Diodes’ application in the inverter

Diodes’ application in the inverter


High efficiency and energy-saving appliances applications are the primary problems. Three-phase brushless DC motors because of its high efficiency and small size advantage has been widely used in consumer electronics devices as well as many other applications. In addition, the use of electronic commutator instead of mechanical reversing device, three-phase brushless DC motors are considered more reliable.

The standard three-phase power stage (power stage) is used to drive a three-phase brushless DC motor, shown in Figure 1. Power level to generate a electric field, in order to make a good motor to work in this field must be maintained with the angle between the rotor magnetic field close to 90 °. Six-step sequence control of the stator magnetic field vector produced six of these vectors must be a specified rotor position to change. Hall-effect sensors scan the rotor position. In order to provide six step rotor current, power level can be used six different specific sequence of switching power MOSFET. The following explains a common switching modes, providing six step currents.

MOSFET Q1, Q3 and Q5 high-frequency (HF) switch, Q2, Q4, and Q6 low-frequency (LF) switch. When a low-frequency MOSFET in the open state, but also a high-frequency MOSFET in a switching state, it will produce a power level.

Step 1) The power level at the same time to two-phase power supply, while the third phase is not receiving power. Assumed power phase of L1, L2, L3 not receiving power. In this case, MOSFET Q1 and Q2 in conduction state, current flows through Q1, L1, L2, and Q4.

Step 2) MOSFET Q1 off. Because the inductance can not be a sudden interruption of current, it will generate additional voltages, until the body diode D2 is directly biased and allows the continued flow of current flow. Freewheeling current path for the D2, L1, L2, and Q4.

Step 3) Q1 opens, the sudden anti-bias body diode D2. Q1 on the current total supply current (as in step 1) and the diode D2 on the recovery currents.

Show that there is a body - drain diode. In Step 2, the current flows into the body - drain diode D2 (see Figure 1), the diode is forward biased, minority carriers injected into the diode area and P area.

When the MOSFET Q1 turns on, the diode D2 is reverse-biased, N region minority-carrier into the P + body area, and vice versa. This led to a large number of fast-moving current flowing through the diode, from the N-epi to the P + area, that is, from drain to source. Inductor L1 flows through Q2 and Q1 for the peak current meter emerged in high impedance. Q1 showed an additional current spikes, an increase of the turn-on switching losses during the period. Figure 4a describes the MOSFET's turn-on process.

In order to improve in these specific applications, body-diode characteristics, R & D staff to develop a fast body diode recovery characteristics of MOSFET. When the diode conduction after reverse-bias, reverse recovery peak current Irrm smaller.

We tested a standard MOSFET contrast and fast recovery MOSFET. ST introduced STD5NK52ZD (SuperFREDmesh series) on Q2 (LF) in as shown in Figure 4b. In Q1 MOSFET (HF) during the conduction, switching losses reduced by 65%. When using STD5NK52ZD efficiency and thermal performance and scored a great increase (without using the free flow of cooler air environment, the shell temperature from 60 ° C down to 50 ° C). In this topology, MOSFET internal body diodes as freewheeling diodes, using a fast MOSFET body diode recovery characteristics more appropriate.

SuperFREDmesh technology make up the existing FDmesh technology with lower on-resistance, zener gate protection and a very high dv / dt performance and uses a fast body - drain recovery diode. N-channel 520V, 1.22 Ohm, 4.4A STD5NK52ZD can provide a variety of packages, including TO-220, DPAK, I2PAK and IPAK package. The device is designed to switch applications for engineers greater flexibility. Other advantages include very high dv / dt, is 100% avalanche tested, with a very low intrinsic capacitance, good repeatable manufacturing, as well as improved ESD performance. In addition, an optional module with the other solutions, compared to discrete solutions offer a flexible PCB, locate the device in order to achieve the optimization of space and access to effective thermal management, so this is a cost-effective solution.

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