Application Notes

AN10781
GreenChip SR TEA1791T integrated synchronous rectification
controller
Rev. 01 — 09 February 2009
Application note
Document information
Info
Content
Keywords
GreenChip SR, TEA1791T, Synchronous rectification, High efficiency,
Flyback.
Abstract
The TEA1791T is a member of the new generation of synchronous rectifier
controller ICs for switched mode power supplies. Its high level of
integration allows the design of a cost effective power supply with a very
low number of external components.
The TEA1791T is a controller IC dedicated for synchronous rectification on
the secondary side of Discontinuous Conduction Mode (DCM) and quasi
resonant flyback converters.
The TEA1791T is fabricated in a Silicon On Insulator (SOI) process. This
NXP SOI process makes a wide voltage range possible.
AN10781
NXP Semiconductors
GreenChip SR integrated synchronous rectifcation controller
Revision history
Rev
Date
Description
01
20090209
First release
Contact information
For more information, please visit: http://www.nxp.com
For sales office addresses, please send an email to: [email protected]
AN10781_1
Application note
© NXP B.V. 2009. All rights reserved.
Rev. 01 — 09 February 2009
2 of 8
AN10781
NXP Semiconductors
GreenChip SR integrated synchronous rectifcation controller
1. Introduction
The TEA1791T is the controller for synchronous rectification to be used in quasi resonant
and discontinuous conduction mode flyback converters.
2. Application schematic TEA1791T
Primary power stage
Secondary output stage
Vo 2
+ Vin
Ns2
C1
C2
R1
C3
Vo 1
Q1
Vout
Ns1
5
6
7
n.c.
DRIVER
n.c.
n.c.
TEA1791
n.c.
GND
8 V
CC
SRSENSE
4
R2
3
R3
2
1
GND
Output voltage regulation feedback stage
CNX82A
TL431
CY
014aaa729
Fig 1.
Example of the TEA1791T in a 90 W adapter
For the relevant component values see Table 1 “Bill of materials” in Section 5.2.
AN10781_1
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Rev. 01 — 09 February 2009
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AN10781
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GreenChip SR integrated synchronous rectifcation controller
3. Functional description and application
3.1 SR control
The TEA1791T uses the SRSENSE pin as an input sense in the control of the
drain-source voltage (VDS) of the MOSFETs. No adjustment is necessary in the
SR-control.
The SR MOSFET is switched on when the drain voltage sensed by pin SRSENSE is
under −310 mV. When the sensed voltage has reached −55 mV, i.e. ID × RDSon, the driver
output will reduce the gate voltage to set the feedback level that regulates the −55 mV
level.
When, at very low drain current, the voltage at the SRSENSE pin rises above
−12 mV (typ), the SR MOSFET is switched off.
If the secondary stroke duration of the flyback converter is less than 2 μs (typ), the driver
output is disabled. This will guarantee stable operation at very low duty cycles. The driver
output is enabled when the secondary stroke time is greater than 2.2 μs (typ).
In Standby mode synchronous rectification remains active as long as the secondary
stroke is less than 2 μs (typ).
For the best performance it is advised that SRSENSE senses the drain of the MOSFET
Q1 directly via the external series resistors. See Section 5.1.
It is not necessary to insert a MOSFET gate series resistor. However, if, because of
switching noise reduction, a gate series resistor is required, the switch-off state of the SR
MOSFET must be checked. Recheck at a high temperature of the MOSFET as well.
When the power MOSFET on the primary side switches on, the drain-source voltage of
the SR MOSFET rises with a high dV/dt. If dV/dt is steep, a capacitive current will flow
from drain to gate through the capacitor Cdg of the MOSFET. This current together with a
gate resistor lifts the gate voltage (VGS). But the voltage rise should remain well below the
threshold voltage (Vth) of the SR MOSFET to prevent switch-on. Therefore the gate series
resistor value is limited and 10 Ω is an indicative value.
3.2 Function of resistors in series with pin SRSENSE
All pins of TEA1791T are protected against ElectroStatic Discharge (ESD) to prevent IC
damage when handled. While operating in the application, ESD protection could possibly
be triggered by an ESD event taking place under testing for instance.
If the ESD protection on the SRSENSE pin is triggered, the pin will be pulled to ground by
the internal ESD protection component. As SRSENSE senses the drain voltage of the
MOSFET, this pin should be protected by a series resistor that limits a possible surge
current initiated by a severe ESD event. Therefore the current limiting resistors R2 and R3
are applied. See Figure 1. A total resistance value of 1 kΩ is sufficient to protect pin
SRSENSE. Two SMD 1206 (see Section 5.2) resistors in series are used to obey the
peak-power rating.
3.3 VCC
Vstartup is 8.5 V typical and the VCC stop voltage is 8.0 V (typ). If necessary, a capacitor C2
can be placed between pin VCC and pin GND to smooth the supply voltage.
AN10781_1
Application note
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Rev. 01 — 09 February 2009
4 of 8
AN10781
NXP Semiconductors
GreenChip SR integrated synchronous rectifcation controller
3.4 VCC auxiliary supply
The IC is supplied by an auxiliary secondary winding which is stacked on the main power
output winding. The IC supply voltage should be greater than 12 V to get full driver output
capability. A supply of circa 15 V can be targeted and is set by the turns ratio between the
power output windings and the AUX windings.
Ns2
V CC = ---------- ⋅ V out – 0.7 V
Ns1
The average IC supply current depends on the dynamic gate charge transfer
characteristic of the applied MOSFET. The gate charge cycle will be under the conditions
of a 10 V gate drive amplitude and a VDS of less than 1 V.In the application example the
MOSFET PSMN015-100P is used. Its total gate charge over 10 V is 75 nC. So at a
switching frequency of 100 kHz, the supply current used for driving is 7.5 mA;
75 nC × 100 kHz. The IC itself uses only 0.95 mA.
3.5 Driver output
Between 0 V and the UnderVoltage LockOut (UVLO), an active current sink circuit keeps
the external MOSFET in the off state.
4. Meeting EMC requirements
By this circuit outline in which the synchronous rectifier MOSFET is placed in the positive
output voltage path, the ElectroMagnetic Compatibility (EMC) requirements can be met by
using a single y-capacitor between the grounds of the primary and secondary buffer
capacitors.
AN10781_1
Application note
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Rev. 01 — 09 February 2009
5 of 8
AN10781
NXP Semiconductors
GreenChip SR integrated synchronous rectifcation controller
5. Appendix
5.1 Layout considerations
The IC ground copper track should be as wide and as low ohmic as possible. The
IC ground is used as reference by the internal circuits, but it also shares the high driver
output current pulses.
The IC ground is used in the control of the SR MOSFET. Direct the IC ground track close
to the MOSFET source and position the IC near the MOSFET.
The SRSENSE pin should be directly connected to the drain pin of the SR MOSFET via
the series resistors. Make the sense track a separate one. This is good practice in proper
sensing and regulating the MOSFET drain-source voltage.
5.2 Bill of materials
Table 1.
Bill of materials
Relevant component values for a 19.5 V / 4.62 A application with the TEA1791T.
Description
Position
Resistor SMD 0805: Thin Film Chip, 47 Ω, 5 %
R1
Resistor SMD 1206: Thin Film Chip, 560 Ω, 5 %
R2, R3
MLCC SMD 0805: 10 mF / 50 V, Y5V
C1
MLCC SMD 0805: 0.1 μF / 50 V, X7R
C2
MLCC SMD 0805: 220 pF / 100 V, X7R
C3
Switching diode, SMD SOD-80: 0.2 A / 100 V, LL4148
D1
MOSFET, TO-220: 75 A / 100 V, 0.015/220p-typ,
PSMN015-100P/NXP
Q1
AN10781_1
Application note
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Rev. 01 — 09 February 2009
6 of 8
AN10781
NXP Semiconductors
GreenChip SR integrated synchronous rectifcation controller
6. Legal information
6.1
Definitions
Draft — The document is a draft version only. The content is still under
internal review and subject to formal approval, which may result in
modifications or additions. NXP Semiconductors does not give any
representations or warranties as to the accuracy or completeness of
information included herein and shall have no liability for the consequences of
use of such information.
6.2
Suitability for use — NXP Semiconductors products are not designed,
authorized or warranted to be suitable for use in medical, military, aircraft,
space or life support equipment, nor in applications where failure or
malfunction of a NXP Semiconductors product can reasonably be expected to
result in personal injury, death or severe property or environmental damage.
NXP Semiconductors accepts no liability for inclusion and/or use of NXP
Semiconductors products in such equipment or applications and therefore
such inclusion and/or use is at the customer’s own risk.
Applications — Applications that are described herein for any of these
products are for illustrative purposes only. NXP Semiconductors makes no
representation or warranty that such applications will be suitable for the
specified use without further testing or modification.
Disclaimers
General — Information in this document is believed to be accurate and
reliable. However, NXP Semiconductors does not give any representations or
warranties, expressed or implied, as to the accuracy or completeness of such
information and shall have no liability for the consequences of use of such
information.
Right to make changes — NXP Semiconductors reserves the right to make
changes to information published in this document, including without
limitation specifications and product descriptions, at any time and without
notice. This document supersedes and replaces all information supplied prior
to the publication hereof.
6.3
Trademarks
Notice: All referenced brands, product names, service names and trademarks
are the property of their respective owners.
GreenChip — is a trademark of NXP B.V.
AN10781_1
Application note
© NXP B.V. 2009. All rights reserved.
Rev. 01 — 09 February 2009
7 of 8
AN10781
NXP Semiconductors
GreenChip SR integrated synchronous rectifcation controller
7. Contents
1
2
3
3.1
3.2
3.3
3.4
3.5
4
5
5.1
5.2
6
6.1
6.2
6.3
7
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Application schematic TEA1791T. . . . . . . . . . .
Functional description and application . . . . . .
SR control . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Function of resistors in series with
pin SRSENSE. . . . . . . . . . . . . . . . . . . . . . . . . .
VCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
VCC auxiliary supply . . . . . . . . . . . . . . . . . . . . .
Driver output . . . . . . . . . . . . . . . . . . . . . . . . . . .
Meeting EMC requirements. . . . . . . . . . . . . . . .
Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Layout considerations. . . . . . . . . . . . . . . . . . . .
Bill of materials . . . . . . . . . . . . . . . . . . . . . . . . .
Legal information. . . . . . . . . . . . . . . . . . . . . . . .
Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Disclaimers . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Trademarks. . . . . . . . . . . . . . . . . . . . . . . . . . . .
Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
3
4
4
4
4
5
5
5
6
6
6
7
7
7
7
8
Please be aware that important notices concerning this document and the product(s)
described herein, have been included in section ‘Legal information’.
© NXP B.V. 2009.
All rights reserved.
For more information, please visit: http://www.nxp.com
For sales office addresses, please send an email to: [email protected]
Date of release: 09 February 2009
Document identifier: AN10781_1
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