Texas Instruments | Supercap last gasp failsafe power for RF comunications in E-Meters | Application notes | Texas Instruments Supercap last gasp failsafe power for RF comunications in E-Meters Application notes

Texas Instruments Supercap last gasp failsafe power for RF comunications in E-Meters Application notes
Solving the backup power supply challenge for electricity
meters with high-power wireless communications
Milen Stefanov, Florian Mueller
Electricity meter designers are continually being
challenged by the demanding requirements from meter
performance during grid outages. Smart meters must
continue to operate during brief power outages and
send notification messages through a wireless radio,
which consumes a significant amount energy. The
Supercapacitor backup power supply for E-Meters
reference design (PMP30528) is an example of two
main power supply rails and two backup rails. The
input DC voltage to this design can be between 7 V
and 17 V. During a grid power failure, an automatic
switch to a single SuperCap backup power occurs
without any external logic or microcontroller (MCU)
involvement. When main power supply rails are
restored, the circuitry automatically reverts back to
them and starts charging the SuperCap.
Figure 2 shows the 3.9 V output, which can be
adjusted between 3 V and 5 V by modifying the output
voltage divider R18, R23 at U2 and R34, R38 at U5.
Figure 3 shows the transition between U2 and U5.
Backup Supply Rails Powered from the SuperCap
The most relevant function of delivering power for the
last gasp RF communication is implemented with the
TPS61022 boost converter (U5) shown in Figure 3. To
maximize the usable energy from the SuperCap, it is
important to discharge the latter to as low voltage as
possible. U5 delivers power over the discharge range
from 2.7 V down to 0.5 V (its minimum input voltage).
Supply Rails in Mains Powered Mode
The two main supply rails with 5 V @ 150 mA and 3.9
V @ 2 A are generated by the buck converters U1
(TPS62173) and U2 (TPS62147), respectively. Both
U1 and U2 are highly efficient and easy-to-use
synchronous step-down DC/DC converters. The
TPS6217x device family supports up to 6 V @ 500
mA, but here, the fixed 5 V output variant is used. See
Figure 1 for a visual. This voltage rail is easily modified
to 1.8 V or 3.3 V if TPS62171 or TPS62172 are used.
Figure 1. 5 V Primary Supply Rail with TPS62173
The TPS62147 provides up to 2 A continuous output
current, which is a perfect fit for supplying RF Power
Amplifiers (PAs) used in 2G, 3G, or LTE modems or
for Wireless M-Bus with up to 1 W transmission power.
Figure 2. 3.9 V Primary Supply Rail with TPS62147
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Figure 3. Boost Rail from SuperCap with D4 and
D6 for Seamless Transition between U2 and U5
Figure 5 shows the circuit that charges the SuperCap
(its capacity can vary depending on the required RF
power and holdup duration) with a constant adjustable
voltage of 2.7 V. When the input voltage fails (Vin = 0
V), U2 stops and the U5 converter seamlessly takes
over the power supply and boosts the SuperCap
output voltage to 3.9 V.
The SuperCap capacitance, its maximum charge
voltage, and the lowest discharge voltage define the
available energy for the RF sub-system backup
feature. This reference design charges a 25 F
SuperCap device to 2.7 V, providing a stable output of
3.9 V @ 200 mA for approximately 70 s. To achieve
an automated (without any MCU interaction) transition
from the normal operating mode (e.g. Vin = 11 V) to
the backup operational mode from SuperCap, the
outputs of U2 and U5 are connected together. Figure 2
shows the D2 diode that blocks the reverse current.
The Zener diode D4 is connected to the feedback pin
of U5 to prevent the controller from switching when the
input voltage is present (7 V < Vin < 12 V).
Solving the backup power supply challenge for electricity meters with highpower wireless communications Milen Stefanov, Florian Mueller
Copyright © 2019, Texas Instruments Incorporated
D2 is directly in series with the load and its conduction
loss is proportional to the output current since the
forward voltage drop is almost a fixed voltage. A diode
reduces the efficiency significantly, generates a lot of
heat, and needs both a large package and a copper
area to get remove the heat. This design uses a
CSD15571Q2 NexFET Power MOSFET (Q2) as an
ideal diode rectifier. When the Q2 conducts, its voltage
drop is proportional to its R_DSon, which is low,
significantly lowers the forward voltage drop of the
diode. This reduces both the power loss and the
temperature rise and significantly improves the overall
efficiency of the power conversion.
TI's P-Channel NexFET Power MOSFET device
CSD25402Q3A (Q1) is the core of the discrete charger
circuit for the SuperCap. Since a P-Channel MOSFET
remains turned off when the gate-source voltage is
zero and turns on when the voltage is negative, a low
cost PNP device Q3 is used to drive Q1. The current
sense resistor R5 connected to Q3 defines the
charging current and an adjustable shunt regulator
TL431 limits the charging voltage. This circuit behaves
like a linear regulator, where the input current is equal
to the output current and therefore the power
dissipation depends on the charging current.
Nearly all of the losses are converted to heat, so the
power dissipation must be kept below the thermal limit.
The current charging time is about 10 minutes for a
SuperCap of 25 F. The two optional resistors, R3 and
R5, can help dissipate the extra heat when a shorter
charging time is needed.
Peak Power Demand
Figure 4. TPS61022 Boost Controller in Shutdown
Mode Through EN Pin
The TPS61022 has a 5.7-V output overvoltage
protection, output short circuit protection, thermal
shutdown protection, and a built-in undervoltage
lockout (UVLO) feature. Figure 4 shows the Alternative
Enable Circuit around transistor Q4 sets the voltage on
the EN pin depending on the Vin voltage.
After the TPS61022 starts and the output voltage is
above 2.2 V, the TPS61022 works with input voltage
as low as 0.5 V for a maximum discharge of the
The most power hungry block is the high transmit
power RF communication sub-system, which can use
a cellular network modem or a wireless MCU, such as
CC1352P, to create a private RF network with
protocols such as Wireless M-Bus or 6LoWPAN. The
common voltage range for the RF power amplifiers is
between 3.3 V and 3.9 V. You choose the voltage
level for the RF PA to work most efficiently and set it
accordingly by modifying the R18, R23, R34 and R38
values. The last gasp support requirement for emeters with wireless communications drives the use of
a SuperCap device, which gets charged while main
power supply rails are present and automatically
delivers power to the RF sub-system when main
power supply is lost.
Optionally, some other e-meter sub-systems (like the
Host MCU or the metrology) can be powered from the
SuperCap as well. The second backup powered rail is
3.3 V @ 50 mA is provided by TPS610981 (U4) boost
device, the pin-compatible derivatives thereof integrate
either a LDO or a load switch to power additional
peripheral devices or sub-systems.
Figure 5. SuperCap C11 with Discrete Charger
When the input voltage Vin of the PMP30528 board is
above a definable threshold, Q4 turns the controller
into shutdown mode. During shutdown, the load is
completely disconnected from the input power and the
TPS61022 is turned off and consumes only 0.25 µA.
The PMP30528 reference design implements a fullyautomated backup power solution for smart e-meters,
which must wirelessly transmit last gasp data after the
main power is lost. By completely avoiding any MCU
interaction for switching from main power supply to
backup power and vice versa, this design is a highly
customizable power supply system with an excellent
power efficiency and multiple options for fine tuning to
meet specific application requirements. A single 2.7 V
SuperCap gets discharged down to 0.5 V by the
TPS61022 boost converter, which eliminates the cost
of implementing two or more stacked SuperCaps and
their associated load balancing circuitry.
Solving the backup power supply challenge for electricity meters with highpower wireless communications Milen Stefanov, Florian Mueller
Copyright © 2019, Texas Instruments Incorporated
SLUA943 – January 2019
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