Power Tip 42 (Part 1): Discrete devices - a good alternative to integrated MOSFET drivers

December 19, 2011 // By Robert Kollman
Robert Kollman, Texas Instruments looks at why discrete devices can make a good alternative to integrated MOSFET drivers.

(Editor's note : to see a linked list of all entries from #1 to the latest one, click here.)

     Many times in power-supply design, an engineer is faced with the problem of limited drive current available from his control IC, or too much power being dissipated in it due to gate-drive losses. To mitigate these issues, external drivers are often used. Semiconductor manufacturers (including TI) have ready-made MOSFET-driver solutions in the form of integrated circuits.

     However, this is not usually the most cost-effective approach. Often a few cents worth of discrete components can suffice.

    The schematic in Figure 1 shows an NPN/PNP emitter follower pair, which can be used to buffer the output of a control IC. This potentially increases the controller's drive capability and moves the drive power dissipation to the external components. Many people believe that this particular circuit will not provide sufficient drive current.

 

Figure 1: A simple buffer can drive more than 2A.

(click here to enlarge.)

      As shown in the h fe curves of Figure 2 , manufacturers do not typically provide data above 0.5A for these low-current devices. However, the circuit can actually provide substantially more than 0.5A current drive, as shown in the scope waveform in Figure 1.

     For this waveform, the buffer was driven with a 50Ω source and was loaded with a 0.01 µF capacitor connected in series with a 1Ω resistor. The trace shows the voltage across the 1Ω resistor so that the scale on the plot is 2A/division. This figure also shows that the MMBT2222A is capable of sourcing almost 3A and the MMBT3906 could sink 2A.

    In reality, the transistors would be paired with their complements (MMBT3904 for the 3906 and MMBT2907 for the 2222). These two different styles were shown for comparison purposes. Devices are also available with higher-current capability and with higher h fe's, such as

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