Transcript 8. Protected Low Side Drivers - Bridging Theory into Practice

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
• 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
MOSFET Review
MOSFET  Metal Oxide Semiconductor Field Effect Transistor
Source
Gate
N-Channel
MOSFET
(Enhancement)
Source
DD
(E
G
G
n+
n+
p+
VGS
S
S
p+
VGS
n-
G
S
S
G
N-Channel
MOSFET
Enhancement) (
Drain
D
n+
P-Channel
MOSFET
(Enhancement)
D
MOSFET Regions 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.
IDS
VGS increases
VDS = VGS-VT
VDS
VGS > 0V
N-Channel
MOSFET
(NMOS)
Low Side Drive
(LSD)Configuration
14V
The switch is
on the “low”
side of the load
Load
MOSFET
Switch
To turn on, the MOSFET
gate is pulled high
Vgate @ 5V to 10V
Vgs = Vg - Vs
Vgs = 5V to 10V
Drain
Gate
Drain voltage is
small – ex. 0.1volt
Source Vs = 0V
HITFET = High Integration Temperature
protected FET
Over Voltage
Protection
Short Circuit
Protection
Current Limit
MOSFET
Over
Temperature
Protection
Diagnostics
Requires external
components
HITFET
High Integration
Temperature protected FET
Vsupply
Over Voltage
Protection
Over voltage
Load
Drain
Protection
Input
ESD
Protection
dv/dt
Limitation
Over
Temperature
Protection
Current
Limitation
S
H
U
N
T
Source
HITFET
Tab connection
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
• 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
Block Diagram
Including Protection Features
Vsupply
Over Voltage
Protection
Over voltage
Load
Drain
Protection
Input
ESD
Protection
dv/dt
Limitation
Over
Temperature
Protection
Current
Limitation
S
H
U
N
T
Source
HITFET
ESD Protection
Maximum Ratings at Tj = 25ºC, Unless Otherwise Specified
Electrostatic discharge voltage VESD
(Human Body Model)
2
KV
IN
ESD structure
Source
ESD structures (Zener) are not designed to conduct continuous DC current
Load Dump Protection
Drive (Vin) Param
*
Voltage Vbatt
Vp
Vs
Pulse
param
Load dump Vld
protection
Vin = low
or high (8V)
80
47
Vload Exponen
dump = tial +
Vp +Vs DC
offset
* Vld = Voltage load dump
13.5
Pulse
Type
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 Protection
Input
Voltage
Vin
Load
Current
Upper thermal hysteresis limit
Junction
Temperature
A
B
C
D
F
E
Lower thermal hysteresis limit
Time
Current Limit Protection
Paramater and Conditions
at Tj=25, Vbb=12V unless otherwise
specified
Symbol Values
Uni
t
min typ max
IDlim
Current Limit
Vin = 10V, Vds = 12V
ID
Time
1
1.5 1.9
A
Thermal Shutdown
Latch Behavior
VBB
Load
Overvoltage
Protection
1
In
Drain
2
dv/dt
limitation
Latch
ESD
Over
temperature
Protection
Short
circuit
protection
Current
Limitation
Source
HITFET
3
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
HITFET Diagnostic
Feedback: Drain Sense
• Diagnostic feedback is sensed at the drain and
applied to a micro processor Analog to Digital Input
Vbatt
• Advantage
• Low cost diagnostic and fault detection
RL
•Disadvantage
• Slower than integrated solution
• Uses one A/D channel per sensed
drain
Micro
Rs
Drive
A/D in
Attenuator
External Drain Sense
HITFET Diagnostic
Feedback Iin current monitor
Vcc
Vs
Id_out
DO
R1
Id_in
D
Iin/Diag
AI
S
Micro
processor
Gnd
+
Vd1
-
HITFET
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
 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?
Period
I1
Ton
Toff
Frequency= 1/Period
Period = Ton + Toff
Duty Cycle = Ton/(Ton+Toff)
I0
T0
T1
T2
T3
T4
HITFET Block Diagram
Vsupply
Over Voltage
Protection
Over voltage
Load
Drain
Protection
Input
ESD
Protection
dv/dt
Limitation
Over
Temperature
Protection
Current
Limitation
S
H
U
N
T
Source
HITFET
Tab connection
HITFET Turn-On/Turn-Off Slew Rate
Controlled
• Turn on / Turn off rate control.
• Slew rate is controlled
Dynamic characteristics
Param
min nom max units
Turn –on time
Vin to 90%ID:
Rl = 22 Ω, Vin = 0 to 10V, Vbb=12V
Ton
--
10
10
μs
Turn-off time
Vin to 10% ID : Rl =
22 Ω, Vin = 0 to 10V, Vbb=12V
Toff
--
10
20
μs
Slew rate on
70 to 50% Vbb:
Rl = 22 Ω, Vin = 0 to 10V, Vbb=12V
-dvds/dton
--
4
10
μs
Slew rate off
70 to 50% Vbb:
Rl = 22 Ω, Vin = 0 to 10V, Vbb=12V
dvds/dtoff
--
4
10
μs
HITFET Turn-on/turn-off Slew Rate
Controlled
100
90
Vdrain/Vsupply %
Turn-Off
Turn-On
70
70
Slew Rate Test Interval
% Change
of
Drain
Voltage
10
50
50
Rise
Time
Interval
10
Fall
Time
Interval
20
10
Time ( µs )
20
Typical HITFET Radiated Emissions Evaluation
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 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
Battery -
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
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!!
High Side Drive
(HSD) Configuration
MOSFET
Switch
The switch is
on the “HIGH”
side of the load
14V
Tothe
turn
on the HSD,
the
MOSFET
If
MOSFET
gate is
pulled
to
is pulled
high
agate
higher
voltage…
28V
VGS ~ 14V
VS ~ 14V
ILOAD
Load
The source voltage is now
approximately Vsupply
The high value of VGS translates
into a large value of ILOAD
(linear region)
Source Follower
Advantage:
Custom edge control (EMC)
Disadvantage
+
Battery
(+12V + Boost)
-- Complexity
Battery
(+12V)
Boost
-- Profet may be better
D
Driver /
Micro
controller
Level
Shifter
Gate
Controller
Vin
S
Load
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
• 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
• External components can
be added to a HITFET to
modify rise and fall time
and slew rate.
+
Load
Impedance
HITFET
R_Series
Driver /
Micro
Gate
Controller
C_Shunt
Simple low pass filter
HITFET Edge Shaping:
Simple Low Pass Filter
Input Voltage
Drain Voltage
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 Capacitor
+
External Miller
Capacitor
Load
Impedance
R_Series
Driver / Micro
controller
Gate Controller
HITFET
Drain feedback (Miller) capacitor method
HITFET Edge
Shaping: Miller Capacitor
Input Voltage
Drain Voltage
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
Test condition
Continuous input current
-0.2V  VIN  10V
VIN <-0.2V or VIN >10V
Parameter Limit
IIN
self limited
|IIN|  2
Unit
ma
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
• 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).
HITFET 5V vs. 10V operation
Additional
Current available
due to full
enhancement
Additional Vin
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
• 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 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 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?
• 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?
• MOSFETs are rated for the max absorbable
energy when turning off inductive loads
 Vcl
E  
 Vcl  Vbat
The equations relate the energy
 1
2 
   L * i pk  absorption Capability regarding a
 Single pulse
 2
Where:
L
= load inductance
Ipk = short circuit load current
Vcl = over voltage clip voltage of HITFET
Vbat = supply voltage
What Is the
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?
• 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?
• 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
• Introduction to Protected Low Side Drivers
• HITFET Protection Features
• HITFET Diagnostic Features
• EMI/EMC Considerations
• System Implementation
• Frequently Asked Questions
Protected Low Side Drivers
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