AN3303
Application note
Secondary-side rectification for an LLC resonant converter
featuring the SRK2000
Introduction
The EVLSRK2000 is a family of demonstration boards designed for the evaluation of the
SRK2000 in LLC resonant converters with synchronous rectification (SR).
The first part of this application note is a brief description of the IC features while the second
is dedicated to the board description. Finally, some considerations regarding circuit
optimization and performance are given.
This board was realized in four different configurations depending on the mounted SR
MOSFETs. Different board codes are shown in
Table 1:
Table 1.
Demonstration board ordering codes
SR MOSFET P/N
STL140N4LLF5
STL85N6F3
STD95N4F3
STS15N4LLF3
MOSFET
package
PowerFLAT™
PowerFLAT™
DPAK
SO-8
MOSFET R
DS(on)
MOSFET BV
DSS
2.75 mΩ
5.70 mΩ
5.80 mΩ
5.00 mΩ
40 V
60 V
40 V
40 V
Ordering code
EVLSRK2000-L-40
EVLSRK2000-L-60
EVLSRK2000-D-40
EVLSRK2000-S-40
Figure 1.
EVLSRK2000: smart driving control for an LLC resonant converter
September 2011
Doc ID 18164 Rev 2
1/27
www.st.com
Contents
AN3303
Contents
1
SRK2000 main characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1
1.2
1.3
1.4
1.5
Drain MOSFET sensing and driving logic . . . . . . . . . . . . . . . . . . . . . . . . . 5
Drain sensing optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Blanking time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Light load operation and sleep mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Enable pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2
3
Electrical diagram description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Sensing optimization by waveform check . . . . . . . . . . . . . . . . . . . . . . 14
3.1
3.2
3.3
Power MOSFET turn-off compensation . . . . . . . . . . . . . . . . . . . . . . . . . . 14
MOSFET turn-on delay compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Sub-harmonic oscillation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4
5
How to implement the board in the converter . . . . . . . . . . . . . . . . . . . 17
Power losses and thermal design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.1
5.2
Power losses calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Thermal design consideration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
6
7
8
9
Layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
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Doc ID 18164 Rev 2
AN3303
List of tables
List of tables
Table 1.
Table 2.
Table 3.
Table 4.
Table 5.
Table 6.
Demonstration board ordering codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
EVLSRK2000-L-40 bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
EVLSRK2000-L-60 bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
EVLSRK2000-S-40 bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
EVLSRK2000-D-40 bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Doc ID 18164 Rev 2
3/27
List of figures
AN3303
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
Figure 18.
Figure 19.
Figure 20.
Figure 21.
Figure 22.
EVLSRK2000: smart driving control for an LLC resonant converter. . . . . . . . . . . . . . . . . . . 1
Block diagram of an LLC converter with synchronous rectification. . . . . . . . . . . . . . . . . . . . 5
Power MOSFET drain voltage sensing and typical waveforms . . . . . . . . . . . . . . . . . . . . . . 7
Parasitic elements model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Effect of parasitic elements on power MOSFET turn-off. . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Effect of parasitic elements on power MOSFET turn-on. . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Blanking time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Duty cycle oscillation when VDVS @ 50 % cycle almost equals VDVS1,2_Off . . . . . . . . . 10
Conduction time sensing during normal operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Conduction time sensing under sleep mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Operation mode transitions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Electrical diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Full load operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Full load operation - detail . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
SR MOSFET turn-off . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
SR MOSFET turn-off - detail. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Operation above resonance frequency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
SR MOSFET turn-on . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
SR MOSFET turn-on - detail. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Duty cycle oscillation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
How to implement the board on an existing converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Board layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
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Doc ID 18164 Rev 2
AN3303
SRK2000 main characteristics
1
SRK2000 main characteristics
The main features of the SRK2000 are described below. The values of the following
parameters are reported in the SRK2000 datasheet (see
Reference1).
The SRK2000 implements a control scheme specific for secondary-side synchronous
rectification in an LLC resonant converter that uses a transformer with center-tap secondary
winding for full-wave rectification. It provides two high-current gate-drive outputs, each
capable of driving one or more N-channel power MOSFETs in parallel. Each gate driver is
controlled separately and an interlocking logic circuit prevents the two synchronous rectifier
MOSFETs from conducting simultaneously.
Figure 2.
Block diagram of an LLC converter with synchronous rectification
1.1
Drain MOSFET sensing and driving logic
The core function of the IC is to switch on each synchronous rectifier MOSFET whenever
the corresponding transformer half-winding starts conducting (i.e. when the MOSFET body
diode starts conducting) and then to switch it off when the flowing current approaches zero.
For this purpose, the IC is provided with two pins (DVS1 and DVS2) able to sense the power
MOSFET drain voltage level. Because each power MOSFET is turned on when its body
diode is conducting, zero voltage turn-on is achieved.
Device operations described below refer to
Figure 3.
a)
When the current ISR1 starts flowing through the body diode, the voltage across
the power MOSFET drain-source becomes negative; as it reaches the negative
threshold V
TH.ON
, the power MOSFET is switched on.
The threshold at which the power MOSFET turns on can be set by the following
formula:
Equation 1
V
THON
=
R
D
⋅
I
DVS1
,
2ON
+
V
DVS1
,
2TH
_
Doc ID 18164 Rev 2
5/27
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