Ch6 Mosfets Hsd_lsd Part 1

Bridging Theory in PracticeTransferring Technical Knowledgeto Practical Applications

MOSFETs, High Side Drivers,and Low Side Drivers


n n

GateSourceSource

n+ substrate

p p

n+ substrate

n

GateSourceSource

n+ substrate

p p

Drain DrainBlocking State Conducting State

n epi n epi

n


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

• 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


Metal Oxide SemiconductorField Effect Transistor

Gate

n+

p+

n+

p+

n+

n-

Source Source

Drain

Vertical Power MOSFETTransistor Process

(n-channel)

Vertical Power MOSFETIntegrated Circuit Process

(n-channel)

p-

n-

Drain

Ground

n+

Gate

p+

n- n-

p+

n-

p+ p+

Source Source

n+ n+

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.

VGS > 0V

N-ChannelMOSFET(NMOS)

IDS

VDS

VG

S i

ncr

ease

s

VDS = VGS-VT

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 ElectricalCharacteristic

Drain-to-Source Breakdown Voltage

Symbol

V(BR)DSS

Condition

VGS = 0V

ID = 1mA

Minimum

25V

Drain

Source

Gate

Low Side Drive (LSD) Configuration

The switch is between the

load and ground

14V

Load

MOSFETSwitch

To turn on the LSD, the MOSFETgate is pulled high

14V

With the MOSFET turned on, thedrain of the MOSFET is at nearground potential

VD ~ 0V

Current flows and the load “turns-on”

ILOAD

High Side Drive (HSD) Configuration

14V

Load

MOSFETSwitch

The switch isbetween theload and supply

To turn on the HSD, the MOSFETgate is pulled high

VS ~ 9V

The drain and gate are assumed toBe at the same potential causing VGS=VDS. The high value of VDS puts the device into the saturationRegion and results in a a small ILOAD.

ILOAD

VGS ~ 5V

14V

High Side Drive (HSD) Configuration

14V

Load

MOSFETSwitch

The switch is on the “HIGH”side of the load

To turn on the HSD, the MOSFETgate is pulled high

28V

The source voltage can now rise to approximately Vsupply

VS ~ 14V

If the MOSFET gate is pulled toa higher voltage than supply

The high value of VGS (and lowVDS) translates into a large valueof ILOAD (linear region)

ILOAD

VGS ~ 14V

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

Low Side Drivers vs.High Side Drivers

H-Bridge Configuration

14V 14V

Load

CW CCW

CCW CW

The load is placedin the middle of a“H” configuration


14V 14V

Load

A CCW

CCW A


To turn the load onin one direction, “CW” is pulled high

28V

14V


14V 14V

Load

CW B

B CW


To turn the load onin one direction, “CW” is pulled high

To turn on in the other direction, “CCW” is pulled high

14V

28V

PROFETs = PROtected FETs

PROFET

MOSFETDiagnostics

Short CircuitProtection

IntegratedCharge Pump

OverVoltage

Protection

Current Limit

OverTemperature

Protection

ReverseBattery

Protection

PROFET - Block Diagram

HITFETs =Highly Integrated, Temperature protected FETs

HITFET

MOSFET

Diagnostics(Requires external

Components)

OverVoltage

Protection

Current LimitShort CircuitProtection

OverTemperature

Protection

HITFET - Block DiagramVSUPPLY

• Introduction

– MOSFET Review










• Power Dissipation (switch applications in linear region)

PD = I2Rdson

• Thermal Impedance

Zthja = Zthjc + Zthca

• Junction Temperature

Tjunction = Tambient + PDZthja

• For static operation Zthja = Rthja

Basic Power Equations

Rdson Equations

• Rearranging, the equations yield:

dsonthja

ambientjunctionLoad RZ

TTI

thjaload

ambientjunctiondson

ZI

TTR 2

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

Datasheet Parameters Affecting Rdson Selection

Rdson Selection Example Calculation

Tambient = 85°C

SOT-223 PackageZthja= 82°C/W

14V

R = 3Iload

To find Iload, initially assumeRdson is 0

Rdson

AV

R

VI

load

battload 67.4

3

14


14V

R = 3

junction,max ambientdson 2

load thja

T - TR

I Z

Rdson can now be calculated fordifferent Tjunction,max

( ) ( )m22=

W/C82A67.4

C85C125≤R 2dson

( ) ( )m36=

W/C82A67.4

C85C150≤R 2dson

Iload

82 C/W

SOT-223 Heatsink

TO-263 Heatsink

Larger PackageLarger Heatsink


Rdson can now be calculated fordifferent Tjunction,max

m76

W/C39A67.4

C85C150R

2dson

m47

W/C39A67.4

C85C125R

2dson

Rthja = 39°C/W with 1 in2 heatsink14V

R = 3Iload

Rdson vs. Package and Heatsink

Package andHeatsink

SOT-223 (0.5 in2)

TO-263 (1 in2)

Rdson atTjunction,max =125C

22 m

47 m

Rdson atTjunction,max =150C

36 m

76 m

Rthja for Various Packages

SO8 SOT-223

TO-263TO-252

• Introduction

– MOSFET Review










Impact of Approximate FETSwitching Loss

0.0 0.2 0.4 0.6 0.8 1.0

1.0

0.8

0.6

0.4

0.2

0.0

Time All values normalized

PROFET Switching LossLamp Turn-On

6m HSDVsupply=13.5VLoad=60W bulbD=0.5, f=100HzCurrent response

approximatelypiece-wise linear

time

• 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)

• 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 = 250s - 45s = 205s

Eloss,approx = (tswitch)(Ploss,approx)

= (205s)(11.1W) = 2.28mJ

PROFET Switching LossLamp Turn-On

PROFET Switching Loss:Lamp Turn-Off

6m HSDVsupply=13.5VLoad=60W bulbD=0.5, f=100HzCurrent response

approximatelylinear


Vbb = 13.5V

Iload = 6.58A

Ploss,approx = (0.125)(Vbb)(Iload)

= (0.125)(13.5V)(6.58A) = 11.1W

tswitch = 205s - 190s = 15s

Eloss,approx = (tswitch)(Ploss,approx)

= (15s)(11.1W) = 0.17mJ

PROFET Switching LossLamp Turn-Off

Eloss,actual measurement = 2.29mJ (total)

% Error = (2.45mJ – 2.29mJ) / 2.29mJ% Error = 7.0%

PROFET Switching Loss:Lamp Turning On and Off


Eloss on,approx = 2.28mJ (turn-on)

Eloss off,approx = 0.17mJ (turn-off)

Eloss,approx = 2.28mJ + 0.17mJ = 2.45mJ (total)

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)

Tjunction,max

Tambient

Rthja

Iload

D (Duty Cycle)

Fswitching

Vsupply

tturn-on

tturn-off

Rdson (FET less expensive) Rdson (FET more expensive)

Decreasesswitching

losses

Tjunction,max

Tambient

Rthja

Iload

D (Duty Cycle)

Fswitching

Vsupply

tturn-on

tturn-off

Increasesswitching

losses

Rdson Calculations for PWM Applications

junction ambient switching supply loadturn-on turn-off

thja

dson,max 2load

T - T F V I- t + t

R 8R

I D

junction ambient switching supply loadturn-on turn-off

thja

dson 2load

T - T F V I- t + t

R 8R

I D

dson 2

100Hz 13.5V 6.57A150C-85C- 155μs+30μs

55K/W 8R

6.57A 0.5


13.5V

2.05

Tambient,max

Tjunction,max

Iload

Ploss

tturn-on

tturn-off

Fswitching

Duty CycleRthja

= 85C= 150C= 6.57A=11.1W= 155s= 30s= 100Hz= 50%= 55°C/W (TO252+1in2)

m45Rdson

Tambient,max

Tjunction,max

Iload

Ploss

tturn-on

tturn-off

Fswitching

Duty CycleRthja

= 85C= 150C= 6.57A=11.1W= 155s= 30s= 100Hz= 50%= 55°C/W (TO252+1in2)

• Introduction

– MOSFET Review










• 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)

Capacitive LoadIn-Rush Current

In Out

23.2

2.803.6mF

• Lamps and RC networks can experience significant “in-rush” current when they are initially turned on

Capacitive LoadIn-Rush Current

600mA

5.5A

• The in-rush current may be 10 times the static (DC) current

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

Lamp In-Rush Current Example

Input voltage

Sense signal

Drain-source voltage

27W lamp in rush current

Driver Pdiss=Vds*Iload

Estimated average power during in-rush (30W)

0.01

0.1

1

10

100

1E-6 1E-5 1E-4 1E-3 1E-2 1E-1 1E0 1E1 1E2 1E3 1E4

tp [s]

Z th

JA [

K/W

]

00,01

0,020,050,10,20,5D=

Zthja Chart for LampIn-Rush Current Example

Single Pulse

2.0°C/W

~3msec

• 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

• Introduction

– MOSFET Review










Switching OFF anInductive Load

OFF

• With inductive loads (for example coils and valves), additional switching losses can occur during turn off

• According to Lenz’s Law:The electromotive force (voltage) and the induced current in an inductor are in a direction as to tend to oppose the change that produced them

• Therefore at turn off, the voltage at the output of the high side driver becomes negative to oppose the decreasing inductor current.

Switching OFF an Inductive Load

VOUT

Vbb

VINVON

• Prior to the PROFET being turned on....

IL

VIN

VOUT

IL


VOUT

Vbb

VINVON

IL

• Initially, the FET is turned on, and IL begins to increase

IL

VIN

VOUT

IL

VOUT

Vbb

VINVON

HIGH

IL

• Eventually, IL reaches it’s DC value


IL

VIN

VOUT

IL

VOUT

Vbb

VINVON

IL

• At some point, the FET is turned off


IL

VIN

VOUT

IL

VOUT

Vbb

VINVON

VON(CL)

• VOUT goes below GND. The zener eventually conducts and supplies gate charge to turn on the FET, clamping VOUT at a "safe" voltage.


IL

VIN

VOUT

IL

IL

• When IL 0A, VOUT will return to GND potential


VOUT

Vbb

VINVON

0V

IL

VIN

VOUT

IL

Safely Clamping VOUT for Inductive Loads

• If VOUT was not clamped, it’s magnitude would increase to the point of the MOSFET avalanche breakdown voltage

Vbreakdown

• Clamping VOUT to a safe value (below avalanche) increases the maximum energy which can be dissipated in the driver during turn off

Silicon Area (mm2)

Abs

orba

ble

Indu

ctor

Ene

rgy

(mJ)

Clamping VOUT Increases the Maximum Inductor Energy

• The maximum safe inductive energy which can be dissipated in the FET is found in the maximum ratings section:

• The clamping voltage is in the electrical characteristics:

Maximum Safe Inductor Energy

Energy Absorbed When Turning Off an Inductive Load

Tj = 150CVsupply = 12VRLOAD = 0

Maximum loadinductance for asingle switch off

• The energy absorbed by the high side driver when an inductive load is turned off (Eloss) is equal to:

Eloss = ESUPPLY + EL - ER

Where:

ESUPPLY is the energy delivered to the MOSFET from

the batteryEL is the energy delivered to the MOSFET from the

inductance (EL ~ LIL2/2)

ER is the energy dissipated by the inductor due to internal self-heating


• This becomes a differential equation:Eloss = ESUPPLY + EINDUCTANCE - EESR = ∫VON(CL)*iL(t) dt

• The solution to this equation can be approximated for RL > 0


L L Lloss SUPPLY OUT(CL)

L OUT(CL)

LI I RE = V + V ln 1+

2R V

What Can Go Wrong? • Protected FET die after the maximum dissipated energy is

exceeded due to switching off an inductive load

Driving a FET with a PWM Input14V

Load

n+

p+

n+

p+

n+

n-

GateSource Source

Drain

Note: Generally, inductive loads are not PWM drivendue to the repetitiveclamping energy / power.

Turning Off an Inductive LoadVINMOSFET is turned off

VOUT

Vsupply = 12V

VINVON

L=630HIL=9.5A

Vsupply = 12V

VAZ = 42V

VINVOUT = Vsupply - VAZ

VOUT = 12V – 42V = -30VVAZ ~ VDS = VON

Turning Off an Inductive Load

VOUT

Vsupply = 12V

VIN

L=630HIL=9.5A

VAZ

VON

VON

IL

di/dt = VOUT / L

toff = L * IL / VOUT

VIN

toff = (630H)(9.5A) / 30Vtoff = 200s

VOUT

Vsupply = 12V

VINVON

L=630HIL=9.5A

VON Vsupply = 12V

VAZ = 42V


VOUT clamped to -30V

Ploss

Eloss

IL di/dt = VOUT / L

toff = 200s

VINArea under the Ploss curve is the dissipated energy

VOUT

Vsupply = 12V

VINVON

L=630HIL=9.5A

VON Vsupply = 12V

VAZ = 42V

Ploss,avg VAZ * IL,max / 2 = (42V)(9.5A) / 2Ploss,avg 200W Eloss (Ploss)(toff) = 40mJ


Low Side Drivers andInductive Loads

• MOSFETs and HITFETs can also be used to drive inductive loads in a low side configuration.

• The low side configuration, however, results in a positive voltage spike at the output

VOUT

VSUPPLY

VIN

VOUT

VSUPPLY

VIN

NegativeVoltageSpike

PositiveVoltageSpike

Switching an Inductive Load

• Initially, the MOSFET is turned on and IL reaches it’s DC value

VOUT

VSUPPLY

VIN

IL

VIN

IL

VOUT

IL


• At some point, the FET is turned off

VOUT

VSUPPLY

VIN

IL

VIN

IL

VOUT

IL

Switching an Inductive Load• VOUT goes above VSUPPLY as the inductor current goes to 0A.

VOUT

VSUPPLY

VIN

IL

VIN

IL

VOUT

IL


• When IL = 0A, VOUT returns back to VSUPPLY

VOUT

VSUPPLY

VIN

IL

VIN

IL

VOUT

IL

• Introduction

– MOSFET Review











n n

GateSourceSource

n+ substrate

p p

n+ substrate

n

GateSourceSource

n+ substrate

p p

Drain DrainBlocking State Conducting State

n epi n epi

n

Thank You!www.btipnow.com

Ch6 Mosfets Hsd_lsd Part 1

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Transcript of Ch6 Mosfets Hsd_lsd Part 1