Detailed MOSFET selection strategy
Time：2020.10.27
View：
Before the introduction of MOSFETs in the late 1970s, thyristors and bipolar junction transistors (BJTs) were the only power switches. BJT is a current control device, and MOSFET is a voltage control device. In the 1980s, the IGBT came on the market and it is still a voltage control device. MOSFET is a positive temperature coefficient device, but IGBT is not necessarily. MOSFET is a majority carrier device, so it is an ideal choice for high frequency applications. An inverter that converts direct current to alternating current can work at ultrasonic frequencies to avoid audio noise. Compared with IGBT, MOSFET also has high avalanche resistance. When choosing a MOSFET, the operating frequency is an important factor. Compared with the same MOSFET, IGBT has a lower clamping capability. When choosing between IGBT and MOSFET, the DC bus voltage, power rating, power topology and operating frequency of the inverter input must be considered. IGBTs are usually used for 200V and above applications, while MOSFETs can be used for applications ranging from 20V to 1000V. Although Fairchild Semiconductor has 300V IGBTs, the switching frequency of MOSFETs is much higher than that of IGBTs.
Newer MOSFETs have lower conduction losses and switching losses, and are replacing IGBTs in medium voltage applications up to 600V. Engineers designing alternative energy power systems, UPS, switching power supplies (SMPS), and other industrial systems are constantly seeking to improve the light-load and full-load efficiency, power density, reliability, and dynamic performance of these systems. Wind energy is one of the fastest growing energy sources. An application example is wind turbine blade control, which uses a large number of MOSFET devices. By catering to different application requirements, application-specific MOSFETs can help achieve these improvements.
Other recent applications that require new and specific MOSFET solutions include electric vehicle (EV) charging systems that are easy to install in home garages and commercial parking lots. These EV charging systems will be operated by photovoltaic (PV) solar systems and the utility grid. Wall-mounted EV charging stations must achieve fast charging. For communication power sources, PV battery charging stations will also become important.
Three-phase motor drives and UPS inverters require the same types of MOSFETs, but PV solar inverters may require different MOSFETs, such as Ultra FRFET MOSFETs and conventional body diode MOSFETs. In recent years, the industry has invested heavily in PV solar power generation. Most of the growth started in residential solar projects, but larger commercial projects are emerging: events such as the price of polysilicon falling from US$400/kg in 2007 to US$70/kg in 2009 have all contributed to huge market growth.
The grid-connected inverter, which is becoming popular, is a special inverter that converts direct current into alternating current and injects it into the existing public grid. DC power is generated by renewable energy sources, such as small or large wind turbines or PV solar panels. This inverter is also called a synchronous inverter. The grid-connected inverter will only work when connected to the grid. The inverters on the market today use different topological designs, depending on the design trade-off requirements. The stand-alone inverters adopt different designs to supply power according to the integral, lagging or leading power factor.
There is already a market demand for PV solar systems, because solar energy can help reduce peak electricity costs, eliminate fuel cost volatility, provide more electricity to the public grid, and promote it as a "green" energy source.
The U.S. government has set a goal to require 80% of the national electricity to come from green energy. The reasons mentioned above, combined with the goals of the US government, PV solar solutions have become a growing market. This has brought about an increasing demand for MOSFET devices. If MOSFET devices of different topologies are optimized, the end product solution can achieve significant efficiency improvements.
High switching frequency applications need to reduce the parasitic capacitance of the MOSFET at the expense of RDSON, while low frequency applications require reducing RDSON as the highest priority. For single-ended applications, the recovery of the MOSFET body diode is not important, but it is very important for double-ended applications because they require low tRR, QRR and softer body diode recovery. In soft-switching double-ended applications, these requirements are extremely important for reliability. In hard-switching applications, as the operating voltage increases, the turn-on and turn-off losses will also increase. To reduce turn-off loss, CRSS and COSS can be optimized according to RDSON.
MOSFET supports zero voltage switching (ZVS) and zero current switching (ZCS) topology, but IGBT only supports ZCS topology. Generally, IGBT is used for high current and low frequency switching, while MOSFET is used for low current and high frequency switching. Mixed-mode simulation tools can be used to design MOSFETs for specific applications. Advances in silicon and trench technology have reduced the on-resistance (RDSON) and other dynamic parasitic capacitances, and improved the body diode recovery performance of the MOSFET. Packaging technology also plays a role in these MOSFETs for specific applications.
Inverter system
DC-AC inverters are widely used in motor drives, UPS and green energy systems. Generally, high-voltage and high-power systems use IGBTs, but for low, medium and high voltages (12V to 400V input DC bus), MOSFETs are usually used. Among high frequency DC-AC inverters used in solar inverters, UPS inverters, and motor drive inverters, MOSFETs have gained popularity. In certain applications where the DC bus voltage is greater than 400V, high-voltage MOSFETs are used for low-power applications. The MOSFET has an inherently poor body diode with poor switching performance, which usually causes high turn-on losses in the complementary MOSFET of the inverter bridge arm. In single-switch or single-ended applications (such as PFC, forward or flyback converters), the body diode is not forward biased, so its presence can be ignored. The low carrier frequency inverter bears the burden of the size, weight and cost of the additional output filter; the advantage of the high carrier frequency inverter is the smaller and lower cost low-pass filter design. MOSFETs are ideal for these inverter applications because they can operate at higher switching frequencies. This can reduce radio frequency interference (RFI) because the switching frequency current component flows inside the inverter and output filter, thereby eliminating the outward flow.
MOSFET requirements for inverter applications include:
The specific on-resistance (RSP) should be small to reduce the conduction loss. The RDSON change from device to device should be small. This has two purposes: the DC component at the output of the inverter is less, and the RDSON can be used for current detection to control abnormal conditions (mainly in low-voltage inverters); The same RDSON, low RSP can reduce wafer size, thereby reducing costs.
When the wafer size is reduced, an Unclamped Inductive Switch (UIS) can be used. A good UIS should be used to design the MOSFET cell structure, and there should not be too many concessions. Generally, for the same wafer size, modern trench MOSFETs have good UIS compared to planar MOSFETs. The thin wafer reduces the thermal resistance (RthJC). In this case, the lower figure of merit (FOM) can be expressed as RSP×RthJC/UIS. 3. Good safe operating area (SOA) and low transconductance.
There will be a small amount of gate-drain capacitance (CGD) (Miller charge), but the CGD/CGS ratio must be low. A moderately high CGD can help reduce EMI. The extremely low CGD increases dv/dt and therefore EMI. The low CGD/CGS ratio reduces the possibility of breakdown. These inverters do not work at high frequencies, thus allowing a slight increase in gate ESR. Because these inverters work at medium frequencies, slightly higher CGD and CGS are allowed.
Even though the operating frequency is already low in this application, reducing COSS helps to reduce switching losses. It also allows a slight increase in COSS.
The sudden change of COSS and CGD during switching will cause grid oscillation and high overshoot, which may damage the grid after a long time. In this case, high source-drain dv/dt will become a problem.
High gate threshold voltage (VTH) can achieve better noise immunity and better MOSFET parallel connection. VTH should exceed 3V.
Body diode recovery: A softer and faster body diode with low reverse recovery charge (QRR) and low reverse recovery time (tRR) is required. At the same time, the softness factor S (Tb/Ta) should be greater than 1. This will reduce the possibility of body diode recovery dv/dt and inverter pass-through. Active body diodes can cause breakdown and high voltage spikes.
In some cases, high (IDM) pulse drain current capability is required to provide high (ISC) short-circuit current immunity, high output filter charging current, and high motor starting current.
By controlling the turn-on and turn-off, dv/dt and di/dt of MOSFET, EMI can be controlled.
Reduce common source inductance by using more wire bonds on the wafer.
"In a fast body diode MOSFET, the charge life cycle of the body diode is shortened, which reduces tRR and QRR, which results in a MOSFET with a body diode similar to an epitaxial diode. This feature makes this MOSFET an excellent choice for high frequency inverters (including solar inverters) for a variety of different applications. As for the inverter bridge arm, the diode is forced to conduct forward due to reactive current, which makes its characteristics more important. Conventional MOSFET body diodes generally have long reverse recovery time and high QRR. If the body diode is forced to conduct forward during the transition of the load current from the diode to the complementary MOSFET of the inverter leg, then the power supply will draw a large current during the entire period of tRR. This increases the power dissipation in the MOSFET and reduces the efficiency. And efficiency is very important, especially for solar inverters.
The active body diode also introduces an instantaneous shoot-through condition. For example, when it recovers under high dv/dt, the displacement current in the Miller capacitor can charge the gate above VTH, while the complementary MOSFET will try to turn on. This may cause an instantaneous short circuit of the bus voltage, increase power dissipation and cause MOSFET failure. To avoid this phenomenon, an external SiC or conventional silicon diode can be connected to the MOSFET in reverse parallel. Because the forward voltage of the MOSFET body diode is low, the Schottky diode must be connected in series with the MOSFET. In addition, anti-parallel SiC must be connected across both ends of the MOSFET and Schottky diode combination. When the MOSFET is reverse-biased, the external SiC diode conducts, and the Schottky diode connected in series does not allow the MOSFET body diode to conduct. This kind of scheme has become very popular in the solar inverter, can improve the efficiency, but increase the cost.
Fairchild Semiconductor's UniFET II MOSFET device using FRFET is a high-voltage MOSFET technology power device suitable for the applications listed above. Compared with UniFET MOSFETs, due to the reduced RSP, the wafer size of UniFET II devices is also reduced, which helps to improve body diode recovery characteristics. There are currently two versions of this device: an F-type FRFET device with a better body diode, and a U-type Ultra FRFET MOSFET with the lowest QRR and tRR on the market. The Ultra FRFET type can eliminate the SiC and Schottky diodes in the inverter bridge arms, while achieving the same efficiency and reducing costs. In this case, QRR has been reduced from 3100nC to 260nC, and diode switching losses have also been significantly reduced.
The conduction propagation delay, current and voltage ringing are reduced, and the conduction loss of the series Schottky diode is also eliminated. Compared with UniFET MOSFET, UniFET II device also has lower COSS, so the switching loss is reduced.
Battery-powered offline UPS inverter
In medium voltage applications, Fairchild Semiconductor’s PowerTrench MOSFET technology is a good solution for this type of inverter.
Compared with the same MOSFET, its turn-on loss is also reduced by about 20%, as shown in Figure 5. The body diode has lower tRR and QRR. According to Table 1, the low QGD/QGS ratio improves the reliability of the inverter. This MOSFET technology supports offline UPS inverters.
switching power supply market
By combining improved power circuit topologies and concepts with improved low-loss power devices, the switching power supply industry is undergoing revolutionary development in terms of increasing power density, efficiency and reliability. Phase-shift-pulse-width modulation-zero-voltage switching-full-bridge (PS-PWM-FB-ZVS) and LLC resonant converter topologies use FRFET MOSFETs as power switches to achieve these goals. LLC resonant converters are generally used for lower power applications, while PS-PWM-FB-ZVS is used for higher power applications. These topologies have the following advantages: reduced switching losses; reduced EMI; reduced MOSFET stress compared to quasi-resonant topologies; increased switching frequency and increased power density, thus reducing the size of the heat sink and transformer.
The MOSFET requirements for phase-shifted full-bridge PWM-ZVS converters and LLC resonant converter applications include: fast soft recovery body diode MOSFETs with lower tRR and QRR and optimal softness, which can improve dv/dt and di/ dt immunity, reduce diode voltage spikes, and increase reliability; low QGD and QGD to QGS ratio: under light load, hard switching will occur, and high CGD*dv/dt will cause breakdown; During the turn-on period, the lower distributed ESR inside the gate is beneficial to ZVS turn-off and uneven current distribution; under light load, low COSS can expand the ZVS switch, at this time the ZVS switch becomes hard switching, and low COSS will reduce hard switching losses ; This topology works at high frequencies and requires an optimized low CISS MOSFET.
FRFET, UniFET II and SupreMOS MOSFET are recommended for the above applications. Conventional MOSFET body diodes can cause failure. For example, SupreMOS MOSFET FRFET MOSFET (FCH47N60NF) is suitable for this topology, because tRR and QRR have been improved. In addition, active diodes that can cause failure have also been improved.
Offline AC/DC
Generally, AC power is rectified and input into a large-capacitor filter, and the current drawn from the power is a large-amplitude narrow pulse. This stage forms the front end of the SMPS. Large amplitude current pulses will generate harmonics, which will cause serious interference to other equipment and reduce the maximum power that can be obtained. Distorted line voltage will cause capacitor overheating, dielectric stress and insulation overvoltage; distorted line current will increase distribution losses and reduce available power. Using power factor correction can ensure compliance with management regulations, reduce device failures caused by the above-mentioned stresses, and improve device efficiency by increasing the maximum power obtained from the power supply.
Power factor correction is a way to make the input as purely resistive as possible. Compared with a typical SMPS with a power factor value of only 0.6 to 0.7, this is very satisfactory because the resistor has an integral power factor. This allows the power distribution system to operate at maximum efficiency.
The requirements of power factor control boost switch include:
low QGD×RSP quality factor. QGD and CGD will affect the switching rate, low CGD and QGD will reduce switching losses, and low RSP will reduce conduction losses.
For hard switching and ZVS switching, low COSS will reduce turn-off loss.
Low CISS will reduce the gate drive power, because PFC usually works at a certain frequency above 100KHz.
High dv/dt immunity to achieve reliable operation.
If MOSFETs need to be connected in parallel, high gate threshold voltage (VTHGS) (3~5V) can help, and the immunity provided by it can withstand the effects of dv/dt reappearance.
During the dynamic switching period, the sudden change of the parasitic capacitance of the MOSFET will cause the gate to oscillate and increase the gate voltage. This will affect long-term reliability.
Gate ESR is very important, because high ESR will increase turn-off loss, especially in ZVS topology.
For this application, UniFET, UniFET II, conventional SuperFET and SupreMOS MOSFET are recommended. FCH76N60N is one of the super junction MOSFETs with the lowest RDS (ON) in TO-247 package on the market. Through SupreMOS technology, design engineers can improve efficiency and power density. FCP190N60 is the latest product added to SuperFET II series MOSFET. Compared to SuperFET I MOSFET, RSP is improved by 1/3, making it an ideal choice for offline AC-DC applications.
Secondary side synchronous rectification: Synchronous rectification is also called "active" rectification, which uses MOSFETs instead of diodes. Synchronous rectification is used to improve rectification efficiency. Generally, the voltage drop of the diode will vary between 0.7V to 1.5V, and a higher power loss occurs in the diode. In low-voltage DC/DC converters, this voltage drop is very significant and will result in a drop in efficiency. Sometimes Schottky rectifiers are used to replace silicon diodes, but as the voltage rises, the forward voltage drop will also increase. In low-voltage converters, Schottky rectification cannot provide sufficient efficiency, so these applications require synchronous rectification.
The RSP of modern MOSFETs has been significantly reduced, and the dynamic parameters of the MOSFET have also been optimized. When the diode is replaced with these active controlled MOSFETs, synchronous rectification can be achieved. Today's MOSFETs can only have an on-resistance of a few milliohms, and can significantly reduce the voltage drop of the MOSFET, even at high currents. Compared to diode rectification, this significantly improves efficiency. Synchronous rectification is not hard switching, it has zero voltage conversion in steady state. During the turn-on and turn-off periods, the MOSFET body diode is turned on, making the voltage drop of the MOSFET negative and causing the CISS to increase. Due to this soft switching, the grid constant voltage (plateau) is converted to zero, thereby effectively reducing the grid charge.
The following are some of the main requirements for synchronous rectification: low RSP; low dynamic parasitic capacitance: this reduces the gate drive power, because synchronous rectification circuits usually work at high frequencies; low QRR and COSS reduce the reverse current. Topological work will become a problem at high switching frequencies. At high switching frequencies, this reverse current acts as a large leakage current; low tRR, QRR and soft diodes are required to avoid instant breakdown and reduce switching losses. Conduction is zero voltage switching. After the MOSFET channel is turned off, the body diode turns on again. When the secondary voltage reverses, the body diode recovers, which will increase the risk of breakdown. Active diodes may require a snubber circuit across each MOSFET; low QGD/QGS ratio.
Using Fairchild Semiconductor’s PowerTrench technology, RSP, COSS, CRSS, and QGD/QGS ratios are all reduced. PowerTrench MOSFET is recommended for secondary active rectification. For the same RDS (ON), the wafer size of PowerTrench is reduced by approximately 30%, and the RSP is reduced by 30%, thus reducing conduction losses in synchronous rectification.
Active OR-ing
The simplest form of OR-ing device is a diode. When the OR-ing diode fails, it will protect it by not allowing current to flow into the input power supply. OR-ing diodes allow current to flow in only one direction. They are used to isolate redundant power supplies, so the failure of one power supply will not affect the entire system. Eliminate single points of failure, allowing the system to use the remaining redundant power supply to keep running. However, achieving this isolation has problems. Once the OR-ing diode is inserted into the current path, additional power loss and efficiency reduction will occur. This power loss will cause the OR-ing diode to heat up, so a heat sink needs to be added to reduce the power density of the system. When the diode is turned off, its reverse recovery becomes a problem-the diode must have soft switching characteristics. To overcome some of these problems, Schottky diodes have been used. An important difference between these diodes and p-n diodes is the reduced forward voltage drop and negligible reverse recovery. The voltage drop of ordinary silicon diodes is between 0.7 and 1.7V; the forward voltage drop of Schottky diodes is between 0.2 and 0.55V. Although the Schottky diode is used as an OR-ing diode, the conduction loss of the system is reduced, but the Schottky diode has a larger leakage current-which will cause conduction loss. This loss is lower than that of silicon diodes.
An alternative solution to this problem is to use power MOSFETs instead of Schottky diodes. This introduces additional MOSFET gate drivers and adds complexity. The RDSON of the MOSFET must be very small, so the voltage drop of the MOSFET is much lower than the forward voltage of the Schottky diode, which can be called active OR-ing. The RDSON of modern low-voltage MOSFETs is very low-even in TO-220 or D2PAK packages, it can be as low as a few milliohms. Fairchild Semiconductor uses the FDS7650 packaged in PQFN56, which can be as small as less than 1 milliohm for a 30V MOSFET. When the OR-ing MOSFET is turned on, it allows current to flow in either direction. In the event of a failure, the redundant power supply will generate a large current, so the OR-ing MOSFET must be turned off quickly. Fairchild Semiconductor’s PowerTrench technology MOSFET is also suitable for this application.
