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FAN3121C Arkusz danych(PDF) 5 Page - Fairchild Semiconductor |
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FAN3121C Arkusz danych(HTML) 5 Page - Fairchild Semiconductor |
5 / 12 page AN-6069 APPLICATION NOTE © 2007 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0.3 • 1/6/10 5 with magnetizing inductance LMAG. In both cases, the DC blocking capacitor CC is large enough so that its voltage is approximately constant. Figure 8. Simplified Pulse Transformer Circuit In Figure 9, the circuit is modified so that the resistor is replaced by the gate-to-source terminal of a MOSFET located on the high side of a bridge circuit. CC VDD T1 NP:NS VS - + VP + - + - VOUT IG IDR IMAG IN LMAG +Bulk Figure 9. Simplified Gate Drive Transformer Circuit 0(a) shows the operational waveforms for the pulse transformer circuit, while 0(b) shows operation in a gate drive application. (a) (b) Figure 10. (a) Pulse Transformer Waveforms and (b) Gate Drive Transformer Waveforms The output of the driver swings from 0V to VDD producing a DC component equal to VDD x duty cycle. If this voltage is applied directly to the primary winding of T1, the transformer would saturate and not be able to transmit useful information. To prevent this, coupling capacitor CC is inserted in series with the primary winding to block the DC voltage while passing the AC portion of the VOUT signal. Transformers designed for pulse and gate drive applications usually specify a voltage-time product the device can withstand without saturating the transformer. In many cases, the same transformer could be used as either a pulse transformer operation or a gate drive transformer. In 0, the major difference between the two applications is in the current waveforms. With a constant drive voltage and magnetizing inductance, LMAG, the magnetizing current IMAG is the same in both circuits. In the pulse transformer waveforms shown in 0(a), the resistor current IR follows the secondary voltage VS, and the driver supplies a current that is the sum of these two components. In the MOSFET gate drive waveforms shown in 0(b), the gate current IG is positive pulses at turn on and negative pulses at turn off. As in the first example, the driver supplies a current that is the sum of these two components, but the waveform has a larger RMS value due to the high- current pulses. It is important to examine the direction of current flow between driver and transformer for the examples of 0. When VOUT swings high as shown Figure 11(a), one might expect the driver to immediately source current. However, the magnetizing current is negative and, if the load current is not larger than the magnetizing current, the driver must sink current until IDR goes positive. The opposite situation exists in Figure 11(b), when VOUT goes from high to low and the driver must source current when expected to operate as a current sink. Figure 11(c) shows additional diodes providing a current path if the driver cannot sink current when VOUT is high or source current when VOUT is low, as found in drivers with a bipolar output stage. VOUT IDR VOUT IDR Pulse Transformer Gate Transformer (a) (b) |
Podobny numer części - FAN3121C |
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Podobny opis - FAN3121C |
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