Electronic Components Datasheet Search |
|
AN-7502 Datasheet(PDF) 1 Page - Fairchild Semiconductor |
|
AN-7502 Datasheet(HTML) 1 Page - Fairchild Semiconductor |
1 / 9 page ©2002 Fairchild Semiconductor Corporation Application Note 7502 Rev. A1 AN-7502 Power MOSFET Switching Waveforms: A New Insight 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 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 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 FIGURE 2. MOS TRANSISTOR WITH CASCODE-CONNECTED JFET AND ALL CAPACITORS 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. SOURCE METAL POLY GATE GLASS GATE OXIDE 0 10 VOLTS DEPLETION EDGE 40 VOLTS n+ DRAIN JFET n- MOS n+ SOURCE p BODY p+ GATE C1 C2 C6 C3 C5 C4 SOURCE DRAIN GATE CGS CDS SOURCE DRAIN Cx Application Note October 1999 /Title AN75 2) Sub- ect Power OS- ET witch ng ave- orms: New nsi ht) Autho () Key- ords Inter- il orpo- ation, emi- on- uctor) Cre- tor () DOCI FO df- ark Page- ode Use- ut- ines |
Similar Part No. - AN-7502 |
|
Similar Description - AN-7502 |
|
|
Link URL |
Privacy Policy |
ALLDATASHEET.NET |
Does ALLDATASHEET help your business so far? [ DONATE ] |
About Alldatasheet | Advertisement | Contact us | Privacy Policy | Link Exchange | Manufacturer List All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |