Application of Fast Diode MOSFETs in Three-Phase Inverter Topologies

High efficiency and energy saving are the primary issues in home appliance applications. Three-phase brushless DC motors are widely used in home appliances and many other applications due to their high efficiency and small size. In addition, three-phase brushless DC motors are considered to be more reliable due to the use of Electronic commutators instead of mechanical commutation devices.

A standard three-phase power stage is used to drive a three-phase brushless DC motor, as shown in Figure 1. The power stage generates an electric field, and for the motor to work well, this electric field must maintain an angle close to 90° to the rotor magnetic field. The six-step sequence control produces six stator field vectors that must be changed at a specified rotor position. Hall Effect Sensor”>sensorScan the position of the rotor. In order to provide 6 step currents to the rotor, the power stage utilizes 6 power MOSFETs which can be switched in different specific sequences” title=”MOSFET”>MOSFET. A common switching pattern is explained below to provide 6 step currents.

Application of Fast Diode MOSFETs in Three-Phase Inverter Topologies
Figure 1: Three-phase inverter” title=”Inverter”>inverter topology diagram

MOSFETs Q1, Q3 and Q5 are high frequency (HF) switching and Q2, Q4 and Q6 are low frequency (LF) switching. A Power stage is created when a low frequency MOSFET is on and a high frequency MOSFET is switched.

Step 1) The power stage energizes two phases simultaneously and leaves the third phase unpowered. It is assumed that the power supply phases are L1, L2, and L3 is not powered. In this case, MOSFETs Q1 and Q2 are on, and current flows through Q1, L1, L2, and Q4.

Step 2) MOSFET Q1 is turned off. Because the Inductor cannot interrupt the current abruptly, it will generate additional voltage until the body diode D2 is directly biased, allowing freewheeling current to flow. The path of the freewheeling current is D2, L1, L2 and Q4.

Step 3) Q1 turns on and body diode D2 is suddenly reverse biased. The total current on Q1 is the sum of the supply current (as in step 1) and the recovery current on diode D2.

Figure 2 shows a cross-sectional view of a MOSFET device showing the body-drain diodes therein. In step 2, current flows into the body-drain diode D2 (see Figure 1), which is forward biased and minority carriers are injected into the diode’s region and P-region.

Application of Fast Diode MOSFETs in Three-Phase Inverter Topologies
Figure 2: Cross-sectional view of a MOSFET device with current flowing through its internal body diode

When MOSFET Q1 is turned on, diode D2 is reverse biased and minority carriers in the N region enter the P+ body region and vice versa. This fast transfer causes a large amount of current to flow through the diode, from the N-epi to the P+ region, that is, from the drain to the source. Inductor L1 exhibits high impedance to the current spikes flowing through Q2 and Q1. Q1 exhibits additional current spikes, increasing switching during on-time >switchloss. Figure 4a depicts the turn-on process of the MOSFET.

To improve the performance of the body diode in these special applications, researchers have developed MOSFETs with fast body diode recovery characteristics. When the diode is reverse biased after conduction, the peak reverse recovery current Irrm is smaller and the time required to complete recovery is shorter (see Figure 3).

Application of Fast Diode MOSFETs in Three-Phase Inverter Topologies
Figure 3: MOSFET with fast body diode recovery, lower peak reverse recovery current and shorter recovery time

We compared standard MOSFETs and fast recovery MOSFETs. The STD5NK52ZD (SuperFREDmesh series) introduced by ST is placed in Q2 (LF), as shown in Figure 4b. During the turn-on operation of the Q1 MOSFET (HF), the switch >switchLoss has been reduced by 65%. Efficiency and thermal performance are greatly improved with the STD5NK52ZD (case temperature is reduced from 60°C to 50°C in a free-flowing air environment without a heatsink). In this topology, the body diode inside the MOSFET acts as a freewheeling diode, and it is more appropriate to use a MOSFET with fast body diode recovery characteristics.

Application of Fast Diode MOSFETs in Three-Phase Inverter Topologies

Figure 4: a) Q2 adopts standard MOSFET on-state operation; b) Q2 adopts ST’s STD5NK52ZD MOSFET on-state operation

SuperFREDmesh technology complements existing FDmesh technology with reduced on-resistance”>resistance, Zener gate protection and very high dv/dt performance with fast body-drain recovery diodes. The N-Channel 520V, 1.22 Ohm, 4.4A STD5NK52ZD is available in a variety of packages including TO-220, DPAK, I2PAK and IPAK. This device designs switches for engineers”>switchApplications provide greater flexibility. Other advantages include very high dv/dt, 100% avalanche tested, and very low intrinsic capacitance”>capacitance, good repeatability, and improved ESD performance. In addition, the use of discrete solutions allows for flexible positioning of components on the PCB for space optimization and efficient thermal management, making it a cost-effective solution compared to alternative modular solutions.

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