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 1. A simple buffer can drive currents above 2Amps.
The schematic in Figure 1 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 think that this special circuit cannot provide enough drive current.
As shown in the hfe curve in Figure 2, manufacturers generally do not provide more than 0.5A for these low current devices. However, the circuit can provide current drive much higher than 0.5A, as shown by the waveform in Figure 1. For this waveform, the buffer is driven by a 50Ω source with a 0.01 uF capacitor in series with a 1Ω resistor. This trace shows the voltage across the 1Ω resistor, so the current on each post is 2A. The figure also shows that the MMBT2222A can provide approximately 3A of current and the MMBT3906 absorbs 2A of current.
In fact, the transistor will be paired with its components (MMBT3904 for 3906 and MMBT2907 for 2222). These two different pairs are for comparison only. These devices also have higher current and higher hfe, such as the FMMT618/718 pair, which has an hfe of 100 at 6A current (see Figure 2). Unlike integrated drivers, discrete devices are a lower cost solution with higher heat and current performance.
Figure 2. Higher current drivers such as the FMMT618 enhance drive capability (up to: MMBT3904/minimum: FMMT618).
Figure 3 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.
If you plan to use an upper drive waveform of less than 50% (see Figure 3), then use a buffer transformer. This helps to avoid any open EFT due to switching ringing. A low-to-zero transition can cause leakage inductance and secondary capacitance, causing ringing and producing a positive voltage outside the transformer.
Figure 3 You can build a standalone drive with several components
In short, discrete devices can help you save money. Discrete devices worth about $0.04 can reduce drive IC costs by a factor of 10. Discrete drives provide more than 2A of current and allow you to get power from the control IC. In addition, the device removes high switching currents in the control IC for improved regulation and noise performance.
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