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FAN3223C Datasheet(PDF) 6 Page - Fairchild Semiconductor |
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FAN3223C Datasheet(HTML) 6 Page - Fairchild Semiconductor |
6 / 12 page AN-6069 APPLICATION NOTE © 2007 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0.3 • 1/6/10 6 (c) Figure 11. Current Flow and Diode Clamp Circuit for Transformer Driver If the transformer is designed with low leakage inductance, the propagation delays through the transformer can be less than 50ns. The GT03 series of transformers from ICE Components [4] is an example of devices with leakage inductance of a few hundred nanoHenries. This is achieved by using tightly coupled windings on a small ferrite core. In the previous transformer examples, the positive and negative peaks vary with duty cycle, while the secondary voltage VS swings around zero volts. In a pulse transformer application, the pulses might feed circuits that cannot accept the negative-going pulses. The circuit in Figure 12 incorporates a clamp circuit consisting of a second coupling capacitor CCS and a diode that restores the DC level of the secondary voltage. Figure 12. Pulse Transformer with DC Restore Circuit Series resistor RS serves to damp the initial transient at startup when CCS is initially uncharged, and is often a discrete resistor in addition to the internal driver impedance. From classical RLC circuit theory, a value of RS for critical damping is approximately: CC MAG S C L 2 R ⋅ = (15) where LMAG is the magnetizing inductance of the transformer. Figure 13 shows a gate drive application circuit that utilizes the DC restore circuit of the previous example with some additional modifications. Figure 13. Improved Gate Drive Transformer Circuit The PNP transistor added at the gate of the MOSFET is turned on when the secondary voltage goes negative to speed up the turn-off time of the MOSFET. Reference [3], “Design and Application Guide for High Speed MOSFET Gate Drive Circuits,” offers further information on transformer-coupled gate drives and should be consulted for detailed design methodology beyond the scope of the present topic. Discrete or Integrated Drivers External drivers can be designed using discrete transistors or integrated circuit solutions that come as predesigned blocks. To select a solution, designers must evaluate the competing size, features, cost, and the overall range of applications to be covered. Regardless of the driver selection, there are some common requirements. Integrated or discrete-design drivers need a local bypass capacitor to supply the high current pulses delivered during the switching intervals and might include a resistor between the driver and the PWM supply VDD. In general, drivers have the greatest impact when located close to the MOSFET gate-source connections to minimize parasitic inductance and resistance effects. Discrete solutions can be designed using bipolar transistors, as shown in Figure 14. The NPN/PNP totem pole features a non-inverting configuration driven by the PWM output. This circuit prevents shoot-through in the bipolar stage because only one of the totem pole devices can be forward biased at a time. In the bipolar common emitter configuration, the driving signal must have fast edges to provide fast switching, and it should be noted that the MOSFET gate is not ohmically connected to the rail when high or low. Figure 14. Discrete Bipolar Transistor Drive Circuit |
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