405 practical considerations
DESCRIPTION
Mosfet driverTRANSCRIPT
20/03/2015
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Practical Considerations in
Implementing Switching Power-Poles
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• Gate Driver ICs
• Design Considerations
– Thermal Considerations
– Magnetic Components
– Capacitors
– Selection of Switching Frequency
• Diode Reverse Recovery Characteristic
– Conduction Losses
– Increase in Switching Losses
Gate Drivers
• In order for PE converters to operate
efficiently, switching losses must be
minimised.
• Resonant converters can help but might
not be possible at high switching
frequencies or if feedback control is
needed
• Need to drive the switches on and off as
quickly as possible
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Initial conditions for drive ccts
• The drive circuit must provide adequate
drive power (e.g., IB for BJTs or Vgs for
MOSFETs) to keep the switch fully in the ON
state where conduction losses are low
• The drive circuit should also provide the
ability to quickly turn the power switch OFF
and keep it OFF even in the presence of
substantial switching noise
• The drive circuit is solely the interface
between the control circuit and the power
switch (we are not considering how to create
the control signal). 3
• The drive circuit may need to output
significant power (compared to control
circuits). e.g., power BJTs have low values of β
(typically around 10), so the IB supplied by the
drive circuit can be substantial if Ic is large.
• The basic form of a drive circuit depends on:
– Does the drive circuit output need to be unipolar or
bipolar (unipolar circuits are simpler than bipolar
circuits)?
– Is the drive output a current source or a voltage
source?
– Is electrical isolation required between the control
signal and the switch drive signal?
– Maximum switching frequency?
– Can it provide the necessary (peak) gate current?4
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• Additional functionality may be required
from the drive circuit:
– Switch over-current protection
– Creation of blanking times
– Wave-shaping of control signals to
enhance switch on and/or switch off
performance
– Number of independent output channels
– Maximum charge per pulse (Qout/pulse);
output caps of the driver must be able to
deliver the gate charge for charge and
discharge5
Choosing the wrong one – Resistor that
is!
• Overheated/burnt – low peak power, no-
surge proof resistor
• EMI – small gate resistors used
• Gate oscillations – high resistors
• Increased Switching losses – High
resistors
• So what values?
– Use switch data sheets – Rt
– Select Rt < Rg <2Rt
– Also Rg(off) = 2Rg(on)6
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MOSFET (& IGBT) drivers
• MOSFETs (& IGBTs) are voltage controlled devices
where an input voltage determines the output current
• Commonly, a VGS that is 5 – 15 V higher than VT is
sufficient to ensure the MOSFET is fully on.
• Switching a MOSFET off requires reducing VGS below
VT
• The charge associated with the gate capacitance can
require high pulse currents for high frequency
applications and drive circuit must be able to source
and sink
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Totem pole
• A very common circuit
configuration used to
provide the high
source and sink
current needed is the
totem pole
arrangement (double
emitter follower; push-
pull buffer)
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Electrically Isolated drivers
• Sometimes need to electrically isolate the
control signal from the output signal to the
switch
• This is usually due to the position of the
switch in a power circuit where the source (or
emitter) terminal voltage shifts high or low
depending on whether it has been switched
on or off.
• To ensure correct operation, the drive signal
must be able to float with the rising and falling
source terminal voltage.9
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Gate Driver Integrated Circuits (ICs) with Built-
in Fault Protection
12extV VCCV
cv
S
12extV VCCV
cv
S
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Optically isolated drivers
• Optocoupler IC with high speed
phototransistor
• The drive cct is then constructed on the
floating side with its own DC Power
supply
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Transformer Isolated
• Transformer isolation – no need for a dc power
source
• Works well for HF
• Core saturation is a problem – secondary V is bipolar
– so need to worry about negative VGS
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Current controlled devices - BJTs
• Power BJTs are current controlled devices. To
ensure a BJT is fully saturated (switched on), the
current through the base must be at least equal to
IC/β.
• Switching speed determined by how quickly the drive
signal can source and sink the charge associated
with the capacitance effect in forward biasing the
base-emitter p-n junction
• For fast switching in power BJTs need a base drive
signal that provides an initial very high current to
quickly charge (or discharge) the base-emitter
capacitance, which falls to a steady-state value
capable of maintaining saturation.13
A simple but effective cct
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• When the drive voltage falls to zero for turn-
off, the charged capacitor creates a high
negative current pulse that quickly discharges
the base-emitter capacitance.
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• Gate driver ICs:
– Save 30% on parts count
– 50% on PCB space
– The input signal Vc from the controller is
referenced to the logic level ground
– Vcc is the logic supply voltage
– Vext to drive the gate is supplied by the isolated
power supply referenced to the MOSFET source
S.
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Quiz #1
In the switching power-pole shown above, the
negative terminal of the input voltage is
grounded and the control voltage to control the
transistor is a logic-level signal generated with
respect to Ground.
In the output stage of the gate-driver IC shown in
the earlier slide, the power supply Vext must be
isolated from ground.
A. True
B. False
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Figure 2-10 Capacitor ESR and ESL.
C ESL ESRC ESL ESR
max max
ˆrms
p
w
LIIA
k J B
,
max max
conv y y rms
p
w s
k V IA
k B J f
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• Switching Frequency
• Selection of Transistors and Diodes
• Magnetic components
• Capacitor Selection
DESIGN CONSIDERATIONS
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PSpice Modeling:
Frequency
1.0KHz 3.0KHz 10KHz 30KHz 100KHz 300KHz 1.0MHz
I(L2) I(L1) -I(V3)
0A
10A
20A
30A
40A
50A
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Simulation Results: Individual and Total Admittances
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Thermal Design
( )j a jc cs sa dissT T R R R P
jcR
dissP
(b)
csR sa
R
aT
aTsTcTjT
(a)
chip
heat sinkisolation pad
jTcT
sTaT
ambient
case
jcR
dissP
(b)
csR sa
R
aT
aTsTcTjT
(a)
chip
heat sinkisolation pad
jTcT
sTaT
ambient
case
Magnetics and
capacitors
size
Heatsink
Sf
Magnetics and
capacitors
size
Heatsink
Sf
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Design Tradeoffs
Size of magnetic components and heat sink
as a function of switching frequency
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Diode Reverse Recovery and Power Losses
Diode Forward Loss: , (1 )diode F FM oP d V I
Diode Reverse Recovery Characteristic:
Diode Switching Losses:, ,
1( )2
diode sw RRM b d neg sP I t V f
0
rrt
at bt
RRMI
t
t0
FMV
,d negV
rrQ
0
rrt
at bt
RRMI
t
t0
FMV
,d negV
rrQ
inV
0
GGV
DiD
0I
Di
S
G DSv
Concept Quiz #2
In the switching power-pole shown in the
earlier slide, what is vd,neg equal to in
magnitude:
A. Essentially Vin
B. 0
C. Depends on the diode characteristic
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DSv
0
0
0 t
t
t
rit
inV
fvt
a rrt t
oI
Di
RRMI
swp
fvt
DSv
0
0
0 t
t
t
rit
inV
fvt
a rrt t
oI
Di
RRMI
swp
fvt
Effect of Diode Reverse Recovery Current:
inV
0
GGV
DiD
0I
Di
S
G DSv
Summary
• Practical Implementation Considerations
– Gate Driver ICs
– Design Considerations
• Thermal Considerations
• Magnetic Components
• Capacitors
• Selection of Switching Frequency
– Diode Reverse Recovery Characteristics
• Conduction Losses
• Increase in Switching Losses
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Quiz #1
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A. Doubles
B. Remains the same
C. Becomes one-half
In a switching converter, the switching power loss in a MOSFET is 100 W.
It is decided to redesign this converter and replace the original MOSFET with a new one
that has twice the switching speed, that is, it has one-half the voltage and current rise-
times and fall-times compared to the original MOSFET.
This new converter is to be operated at twice the switching-frequency compared to the
original converter.
Question: What happens to the switching power loss in the MOSFET of the new
converter, compared to that in the original converter?
Quiz #2
In the MOSFET of a switching power-pole, the average power dissipation is 30 Watts. The junction temperature of this MOSFET must not exceed 1000C, and the ambient temperature is given as 400C. Calculate the maximum thermal resistance that can be present between the ambient and the MOSFET junction.
A. 20 C/W
B. 3.330 C/W
C. 4.660 C/W
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