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Bridging Theory in Practice

Bridging Theory in Practice. Transferring Technical Knowledge to Practical Applications. MOSFETs, High Side Drivers, and Low Side Drivers. MOSFETs, High Side Drivers, and Low Side Drivers. Gate. Gate. Source. Source. Source. Source. n. n. n. n. p. p. p. p. n epi. n epi.

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Bridging Theory in Practice

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  1. Bridging Theory in Practice Transferring Technical Knowledge to Practical Applications

  2. MOSFETs, High Side Drivers,and Low Side Drivers

  3. MOSFETs, High Side Drivers,and Low Side Drivers Gate Gate Source Source Source Source n n n n p p p p n epi n epi n+ substrate n+ substrate n+ substrate Drain Drain Blocking State Conducting State

  4. MOSFETs, High Side Drivers,and Low Side Drivers Intended Audience: • Electrical engineers with a knowledge of simple electrical circuits • A basic understanding of thermal design is required • A simple, functional understanding of capacitive and inductive loads is assumed Topics Covered: • What is a MOSFET, a High Side Driver, and a Low Side Driver? • How do you select a MOSFET with the correct on-resistance (Rdson)? • How does capacitive load in-rush current affect designs? • What precautions need to be taken with an inductive load? Expected Time: • Approximately 90 minutes

  5. MOSFETs, High Side Drivers,and Low Side Drivers • Introduction • MOSFET Review • Low Side, High Side, and H-Bridge Drivers • PROFET Introduction • HITFET Introduction • Selecting the Correct Rdson • Static Operation • Dynamic Operation and the Impact of Switching Losses • Capacitive Load In-Rush Current • Switching Off an Inductive Load

  6. Gate Drain Gate Source Source Source Source n+ n+ n+ n+ p+ p+ p+ p+ p+ p+ n- n- n- n- n- n+ n+ p- Ground Drain Metal Oxide SemiconductorField Effect Transistor Vertical Power MOSFET Integrated Circuit Process (n-channel) Vertical Power MOSFET Transistor Process (n-channel)

  7. VGS increases MOSFETRegions of Operation • A positive (for N-Channel) or negative (for P-Channel) VGS produces a conducting channel between the Drain and Source • The MOSFET is then able to operate in two regions: • 1) Linear region: The MOSFET behaves like a resistance. • 2) Saturation region: The MOSFET behaves like a current source. VDS = VGS-VT VGS > 0V N-Channel MOSFET (NMOS) IDS VDS

  8. MOSFET Breakdown The breakdown voltage, V(BR)DSS, is the voltage at which current will begin to flow from drain-source in OFF-state due to avalanche breakdown process For Drain-Source voltages above V(BR)DSS, significant current can flow through the MOSFET, even when it is turned off V(BR)DSS ID Drain Electrical Characteristic Drain-to-Source Breakdown Voltage Symbol V(BR)DSS Condition VGS = 0V ID = 1mA Minimum 25V Gate Source

  9. Low Side Drive (LSD) Configuration ILOAD To turn on the LSD, the MOSFET gate is pulled high With the MOSFET turned on, the drain of the MOSFET is at near ground potential VD ~ 0V 14V Current flows and the load “turns-on” The switch is between the load and ground 14V Load MOSFET Switch

  10. High Side Drive (HSD) Configuration To turn on the HSD, the MOSFET gate is pulled high VS ~ 9V VGS ~ 5V The switch is between the load and supply 14V MOSFET Switch 14V The drain and gate are assumed to Be at the same potential causing VGS=VDS. The high value of VDS puts the device into the saturation Region and results in a a small ILOAD. ILOAD Load

  11. To turn on the HSD, the MOSFET gate is pulled high 28V VS ~ 14V VGS ~ 14V The source voltage can now rise to approximately Vsupply ILOAD The high value of VGS (and low VDS) translates into a large value of ILOAD (linear region) High Side Drive (HSD) Configuration The switch is on the “HIGH” side of the load 14V MOSFET Switch If the MOSFET gate is pulled to a higher voltage than supply Load

  12. Low Side Drivers vs.High Side Drivers In a Low Side Drive configuration: • More robust with simple ground • Simpler, lower price driver • 2 wires in system • Short to ground can destroy load • Possible load corrosion (connected to VSUPPLY) In a High Side Drive configuration: • 1 wire in system • Short to ground can not destroy load • Load corrosion unlikely (connected to GND) • Less robust with distributed ground • More complex, higher price driver

  13. H-Bridge Configuration 14V The load is placed in the middle of a “H” configuration 14V CW CCW Load CCW CW

  14. 14V 28V 14V H-Bridge Configuration The load is placed in the middle of a “H” configuration 14V CCW A To turn the load on in one direction, “CW” is pulled high Load CCW A

  15. 14V H-Bridge Configuration The load is placed in the middle of a “H” configuration 14V CW 28V B To turn the load on in one direction, “CW” is pulled high Load CW 14V B To turn on in the other direction, “CCW” is pulled high

  16. PROFETs = PROtected FETs Over Voltage Protection Reverse Battery Protection Integrated Charge Pump Current Limit Diagnostics Over Temperature Protection Short Circuit Protection MOSFET PROFET

  17. PROFET - Block Diagram

  18. HITFETs =HighlyIntegrated, Temperature protectedFETs Diagnostics (Requires external Components) Short Circuit Protection Current Limit Over Voltage Protection Over Temperature Protection MOSFET HITFET

  19. HITFET - Block Diagram VSUPPLY

  20. MOSFETs, High Side Drivers,and Low Side Drivers • Introduction • MOSFET Review • Low Side, High Side, and H-Bridge Drivers • PROFET Introduction • HITFET Introduction • Selecting the Correct Rdson • Static Operation • Dynamic Operation and the Impact of Switching Losses • Capacitive Load In-Rush Current • Switching Off an Inductive Load

  21. Basic Power Equations • Power Dissipation (switch applications in linear region) PD = I2Rdson • Thermal Impedance Zthja = Zthjc + Zthca • Junction Temperature Tjunction = Tambient + PDZthja • For static operation Zthja = Rthja

  22. Rdson Equations • Rearranging, the equations yield:

  23. Parameters Affecting Rdson Selection • Typically, the following parameters are set by the device: Tjunction,max - Usually 150°C Rdson - Function of the silicon die and package Zthjc - Function of the package type (and die size) • Typically, the following parameters are set by the application: Tambient - Usually 85°C, 105°C, or 125°C Iload - Function of the load resistance Zthca - Function of the external heatsink

  24. Datasheet Parameters Affecting Rdson Selection

  25. Rdson Selection Example Calculation 14V Tambient = 85°C SOT-223 Package Zthja= 82°C/W Rdson To find Iload, initially assume Rdson is 0 Iload R = 3

  26. Iload Rdson Selection Example Calculation Rdson can now be calculated for different Tjunction,max 14V R = 3

  27.  82 C/W SOT-223 Heatsink

  28. Larger Package Larger Heatsink TO-263 Heatsink

  29. 14V Iload R = 3 Rdson Selection Example Calculation Rthja = 39°C/W with 1 in2 heatsink Rdson can now be calculated for different Tjunction,max

  30. Rdson vs. Package and Heatsink Package and Heatsink SOT-223 (0.5 in2) TO-263 (1 in2) Rdson at Tjunction,max =125C 22 m 47 m Rdson at Tjunction,max =150C 36 m 76 m

  31. SOT-223 SO8 TO-252 TO-263 Rthja for Various Packages

  32. MOSFETs, High Side Drivers,and Low Side Drivers • Introduction • MOSFET Review • Low Side, High Side, and H-Bridge Drivers • PROFET Introduction • HITFET Introduction • Selecting the Correct Rdson • Static Operation • Dynamic Operation and the Impact of Switching Losses • Capacitive Load In-Rush Current • Switching Off an Inductive Load

  33. Impact of Approximate FETSwitching Loss 1.0 0.8 0.6 0.4 0.2 0.0 0.0 0.2 0.4 0.6 0.8 1.0 Time All values normalized

  34. PROFET Switching LossLamp Turn-On 6m HSDVsupply=13.5VLoad=60W bulbD=0.5, f=100Hz Current response approximately piece-wise linear time

  35. For a resistive load (with a piecewise linear current and voltage response), the approximate FET switching loss is: Approximate FETSwitching Loss Ploss~ (0.125)(VDSIDS) Eloss = (Ploss)(tswitch)

  36. PROFET Switching LossLamp Turn-On • Approximate Switching Energy Loss Vsupply = 13.5V Iload = 6.58A Ploss,approx = (0.125)(Vsupply)(Iload) = (0.125)(13.5V)(6.58A) = 11.1W tswitch = 250s - 45s = 205s Eloss,approx = (tswitch)(Ploss,approx) = (205s)(11.1W) = 2.28mJ

  37. PROFET Switching Loss:Lamp Turn-Off 6m HSDVsupply=13.5VLoad=60W bulbD=0.5, f=100Hz Current response approximately linear

  38. PROFET Switching LossLamp Turn-Off • Approximate Switching Energy Loss Vbb = 13.5V Iload = 6.58A Ploss,approx = (0.125)(Vbb)(Iload) = (0.125)(13.5V)(6.58A) = 11.1W tswitch = 205s - 190s = 15s Eloss,approx = (tswitch)(Ploss,approx) = (15s)(11.1W) = 0.17mJ

  39. PROFET Switching Loss:Lamp Turning On and Off Eloss,actual measurement = 2.29mJ (total) % Error = (2.45mJ – 2.29mJ) / 2.29mJ % Error = 7.0% • Approximate Switching Energy Loss Eloss on,approx = 2.28mJ (turn-on) Eloss off,approx = 0.17mJ (turn-off) Eloss,approx = 2.28mJ + 0.17mJ = 2.45mJ (total)

  40. Rdson Calculations for PWM Applications The power dissipated in a PWM application is given by: PD = Pswitching + Pon Pswitching = (Fswitching)(Ploss-ontturn-on + Ploss-offtturn-off) Ploss-off (0.125)(VsupplyIload) Ploss-on  (0.125)(VsupplyIload) Pon = (Iload2)(Rdson)(tpulse-on)(Fswitching) Tjunction = Tambient + PDRthja D = (tpulse-on)(Fswitching) = (tpulse-on) / (TPeriod)

  41.  Rdson (FET less expensive)  Rdson (FET more expensive) Rdson Calculations for PWM Applications •  Tjunction,max •  Tambient •  Rthja •  Iload •  D (Duty Cycle) •  Fswitching •  Vsupply •  tturn-on •  tturn-off •  Tjunction,max •  Tambient •  Rthja •  Iload •  D (Duty Cycle) •  Fswitching •  Vsupply •  tturn-on •  tturn-off Decreases switching losses Increases switching losses

  42. 13.5V 2.05 Rdson Selection Example Calculation Tambient,max Tjunction,max Iload Ploss tturn-on tturn-off Fswitching Duty Cycle Rthja Tambient,max Tjunction,max Iload Ploss tturn-on tturn-off Fswitching Duty Cycle Rthja = 85C = 150C = 6.57A =11.1W = 155s = 30s = 100Hz = 50% = 55°C/W (TO252+1in2) = 85C = 150C = 6.57A =11.1W = 155s = 30s = 100Hz = 50% = 55°C/W (TO252+1in2)

  43. MOSFETs, High Side Drivers,and Low Side Drivers • Introduction • MOSFET Review • Low Side, High Side, and H-Bridge Drivers • PROFET Introduction • HITFET Introduction • Selecting the Correct Rdson • Static Operation • Dynamic Operation and the Impact of Switching Losses • Capacitive Load In-Rush Current • Switching Off an Inductive Load

  44. Capacitive LoadIn-Rush Current • Lamps and RC networks can experience significant “in-rush” current when they are initially turned on • When a lamp initially turns on, the filament is cold, and has a relatively low resistance • As the filament warms up, the resistance increases dramatically (often by an order of magnitude) 23.2 In Out 2.80 3.6mF

  45. The in-rush current may be 10 times the static (DC) current 5.5A 600mA Capacitive LoadIn-Rush Current • Lamps and RC networks can experience significant “in-rush” current when they are initially turned on

  46. Standard Current Limiting When the load resistance is lower than expected, PROFETs/HITFETs can go into a protective current limiting mode Current limiting is considered a FAULT condition – devices are not designed for prolonged use in this mode of operation Care must be taken to keep in-rush current levels below the device’s current limit threshold

  47. Lamp In-Rush Current Example Input voltage Sense signal Drain-source voltage Estimated average power during in-rush (30W) 27W lamp in rush current Driver Pdiss=Vds*Iload

  48. Zthja Chart for LampIn-Rush Current Example Single Pulse 2.0°C/W ~3msec

  49. Approximate junction temperature increase (using Zth diagram and estimated rectangular average in-rush power) tin-rush 3msec Zthja  2.0°C/W Ploss,ave  30W (estimated from oscilloscope) Tjunction = ZthjaPloss = (2.0°C/W)(30W) = 60°C Lamp In-Rush CurrentExample Calculations

  50. MOSFETs, High Side Drivers,and Low Side Drivers • Introduction • MOSFET Review • Low Side, High Side, and H-Bridge Drivers • PROFET Introduction • HITFET Introduction • Selecting the Correct Rdson • Static Operation • Dynamic Operation and the Impact of Switching Losses • Capacitive Load In-Rush Current • Switching Off an Inductive Load

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