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SLUS577C − SEPTEMBER, 2003 − REVISED MARCH 2009
   
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FEATURES
DControls Boost Preregulator to Near-Unity
Power Factor
DLimits Line Distortion
DWorld Wide Line Operation
DOver-Voltage Protection
DAccurate Power Limiting
DAverage Current Mode Control
DImproved Noise Immunity
DImproved Feed-Forward Line Regulation
DLeading Edge Modulation
D150-µA Typical Start-Up Current
DLow-Power BiCMOS Operation
D12-V to 17-V Operation
DFrequency Range of 6 kHz to 220 kHz
DESCRIPTION
The UCC3817A and the UCC3818A family
provides all the functions necessary for active
power factor corrected preregulators. The
controller achieves near unity power factor by
shaping the ac input line current waveform to
correspond to that of the ac input line voltage.
Average current mode control maintains stable,
low distortion sinusoidal line current.
Designed i n Texas Instrument’s BiCMOS process,
the UCC3817A/UCC3818A offers new features
such as lower start-up current, lower power
dissipation, overvoltage protection, a shunt UVLO
detect circuitry, a leading-edge modulation
technique to reduce ripple current in the bulk
capacitor and an improved, low-offset (±2 mV)
current amplifier to reduce distortion at light load
conditions.
BLOCK DIAGRAM
UDG-03122
VREF9
2
16
1
15
10
5
4
DRVOUT
GND
CAI
VCC
OVP/EN
VAOUT
1.9 V
PKLMT
7.5 V
REFERENCE
UVLO
16 V/10 V (UCC3817A)
10.5 V/10 V (UCC3818A)
VCC
3
OSCILLATOR
12
RT
14
CT
SQ
R
PWM
LATCH
+
PWM
CAOUT
+
+
+
SS
VOLTAGE
ERROR AMP 8.0 V
13
7
11VSENSE
VFF 8
IAC 6
MOUT
MIRROR
2:1
X2
+
7.5 V
ENABLE
OVP
÷
X
XMULT
OSC
CLK
CLK
CURRENT
AMP
16 V (FOR UCC3817A ONLY)
+
0.33 V ZERO POWER
R
+
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Copyright 2006 − 2009, Texas Instruments Incorporated
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
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DESCRIPTION (CONTINUED)
The UCC3817A/18A family of PFC Controllers is directly pin for pin compatible with the UCC3817/18 family of
devices. Only the output stage of UCC3817A family has been modified to allow use of a smaller external gate
drive resistor values. For some power supply designs where an adequately high enough gate drive resistor can
not be used, the UCC3817A/18A family offers a more robust output stage at the cost of increasing the internal
gate resistances. The gate drive of the UC3817A/18A family however remains strong at ±1.2 A of peak current
capability.
UCC3817A offers an on-chip shunt regulator with low start-up current, suitable for applications utilizing a
bootstrap supply. UCC3818A is intended for applications with a fixed supply (VCC). Both devices are available
in the 16-pin D, N and PW packages.
PIN CONNECTION DIAGRAM
D, N, AND PW PACKAGES
(TOP VIEW)
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
GND
PKLMT
CAOUT
CAI
MOUT
IAC
VAOUT
VFF
DRVOUT
VCC
CT
SS
RT
VSENSE
OVP/EN
VREF
AVAILABLE OPTIONS TABLE
PACKAGE DEVICES
T = T
SOIC (D) PACKAGE(1) PDIP (N) PACKAGE TSSOP (PW) PACKAGE(1)
TA = TJTurn-on
Threshold
16 V
Turn-on
Threshold
10.2 V
Turn-on
Threshold
16 V
Turn-on
Threshold
10.2 V
Turn-on
Threshold
16 V
Turn-on
Threshold
10.2 V
−40°C to 85°C UCC2817AD UCC2818AD UCC2817AN UCC2818AN UCC2817APW UCC2818APW
0°C to 70°C UCC3817AD UCC3818AD UCC3817AN UCC3818AN UCC3817APW UCC3818APW
NOTES: (1) The D and PW packages are available taped and reeled. Add R suffix to the device type (e.g. UCC3817ADR) to order quantities
of 2,500 devices per reel (D package) and 2,000 devices per reel (for PW package). Bulk quantities are 40 units (D package) and
90 units (PW package) per tube.
THERMAL RESISTANCE TABLE
PACKAGE θjc(°C/W) θja(°C/W)
SOIC−16 (D) 22 40 to 70 (1)
PDIP−16 (N) 12 25 to 50 (1)
TSSOP−16 (PW) 14 (2) 123 to 147 (2)
NOTES: (1) Specified θja (junction to ambient) is for devices mounted to 5-inch2 FR4 PC board with one ounce copper
where noted. When resistance range is given, lower values are for 5 inch2 aluminum PC board. Test PWB
was 0.062 inch thick and typically used 0.635-mm trace widths for power packages and 1.3-mm trace
widths for non-power packages with a 100-mil x 100-mil probe land area at the end of each trace.
(2) Modeled data. If value range given for θja, lower value is for 3x3 inch. 1 oz internal copper ground plane,
higher value is for 1x1-inch. ground plane. All model data assumes only one trace for each non-fused
lead.
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ABSOLUTE MAXIMUM RATINGS
over operating free-air temperature (unless otherwise noted)
UCCx81xA UNIT
Supply voltage VCC 18 V
Supply current ICC 20 mA
Gate drive current, continuous 0.2
A
Gate drive current 1.2 A
Input voltage, CAI, MOUT, SS 8
Input voltage, PKLMT 5V
Input voltage, VSENSE, OVP/EN 10
V
Input current, RT, IAC, PKLMT 10
mA
Input current, VCC (no switching) 20 mA
Maximum negative voltage, DRVOUT, PKLMT, MOUT −0.5 V
Power dissipation 1 W
Junction temperature, TJ−55 to 150
Storage temperature, Tstg −65 to 150 °C
Lead temperature, Tsol (soldering, 10 seconds) 300
C
Power dissipation 1 W
Stresses beyond those listed under “absolute maximum ratings” may cause permanent damage to the device. These are stress ratings only, and
functional operation of the device at these or any other conditions beyond those indicated under recommended operating conditions is not implied.
Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
TA = 0°C to 70°C for the UCC3817A and TA = −40°C to 85°C for the UCC2817A, TA = TJ, VCC = 12 V, RT = 22 k,
CT = 270 pF, (unless otherwise noted)
PARAMETER TEST CONDITIONS MIN TYP MAX UNITS
Supply Current Section
Supply current, off VCC = (VCC turn-on threshold −0.3 V) 150 300 µA
Supply current, on VCC = 12 V, No load on DRVOUT 2 4 6 mA
UVLO Section
VCC turn-on threshold (UCCx817) 15.4 16 16.6
VCC turn-off threshold (UCCx817) 9.4 9.7
UVLO hysteresis (UCCx817) 5.8 6.3
Maximum shunt voltage (UCCx817) IVCC = 10 mA 15.4 17 17.5 V
VCC turn-on threshold (UCCx818) 9.7 10.2 10.8
V
VCC turn-off threshold (UCCx818) 9.4 9.7
UVLO hysteresis (UCCx818) 0.3 0.5
Voltage Amplifier Section
Input voltage
TA = 0°C to 70°C 7.387 7.5 7.613
V
Input voltage TA = −40°C to 85°C 7.369 7.5 7.631 V
VSENSE bias current VSENSE = VREF, VAOUT = 2.5 V 50 200 nA
Open loop gain VAOUT = 2 V to 5 V 50 90 dB
High-level output voltage IL = −150 µA 5.3 5.5 5.6 V
Low-level output voltage IL = 150 µA 0 50 150 mV
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ELECTRICAL CHARACTERISTICS
TA = 0°C to 70°C for the UCC3817A and TA = −40°C to 85°C for the UCC2817A, TA = TJ, VCC = 12 V, RT = 22 k,
CT = 270 pF, (unless otherwise noted)
PARAMETER TEST CONDITIONS MIN TYP MAX UNITS
Over Voltage Protection and Enable Section
Over voltage reference VREF
+0.48 VREF
+0.50 VREF
+0.52 V
Hysteresis 300 500 600 mV
Enable threshold 1.7 1.9 2.1
V
Enable hysteresis 0.1 0.2 0.3 V
Current Amplifier Section
Input offset voltage VCM = 0 V, VCAOUT = 3 V −3.5 0 2.5 mV
Input bias current VCM = 0 V, VCAOUT = 3 V −50 −100
nA
Input offset current VCM = 0 V, VCAOUT = 3 V 25 100 nA
Open loop gain VCM = 0 V, VCAOUT = 2 V to 5 V 90
dB
Common-mode rejection ratio VCM = 0 V to 1.5 V, VCAOUT = 3 V 60 80 dB
High-level output voltage IL = −120 µA 5.6 6.5 6.8
V
Low-level output voltage IL = 1 mA 0.1 0.2 0.5 V
Gain bandwidth product (1) 2.5 MHz
Voltage Reference Section
Input voltage
TA = 0°C to 70°C 7.387 7.5 7.613
V
Input voltage TA = −40°C to 85°C 7.369 7.5 7.631 V
Load regulation IREF = 1 mA to 2 mA 0 10
mV
Line regulation VCC = 10.8 V to 15 V(2) 0 10 mV
Short-circuit current VREF = 0 V −20 −25 −50 mA
Oscillator Section
Initial accuracy TA = 25°C 85 100 115 kHz
Voltage stability VCC = 10.8 V to 15 V −1 1 %
Total variation Line, temp 80 120 kHz
Ramp peak voltage 4.5 5 5.5
Ramp amplitude voltage
(peak to peak) 3.5 4 4.5 V
Peak Current Limit Section
PKLMT reference voltage −15 15 mV
PKLMT propagation delay 150 350 500 ns
NOTES: 1. Ensured by design, not production tested.
2. Reference variation for VCC < 10.8 V is shown in Figure 8.
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ELECTRICAL CHARACTERISTICS
TA = 0°C to 70°C for the UCC3817A and TA = −40°C to 85°C for the UCC2817A, TA = TJ, VCC = 12 V, RT = 22 k,
CT = 270 pF, (unless otherwise noted)
PARAMETER TEST CONDITIONS MIN TYP MAX UNITS
Multiplier Section
IMOUT, high line, low power output
current, (0°C to 85°C) IAC = 500 µA, VFF = 4.7 V, VAOUT = 1.25 V 0 −6 −20
IMOUT, high line, low power output
current, (−40°C to 85°C) IAC = 500 µA, VFF = 4.7 V, VAOUT = 1.25 V 0 −6 −23
IMOUT, high line, high power output
current IAC = 500 µA, VFF = 4.7 V, VAOUT = 5 V −70 −90 −105
µA
IMOUT, low line, low power output
current IAC = 150 µA, VFF = 1.4 V, VAOUT = 1.25 V −10 −19 −50
µA
IMOUT, low line, high power output
current IAC = 150 µA, VFF = 1.4 V, VAOUT = 5 V −268 −300 −345
IMOUT, IAC limited output current IAC = 150 µA, VFF = 1.3 V, VAOUT = 5 V −250 −300 −400
Gain constant (K) IAC = 300 µA, VFF = 3 V, VAOUT = 2.5 V 0.5 1 1.5 1/V
IMOUT, zero current
IAC = 150 µA, VFF = 1.4 V, VAOUT = 0.25 V 0 −2
IMOUT, zero current IAC = 500 µA, VFF = 4.7 V, VAOUT = 0.25 V 0 −2
A
IMOUT, zero current, (0°C to 85°C) IAC = 500 µA, VFF = 4.7 V, VAOUT = 0.5 V 0 −3 µA
IMOUT, zero current, (−40°C to 85°C) IAC = 500 µA, VFF = 4.7 V, VAOUT = 0.5 V 0 −3.5
Power limit (IMOUT x VFF) IAC = 150 µA, VFF = 1.4 V, VAOUT = 5 V −375 −420 −485 µW
Feed-Forward Section
VFF output current IAC = 300 µA −140 −150 −160 µA
Soft Start Section
SS charge current −6 −10 −16 µA
Gate Driver Section
Pullup resistance IO = –100 mA to −200 mA 9 12
Pulldown resistance IO = 100 mA 4 10
Output rise time CL = 1 nF, RL = 10 Ω, VDRVOUT = 0.7 V to 9.0 V 25 50
ns
Output fall time CL = 1 nF, RL = 10 Ω, VDRVOUT = 9.0 V to 0.7 V 10 50 ns
Maximum duty cycle 93% 95% 99%
Minimum controlled duty cycle At 100 kHz 2%
Zero Power Section
Zero power comparator threshold Measured on VAOUT 0.20 0.33 0.50 V
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PIN ASSIGNMENTS
TERMINAL
I/O
DESCRIPTION
NAME NO.
I/O
DESCRIPTION
CAI 4 I Current amplifier noninverting input
CAOUT 3 O Current amplifier output
CT 14 I Oscillator timing capacitor
DRVOUT 16 O Gate drive
GND 1 Ground
IAC 6 I Current proportional to input voltage
MOUT 5 I/O Multiplier output and current amplifier inverting input
OVP/EN 10 I Over-voltage/enable
PKLMT 2 I PFC peak current limit
RT 12 I Oscillator charging current
SS 13 I Soft-start
VAOUT 7 O Voltage amplifier output
VCC 15 I Positive supply voltage
VFF 8 I Feed-forward voltage
VSENSE 11 IVoltage amplifier inverting input
VREF 9 O Voltage reference output
Pin Descriptions
CAI: Place a resistor between this pin and the GND side of current sense resistor. This input and the inverting
input (MOUT) remain functional down to and below GND.
CAOUT: This is the output of a wide bandwidth operational amplifier that senses line current and commands
the PFC pulse-width modulator (PWM) to force the correct duty cycle. Compensation components are placed
between CAOUT and MOUT.
CT: A capacitor from CT to GND sets the PWM oscillator frequency according to:
f[ǒ0.6
RT CTǓ
The lead from the oscillator timing capacitor to GND should be as short and direct as possible.
DRVOUT: The output drive for the boost switch is a totem-pole MOSFET gate driver on DRVOUT. To av o i d t h e
excessive overshoot of the DRVOUT while driving a capacitive load, a series gate current-limiting/damping
resistor is recommended to prevent interaction between the gate impedance and the output driver. The value
of the series gate resistor is based on the pulldown resistance (Rpulldown which is 4 typical), the maximum
VCC voltage (VCC), and the required maximum gate drive current (IMAX). Using the equation below, a series
gate resistance of resistance 11 would be required for a maximum VCC voltage of 18 V and for 1.2 A of
maximum sink current. The source current will be limited to approximately 900 mA (based on the Rpullup of 9-
typical).
RGATE +VCC *ǒIMAX RpulldownǓ
IMAX
GND: All voltages measured with respect to ground. VCC and REF should be bypassed directly to GND with
a 0.1-µF or larger ceramic capacitor.
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Pin Descriptions (cont.)
IAC: This input to the analog multiplier is a current proportional to instantaneous line voltage. The multiplier
is tailored for very low distortion from this current input (IIAC) to multiplier output. The recommended maximum
IIAC is 500 µA.
MOUT: The output of the analog multiplier and the inverting input of the current amplifier are connected together
at MOUT. As the multiplier output is a current, this is a high-impedance input so the amplifier can be configured
as a differential amplifier. This configuration improves noise immunity and allows for the leading-edge
modulation operation. The multiplier output current is limited to ǒ2 IIACǓ. The multiplier output current is given
by the equation:
IMOUT +IIAC (VVAOUT *1)
VVFF2 K
where K +1
V is the multiplier gain constant.
OVP/EN: A window comparator input that disables the output driver if the boost output voltage is a programmed
level above the nominal or disables both the PFC output driver and resets SS if pulled below 1.9 V (typ).
PKLMT: The threshold for peak limit is 0 V. Use a resistor divider from the negative side of the current sense
resistor to VREF to level shift this signal to a voltage level defined by the value of the sense resistor and the
peak current limit. Peak current limit is reached when PKLMT voltage falls below 0 V.
RT: A resistor from RT to GND is used to program oscillator charging current. A resistor between 10 k and
100 k is recommended. Nominal voltage on this pin is 3 V.
SS: VSS is discharged for VVCC low conditions. When enabled, SS charges an external capacitor with a current
source. This voltage is used as the voltage error signal during start-up, enabling the PWM duty cycle to increase
slowly. I n the event of a VVCC dropout, the OVP/EN is forced below 1.9 V (typ), SS quickly discharges to disable
the PWM.
Note: In an open-loop test circuit, grounding the SS pin does not ensure 0% duty cycle. Please see the
application section for details.
VAOUT: This is the output of the operational amplifier that regulates output voltage. The voltage amplifier output
is internally limited to approximately 5.5 V to prevent overshoot.
VCC: Connect to a stable source of at least 20 mA between 10 V and 17 V for normal operation. Bypass VCC
directly to GND to absorb supply current spikes required to charge external MOSFET gate capacitances. To
prevent inadequate gate drive signals, the output devices are inhibited unless VVCC exceeds the upper
under-voltage lockout voltage threshold and remains above the lower threshold.
VFF: The RMS voltage signal generated at this pin by mirroring 1/2 of the IIAC into a single pole external filter.
At low line, the VFF voltage should be 1.4 V.
VSENSE: This is normally connected to a compensation network and to the boost converter output through a
divider network.
VREF: VREF is the output of an accurate 7.5-V voltage reference. This output is capable of delivering 20 mA
to peripheral circuitry and is internally short-circuit current limited. VREF is disabled and remains at 0 V when
VVCC is below the UVLO threshold. Bypass VREF to GND with a 0.1-µF or larger ceramic capacitor for best
stability. Please refer to Figures 8 and 9 for VREF line and load regulation characteristics.
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APPLICATION INFORMATION
The UCC3817A is a BiCMOS average current mode boost controller for high power factor, high efficiency
preregulator power supplies. Figure 1 shows the UCC3817A in a 250-W PFC preregulator circuit. Off-line
switching power converters normally have an input current that is not sinusoidal. The input current waveform
has a high harmonic content because current is drawn in pulses at the peaks of the input voltage waveform.
An active power factor correction circuit programs the input current to follow the line voltage, forcing the
converter to look like a resistive load to the line. A resistive load has 0° phase displacement between the current
and voltage waveforms. Power factor can be defined in terms of the phase angle between two sinusoidal
waveforms of the same frequency:
PF +cosQ
Therefore, a purely resistive load would have a power factor of 1. In practice, power factors of 0.999 with THD
(total harmonic distortion) of less than 3% are possible with a well-designed circuit. Following guidelines are
provided to design PFC boost converters using the UCC3817A.
NOTE: Schottky diodes, D5 and D6, are required to protect the PFC controller from electrical over stress during
system power up.
UDG-98183
1
11
7
16GND DRVOUT
R17
20
15
C3
1µF CER
VCC
C2
100µF AI EI
14
C1
560pF
13 C4 0.01µF
12 R1 12k
R3 20k R2
499k
R4
249k
R5
10k
C5 1µF
9
4
10
VREF
VCC
CT
SS
RT
VSENSE
OVP/EN
VREF
VAOUT
3
8
2
VFF
C6 2.2µF
C7 150nF
R7 100k
6
5
R9
4.02k
C8 270pF
R8 12k
D6
R10
4.02k
D5
R11
10k
R12
2k
R14
0.25
3W
C13
0.47µF
600V
C14
1.5µF
400V
R13
383k
IAC R18
24k
R15
24k
R16
100
VCC
C10
1µFC11
1µF
D7 D8
L1
1mH
D2
6A, 600V
D1
8A, 600V
C12
220µF
450V
VOUT
385V−DC
+
PKLIMIT
CAOUT
CAI
MOUT
IAC
VO
UCC3817A
VLINE
85−270 V
AC
VREF
C9 1.2nF
R6 30k
6A 600V
D3 Q1
F1
VO
D4
R19
499k
R20 274k
R21
383k
AC2
AC1
C15 2.2µF
IRFP450
Figure 1. Typical Application Circuit
(1)
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APPLICATION INFORMATION
Power Stage
LBOOST : The boost inductor value is determined by:
LBOOST +ǒVIN(min) DǓ
(DI fs)
where D is the duty cycle, I is the inductor ripple current and fS is the switching frequency. For the example
circuit a switching frequency of 100 kHz, a ripple current of 875 mA, a maximum duty cycle of 0.688 and a
minimum input voltage of 85 VRMS gives us a boost inductor value of about 1 mH. The values used in this
equation are at the peak of low line, where the inductor current and its ripple are at a maximum.
COUT : Two main criteria, the capacitance and the voltage rating, dictate the selection of the output capacitor.
The value of capacitance is determined by the holdup time required for supporting the load after input ac voltage
is removed. Holdup is the amount of time that the output stays in regulation after the input has been removed.
For this circuit, the desired holdup time is approximately 16 ms. Expressing the capacitor value in terms of output
power, output voltage, and holdup time gives the equation:
COUT +ǒ2 POUT DtǓ
ǒVOUT2*VOUT(min)2Ǔ
In practice, the calculated minimum capacitor value may be inadequate because output ripple voltage
specifications limit the amount of allowable output capacitor ESR. Attaining a suf ficiently low value of ESR often
necessitates the use of a much larger capacitor value than calculated. The amount of output capacitor ESR
allowed can be determined by dividing the maximum specified output ripple voltage by the inductor ripple
current. In this design holdup time was the dominant determining factor and a 220-µF, 450-V capacitor was
chosen for the output voltage level of 385 VDC at 250 W.
(2)
(3)
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APPLICATION INFORMATION
Power switch selection: As in any power supply design, tradeoffs between performance, cost and size have
to be made. When selecting a power switch, it can be useful to calculate the total power dissipation in the switch
for several different devices at the switching frequencies being considered for the converter. Total power
dissipation in the switch is the sum of switching loss and conduction loss. Switching losses are the combination
of the gate charge loss, COSS loss and turnon and turnoff losses:
PGATE +QGATE VGATE fS
PCOSS +1
2 COSS V2OFF fS
PON )POFF +1
2 VOFF IL ǒtON )tOFFǓ fS
where Q GATE is the total gate charge, VGATE is the gate drive voltage, fS is the clock frequency, C OSS is the drain
source capacitance of the MOSFET, IL is the peak inductor current, tON and tOFF are the switching times
(estimated using device parameters RGATE, QGD and VTH) and VOFF is the voltage across the switch during the
off time, in this case VOFF = VOUT.
Conduction loss is calculated as the product of the RDS(on) of the switch (at the worst case junction temperature)
and the square of RMS current:
PCOND +RDS(on) K I2RMS
where K is the temperature factor found in the manufacturer’s RDS(on) vs. junction temperature curves.
Calculating these losses and plotting against frequency gives a curve that enables the designer to determine
either which manufacturer’s device has the best performance at the desired switching frequency, or which
switching frequency has the least total loss for a particular power switch. For this design example an IRFP450
HEXFET from International Rectifier was chosen because of its low RDS(on) and its VDSS rating. The IRFP450’s
RDS(on) of 0.4 and the maximum VDSS of 500 V made it an ideal choice. An excellent review of this procedure
can be found in the Unitrode Power Supply Design Seminar SEM1200, Topic 6, Design Review: 140 W, [Multiple
Output High Density DC/DC Converter].
Softstart
The softstart circuitry is used to prevent overshoot of the output voltage during start up. This is accomplished
by bringing up the voltage amplifier’s output (VVAOUT) slowly which allows for the PWM duty cycle to increase
slowly. Please use the following equation to select a capacitor for the softstart pin.
In this example tDELAY is equal to 7.5 ms, which
would yield a CSS of 10 nF.
CSS +10 mA tDELAY
7.5 V
In an open-loop test circuit, shorting the softstart pin to ground does not ensure 0% duty cycle. This is due to
the current amplifiers input offset voltage, which could force the current amplifier output high or low depending
on the polarity of the offset voltage. However, in the typical application there is sufficient amount of inrush and
bias current to overcome the current amplifier’s offset voltage.
(4)
(5)
(6)
(7)
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APPLICATION INFORMATION
Multiplier
The output of the multiplier of the UCC3817A is a signal representing the desired input line current. It is an input
to the current amplifier, which programs the current loop to control the input current to give high power factor
operation. As such, the proper functioning of the multiplier is key to the success of the design. The inputs to the
multiplier are VAOUT, the voltage amplifier error signal, IIAC, a representation of the input rectified ac line
voltage, and an input voltage feedforward signal, VVFF. The output of the multiplier, IMOUT, can be expressed
as:
IMOUT +IIAC ǒVVAOUT *1Ǔ
K VVFF2
where K is a constant typically equal to 1
V.
The electrical characteristics table covers all the required operating conditions for designing with the
multiplier. Additionally, curves in Figures 10, 11, and 12 provide typical multiplier characteristics over its entire
operating range.
The IIAC signal is obtained through a high-value resistor connected between the rectified ac line and the IAC
pin of the UCC3817A/18A. This resistor (RIAC) is sized to give the maximum IIAC current at high line. For the
UCC3817A/18A the maximum IIAC current is about 500 µA. A higher current than this can drive the multiplier
out of its linear range. A smaller current level is functional, but noise can become an issue, especially at low
input line. Assuming a universal line operation of 85 VRMS to 265 VRMS gives a RIAC value of 750 k. Because
of voltage rating constraints of standard 1/4-W resistor, use a combination of lower value resistors connected
in series to give the required resistance and distribute the high voltage amongst the resistors. For this design
example two 383-k resistors were used in series.
The current into the IAC pin is mirrored internally to the VFF pin where it is filtered to produce a voltage feed
forward signal proportional to line voltage. The VFF voltage is used to keep the power stage gain constant; and
to provid input power limiting. Please refer to Texas Instruments application note SLUA196 for detailed
explanation on how the VFF pin provides power limiting. The following equation can be used to size the VFF
resistor ( R VFF) to provide power limiting where VIN(min) is the minimum RMS input voltage and RIAC is the total
resistance connected between the IAC pin and the rectified line voltage.
RVFF +1.4 V
VIN(min) 0.9
2 RIAC
[30 kW
(9)
(10)
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APPLICATION INFORMATION
Because the VFF voltage is generated from line voltage it needs to be adequately filtered to reduce total
harmonic distortion caused by the 120 Hz rectified line voltage. Refer to Unitrode Power Supply Design
Seminar, SEM−700 Topic 7, [Optimizing the Design of a High Power Factor Preregulator.] A single pole filter
was adequate for this design. Assuming that an allocation of 1.5% total harmonic distortion from this input is
allowed, and that the second harmonic ripple is 66% of the input ac line voltage, the amount of attenuation
required by this filter is:
1.5 %
66 % +0.022
With a ripple frequency (fR) of 120 Hz and an attenuation of 0.022 requires that the pole of the filter (fP) be placed
at:
fP+120 Hz 0.022 [2.6 Hz
The following equation can be used to select the filter capacitor (CVFF) required to produce the desired low pass
filter.
CVFF +1
2 p RVFF fP[2.2 mF
The R MOUT resistor is sized to match the maximum current through the sense resistor to the maximum multiplier
current. The maximum multiplier current, or IMOUT(max), can be determined by the equation:
IMOUT(max) +IIAC@VIN(min) ǒVVAOUT(max) *1V
Ǔ
K VVFF2(min)
IMOUT(max) for this design is approximately 315 µA. The RMOUT resistor can then be determined by:
RMOUT +VRSENSE
IMOUT(max)
In this example VRSENSE was selected to give a dynamic operating range of 1.25 V, which gives an RMOUT of
roughly 3.91 k.
(11)
(12)
(13)
(14)
(15)
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APPLICATION INFORMATION
Voltage Loop
The second major source of harmonic distortion is the ripple on the output capacitor at the second harmonic
of the line frequency. This ripple is fed back through the error amplifier and appears as a 3rd harmonic ripple
at the input to the multiplier. The voltage loop must be compensated not just for stability but also to attenuate
the contribution of this ripple to the total harmonic distortion of the system. (refer to Figure 2).
RIN
RD+
Rf
Cf
VREF
VOUT
CZ
Figure 2. Voltage Amplifier Configuration
The gain of the voltage amplifier, GVA, can be determined by first calculating the amount of ripple present on
the output capacitor. The peak value of the second harmonic voltage is given by the equation:
VOPK +PIN
ǒ2p fR COUT VOUTǓ
In this example VOPK is equal to 3.91 V. Assuming an allowable contribution of 0.75% (1.5% peak to peak) from
the voltage loop to the total harmonic distortion budget we set the gain equal to:
GVA +ǒDVVAOUTǓ(0.015)
2 VOPK
where VVAOUT is the effective output voltage range of the error amplifier (5 V for the UCC3817A). The network
needed to realize this filter is comprised of an input resistor, R IN, and feedback components Cf, CZ, and Rf. The
value of RIN is already determined because of its function as one half of a resistor divider from VOUT feeding
back to the voltage amplifier for output voltage regulation. In this case the value was chosen to be 1 M. This
high value was chosen to reduce power dissipation in the resistor. In practice, the resistor value would be
realized by the use of two 500-k resistors in series because of the voltage rating constraints of most standard
1/4-W resistors. The value of Cf is determined by the equation:
Cf+1
ǒ2p fR GVA RINǓ
(16)
(17)
(18)
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APPLICATION INFORMATION
In this example Cf equals 150 nF. Resistor Rf sets the dc gain of the error amplifier and thus determines the
frequency of the pole of the error amplifier. The location of the pole can be found by setting the gain of the loop
equation to one and solving for the crossover frequency. The frequency, expressed in terms of input power , can
be calculated by the equation:
fVI2+PIN
ǒ(2p)2 DVVAOUT VOUT RIN COUT CfǓ
fVI for this converter is 10 Hz. A derivation of this equation can be found in the Unitrode Power Supply Design
Seminar SEM1000, Topic 1, [A 250-kHz, 500-W Power Factor Correction Circuit Employing Zero Voltage
Transitions].
Solving for Rf becomes:
Rf+1
ǒ2p fVI CfǓ
or Rf equals 100 k.
Due to the low output impedance of the voltage amplifier, capacitor CZ was added in series with RF to reduce
loading on the voltage divider. To ensure the voltage loop crossed over at fVI, CZ was selected to add a zero
at a 10th of fVI. For this design a 2.2-µF capacitor was chosen for CZ. The following equation can be used to
calculate CZ.
CZ+1
2 p fVI
10 Rf
Current Loop
The gain of the power stage is:
GID(s) +ǒVOUT RSENSEǓ
ǒs LBOOST VPǓ
RSENSE has been chosen to give the desired differential voltage for the current sense amplifier at the desired
current limit point. In this example, a current limit of 4 A and a reasonable dif ferential voltage to the current amp
of 1 V gives a RSENSE value of 0.25 . VP in this equation is the voltage swing of the oscillator ramp, 4 V for
the UCC3817A. Setting the crossover frequency of the system to 1/10th of the switching frequency, or 10 kHz,
requires a power stage gain at that frequency of 0.383. In order for the system to have a gain of 1 at the crossover
frequency, the current amplifier needs to have a gain of 1/GID at that frequency. GEA, the current amplifier gain
is then:
GEA +1
GID +1
0.383 +2.611
(19)
(20)
(21)
(22)
(23)
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APPLICATION INFORMATION
RI is the RMOUT resistor, previously calculated to be 3.9 k. (refer to Figure 3). The gain of the current amplifier
is Rf/RI, so multiplying RI by GEA gives the value of Rf, in this case approximately 12 k. Setting a zero at the
crossover frequency and a pole at half the switching frequency completes the current loop compensation.
CZ+1
2 p Rf fC
CP+1
2 p Rf fs
2
RI
+
Rf
CP
CAOUT
CZ
Figure 3. Current Loop Compensation
The UCC3817A current amplifier has the input from the multiplier applied to the inverting input. This change
in architecture from previous Texas Instruments PFC controllers improves noise immunity in the current
amplifier. It also adds a phase inversion into the control loop. The UCC3817A takes advantage of this phase
inversion to implement leading-edge duty cycle modulation. Synchronizing a boost PFC controller to a
downstream dc-to-dc controller reduces the ripple current seen by the bulk capacitor between stages, reducing
capacitor size and cost and reducing EMI. This is explained in greater detail in a following section. The
UCC3817A current amplifier configuration is shown in Figure 4.
+
+
RSENSE +
MULT
ZfPWM
COMPARATOR
CA
QBOOST
LBOOST
VOUT
Figure 4. UCC3817A Current Amplifier Configuration
(24)
(25)
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APPLICATION INFORMATION
Start Up
The UCC3818A version of the device is intended to have VCC connected to a 12-V supply voltage. The
UCC3817A has an internal shunt regulator enabling the device to be powered from bootstrap circuitry as shown
in the typical application circuit of Figure 1. The current drawn by the UCC3817A during undervoltage lockout,
or start-up current, is typically 150 µA. Once VCC is above the UVLO threshold, the device is enabled and draws
4 mA typically. A resistor connected between the rectified ac line voltage and the VCC pin provides current to
the shunt regulator during power up. Once the circuit is operational, the bootstrap winding of the inductor
provides the VCC voltage. Sizing of the start-up resistor is determined by the start-up time requirement of the
system design.
IC+CDV
Dt
R+VRMS (0.9)
IC
Where IC is the charge current, C is the total capacitance at the VCC pin, V is the UVLO threshold and t is
the allowed start-up time.
Assuming a 1 second allowed start-up time, a 16-V UVLO threshold, and a total VCC capacitance of 100 µF,
a resistor value of 51 k is required at a low line input voltage of 85 VRMS. The IC start-up current is sufficiently
small as to be ignored in sizing the start-up resistor.
Capacitor Ripple Reduction
For a power system where the PFC boost converter is followed by a dc-to-dc converter stage, there are benefits
to synchronizing the two converters. In addition to the usual advantages such as noise reduction and stability,
proper synchronization can significantly reduce the ripple currents in the boost circuit’s output capacitor.
Figure 5 helps illustrate the impact of proper synchronization by showing a PFC boost converter together with
the simplified input stage of a forward converter. The capacitor current during a single switching cycle depends
on the status of the switches Q1 and Q2 and is shown in Figure 6. It can be seen that with a synchronization
scheme that maintains conventional trailing-edge modulation on both converters, the capacitor current ripple
is highest. The greatest ripple current cancellation is attained when the overlap of Q1 offtime and Q2 ontime
is maximized. One method of achieving this is to synchronize the turnon of the boost diode (D1) with the turnon
of Q2. This approach implies that the boost converter’s leading edge is pulse width modulated while the forward
converter is modulated with traditional trailing edge PWM. The UCC3817A is designed as a leading edge
modulator with easy synchronization to the downstream converter to facilitate this advantage. Table 1 compares
the ICB(rms) for D1/Q2 synchronization as offered by UCC3817A vs. the ICB(rms) for the other extreme of
synchronizing the turnon of Q1 and Q2 for a 200-W power system with a VBST of 385 V.
(26)
(27)
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APPLICATION INFORMATION
UDG-97130-1
Figure 5. Simplified Representation of a 2-Stage PFC Power Supply
UDG-97131
Figure 6. Timing Waveforms for Synchronization Scheme
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APPLICATION INFORMATION
Table 1 illustrates that the boost capacitor ripple current can be reduced by about 50% at nominal line and about
30% at high line with the synchronization scheme facilitated by the UCC3817A. Figure 7 shows the suggested
technique for synchronizing the UCC3817A to the downstream converter. With this technique, maximum ripple
reduction as shown in Figure 6 is achievable. The output capacitance value can be significantly reduced if its
choice is dictated by ripple current or the capacitor life can be increased as a result. In cost sensitive designs
where holdup time is not critical, this is a significant advantage.
Table 1. Effects of Synchronization on Boost Capacitor Current
VIN = 85 V VIN = 120 V VIN = 240 V
D(Q2) Q1/Q2 D1/Q2 Q1/Q2 D1/Q2 Q1/Q2 D1/Q2
0.35 1.491 A 0.835 A 1.341 A 0.663 A 1.024 A 0.731 A
0.45 1.432 A 0.93 A 1.276 A 0.664 A 0.897 A 0.614 A
An alternative method of synchronization to achieve the same ripple reduction is possible. In this method, the
turnon of Q 1 i s synchronized to the turnoff of Q2. While this method yields almost identical ripple reduction and
maintains trailing edge modulation on both converters, the synchronization is much more dif ficult to achieve and
the circuit can become susceptible to noise as the synchronizing edge itself is being modulated.
CT
RT
RT
CT
D2
D1
C1
Gate Drive
From Down
Stream PWM
UCC3817A
Figure 7. Synchronizing the UCC3817A to a Down-Stream Converter
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APPLICATION INFORMATION
Figure 8
141210
7.45
7.50
7.55
7.60
7.40
VCC − Supply Voltage − V
13119
VREF − Reference Voltage − V
REFERENCE VOLTAGE
vs
SUPPLY VOLTAGE
Figure 9
REFERENCE VOLTAGE
vs
REFERENCE CURRENT
0 5 10 15 20 25
7.495
7.500
7.505
7.510
7.490
VREF − Reference Voltage − V
IVREF − Reference Current − mA
Figure 10
MULTIPLIER OUTPUT CURRENT
vs
VOLTAGE ERROR AMPLIFIER OUTPUT
0.0 1.0 2.0 3.0 4.0 5.0
50
200
250
350
0
100
300
150
I
MOUT
- Multiplier Output Current
µ
A
VAOUT − Voltage Error Amplifier Output − V
IAC = 150 µA
VFF = 1.4 V
IAC = 300 µA
VFF = 3.0 V
IAC = 500 µA
VFF = 4.7 V
Figure 11
MULTIPLIER GAIN
vs
VOLTAGE ERROR AMPLIFIER OUTPUT
1.0 2.0 3.0 4.0 5.0
0.7
1.1
1.3
1.5
0.5
0.9 IAC = 300 µA
IAC = 500 µA
IAC = 150 µA
Multiplier Gain − K
VAOUT − Voltage Error Amplifier Output − V
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APPLICATION INFORMATION
Figure 12
VFF − Feedforward Voltage − V
1.0 2.0 3.0 4.0 5.0
100
300
400
500
0
200
VAOUT = 3 V
VAOUT = 2 V
VAOUT = 4 V
VAOUT = 5 V
(VFF × IMOUT) − µW
MULTIPLIER CONSTANT POWER PERFORMANCE
0.0
References and Resources:
Application Note, Differences Between UCC3817A/18A/19A and UCC3817/18/19, Texas Instruments
Literature Number SLUA294
Evaluation Module, UCC3817EVM, 385V, 250W PFC Boost Converter
User’s Guide, UCC3817 BiCMOS Power Factor Preregulator Evaluation Board, Texas Instruments Literature
Number SLUU077
Application Note, Synchronizing a PFC Controller from a Down Stream Controller Gate Drive, Texas
Instruments Literature Number SLUA245
Seminar topic, High Power Factor Switching Preregulator Design Optimization, L.H. Dixon, SEM−700,1990.
Seminar topic, High Power Factor Preregulator for Off−line Supplies, L.H. Dixon, SEM−600, 1988.
Related Products
DEVICE DESCRIPTION CONTROL METHOD TYPICAL POWER LEVEL
UC3854 PFC controller ACM(2) 200 W to 2 kW+
UC3854A/B Improved PFC controller ACM(2) 200 W to 2 kW+
UC3855A/B High performance soft switching PFC controller ACM(2) 400 W to 2 kW+
UCC38050/1 Transition mode PFC controller CRM(1) 50 W to 400 W
UCC3819 Tracking boost PFC controller ACM(2) 75 W to 2 kW+
UCC28510/11/12/13 Advanced PFC+PWM combo controller ACM(2) 75 W to 1kW+
UCC28514/15/16/17 Advanced PFC+PWM combo controller ACM(2) 75 W to 1kW+
NOTES: (1). Critical conduction mode
(2). Average current mode