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Texas Instruments UCC28722, Bias Supply Design Considerations for Intelligent eMeter Applications Application notes
Application Report
SLUA781 – August 2016
UCC28722, Bias Supply Design Considerations for
Intelligent eMeter Applications
Michael O'Loughlin
1
eMeters and the UCC28722
The old analog power meters that were used to measure commercial and residential power usage are
being replaced with more intelligent electronic meters (eMeters). These eMeters communicate either
through wired or wireless communication to the power company. These meters are designed for single,
dual, and/or three phase operation and use a 2-W to 7-W offline buck or flyback converter for the power
supply. Figure 1 shows a functional block diagram of a two phase eMeter. This application note reviews
design considerations for eMeter applications using the UCC28722 primary-side regulation (PSR)
controller. This information is supplemental to the UCC28722/20 Design Example/Application Note
(UCC28722 Data Sheet SLUSBL7 and UCC28722/UCC28720 5W Design Example SLAU700).
L1
Battery/SuperCap
Management
N
Regulator
2-6 Sockets
85-264 V AC
UART
15V
15V
DC/DC Buck
5.0 V
UART
DC/DC Buck
3.3 V
USB
LDO
1.8 V
Protection
15V
PWM
Controller
Voltage
Reference
Comparator
3.3 V
Opto
feedback
Wired Communication
15 V
Battery/SuperCap
RMII/MII
3.3V
Ethernet
Cu
3.3V
PMU
Hybrid RF+PLC
Wireless Communication
RTC
I2C
EEPROM
UART
Keypad
DDR
UART
LCD Driver
FLASH
Case Open
Detect
SD/ SDHC
Crystal
15 V
Battery
SVS/RESET
Relay
USB
3.3V
Relay Driver
Relay
3.3/5V
PLC
3.3V
Host Processing Subsystem
MPU/MCU
UART
<GHz RF
ZigBee
WMBUS
UART
Bluetooth
14 Sockets
UART
Current_N
Load 2
Relay
Measurement Subsystem
1¶
L1¶
ESD/EMI
Protection
Sensors
Digital
Isolation
DSP/MCU
NFC
UART
Wi-Fi
UART
6LoWPAN
UART
L2¶
Current_L2
Interrupt GPIO
UART
Current_L1
Relay
RS-485
3.3/5V
UART
PFI
1-6 Sockets
UART
Load 1
RS-232
ESD
Power Supply
L2
Infrared
UART
5 Sockets
ADC
Hybrid RF+PLC
UART
GPIO
Copyright © 2016, Texas Instruments Incorporated
Figure 1. Functional Block Diagram of a 2-Phase eMeter
All trademarks are the property of their respective owners.
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eMeters are used universally and are designed for a wide variety of input voltage ranges. For single phase
applications the input voltages generally range from 85 V AC to 265 V AC. One eMeter design for India,
however, has input specifications of 48 V AC to 270 V AC. Some design requirements may require the
eMeter bias supply to work with a single phase or multi-phase systems, where the input range is 85 V AC
to 480 V AC.
At 480-V AC input, the eMeter’s internal bias supply could see a peak-input voltage of roughly 679-V peak
(VIN). To reduce the voltage stress on the input bulk capacitor (CIN), designers have used two 400-V
capacitors in series, which requires each capacitor to have twice the capacitance needed for input ripple
voltage and holdup requirements. To reduce the voltage stress on the eMeter’s step down converter’s
main switch (Q2), some designers use a simple series pass regulator (R1, D1 and Q1). Refer to Figure 2
for a functional schematic.
NOTE: Even though the series pass regulator reduces the voltage stress on the eMeter’s bias
supply switch (Q2), this circuitry will hurt the bias supply’s overall efficiency.
Due to this reduction in system efficiency, it is not recommended to use a series pass regulator before the
eMeter’s bias supply. It would be better to find a higher voltage rated FET, or even better, a power supply
controller that can drive a bipolar junction transistor (BJT) for the switch.
eMeter Stepdown/Bias Supply
VBULK
VIN = 480 V × 2½ C 679 V
Q1
+VOUT
R1
CB1
CIN1
COUT
VOUT = 15 V
-VOUT
D1
CIN2
CB2
RCS
Netral
Figure 2. Schematic of eMeter Bias Supply with Series Pass Pre-regulator (R1, D1, Q1)
Both the offline flyback converter and offline buck converter use a bridge rectifier to convert the AC line
voltage to a DC voltage. These converters typically have a power factor (PF) of 0.45. There is an ANSI
standard that requires the input power of an eMeter to be less than 20 VA. To meet this standard, it is
required that the load of the bias supply be much less than 20 VA. If an eMeter’s output power
requirement (POUT) is 6 W, the converter needs to be designed for greater than 67% efficiency (ƞ) at full
load based on the ANSI standard and the PF of the offline step-down converter. To make it easier to meet
this efficiency requirement it is advisable to not use the series-pass regulator circuit as presented in
Figure 2.
POUT
P
´ 100
h
PF =
= 0.45
h ³ OUT
» 67%
PF ´ 20VA
20VA
(1)
2
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To keep the bias supply design cost effective, green, and efficient, it is ideal to use TI’s UCC28722
primary-side regulated (PSR) controller. One reason for using this controller is that it uses the auxiliary to
secondary-side transformer turns ratio to sense the output voltage. This removes the need of an optoisolator feedback in the design, which improves the reliability of the system, and reduces standby power.
Removing the opto-isolator feedback network also reduces the design’s List of Materials component count
and total design cost. Also, the UCC28722 is designed to drive bipolar junction transistors which are
easier to find with high-voltage ratings and are less expensive than FETs for the same power rating, which
makes it an ideal choice for eMeter applications. Refer to Figure 3 for a functional schematic.
VBULK
LA
DG
T1
DA
470PH
DC
DZ
CB
CA
DD
DB
VIN
NP
RT
RS
VOUT
NS
DF
RZ
COUT
3.01 k
CS
RL
RG1
VDD
10
DE
QA
VOUT
RF1
10
RG2
10 k
DRV
CDD
RCS
R1
R2
VS
UCC28722
Opto
RS1
T1
NA
2 VDD
DRV 3
6 VS
CS 4
5 GND
R3
R4
C1
RLC
CBC 1
RS2
RCBC
R5
Removing Opto TL431 Feedback
Reduces Design Costs
Figure 3. Offline Flyback Converter with UCC28722 PSR Controller
The UCC28722 is designed for discontinuous conduction mode Flyback converters. The power supply
controller uses an FM/AM/FM modulation scheme to control duty cycle and improve overall efficiency; as
well as reduce standby power. Please refer to Figure 4 for the UCC28722’s internal control feedback block
diagram.
VA
Zero Current Detect
Near Valley Switching
RS1
4V
VS
SAMPLER
RS2
+
VEA
S
Q
R
Q
QA
DRV
Control Law
+
IPP
VRCS
RCS
CS
Figure 4. UCC28722 Control Bock Diagram
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Figure 5 show the UCC28722 control law profile and shows the converter frequency (fSW) and primary
peak current (IPP) is modulated based on the UCC28722’s error amplifier (E/A) output voltage. Please
refer to the UCC28722’s data sheet for more detailed information.
Control Law Profile in Constant Voltage (CV) Mode
84 kHz
IPP (peak primary current)
fSW (1 / MINP)
IPP(max)
IPP
fSW
28 kHz
FM
AM
IPP(max) / 4
FM
fSW(min) = 680 Hz
4.4 kHz
0.75 V
1.3 V
2.2 V
3.55 V
5V
Control Voltage, E/A Output - VCL
Figure 5. UCC28722 Control Law Profile
Previously it was mentioned that different eMeters have different input voltage ranges that vary from 48 V
AC to 270 V AC, 85 V AC to 265 V AC, and 85 V AC to 480 V AC. These input ranges are extremely wide
and it is not recommended to design one eMeter bias supply to meet every condition. For example
designing for 48 V AC to 480 V AC input range is not feasible by using UCC28722. In this example the
minimum input where the bias supply would have to operate is about 66% of the minimum peak input
voltage, which is about 45 V. The maximum peak input voltage would be roughly 679 V. The bias supply
would have to operate with an input range of 15:1, which PWM controllers cannot handle due to limitations
in duty cycle. The UCC28722 can be designed for a wide input range 9.8:1 down to 4.9:1 but this range
varies with the maximum switching frequency (fSW(max)) chosen for the design, as well as with the switchnode resonant-ringing frequency (fLC).
fLC is determined by the magnetizing inductance (LM) and the switch node capacitance (CSW). For this
evaluation we estimate the resonant frequency to be 500 kHz.
NOTE: Frequency varies based on power level, layout and semiconductors chosen, etc. for the
design.
f LC =
1
2 ´ π LM ´ CSW
» 500kHz
(2)
The maximum duty cycle (DMAX) of the converter happens at the minimum input voltage and full load
condition. It is limited by the resonant frequency (fLC), maximum allowable switching frequency (fSW(max))
and allowing the flyback rectifier diode to conduct for at least 42.5% of the switching period during DMAX.
NOTE: the UCC28722 control law limits the output rectifier conduction time to less than 42.5% of
the switching period.
DMAX ( f SWMAX ) = 1 - 0.425 -
f SWMAX
2 ´ f LC
(3)
The primary magnetizing inductance (LPM) of the transformer in Figure 3 sets the converter’s maximum
switching (fSW(max)) and is selected based on estimated/actual systems efficiency (ƞ), output power, the
transformer primary peak current (IPPK), and the converter’s minimum input bulk voltage.
4
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NOTE: Due to limitations imposed by EMI jitter and absolute maximum frequency ratings of the
device it is recommended that the converter be designed for fSW(max) between 38 kHz and 72
kHz.
This is within the 28-kHz to 80-kHz range for the upper FM region in the control law presented in Figure 5.
38 kHz £ f SWMAX £ 72 kHz
(4)
2 ´ POUT
I PPK =
h´ DMAX ´ VBULK (MIN )
(5)
LM =
2 ´ POUT
h
(I PPK )2 ´ f SWMAX
(6)
NOTE: For the controller to operate correctly over the entire load range at maximum input voltage,
the minimum duty cycle at maximum load and maximum switching frequency
(DMIN@fSW(max)(fSW(max))) needs to be 4 times greater than the UCC28722 current sense leading
edge blanking timer (TCSLEB) times the maximum switching frequency.
This ensures the converter operates in the AM range allowing the peak primary current (IPPK) to be
modulated from its maximum value down to its minimum value of IPPK/4. This in turn limits the input voltage
range (Range(fSW(max))) the converter can be designed for.
TCSLEB = 355ns
(7)
DMIN @ fswmax ( f SWMAX ) = 4.05 ´ TCSLEB ´ f SWMAX
(8)
In this equation DMIN@fSW(max)(fSW(max)) is the primary-side switch duty cycle at maximum input voltage and
full-load condition.
In the following equation, VBULK(max) is the maximum input voltage and VBULK(min) is the minimum input
voltage to the eMeter bias supply.
VBULK (MAX )
DMAX ( f SWMAX )
Range ( f SWMAX ) =
=
VBULK (MIN ) DMIN @ fswmax ( f SWMAX )
(9)
With the equation for Range(fSW(max)), the input range was plotted versus frequency in Figure 6.
NOTE: This is based on an estimated fLC and varies with actual magnetizing inductance and switch
node capacitance.
From Figure 6 it can be observed that, for the fSW(max) chosen for the design, the maximum input voltage
range can vary from 9.8:1 down to 4.8:1.
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10
VBULK((MAX)/VBULK(MIN) (V)
9
8
7
6
5
4
38000
48000
58000
fSWMAX (Hz)
68000
78000
D001
Figure 6. Input Range vs. Frequency
An eMeter bias supply using the UCC28722 flyback PSR controller will be evaluated to determine if it can
be used for an aggressive input voltage range (VIN) of 85 VRMS to 480 VRMS. For this design, the minimum
bulk voltage (VBULK(min)) is limited to 72 V and the maximum input voltage (VBULK(max)) is the peak of highline
operation which is roughly 679 V. This requires an input range of roughly 9.4 to 1.
VBULK (MAX ) 679V
Range =
=
= 9.4
VBULK (MIN )
72V
(10)
By evaluating the graph in Figure 6 and the equation for Range(fSW(max)) it can be observed that a 9.4:1
input range is obtainable at a switching frequency of 39.5 kHz.
NOTE: The allowable input range will not just vary based on fSW(max), it also varies based on CSW and
LM.
For this reason it is a good idea to leave some margin and not design for the maximum input range that is
possible (9.8:1).
VIN (MAX )
DMAX (39.5kHz )
Range (39.5kHz ) =
=
= 9.4
VBULK (MIN ) 4.05 ´ TCSLEB ´ 39.5kHz
(11)
Once the maximum frequency is determined, the primary magnetizing inductance (LM) of the transformer
in Figure 3 can be calculated, based on an estimated efficiency of the design. For these high input voltage
range eMeter designs, I have observed the efficiency at the peak of line to be roughly 70%. For this
design we selected the maximum switching frequency to be 39 kHz and estimated the efficiency to be
70%, which gave us an LM of roughly 2.7 mH. For a 5-W, 15-V design:
2 ´ POUT
2 ´ 5W
I PPK =
=
» 0.36 A
h´ DMAX (39kHz )´ VBULK (MIN ) 0.7 ´ 0.54 ´ 0.72
(12)
LM =
2 ´ POUT
h
(I PPK )2 ´ f SWMAX
2 ´ 5W
0.7
=
» 2.7 mH
(0.367 A)2 ´ 39kHz
(13)
The transformer primary-to-secondary turns (NPS) is calculated based on volt-second balance.
NOTE: In the equation for NPS, variable VQA(sat) is the bipolar transistor saturation and VRCS is the peak
voltage drop across the current sense resistor.
VDG is the voltage drop across the output rectifier diode (DG) of the flyback converter in Figure 3.
6
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N PS £
N PS £
(
N P DMAX ( f SWMAX )´ VBULK (MIN ) - VQA(SAT ) - VRCS
=
0.425 ´ (VOUT + VDG )
NS
NP
=
NS
DMAX (39kHz )´ (72V - 0.6V - 0.75V )
0.425 ´ (15V + 0.6V )
)
(14)
= 5.7
where
•
NPS = 5, turns ratio was rounded down to a whole number.
(15)
The primary-to-auxiliary turns ratio (NPA) can be calculated based on the minimum input bulk voltage and
the UCC28722 UVLO turn on voltage (VDD(on)).
NOTE: This turns ratio is set higher to combat magnetic tampering by means of VDD over voltage
shutdown protection, as discussed later in the application note.
N PA =
N P VBULK (MIN ) 72V
=
=
» 3.6
NA
VDD(on )
21V
(16)
For this design example we set NPA to 3.5.
N
N PA = P = 3.5
NA
(17)
The output capacitor is selected based on holdup requirement.
P
5W
2ms ´ OUT
2ms ´
VOUT
V = 222uF
15
COUT ³
=
VOUT - VOUT ´ 0.8 15V - 15V ´ 0.8
where
•
COUT = 270 µF, standard output capacitance was chosen for the design.
(18)
There is one social issue with eMeter usage and that is with magnetic tampering to interfere with metering.
If a strong magnet is applied near the flyback transformer it saturates the transformer causing sampling
errors in the output voltage sensing. This causes the UCC28722 controller to demand more duty cycle
than it should, which causes the output voltage to increase because of the incorrect output voltage
sampling and most likely would damage the eMeter bias supply. The inexpensive, simple circuitry
presented in Figure 7 can be used to disable the UCC28722 BJT driver output if such an event should
occur.
VDD
DRV
QA
DO
22 V
QB
ROVP
2.05 k
Figure 7. VDD Over Voltage Protection (OVP)
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Reference Material
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The VDD voltage tracks the output voltage through the auxiliary-to-secondary transformer turns ratio (NAS).
The circuitry in Figure 7 is just a VDD/VOUT over-voltage protection circuit. The VDD over voltage threshold
VDD(OVP) will be FET QB’s gate-to-source threshold (VGS(th)) plus diode DO’s zener voltage. In this
example, if a logic level FET of with a VGS(th) of 2 V was used with a VDO of 22 V, FET QB will turn on,
stealing the base drive current from QA, preventing the PSR flyback converter from switching. This also
triggers a current sense short circuit fault, which disables driver switching and discharges the VDD
capacitor to the UVLO turn-off voltage and initiates a restart after the VDD capacitor has been charged up
with a trickle charge resistor to the UVLO turn on voltage.
VVDD(OVP ) = Vgs (th ) + VDO » 2V + 22V » 24V
(19)
NOTE: The VDD(OVP) threshold varies with changes in VGS(th) and variations in zener diode
voltages. In this example, the VDD over voltage trip point would be between 23 V and 25 V.
VVDD(OVP ) = 23V to 25V
(20)
The UCC28722 BJT driver (DRV) is current limited (IDRS(max)) to 42 mA. To ensure that the BJT does not
start switching in case of an over voltage event, it is recommended to select a FET for QB with resistance
(RDS(on)) that is low enough to prevent BJT QA from turning on.
0.3V
0.3V
Rds (on ) £
=
» 7W
I DRS (max ) 42mA
(21)
NOTE: The information in this application note reviews special considerations and techniques for
designing the UCC28722 into an eMeter bias supply.
This includes tips for designing for large input voltage ranges, as well as magnetic tampering protection
circuitry. However, this is supplemental information and should be used in conjunction with the
UCC28722/UCC28720 5-W design example/application note (SLUA700) to complete the eMeter bias
supply design.
2
Reference Material
1.
2.
3.
4.
5.
6.
8
UCC28722 Data Sheet, http://www.ti.com/lit/gpn/ucc28722
UCC28722/UCC28720 5W Design Example, http://www.ti.com/lit/pdf/slua700
UCC28722 MathCAD Flyback Design Tool, http://www.ti.com/lit/zip/sluc528
UCC28722 Excel Design Tool, http://www.ti.com/lit/zip/sluc529
Universal AC Input 7.5W Adapter (flyback), TI Reference Design, http://www.ti.com/tool/PMP4391
Wide Input 2W Bias Power Supply (Buck), TI Reference Design, http://www.ti.com/tool/PMP7668
UCC28722, Bias Supply Design Considerations for Intelligent eMeter
Applications
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