How to choose MOS tube for switching power supply? Parameters have the final say

In a switching power supply , the switch-off and switch-on time of the switch tube affects the working efficiency of the switching power supply, and some parameters of the MOS tube play a decisive role. So what are the techniques for selecting MOS tubes?

Since MOS tubes have great benefits for the output of the circuit, they are often used as switching elements in power supplies. 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 the power supplies 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.

Because the power supply is constantly working in the server, the MOS tube, as a switching device, is 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 who are engaged in 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, ORing FET application designers are most concerned about the minimum conduction loss.

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 you want to design a smaller and lower-cost power supply, you need to pay full attention to low on-resistance. In power supply design, each power supply often requires multiple ORing MOS tubes to work in parallel, and multiple devices are required to transmit current to the load. In many cases, designers must connect MOS tubes in parallel to effectively reduce RDS(ON).

It should be noted that when in a DC circuit, the equivalent impedance of parallel resistive loads is less than the impedance of each load individually. For example, two 2Ω resistors in parallel are equivalent to a 1Ω resistor. Therefore, generally speaking, a MOS tube with a low RDS(ON) value and a large rated current can allow designers to minimize the number of MOS tubes used in the power supply.

In addition, there are some parameters that must be taken into consideration when selecting a MOS tube. 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. Basically, SOA defines the supply voltage and current at which the MOSFET can operate safely. In ORing FET applications, the primary issue 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.

When the circuit design goal is to achieve hot-swap function, the SOA curve can play a better role. In this case, the MOS tube needs to be partially turned on. The SOA curve defines the current and voltage limits during different pulses.

By the way, the rated current mentioned above is also a thermal parameter that needs to be considered. Because the MOS tube that is always turned on is easy to 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 capacity of the semiconductor junction of the MOS tube package . The simplest definition of RθJC is the thermal impedance from junction to shell. In detail, in actual measurement, it represents the thermal impedance from the device junction (for a vertical MOS tube, that is, near the upper surface of the die) to the outer surface of the package, which is described in the data sheet. If the PowerQFN package is used, the shell is defined as the center of this large drain piece. Therefore, RθJC defines the thermal effect of the die and the packaging system. RθJA defines the thermal impedance from the die surface 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 tube in switching power supply

Now let’s consider switching power applications and why this type of application requires a different view of the data sheet. By definition, this type of application requires the MOS tube to be turned on and off periodically. While there are dozens of topologies that can be used for switching power supplies, consider a simple example here. The basic buck converter commonly used in DC-DC power supplies relies on two MOS tubes to perform the switching function (Figure 2), which alternately store energy in the inductor and then release the energy to the load. Today, 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 applications. (DC-DC controller)

The reason why there are so many articles about the selection of MOS tubes in switching power supplies is that the design of switching power supplies is complex, but there is no calculation formula suitable for MOS tube selection. So at this time, it is worth considering some key parameters and why these parameters are so important. Traditionally, many power supply designers use a comprehensive quality factor [gate charge QG × on-resistance RDS(ON)] to evaluate or grade MOS tubes.

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, 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.

This article gives some suggestions on the selection of MOS tube parameters in switching power supplies. In particular, some important parameters are explained in detail. By determining the parameters, we can choose the appropriate MOS tube for the switching power supply more quickly and accurately.