In the field of power electronics, inverters, as key devices for converting direct current (DC) to alternating current (AC), are widely used in solar power generation, electric vehicles, uninterruptible power supplies (UPS), and many other areas. The push-pull output structure of the inverter's front-end is favored due to its simple structure and high efficiency. Among these components, the MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) plays a crucial role as a power switching element in the push-pull output. This article will provide an in-depth analysis of the working principle of the MOSFET in the push-pull output of the inverter's front end.
I. Basic Principles of Push-Pull Output
Push-pull output is a bipolar output structure that uses two complementary switching transistors (usually MOSFETs) to alternately turn on and off, thereby achieving the transmission and conversion of electrical energy. In a push-pull output inverter front-end, two MOSFETs, Q1 and Q2, are typically used as switching elements, connected to the two primary windings of the transformer. When Q1 is on, Q2 is off, and current flows from the primary winding containing Q1 to the secondary winding; conversely, when Q2 is on, Q1 is off, and current flows from the primary winding containing Q2 to the secondary winding. This alternating on and off behavior allows the transformer secondary winding to output alternating current.
II. Basic Structure and Working Principle of MOSFETs
A MOSFET is a voltage-controlled semiconductor device whose operating principle is based on the fact that the conductivity of the semiconductor surface can be controlled by an applied voltage. A MOSFET consists of three electrodes: a gate (G), a source (S), and a drain (D). The gate and source are isolated by an insulating layer (usually silicon dioxide). When a positive voltage is applied between the gate and source, a conductive channel is formed beneath the insulating layer, allowing conduction between the drain and source. Conversely, when the gate voltage is zero or negative, the conductive channel disappears, and conduction is cut off between the drain and source.
III. Working process of MOSFETs in the push-pull output stage of the inverter
In the push-pull output stage of the inverter, the operation of MOSFETs Q1 and Q2 can be divided into the following stages:
Q1 conduction phase:
When the gate of Q1 receives a sufficient positive drive voltage, Q1 turns on, forming a current path from the positive terminal of the power supply through Q1 to the primary winding of the transformer and back to the negative terminal of the power supply. At this time, Q2 is in the off state and does not participate in conduction. During this stage, the current in the primary winding of the transformer gradually increases, while storing energy.
Q1 cutoff, Q2 conduction phase:
As the gate voltage of Q1 decreases or becomes negative, Q1 gradually turns off, while the gate of Q2 receives sufficient positive drive voltage and turns on. At this time, the current path changes to run from the positive terminal of the power supply through Q2 to the other primary winding of the transformer and back to the negative terminal of the power supply. During this process, the current direction in the primary winding of the transformer reverses, while energy continues to be stored.
Dead time:
There is a brief dead time between the turn-off of Q1 and the turn-on of Q2, and between the turn-off of Q2 and the turn-on of Q1. During this period, both Q1 and Q2 are in the off state, and no current flows through the primary winding of the transformer. However, due to the distributed inductance and leakage inductance in the primary winding of the transformer, these inductances will generate a reverse induced electromotive force when the current is suddenly interrupted, causing the drain voltage of the MOSFET to rise instantaneously, forming a voltage spike. This voltage spike may damage the MOSFET and affect the performance of the inverter.
IV. Causes and Elimination of Voltage Spikes
Voltage spikes are primarily caused by the distributed inductance and leakage inductance of the transformer's primary winding. When the MOSFET is turned off, the current in these inductors cannot change abruptly, generating a reverse induced electromotive force that hinders the decrease in current. To eliminate voltage spikes, the following measures can be taken:
Add an absorption circuit:
An absorption circuit consisting of a resistor, capacitor, and diode is added between the drain of the MOSFET and the power supply. When the MOSFET is turned off, the absorption circuit can absorb the energy released in the inductor, thereby limiting the rise of the drain voltage.
Optimize circuit design:
By optimizing the layout and routing of the transformer's primary winding, distributed inductance and leakage inductance are reduced. Simultaneously, the drive circuit and timing control of the MOSFETs are rationally designed to ensure sufficient overlap time between the two MOSFETs during switching, preventing excessively long dead times that could lead to voltage spikes.
Select high-performance MOSFETs:
Using high-performance MOSFETs with fast switching speed and low on-resistance can reduce energy loss and noise interference during the switching process and improve the overall performance of the inverter.
V. Conclusion
The operating principle of MOSFETs in the push-pull output stage of an inverter involves complex electromagnetics and semiconductor physics. By deeply understanding the basic structure and operating principle of MOSFETs, as well as the working process of push-pull output and the causes and elimination methods of voltage spikes, we can better design and optimize inverter circuits, improving inverter performance and reliability. In the future, with the continuous emergence of new materials, processes, and technologies, we have reason to believe that inverter technology will usher in an even broader prospect for development.
