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
Source
Source
Source
n
n
n
p
p
Gate
n
p
p
n epi
n+ substrate
Drain
Blocking State
Source
n epi
n+ substrate
Drain
Conducting State
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
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
Metal Oxide Semiconductor
Field Effect Transistor
Vertical Power MOSFET
Transistor Process
Vertical Power MOSFET
Integrated Circuit Process
(n-channel)
(n-channel)
Source
Source
n+
n+
p+
Source
Source
n+
n+
p+
p+
p+
n-
n+
p-
Drain
Ground
p+
p+
n-
n-
nn+
Gate
Drain
Gate
n-
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)
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
Symbol Condition Minimum
Gate
Drain-to-Source
V(BR)DSS VGS = 0V
Breakdown Voltage
ID = 1mA
Source
25V
Low Side Drive (LSD) Configuration
The switch is
between the
load and ground
14V
ILOAD
Load
MOSFET
Switch
VD ~ 0V
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
14V
Current flows and the load
“turns-on”
High Side Drive (HSD) Configuration
MOSFET
Switch
The switch is
between the
load and supply
14V
To turn on the HSD, the MOSFET
gate is pulled high
14V
VGS ~ 5V
VS ~ 9V
ILOAD
Load
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.
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
than supply
28V
VGS ~ 14V
VS ~ 14V
ILOAD
Load
The source voltage can now rise
to approximately Vsupply
The high value of VGS (and low
VDS) translates into a large value
of ILOAD (linear region)
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
H-Bridge Configuration
14V
14V
CW
CCW
The load is placed
in the middle of a
“H” configuration
CCW
Load
CW
H-Bridge Configuration
14V
The load is placed
in the middle of a
“H” configuration
14V
CCW
28V
A
To turn the load on
in one direction,
“CW” is pulled high
CCW
Load
A14V
H-Bridge Configuration
14V
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
14V
B
Load
CW
To turn on in the
other direction,
“CCW” is pulled high
PROFETs = PROtected FETs
Integrated
Charge Pump
Diagnostics
Short Circuit
Protection
Over
Voltage
Protection
Reverse
Battery
Protection
MOSFET
Current Limit
PROFET
Over
Temperature
Protection
PROFET - Block Diagram
HITFETs =Highly Integrated,
Temperature protected FETs
Diagnostics
(Requires external
Components)
Over
Voltage
Protection
Current Limit
MOSFET
HITFET
Short Circuit
Protection
Over
Temperature
Protection
HITFET - Block Diagram
VSUPPLY
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
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
Rdson Equations
• Rearranging, the equations yield:
I Load 
Rdson 
T junction  Tambient
Z thja Rdson
T junction  Tambient
2
I load Z thja
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
14V
Rdson
Tambient = 85°C
SOT-223 Package
Zthja= 82°C/W
To find Iload, initially assume
Rdson is 0
Iload
R = 3
I load
Vbatt 14V


 4.67A
Rload 3
Rdson Selection
Example Calculation
Rdson can now be calculated for
different Tjunction,max
14V
Rdson 
Iload
Tjunction,max - Tambient
Iload2 Zthja
Rdson
150C 85C
≤
= 36m
2
(4.67A ) (82C / W )
Rdson
125C 85C
≤
= 22m
2
(4.67A ) (82C / W )
R = 3
SOT-223 Heatsink
 82 C/W
TO-263 Heatsink
Larger Package
Larger Heatsink
Rdson Selection
Example Calculation
Rthja = 39°C/W with 1 in2 heatsink
14V
Rdson can now be calculated for
different Tjunction,max
Rdson 
Iload
R = 3
Rdson 
150C  85C
4.67A  39C / W 
2
125C  85C
4.67A  39C / W 
2
 76m
 47m
Rdson vs. Package
and Heatsink
Package and
Heatsink
Rdson at
Rdson at
Tjunction,max =125C Tjunction,max =150C
SOT-223 (0.5 in2)
22 m
36 m
TO-263 (1 in2)
47 m
76 m
Rthja for Various Packages
SO8
TO-252
SOT-223
TO-263
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
Impact of Approximate FET
Switching Loss
1.0
0.8
0.6
0.4
0.2
0.0
0.0
0.2
0.4
Time
0.6
0.8
1.0
All values normalized
PROFET Switching Loss
Lamp Turn-On
Current response
approximately
piece-wise linear
6m HSD
Vsupply=13.5V
Load=60W bulb
D=0.5, f=100Hz
time
Approximate FET
Switching Loss
• For a resistive load (with a piecewise linear current and
voltage response), the approximate FET switching loss is:
Ploss ~ (0.125)(VDSIDS)
Eloss = (Ploss)(tswitch)
PROFET Switching Loss
Lamp 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
PROFET Switching Loss:
Lamp Turn-Off
Current response
approximately
linear
6m HSD
Vsupply=13.5V
Load=60W bulb
D=0.5, f=100Hz
PROFET Switching Loss
Lamp 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
PROFET Switching Loss:
Lamp Turning On and Off
• Approximate Switching Energy Loss
Eloss on,approx = 2.28mJ
Eloss off,approx = 0.17mJ
Eloss,approx
= 2.28mJ + 0.17mJ = 2.45mJ
(turn-on)
(turn-off)
(total)
Eloss,actual measurement
(total)
% Error
% Error
= 2.29mJ
= (2.45mJ – 2.29mJ) / 2.29mJ
= 7.0%
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)
Rdson Calculations for
PWM Applications
Rdson,max
 Tjunction - Tambient Fswitching VsupplyIload

 t turn-on + t turn-off 

Rthja
8



Iload2D
 Rdson (FET more expensive)
 Tjunction,max
 Tambient
 Rthja
 Iload
 D (Duty Cycle)
 Fswitching
Increases
 Vsupply
switching
 tturn-on
losses
 tturn-off
 Rdson (FET less expensive)
 Tjunction,max
 Tambient
 Rthja
 Iload
 D (Duty Cycle)
 Fswitching
Decreases
 Vsupply
switching
 tturn-on
losses
 tturn-off
Rdson Selection
Example Calculation
Tambient,max
Tjunction,max
Iload
Ploss
tturn-on
tturn-off
Fswitching
Duty Cycle
Rthja
13.5V
2.05
Rdson
= 85C
= 150C
= 6.57A
=11.1W
= 155s
= 30s
= 100Hz
= 50%
= 55°C/W (TO252+1in2)
 Tjunction - Tambient Fswitching VsupplyIload

 t turn-on + t turn-off 

Rthja
8




Iload2D
150C - 85C 100Hz 13.5V  6.57A 

155μs + 30μs 

55K/W
8


Rdson 
2
 6.57A   0.5 
Rdson  45m
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
Capacitive Load
In-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
3.6mF
2.80
Capacitive Load
In-Rush Current
• Lamps and RC networks can experience significant “inrush” current when they are initially turned on
• The in-rush current may be 10 times the static (DC) current
5.5A
600mA
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
Estimated average power during in-rush (30W)
27W lamp in rush current
Driver Pdiss=Vds*Iload
Zthja Chart for Lamp
In-Rush Current Example
100
10
Single Pulse
ZthJA [K/W]
2.0°C/W
1
D=
0,5
0,2
0,1
0,05
0,02
0,01
0
0.1
0.01
1E-6
1E-5
1E-4
1E-3
1E-2
~3msec
1E-1
tp [s]
1E0
1E1
1E2
1E3
1E4
Lamp In-Rush Current
Example Calculations
• Approximate junction temperature increase
(using Zth diagram and estimated rectangular
average in-rush power)
tin-rush
Zthja
 3msec
 2.0°C/W
Ploss,ave
 30W (estimated from
oscilloscope)
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
Switching OFF an
Inductive Load
• 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
• Prior to the PROFET being turned on....
Vbb
VIN
VIN
VON
VOUT
VOUT
IL
Switching OFF an Inductive Load
• Initially, the FET is turned on, and IL begins to
increase
Vbb
VIN
VON
VIN
VOUT
VOUT
IL
IL
Switching OFF an Inductive Load
• Eventually, IL reaches it’s DC value
Vbb
VIN
VON
VIN
HIGH
VOUT
VOUT
IL
IL
Switching OFF an Inductive Load
• At some point, the FET is turned off
Vbb
VIN
VON
VIN
VOUT
VOUT
IL
IL
Switching OFF an Inductive Load
• VOUT goes below GND. The zener eventually conducts and supplies
gate charge to turn on the FET, clamping VOUT at a "safe" voltage.
Vbb
VIN
VON
VIN
VOUT
VON(CL)
VOUT
IL
IL
Switching OFF an Inductive Load
• When IL  0A, VOUT will return to GND potential
Vbb
VIN
VIN
VON
0V
VOUT
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 Increases the
Maximum Inductor Energy
Absorbable Inductor
Energy (mJ)
• 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)
Maximum Safe 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:
Energy Absorbed When
Turning Off an Inductive Load
Maximum load
inductance for a
single switch off
Tj = 150C
Vsupply = 12V
RLOAD = 0
Energy Absorbed When
Turning Off an Inductive Load
• 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 battery
EL 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
Energy Absorbed When
Turning Off an Inductive Load
• 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

LIL
ILRL
Eloss =
VSUPPLY + VOUT(CL) ln 1+

2RL
VOUT(CL)







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 Input
14V
Note: Generally,
inductive loads are
not PWM driven
due to the repetitive
clamping energy /
power.
Source
Gate
n+
Source
n+
p+
p+
nn+
Load
Drain
Turning Off an Inductive Load
MOSFET is turned off
Vsupply = 12V
VIN
VON
VOUT
L=630H
IL=9.5A
VIN
Turning Off an Inductive Load
VOUT = Vsupply - VAZ
VOUT = 12V – 42V = -30V
VAZ ~ VDS = VON
VAZ = 42V
VON
Vsupply = 12V
VAZ
VIN
VIN
VON
VOUT
L=630H
IL=9.5A
Vsupply = 12V
Turning Off an Inductive Load
VOUT clamped to -30V
VAZ = 42V
Vsupply = 12V
VIN
VIN
VON
Vsupply = 12V
VON
IL
VOUT
L=630H
IL=9.5A
di/dt = VOUT / L
toff = L * IL / VOUT
toff = (630H)(9.5A) / 30V
toff = 200s
Turning Off an Inductive Load
Area under the Ploss curve
is the dissipated energy
VAZ = 42V
Vsupply = 12V
VIN
VIN
VON
Vsupply = 12V
VON
IL
VOUT
L=630H
IL=9.5A
Ploss
di/dt = VOUT / L
toff = 200s
Eloss
Ploss,avg  VAZ * IL,max / 2 = (42V)(9.5A) / 2
Ploss,avg  200W
Eloss  (Ploss)(toff) = 40mJ
Low Side Drivers and
Inductive 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
VSUPPLY
VIN
VSUPPLY
Negative
Voltage
Spike
Positive
Voltage
Spike
VOUT
VOUT
VIN
Switching an Inductive Load
• Initially, the MOSFET is turned on and IL reaches it’s DC
value
VSUPPLY
VIN
IL
VOUT
VOUT
VIN
IL
IL
Switching an Inductive Load
• At some point, the FET is turned off
VSUPPLY
VIN
IL
VOUT
VOUT
VIN
IL
IL
Switching an Inductive Load
• VOUT goes above VSUPPLY as the inductor current goes to 0A.
VSUPPLY
VIN
IL
VOUT
VOUT
VIN
IL
IL
Switching an Inductive Load
• When IL = 0A, VOUT returns back to VSUPPLY
VSUPPLY
VIN
IL
VOUT
VOUT
VIN
IL
IL
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
MOSFETs, High Side Drivers,
and Low Side Drivers
Gate
Source
Source
Source
n
n
n
p
p
Gate
n
p
p
n epi
n+ substrate
Drain
Blocking State
Source
n epi
n+ substrate
Drain
Conducting State
Thank You!
www.btipnow.com