PDF《Application Report》

40 35 DC Restore Circuit in Transformer-Coupled Gate Drive

41 36 Gate-Drive Transformer Volt-second Product vs. Duty Ratio

42 37 Power and Control Transmission With One Transformer.43

38 Power and Control Transmission With One Transformer.43

39 Push-Pull Type Half-Bridge Gate Drive

44 40 Push-Pull Type Half-Bridge Gate Drive

45 Trademarks All trademarks are the property of their respective owners.

1 Introduction MOSFET C is an acronym for Metal Oxide Semiconductor Field Effect Transistor and it is the key component in high frequency, high efficiency switching applications across the electronics industry. It might be surprising, but FET technology was invented in 1930, some

20 years before the bipolar transistor. The first signal level FET transistors were built in the late 1950'

s while power MOSFETs have been available from the mid 70'

s. Today, millions of MOSFET transistors are integrated in modern electronic components, from microprocessors, through discrete power transistors. The focus of this topic is the gate drive requirements of the power MOSFET in various switch mode power conversion applications.

2 MOSFET Technology The bipolar and the MOSFET transistors exploit the same operating principle. Fundamentally, both type of transistors are charge controlled devices, which means that their output current is proportional to the charge established in the semiconductor by the control electrode. When these devices are used as switches, both must be driven from a low impedance source capable of sourcing and sinking sufficient current to provide for fast insertion and extraction of the controlling charge. From this point of view, the MOSFETs have to be driven just as hard during turn-on and turn-off as a bipolar transistor to achieve comparable switching speeds. Theoretically, the switching speeds of the bipolar and MOSFET devices are close to identical, determined by the time required for the charge carriers to travel across the semiconductor region. Typical values in power devices are approximately

20 to

200 picoseconds depending on the size of the device. www.ti.com MOSFET Technology

3 SLUA618ACMarch 2017CRevised October

2018 Submit Documentation Feedback Copyright ? 2017C2018, Texas Instruments Incorporated Fundamentals of MOSFET and IGBT Gate Driver Circuits The popularity and proliferation of MOSFET technology for digital and power applications is driven by two of their major advantages over the bipolar junction transistors. One of these benefits is the ease of use of the MOSFET devices in high frequency switching applications. The MOSFET transistors are simpler to drive because their control electrode is isolated from the current conducting silicon, therefore a continuous ON current is not required. Once the MOSFET transistors are turned-on, their drive current is practically zero. Also, the controlling charge and accordingly the storage time in the MOSFET transistors is greatly reduced. This basically eliminates the design trade-off between on state voltage drop, which is inversely proportional to excess control charge, and turn-off time. As a result, MOSFET technology promises to use much simpler and more efficient drive circuits with significant economic benefits compared to bipolar devices. Furthermore, it is especially important to highlight for power applications, that MOSFETs have a resistive nature. The voltage drop across the drain source terminals of a MOSFET is a linear function of the current flowing in the semiconductor. This linear relationship is characterized by the RDS(on) of the MOSFET and known as the on-resistance. On-resistance is constant for a given gate-to-source voltage and temperature of the device. As opposed to the -2.2mV/°C temperature coefficient of a p-n junction, the MOSFETs exhibit a positive temperature coefficient of approximately 0.7%/°C to 1%/°C. This positive temperature coefficient of the MOSFET makes it an ideal candidate for parallel operation in higher power applications where using a single device would not be practical or possible. Due to the positive TC of the channel resistance, parallel connected MOSFETs tend to share the current evenly among themselves. This current sharing works automatically in MOSFETs since the positive TC acts as a slow negative feedback system. The device carrying a higher current will heat up more C don'