Texas Instruments | UCC28056,Using Auxiliary winding voltage for driving ZCD/CS Pin (Rev. B) | Application notes | Texas Instruments UCC28056,Using Auxiliary winding voltage for driving ZCD/CS Pin (Rev. B) Application notes

Texas Instruments UCC28056,Using Auxiliary winding voltage for driving ZCD/CS Pin (Rev. B) Application notes
Application Report
SLUA920B – October 2018 – Revised October 2019
UCC28056x, Using Auxiliary Winding Voltage for Driving
ZCD/CS Pin
Sonal Singh
ABSTRACT
This application report provides an alternate approach to sensing the zero crossing detection aimed for
customers who prefer using an auxiliary winding. The advantage of using the aux winding approach is that
UCC28056x application designs are more robust and have better immunity to interference from adjacent
nodes. UCC28056x is a single-phase PFC boost stage working on an innovative mode method, operating
in transition mode (TM) during full load, and transitioning to discontinuous conduction mode (DCM) during
reduced load. This application report provides a comparative analysis of the current method of biasing the
ZCD/CS pin voltage across the MOSFET versus the modified approach of introducing an auxiliary winding
as shown on a high level in Figure 1 and Figure 2.
The new variants of the UCC28056x devices help resolve the noise interference in the application designs
by introducing a second level of comparators for the ZCD detection. Table 1 shows the feature description
of the UCC28056x devices. Refer to the UCC28056 6-Pin High Performance CRM/DCM PFC Controller
Data Sheet for a full feature comparison.
Table 1. Feature Description
FEATURE
ZCD improvements
UCC28056
UCC28056A
UCC28056B
UCC28056C
No
Yes
Yes
Yes
Figure 1. Direct Drain Approach
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Figure 2. Aux Winding Approach
UCC28056, Using Auxiliary Winding Voltage for Driving ZCD/CS Pin
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Contents
Introduction ................................................................................................................... 3
Advantages of Using the AUX Winding Approach ...................................................................... 3
Calculations for AUX Winding .............................................................................................. 3
Results ....................................................................................................................... 5
Layout Guidelines ......................................................................................................... 11
1
Direct Drain Approach
List of Figures
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
...................................................................................................... 1
Aux Winding Approach ..................................................................................................... 1
Direct Drain Approach without Aux Winding ............................................................................ 3
Aux Winding Approach ...................................................................................................... 3
ZCD/CS Original EVM Waveform ........................................................................................ 5
ZCD/CS Modified EVM Waveform ........................................................................................ 5
AUX Winding Approach with Reference Voltage Points ............................................................... 6
Standby Power Comparison for the UCC28056 ........................................................................ 8
85 Vinac at NL.................................................................................................................. 8
85 Vinac at FL .................................................................................................................. 8
115 Vinac at NL ................................................................................................................ 8
115 Vinac at FL ................................................................................................................ 8
230 Vinac at NL ................................................................................................................ 8
230 Vinac at FL ................................................................................................................ 8
265 Vinac at NL ................................................................................................................ 9
265 Vinac at FL ................................................................................................................ 9
85 Vinac at 60 mA Load ..................................................................................................... 10
115 Vinac at 60 mA Load ................................................................................................... 10
230 Vinac at 100 mA Load .................................................................................................. 10
265 Vinac at 100 mA Load .................................................................................................. 10
No-Load to 200 mA at 90 Vac ............................................................................................ 10
No-Load to 400 mA at 90 Vac ............................................................................................ 10
Low-line Voltage and Current Waveforms .............................................................................. 11
High-line Voltage and Current Waveform ............................................................................... 11
Layout Example; EVM Bottom Layer ................................................................................... 12
Modified EVM Schematic.................................................................................................. 13
List of Tables
1
Feature Description.......................................................................................................... 1
2
Brown-in Threshold Comparison
3
4
5
..........................................................................................
Overvoltage Protection Threshold Comparison for the UCC28056 ..................................................
Overcurrent Protection Threshold Comparison .........................................................................
Total Standby Power .......................................................................................................
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6
7
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Trademarks
All trademarks are the property of their respective owners.
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UCC28056, Using Auxiliary Winding Voltage for Driving ZCD/CS Pin
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Introduction
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1
Introduction
UCC28056x has a ZCD/CS pin which combines the functionality of current sense and the zero crossing
detection into a single pin. This application report talks in depth about the more conventional method of
ZCD sensing by using a separate winding. This approach is easier to implement and the signal has more
immunity to the surrounding interference because the signal is not sensed across the MOSFET.
Figure 3. Direct Drain Approach without Aux Winding
Figure 4. Aux Winding Approach
The approach shown in Figure 4 can be used to replace Figure 3 in order to avoid any sensitivity at the
ZCD/CS pin. When the controller is not switching, the voltage across the auxiliary winding is zero, so only
the pullup resistor contributes to the standby power. The impedance of the ZCD divider is high and placing
it close to the ZCD/CS pin of the controller makes the signal more immune to the disturbances by the
capacitive coupling from the nearby switching nodes. Although the direct drain approach eliminates the
need of an aux winding, the signal sensing is more complicated and prone to increased noise
susceptibility. By using an aux winding approach across the inductor, these issues can be mitigated.
2
Advantages of Using the AUX Winding Approach
This section highlights the advantages of using the proposed auxiliary winding approach.
• All the nodes are low impedance nodes and are relatively insensitive to layout, except the ZCD/CS pin.
• Only small, low-voltage resistors are required at the ZCD/CS pin, therefore, the pin has a smaller
parasitic capacitance to surrounding switched nodes.
• High voltage cap is not required.
• Lower cost
3
Calculations for AUX Winding
This section contains the calculations for the component values for this approach mentioned in Figure 4.
3.1
ZCD Divider
The resistor divider for ZCD/CS pin can be calculated as:
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Calculations for AUX Winding
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where
•
Vaux is the voltage across the aux winding, which can be calculated as:
•
Npa is the primary to aux turn ratio; Npa equals 10.4 for the modified EVM.
•
RZCD2 = 20 kΩ and VZCBoRise = 0.3 V from the electrical characteristics table in the data sheet.
(2)
(3)
(3)
If you substitute all the above values to solve for RZCD1 from Equation 1, you get RZCD1 = 750 kΩ.
VZCBoRise is used to ensure that the attenuation factor of the ZCD and Rvin divider matches the attenuation
factor on the original EVM.
3.2
Ratio Check for ZCD Divider
There are a number of internal voltage thresholds driven by the attenuated drain voltage signal supplied to
the ZCD/CS pin.
(4)
If you substitute the RZCD1 and RZCD2 from Equation 4, you get KZC = 400.4.
Having this ratio (KZC) the same as the original EVM ensures that the other thresholds like brown-out
(VZCBoRise), line feedforward rise and fall (VFFxRise, VFFxFall), and the second overvoltage protection (VOVP2th)
are still at the same levels. There is a limited scope to vary the attenuation ratio because it impacts all of
these threshold values.
3.3
Rvin Divider
The resistor divider from the rectified voltage line to ensure start-up can be calculated as:
(5)
VZCBoRise is used to ensure that the attenuation factor of the ZCD and Rvin divider matches the attenuation
factor on the original EVM.
Substituting the values of RZCD1 and RZCD2 from Equation 5, Rvin = 7.2 MΩ.
Two resistors in series valued at 4.7 MΩ and 2.7 MΩ were used on the modified EVM.
3.4
Ratio Check for Rvin divider
Check the ratio for the attenuation divider to ensure that it matches to the original EVM. This can be done
using Equation 6:
(6)
If you substitute the values in Equation 6, you get:
(7)
3.5
Caux
Caux charges up during the TON to Vcaux, which can be defined from Equation 8:
where
•
Vfd is the forward diode voltage drop, which is 0.6 V
(8)
Ideally the Caux discharge is fast enough to trace the Vaux voltage. The value of the Caux voltage
discharge slope when Vin is falling is greater than the slope of Vaux. The following time constant can be
used to ensure this slope.
4
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Calculations for AUX Winding
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where
•
RZCD is total resistance feeding into the ZCD pin (RZCD = RZCD1 + RZCD2)
(9)
(10)
If you substitute the value of the selected ZCD divider resistance, you can calculate the value of the
auxiliary capacitance as:
(11)
270 pF of Caux was selected on the modified EVM.
3.6
Raux
The value of Raux and Caux are chosen to ensure that the Caux charging occurs very quickly cycle by
cycle. Use Equation 12 to select the time constant during the charging period.
(12)
If you substitute the value of Caux from Equation 12, you get:
(13)
390 Ω of Raux was used on the modified EVM.
3.7
Daux
It is recommended to use a diode with small reverse leakage current and forward voltage drop. A small
signal diode can be a good choice. A bigger voltage drop is replicated in the VOVP2 threshold as
mentioned in Equation 15. It is recommended to use a 1N4148 or a similar diode.
4
Results
The section shows comparative data between the direct drain and the aux winding approach including the
waveform and the calculations supporting the difference in the results.
4.1
ZCD/CS Waveform Comparison
Figure 5 and Figure 6 show the ZCD/CS waveform of the original and the modified EVM taken under 90
Vac input at a 100 mA load condition.
Figure 5. ZCD/CS Original EVM Waveform
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Figure 6. ZCD/CS Modified EVM Waveform
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Results
4.2
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Brown-in Threshold
Table 2 shows the brown-in threshold levels of the original EVM and the modified EVM. Table 2 shows
that the modification does not affect this threshold.
Table 2. Brown-in Threshold Comparison
THRESHOLD LEVELS
BROWN-IN THRESHOLD
4.3
ORIGINAL EVM
MODIFIED EVM
84 Vac
83 Vac
OVP2 Threshold
The UCC28056 has an OVP2 comparator with a fixed internal threshold, Vovp2th, that monitors the
ZCD/CS pin voltage during the discharge time. The OVP2 threshold is 1.125 V as specified in the
UCC28056 6-Pin High Performance CRM/DCM PFC Controller Data Sheet. Remember to factor in the 0.6
V diode drop from Daux.
You can calculate the value of the output voltage at which the OVP2 level would trip from this information.
The following equations are working backwards from the ZCD/CS pin.
Figure 7. AUX Winding Approach with Reference Voltage Points
(14)
Accounting for the diode drop, you can say:
(15)
Vt1 in Equation 15 is scaled by the turn ratio of the inductor and reflected at the output during the
discharge period when the boost diode is conducting.
(16)
Table 3 compares the theoretical Vout value to the original and the modified EVM.
Table 3. Overvoltage Protection Threshold Comparison for the UCC28056
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THRESHOLD LEVELS
ORIGINAL EVM
MODIFIED EVM
OVP2_UCC28056
423 V
443 V
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Refer to the UCC28056x selection guide for a feature comparison.
4.4
OCP1 Threshold
The OCP1 level is a cycle-by-cycle peak current protection that terminates the Ton duration early if the
current sense-to-pin voltage rises above 0.5 V. This is achieved by limiting the peak inductor current,
which avoids inductor saturation.
This can be checked by increasing the RCS value:
(17)
The normal switching operation continues and the controller is able to maintain the output regulation at full
load condition 423 mA for an Rcs value of 65 mΩ. This also ensures the required peak inductor current
does not cause the early termination of the TON period.
Changing Rcs to 3.54 times the original value causes the TON period to terminate early by a factor
calculated by Equation 18.
(18)
Table 4 shows the OCP1 threshold comparison of the original and the modified EVM. A resistor value of
230 mΩ was selected for the Rcs.
Table 4. Overcurrent Protection Threshold Comparison
4.5
THRESHOLD LEVELS
ORIGINAL EVM
MODIFIED EVM
OCP1
140 mA load
130 mA load
Standby Power Comparison
Equation 19 and Equation 20 show the contribution of the aux winding approach to the total standby
power measurements. The electronic load has been disconnected for this test. The average input power is
measured across the input and external VCC over a 10 min interval.
where
•
•
Vinac is the peak of line bulk voltage
R is the total resistance in the impedance path
(19)
(20)
Table 5 contains the modified EVM standby power measurements.
Table 5. Total Standby Power
INPUT VOLTAGE
(Vrms)
INPUT POWER (mW)
VCC VOLTAGE (V)
VCC CURRENT (µA)
TOTAL STANDBY
POWER: MODIFIED
EVM (mW)
85
27.36
12.01
102.89
28.596
115
28.44
12.01
102.98
29.677
230
64.7
12.01
103.12
65.938
265
69.52
12.01
103.5
70.763
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Figure 8 shows the comparative difference in standby power between the original UCC28056 and the
modified EVM UCC28056..
Figure 8. Standby Power Comparison for the UCC28056
4.6
Start-up
Figure 9 through Figure 16 show the output voltage behavior when the line voltage has already been
applied and the instant the VCC voltage exceeds the start-up threshold on the EVM with the new
approach. CH1 is Vout, CH4 is Vin_AC, and CH3 is Vcc.
Figure 10. 85 Vinac at FL
Figure 9. 85 Vinac at NL
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Figure 11. 115 Vinac at NL
Figure 12. 115 Vinac at FL
Figure 13. 230 Vinac at NL
Figure 14. 230 Vinac at FL
Figure 15. 265 Vinac at NL
Figure 16. 265 Vinac at FL
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4.7
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Valley Switching
Figure 17 through Figure 20 show drain-to-source voltage of the MOSFET and the valley switching action
on the EVM with the modified EVM.
4.8
Figure 17. 85 Vinac at 60 mA Load
Figure 18. 115 Vinac at 60 mA Load
Figure 19. 230 Vinac at 100 mA Load
Figure 20. 265 Vinac at 100 mA Load
Transient Response
Figure 21 and Figure 22 show the transient response of the EVM with the aux winding approach where
CH1 is Vout and CH4 is Iout.
10
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Figure 21. No-Load to 200 mA at 90 Vac
4.9
Figure 22. No-Load to 400 mA at 90 Vac
Line Voltage and Line Current
Figure 23 and Figure 24 illustrate the low-line, high-line voltage, and current waveforms where CH4 is Vinac
and CH2 is Iinac.
Figure 23. Low-line Voltage and Current Waveforms
5
Figure 24. High-line Voltage and Current Waveform
Layout Guidelines
The ZCD/CS resistor network is very sensitive to parasitic capacitance. This capacitance can be present
between the RZCD1 or RZCD2 resistor nodes and the surrounding high voltage switching nodes. The
presence of this parasitic capacitance can cause false fault detection triggering of OCP2 and OVP2,
causing the controller to shut down and operate in hiccup mode with an interval of 1 sec when attempting
to turn on.
• RZCD2 must be placed as close as possible to the ZCD/CS pin on the controller.
• Rvin and RZCD1 must be placed as close as possible to the RZCD2 resistor.
• PCB traces connecting RZCD1 and Rvin must be kept as short as possible. The width of these traces
must be very narrow to allow for the minimum parasitic capacitance.
• The traces connecting the high-valued resistors feeding to the ZCD/CS pin, DRV pin, and Vrect must
be at least 1 cm apart to avoid coupling.
• It is important to keep the traces feeding the ZCD/CS pin as far as possible to the high switching traces
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Layout Guidelines
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(VDrain).
Figure 25 shows an example layout of the modified EVM bottom layer with highlighted circuit.
Figure 25. Layout Example; EVM Bottom Layer
See Figure 25 to identify the modified circuit elements. Figure 26 is an updated schematic diagram with
the proposed approach.
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Figure 26. Modified EVM Schematic
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Revision History
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Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from B Revision (June 2019) to C Revision .................................................................................................... Page
•
•
Updated to reflect the new inductance value .......................................................................................... 2
Updated to reflect the new inductance value ........................................................................................ 13
Changes from Original (October 2018) to B Revision .................................................................................................... Page
•
•
•
•
•
•
14
Edited application report for clarity ......................................................................................................
Added UCC28056x variant description .................................................................................................
Changed specifics to UCC28056 .......................................................................................................
Added Specific to UCC28056 ...........................................................................................................
Added selection guide reference ........................................................................................................
Added specifics for UCC28056 controller ..............................................................................................
Revision History
1
1
6
6
7
8
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