Transcript Project Proposal on DCDC converters for the 42V Automotive

The Active Diode
A Current Driven Synchronous Rectifier
W2-tech Inc.
Demand for Low Voltage High
Current Power Converters


Modern Microprocessor
operates at low voltage
and high current
The future demand will go
for less than 1 V and more
than 150A
Reasons for low voltage &
high current




Switching time is shorter
between close voltage
levels
Less loss due to capacitance
IC sub micron technology
requires low operating
voltage
Large number of devices
need high current
Distributed Power Structure
Synchronous Rectification is
needed


In order to handle high current at low voltages,
SR is needed
Low Rdson MOSFET greatly reduces losses at the
output rectifier
There are many problems with
conventional SR


Different topologies need different drive circuit design
Active clamp on the primary side is often needed
Problems with conventional
SR …cont’d
Input voltage
120V to 380V



MOSFET driving voltage is directly coupled to the input
voltage
Gate voltage limits input voltage range
Gate drive voltage not optimized
Problems with conventional
SR …cont’d
Body diode conduction
T1
T1 I/P voltage
M1
M2 gate
M2



Leakage inductance
produces long body diode
conduction period
This increases dissipation
and greatly reduces
efficiency
Bad at high frequencies
M1 gate
0.6V
T1 O/P voltage
M1 M2 conduction
voltage
0.1V
M1, M2 power loss
M1 current
M2 current
Problems with conventional
SR …cont’d

MOSFET is a bi-directional switch
 Converters with SR cannot be connected in
parallel, as reverse current will flow between
Vo
converters
Vo-
Vo+
Problems with conventional
SR …cont’d
iL

MOSFET is a bi-directional switch


No discontinuous mode
Poor light load efficiency because of
current peaks
Problems with conventional
SR …cont’d


The gate drive will be lost after the
transformer is reset in a forward converter
Active clamp on the primary side is often needed

More components and violation of patents
SR1
SR1
SR2
SR2
Vgs(SR1)
Vgs(SR2)
Desired Solution



The SR should turn on and off according to current flow
This makes the SR behaves like a diode
Solves all aforementioned problems
The Active Diode – Basic
configuration
Current sense
circuit
M1
N1
N2
N3
D1
Amplifier
N4
D2


Energy recovery
circuit
Reset circuit

N1 is the current sense winding
N2 amplify voltage at N1
N1 N3 & D1 form energy recovery
circuit

N4 & D2 form reset circuit
Basic Operation of the Active
Diode
Vcs  Vo
N1
N3
Voltage drop Vcs across current sense winding N1 is
depend on the winding ratio of N1 to N3 and
voltage source Vo
Ii
Voltage source Vo
can be any voltage
source in a converter,
e.g. output voltage
M1
N1
N2
T1
N3
D1
and voltage Vo
N4
Vo
Voltage across winding N2 or
gate drive voltage Von of SR
depends on ratio of N2 to N3
D2
Von  Vo
N2
N3
Waveforms
Ton_d
Toff
Ii
M1
N1
Ii
N2
Von
VN2
T1
N3
D1
N4
Vo
Vo
VN3
D2
Vo
VN4
Toff_d
Active Diode –
K
A
the way to a perfect diode
K
AD
K
A
A
It is better than Synchronous Rectifier
Sync Rect
Active Diode
AD
AD
•
Complicated primary circuit
• Converter cannot be paralleled – Reverse
current
• Poor efficiency at low load
• Special driving circuits SR are needed for
different topologies
• Sensitive to transformer leakage inductance
• Limited input voltage range
• Simple primary circuit
• Discontinuous mode is allowed
• Good low load efficiency
• Converter can be paralleled
• Works just like a diode
It is far better than Schottky diode
K
A
K
K
A
K
AD
• Inherent forward Volt. drop
• No inherent Volt. drop
• Low reverse voltage
• high reverse voltage
• No or little avalanche rated
• 100% avalanche guaranteed
A
A
SCK Diode or Active Diode?
K
0.24 V
9 m
A
K
7.8 m A
SCK Diode VS Active Diode
SCK 2.8W losses @ 10A
3
2.5
W
2
1.5
1
AD 0.7W losses @ 10A
0.5
0
AD
0
2
4
6
8
10
A
STP S30 L30
SO8 Act iv e Di o de
Lo sses comp ari so n b etw een SCK and AD
Comparisons of Sync-Rect, SCK and Active
Diode
Schottky
Sync-Rect
Active Diode
Losses
Bad 
Good 
Good 
Avalanche
guaranteed
No 
Yes 
Yes 
Topologies
Independent
Yes 
No 
Yes 
Low loading Eff.
Good 
Bad 
Good 
High reverse Volt.
Bad 
Good 
Good 
Operating Temp.
Bad 
Good 
Good 
Overall cost
Good 
Bad 
Good 
Commutation
conduction
Good 
Bad 
Good 
Design engineer’s consideration
AD
IR1176
AD
Self voltage driven approach
SR IC driven approach
Active Diode approach
Detail
Cost (USD)
Detail
Cost (USD)
Detail
Cost (USD)
Magnetic
1
Magnetic
1
Magnetic
1
Main Pri MOSFET
0.8
Main Pri MOSFET
0.8
Main Pri MOSFET
0.8
Aux. Pri MOSFET
0.4
IR1176 SR IC
1.2
Aux. MOSFET
0
Aux. Cap.
0.05
SR IC aux. circuit
0.1
Aux. Circuit
0
SR MOSFETs
1.0
SR MOSFETs
1.0
Active Diodes
1.72
O/P Cap.
0.5
O/P Cap.
0.6
O/P Cap.
0.6
Design Cost
0.3
Design Cost
0.1
Design Cost
0.1
Total
4.05
Total
4.8
Total
4.22
The Active Diode is
• 5 times lower losses than state of the art Schottky diode
• 50 times lower losses is also possible
• 100% avalanche guaranteed
• Only MOSFET solution can ensure important no load power <0.3W
• Cheapest solution compared with other Sync-Rect solution
• Replace diode on all old and new converter designs
• Much higher operating temperature than Schottky diode
•Wide frequency & voltage range from 50 Hz to 500kHz and 12V to
1000V
The Active Diode works in all
topologies
Lf
+
+
+
Co
Vo
+
Vin
-
S
Vin
Flyback SR
-
(a)
Flyback
Freewheel
SR
Magnetic
Reset
Co
Vo
-
S
Forward SR
-
(a)
Forward
It works just like a low loss diode
Lf1
Lf
+
S1
SR1
C1
Vin
-
S2
C2
SR2
(a)
Half Bridge centre tap
+
Co Vo
-
+
S1
C1
SR1
S2
C2
SR2
Lf2
Vin
+
Vo -
(a)
Current Doubler
Co
……. in different topologies
SR1
+
Co Vo
-
ISIN
SR2
(a)
Resonant converter
and many others….
Conclusions



A new “Active Diode” technology is
presented
A kind of current driven synchronous
rectifier which solves many problems
of the conventional Sync Rect
Well proven by many converter designs