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Texas Instruments Non-Isolated High-Side Buck Converter With UCC28910 Application notes
Application Report
SNVA750 – June 2016
Non-Isolated High-Side Buck Converter with UCC28910
David Ji
ABSTRACT
The non-isolated Buck topology is widely applied in the LED driver and low power products. A buck
converter can obtain smaller size and fewer components compared to a flyback. This paper discusses the
Buck converter design steps and theoretical analysis with wide input voltage range. It is practical to solve
the limit of input voltage hysteresis by adding a few components. Electrical performance is tested and
presented.
1
2
3
4
5
6
Contents
Introduction ................................................................................................................... 1
Design Parameters Consideration ......................................................................................... 2
Test Result Waveform ...................................................................................................... 7
Layout Suggestion ......................................................................................................... 12
Conclusion .................................................................................................................. 12
Reference ................................................................................................................... 12
List of Figures
1
1
Schematic ..................................................................................................................... 2
2
Buck Converter Function Diagram
3
Bulk Capacitor Ripple Voltage ............................................................................................. 4
4
Add Resistor Solution Schematic .......................................................................................... 5
5
Add Diode and Resistor Schematic ....................................................................................... 6
6
MOSFET Voltage Stress.................................................................................................... 7
7
Efficiency Comparison ...................................................................................................... 7
8
Standby Power ............................................................................................................... 8
9
Dynamic Response .......................................................................................................... 8
10
115 Vin Output Ripple Voltage at No Load and Full Load ............................................................. 9
11
230 Vin Output Ripple Voltage at No Load and Full Load ............................................................. 9
12
85 Vin Thermal Performance ............................................................................................. 10
13
230 Vin Thermal Performance............................................................................................ 10
14
230 Vin EMI Performance ................................................................................................. 11
15
115 Vin EMI Performance ................................................................................................. 11
........................................................................................
2
Introduction
This paper is based on a non-isolated high-side Buck design using the UCC28910. The UCC28910
integrates an internal 700-V MOSFET, which enables universal AC input operation. It is also easier to use
fewer components through the internal, integrated control loop compensation. Fewer components make a
low-cost solution possible. The UCC28910 uses peak current control and DCM mode operation only,
which avoids the extra slope compensation requirement. Also, the IC integrates various protection
features to meet more application requirements. This topology using an inductor can save size and lower
cost compared to a flyback transformer. It can also achieve higher efficiency compared to HV LDO
topology. Therefore, non-isolated high-side Buck is suitable for limited size and cost applications.
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1
Design Parameters Consideration
2
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Design Parameters Consideration
Specifications
Input voltage: 85 Vac–265 Vac. Output voltage: 20 V, Output current: 100 mA
Requirements: When input voltage is in the brownout condition, output voltage will not bounce.
Output ripple voltage < 200 mV
Dynamic response requirements: < +1 V / –1 V
UCC28910 operation scheme in Buck mode:
As the input voltage increases, the UCC28910 internal high voltage current source charges the VDD
capacitor and output capacitor. When the VDD reaches the startup threshold, the internal MOSFET turns
on for three pulses. These three pulses detect the line brownout, output short circuit, and output over
voltage conditions to prevent converter operation in a fault condition. The initial three current pulses keep
the MOSFET running with one-third maximum current limit. Each switching cycle, when the integrated
MOSFET turns off, freewheel diode forms the current flowing path for the output inductor.
D3
US1G-E3/5AT
R2
336 lQ
L1
1 mH
5
6
VS
IPK
GND
VDD
GND
8
DRAN
GND
4
3
2
R4
1.30 lQ
1
C8
100 pF
C6
0.1 µF
R3
84.5 lQ
C7
47 µF
L2
220 µH
RF1
RV1
AC
D1
RH06
C1
0.1 µF
D2
US1G-E3/5AT
C2
10 µF
C4
0.1 µF
C3
100 µF
R1
20.0 kQ
Copyright © 2016, Texas Instruments Incorporated
Figure 1. Schematic
UCC28910
IVSL
VS
DRAIN
R2
R1
0.25-V
Clamp
GND
VO
Vin
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Figure 2. Buck Converter Function Diagram
2
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Design Parameters Consideration
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2.1
Output Inductor
Considering the output inductor size and output capacitance, it is desired to set the controller switching
frequency to 60 kHz. Discontinuous conduction mode (DCM) with valley switching is used to reduce
switching losses. Assuming the converter is working in the boundary conduction mode at lowest input
voltage and highest load condition, the output inductor value is designed by the following equations:
Vout
D=
Vbrownout
where
•
•
Vbrownin = 72 V
Vbrownout = 43 V
Vout = L ´
(1)
I pk
(1 - D) T
where
•
L = 290 µH
(2)
To keep the converter working in DCM, TDK B82464P4224M000 is selected. The saturation current is
0.75 A.
2.2
Output Capacitor
Output capacitance is a key factor to transient response, output voltage ripple, and system stability. Due to
the internal compensation of the UCC28910, the loop gain and phase cannot be obtained through
measurements. However, the load transient test can be used to verify the loop stability. After verification, a
100-µF or more Aluminum capacitor is proved to allow stable operation.
2.3
VS pin: Divided Resistors
When MOSFET is off and inductor current comes to zero, the UCC28910 VS pin is used to detect the
output voltage. Select the voltage divider ration to meet equation Equation 3:
R + R3
Vout = VVSR ´ 2
R2
where
•
2.4
VVSR = 4.05 V
(3)
Startup Procedure
Through internal integrated high voltage current source, the current source charges the VDD capacitor
and output capacitor. When the VDD reaches the startup threshold, the internal MOSFET turns on for
three pulses. The controller sends out three pulses for detecting the UVLO, output short circuit, and output
overvoltage protection. The initial three current pulses keep in one-third maximum current limit.
2.5
Current Limit
Use Equation 4 to calculate the current limit.
V
I pk = CCR
R pk
where
•
VCCR = 540 V
(4)
To avoid the noise interference, the Ipk pin is preferred to be shorted to GND of the chip.
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Design Parameters Consideration
2.6
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Cin
For lower system cost, the half-waveform rectifier is used. However, it needs more input bulk capacitor
due to bigger ripple voltage on the bulk capacitor compared to the full wave rectifier. Regardless of
whether the designer uses a half wave or full wave rectifier, the line under voltage protection hysteresis
must be considered. During start up, the output voltage can bounce if the hysteresis is not enough.
Equation 5 is used to calculate Cin.
Vp
Vbulk
∆V
t1
t2
Figure 3. Bulk Capacitor Ripple Voltage
In the period: t1 + t2, the output power is provided by bulk capacitor Cin. To allow a 30-V line brownout
protection hysteresis to work, the voltage ripple on Cin must meet Equation 5:
2ù
Pout
1
é
(t1 + t 2 ) = Cmin ê Vp2 - Vp - DV ú
eff
2
ë
û
(
)
where
•
•
•
Eff = 0.75
Vp = 76 V
∆V = 30 V
(5)
The minimum input capacitor is calculated as 10 µF. The line brownout protection is calculated based on
Equation 6 and Equation 7:
Vbrownin = I VSL(run) ´ R2
(6)
Vbrownout = I VSL(stop) ´ R2 + Vout
(7)
Considering the IC parameter tolerances, two methods can be used to achieve wider line brownout
protection to allow the system to use smaller input capacitors.
4
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Design Parameters Consideration
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2.6.1
Add a Resistor Between VS and Output Capacitor Negative Terminal
A resistor can be added between the UCC28910 VS pin and the output ground as Figure 4 shows. The
UCC28910 measures the current flowing out of the VS pin to determine the input voltage. When the
internal MOSFET turns on, the IC senses both the current flowing through R2 and R5 and this current is
sensed as an indication of input voltage. During start up, output voltage is zero. After start up, the output
voltage stays high. Keeping R5 to ground adds some offset to the line brownout protection hysteresis.
D3
US1G-E3/5AT
R2
336 kQ
L1
1 mH
5
6
VS
VDD
IPK
GND
GND
8
DRAN
GND
4
3
2
1
R4
1.30 kQ
C8
100 pF
C6
0.1 µF
C7
47 µF
R5
336 kQ
R3
84.5 kQ
L2
220 µH
RF1
RV1
AC
D1
RH06
C1
0.1 µF
D2
US1G-E3/5AT
C2
10 µF
C4
0.1 µF
C3
100 µF
R1
20.0 kQ
Copyright © 2016, Texas Instruments Incorporated
Figure 4. Add Resistor Solution Schematic
Equation 8 through Equation 10 show the new brownout protection and recovery point calculation
equations:
Vbrownin = I VSL(run) ´ (R2 / / R5 )
æ
ö
V
Vbrownout = ç I VSL(stop) + out ÷ ´ (R2 / / R5 )
R2 ø
è
R + R3 / / R5
Vout = VVSR ´ 2
R3 / / R5
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(8)
(9)
(10)
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5
Design Parameters Consideration
2.6.2
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Add a Diode and a Resistor
Similar to adding s resistor, adding a diode and resistor separates the input voltage sensing and output
voltage sensing as shown in Figure 5. When the internal MOSFET is turned on, the input voltage is
sensed through R2 and R5 by the current flowing out of the VS pin. However, when the MOSFET is
turned off, output voltage is sensed through the R2 and R3 voltage divider using diode D4. This way, the
input voltage is always sensed without the influence of the output voltage.
D3
US1G-E3/5AT
R2
336 lQ
L1
1 mH
5
6
VS
VDD
IPK
GND
GND
8
DRAN
GND
R5
336 lQ
4
3
2
1
R4
1.30 lQ
C8
100 pF
C6
0.1 µF
C7
47 µF
R3
84.5 lQ
L2
220 µH
D4
US1G-E3/5AT
RF1
RV1
AC
D1
RH06
C1
0.1 µF
C2
10 µF
D2
US1G-E3/5AT
C4
0.1 µF
C3
100 µF
R1
20.0 lQ
Copyright © 2016, Texas Instruments Incorporated
Figure 5. Add Diode and Resistor Schematic
Equation 11 and Equation 12 show the new brownout protection and recovery point calculation equations.
Vbrownin = I VSL(run) ´ (R2 + R5 )
(11)
Vbrownout = I VSL(stop) ´ (R2 + R5 )
(12)
From these two solutions, the operation principles are about the same. The purpose is to avoid the
influence of the output voltage. The solution in Section 2.6.1 cannot fully eliminate the influence of the
output voltage at startup. The solution in Section 2.6.2 can fully eliminate the influence of the output
voltage at the startup.
6
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Test Result Waveform
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3
Test Result Waveform
3.1
MOSFET Stress
Due to the Buck configuration, the MOSFET voltage stress is about the same as input voltage. No voltage
spike is observed. Figure 6 illustrates MOSFET voltage stress.
Figure 6. MOSFET Voltage Stress
3.2
Efficiency
Efficiency
Efficiency
Due to the nature of low duty cycle, the Buck converter is not able to achieve very high efficiency.
However, the simple topology and few-external components still make it an attractive solution for certain
applications. Figure 7 illustrates comparisons in efficiency.
115 V
10 mA
50 mA
230 V
30 mA
100 mA
Input Voltage (V)
Load (mA)
Figure 7. Efficiency Comparison
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Test Result Waveform
3.3
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Standby Power
In a PLC application, the power supply works in standby mode most of the time. Achieving low standby
power is critical. The UCC28910 is able to obtain low standby power through variable frequency control.
Figure 8. Standby Power
3.4
Dynamic Response
According to the +1V/–1V dynamic response requirement, the design meets the requirement, see
Figure 9.
Figure 9. Dynamic Response
8
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Test Result Waveform
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3.5
Output Ripple Voltage
According to 200-mV output voltage ripple requirement, the design meets the requirement.
Figure 10. 115 Vin Output Ripple Voltage at No Load and Full Load
Figure 11. 230 Vin Output Ripple Voltage at No Load and Full Load
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Test Result Waveform
3.6
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Thermal Test at 25°C
According to the 45° temperature rise requirement, the design meets the requirement.
Figure 12. 85 Vin Thermal Performance
Figure 13. 230 Vin Thermal Performance
10
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Test Result Waveform
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3.7
EMI Test
According to the EN55022B EMI limit, the design meets the requirement. Note: Output is not grounded.
Figure 14. 230 Vin EMI Performance
Figure 15. 115 Vin EMI Performance
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Layout Suggestion
4
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Layout Suggestion
To increase the reliability and feasibility of the design, TI recommends the layout uses the following
guidelines:
1. Place the Ripk resistor as close as possible to the UCC28910 with the shortest traces possible.
2. Try to minimize the area of DRAIN trace, this helps keep EMI disturbance low.
3. A copper area connected to the GND pins improves heat sinking thermal performance.
4. Place the auxiliary voltage sense resistor divider directly on the VS pin keeping traces as short as
possible.
5
Conclusion
Through test and optimization, the non-isolated Buck converter achieves the performance meeting the
specifications. This paper also details two methods for improving the hysteresis for line brownout
protection. EMI standard EN55022B is also compliant. The non-isolated Buck converter is suitable for
industrial and appliance applications.
6
Reference
1. UCC28910: High-Voltage, Flyback Switcher with Primary-Side Regulation and Output Current Control
(SLUS769)
2. PMP4443: Universal AC input, 20V/100mA Non-isolated High-side Buck Converter
12
Non-Isolated High-Side Buck Converter with UCC28910
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