MOSFET selection strategy for unused applications
Time:2022-04-20
Views:2231
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, IGBT came into the market, and it is still a voltage control device. MOSFET is a positive temperature coefficient device, while IGBT is not necessarily. MOSFET is most carrier devices, so it is an ideal choice for high frequency applications. The inverter that converts DC to AC can work at ultrasonic frequency to avoid audio noise. Compared with IGBT, MOSFET also has high avalanche resistance. When choosing MOSFET, the operating frequency is an important factor. Compared with the same MOSFET, IGBT has lower clamping ability. When selecting between IGBT and MOSFET, the DC bus voltage, power rating, power topology and working frequency input by the inverter must be considered. IGBT is usually used in applications of 200V and above, while MOSFET can be used in applications from 20V to 1000V. Although Fairchild Semiconductor has 300V IGBT, the switching frequency of MOSFET is much higher than that of IGBT.
With lower conduction loss and switching loss, newer MOSFETs are replacing IGBT in medium voltage applications up to 600V. Engineers who design alternative energy power systems, UPS, SMPS and other industrial systems are constantly trying 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, in which a large number of MOSFET devices are used. 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 operate through photovoltaic (PV) solar systems and utility grids. The wall mounted EV charging station must realize fast charging. For communication power supply, PV battery charging station will also become important.
Three phase motor drive and UPS inverter need the same type of MOSFET, but PV solar inverter may need different MOSFET, such as ultra frfet and conventional body diode MOSFET. In recent years, the industry has invested heavily in PV solar power generation. Most of the growth began with residential solar projects, but larger commercial projects are emerging: events such as the price of polysilicon falling from $400 / kg in 2007 to $70 / kg in 2009 have contributed to huge market growth.
The popular grid connected inverter is a special inverter that converts DC into AC and injects it into the existing public power grid. DC power is generated from renewable energy sources, such as small or large wind turbines or PV solar panels. The inverter is also called synchronous inverter. The grid connected inverter works only when connected to the grid. Inverters on the market today adopt different topology designs, depending on the trade-off requirements of the design. The independent inverter adopts different designs to supply power according to the integral, lagging or leading power factor.
The market demand for PV solar system has long existed, because solar energy can help reduce the peak power cost, eliminate the volatility of fuel cost, provide more power for the public grid, and promote it as a "green" energy.
The US government has set a target that requires 80% of the country‘s electricity to come from green energy. As mentioned above, combined with the objectives of the U.S. government, PV solar solutions have become a growing market. This brings a growing demand for MOSFET devices. If MOSFET devices with different topologies are optimized, the solution of terminal products can achieve significant efficiency improvement.
High switching frequency applications need to reduce the parasitic capacitance of MOSFET at the expense of RDSON, while low-frequency applications require reducing RDSON as the highest priority. For single ended applications, MOSFET body diode recovery is not important, but it is very important for double ended applications because they require low TRR, qrr and softer body diode recovery. These requirements are extremely important for reliability in soft switching two terminal applications. In hard switching applications, as the working voltage increases, the on and off losses will also increase. In order to reduce shutdown loss, CRSs and coss can be optimized according to RDSON.
MOSFET supports zero voltage switching (ZVS) and zero current switching (ZCS) topologies, but IGBT only supports ZCS topology. Generally, IGBT is used for high current and low frequency switches, while MOSFET is used for low current and high frequency switches. Mixed mode simulation tools can be used to design MOSFETs for specific applications. Advances in silicon and trench technology have reduced on resistance (RDSON) and other dynamic parasitic capacitances, and improved the bulk diode recovery performance of MOSFETs. Packaging technology also plays a role in MOSFETs for these specific applications.
Inverter system
DC-AC inverter is widely used in motor drive, ups and green energy system. Generally, IGBT is used for high voltage and high power systems, but MOSFET is usually used for low voltage, medium voltage and high voltage (12V to 400V input DC bus). MOSFET has been widely used in high-frequency DC-AC inverters for solar inverter, UPS inverter and motor-driven inverter. In some applications where the DC bus voltage is greater than 400V, high voltage MOSFETs are used in low-power applications. MOSFET has an inherent body diode with poor switching performance, which usually brings high turn-on loss 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 existence 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 high carrier frequency inverter is the design of smaller and lower cost low-pass filter. MOSFETs are ideal for these inverter applications because they can operate at higher switching frequencies. This reduces radio frequency interference (RFI) because the switching frequency current component flows inside the inverter and output filter, eliminating outward flow.
The requirements for MOSFETs for inverter applications include:
The specific on resistance (RSP) should be small to reduce the on loss. The change of RDSON from device to device should be small, which has two purposes: less DC component at the output of the inverter, and the RDSON can be used for current detection to control abnormal conditions (mainly in low-voltage inverter); For the same RDSON, low RSP can reduce the wafer size and reduce the cost.
When the wafer size is reduced, a non clamped inductive switch (UIS) can be used. A good UIs should be used to design the MOSFET cell structure without too many concessions. Generally, for the same wafer size, compared with planar MOSFET, modern trench MOSFET has good UIs. Thin wafers reduce the thermal resistance (rthjc), in which case the lower quality factor (FOM) can be expressed as RSP × RthJC/UIS。 3. Good safe working 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. Moderately high CGD can help reduce EMI. Very low CGD increases DV / dt and therefore EMI. Low CGD / CGS ratio reduces the possibility of breakdown. These inverters do not operate at high frequencies, thus allowing a slight increase in grid ESR. Because these inverters work at medium frequency, slightly higher CGD and CGS can be allowed.
Even if the operating frequency is low in this application, reducing coss helps to reduce switching loss. It is also allowed to increase coss slightly.
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 parallel MOSFET. Vth should exceed 3V.
Bulk diode recovery: a softer and faster bulk diode with low reverse recovery charge (qrr) and low reverse recovery time (TRR) is required. Meanwhile, the softness factor s (TB / TA) shall be greater than 1. This will reduce the possibility of body diode recovery DV / dt and inverter direct connection. Active bulk 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.
EMI can be controlled by controlling the on and off of MOSFET, DV / dt and di / dt.
Reduce the common source inductance by using more wire welding on the wafer.
In the fast body diode MOSFET, the charge life cycle of the body diode is shortened, resulting in the reduction of TRR and qrr, which makes the MOSFET with body diode similar to the epitaxial diode. This feature makes the MOSFET an excellent choice for high-frequency inverters (including solar inverters) for various applications. As for the bridge arm of the inverter, the diode is forced to conduct forward due to reactive current, which makes its characteristics more important. Conventional MOSFET body diodes usually have long reverse recovery time and high qrr. If the body diode is forced to conduct in the forward direction during the conversion of the load current from the diode to the complementary MOSFET of the inverter bridge arm, the power supply will draw a large current during the whole time period of TRR. This increases the power dissipation in the MOSFET and reduces the efficiency. Efficiency is very important, especially for solar inverter.
The active body diode also introduces an instantaneous through condition. For example, when it recovers at high DV / DT, the displacement current in the Miller capacitor can charge the gate above Vth, and the complementary MOSFET will try to turn on. This may cause transient short circuit of bus voltage, increase power dissipation and cause MOSFET failure. To avoid this phenomenon, an external SiC or conventional silicon diode can be connected in reverse parallel with the MOSFET. Because the forward voltage of MOSFET body diode is low, Schottky diode must be connected in series with MOSFET. In addition, anti parallel SiC must be connected across both ends of the combination of MOSFET and Schottky diode (Fig. 1). When the MOSFET is reverse biased, the external SiC diode is on, and the series Schottky diode does not allow the MOSFET body diode to be on. This scheme has become very popular in solar inverter, which can improve efficiency, but increase the cost.
Fairchild‘s unifet II MOSFET device using frfet is a high-voltage MOSFET technology power device, which is suitable for the applications listed above. Compared with unifet MOSFET, the wafer size of unifet II device is also reduced due to the reduction of RSP, which helps to improve the recovery characteristics of bulk diode. There are currently two versions of this device: F-type frfet device with good bulk diode and U-type ultra frfet MOSFET with the lowest qrr and TRR in the market. Ultra frfet can eliminate SiC and Schottky diodes in the inverter bridge arm, achieve the same efficiency and reduce the cost. Figure 2 shows the efficiency comparison of ultra frfet MOSFET, standard MOSFET structure (as shown in Figure 1b) and SiC structure (as shown in Figure 1a). Figure 3 shows a diode recovery comparison between ultra frfet unifet II MOSFET and conventional unifet MOSFET devices. In this case, the qrr has been reduced from 3100nc to 260nc, and the diode switching loss has been significantly reduced.
Figure 4 shows that using ultra frfet can reduce the on loss by about 75% compared with the standard unifet II MOSFET. The conduction propagation delay, current and voltage ringing are reduced, and the conduction loss of series Schottky diodes is 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‘s powertrench MOSFET technology is a good solution for this kind of inverter.
Figures 5 and 6 show the switching performance of Fairchild Semiconductor powertrench MOSFET technology in inverter applications using fdp023n08. As shown in Figure 6, the turn-off energy is reduced by about 30% compared with the best MOSFET on the market. At the same time, compared with the same MOSFET, its turn-on loss is also reduced by about 20%, as shown in Figure 5. The bulk diode has low TRR and qrr. According to table 1, low QGD / Qgs ratio improves the reliability of the inverter. This MOSFET technology supports off-line UPS inverters.
Switching power supply market
By combining the improved power circuit topology and concept with the improved low loss power devices, the switching power supply industry is undergoing revolutionary development in improving 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 MOSFET as power switch to achieve these goals. LLC resonant converters are typically used for lower power applications, while ps-pwm-fb-zvs is used for higher power applications. These topologies have the following advantages: reducing switching loss; Reduced EMI; Compared with the quasi resonant topology, the stress of MOSFET is reduced; Because the switching frequency is increased and the power density is increased, the size of the radiator and the size of the transformer are reduced.
MOSFET requirements for phase shifted full bridge pwm-zvs converter and LLC resonant converter applications include: fast soft recovery diode MOSFET with low TRR and qrr and optimal flexibility, which can improve DV / dt and di / dt immunity, reduce diode voltage spike and increase reliability; Low ratio of QGD and QGD to Qgs: under light load, hard switch will appear, and high CGD * DV / dt will cause breakdown; During turn off and turn on, the low distribution ESR in the grid is beneficial to ZVS turn off and uneven current distribution; Under light load, low coss can expand ZVS switch. At this time, ZVS switch becomes hard switch, and low coss will reduce hard switch loss; The topology works at high frequency and needs to be optimized for 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 cause failure have also been improved.
Off line AC / DC
Generally, the AC power supply is input into the large capacitance filter through rectification, and the current extracted from the power supply is a large amplitude narrow pulse. This stage forms the front end of SMPS. Large amplitude current pulse will produce harmonics, 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 currents will increase distribution losses and reduce available power. Using power factor correction can ensure compliance with management specifications, reduce device failure caused by the above stress, and improve device efficiency by increasing the maximum power obtained from the power supply.
Power factor correction is a method to make the input as purely resistive as possible. Compared with a typical SMPS with a power factor of only 0.6 to 0.7, this is very satisfactory because the resistance has an integral power factor. This enables the distribution system to operate with maximum efficiency.
The requirements of power factor control step-up switch include:
Low QGD × RSP quality factor. QGD and CGD will affect the switching rate, low CGD and QGD will reduce the switching loss, and low RSP will reduce the conduction loss.
For hard switches and ZVS switches, low coss will reduce shutdown loss.
Low CISS will reduce gate drive power because PFC usually operates at a frequency above 100kHz.
High DV / dt immunity for reliable operation.
If MOSFETs need to be connected in parallel, high gate threshold voltage (vthgs) (3 ~ 5V) can help, and its immunity can withstand the impact of the recurrence of DV / dt conditions.
During dynamic switching, the sudden change of parasitic capacitance of MOSFET will lead to gate oscillation and increase gate voltage. This will affect long-term reliability.
Gate ESR is very important because high ESR increases the 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 the 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 with 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 known as "active" rectification, which uses MOSFET instead of diode. Synchronous rectification is used to improve rectification efficiency. Generally, the voltage drop of the diode will vary from 0.7V to 1.5V, resulting in higher power loss in the diode. In the low-voltage DC / DC converter, the voltage drop is very significant, which will lead to the decrease of efficiency. Schottky rectifier is sometimes used to replace silicon diode, but its forward voltage drop will also increase due to the increase of voltage. In low voltage converters, Schottky rectification cannot provide sufficient efficiency, so these applications need synchronous rectification.
The RSP of modern MOSFET has been significantly reduced, and the dynamic parameters of MOSFET have been optimized. When the diodes are replaced with these active controlled MOSFETs, synchronous rectification can be realized. Today‘s MOSFETs can have on resistance of only a few milliohms and can significantly reduce the voltage drop of MOSFETs, even under high current. This significantly improves efficiency compared to diode rectification. Synchronous rectifier is not a hard switch. It has zero voltage conversion in steady state. During on and off, the MOSFET body diode is on, which makes the voltage drop of MOSFET negative and causes the increase of CISS. Due to this soft switch, the gate constant voltage (plateau) changes to zero, which effectively reduces the gate charge.
The following are some main requirements for synchronous rectification: low RSP; Low dynamic parasitic capacitance: this reduces the grid driving power, because the synchronous rectifier circuit usually works at high frequency; Low qrr and coss reduce the reverse current. When the topology works at high switching frequency, it will become a problem. At high switching frequency, the reverse current acts as a large leakage current; Low TRR, qrr and soft diode are needed to avoid transient breakdown and reduce switching loss. The conduction is zero voltage switch. After the MOSFET channel is turned off, the body diode is turned on again. When the secondary voltage is reversed, the body diode recovers, which will increase the risk of breakdown. The active diode may need to jumper a buffer circuit on each MOSFET; Low QGD / Qgs ratio.
Table 1 illustrates the performance differences between powertrench MOSFET devices and leading competitive products. Using Fairchild powertrench technology, RSP, coss, CRSs, and qgd/qgs ratios are reduced. Powertrench MOSFET is recommended for secondary active rectification. For the same RDS (on), the wafer size of powertrench is reduced by about 30% and RSP is reduced by 30%, so the conduction loss is reduced in synchronous rectification.
Active or ing
The simplest form of or ing device is a diode. When the or ing diode fails, it will be protected by not allowing current to flow into the input power supply. The or ing diode allows 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 whole system. Eliminate single point of failure and allow the system to use the remaining redundant power supply to maintain operation. However, there are difficulties in achieving this isolation. 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 it is necessary to increase the radiator and 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 diode is between 0.7 and 1.7V; The forward voltage drop of Schottky diode is between 0.2 and 0.55v. Although the conduction loss of the system is reduced when Schottky diodes are used as or ing diodes, Schottky diodes have large leakage current - which will lead to conduction loss. This loss is lower than that of silicon diodes.
An alternative solution to this problem is to use a power MOSFET instead of a Schottky diode. This introduces additional MOSFET gate drivers and adds complexity. The RDSON of MOSFET must be very small, so the voltage drop of MOSFET is much lower than the forward voltage of Schottky diode, which can be called active or ing. The RDSON of modern low-voltage MOSFETs is very low - it can be as low as a few milliohms even in TO-220 or D2Pak packages. Fairchild Semiconductor adopts fds7650 in pqfn56 package, which can be as small as less than 1 milliohm for 30V MOSFET. When the or ing MOSFET is turned on, it allows the current to flow in either direction. In case of failure, the redundant power supply will generate large current, so the or ing MOSFET must be turned off quickly. Fairchild‘s powertrench technology MOSFET is also suitable for this application.
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