How to select MOS tubes for switching power supplies-EEWORLD

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The most common application of MOS tubes may be the switching elements in power supplies. In addition, they are also very beneficial to power output. Applications such as servers and communication equipment are generally configured with multiple parallel power supplies to support N+1 redundancy and continuous operation (Figure 1). Each parallel power supply shares the load evenly, ensuring that the system can continue to work even if one power supply fails. However, this architecture also requires a way to connect the outputs of parallel power supplies together and ensure that the failure of one power supply will not affect other power supplies. At the output of each power supply, there is a power MOS tube that allows the power supplies to share the load while isolating each power supply from each other. MOS tubes that play this role are called "ORing" FETs because they essentially connect the outputs of multiple power supplies with "OR" logic.

Figure 1 MOS tube for parallel power control for N+1 redundant topology In ORing FET applications, the MOS tube acts as a switch device, but because the power supply in server applications works continuously, this switch is actually always in the on state. Its switching function is only used when starting and shutting down, and when the power supply fails.

    Compared with designers working on switch-centric applications, ORing FET application designers obviously have to pay attention to the different characteristics of MOS tubes. Taking the server as an example, during normal operation, the MOS tube is only equivalent to a conductor. Therefore, the ORing FET application designer is most concerned about the minimum conduction loss. Low RDS(ON) can minimize BOM and PCB size .     Generally speaking, MOS tube manufacturers use the RDS(ON) parameter to define the on-resistance; for ORing FET applications, RDS(ON) is also the most important device characteristic. The data sheet defines RDS(ON) as related to the gate (or drive) voltage VGS and the current flowing through the switch, but for sufficient gate drive, RDS(ON) is a relatively static parameter.     If designers try to develop the smallest size and lowest cost power supply, low on-resistance is doubly important. In power supply design, each power supply often requires multiple ORing MOS tubes to work in parallel, and multiple devices are required to deliver current to the load. In many cases, designers must connect MOS tubes in parallel to effectively reduce RDS(ON).     Keep in mind that in a DC circuit, the equivalent impedance of a parallel resistive load is less than the impedance of each load individually. For example, two 2Ω resistors in parallel are equivalent to a 1Ω resistor. Therefore, in general, a MOS tube with a low RDS(ON) value and a large current rating allows designers to minimize the number of MOS tubes used in the power supply.     In addition to RDS(ON), there are several MOS tube parameters that are also very important to power supply designers during the MOS tube selection process. In many cases, designers should pay close attention to the safe operating area (SOA) curve on the data sheet, which describes the relationship between drain current and drain-source voltage at the same time. Basically, SOA defines the supply voltage and current at which the MOSFET can safely operate. In ORing FET applications, the first question is: the current delivery capability of the FET in the "fully on state". In fact, the drain current value can be obtained without the SOA curve.     If the design is to implement hot-swap function, the SOA curve may be more useful. In this case, the MOS tube needs to be partially turned on. The SOA curve defines the current and voltage limits during different pulses.     Note the current rating just mentioned, which is also a thermal parameter worth considering, because a MOS tube that is always on can easily heat up. In addition, the increasing junction temperature will also lead to an increase in RDS(ON). The MOS tube data sheet specifies the thermal impedance parameter, which is defined as the heat dissipation capability of the semiconductor junction of the MOS tube package. The simplest definition of RθJC is the thermal impedance from junction to case. In detail, in actual measurement, it represents the thermal impedance from the device junction (for a vertical MOS tube, that is, near the top surface of the die) to the outer surface of the package, which is described in the data sheet. In the case of the PowerQFN package, the case is defined as the center of this large drain piece. Therefore, RθJC defines the thermal effect of the die and the package system. RθJA defines the thermal impedance from the surface of the die to the surrounding environment, and is generally indicated by a footnote to indicate the relationship with the PCB design, including the number and thickness of copper plating. MOS tubes in switching power supplies     Now let's consider switching power supply applications and how this application requires a different perspective on the data sheet. By definition, this application requires the MOS tube to be turned on and off regularly. Meanwhile, there are dozens of topologies available for switching power supplies, so let's consider a simple example. The basic buck converter commonly used in DC-DC power supplies relies on two MOS tubes to perform switching functions (Figure 2), which alternately store energy in the inductor and then release the energy to the load. Currently, designers often choose frequencies of hundreds of kHz or even above 1 MHz, because the higher the frequency, the smaller and lighter the magnetic components can be.

Figure 2 MOS transistor pair for switching power supply application (DC-DC controller) Obviously, power supply design is quite complex, and there is no simple formula for evaluating MOS transistors. But let's consider some key parameters and why they are important. Traditionally, many power supply designers use a comprehensive quality factor (gate charge QG × on-resistance RDS(ON)) to evaluate or grade MOS transistors.

Gate charge and on-resistance are important because they both have a direct impact on the efficiency of the power supply. The losses that affect efficiency are mainly divided into two forms - conduction losses and switching losses.

Gate charge is the main cause of switching loss. The unit of gate charge is nanocoulomb (nc), which is the energy required to charge and discharge the gate of the MOS tube. Gate charge and on-resistance RDS(ON) are interrelated in semiconductor design and manufacturing processes. Generally speaking, the lower the gate charge value of the device, the higher its on-resistance parameter. The second most important MOS tube parameters in the switching power supply include output capacitance, threshold voltage, gate impedance and avalanche energy.

Certain special topologies also change the relative qualities of different MOS tube parameters. For example, a traditional synchronous buck converter can be compared with a resonant converter. The resonant converter only switches the MOS tube when VDS (drain-source voltage) or ID (drain current) passes zero, thereby minimizing switching losses. These techniques are called soft switching or zero voltage switching (ZVS) or zero current switching (ZCS) techniques. Since switching losses are minimized, RDS(ON) becomes more important in this type of topology.

Low output capacitance (COSS) values are a great benefit for both types of converters. The resonant circuit in a resonant converter is mainly determined by the leakage inductance of the transformer and COSS. In addition, the resonant circuit must allow COSS to be fully discharged during the dead time when the two MOS tubes are turned off.

Low output capacitance also benefits conventional buck converters (sometimes called hard-switching converters), but for different reasons. This is because the energy stored in the output capacitor is lost with each hard-switching cycle, whereas in a resonant converter the energy is recycled over and over again. Therefore, low output capacitance is especially important for the low-side switch of a synchronous buck regulator.

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