This tutorial installment is: Power Semiconductor Switches | Power MOSFETs and Schottky Diodes. This topic answers the following questions:
- What are the majority carrier power semiconductor switches used today?
- What are the typical applications for these switches?
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As shown in the installment of this tutorial entitled Power Semiconductor Switches, Classification, power semiconductor switches can be classified by the type of charge carriers: minority carrier devices or majority carrier devices. This topic will discuss majority carrier devices. For a discussion of minority carrier devices, please see the topic Power Semiconductor Switches, PIN Diode, BJT, IGBT, Thyristor.
Majority Carrier Device Family
Majority carrier devices do not store significant minority carriers and therefore have turn on and turn off times an order of magnitude faster than minority carrier devices. Switching times of majority carrier devices are less than 200nS and frequently very much less. Consequently, the maximum practical switching frequency of a power supply with majority carrier active switches can reach 1MHz and beyond.
Power Metal Oxide Semiconductor Field Effect Transistor (MOSFET)
We will discuss the N-channel enhancement mode power MOSFET for this tutorial topic. There is also a P-channel enhancement mode power MOSFET for some power supply applications.
The power MOSFET has three terminals: drain, gate, and source. The gate to source voltage controls the conduction state of the power MOSFET. There is an insulating layer between the gate and an electrostatically controlled conduction channel between the drain and the source. Application of a gate to source voltage greater than the device threshold voltage will cause the power MOSFET to turn on by modulating the geometry of the electrostatic conduction channel.
Power MOSFETs also contain a slow reverse recovery time anti-parallel body diode from the drain to the source.
Parasitic parameters of the power MOSFET:
- Rds, the on state drain to source resistance, which causes conduction losses and non-zero voltage drop across the on state switch,
- Leakage resistance during the off state,
- Cdg, the Miller, drain to gate, or reverse transfer capacitance, which slows the turn-on and turn-off transition time of the transistor switch as well as potentially being a source of instability and dV/dT turn on problems,
- Cgs, the gate to source capacitance, which contributes a delay time to the turn-on and turn-off of the transistor switch.
Advantages of the power MOSFET:
- Voltage controlled, high input impedance device, easier than current control of BJT.
- Fast switching speed because they are free from minority carrier stored charge.
Disadvantages of the power MOSFET:
- Can be susceptible to thermal runaway in power supply applications requiring constant current.
- Fast transient switching induces greater radiated and conducted emissions (EMI).
- Devices with breakdown voltages of 600V or higher typically have greater on-state voltages than comparable minority carrier devices.
Power MOSFET maximum ratings available from Digi-Key as of 3/2011:
- Maximum drain to source breakdown voltage, (VDS): 4000 volts.
- Maximum drain current, (ID): 600A.
- Maximum apparent power, (VDS*ID): 56kVA1
Silicon Carbide Power MOSFET
Silicon carbide (SiC) is an emerging semiconductor material for use with power MOSFET’s. These devices have an RDSon about one third that of comparable silicon power MOSFETs, allowing for on state voltages comparable to IGBT’s. This feature opens the door to high power applications for power MOSFET’s with breakdown voltages over 600V. Since the silicon carbide power MOSFET is a majority carrier device, there is no associated storage time to cause the well know current tail characteristic of IGBT’s. Consequently, silicon carbide power MOSFET’s enable high voltage switching at frequencies greater than 50kHz.
Additionally, the total gate charge on a silicon carbide power MOSFET is actually about 3 times less than for a comparable silicon power MOSFET, resulting in yet further gains in the upper frequency limit and/or reduction of switching loss.
Furthermore, the silicon carbide material is relatively insensitive to operating temperature, allowing an RDSon which is stable over the operating temperature. The maximum junction temperature is 200 degrees C.
Advantages of the silicon carbide power MOSFET:
- Low on-state drain to source voltage due to low RDSon
- Low total gate charge
- RDSon changes little as temperature is increased
- Higher maximum junction temperature
Disadvantages of the silicon carbide power mosfet:
- Gate drive requires greater voltage for full enhancement, and slightly negative values for reliable cutoff.
- Thoughtful consideration of parasitic layout parameters required due to very fast switch transitions.
- Likely to generate greater emissions due to rapid switch transitions.
- Cost
As of 3/2011 manufacturers of SiC MOSFETs include Cree and Powerex.
Gallium Nitride MOSFETs
Gallium Nitride (GaN) is another emerging semiconductor material for use with power MOSFET’s with breakdown ratings of 200V or less. These devices have a total gate charge approximately one fifth that of comparable silicon MOSFETs and an RDSon about half or less. This feature opens the door to switching applications well above 2MHz in frequency, greatly reducing component size for non-isolated topologies and facilitating large step down ratios in buck converters.
Advantages of the Gallium Nitride power MOSFET:
- Very low total gate charge
- Low RDSon resulting in lower conduction losses
- RDSon changes less than silicon power MOSFETs as temperature is increased
- Device is fully enhanced with a gate to source voltage of 5 volts.
Disadvantages of the Gallium Nitride power mosfet:
- Thoughtful consideration of parasitic layout parameters required due to very fast switch transitions.
- Likely to generate greater emissions due to rapid switch transitions.
Gallium Nitride Power MOSFET maximum ratings available from Digi-Key as of 3/2011:
- Maximum drain to source breakdown voltage, (VDS): 200 volts. (Update on 5/27/2011: EPC gave a GaN overview and roadmap presentation on May 11th at the IBM Power Symposium. Plans were announced for availability of 600V GaN devices sometime in 2011, and possible availability of 1200V devices in 2012.)
- Maximum drain current, (ID): 33A.
- Maximum apparent power, (VDS*ID): 6.6kVA1
As of 3/2011 manufacturers of GaN MOSFETs include Efficient Power Conversion (EPC).
Silicon Schottky Diodes
The power versions of the silicon Schottky diode also have broad applicability in switching power supplies. Being majority carrier devices, silicon Schottky diodes do not have minority stored charge and therefore have zero reverse recovery time. However, silicon Schottky diodes have roughly 10 times as much junction capacitance which can have similar effects as reverse recovery time as well as ringing with parasitic inductance in the circuit.
Silicon Schottky diodes have substantially less forward voltage drop than their non-Schottky counterparts, which can be used advantageously to reduce conduction losses or to tweak output voltage for certain topologies.
An undesirable characteristic of silicon Schottky diodes is much greater reverse leakage current which can be problematic at elevated temperatures, causing substantial dissipation on the device and loss of efficiency.
Advantages of silicon Schottky diodes:
- Zero reverse recovery time
- Substantially less forward voltage drop compared to conventional diodes
Disadvantages of silicon Schottky diodes:
- Ten times greater junction capacitance than conventional diodes
- Substantially greater leakage current compared to conventional diodes
Silicon Schottky Diode Maximum ratings available from Digi-Key as of 3/2011:
- Maximum collector to emitter breakdown voltage, (VRRM): 250 volts2
- Maximum collector current, (IF): 140A
- Maximum apparent power (VRRM * IR): 24kVA1
Silicon Carbide Schottky Diodes
Silicon carbide Schottky diodes, being majority carrier devices, do not store minority carrier charge and therefore do not have a reverse recovery time. Additionally, the silicon carbide semiconductor has excellent stability characteristics over the operating temperature range as well as increased maximum junction temperature. Unlike the silicon Schottky diode, the forward voltage drop and junction capacitance of a silicon carbide Schottky diode are comparable to the non-Schottky diodes. Like the silicon Schottky diodes, the silicon carbide schottky diodes have substantially greater leakage current compared to conventional diodes.
Advantages of silicon Carbide Schottky diodes:
- Zero reverse recovery time
- Relatively insensitive to operating temperature change
- Greater maximum junction temperature
Disadvantages of silicon carbide Schottky diodes:
- Substantially greater leakage current compared to conventional diodes
Silicon Carbide Schottky Diode Maximum ratings available from Digi-Key as of 3/2011:
- Maximum breakdown voltage, (VRRM): 1700 volts
- Maximum forward current, (IF): 25A
- Maximum apparent power (VRRM * IF): 42.5kVA1
Gallium Arsenide (GaAs) Schottky Diode
Gallium arsenide Schottky diodes have the lowest junction capacitance of all power semiconductor diodes. Consequently, they are able to operate at switching frequencies of 5MHz and beyond.
Advantages of GaAs Schottky Diodes:
- Lowest junction capacitance
- Temperature stable
Disadvantages of GaAs Schottky Diodes:
- Limited documentation
GaAs Schottky Diode Maximum ratings available from Digi-Key as of 3/2011:
- Only one breakdown rating available: 300V
- Maximum forward current, (IF): 29A
- Maximum apparent power (VRRM * IF): 8.7kVA1
Next Topic
The next tutorial installment is Conduction Modes. This topic answers the following questions:
- What are power supply conduction modes?
- What are the effects of conduction modes on power supply performance?
If you need assistance with power electronics design, call or email us today for help with your requirements. You can also go to our power electronics consultant website for more information about our services for business clients. Thank you for reading this tutorial article entitled “Power Semiconductor Switches | Power MOSFETs and Schottky Diodes”
Next topic: Power Supply Conduction Modes
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Notes:
1 As used in this article apparent power is a relative figure of merit for power handling comparison. It does not indicate the absolute power through put capability of the device.
2 Silicon Schottky diodes with breakdown voltages greater than 100 volts exhibit some reverse recovery time even though they are still classified as Schottky devices.
References
[1] Reid L. Sprite, “Power Semiconductors: The BJT, MOSFET, and IGBT”, December, 2004.
[2] Ned Mohan, Tore M. Underland, and William P. Robbins, Power Electronics, 2nd Edition, New York, NY, John Wiley & Sons, 1995.
[3] Robert W. Erickson, Dragan Maksimovic, Fundamentals of Power Electronics, 2nd Edition, Norwell, MA, Kluwer Academic Publishers, 2001.