Power Design Tip 42: Discrete Alternatives to Integrated MOSFETs

In power supply design, engineers often face the problem of insufficient drive current of the control IC, or the problem of excessive power consumption of the control IC due to gate drive losses. To alleviate this problem, engineers often resort to external drives. semiconductor manufacturers (including TI) have off-the-shelf MOSFET IC driver solutions, but this is usually not the lowest cost solution. Usually a few cents worth of discrete components are chosen.

picture1 Simple buffers can drive2 Amps above current.

picture1 The schematic in , shows an NPN/PNP transmit follower pair that can be used to buffer the output of the control IC. This may increase the drive capability of the controller and transfer drive losses to external components. Many people thought that this particular circuit could not provide enough drive current.

likepicture2 h_fe As the curve shows, manufacturers typically do not supply currents higher than 0.5A for these low-current devices.However, this circuit can provide much higher current drive than 0.5A, such aspicture1shown in the waveform. For this waveform, the buffer is driven by a 50Ω source and loaded with a 0.01 uF capacitor in series with a 1Ω resistor. The trace shows the voltage across the 1Ω resistor, so the current on each piece of binding post is 2A. The numbers also show that the MMBT2222A can source about 3A and the MMBT3906 sinks 2A.

In fact, the transistors will be paired with their components (MMBT3904 for 3906, MMBT2907 for 2222). These two different pairings are for comparison only. These devices also have higher current and higher h_fesuch as the FMMT618/718 pair, which has an h of 100 at 6 A_fe(Seepicture2). Unlike integrated drivers, discrete devices are lower cost solutions with higher thermal and current performance.

picture2 such asFMMT618Higher current drivers for enhanced drive capability (up to:MMBT3904 / lowest:FMMT618).

picture3Shows a simple buffer variable case that lets you cross isolation boundaries. A signal level transformer is driven by a symmetrical bipolar drive signal. The transformer secondary winding is used to generate the buffer power and provide the input signal to the buffer. Diodes D1 and D2 regulate the voltage from the transformer, while transistors Q1 and Q2 buffer the transformer output impedance to provide high current pulses to charge and discharge the FET connected to the output.This circuit is very efficient and has a 50% duty cycle input (seepicture3lower drive signal) because it drives the FET gate negative and provides fast switching, minimizing switching losses. This is ideal for phase-shifted full-bridge converters.

If you plan to use an upper drive waveform less than 50% (seepicture3), then a snubber transformer should be used. Doing this helps to avoid arbitrarily turning on the EFT due to transition ringing. A low-to-zero transition can cause leakage inductance and secondary capacitance, causing ringing and a positive voltage outside the transformer.

picture3 With just a few parts you can build a self-contained drive

In short, discrete devices can help you save money. Discretes costing about $0.04 can reduce driver IC costs by a factor of 10. Discrete drivers can supply more than 2A and allow you to draw power from the control IC. Additionally, the device removes high switching currents in the control IC, improving regulation and noise performance.

Stay tuned for our next discussion of simple FET gate drive circuits and synchronous rectifier circuits.

For more details on this and other power solutions, please visit: www.ti.com.cn/power.