Fairchild AN-7502 manual Device Models, Gate Drive Constant Voltage or Constant Current

Power MOSFET Switching Waveforms:

A New Insight

Application Note October 1999 AN-7502

Title N75

The examination of power MOSFET voltage and current waveforms during switching transitions reveals that the device characterization now practiced by industry is inade- quate. In this Note, device waveforms are explained by con- sidering the interaction of a vertical JFET driven in cascode

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from a lateral MOSFET in combination with the interelec- trode capacitances. Particular attention is given to the drain-voltage waveform and its dual-slope nature. The three terminal capacitances now published by the industry are shown to be valid only for zero drain current. For cases where the gate drive is a voltage step generator with inter- nal fixed resistance, the drain voltage characteristics are inferred from the gate current drive behavior and compared

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to observed waveforms. The nature of the “asymmetric switching times” is explained.

A waveform family is proposed as a more descriptive and accurate method of characterization. This new format is a plot of drain voltage and gate voltage versus normalized time. A family of curves is presented for a constant load resistance with VDS varied. Gate drive during switching transitions is a constant current with voltage compliance limits of 0 and 10 volts. Time is normalized by the value of gate driving current. The normalization shows excellent agreement with data over five orders of magnitude, and is bounded on one extreme by gate propagation effects and on the other by transition time self-heating (typically tens of nanoseconds to hundreds of microseconds).

Device Models

The keystone of an understanding of power MOSFET switching performance is the realization that the active device is bimodal and must be described using a model that accounts for the dual nature. Buried in today’s power MOS- FET devices is the equivalent of a depletion layer JFET that contributes significantly to switching speed. Figure 1 is a cross-sectional view of a typical power MOSFET, with MOS- FET/JFET symbols superimposed on the structure.

Figure 2 is obtained by taking the lateral MOS and vertical JFET from this conception and adding all the possible node- to-node capacitances. Computed values of the six capaci- tances for a typical device structure suggest that device behavior may be adequately modeled using only three capacitors in the manner of Figure 3. This is the model to be employed for analysis and study.

FIGURE 1. CROSS-SECTION VIEW OF MOSFET SHOWING EQUIVALENT MOS TRANSISTOR AND JFET

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C2	C3	DRAIN
GATE
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	SOURCE

FIGURE 2. MOS TRANSISTOR WITH CASCODE-CONNECTED JFET AND ALL CAPACITORS

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	CGS

SOURCE

FIGURE 3. FIGURE 2 SIMPLIFIED

Gate Drive: Constant Voltage or Constant Current

Before moving on to the study of the equivalent circuit states of the model, a gate-drive forcing function which is easy to represent, relates to reality, and best illustrates device behavior must be chosen. The choice may be immediately narrowed to two:

(1)An instantaneous step voltage with internal resistance R, Figure 5.

(2)An instantaneous step current with infinite internal resis- tance, Figure 6.

©2002 Fairchild Semiconductor Corporation Application Note 7502 Rev. A1