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FAN3225T Scheda tecnica(PDF) 1 Page - Fairchild Semiconductor |
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FAN3225T Scheda tecnica(HTML) 1 Page - Fairchild Semiconductor |
1 / 12 page www.fairchildsemi.com © 2007 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0.3 • 1/6/10 AN-6069 Application Review and Comparative Evaluation of Low-Side Gate Drivers Summary Power MOSFETs require a gate drive circuit to translate the on/off signals from an analog or digital controller into the power signals necessary to control the MOSFET. This paper provides details of MOSFET switching action in applications with clamped inductive load, when used as a secondary synchronous rectifier, and driving pulse/gate drive transformers. Potential driver solutions, including discrete and integrated driver designs, are discussed. MOSFET driver datasheet current ratings are examined and circuits are presented to assist with evaluating the performance of drivers on the lab bench. Introduction In many low-to-medium power applications, a low-side (ground referenced) MOSFET is driven by the output pin of a PWM control IC to switch an inductive load. This solution is acceptable if the PWM output circuitry can drive the MOSFET with acceptable switching times without dissipating excessive power. As the system power requirements grow, the number of switches and associated drive circuitry increases. As control circuit complexity increases, it is becoming more common for IC manufacturers to omit onboard drivers because of grounding and noise problems. Synchronous rectifiers (SRs) are increasingly used to replace standard rectifiers when high efficiency and increased power density are important. It is common for isolated power stages delivering tens of amps to parallel two or more low- resistance MOSFETs in each rectifying leg, and these devices require current pulses reaching several amps to switch the devices in the sub-100ns timeframe desired. External drivers can provide these high-current pulses and a means to implement timing to eliminate shoot-through and optimize efficiency to control the SR operation. In addition, drivers can translate logic control voltages to the most effective MOSFET drive level. Low-side drivers are also used to drive transformers, which provide isolated MOSFET gate drive circuits or communication across the power supply isolation boundary. In these applications, a driver is required to handle concerns specific to transformer drive, discussed later. Low-side drivers may seem a mundane topic; several papers have been written on the subject. Though often presented as an ideal voltage source that can source or sink current determined by the circuit’s series impedance, the current available from a driver is, in fact, limited by the discrete or integrated circuit design. This note reviews the basic requirements of drivers from an application viewpoint, then investigates methods for testing and evaluating the current capability of drivers on the lab bench. Clamped Inductive Switching The simplified boost converter in Figure 1 provides the schematic for a typical power circuit with a clamped inductive load. When the MOSFET Q is turned on, the input voltage VIN is applied across inductor L and the current ramps up in a linear fashion to store energy in the inductor. When the MOSFET turns off, the inductor current flows through diode D1 and delivers energy to COUT and RLOAD at voltage VDC. The inductor is assumed large enough to maintain current constant during the switching interval. VDC L VDD VIN RG CBYP VOUT D1 Q RLOAD COUT IG Figure 1. Simplified Boost Converter |
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