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Texas Instruments Analysis of Starting Up Performance for UCC28700 Application notes
Application Report
SLUA706 – June 2014
Analysis of Start-Up Performance for UCC28700
Max Han
Power Management/Field Application
ABSTRACT
The UCC28700 device is a primary controlled fly-back power supply controller, which
provides both constant voltage and constant current regulation. The device has high
resolution for voltage and current regulation, and it has very low no-load power
consumption and good start-up performance. As a result, the UCC28700 device is highly
suited for low-power adapter and auxiliary power supply application. Compared with other
competitors, the UCC28700 device has better performance and needs a smaller VDD
capacitor. A customer may experience a situation in which the UCC28700 device cannot
start a constant current full load, but can start at resistance full load. The real reason is
that the value of the VDD capacitor is not sufficient and primary peak current is designed
too small. This paper analyzes the design of primary peak current and VDD capacitor. An
experiment result validates the theoretical analysis.
Contents
1. Introduction ...................................................................................................................................... 2
2. Analysis ............................................................................................................................................ 2
3. Design ............................................................................................................................................... 4
4. Experiment ....................................................................................................................................... 6
5. Conclusion ....................................................................................................................................... 8
6. References........................................................................................................................................ 8
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figures
UCC28700 Application Circuit ........................................................................................ 2
UCC28700 Secondary Side Circuit ................................................................................. 3
Transformer Current ........................................................................................................ 4
UCC28700 Start-up Waveform ........................................................................................ 6
UCC28700 Start-up Waveform with CDD = 4.7 µF, RCS = 2.05 Ω ...................................... 7
UCC28700 Start-up Waveform with CDD = 4.7 µF, RCS = 1.8 Ω ........................................ 7
UCC28700 Start-up Waveform with CDD = 1 µF, RCS = 1.8 Ω ........................................... 8
Table 1.
Tables
Parameters Comparison Table ....................................................................................... 6
1
SLUA706
1. Introduction
The UCC28700 device is a constant voltage, constant current fly-back controller with primary
side regulation without the use of an optical coupler. Figure 1 shows the UCC28700 application
circuit.
Figure 1.
UCC28700 Application Circuit
In Figure 1:
•
RSTR is high-voltage start-up resistance.
•
CDD is energy storage capacitor on the VDD pin.
•
RS1 is high-side feedback resistance.
•
RS2 is low-side feedback resistance.
•
RCBC is programming cable compensation resistance.
•
RCS is primary peak current programming resistance.
•
RLC is MOSFET turn-off delay compensation programming resistance.
Primary peak current is a critical factor for the UCC28700 starting up at constant current full load.
The following section provides a detailed analysis.
2. Analysis
Figure 2 shows the UCC28700 secondary side circuit, I=
I C + I L . If the load of the UCC28700
S
device is resistance at the beginning of startup, VO ramps up from zero and IL is low enough so
that high IS is not required. However, if the load of the device is constant current and the load
current is large, high IS is required to make IC positive and to reduce the time that output voltage
ramps up from 0 to VOCC. VOCC is the target lowest converter output voltage, which makes
auxiliary turn voltage equal to UVLO turn-off voltage on the VDD pin.
2
Analysis of Start-Up Performance for UCC28700
SLUA706
IS
D
VS
Figure 2.
IL
IC
CO
VO
UCC28700 Secondary Side Circuit
To CDD, CO, and the transformer, the following equations are achieved. In Equation 4, a 1-mA
current margin is provided.
NOTE: NP is primary turns of transformer, NS is secondary turns, and NA is auxiliary turns.
I C= I S − I L
VOCC = VDD ( off )
ta = CO
(1)
NS
NA
(2)
VOCC
IC
(3)
( I run + 1mA)
∆VDD =
ta
CDD
(4)
=
VDD VDD ( on ) − ∆VDD
(5)
Where:
•
VDD(off) is UVLO turn-off voltage.
•
VDD(on) is UVLO turn-on voltage.
•
Irun is supply current on the VDD pin when UCC28700 works.
•
VDD is the voltage of CDD.
•
ΔVDD is the decreased voltage on CDD.
•
ta is the time that output voltage ramps up from 0 to VOCC.
According to the preceding equations, if the value of IS is low, IC will be small, so ta becomes a
long time when output voltage ramps up to VOCC; however, during this period, VDD may decrease
below VDD(off) and the UCC28700 device may enter UVLO state and stop switching. Then the
current through RSTR charges CDD; when VDD is higher than VDD(on), the device restarts. Although
faulty startup continues, the UCC28700 device cannot enter normal state.
In Equation 4, if CDD is large enough, ΔVDD will be small for certain ta. So, both a large value CDD
and a high primary peak current can make the UCC28700 device start well. However, large
value CDD means higher price and larger size, and high primary peak current increases power
loss and increases transformer size. Consequently, choosing CDD and primary peak current is a
trade-off.
Analysis of Start-Up Performance for UCC28700
3
SLUA706
In normal operation, auxiliary winding voltage dominates VDD. If VO reaches its maximum value,
VDD will also match its maximum value. The relation is shown in Equation 6.
VDD max = VO max
NA
NS
(6)
From Equation 2, 3, and 6, if NA increases, ta is reduced, that’s good for UCC28700 starting up.
So a large value should be chosen for NA, and it also must provide voltage margin for VDD.
3. Design
All device values, except CDD and RCS, are the same as the UCC28700EVM-068 5-W USB
adapter [1] schematic. Figure 3 is shown in the UCC28700 data sheet [2]. IS is deduced as
Equation 7, where ηXFMR is estimated transformer efficiency.
Transformer efficiency is influenced by the core and winding losses, leakage inductance ratio,
and bias power ratio to rated output power. For a 5-V, 1-A charger example, bias power of 1.5%
is a good estimate [1]. An overall transformer efficiency of 0.9 is an approximate estimate to
include 3.5% leakage inductance, 5% core and winding loss, and 1.5% bias power [1].
Maximum primary peak current IPP is achieved at the beginning of startup, and the UCC28700
device enters constant current regulation with maintaining constant secondary diode conduction
duty cycle, 0.425.
The transformer is WE 750312723 on EVM, NP/NS = 15.33, NP/NA = 3.83, and saturation current
is 440 mA.
Figure 3.
IS =
4
Transformer Current
I PP N P t DM
η XFMR
2 N S TSW
Analysis of Start-Up Performance for UCC28700
(7)
SLUA706
At the beginning of startup, average charging current of output capacitor is positive, the charge
current equals (IS-IL) as Equation 1 shows. Before VO ramps up to VOCC, the auxiliary turn voltage
is lower than VDD, and CDD cannot be charged by auxiliary turns. However, CDD is discharged by
Irun and gate drive current during this period; if VDD is lower than VDD(off), the UCC28700 device
shuts down. To ensure the device starts up well, VDD must be larger than VDD(off) during ta. In
Equations 8 and 9, a critical condition is applied; tstart is the time that VO ramps up from 0 to VOCC.
Equation 2 shows the relation of VOCC and VDD(off). An estimated 1 mA of gate-drive current exists
in Equation 8 and 1 V of margin is added to VDD. VCST is chip select threshold voltage. At the
beginning of startup, voltage on the UCC28700 VS pin is low, so VCST stays at its maximum
value.
CDD =
tstart ( I run + 1mA)
(VDD ( on ) − VDD ( off ) ) − 1V
(8)
tstart =
COVOCC
IS − IL
(9)
RCS =
VCST
I PP
(10)
In the UCC28700 device, VDD(on) = 21 V, VDD(off) = 8.1 V, so according to Equation 2, VOCC = 2.02
V. Irun = 2.1 mA in data sheet, CDD is chosen as 4.7 µF, tstart = 18.04 ms is deduced by Equation 8.
UCC28700 EVM is 5-V, 1-A adapter, so full load current IL is 1 A. Output capacitors are two
paralleled 560-µF capacitors, so CO = 1120 µF. tstart, IL, and CO are substituted into Equations 7
and 9, IPP is obtained as 383.85 mA. From Equation 10, RCS = 1.95 Ω. To add margin on VDD,
1.8 Ω is selected for RCS.
As shown in Table 1, the UCC28700 device has better constant current (CC) regulation
performance; a higher max operation frequency, which can minimize the solution size; standby
power is less than 30 mW, which is for 5-star rating; and higher max VDD, which can reduce VDD
capacitor value. Of the three products highlighted in Table 1, the UCC28700 device is the best
choice when designing 5-V adapters. The UCC28700 device can choose higher NA/NS because
it has higher max VDD according to Equation 2, and smaller tstart is achieved (see Equation 9). In
Equation 8, tstart is proportional to CDD, so smaller CDD is required during design.
Analysis of Start-Up Performance for UCC28700
5
SLUA706
Table 1.
Product Number
Parameters Comparison Table
UCC28700
OB2520M
iW1680
CV (constant voltage)
5%
5%
null
CC (constant current)
5%
6%
null
Max Operation Frequency
130 kHz
100 kHz
72 kHz
Standby Power
<30 mW
<200 mW
<30 mW
38 V
28 V
25 V
Max VDD
4. Experiment
To validate the preceding analysis, a UCC28700EVM-068 5-W USB adapter is used. All device
values are kept the same except CDD and RCS, CDD = 4.7 µF, RCS = 1.8 Ω. The load is constant
current as 1 A.
Figure 4 is a UCC28700 start-up waveform. CH1 is a MOSFET gate-drive signal and CH3 is
output voltage. The device starts up smoothly, with no overshoot and audible noise. The figure
shows the UCC28700 device has a very good start-up performance. In Figure 4, tstart
approximates 18 ms, which meets the calculated result.
tstart
Figure 4.
UCC28700 Start-up Waveform
Figure 5, Figure 6, and Figure 7 represent a compared experiment. CH1 is VDD voltage and CH3
is output voltage.
6
Analysis of Start-Up Performance for UCC28700
SLUA706
•
In Figure 5, CDD = 4.7 µF, RC S= 2.05 Ω: because primary peak current is not large enough,
VDD decreases below VDD(off), thus the UCC28700 device cannot start.
•
In Figure 6, CDD = 4.7 µF, RCS = 1.8 Ω: primary peak current is increased, so a good start-up
performance is observed.
•
In Figure 7, CDD = 1 µF, RCS = 1.8 Ω: the UCC28700 device cannot start because CDD is not
large enough to provide sufficient energy.
The results of the experiment reveal that both high primary peak current and large volume CDD
can make the UCC28700 device start successfully at constant current full load. These findings
verify the preceding analysis.
Figure 5.
UCC28700 Start-up Waveform with CDD = 4.7 µF, RCS = 2.05 Ω
Figure 6.
UCC28700 Start-up Waveform with CDD = 4.7 µF, RCS = 1.8 Ω
Analysis of Start-Up Performance for UCC28700
7
SLUA706
Figure 7.
UCC28700 Start-up Waveform with CDD = 1 µF, RCS = 1.8 Ω
5. Conclusion
Comparison results indicate the UCC28700 device has better performance in CV and CC
regulation, solution size, standby power, and VDD capacitor value. In the course of this study,
primary peak current and VDD capacitor were analyzed and calculated. Proper parameters were
chosen according to equations, and the analysis was verified by experiment results.
6. References
[1] UCC28700EVM-068 5-W USB Adapter. Texas Instruments User’s Guide, SLUU968,
July 2012
[2] Constant-Voltage, Constant-Current Controller With Primary-Side Regulation. Texas
Instruments UCC2870x data sheet, SLUSB41, July 2012
8
Analysis of Start-Up Performance for UCC28700
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