Power Tip 42 (Part 1): Discrete devices—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 the FMMT618/718 pair which has h_fe’s near 100 at 6A (see Figure 2). While not being as elegant as an integrated driver, discretes can deliver a lower-cost solution with improved thermal and current capabilities.

Figure 2: Higher-current drivers, like the FMMT618,

can beef up the drive (Top: MMBT3904 / Bottom: FMMT618).

(Click here to enlarge.)

     Figure 3 shows a variation to the simple buffer that allows you to cross an isolation boundary. A signal-level transformer is driven with a symmetrically bipolar-drive signal. The secondary of the transformer is used to generate power for the buffer as well as provide the input signal to the buffer. Diodes D1 and D2 rectify the voltage from the transformer, while transistors Q1 and Q2 buffer the transformer output impedance to provide large current pulses to charge and discharge the gate of a FET connected across the output.

Figure 3: With a few more parts, you can build an isolated driver.

(click here to enlarge.)

     This circuit is extremely effective with a 50% duty cycle input (see lower drive signal in Figure 3) as it will drive the gate of the FET negative and provide rapid turn-off, minimizing switching losses. This makes it ideal for the phase-shifted full-bridge converter.

     If you are using an upper-drive waveform of less than 50% (Figure 3), consider snubbing the transformer. This helps to avoid inadvertently turning the FET on due to ringing after the transitions. A low-to-zero transition may cause the leakage inductance and secondary capacitance to ring and produce a positive voltage out of the transformer.

     To summarize, discrete drivers can save you money. About $0.04 worth of discretes can replace over ten times that much cost in driver ICs. The discrete drivers are capable of providing drive currents in excess of 2A and allow you to move power out of the control IC. Also, they remove the high switching currents from the control ICs, which can improve regulation and noise performance.

     Please join us next month when we will continue our discussion of simple FET gate drive circuits and look at synchronous rectifier drives.

Robert Kollman is a Senior Applications Manager and Distinguished Member of Technical Staff at Texas Instruments. He has more than 30 years of experience in the power electronics business and has designed magnetics for power electronics ranging from sub-watt to sub-megawatt with operating frequencies into the megahertz range. Robert earned a BSEE from Texas A&M University, and a MSEE from Southern Methodist University.

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