Texas Instruments | AC-DC Non-Isolated SMPS for Single-Phase Smart Meters Based on UCC28722 (Rev. A) | Application notes | Texas Instruments AC-DC Non-Isolated SMPS for Single-Phase Smart Meters Based on UCC28722 (Rev. A) Application notes

Texas Instruments AC-DC Non-Isolated SMPS for Single-Phase Smart Meters Based on UCC28722 (Rev. A) Application notes
Application Report
SLUA721A – June 2014 – Revised November 2017
AC-DC Non-Isolated SMPS for Single Phase Smart Meters
Based on UCC28722
Harmeet Singh, Salil Chellappan, Sanjay Dixit ....................................................................... Power Stage
ABSTRACT
Single phase prepaid/smart meters need very low cost, low wattage (< 2.5 W) non-isolated power supply.
The meters should be capable of working with an input voltage range of 100 VAC on the lower side, given
poor utility conditions in some regions, to 415 VAC on the higher side so as to sustain phase-phase
voltage as well. The real voltage levels can touch 500 VAC (415 VAC +20%).
The application report describes an innovative application of the UCC28722 flyback controller used in a
buck configuration tailored for a smart energy metering application. This serves as superior alternative to
existing power supplies with integrated MOSFET technology. The following list shows the benefits:
• Works for a wide input range (110 VAC–500 VAC)
• Uses lowest cost BJT making the overall solution cheap
• High conversion efficiency because of combination of frequency and peak current modulation
• Ultra low standby power of 75 mW
• Features drum core inductor for smallest form factor
This application report goes through the step-by-step procedures a designer must follow to complete an
AC-DC Buck switch mode power supply unit (SMPS) using the UCC28722.The board has been built and
tested based on this application report and is available for download as PMP7668 on www.ti.com.
Figure 1. AC-DC Buck Design Using UCC28722 (PMP7668)
SLUA721A – June 2014 – Revised November 2017
Submit Documentation Feedback
AC-DC Non-Isolated SMPS for Single Phase Smart Meters Based on
UCC28722
Copyright © 2014–2017, Texas Instruments Incorporated
1
www.ti.com
Contents
SMPS Design Specifications ............................................................................................... 3
Basic Principle of Buck Topology .......................................................................................... 3
2.1
Buck Steady-State Discontinuous Conduction Mode Analysis ............................................... 4
3
Step-by-Step Design Procedure of UCC28722-Based Buck Converter .............................................. 7
3.1
AC Input Stage Components ..................................................................................... 7
3.2
Feedback Resistors ................................................................................................ 7
3.3
VDD Capacitance .................................................................................................. 8
3.4
Startup Resistors and Startup Time .............................................................................. 8
3.5
Current Sense Resistor ............................................................................................ 9
3.6
Output Inductor .................................................................................................... 10
3.7
Freewheeling Diode and Vdd Diode ............................................................................ 10
3.8
Output Capacitor .................................................................................................. 10
3.9
Bipolar Junction Transistor ....................................................................................... 11
3.10 LDO for 3.3 V ...................................................................................................... 12
4
Bill of Materials ............................................................................................................. 13
5
PCB Details ................................................................................................................. 14
6
Conclusion .................................................................................................................. 15
7
References .................................................................................................................. 15
Appendix A
Experimental Results ............................................................................................. 16
1
2
List of Figures
24
.................................................................... 1
............................................................................................. 3
Buck Power Stages in Discontinuous Mode ............................................................................. 5
Discontinuous Mode Power Stage Buck Waveforms ................................................................... 6
Timing Diagram of Startup Sequence in UCC28722 ................................................................... 8
Discontinuous Current Waveform in UCC28722 ........................................................................ 9
Complete Schematic of the PMP7668 Non-Isolated Buck Converter ............................................... 12
PCB Top Silk Screen ...................................................................................................... 14
PCB Bottom Silk Screen .................................................................................................. 14
PCB Composite Layout.................................................................................................... 14
Efficiency vs AC I/p Voltage (110 V–275 V) ............................................................................ 16
Efficiency vs DC I/p Voltage (400 V–705 V) ............................................................................ 16
Efficiency vs O/p Load Current at 325 VDC ............................................................................ 17
O/p Voltage Variation vs O/p Load Current at 325 VDC .............................................................. 17
SW Node Waveform and Collector Current at Vin = 110 VAC, Full Load .......................................... 18
Zoomed SW Waveform and Collector Current at Vin = 110 VAC, Full Load ...................................... 18
SW node Waveform and Collector Current at Vin = 275 VAC, Full Load .......................................... 19
Zoomed SW Waveform and Collector Current at Vin = 275 VAC, Full Load ...................................... 19
Vout Ripple at Vin = 110 VAC, Full Load ............................................................................... 20
Vout Ripple at Vin = 275 VAC, Full Load ............................................................................... 20
Vout Turn On Waveform at Vin = 110 VAC, Full Load ................................................................ 21
Vout Turn On Waveform at Vin = 275 VAC, Full Load ................................................................ 22
Vout Waveform at Vin = 230 VAC, Load Transient from 2 mA to 200 mA ......................................... 23
Conducted Emissions as per EN55022 Class B ....................................................................... 24
1
AC-DC Non-Isolated Single Phase Smart Meter SMPS Design Specifications ..................................... 3
2
Bill of Materials of Complete Schematic................................................................................. 13
1
AC-DC Buck Design Using UCC28722 (PMP7668)
2
Buck Power Stage Schematic
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
List of Tables
2
AC-DC Non-Isolated SMPS for Single Phase Smart Meters Based on
UCC28722
SLUA721A – June 2014 – Revised November 2017
Submit Documentation Feedback
Copyright © 2014–2017, Texas Instruments Incorporated
SMPS Design Specifications
www.ti.com
Trademarks
All trademarks are the property of their respective owners.
1
SMPS Design Specifications
Table 1. AC-DC Non-Isolated Single Phase Smart Meter SMPS Design Specifications
The main specifications of the SMPS are listed below:
Input
Voltage
110–500 VAC
Frequency
48–52 Hz
Output
10 V ±1 V
3.3 V ±1%
200 mA output current on 10 V and 2 mA on 3.3 V for full input voltage range
Standby Power(W/VA)
< 75 mW/300 mVA at 240 VAC (2 mA on 3.3 V and no load on 10 V)
Efficiency
> 65%
Overload/Short-circuit
Protected
Output overvoltage
Protected
PCB type and size
FR4
Single sided 35 μm
69 mm × 26 mm
2
Isolation
Non isolated - N connected to output GND
EMI
In accordance with EN55022 - class B
EMC
Surge - IEC 61000-4-5 - 4kV
EMC
EFT - IEC 61000-4-4 - 4kV
Basic Principle of Buck Topology
Figure 2 shows a simplified schematic of the buck power stage with a drive circuit block included. The
power switch, Q1, is a NPN bipolar transistor. The diode, D1, is usually called the freewheeling diode. The
inductor, L, and capacitor, C1, make up the output filter. Resr represents the capacitor ESR and RL
represents the inductor DC resistance. The resistor, RLoad, represents the load seen by the power stage
output.
Figure 2. Buck Power Stage Schematic
During normal operation of the buck power stage, Q1 is repeatedly switched on and off with the on and off
times governed by the control circuit. This switching action causes a train of pulses at the junction of Q1,
D1, and L which is filtered by the L/C1 output filter to produce a DC output voltage, Vo.
SLUA721A – June 2014 – Revised November 2017
Submit Documentation Feedback
AC-DC Non-Isolated SMPS for Single Phase Smart Meters Based on
UCC28722
Copyright © 2014–2017, Texas Instruments Incorporated
3
Basic Principle of Buck Topology
www.ti.com
A power stage can operate in continuous or discontinuous inductor current mode. Continuous inductor
current mode is characterized by current flowing continuously in the inductor during the entire switching
cycle in steady-state operation. Discontinuous inductor current mode is characterized by the inductor
current being zero for a portion of the switching cycle. It starts at zero, reaches a peak value, and returns
to zero during each switching cycle. It is very desirable for a power stage to stay in only one mode over its
expected operating conditions, because the power stage frequency response changes significantly
between the two modes of operation.
2.1
Buck Steady-State Discontinuous Conduction Mode Analysis
The following is a description of steady-state operation in discontinuous conduction mode – as UCC28722
operates only in this mode. A power stage operating in discontinuous conduction mode has three unique
states during each switching cycle, shown in Figure 2. The ON state is when Q1 is ON and D1 is OFF.
The OFF state is when Q1 is OFF and D1 is ON. The IDLE state is when both Q1 and D1 are OFF A
simple linear circuit can represent each of the three states where the switches in the circuit are replaced
by their equivalent circuits during each state. The circuit diagram for each of the three states is shown in
Figure 3.
The duration of the ON state is TON = D × TS where D is the duty cycle, set by the control circuit,
expressed as a ratio of the switch ON time to the time of one complete switching cycle, Ts . The duration
of the OFF state is TOFF = D2 × TS. The IDLE time is the remainder of the switching cycle and is given
as TS – TON – TOFF = D3 × TS. These times are shown with the waveforms in Figure 4.
The main result of this section is a derivation of the voltage conversion relationship for the discontinuous
conduction mode buck power stage. In addition, the dc resistance of the output inductor, the output diode
forward voltage drop, and the power BJT Vce drop are all assumed to be small enough to omit in analysis.
Referring to Figure 3, during the ON state, the voltage applied to the right-hand side of L is simply the
output voltage, Vo. The inductor current, IL, flows from the input source, VI, through Q1 and to the output
capacitor C1 and load resistor RLoad combination. During the ON state, the voltage applied across the
inductor is constant and equal to VI – Vo. Adopting the polarity convention for the current IL shown in
Figure 3, the inductor current increases as a result of the applied voltage. Also, since the applied voltage
is essentially constant, the inductor current increases linearly. This increase in inductor current during
TON is illustrated in Figure 4.
The amount that the inductor current increases can be calculated by using a version of the familiar
relationship:
diL
VL
VL = L ´
® DIL =
´ DT
dt
L
(1)
The inductor current increase during the ON state is given by:
VI - Vo
VI - Vo
DIL( + ) =
´ Ton =
´ D ´ Ts = IPK
L
L
(2)
The ripple current magnitude, ΔIL (+), is also the peak inductor current, IPK.
4
AC-DC Non-Isolated SMPS for Single Phase Smart Meters Based on
UCC28722
SLUA721A – June 2014 – Revised November 2017
Submit Documentation Feedback
Copyright © 2014–2017, Texas Instruments Incorporated
Basic Principle of Buck Topology
www.ti.com
Figure 3. Buck Power Stages in Discontinuous Mode
Referring to Figure 3, when Q1 is OFF, it presents high impedance from its collector to emitter. Therefore,
since the current flowing in the inductor L cannot change instantaneously, the current shifts from Q1 to D1.
Due to the decreasing inductor current, the voltage across the inductor reverses polarity until rectifier D1
becomes forward biased and turns ON. The voltage on the left-hand side of L becomes zero if we neglect
the forward voltage drop of diode D1 and the drop across DC resistance of the inductor. The voltage
applied to the right-hand side of L is still the output voltage, Vo. The inductor current, IL, now flows from
ground through D1 and to the output capacitor C1 and load resistor RLoad combination. During the OFF
state, the magnitude of the voltage applied across the inductor is constant and equal to Vo. Maintaining
our same polarity convention; this applied voltage is negative (or opposite in polarity from the applied
voltage during the ON time). Hence, the inductor current decreases during the OFF time. Also, since the
applied voltage is essentially constant, the inductor current decreases linearly. This decrease in inductor
current during TOFF is illustrated in Figure 4.
The inductor current decrease during the OFF state is given by:
Vo
DIL (- ) =
´ TOFF
L
(3)
This quantity, ΔIL (–), is also referred to as the inductor ripple current.
In steady state conditions, the current increase, ΔIL (+), during the ON time and the current decrease
during the OFF time, ΔIL(–), must be equal. Otherwise, the inductor current would have a net increase or
decrease from cycle to cycle which would not be a steady state condition. Therefore, Equation 2 and
Equation 3 can be equated and solved for Vo to obtain the first of two equations to be used to solve for
the voltage conversion ratio:
TON
D
Vo = VI ´
= VI ´
TON + TOFF
D + D2
(4)
Now we calculate the output current (the output voltage Vo divided by the output load RLoad). It is the
average of the inductor current.
SLUA721A – June 2014 – Revised November 2017
Submit Documentation Feedback
AC-DC Non-Isolated SMPS for Single Phase Smart Meters Based on
UCC28722
Copyright © 2014–2017, Texas Instruments Incorporated
5
Basic Principle of Buck Topology
Io = IL (avg) =
www.ti.com
Vo
IPK
D ´ Ts + D2 ´ Ts
=
´
RLoad
2
Ts
(5)
Now, substitute the relationship for IPK into the above equation to obtain:
Vo
D ´ Ts
= (VI - Vo ) ´
´ (D + D2 )
Io =
RLoad
2´L
(6)
We now have two equations, the one for the output current just derived and the one for the output voltage
(above), both in terms of VI, D, and D2. We now solve each equation for D2 and set the two equations
equal to each other. Using the resulting equation, an expression for the output voltage, Vo, can be
derived.
The discontinuous conduction mode buck voltage conversion relationship is given by:
2
Vo = VI ´
4 ´ K
1+ 1+
D2
(7)
Where K is defined as:
K=
(2
´ L)
RLoad ´ Ts
(8)
IQ1
IPK
Id1
IPK
IL Solid
IO Dashed
IL
VSW Solid
VO Dashed
D x TS
D2 x TS
TS
Figure 4. Discontinuous Mode Power Stage Buck Waveforms
6
AC-DC Non-Isolated SMPS for Single Phase Smart Meters Based on
UCC28722
SLUA721A – June 2014 – Revised November 2017
Submit Documentation Feedback
Copyright © 2014–2017, Texas Instruments Incorporated
Step-by-Step Design Procedure of UCC28722-Based Buck Converter
www.ti.com
3
Step-by-Step Design Procedure of UCC28722-Based Buck Converter
We will be referring to Figure 6 for discussion on choosing the various components for the design.
3.1
AC Input Stage Components
The input stage consists of fusible resistance FR1, Varistor RT1, input rectification diode D1, the line filter
network L1/ C1 and the main bulk electrolytic capacitors C2 and C3.
The input resistance FR1 provides three important functions:
• Acts as a fuse in case of any short in the power supply
• Controls the inrush current going into bulk capacitors
• Aids in differential mode attenuation
As it has to perform these three functions so flame proof and film type resistance or WWR surge
resistance is recommended.
For designs up to 2 W of output power, 8.2–10 Ω 3 W is recommended for FR1.
Regulation IEC 61000-4-5 defines the surge immunity test as high power spikes caused by large inductive
devices in mains. The input of the SMPS is coupled by a short duration (1.2/50 μs) pulses but high voltage
(up to 4 kV). The pulses are applied between L-N and between L (N) – PE at different angles 0°, 90°,
180°, 270°, 360° of the ac voltage. For the energy meter it is applied between L-N as there is no earth.
The surge pulse causes high inrush current, quickly charging the storage capacitor in a standard SMPS.
The major risk is overvoltage for input components - bulk capacitors, rectifier diode and the main BJT
switch. The inrush current can damage the components- rectifier diode, fusible resistance in series in the
input section. Typically, the varistors are used to absorb part of the energy and the rest is absorbed by the
bulk capacitors used.
As the input AC voltage can go as high as 500 VAC, so a 510 VAC, 10-mm varistor RT1 is recommended
for the design for surges up to the 4-kV level. If the application requires surge immunity up to higher levels
than appropriate values and diameter of the varistor along with the input resistance should be chosen.
This topic is beyond the scope of this application note.
As the output ground of the system will be same as the input neutral in case of energy meters, a halfwave rectifier comprising of D1, C2 and C3 are used. As the input AC voltage can go up to 500 VAC, so
the DC voltage can reach voltage levels of up to 707 VDC. The input bulk capacitor must be able to
sustain such voltage levels. There are no standard aluminum capacitors in the market suitable for this
voltage, so two capacitors connected in series have been used. For a half-wave rectifier, choose net
capacitance between 6–8 µF per Watt of output power. Keeping the cost and size in mind, C2 = C3 = 10
µF 400-V capacitors are chosen. TI recommends 1 A, 1000 V, 1N4007 for D1.
L1 and C1 form a differential filter attenuating the differential noise produced by the UCC28722-based
buck converter. The recommended value for L1 is 2.2–4.7 mH and C1 is 47–100 nF.
3.2
Feedback Resistors
The VS divider resistors R1 and R2 determine the output voltage regulation point of the buck converter.
The high-side divider resistor (R1) determines the line voltage at which the controller enables continuous
DRV operation. R1 is initially determined based on desired input voltage operating threshold.
R1 =
VINmin ´ 2
IVSL(run)
(9)
Where
VINmin is the AC RMS voltage to enable turn-on of the controller (run),
IVSL(run) is the run-threshold for the current pulled out of the VS pin during the switch on time
Keep the impedance at Vs pin low, < 100 kΩ, so as to be not effected by switching noise. So substituting
VINmin as 25 VAC instead of 100 VAC and IVSL (run) as 225 µA, R1 comes out to be 157 kΩ. A standard
value of 150 kΩ is chosen.
The low-side VS pin resistor R2 is selected based on desired Vo regulation voltage.
SLUA721A – June 2014 – Revised November 2017
Submit Documentation Feedback
AC-DC Non-Isolated SMPS for Single Phase Smart Meters Based on
UCC28722
Copyright © 2014–2017, Texas Instruments Incorporated
7
Step-by-Step Design Procedure of UCC28722-Based Buck Converter
R2 =
www.ti.com
R1 ´ VVSR
(Vo + VF ) - VVSR
(10)
Where
Vo is the converter regulated output voltage,
VF is the output rectifier forward drop at near-zero current,
R1 is the VS divider high-side resistance,
VVSR is the CV regulating level at the VS input
Substituting Vo as 10 V, VF as 0.7 V, R1 as 150 kΩ, and VVSR as 4.05 V, we get R2 as 91 kΩ so
standard value of 100 kΩ is chosen.
3.3
VDD Capacitance
The capacitance C4 on VDD needs to supply the device operating current until the output of the converter
reaches the target minimum operating voltage in constant-current regulation. At this time, the output
voltage can sustain the voltage to the UCC28722. The total output current available to the load and to
charge the output capacitors is the constant-current regulation target, IOCC. Equation 11 is used to
calculate the value of Capacitance required at the VDD pin:
C5 ´ Vo
æ
æ
öö
Vo
Iocc
Cdd = çç Irun + ç Idrsmax ´
÷ ÷÷ ´
Vddon
Vddoff - 1V )
VINmin ´ 2 ø ø (
è
è
(11)
Where
Idrsmax is the maximum driver source current
Vddoff is the UVLO turn-off voltage
Vddon is the UVLO turn-on voltage
C5 is the output capacitor used
Irun is the supply current in run state with Idrv = 0
Substituting Irun as 2.65 mA, Idrsmax as 41 mA, Vo as 10 V, VINmin as 100 V, C5 as 220 µF, Iocc as
220 mA, Vddon as 21 V, and Vddoff as 8 V we get Cdd as 4.58 µF. So a standard value of 4.7 uF is
chosen. The timing diagram illustrating the startup of UCC28722 is shown in Figure 5.
Vcc
Irun + Idrsmax x
Design
Margin
Vo
VINminxÖ2
UVLOhigh (21 V)
Iv
DV
UVLOlow (8 V)
O/p Voltage
Takes Over
Istr
Power Supply
Regulates
t2
t1
Total Startup Time
Figure 5. Timing Diagram of Startup Sequence in UCC28722
3.4
Startup Resistors and Startup Time
An external resistor connected from the bulk capacitor voltage to the VDD pin charges the VDD capacitor.
The amount of startup current that is available to charge the VDD capacitor is dependent on the value of
this external startup resistor. Smaller values supply more current and decrease startup time but at the
expense of increasing standby power and decreasing efficiency, particularly at high input voltage and light
loading.
8
AC-DC Non-Isolated SMPS for Single Phase Smart Meters Based on
UCC28722
SLUA721A – June 2014 – Revised November 2017
Submit Documentation Feedback
Copyright © 2014–2017, Texas Instruments Incorporated
Step-by-Step Design Procedure of UCC28722-Based Buck Converter
www.ti.com
When VDD reaches the 21-V UVLO turn-on threshold, the controller is enabled and the converter starts
switching. The initial three cycles are limited to IPP(min). After the initial three cycles at minimum IPP(min), the
controller responds to the condition dictated by the control law. The converter will remain in discontinuous
mode during charging of the output capacitor(s), maintaining a constant output current until the output
voltage is in regulation.
Once the VDD capacitor is known, there is a tradeoff to be made between startup time and overall
standby input power to the converter. Faster startup time requires a smaller startup resistance, which
results in higher standby input power.
Rstr =
2 ´ VINmin
VDDon ´ Cdd
Istart +
Tstr
(12)
Where:
VINmin is the minimum voltage at which the converter should work
Istart is the startup current of UCC28722
VDDon is the UVLO turn on threshold
Tstr is the time in which the power supply should be stable at desired O/p voltage
Cdd is the capacitance value at VDD pin
Substituting VINmin as 110 V, Istart as 1.5 µA, VDDon as 21 V, Tstr as 3 s, Cdd as 4.7 µF, Rstr comes
out to be 4.09 MΩ. So we split the required resistance into R4 = R5 = R6 = R7 = 1 MΩ. Splitting the
resistance into four helps meet the required voltage rating.
R5 and R6 resistance midpoint is attached to the midpoint of series capacitor C2 and C3 .This concept
serves to keep the center point of capacitors at exactly half voltage of Vbulk by acting as balancing
resistors.
3.5
Current Sense Resistor
During startup, the converter remains in constant current mode charging the output capacitor until it
comes into voltage loop or regulation. This constant current should be more than the output current
required in order to charge the o/p capacitor at full load. The inductor and the output current is shown in
Figure 6 for UCC28722 in buck discontinuous mode.
Figure 6. Discontinuous Current Waveform in UCC28722
Using Equation 5 for calculation of output current in constant current (CC) mode, substituting Io = Iocc, D2
= toff/T = Dmagcc and ignoring D (ton << toff), as it is very small due to the large differential between VIN
and Vo:
IPK
Iocc =
´ Dmagcc
(13)
2
Reorganizing the Equation 13, the peak inductor current required is given by following equation:
2 ´ Iocc
Ipk =
Dmagcc
SLUA721A – June 2014 – Revised November 2017
Submit Documentation Feedback
AC-DC Non-Isolated SMPS for Single Phase Smart Meters Based on
UCC28722
Copyright © 2014–2017, Texas Instruments Incorporated
(14)
9
Step-by-Step Design Procedure of UCC28722-Based Buck Converter
www.ti.com
Where
Iocc is the converter output constant-current target
Dmagcc is the maximum demagnetization duty cycle
The UCC28722 constant-current regulation is achieved by maintaining a maximum Dmag duty cycle of
0.425 at the maximum inductor current setting. Substituting Iocc as 220 mA (10% more than Io) and
Dmagcc as 0.425, Ipk comes out to be 1.035 A.
During constant current mode the voltage drop across R3 is maintained at 0.78 V. The current sense
resistance R3 is calculated using Equation 15.
0.78
R3 =
Ipk
(15)
Substituting the Ipk value gives R3 as 0.75 Ω.
We can estimate the switching frequency during the exit from CC to CV mode during the start up phase.
Using Equation 3 and substituting ∆IL(-) = IPK and reorganizing the equation, TOFF is given by:
IPK
TOFF =
´ L
Vo
(16)
Substituting IPK as 1.035 A, Vo = 10 V, L = 800 µH, TOFF comes out to be 82.8 µs.
The switching frequency, Fsw, is given by Equation 17.
Dmagcc
Fsw = 1/ Tsw =
TOFF
(17)
Substituting Dmagcc = 0.425 and TOFF as 82.8 µs, Fsw comes out to 5.1 kHz.
3.6
Output Inductor
The output inductor can be calculated by reshuffling Equation 2 as seen in Equation 18:
L2 =
(Ö 2 ´ VINmax) - Vo
´ Ton
Ipk
(18)
Now there is an internal leading-edge blanking time of approximately 300 ns to eliminate sensitivity to the
turn-on current spike. So the worst case is that at maximum input voltage and light or no load the on time
required should be > 300 ns, that is, should be 500 ns with margin added. This implies that at maximum
input voltage and maximum load Ton should be selected as 1.2 µs so that it meets the minimum blanking
time criteria at minimum load.
Substituting VIN max as 500 VAC, Ipk as 1.035 A and Ton as 1.2 µs, Vo as 10 V, L comes out to be799
µH so a standard value of 800 µH is chosen.
3.7
Freewheeling Diode and Vdd Diode
Select D2, a fast rectifier diode with PIV > 1.25 × VINmax and IF >1.5 × Io.
So 1 A, 1000 V, FR107 is chosen for the application.
The worst-case voltage across Vdd diode D3 is the maximum input voltage 707 VDC, so keeping margin 1
A, 1000 V, FR107 is recommended.
3.8
Output Capacitor
In switching power supply power stages, the function of output capacitance is to store energy. The energy
is stored in the capacitor’s electric field due to the voltage applied. Thus, qualitatively, the function of a
capacitor is to attempt to maintain a constant voltage.
The value of output capacitance of a Buck power stage is generally selected to limit output voltage ripple
to the level required by the specification. Since the ripple current in the output inductor is usually already
determined, the series impedance of the capacitor primarily determines the output voltage ripple. The
three elements of the capacitor that contribute to its impedance (and output voltage ripple) are equivalent
series resistance (ESR), equivalent series inductance (ESL), and capacitance (C).
10
AC-DC Non-Isolated SMPS for Single Phase Smart Meters Based on
UCC28722
SLUA721A – June 2014 – Revised November 2017
Submit Documentation Feedback
Copyright © 2014–2017, Texas Instruments Incorporated
Step-by-Step Design Procedure of UCC28722-Based Buck Converter
www.ti.com
For discontinuous inductor current mode operation, to determine the amount of capacitance needed as a
function of inductor current ripple, ΔIL , output current Io, switching frequency, Fsw, and output voltage
ripple, ΔVo, the Equation 16 is used assuming all the output voltage ripple is due to the capacitor’s
capacitance.
2
Io ö
æ
ç 1 - DIL ÷
ø
C ³ Io ´ è
Fsw ´ DVo
(19)
where ΔIL is the inductor ripple current defined as per Equation 2.
In many practical designs, to get the required ESR, a capacitor with much more capacitance than is
needed must be selected.
Assuming there is enough capacitance such that the ripple due to the capacitance can be ignored, the
ESR needed to limit the ripple to ΔVo V peak-to-peak is:
DVo
ESR £
Ipk
(20)
As 10-V output is targeted at relay operation, so taking ΔVo = 400 mV (< 4% of Vo) and Ipk as 1.035 A,
ESR of capacitor required is < 0.386 Ω. So 220 µF, 25 V with impedance of 0.13 Ω is chosen for the
application.
3.9
Bipolar Junction Transistor
Bipolar junction transistor is selected based on three main specifications:
• Minimum current gain: hfe
• VCE(sus) breakdown
• Current rating
The current gain required is calculated by Equation 21:
hfe = IPK / Idrs
(21)
Where
IPK is the peak current in constant current mode
Idrs is the source current of the drive
Substituting Ipk = 1.035 A and Idrs as 37 mA, we get hfe as 27.9. The current rating of the BJT should be
> 1.5 × IPK and voltage should be > 1.1 × VINmax.
The KSC5026MOS, BJT is chosen for Q1 which satisfies all the above three criteria.
SLUA721A – June 2014 – Revised November 2017
Submit Documentation Feedback
AC-DC Non-Isolated SMPS for Single Phase Smart Meters Based on
UCC28722
Copyright © 2014–2017, Texas Instruments Incorporated
11
Step-by-Step Design Procedure of UCC28722-Based Buck Converter
www.ti.com
3.10 LDO for 3.3 V
3.3 V is required for powering the microcontroller and its peripherals in energy meters. As the meter will be working mostly at 3.3 V at 2–3 mA with
no load on 10 V, low quiescent current LDO is required in order to target low standby power. The TLV70433DBVR with quiescent current of 3.2 µA
is chosen for the application.
Figure 7 shows the schematic of the PMP7668 non-isolated buck converter.
FR1
10, 3W
D1
L1
1N4007
4.7mH
Q1
KSC5026M
R3
L2
+
10V
0.75
820uH
R4
1M
AC
RT1
510V
AC IN
80 - 500 Vac
U2
TLV70433
+
C2
10uF
C1
0.1uF
GND
1
IN
2
OUT
3
+
R6
R7
3 DRV
CS
2 VDD
AC
1M
1M
VS
6
D2
FR107
+
C5
220 uF
R8
68.1K
C6
4 NC
3.3V
0.1uF
C7
D3
FR107
C3
10uF
GND
4
5
1 CBC/NTC
0.1uF
C4
+
+
5 NC
U1
UCC28722
R5
1M
10uF
R1
R2
150K
100K
RTN
-
Figure 7. Complete Schematic of the PMP7668 Non-Isolated Buck Converter
12
AC-DC Non-Isolated SMPS for Single Phase Smart Meters Based on
UCC28722
Copyright © 2014–2017, Texas Instruments Incorporated
SLUA721A – June 2014 – Revised November 2017
Submit Documentation Feedback
Bill of Materials
www.ti.com
4
Bill of Materials
Table 2 lists the bill of materials.
Table 2. Bill of Materials of Complete Schematic
Qty
Reference
Value
Description
Part Number
Manufacturer
Size
1
C1
0.1µF
Capacitor, Leaded, 760 VDC, ±10%
PHE840MA6100KA04R17
Kemet
0.157 × 0.512 inch
2
C2, C3
10µF
Capacitor, Alum Electrolytic 400V, ±20%
UCA2G100MPD1TD
Nichicon
10.00 mm Dia
1
C4
10µF
Capacitor, Alum Elect, 25V, ±20%
Std
Std
5 × 11 mm
1
C5
220µF
Capacitor, Alum Electrolytic, 25V, ±20%
25YXG220MEFC8X11.5
Rubycon
8 × 11.5 mm
2
C6, C7
0.1µF
Capacitor, Ceramic Chip, X7R, 50V, ±10%
Std
Std
805
1
D1
1N4007
Diode, Rectifier, 1000V, 1A
1N4007
Diodes
DO-41
2
D2, D3
FR107
Diode, Fast, 1000V, 1A
FR107
Fairchild
DO-41
1
FR1
10
Film resistor, 3W
Std
Std
Std
1
L1
4.7mH
Inductor, 150mA, 25Ω
744741472
Wurth Elektronik
8.5 × 5.5mm
1
L2
820µH
Inductor, 0.7A, 1.56Ω
7447728215
Wurth Elektronik
7.8 × 9.5mm
1
Q1
KSC5026M
Trans, NPN Medium Power, 800V, 1.5A
KSC5026MOS
Fairchild
TO-126
1
R1
150K
Resistor, Chip, 1/4W, 1%
Std
Std
1206
1
R2
100K
Resistor, Chip, 1/8W, 1%
Std
Std
805
1
R3
0.75
Resistor, Chip, 1/2W, 5%
CRL1206-FW-R750ELF
Bourns
1206
4
R4–R7
1M
Resistor, Chip, 1/4W, 5%
Std
Std
1206
1
R8
68.1K
Resistor, Chip, 1/8W, 1%
Std
Std
805
1
RT1
510V
MOV, 510VAC
MOV-10D821KTR
Bourns
10mm dia
UCC28722DBV
TI
SOT-23
TLV70433DBV
TI
SOT-23
1
U1
UCC28722
IC, CV/CC PWM With Primary Side
Regulation
1
U2
TLV70433
IC, 24-V Input, 150 mA, Ultralow IQ LDO
Regulator
SLUA721A – June 2014 – Revised November 2017
Submit Documentation Feedback
AC-DC Non-Isolated SMPS for Single Phase Smart Meters Based on
UCC28722
Copyright © 2014–2017, Texas Instruments Incorporated
13
PCB Details
5
www.ti.com
PCB Details
The converter is assembled on a single layer 69 × 26 mm, 35 μm, FR4 PCB. The PCB top silk screen
(Figure 8), PCB bottom silk screen (Figure 9), and PCB composite layout (Figure 10) along with the
position of the components are illustrated in the following images.
Figure 8. PCB Top Silk Screen
Figure 9. PCB Bottom Silk Screen
Figure 10. PCB Composite Layout
14
AC-DC Non-Isolated SMPS for Single Phase Smart Meters Based on
UCC28722
SLUA721A – June 2014 – Revised November 2017
Submit Documentation Feedback
Copyright © 2014–2017, Texas Instruments Incorporated
Conclusion
www.ti.com
6
Conclusion
This document shows that it is possible to implement a low power, non-isolated SMPS operating in a buck
converter topology for wide input range (100–500 VAC), thanks to the new PSR CV CC PWM controller,
UCC28722 which can drive a low cost BJT.
7
References
1. UCC28722 Constant-Voltage, Constant-Current Controller With Primary-Side Regulation, BJT Drive
datasheet (SLUSBL7A)
2. Understanding Buck Power Stages in Switch mode Power Supplies (SLVA057)
3. Application Report-UCC28722/UCC28720 5W USB BJT Flyback Design (SLUA0700)
SLUA721A – June 2014 – Revised November 2017
Submit Documentation Feedback
AC-DC Non-Isolated SMPS for Single Phase Smart Meters Based on
UCC28722
Copyright © 2014–2017, Texas Instruments Incorporated
15
Appendix A
SLUA721A – June 2014 – Revised November 2017
Experimental Results
Efficiency was measured for 10 V/200 mA for input 110–275 VAC and 400–705 VDC.
A.1
Efficiency and Regulation Performance Data
Figure 11. Efficiency vs AC I/p Voltage (110 V–275 V)
Figure 12. Efficiency vs DC I/p Voltage (400 V–705 V)
Efficiency was measured for variation in load from 2 mA to 220 mA on 10-V output at 325 VDC.
16
AC-DC Non-Isolated SMPS for Single Phase Smart Meters Based on
UCC28722
SLUA721A – June 2014 – Revised November 2017
Submit Documentation Feedback
Copyright © 2014–2017, Texas Instruments Incorporated
Efficiency and Regulation Performance Data
www.ti.com
Figure 13. Efficiency vs O/p Load Current at 325 VDC
Output 10-V variation was plotted with change in load current from 2 mA to 220 mA at 325 V DC input.
Figure 14. O/p Voltage Variation vs O/p Load Current at 325 VDC
SLUA721A – June 2014 – Revised November 2017
Submit Documentation Feedback
AC-DC Non-Isolated SMPS for Single Phase Smart Meters Based on
UCC28722
Copyright © 2014–2017, Texas Instruments Incorporated
17
Switch Node Waveforms
A.2
www.ti.com
Switch Node Waveforms
Waveform at SW node was observed along with the collector current for 110 VAC input and 10-V output
loaded to 200 mA. The settings of the oscilloscope are as follows:
Red trace: SW node voltage, 50 V/div; Blue trace: collector current, 500 mA/div.
Figure 15. SW Node Waveform and Collector Current at Vin = 110 VAC, Full Load
Figure 16. Zoomed SW Waveform and Collector Current at Vin = 110 VAC, Full Load
18
AC-DC Non-Isolated SMPS for Single Phase Smart Meters Based on
UCC28722
SLUA721A – June 2014 – Revised November 2017
Submit Documentation Feedback
Copyright © 2014–2017, Texas Instruments Incorporated
Switch Node Waveforms
www.ti.com
Waveform at SW node was observed along with the collector current for 275 VAC input and 10-V output
loaded to 200 mA. The settings of the oscilloscope are as follows:
Red trace: SW node voltage, 100 V/div; Blue trace: collector current, 500 mA/div.
Figure 17. SW node Waveform and Collector Current at Vin = 275 VAC, Full Load
Figure 18. Zoomed SW Waveform and Collector Current at Vin = 275 VAC, Full Load
SLUA721A – June 2014 – Revised November 2017
Submit Documentation Feedback
AC-DC Non-Isolated SMPS for Single Phase Smart Meters Based on
UCC28722
Copyright © 2014–2017, Texas Instruments Incorporated
19
Vout Ripple
A.3
www.ti.com
Vout Ripple
Ripple is observed at 10-V output loaded to 200 mA at 110 VAC and 275 VAC.
Figure 19. Vout Ripple at Vin = 110 VAC, Full Load
Figure 20. Vout Ripple at Vin = 275 VAC, Full Load
20
AC-DC Non-Isolated SMPS for Single Phase Smart Meters Based on
UCC28722
SLUA721A – June 2014 – Revised November 2017
Submit Documentation Feedback
Copyright © 2014–2017, Texas Instruments Incorporated
Vout Turn On Characteristics
www.ti.com
A.4
Vout Turn On Characteristics
The 10-V output turn on at 200 mA was recorded at 110 VAC along with DC bus. The settings of CRO are
as follows:
Red trace: Input DC bus, 50 V/div; Yellow trace: Output voltage, 5 V/div.
Figure 21. Vout Turn On Waveform at Vin = 110 VAC, Full Load
The 10-V output turn on at 200 mA was recorded at 275 VAC along with DC bus. The settings of CRO are
as follows:
Red trace: Input DC bus, 100 V/div; Yellow trace: Output voltage, 2 V/div.
SLUA721A – June 2014 – Revised November 2017
Submit Documentation Feedback
AC-DC Non-Isolated SMPS for Single Phase Smart Meters Based on
UCC28722
Copyright © 2014–2017, Texas Instruments Incorporated
21
Vout Turn On Characteristics
www.ti.com
Figure 22. Vout Turn On Waveform at Vin = 275 VAC, Full Load
22
AC-DC Non-Isolated SMPS for Single Phase Smart Meters Based on
UCC28722
SLUA721A – June 2014 – Revised November 2017
Submit Documentation Feedback
Copyright © 2014–2017, Texas Instruments Incorporated
Vout Transient Response
www.ti.com
A.5
Vout Transient Response
Vin = 230 VAC, load transient from 2 mA to 200 mA at 10-V output.
Figure 23. Vout Waveform at Vin = 230 VAC, Load Transient from 2 mA to 200 mA
A.6
Conducted Emissions
230 VAC input, 200-mA resistive load connected to PSU with short leads. The conducted emissions in a
pre-compliance test set-up were compared against EN55022 class B limits and found to meet them
comfortably.
SLUA721A – June 2014 – Revised November 2017
Submit Documentation Feedback
AC-DC Non-Isolated SMPS for Single Phase Smart Meters Based on
UCC28722
Copyright © 2014–2017, Texas Instruments Incorporated
23
Standby Power and VA
www.ti.com
Figure 24. Conducted Emissions as per EN55022 Class B
A.7
Standby Power and VA
The standby power and VA was noted at 240 VAC input with a 2-mA load on 3.3 V and no load on 10 V.
The results were in accordance with the target specifications.
Power consumption = 59 mW
VA = 242 mVA
A.8
Surge Test as per IEC61000-4-5
The 4-kV surge test as per IEC61000-4-5 was performed on the board with no failures.
A.9
EFT Test as per IEC61000-4-4
IEC 61000-4-4 defines the burst immunity test as fast switching disturbance presented in the mains. This
test means the high frequency, high voltage 4 kV, very short pulses (50 ns) are applied between the input
line and neutral. The possible impact seen in the SMPS is unstable operation, that is, restarting of the
SMPS or latching of the SMPS.
The 4-kV burst test was performed on the board with no failures.
24
AC-DC Non-Isolated SMPS for Single Phase Smart Meters Based on
UCC28722
SLUA721A – June 2014 – Revised November 2017
Submit Documentation Feedback
Copyright © 2014–2017, Texas Instruments Incorporated
Revision History
www.ti.com
Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Original (June 2014) to A Revision ......................................................................................................... Page
•
Changed incorrect part number to PMP7668 globally. ............................................................................... 1
SLUA721A – June 2014 – Revised November 2017
Submit Documentation Feedback
Copyright © 2014–2017, Texas Instruments Incorporated
Revision History
25
IMPORTANT NOTICE FOR TI DESIGN INFORMATION AND RESOURCES
Texas Instruments Incorporated (‘TI”) technical, application or other design advice, services or information, including, but not limited to,
reference designs and materials relating to evaluation modules, (collectively, “TI Resources”) are intended to assist designers who are
developing applications that incorporate TI products; by downloading, accessing or using any particular TI Resource in any way, you
(individually or, if you are acting on behalf of a company, your company) agree to use it solely for this purpose and subject to the terms of
this Notice.
TI’s provision of TI Resources does not expand or otherwise alter TI’s applicable published warranties or warranty disclaimers for TI
products, and no additional obligations or liabilities arise from TI providing such TI Resources. TI reserves the right to make corrections,
enhancements, improvements and other changes to its TI Resources.
You understand and agree that you remain responsible for using your independent analysis, evaluation and judgment in designing your
applications and that you have full and exclusive responsibility to assure the safety of your applications and compliance of your applications
(and of all TI products used in or for your applications) with all applicable regulations, laws and other applicable requirements. You
represent that, with respect to your applications, you have all the necessary expertise to create and implement safeguards that (1)
anticipate dangerous consequences of failures, (2) monitor failures and their consequences, and (3) lessen the likelihood of failures that
might cause harm and take appropriate actions. You agree that prior to using or distributing any applications that include TI products, you
will thoroughly test such applications and the functionality of such TI products as used in such applications. TI has not conducted any
testing other than that specifically described in the published documentation for a particular TI Resource.
You are authorized to use, copy and modify any individual TI Resource only in connection with the development of applications that include
the TI product(s) identified in such TI Resource. NO OTHER LICENSE, EXPRESS OR IMPLIED, BY ESTOPPEL OR OTHERWISE TO
ANY OTHER TI INTELLECTUAL PROPERTY RIGHT, AND NO LICENSE TO ANY TECHNOLOGY OR INTELLECTUAL PROPERTY
RIGHT OF TI OR ANY THIRD PARTY IS GRANTED HEREIN, including but not limited to any patent right, copyright, mask work right, or
other intellectual property right relating to any combination, machine, or process in which TI products or services are used. Information
regarding or referencing third-party products or services does not constitute a license to use such products or services, or a warranty or
endorsement thereof. Use of TI Resources may require a license from a third party under the patents or other intellectual property of the
third party, or a license from TI under the patents or other intellectual property of TI.
TI RESOURCES ARE PROVIDED “AS IS” AND WITH ALL FAULTS. TI DISCLAIMS ALL OTHER WARRANTIES OR
REPRESENTATIONS, EXPRESS OR IMPLIED, REGARDING TI RESOURCES OR USE THEREOF, INCLUDING BUT NOT LIMITED TO
ACCURACY OR COMPLETENESS, TITLE, ANY EPIDEMIC FAILURE WARRANTY AND ANY IMPLIED WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, AND NON-INFRINGEMENT OF ANY THIRD PARTY INTELLECTUAL
PROPERTY RIGHTS.
TI SHALL NOT BE LIABLE FOR AND SHALL NOT DEFEND OR INDEMNIFY YOU AGAINST ANY CLAIM, INCLUDING BUT NOT
LIMITED TO ANY INFRINGEMENT CLAIM THAT RELATES TO OR IS BASED ON ANY COMBINATION OF PRODUCTS EVEN IF
DESCRIBED IN TI RESOURCES OR OTHERWISE. IN NO EVENT SHALL TI BE LIABLE FOR ANY ACTUAL, DIRECT, SPECIAL,
COLLATERAL, INDIRECT, PUNITIVE, INCIDENTAL, CONSEQUENTIAL OR EXEMPLARY DAMAGES IN CONNECTION WITH OR
ARISING OUT OF TI RESOURCES OR USE THEREOF, AND REGARDLESS OF WHETHER TI HAS BEEN ADVISED OF THE
POSSIBILITY OF SUCH DAMAGES.
You agree to fully indemnify TI and its representatives against any damages, costs, losses, and/or liabilities arising out of your noncompliance with the terms and provisions of this Notice.
This Notice applies to TI Resources. Additional terms apply to the use and purchase of certain types of materials, TI products and services.
These include; without limitation, TI’s standard terms for semiconductor products http://www.ti.com/sc/docs/stdterms.htm), evaluation
modules, and samples (http://www.ti.com/sc/docs/sampterms.htm).
Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265
Copyright © 2017, Texas Instruments Incorporated
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Related manuals

Download PDF

advertising