tea1781 NXP Semiconductors, tea1781 Datasheet - Page 7

• Manufacturer Part Number: tea1781
• Description: Greenchip Pc Secondary Control Ics
• Manufacturer: NXP Semiconductors

NXP Semiconductors
TEA1781_82_1
Preliminary data sheet
7.2 Supply
The freewheel switch behaves like an ideal diode so the setup runs in a discontinuous
mode at low loads. Negative currents are avoided by the control ICs through this switch.
Replacement of the rectifying Schottky diode by two switches in anti-series allows:
The supply voltages can be individually and more accurately regulated, eliminating
ubiquitous post regulators.
To fulfill the ATX12V specification on standby efficiency requirements, the system has a
Standby mode in which the frequency, duty cycle and supply currents are reduced.
The TEA1781T and TEA1782T are secondary control ICs. Together with the TEA1771T
primary controller they form a unique system that fulfills the 80+ gold requirements at
minimum costs, eliminating any cross regulation and maximizing standby efficiency.
When the voltage on the primary side from the rectified input mains reaches its starting
value, the primary side starts up and switches the primary main switch. From that
moment, the secondary rectified transformer voltage at V
the input voltage.
Fig 5.
•
•
•
Integration of the 5 V standby supply, because in Standby mode the main secondary
outputs can be switched off.
As the output voltages is now regulated by the secondary switches, the duty cycle of
the primary control switch can be kept constant, preventing ringing at the primary
reset voltage. A lower reset voltage results in lower required breakdown voltages of all
applied FETs, both primary and secondary. This reduces costs.
The use of Zero Voltage Switching (ZVS) at the primary side to fulfill the required
standby efficiency.
Start-up sequence TEA1771T
TEA1781
Start-up
Rev. 01 — 13 February 2009
VSTARTUP
VDD
TEA1781T; TEA1782T
VDD
GreenChip PC secondary control ICs
V
startup(VDD)
startup
TEA1782
Start-up
12 V
, see
UVLO
014aaa185
V
Figure
VOUT_STB
© NXP B.V. 2009. All rights reserved.
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