Presentation on assignment on MOSFET.pptx

Assignment presentation

1. 1 Abdul Qavi Bahashwan M.Tech -EPS ME21F12F002 Advanced Power Electronics Government Engineering College Aurangabad (GECA)

2. MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor. It's a voltage controlled device with 3 terminals: G ate (electrically insulated from the semiconductor) D rain S ource 2 Power MOSFET Basics MOSFET circuit symbol

3. When a voltage applied between the  G ate and the  S ource reaches a certain threshold ( V GS( th )  or  threshold G-S voltage ), the device is able to support current conduction between the  D rain and the  S ource ( I D  or  drain current ). When a voltage applied between the  G ate and the  S ource is below V GS( th ) , the device will withstand a voltage up to  BV DSS  (or  breakdown voltage ). MOSFETs can be used as a  signal amplifier  (linear operation) or  as a switch  in power applications . 3

4. 4 Review of the basic structure of low-power MOSFETs. D S G + P - Substrate N + N + SiO 2 Ohmic c ontact Metal G S D Name Metal O xide Semiconductor S tructure N-channel enhancement MOSFET G (Gate) D (Drain) S (source) Substrate Schematic symbol G D S P-channel enhancement MOSFET Schematic symbol

5. 5 Review of the operation of low-power MOSFETs (I). ++ ++ G D S + P - Substrate N + N + - - - - G D S + P - Substrate N + N + V 1 + + + + - - - - Depletion layer ( space charge) V 2 > V 1 + + + + +++ +++ - - - - - - A thin layer containing mobile electrons (minority carriers) is induced

6. 6 Review of the operation of low-power MOSFETs (II). V 3 = V TH > V 2 G D S + P - Substrate N + N + ++++ ++++ - - - - - - - - This thin layer containing mobile electrons (minority carriers) is called inversion layer This is a depletion layer (without carriers) When the electron concentration in the new thin layer of electrons is the same as the hole concentration in the substrate, we say that the inversion process has started. The gate voltage corresponding to this situation is the threshold voltage, V TH . It should be noted that the inversion layer is like a new N-type region artificially created by the gate voltage.

7. 7 Review of the operation of low-power MOSFETs (III). V 4 > V TH G D S P P - Substrate N + N + +++++ +++++ - - - - - - - - - - Inversion layer when the voltage between gate and substrate is higher than V TH . v GS G D S P - Substrate N + N + +++++ +++++ - - - - - - - - - - v DS Now, we connect terminal source to terminal substrate . Moreover, we connect a voltage source between terminals drain and source. How is the drain current i D now? i D

8. 8 Review of the operation of low-power MOSFETs (IV). Now , t here is a N-type channel due to the inversion layer. This channel allows the current to pass from the drain terminal to the source terminal. With low values of v DS (i.e., v DS << v GS ), the channel shape is uniform . v GS G D S P - Substrate N + N + +++++ +++++ - - - - - v DS = v DS1 > 0 i D - - - - - If the value of v DS approaches v GS , then the channel shape is not uniform any more. This is the normal situation when the MOSFET is working in linear applications, which is very different from the case of switching applications.

9. 9 Review of the operation of low-power MOSFETs (V). Drain current i D is practically zero when v GS = 0, because no channel exists between drain and source. The same occurs for any v GS < V TH (i.e., i D » 0). v DS1 G D S P - Substrate N + N + i D » G D S P - Substrate N + N + i D » v DS2 > v DS1 Operation when v GS = 0.

10. 10 Review of the operation of low-power MOSFETs (VI) v DS [V] i D [mA] 4 2 8 4 12 v GS = 2.5V v GS = 3V v GS = 3.5V v GS = 4V v GS = 4.5V V GS = 0V < 2.5V < 3V < 3.5V < 4V Resistive behaviour Behaviour as current source v GS < V TH = 2V < 4.5V Open-circuit behaviour + - v DS i D + - v GS 2.5k W 10V G D S Graphical analysis. Load line

11. 11 Review of the operation of low-power MOSFETs (VII) Parasitic diode. G D S There is a parasitic diode between drain terminal and source terminal due to the internal connection between substrate and source. D S G + P - Substrate N + N +

12. 12 Internal structure of power MOSFETs (I). G D S A typical transistor is constituted of several thousand cells. As all the FET devices, MOSFET can be easily connected in parallel. They are vertical MOSFETs . Examples of cells: V- groove MOS ( VMOS ) Drain N + N - P N + Source Gate Body Source-body connection N + N - P N + N + Double diffused MOSFET ( D MOS ) Drain Source Gate Body

13. Internal structure of power MOSFETs (II). Other structures (I): MOSFET with trench gate ( U MOSFET) Drain N + N - P N + Source Body Gate MOSFET with extending trench gate ( EXT FET) Drain N + N - P N + Source Body Gate Source-body connection The breakdown voltage is limited to 25 V. 13

14. Internal structure of power MOSFETs (III). Other structures (II): MOSFET with graded doped (GD) and trench gate Drain N + N P N + Source Body Gate Source-body connection Also for low voltage (breakdown voltage is about 50 V). 14 N D-source N D-drain- N D-drain+ N A-body Doping Structure with charge coupled PN super-junction in the drift region ( CoolMOS TM ) N + N - N + N + P + P - Drain Source Gate Body Source-body connection 3 times better for 600-800 V devices.

15. 15 Internal structure of power MOSFETs (IV). N + N - P N + N + Tridimensional structure of a DMOS : Drain Drift region Body Gate Source

16. 16 Packages for power MOSFETs (I). Packages are similar to those of power diodes (except axial leaded through-hole packages). There are many different packages. Examples: 60V MOSFETs . R DS(on) = 9.4 m W , I D = 12 A R DS(on) = 12 m W , I D = 57 A R DS(on) = 9 m W , I D = 93 A R DS(on) = 5.5 m W , I D = 86 A R DS(on) = 1.5 m W , I D = 240 A

17. 17 Information given by the manufacturers. Static characteristics: - Drain-source breakdown voltage. - Maximum drain current. - Drain-source on-state resistance. Gate threshold voltage. Maximum gate to source voltage. Dynamic characteristics: - Parasitic capacitances.

18. 18 It is the drain-source breakdown voltage when the gate terminal is connected to the source terminal. It corresponds to a specific value of the drain current (for example, 0.25 mA ). Drain-source breakdown voltage, V (BR) DSS . I D = 0.25 mA V (BR) DSS G D S

19. 19 Maximum drain current. I D depends on the mounting base ( case) temperature . At 100 o C , I D = 23·0.7 = 16.1 A P ro Manufacturers vide two different values (at least) : - Maximum continuous drain current , I D . - Maximum pulse drain current , I D M .

20. 20 Drain-source on-state resistance, R DS (ON) (I). It is one of the most important characteristics in a power MOSFET. The lower, the better. For a given device, its on-resistance decreases with the gate voltage, at least until this voltage reaches a specific value. Po wer MOSFETs typically increase their on-resistance with temperature. Under some circumstances, power dissipated in this resistance causes more heating of the junction, which further increases the junction temperature, in a positive feedback loop. If a MOSFET transistor produces more heat than the heat sink can dissipate, then thermal runaway can still destroy the transistors. This problem can be alleviated by lowering the thermal resistance between the transistor die and the heat sink. Thermal runaway refers to a situation where an increase in temperature changes the conditions in a way that causes a further increase in temperature, often leading to a destructive result. It is a kind of uncontrolled positive feedback. However, the increase of on-resistance with temperature helps balance current across multiple MOSFETs (and MOSFET cells) connected in parallel, so current hogging does not occur.

21. Drain-source On Resistance, R DS(on) (Ohms) 21 Drain-source on-state resistance, R DS (ON) (II). R DS (ON) increases with temperature. R DS(ON) decreases with V GS .

22. 22 Drain-source on-state resistance, R DS (ON) (III). Comparing different devices with a given value of I D , the value of R DS (on) increases with V (BR) DSS .

23. 23 Drain-source on-state resistance, R DS (ON) (IV). The use of new internal structures (such as charge coupled PN super-junction in the drift region) has improved the value of R DS (ON) in devices in the range of 600-1000 V. Year 2000 Year 1984

24. 24 Gate threshold voltage, V GS (TO) (I). Manufacturers define V GS(TO) with the gate terminal connected to the drain terminal and at a specific value of I D (e.g., 0.25 mA or 1 mA) Standard values of V GS(TO) are in the range of 2-4 V. I D = 1 mA V GS (TO) G D S

25. 25 Gate threshold voltage, V GS (TO) (II). V GS (TO) depends on the temperature:

26. 26 Maximum gate to source voltage, V GS . Frequently, t his value is ± 20V .

27. 27 Parasitic capacitances in power MOSFETs (I). Power MOSFETs are faster than other power devices ( such as bipolar transistors, IGBTs , thyristors , etc.). This is because MOSFETs are u nipolar devices ( no minority carriers are stored at the edges of PN junctions). The switching speed is limited by parasitic capacitances. T hree parasitic capacitances should be taken into account: - C gs , which is a quite linear capacitance. - C ds , which is a non-linear capacitance. S D G C dg C gs C ds - C dg , Miller capacitance, which is also a non-linear capacitance.

28. 28 Parasitic capacitances in power MOSFETs (II). Manufacturers provide information about three capacitances, which are different from the ones mentioned in the previous slide (however, they are directly related with them) : - C iss = C gs + C gd with V ds =0 ( input capacitance ). C iss » C gs . - C rss = C dg ( Miller or feedback capacitance ). - C oss = C ds + C dg ( output capacitance ). C oss » C ds . C iss C oss S D G C dg C gs C ds S D G C dg C gs C ds

29. 29 Parasitic capacitances in power MOSFETs (III). Example of information provided by manufacturers. C iss = C gs + C gd C rss = C dg C oss = C ds + C dg

30. 30 Switching process in power MOSFETs (I). Analysis of the switching process assuming: - Inductive load (frequent situation in power electronics) . - C lamping (or free wheeling) diode. - Ideal diode. C dg C gs C ds V 1 R V 2 I L

31. 31 Switching process in power MOSFETs (II). Starting situation: - The power transistor is off and power diode is on. - Therefore: v DG = V 2 , v DS = V 2 and v GS = 0. i DT = 0 and i D = I L . + - v DS v GS + - + - v DG C dg C gs C ds V 1 R V 2 I L i DT i D B A - From this situation, the mechanical switch changes from position “B” to “A”. + - + -

32. 32 Switching process in power MOSFETs (III). i DT = 0 while v GS < V GS(TO) v DS = V 2 while i DT < I L (the diode is on). + - v DS v GS + - + - v DG C dg C gs C ds V 1 R V 2 I L i DT i D B A V GS ( TO ) v DS i DT v GS B ® A I L V 2 This slope depends on R, C gs and C dg . + - + - + -

33. 33 Switching process in power MOSFETs (IV). The current provided by V 1 through R is mainly used to discharge C dg Þ almost no current is used to charge C gs Þ v GS = constant. As a consequence, the Miller plateau appears. + - v DS v GS + - + - v DG C dg C gs C ds V 1 R V 2 I L i DT B A V GS(TO) v DS i DT v GS B ® A I L + - + - + -

34. 34 Switching process in power MOSFETs (V). C gs y C dg complete the charging process. V GS(TO) v DS i DT v GS B ® A I L + - v DS v GS + - + - v DG C dg C gs C ds V 1 R V 2 I L i DT B A + - V 1 The time constant depends on R, C gs and C dg . + -

35. 35 Switching process in power MOSFETs (VI). Computing losses between t and t 2 : - The control voltage source V 1 has to charge C gs (large) from 0 to V M and discharge C dg (small) from V 2 to V 2 -V M . - There is high voltage (V 2 ) and increasing current (from 0 to I L ) in the MOSFET at the same time (from t 1 to t 2 ). i DT + - v DS v GS + - C dg C gs C ds V 2 + - + - + - t t 1 t 2 t 3 V GS(TO) v DS i DT v GS B ® A I L V 1 V M P VI

36. 36 Switching process in power MOSFETs (VII). Computing losses between t 2 and t 3 : - The control voltage source V 1 has to discharge C dg from V 2 -V M to -V M . - There is high current (I L ) and decreasing voltage (from V 2 to 0) in the MOSFET at the same time (from t 2 to t 3 ). V 1 V M t t 1 t 2 t 3 V GS(TO) v DS i DT v GS B ® A I L P VI i DT = I L + - v DS v GS + - C dg C gs C ds + - + - + - I L

37. 37 Switching process in power MOSFETs (VIII). Computing losses after t 3 : - The control voltage source V 1 has to charge C gs from V M to V 1 and C dg from - V M to - V 1 . - There is high current (I L ), but the voltage is very low. Therefore, there are only conduction losses. i DT = I L + - v DS v GS + - C dg C gs C ds + - + - + - I L V 1 V M t t 1 t 2 t 3 V GS(TO) v DS i DT v GS B ® A I L P VI

38. 38 Switching process in power MOSFETs (IX). The switching speed strongly depends on the gate charge Q g . The gate charge is the electric charge that the driving circuitry must provide for switching the MOSFET . - The driving circuitry must provide the gate-source charge Q gs from t to t 2 . - The driving circuitry must provide the gate-drain charge Q gd from t 2 to t 3 . - The driving circuitry must provide more electric charge for the gate voltage to reach the final value V 1 . The gate charge Q g includes this charge and the addition of Q gs + Q gd . - For a given driving circuitry, the lower Q g , the faster the switching process. - Obviously, t 2 -t » Q gs R / V 1 , t 3 -t 2 » Q dg R / V 1 and P V1 = V 1 Q g f S , where f S is the switching frequency. v GS i V1 t t 2 t 3 V 1 i V1 R Q gs Q gd Q g

39. Year 2000 39 Switching process in power MOSFETs (X). IRF 540 BUZ80 Year 1984 Example of information provided by manufacturers:

40. 40 Power losses in power MOSFETs (I). Static losses: Reverse losses Þ negligible in practice due to the low value of the drain current at zero gate voltage, I DSS . Conduction losses, due to R DS (on) : P MOS_cond = R DS (ON) · I D_ RMS 2 , where I D_ RMS is the RMS value of the drain current. Switching (dynamic) losses: Turn-on losses and turn-off losses. Driving losses.

41. v DS i DT v GS P VI 41 Power losses in power MOSFETs (II). Conduction losses Switching losses P MOS_cond = R DS ( on ) · I D_RMS 2 W on W off P MOS_S = f S (w on + w off )

42. 42 Power losses in power MOSFETs (III). Driving losses. v GS i V1 t t 2 t 3 Q gs Q gd Q g P V1 = V 1 Q g f S V 1 i V1 R Equivalent circuit V 1 i V1 R B Actual circuit to have low R equivalent values It should be noted that for a given MOSFET , the switching times decrease when R decreases, thus allowing higher values of i V1 . Moreover, the lower the switching times, the lower the switching losses.

43. 43 The parasitic diode in power MOSFETs (I). It usually is a slow diode, especially in the case of high voltage MOSFETs. G D S IRF 540

44. 44 The parasitic diode in power MOSFETs (II). Case of a high voltage MOSFET structure (e.g., charge coupled PN super-junction in the drift region).