DT_TDA4863_Driving_low_Rdson_MOSFET

Design Note, V1.0, Mar. 2003
T D A4 8 6 3
DN-PFC-TDA4863-1
TDA4863 Driving MOSFET with large
Capacitances
Author: Wolfgang Frank
http://www.infineon.com/pfc
Power Management & Supply
N e v e r
s t o p
t h i n k i n g .
TDA4863 Driving MOSFET with large Capacitances
Revision History:
2003-03
V1.0
Previous Version:
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Subjects (major changes since last revision)
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Edition 2003-03
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TDA4863 Driving MOSFET with large
Capacitances
1
DN-PFC-TDA4863-1
Large Capacitances
In adapters MOSFET with a lower on-state resistances RDS(on) are often used in order to
reduce power losses. But such transistors have typically large capacitances Ciss, Coss,
and Crss according to Figure 1. Especially in power factor correction (PFC)
preconverters this issue is even more dramatically, because there are points of
operation, at which the drain-source-voltage is very low or even zero. At those points,
the parasitic drain-gate-capacitance Crss(VDS) (“Miller-capacitance”) increases highly
nonlinearly. This can be easily seen in the datasheets of the MOSFET, as it is shown in
figure 24 of [2].
Lboost
ton
toff
tosc
TP
VCC
idisplace
iL
D
Crss
vDS
RG
idisplace
Gate
Coss
G
Q1
Ciss
S
vGate
RG
Figure 1
Equivalent Circuit of a MOSFET with parasitic Capacitors
In discontinuous conduction mode (DCM) the drain-source-voltage swings down to zero
by system, if the input voltage is lower than 50% of the output voltage even without the
MOSFET being switched on. This means that the drain potential also goes down to zero
which will cause a capacitive current flowing into the gate pin of the MOSFET and
through the capacitor Crss.
The larger the capacitance Crss the larger is the amplitude of the capacitive current. This
may reverse bias the lower gate drive transistor and may lead to substrate currents in
the control IC of the MOSFET and may cause malfunction. Substrate current can be
Design Note
6
V1.0, 2003-03
TDA4863 Driving MOSFET with large Capacitances
Large Capacitances
detected easily by measuring the voltage at the gate drive pin. Substrate currents cause
a voltage of about -0,7 V.
This effect is well known. Usually schottky diodes are used directly at the gate drive pin
to ground according to Figure 2 in order to clamp the gate drive voltage of -0,3 V
minimum.
MUR460
D5
TDA4863
5
6
Figure 2
7
1
2
4
VOUT
R9
33k
R4A
Q1
SPP20N60 422k
C3
R10
R4
422k
12Ω
Schottky
C8
C2
1µ
R2
33k
C1
2,2µ
R11
0R47||
0R47
100µF
450V
R5
5k1
C1
µ1
GND
Gate Drive Design of TDA4863 with Schottky Clamp Diode
The rating of the schottky diode depends on the peak value and the rms value of the
clamp current. But typically small signal schottky diodes with a forward current capability
of approximately 100 mA are already sufficient.
Design Note
7
V1.0, 2003-03
TDA4863 Driving MOSFET with large Capacitances
Summary of Used Nomenclature
2
Summary of Used Nomenclature
Physics:
General identifiers:
Special identifiers:
A .........cross area
b, B .....magnetic inductance
c, C .....capacitance
d, D .....duty cycle
f...........frequency
i, I ........current
l, L .......inductance
N .........number of turns
p, P .....power
t, T.......time, time-intervals
v, V......voltage
W ........energy
h..........efficiency
AL ........... inductance factor
V(BR)CES .. collector-emitter breakdown
voltage of IGBT
VF........... forward voltage of diodes
Vrrm .......... maximum reverse voltage of diodes
big letters:
constant values and time
intervals
small letters: time variant values
K1, K2 ..ferrite core constants
Components:
C .........capacitor
D .........diode
IC ........integrated circuit
L..........inductor
R .........resistor
TR .......transformer
Indices:
AC.......alternating current value
DC.......direct current value
BE .......basis-emitter value
CS.......current sense value
OPTO..optocoupler value
P .........primary side value
Pk........peak value
R........... reflected from secondary to primary side
S .........secondary side value
Sh .......shunt value
UVLO ..undervoltage lockout value
Z..........zener value
Design Note
8
fmin ......... value at minimum pulse frequency
i ..............running variable
in ............input value
max ........maximum value
min .........minimum value
off ...........turn-off value
on ...........turn-on value
out ..........output value
p .............pulsed
rip ...........ripple value
1, 2, 3 .....on-going designator
V1.0, 2003-03
TDA4863 Driving MOSFET with large Capacitances
References
3
References
[1]
Infineon Technologies AG: TDA4863 - Power factor controller; Preliminary
Data sheet; Infineon Technologies AG ; Munich; Germany; 02 / 02.
[2]
Infineon Technologies AG: SPP20N60C3 CoolMOS - Power Transistor; Data
sheet; Infineon Technologies AG ; Munich; Germany; 10 / 02.
Design Note
9
V1.0, 2003-03
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