Texas Instruments | Charging Supercapacitor Banks from USB Type C Port With Boost Converter | Application notes | Texas Instruments Charging Supercapacitor Banks from USB Type C Port With Boost Converter Application notes

Texas Instruments Charging Supercapacitor Banks from USB Type C Port With Boost Converter Application notes
Application Report
SLVA981 – July 2018
Charging Supercapacitor Banks from USB Type C Port
With Boost Converter
Helen Chen .............................................................................. Boost Converter and Controller Solutions
ABSTRACT
The maximum allowed output current from the USB type C port is 5V3A by default. Most of the boost
converter cannot limit the input current below 3A when charging big capacitance. Because when charging
a big capacitance, it will take a relatively long time before the output voltage reaches the input voltage.
The boost converter works with maximum duty. So the input current is uncontrollable and will rise to a
very high value during this period. This application note gives out a simple, reliable and low cost solution
for the big capacitance charge circuit. It is especially fit for the USB type C port supplied system as the
input current can be well limited below 3A. The charging speed can be adjusted by using different fly-back
transformers. It is also fit for some critical applications where the input current need to be limited below
200-500mA by choosing a small current rating boost converter.
1
2
3
Contents
Introduction ................................................................................................................... 1
Device Overview ............................................................................................................. 2
Test Results .................................................................................................................. 3
List of Figures
1
Application Schematic....................................................................................................... 2
2
Waveforms of VO, IIN at NP/NS=0.25 ...................................................................................... 3
3
Waveforms of VO, IIN at NP/NS=0.6
4
Waveforms of VO and the Charge Current into the Supercapacitor (NP/NS=0.25) ................................. 3
5
Waveforms of VO and the Charge Current into the Supercapacitor (NP/NS=0.6) ................................... 3
.......................................................................................
3
List of Tables
1
Performance Specification Summary
.....................................................................................
2
Trademarks
All trademarks are the property of their respective owners.
1
Introduction
There are various methodologies on how to charge a big cap like super capacitor (SC). Constant current
and constant voltage (CICV) is one of the most commonly used and preferred solution. At the beginning of
the charging cycle, the charging device charges the SC with a constant current. When the SC is charged
to the target value, constant voltage loop becomes active and prevent the SC from over charging. This
kind of charging IC, especially the boost charging IC is less common and much expensive than the normal
boost converter.
This application note delivers a very simple and low cost solution on how to charge the SC banks from a
USB type C port. By choosing an appropriate boost converter and connecting it in the fly-back topology,
the input current can be effectively limited below 3A. The maximum charge current into the SC banks is
also limited below a certain value, which can reduce the heat generation and extend the life of SC.
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Charging Supercapacitor Banks from USB Type C Port With Boost Converter
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1
Device Overview
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2
Device Overview
2.1
Specification
Table 1 shows the specification of this application report. The maximum input current is limited below 3A
during the startup and during the normal operation.
Table 1. Performance Specification Summary
PARAMETER
2.2
TEST CONDITIONS
VIN
Input Voltage
IIN_peak
Peak Input Current
Anytime during charging the SC banks
VSC_target
The voltage across the SC
banks after fully charged
VIN=5V
MIN
TYP
MAX
UNIT
4.5
5.0
5.5
V
3
A
13.5
V
Schematic
D3
T2
1
MBR130T1G
Vout1
6
1
2
3
J11
3
2
1
Vin1
3
2
C12
10µF
C11
100µF
J12
R13
220
C14
0.1µF
J13
4
NC
NC
5
750343874
C15
10µF
3
2
1
R11
158k
D4
GND1
C17
4700µF
C16
10µF
1
2
3
GND1
U2
GND1
2
1
SH-JP11
8
IN
9
FREQ
3
JP11
SS1 10
COMP1 1
SW
SW
FB
6
7
SW1
2
FB1
GND1
GND1
EN
SS
COMP
TP
AGND
PGND
11
4
5
R12
16.2k
TPS61087DRCR
0.1µF
C13
J14
GND1
Rc1
39k
GND1
Cc1
3300pF
GND1
GND1
GND1
Figure 1. Application Schematic
Figure 1 shows the schematic for this application report. The circuit is of fly-back topology. The boost
converter, TPS61087, always works at the current-limit mode during start-up before the output voltage
reaches the target value. The peak primary inductor current is limited at the peak current limit point of
TPS61087, which is around 4A. Accordingly, the peak secondary inductor current is limited at the primary
current times the transformer turns ratio. After the output capacitor is fully charged, the TPS61087 device
works in the skip mode.
2.3
Parameter Calculation
The maximum allowable turns ratio NP/NS is determined by the input voltage (VIN), output voltage (VO),
and the maximum switch node voltage (VSWmax).
NP VSWmax F VIN
Nmax =
=
NS
VO
where
•
•
2
VO=13.5V
VIN=5V
(1)
Charging Supercapacitor Banks from USB Type C Port With Boost Converter
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Test Results
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If we set the maximum switch node voltage VSWmax at 14V, which is 70% of the absolute maximum rating
voltage, then the maximum turns ratio Nmax= 0.65. So the turns ratio of the flyback transformer should be
lower than this value to meet the design target. Larger inductance means larger inductor size. In AN
SLVA981, TPS61087 works in the DCM and CCM boundary when the output voltage reaches the target
value. So the inductance will be neither too large nor too small.
If NP/NS=0.25, the maximum duty cycle Dmax1 can be calculated with Equation 2.
0.25 ® VO
Dmax1 =
N 0.44
0.25 ® VO + š ® Vin
where
•
ɧ is the conversion efficiency, ɧ=0.85
If NP/NS=0.6, the maximum duty cycle, Dmax2, can be calculated with Equation 3.
0.6 ® VO
Dmax2 =
N 0.66
0.6 ® VO + š ® Vin
(2)
(3)
As the TPS61087 works in the current limit mode, so the maximum average input current under different
duty cycle is different. The typical current limit value ILIM of TPS61087 is 4A according to the datasheet. So
the maximum average input current under different turns ratio can be calculated by the following equation:
Dmax1
IIN _1 = ILIM ®
= 0.88A
2
(4)
IIN _2 = ILIM ®
Dmax2
= 1.32A
2
(5)
The current limit value ILIM has ±20% tolerance, so the average input current will also has ±20% tolerance.
The secondary current is determined by the primary current limit ILIM times the turns ratio. So the maximum
charge current under different turns ratio is:
Icharge _sup1 = ILIM ® 0.25 = 1A
(6)
Icharge _sup 2 = ILIM ® 0.6 = 2.4A
(7)
So the transformer turns ratio can be chosen according to the maximum current rating of the
supercapacitor.
3
Test Results
Figure 2 and Figure 3 show the waveforms of the output voltage VO and the input current IIN during
charging a 70mF cap. When the turns ratio is 0.25, the charging period is around 2.2 seconds; when the
turns ratio is 0.6, the charging period is around 1.2 seconds.
Figure 4 and Figure 5 show the waveforms of the current flowing into the output cap during charging up.
The charging current is larger with a larger turns ratio transformer as shown in these waveforms.
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3
Test Results
4
www.ti.com
Figure 2. Waveforms of VO, IIN at NP/NS=0.25
Figure 3. Waveforms of VO, IIN at NP/NS=0.6
Figure 4. Waveforms of VO and the Charge Current into the
Supercapacitor (NP/NS=0.25)
Figure 5. Waveforms of VO and the Charge Current into the
Supercapacitor (NP/NS=0.6)
Charging Supercapacitor Banks from USB Type C Port With Boost Converter
Copyright © 2018, Texas Instruments Incorporated
SLVA981 – July 2018
Submit Documentation Feedback
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