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Power Management
Texas Instruments Incorporated
A low-cost, non-isolated AC/DC buck
converter with no transformer
By Jeff Falin, Senior Applications Engineer,
and Dave Parks, Senior Member, Technical Staff
Introduction
Off-line equipment such as a smart meter or a power
monitor has electronics that require non-isolated DC power
under 10 W. Until recently, the only practical options for
providing a low-power DC power rail from an AC source
were to use an extremely inefficient, unregulated resistive/
capacitive divider following the rectifier, or a flyback DC/DC
converter that was cumbersome to design. Advances in
MOSFET technology and an innovative gate-drive circuit
for a hysteretic buck controller have resulted in an ultra-
low-cost DC power rail.
Figure 1 shows the entire converter. The rectifier circuit
uses a standard, fast-switching rectifier diode bridge (D1)
and an LC filter (L1 and C2). The remaining components
will be explained in more detail.
The basic buck converter
The TPS64203 is a hysteretic buck controller designed to
drive a high-side pFET and has minimum turn-on and
minimum turn-off switching-time requirements. Unlike a
traditional hysteretic converter with a switching frequency
that varies with load current, the minimum on and off
times essentially clamp the switching frequency when the
converter begins to run in continuous-conduction mode at
high output-power levels. Other members of the TPS6420x
family actively avoid switching in the audible frequency
range, effectively having a maximum on and off time.
Originally designed for battery-powered applications, the
TPS6420x family has an input-voltage range of 1.8 V to
6.5 V and very low quiescent current (35 µA maximum).
During start-up, the TPS64203 is biased by Zener diode
Figure 1. AC/DC buck-converter circuit
L1
470 µH
V IN
1
2
+
C5
0.1 µF
400 V
D1
HD04
+
C2
2.2 µF
R5
200
Q2
C4
0.1 µF
3904
AC
AC
C1
0.01 µF
400 V
120 to
230 V RMS
S
–
FQD2P40
Q4
Q3
G
3906
L2
470 µH
F1
1-A Fuse
R2
200 k
D
D5
ES1G
V
5 VDC at
750 mA
+
D3
C6
68 µF
OUT
R3
200 k
BAS16
D4
BAT54
D2
5.1 V
Q1
FCX658ATA
R6
3.74 k
U1
TPS64203DBV
R4
66.5
1
2
3
6
5
4
EN
GND
FB
SW
VIN
ISENSE
R1
1.21 k
C3
1 µF
16
High-Performance Analog Products
4Q 2010
Analog Applications Journal
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Texas Instruments Incorporated
Power Management
D2 and high-voltage resistors R2 and R3. After the 5-V rail
is up, Schottky diode D4 allows the 5-V output rail to
power the controller.
Power FET Q4 must have a high enough V DS voltage
rating not to be damaged by the input voltage, and a high
e nough current rating to handle I PMOS(RMS) = I OUT(max) ×
√D max . It must also be in a package capable of dissipating
P Cond = (I OUT(max) × √D max ) 2 × R DS(on) . Traditionally, high-
voltage p-channel FETs have had a gate capacitance or
turn-on/off times that were too large, a drain-to-source
resistance (R DS(on) ) that was too high, a threshold voltage
(V TH ) that was too large, and/or have simply been too
expensive to make a circuit like the one in Figure 1 practi-
cal (i.e., efficient enough relative to cost). Since the high
line of 230 V RMS + 10% tolerance comes from the 350-V PK
AC line, the FET, filter, and input capacitors need to be
rated for 400 V.
The FQD2P40 is a relatively new, 400-V p-channel
MOSFET. With an R DS(on) of 5.0 Ω from a 10-V gate drive
and a total gate charge of less than 13 nC, this FET can
easily be switched by the controller—with relatively fewer
conductive and switching losses than older FETs—with the
help of the innovative drive circuit consisting of Q2, Q3, C4,
and D3. The converter’s rectifying Schottky diode, D5, is
selected with a voltage rating capable of blocking the input
voltage, a peak-current rating slightly higher than the out-
put voltage, and an average current rating of I Diode(Avg) =
(1 – D) × I OUT(max). With a D max of 5 V/120 V = 0.04 and
such low output power, the peak-current rating and the
power dissipation are not a concern in either switch.
The buck power stage’s LC filter is designed as explained
in the TPS6420x family data sheet. With the input voltage
being much larger than the output voltage, all of the
TPS6420x controllers will run in minimum-on-time mode.
Equation 1 computes the recommended buck-converter
inductance at high line, assuming that K = 0.4 for the
inductor’s ripple-current factor.
(V
−
V
)
×
t
( 230 V
−
5 V )
×
0.65 s
µ
IN
OUT
on (min)
L
=
=
∆
I
0.4
×
0.750 A
(1)
L
= µ→ µ
488 H
470 H
The relatively high K value minimizes inductor size and
proves to be acceptable because the steady-state output-
ripple requirement for this particular application was no
larger than 0.02 × V OUT , or 100 mV PP at high load. Being
hysteretic, the TPS6420x controllers typically work best
with some ripple on the output voltage. An output capaci-
tor with at least 50-mΩ ESR is recommended and would
produce a ripple voltage of ∆V PP(ESR) = ∆I L × R ESR , which
typically far exceeds the capacitive component of the volt-
age ripple. The measured ripple for this application is
shown in Figure 2.
Because the TPS64203 is hysteretic, its output voltage
will have higher ripple at lower output power when it is
running in pulsed-frequency mode. The measured operat-
ing frequency of the converter is approximately 32 kHz,
which agrees with the predicted value of
D
5 V/250 V
min
f
=
=
=
31 kHz.
SW
t
0.65 s
µ
on (min)
How the drive circuit works
Bipolar transistor Q1 and resistors R4 and R5 form a
constant-current-driven level shifter that allows the low-
voltage TPS64203 controller to operate the discrete gate-
drive circuit formed by Q2 and Q3. Like the controller, the
level shifter is powered by Zener diode D2 at start-up and
the regulated 5-V rail, through Schottky diode D4, after
start-up. Power FET Q4’s gate must be overdriven just
enough to provide the required output current with an
acceptable R DS(on) . Too much drive increases switching
losses, while too little increases conduction losses. From a
review of the FQD2P40 data sheet and some trial and
error, V GS ≅ 12 V was selected.
Capacitor C4 and diode D3 are critical to the drive
circuit’s functionality. Resistor R5 is selected to set the
gate-drive level of 12 V below the voltage at the rectifier’s
output. Diode D3 clamps capacitor C4 to this level. Specif-
ically, when U1’s switch pin outputs a low signal to turn on
the power FET, the signal gets level shifted to the base of
Q3. Transistor Q3 turns on and quickly charges Q4’s gate-
to-source capacitance, C GS , to 12 V. Without C4 and D3,
turning off Q4 would have required Q3 to be an expensive,
high-voltage bipolar transistor with its drain tied to ground.
When U1’s switch pin outputs a high signal to turn off the
power FET, the signal gets level shifted to the base of Q2.
Q2 turns on, effectively tying Q4’s gate to the input voltage.
It is important to note that without capacitor C4 acting as
a local power supply, transistors Q2 and Q3 would not be
able to provide the fast current spikes necessary to quickly
—and therefore efficiently—pull up or pull down Q4’s gate
Figure 2. Output ripple at V IN = 250 VDC and
I OUT = 500 mA
Time (5 µs/div)
17
Analog Applications Journal
4Q 2010
High-Performance Analog Products
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Power Management
Texas Instruments Incorporated
capacitance. Also, the level shifter’s current, I LS , set by R4,
must be high enough to move Q4’s gate charge, Q Gate ,
during the t on(min) . That is,
Figure 3. Q4 gate and drain voltages during
one switching cycle
I
=
V
−
V
Q
LS
Z( D 4 )
BE
Gate
on (min)
>>
.
R4
t
Capacitor C4 is sized to be much larger than Q4’s gate
capacitance, but it must be small enough that it can be
recharged during the shorter of the controller’s minimum
on and off times. Figure 3 shows the gate and drain turn-
on/off times during one switching cycle with an input
voltage of 300 V and a 500-mA load. Measured conversion
efficiency is shown in Table 1.
Current limit and soft start
In low-voltage applications, the TPS6420x uses a high-
side current-limit circuit to compare the drop across
a current-sense resistor, placed between the VIN and
ISENSE pins, to a reference voltage. If the voltage across
the sense resistor exceeds that voltage, the circuit turns
off the switch, thereby implementing a pulse-by-pulse
current limit. In a high-voltage application, the current-
limit circuit cannot be used without overvoltage on the
ISENSE pin, so the ISENSE pin is tied high to VIN. There-
fore, the circuit in Figure 1 does not have a current limit.
A high-side series fuse is recommended to provide short-
circuit protection.
In typical applications during start-up, the TPS64203’s
current-limit value is slowly ramped up to provide a
current-limited, controlled soft start. In this application,
the current-limit circuit and therefore the soft start are
disabled; therefore, the start-up inrush current may be
large and the output voltage may overshoot slightly, as
shown in Figure 4.
Conclusion
Using a level shifter and gate driver with a localized power
source allows the use of a low-voltage buck controller to
provide a DC voltage from an AC power source. Conversion
efficiency near 60% can be achieved by using a simple
circuit and no transformer. This circuit can also be used
for DC/DC conversion where the input DC voltage is above
the maximum rating of the TPS6420x.
Related Web sites
Q4 Gate Voltage
( 5 V/div )
Q4 Drain Voltage
( 100 V/div )
Time(0.1 µs/div)
Table 1. Measured conversion efficiency
V IN
(V)
I IN
(A)
P IN
(W)
I OUT
(A)
V OUT
(V)
P OUT
(W)
EFFICIENCY
(%)
100
0.043
4.3
0.5
5.023
2.5115
58.40698
200
0.021
4.2
0.5
5.023
2.5115
59.79762
300
0.015
4.5
0.5
5.023
2.5115
55.81111
100
0.066
6.6
0.75
5.023
3.76725
57.07955
200
0.031
6.2
0.75
5.023
3.76725
60.7621
300
0.022
6.6
0.75
5.023
3.76725
57.07955
Figure 4. Start-up into a 10- Ω load with
V IN = 300 V
Time ( 1 ms/div )
18
High-Performance Analog Products
4Q 2010
Analog Applications Journal
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SLYT391
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