Power Design Tip 42: A discrete device that can replace an integrated MOSFET (PDF with circuit diagram!)

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In power supply design, engineers often face problems with controlling IC drive current shortages or face excessive power consumption due to gate drive losses. To alleviate this problem, engineers often use external drives. Semiconductor manufacturers (including TI) have off-the-shelf MOSFET integrated circuit driver solutions, but this is usually not the lowest cost solution. Discrete devices worth a few cents are usually chosen.

Figure 3 in this article shows a simple buffer variable case that allows you to cross the isolation boundary. A signal level transformer is driven by a symmetrical bipolar drive signal. The transformer secondary winding is used to generate buffer power and provide an input signal to the buffer. Diodes D1 and D2 adjust the voltage from the transformer, while transistors Q1 and Q2 are used to buffer the transformer output impedance to provide large current pulses to charge and discharge the FETs connected to the output. The circuit is extremely efficient and has a 50% duty cycle input (see the lower drive signal in Figure 3) because it will drive the FET gate negative and provide fast switching to minimize switching losses. This is ideal for phase-shifted full bridge converters.

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