UCC28720_15 Datasheet, PDF (17/33 Pages) Texas Instruments – UCC28720 Constant-Voltage, Constant-Current Controller With Primary-Side Regulation

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UCC28720
SLUSBE8B – MAY 2013 – REVISED SEPTEMBER 2015
Typical Application (continued)
8.2.1 Design Requirements
The design parameters are listed in Table 1.
Table 1. Design Parameters
PARAMETER
INPUT CHARACTERISTICS
V_IN
RMS Input Voltage
f_LINE
Line Frequency
P_{SB_CONV} No Load Input Power
V_IN(RUN)
Brownout Voltage
OUTPUT CHARACTERISTICS
V_OCV
V_RIPPLE
I_OUT
Output Voltage
Output Voltage Ripple
Output Current
Output OVP
Transient Response
Load Step (I_TRAN = 0.6 A)
SYSTEMS CHARACTERISTICS
f_MAX
Switching Frequency
Full Load Efficiency (115/230 V RMS
ƞ
input)
TEST CONDITIONS
V_IN = Nom, I_OUT = 0 A
I_OUT = Nom
V_IN = Nom, I_OUT = NOM
V_IN = Nom, I_O = Max
V_IN = Min to Max
I_OUT = Min to Max
(0.1 to 0.6 A) or (0.6 to 0.1 A)
V_OΔ= 0.9 V for Calculations
I_OUT = 1 A
MIN NOM MAX UNIT
100 (V_IN(MIN)) 115/230
47 50/60
70
240 V
64 Hz
10 mW
V
4.75
5 5.25 V
0.1 V
1 1.05 A
5.75
V
4.1
5
5V
70
kHz
74%
8.2.2 Detailed Design Procedure
This procedure outlines the steps to design a constant-voltage, constant-current flyback converter using the
UCC28720 controller. Refer to the Figure 20 for component names and network locations. The design procedure
equations use terms that are defined below.
8.2.2.1 Stand-by Power Estimate
Assuming no-load stand-by power is a critical design parameter, determine estimated no-load power based on
target converter maximum switching frequency and output power rating.
The following equation estimates the stand-by power of the converter.
P_{SB _ CONV}
=
P_OUT ´ f_MIN
h_SB
´
K
2
AM
´
f_MAX
(7)
For a typical USB charger application, the bias power during no-load is approximately 2.5 mW. This is based on
25-V VDD and 100-µA bias current. The output preload resistor can be estimated by V_OCV and the difference in
the converter stand-by power and the bias power. The equation for output preload resistance accounts for bias
power estimated at 2.5 mW.
R_PL
=
V_OCV ²
P_{SB _ CONV} - 2.5 mW
(8)
The capacitor bulk voltage for the loss estimation is the highest voltage for the stand-by power measurement,
typically 325 V_DC.
For the total stand-by power estimation add an estimated 2.5 mW for snubber loss to the converter stand-by
power loss.
P_SB = P_{SB _ CONV} + 2.5 mW
(9)
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