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Bridging Theory in Practice. Transferring Technical Knowledge to Practical Applications. Protected Low Side Drivers. Protected Low Side Drivers. Protected Low Side Drivers. Intended Audience: Electrical engineers with a knowledge of simple electrical circuits

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Bridging theory in practice

Bridging Theory in Practice

Transferring Technical Knowledge

to Practical Applications




Protected low side drivers2
Protected Low Side Drivers

Intended Audience:

Electrical engineers with a knowledge of simple electrical circuits

An understanding of MOSFETs and low side drivers is assumed

Topics Covered:

What is a Protected Low Side Driver?

What type of protection does a HITFT have?

What type of diagnostics does a HITFET have?

How does a HITFET impact system EMI?

How is a HITFET circuit implemented?

HITFET Selection Questions

Expected Time:

Approximately 90 Minutes


Protected Low Side Drivers

  • Introduction to Protected Low Side Drivers

  • HITFET Protection Features

  • HITFET Diagnostic Features

  • EMI/EMC Considerations

  • System Implementation

  • Frequently Asked Questions


Protected Low Side Drivers

  • Introduction to Protected Low Side Drivers

  • HITFET Protection Features

  • HITFET Diagnostic Features

  • EMI/EMC Considerations

  • System Implementation

  • Frequently Asked Questions


Gate

Source

Source

n+

n+

p+

p+

n-

n+

Drain

MOSFET Review

MOSFET MetalOxideSemiconductorFieldEffectTransistor

D

G

VGS

S

VGS

S

S

G

G

P-Channel

MOSFET

(Enhancement)

(

Enhancement)

MOSFET

D

D

N-Channel


MOSFET Regions of Operation

VGS increases

  • 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


To turn on, the MOSFET

gate is pulled high

Low Side Drive (LSD)Configuration

The switch is

on the “low”

side of the load

14V

Load

MOSFET

Switch

Drain

Vgate @ 5V to 10V

Vgs = Vg - Vs

Vgs = 5V to 10V

Drain voltage is small – ex. 0.1volt

Gate

Source Vs = 0V


Hitfet h igh i ntegration t emperature protected fet
HITFET = High Integration Temperature protected FET

Diagnostics Requires external components

Current Limit

Over Voltage Protection

MOSFET

Over

Temperature

Protection

Short Circuit

Protection

HITFET


H igh i ntegration t emperature protected fet
High Integration Temperature protected FET


Protected Low Side Drivers

  • Introduction to Protected Low Side Drivers

  • HITFET Protection Features

  • HITFET Diagnostic Features

  • EMI/EMC Considerations

  • System Implementation

  • Frequently Asked Questions


Rugged vs protected
Rugged vs. Protected

Rugged

MOSFETs

Achieved through process & manufacturing technology

Protection Not Built in

Protected

  • HITFETs

  • Achieved through design and utilization of more advanced integrated circuit technologies

  • Available CMOS, DMOS and Bipolar devices allow for the integration of ESD protection, active clamping, current limit, temperature sensing, etc.

  • Protection Built in


Hitfet protection features

Electrostatic Discharge (ESD) Protection

Load Dump Tolerant

Inductive and Over voltage Output Clamp Protection

Current Limit Protection

Thermal Shutdown Protection

HITFET Protection Features


Block diagram including protection features
Block Diagram Including Protection Features


Esd protection
ESD Protection

Maximum Ratings at Tj = 25ºC, Unless Otherwise Specified


Load Dump Protection

* Vld = Voltage load dump


Inductive and over voltage clamp
Inductive And Over Voltage Clamp

Inductive and Over

Voltage Output Clamp

Over voltage condition

usually occurs in the

presence of an

inductive switching

action




Thermal shutdown latch behavior
Thermal Shutdown Latch Behavior


Thermal shutdown latch behavior1
Thermal Shutdown Latch Behavior

Input Vin is driven high

Input current reflects latch

current consumption

Current is switched off when latch

engages

Silicon temperature drops when

current is switched off


Protected Low Side Drivers

  • Introduction to Protected Low Side Drivers

  • HITFET Protection Features

  • HITFET Diagnostic Features

  • EMI/EMC Considerations

  • System Implementation

  • Frequently Asked Questions


HITFET Diagnostic Feedback

  • HITFET does not contain internal structures that Are specifically intended for diagnostic feedback

  • Diagnostic feedback can be obtained by using external Components in conjunction with:

    • Drain – high current output pin – true conductive state of device can be estimated by monitoring the drain voltage

    • Iin – input drive bias can be detected and evaluated against over current or thermal shutdown bias current– this method requires that input current be evaluated.

  • NOTE: Care must be taken so as not to significantly reduce

    the available Vin voltage

    • Drain – high current output pin – true conductive state of device can be

      estimated by monitoring the drain voltage

    • Vin – input drive bias can be detected and evaluated against over current ,or thermal shutdown bias current– this method requires that input current be evaluated.

    • NOTE: Care must be taken so as not to significantly reduce

      the available Vin voltage

      Diagnostic feedback can be obtained by using external Components in

      conjunction with:

    • Drain – high current output pin – true conductive state of device can be

      estimated by monitoring the drain voltage

    • Vin – input drive bias can be detected and evaluated against over current ,or thermal shutdown bias current– this method requires that input current be evaluated.

    • NOTE: Care must be taken so as not to significantly reduce

      the available Vin voltage


Hitfet diagnostic feedback drain sense
HITFET Diagnostic Feedback: Drain Sense

  • Diagnostic feedback is sensed at the drain and applied to a micro processor Analog to Digital Input

  • Advantage

    • Low cost diagnostic and fault detection

  • Disadvantage

    • Slower than integrated solution

    • Uses one A/D channel per sensed

    • drain


Hitfet diagnostic feedback iin current monitor
HITFET Diagnostic Feedback Iin current monitor

Vcc

Vs

Id_out

R1

Id_in

D

DO

Iin/Diag

AI

S

+

Micro processor

HITFET

Vd1

-

Gnd


Protected Low Side Drivers

  • Introduction to Protected Low Side Drivers

  • HITFET Protection Features

  • HITFET Diagnostic Features

  • EMI/EMC Considerations

  • System Implementation

  • Frequently Asked Questions


Pwm definitions
PWM Definitions

Frequency – (frequency domain) What is the rate of repetition of a wave form?

Duty cycle – (Time domain) What amount time is spent on with respect to what amount of time is spent off?



Hitfet turn on turn off slew rate controlled
HITFET Turn-On/Turn-Off Slew Rate Controlled

Turn on / Turn off rate control.

Slew rate is controlled




Protected Low Side Drivers

  • Introduction to Protected Low Side Drivers

  • HITFET Protection Features

  • HITFET Diagnostic Features

  • EMI/EMC Considerations

  • System Implementation

  • Frequently Asked Questions


Reverse battery tolerance
Reverse Battery Tolerance

  • Reverse load current through the intrinsic drain diode in series with the

  • load.

  • Power dissipation is Higher compared to normal operating

  • conditions due to the voltage drop across the drain to source diode

  • Source diode current is limited by the load


Reverse battery tolerance normal operation
Reverse Battery Tolerance:Normal Operation

Recall:

TJ = TAmbient + PD* RTHJA V = I*R

PD = ILOAD2RDS(on)

Therefore:

ILOAD = ((TJ –TAmbient) / (RDS(on)* RTHJA )1/2

Given:

TJMAX = 150 C °

TAmbient = 95 C °

RDS(on) = 0.068 Ω

Rthja = 55 C/W

VBattery = 14 V

Results:

ILOAD_MAX = 3.8A and RLOAD_MIN = 3.6 Ω


Reverse battery tolerance body diode
Reverse Battery Tolerance:Body Diode

Recall:

PD = VDiode* ILOAD

Therefore:

ILOAD = ((TJ –TAmbient) / (RTHJA * VDiode)

Given:

TJMAX = 150 C °

TAmbient = 95 C °

VDiode = 0.7 V

Rthja = 55 C/W

VBattery = 14 V

Results:

ILOAD_MAX = 1.4 A and RLOAD_MIN = 9.5 Ω!!

Compared to 3.8A and 3.6 Ω when used in normal operation!!


To turn on the HSD, the MOSFET

gate is pulled high

28V

VS ~ 14V

VGS ~ 14V

The source voltage is now

approximately Vsupply

ILOAD

The high value of VGS 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…

Load


Source follower
Source Follower

  • Advantage:

  • Custom edge control (EMC)

  • Disadvantage

    -- Complexity

    -- Profet may be better


Hitfet edge shaping
HITFET Edge Shaping

Edge rise and fall time can only be increased by the addition of external components

Slew rate can not be made faster by the addition of external components

Potentially can modify EMC characteristics (Electro Magnetic emissions)

Allows for symmetrical or asymmetrical adjustment to rise and fall times as well as slew rate modification

Additional power is consumed by changing transition times (operation in linear region)


Hitfet edge shaping simple low pass filter
HITFET Edge Shaping:Simple low pass filter

Advantages

– Simple in terms of calculating RC values

– Is effective at controlling rise and fall time of the device

Disadvantage:

– Adding a low pass filter to the input

– Will insert a turn on delay and a turn off delay (dead time) which may modify the intent of PWM applications


Hitfet edge shaping simple rc
HITFET Edge Shaping: Simple RC

External components can be added to a HITFET to modify rise and fall time and slew rate.

HITFET

Simple low pass filter


Hitfet edge shaping simple low pass filter1
HITFET Edge Shaping: Simple Low Pass Filter

Input Voltage

Drain Voltage


Hitfet edge shaping miller capacitor
HITFET Edge Shaping: Miller Capacitor

Advantages

Is effective at controlling rise and fall time of the device

Does not insert a significant turn on or turn off delay

Disadvantages

Calculation or the RC components is more complicated

Must now consider the resistance of the load


Hitfet edge shaping miller capacitor1
HITFET Edge Shaping: Miller Capacitor

HITFET

Drain feedback (Miller) capacitor method


Hitfet edge shaping miller capacitor2
HITFET Edge Shaping: Miller Capacitor

Input Voltage

Drain Voltage


Hitfet edge shaping1
HITFET Edge Shaping

Edge shaping can only be used to increase the rise and fall time of the respective edge

Slowing the edge rates will result in additional heat being dissipated in the part

In both simple and miller edge shaping approaches, the series limiting resistor must be sized to allow proper bias of the over current protection functions


HITFET Input protection

  • Input protection usually not needed if driven directly from a micro processor

  • Input protection is needed in cases were drive is sourced from a non regulated or out boarded signal source


Hitfet input protection
HITFET Input protection

The HITFET input drive circuitry must provide adequate voltage to the gate (4.5V or more) and must not exceed the maximum allowable input voltage (typically 10V).

The maximum specified current allowed to sink or source from the HITFET in pin is 2.0 mA. Current up to 2mA may be required to operate internal HITFET input protection circuitry.


Hitfet 5 volt versus 10 volt operation
HITFET 5 volt versus 10 volt operation

HITFETs comply to a specification which uses a nominal 5 Vin drive voltage as a specified operating point.

Further device enhancement – lower Rdson and higher output current may be achieved by operating the device at a higher Vin voltage (10V).



Protected Low Side Drivers

  • Introduction to Protected Low Side Drivers

  • HITFET Protection Features

  • HITFET Diagnostic Features

  • EMI/EMC Considerations

  • System Implementation

  • Frequently Asked Questions


Frequently asked questions
Frequently Asked Questions

What is the load current?

Is the load capacitive and what is the inrush current?

Is the load inductive and the inductance and/or energy during turn-off?

Will load be on/off or PWM? What is PWM frequency (load states) ?

What is ambient temperature?

Can a HITFET be operated as a high side switch?


Frequently asked questions1
Frequently Asked Questions

What happens if ground (drain leg) opens?

What type of package - surface mount or through-hole?

If surface mount, how much copper area for Vbb / tab connection?

How is inductive energy evaluated and controlled by the HITFET?

If through-hole, what type of heat sink will be provided for package?

What diagnostics are needed?

What application extremes will the device / system be subjected to (reverse battery, load dump, over voltage etc.)?


What is the load current
What Is the Load Current?

What is the maximum load current?

When does the maximum occur?

What is the typical load current?

Alternative Question: What is the load resistance?

Alternative Question: If the load is a lamp, what is it’s wattage?

Recall, the load current is fundamental in determining the Rdson value…


Is the load capacitive what is the in rush current
Is the Load Capacitive?What Is the In-rush Current?

Recall, the in rush current for lamps and RC networks may be an order of magnitude higher than the steady state current

5.5A

500mA


Is the load inductive inductance and or energy during turn off
Is the Load Inductive? Inductance and/or Energy During Turn-Off?

MOSFETs are rated for the max absorbable energy when turning off inductive loads

The equations relate the energy absorption Capability regarding a

Single pulse

Where:

L = load inductance

Ipk = short circuit load current

Vcl = over voltage clip voltage of HITFET

Vbat = supply voltage


What is the ambient temperature
What Is the Turn-Off?Ambient Temperature?

Minimum automotive ambient temperatures is usually -40C

Maximum ambient temperature ranges from 85C to 125C for most applications:

85C for most non - power train applications

105C for some in - dashboard applications

125C for most power train applications


What type of package surface mount or through hole
What Type of Package? Turn-Off?Surface Mount or Through-hole?

Many applications require all surface mount components

Surface mount components typically only have excess copper board space heat sinks

Through-hole components can have large heat sinks for improved power dissipation


If surface mount how much board area is available for heat sinks
If Surface Mount - How Much Board Area Is Available for Heat sinks?

Engineers must trade-off the cost and size of the heatsink vs. the Rdson (and hence, the cost) of the HITFET


Protected Low Side Drivers sinks?

  • Introduction to Protected Low Side Drivers

  • HITFET Protection Features

  • HITFET Diagnostic Features

  • EMI/EMC Considerations

  • System Implementation

  • Frequently Asked Questions



Thank you
Thank You! sinks?

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