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FAN3268T Datasheet(PDF) 3 Page - Fairchild Semiconductor |
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FAN3268T Datasheet(HTML) 3 Page - Fairchild Semiconductor |
3 / 12 page AN-6069 APPLICATION NOTE © 2007 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0.3 • 1/6/10 3 For a practical example, the gate-source voltage versus total gate charge is reproduced from the Fairchild FCP20N60 power MOSFET datasheet in Figure 4. The curve was produced using a test circuit that drives the gate of the Device Under Test (DUT) with a small current source of 3mA. In this example, the gate charge needed to reach the threshold voltage of 3V is approximately 7nC. The charge required during interval t2, QGS2, is found to be 14nC – 7nC = 7nC. In interval t3, the value of QGD is found to be QGD = 46nC – 14nC = 32nC. In this typical case, the effect of QGD on the switching loss is more significant than the contribution resulting from QGS2. t4 t3 t2 t1 FCP20N60 Figure 4. VGS vs. Qg for FCP20N60 With VGS at final drive level, the value for QG,total is known. To find the average current required from the bias supply: SW G DD f Q I ⋅ = (6) where fsw is the switching frequency of the power stage. With the average current requirement known, the input power drawn from the VDD bias supply can be found as: SW G DD DD DD dr f Q V I V P ⋅ ⋅ = ⋅ = (7) The circuit waveforms and current paths during inductive load turn off are similar to those for turn on, but taken in a reverse order. For brevity, the circuit waveforms are indicated in Figure 5, but the current paths are not shown. t8 t7 t6 t5 time V DD V PL V TH I L V o -I PK -I PL V DS I DS V GS I G Figure 5. MOSFET Turn Off with Inductive Load In the t5 interval, IG rises to discharge VGS from VDD to the plateau level defined by (2). In the t6 interval, VGS remains at the plateau voltage while VDS rises to the off state voltage. The t6 interval lasts for a time approximated by: G GD rise , VDS I Q t 6 t = = (8) In interval t7, the drain current IDS falls from the value of IL to 0 while VGS falls from VPL to VTH. This time interval is given by: G 2 , GS fall , IDS I Q t 7 t = = (9) In the t8 interval, VGS is discharged from the threshold voltage to zero. An equation relating IG to the switching loss during the turn off interval is given as: () ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ + ⋅ ⋅ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ × = 7 t , G 2 GS 6 t , G GD SW LOAD IN OFF . SW I Q I Q f 2 I V P (10) |
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