In the first part of this series, we discussed how high current gate drivers can help systems achieve higher efficiencies. The same effect can be achieved with a high speed gate driver.
High-speed gate drivers can increase efficiency by reducing the body diode power dissipation of the FET. Body diodes are parasitic diodes that are inherent in most types of FETs. It is formed by a pn junction and is located between the drain and the source. Figure 1 shows the body diodes shown in the typical MOSFET circuit symbol.
Figure 1: MOSFET symbol includes an intrinsic body diode
Limiting the on-time of the body diode will in turn reduce the power consumed at both ends. This is because when the MOSFET is turned on, the voltage drop across the body diode is typically higher than the voltage across the MOSFET. For the same current level, P = I&TImes; V (where P is the power dissipation, I is the current, and V is the voltage drop), and the conduction loss through the MOSFET channel is significantly lower than the conduction loss through the body diode.
These concepts play a role in the synchronous rectification of power electronic circuits. Synchronous rectification increases the efficiency of the circuit by replacing the diode with an active control device such as a power MOSFET. Reducing body diode conduction maximizes the benefits of this technology.
Consider the case of a synchronous buck converter. When the high side FET is turned off and current is still present in the inductor, the body diode of the low side FET becomes forward biased. Short dead time is necessary to avoid straight through. After this, the low side FET turns on and begins to conduct through its channel. The same principles apply to other synchronous half-bridge configurations typically found in DC/DC power and motor drive designs.
For high speed switching, an important parameter of the gate driver is the conduction propagation delay. This is the time when the signal is applied to the input of the gate driver until the output begins to go high. This situation is shown in Figure 2. When the FET is turned back on, the body diode will turn off. Fast turn-on propagation delays turn the FETs on faster, minimizing the on-time of the body diodes and minimizing losses.
Figure 2: Time diagram, t_PDLH is the conduction propagation delay
TI's product portfolio includes gate drivers with industry-leading high-speed conduction propagation delays. See Table 1.
Table 1: High Speed ​​Drives
System efficiency is the result of a team effort. This blog series introduces high-speed and high-current gate drivers as key components. Start by designing your efficient system at www.TI.com/gatedrivers.
other information· High-speed gate drivers in high-efficiency systems are shown in the TI Designs Reference Design Library:
· "Isolated GaN Driver Reference Design."
· “1 kW three-way isolated DC/DC digital power supply for telecommunications (-8V @ .25A)â€.
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