing gate charge (Qg) against RDS(on)

VRM: Voltage regulator module is a buck converter that provides a microprocessor the appropriate supply voltage.

Now we’ll look at three examples of Power MOSFETs for your consideration. All three are available on our website with real-time distributor sourcing and costing information in addition to links to datasheets.

Vishay Si4800BDY N-Channel Reduced Qg, Fast Switching MOSFET 30V

This is an N-MOSFET from Vishay’s Low-Voltage TrenchFET^® product line. It boasts the industry’s lowest RDS(on) and gate charge Qg to improve efficiency for switch-mode power supplies. It is suitable for low-voltage applications needing less than 30V. With higher efficiency and increased power density, there is a need for heat sinking. The packages have been designed to be small and with a large heat sink pad for a direct tie to the PCB’s ground plane for thermal release. High RDS(on) to gate charge Qg (0.030 Ω to 8.7 nC).

Single MOSFET SO-8 pad pattern showing thermal heatsink

These N-FETs are designed for stable RDS(on) over their VDS operating range, as shown in the graph below. The material used to make these transistors was designed to maximize the tradeoffs between on-resistance, gate charge, and parasitic capacitances. If you have need for a 30V Power MOSFET, to deliver about 9A of load current, consider Vishay’s Si4800BDY.

Stable RDSON over drain current load demands

Applications suitable for this type of low-voltage MOSFET are consumer electronics, including computers and cellphones, Servers, Telecom equipment, and drones.

Infineon BSC026N04LS N-Channel OptiMOS^™ Power MOSFET 40V

This is an N-Channel MOSFET from Infineon’s Low-Voltage OptiMOS^™ product line. It offers excellent RDS(on) vs. Qg (2.6 mΩ to 32nC) to improve efficiency for switch-mode power supplies. This newest generation of material has achieved a 38% reduction in RDS(on) making it ideal for lowering power losses during operation.

Infineon’s Newest Power MOSFET technology reduces RDS(on) by 38%

The packages are designed with planar heat sink pads for a direct tie to your PCB’s ground plane. More copper allows optimum thermal relief reducing thermal losses during operation.

OptiMOS^™ 40V is a perfect choice for synchronous rectification in switched mode power supplies (SMPS) such as those found in servers and desktops. In addition, these devices can be used for a broad range of industrial applications including motor control, Telecom, Solar micro inverter, and fast switching DC-DC converters.

The graphs below show that this MOSFET has options for gate biasing from 4V to 10V to minimize power dissipation across the part during operation. Further, gate voltages above 5V are best for maintaining stable RDS(on). Having high gate voltages for activation of the MOSFET is beneficial for reducing effects due to transients on the gate. Reducing transients on the gate during startup minimize or alleviate false turnons of the part.

Infineon BSC026N04LS I-to-V and RDS(on) output characteristic graphs

Infineon targets this Power MOSFET for synchronous rectification, isolated DC-DC converters, motor control, and Or-ing switches. If you are looking for a 40V Power MOSFET, to deliver about 50A of load current, consider the Infineon BSC026N04LS.

On-Semiconductor FDMS8020 N-Channel Power Trench^® MOSFET 30V

This is an N-Channel MOSFET from On-Semiconductor’s (formerly Fairchild) Low-Voltage PowerTrench^® product line. It specifies gate RDS(on) vs. Qg of 3.6 mΩ to 43 nC.

This N-Channel MOSFET has been designed specifically to improve the overall efficiency and to minimize switch node ringing of DC/DC converters using either synchronous or conventional switching PWM controllers. It has been optimized for low gate charge, low RDS(on), fast switching speed and body diode reverse recovery performance.

As with most Power MOSFETs manufactured in small surface mount packaging, ample copper pads are provided for thermal relief.

On-Semiconductor FDMS8020 thermal package and footprint

Applications suitable for this type of Power MOSFET include VRM Vcore switching for desktop and server, OringFET and/or load switching, DC-DC conversion, and for use as a motor bridge switch. If you are looking for a 30V Power MOSFET, to deliver 26A load current, consider the On-Semiconductor FDMS8020.

Power MOSFETs are refined components dedicated for use in small form factor power applications where size and cost are important. You’ll want to consider on-resistance, gate capacitance, gate charge, and switching speeds when biasing for optimal performance within your system. We looked at three offerings by three separate vendors that may work for your application. Octopart has a large selection of parts to suit your design needs.

If you have comments or suggestions on Power MOSFET part selection, drop us a note in comments below.

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