UCC28704 Datasheet, PDF (34/44 Pages) Texas Instruments – High-Efficiency Off-Line CV

UCC28704
SLUSCA8A – FEBRUARY 2016 – REVISED FEBRUARY 2016
www.ti.com
9 Power Supply Recommendations
The UCC28704 is intended for AC-to-DC adapters and chargers with universal input voltage range of 85 V_RMS to
265 V_RMS, 47 Hz to 63 Hz, using flyback topology. It can also be used in other applications and converter
topologies with different input voltages. Be sure that all voltages and currents are within the recommended
operating conditions and absolute maximum ratings of the device.
10 Layout
10.1 Layout Guidelines
In order to increase the reliability and feasibility of the project it is recommended to adhere to the following
guidelines for PCB layout. In Figure 32, a typical 5-V/2-A USB adapter design schematic is shown in Figure 32.
• Minimize stray capacitance on the VS node. Place the voltage sense resistors (R_S1 and R_S2 in Figure 24)
close to the VS pin.
• Arrange the components to minimize the loop areas of the switching currents as much as possible. These
areas include such loops as the transformer primary winding current loop (a), the MOSFET gate-drive loop
(b), the primary snubber loop (c), the auxiliary winding loop (d) and the secondary output current loop (e). In
practice, trade-offs may have to be made. Loops with higher current should be minimized with higher priority.
As a rule of thumb, the priority goes from high to low as (a) – (e) – (c) – (d) – (b).
• The R_LC resistor location is critical. To avoid any dv/dt induced noise (for example MOSFET drain dv/dt)
coupled onto this resistor, it is better to place R_LC closer to the controller and avoid nearby the MOSFET.
• To improve thermal performance increase the copper area connected to GND pins.
L
V_AC
N
+
C_B1
D_IN
V_AUX
N_A
D2
R_S1
V_BLK
C_B2
(a)
R_STR
R_SN1
C_{SN1 (C)}
T₁
D1
N_P
N_{S (e)}
C_OUT
+
R_PL VOUT
-
D_SN1
R_SN2
Optional, short
across if not used
UCC28704
2 VDD
C_DD
DRV 3
Q1
R_G1
R_S2 ^(d)
-t°
6 VS
(B) _RLC
CS 4
1 NTC/SU
R_CS
R_NTC
GND
5
Isolation
Boundary
C_Y
Figure 32. 10-W, 5-V/2-A USB Adapter Schematics
34
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