TI UC1855

UC1855A/B
UC2855A/B
UC3855A/B
High Performance Power Factor Preregulator
FEATURES
DESCRIPTION
• Controls Boost PWM to Near Unity
Power Factor
The UC1855A/B provides all the control features necessary for high
power, high frequency PFC boost converters. The average current mode
control method allows for stable, low distortion AC line current programming without the need for slope compensation. In addition, the UC1855
utilizes an active snubbing or ZVT (Zero Voltage Transition technique) to
dramatically reduce diode recovery and MOSFET turn-on losses, resulting in lower EMI emissions and higher efficiency. Boost converter switching frequencies up to 500kHz are now realizable, requiring only an
additional small MOSFET, diode, and inductor to resonantly soft switch
the boost diode and switch. Average current sensing can be employed using a simple resistive shunt or a current sense transformer. Using the current sense transformer method, the internal current synthesizer circuit
buffers the inductor current during the switch on-time, and reconstructs the
inductor current during the switch off-time. Improved signal to noise ratio
and negligible current sensing losses make this an attractive solution for
higher power applications.
• Fixed Frequency Average Current
Mode Control Minimizes Line Current
Distortion
• Built-in Active Snubber (ZVT) allows
Operation to 500kHz, improved EMI
and Efficiency
• Inductor Current Synthesizer allows
Single Current Transformer Current
Sense for Improved Efficiency and
Noise Margin
• Accurate Analog Multiplier with Line
Compensator allows for Universal
Input Voltage Operation
• High Bandwidth (5MHz), Low Offset
Current Amplifier
• Overvoltage and Overcurrent
protection
• Two UVLO Threshold Options
• 150µA Startup Supply Current Typical
• Precision 1% 7.5V Reference
The UC1855A/B also features a single quadrant multiplier, squarer, and
divider circuit which provides the programming signal for the current loop.
The internal multiplier current limit reduces output power during low line
conditions. An overvoltage protection circuit disables both controller outputs in the event of a boost output OV condition.
Low startup supply current, UVLO with hysteresis, a 1% 7.5V reference,
voltage amplifier with softstart, input supply voltage clamp, enable comparator, and overcurrent comparator complete the list of features. Available packages include: 20 pin N, DW, Q, J, and L.
BLOCK DIAGRAM
License Patent from Pioneer Magnetics. Pin numbers refer to DIL-20 J or N packages.
6/98
UDG-94001-2
UC1855A/B
UC2855A/B
UC3855A/B
CONNECTION DIAGRAMS
ABSOLUTE MAXIMUM RATINGS
Supply Voltage VCC . . . . . . . . . . . . . . . . . . . . . . . . . . Internally Limited
VCC Supply Clamp Current . . . . . . . . . . . . . . . . . . . . . . . 20mA
PFC Gate Driver Current (continuous) . . . . . . . . . . . . . . ± 0.5A
PFC Gate Driver Current (peak) . . . . . . . . . . . . . . . . . . . ± 1.5A
ZVT Drive Current (continuous) . . . . . . . . . . . . . . . . . . . ± 0.25A
ZVT Drive Current (peak) . . . . . . . . . . . . . . . . . . . . . . . ± 0.75A
Input Current (IAC, RT, RVA) . . . . . . . . . . . . . . . . . . . . . . . 5mA
Analog Inputs (except Peak Limit) . . . . . . . . . . . . . . −0.3 to 10V
Peak Limit Input . . . . . . . . . . . . . . . . . . . . . . . . . . . −0.3 to 6.5V
Softstart Sinking Current . . . . . . . . . . . . . . . . . . . . . . . . . 1.5mA
Storage Temperature . . . . . . . . . . . . . . . . . . . −65°C to +150°C
Junction Temperature . . . . . . . . . . . . . . . . . . . −55°C to +150°C
Lead Temperature (Soldering, 10 sec.) . . . . . . . . . . . . . +300°C
PLCC-20 & LCC-20 (Top View)
Q or L Package
Currents are positive into, negative out of the specified terminal. Consult Packaging Section of Databook for thermal limitations and considerations of packages. All voltages are
referenced to GND.
DIL–20 (Top View)
J or N Package
SOIC-20 (Top View)
DW Package
ELECTRICAL CHARACTERISTICS: Unless otherwise specified: VCC = 18V, RT = 15k, RVS = 23k, CT = 470pF, CI =
150pF, VRMS = 1.5V, IAC = 100µA, ISENSE = 0V, CAOUT = 4V, VAOUT= 3.5V, VSENSE = 3V. TA = TJ. TA = –55°C to 125°C
(UC1855A/B), –40°C to 85°C (UC2855A/B), 0°C to 70°C (UC3855A/B).
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX UNITS
Overall
CAO, VAOUT = 0V, VCC = UVLO −0.3V
150
VCC Turn-On Threshold
UC1855A
VCCTurn-Off Threshold
UC1855A,B
VCC Turn-On Threshold
UC1855B
VCC Clamp
I(VCC) = ICC(on) + 5mA
Supply Current, OFF
Supply Current, OPERATING
9
500
µA
17
25
mA
15.5
17.5
V
10
V
10.5
10.8
V
20
22
V
3.1
V
−500
25
500
nA
18
Voltage Amplifier
Input Voltage
2.9
VSENSE Bias Current
Open Loop Gain
VOUT = 2 to 5V
65
80
VOUT High
ILOAD = –300µA
5.75
6
6.25
V
VOUT Low
ILOAD = 300µA
0.3
0.5
V
Output Short Circuit Current
VOUT = 0V
0.6
3
mA
2
dB
UC1855A/B
UC2855A/B
UC3855A/B
ELECTRICAL CHARACTERISTICS: Unless otherwise specified: VCC = 18V, RT = 15k, RVS = 23k, CT = 470pF, CI =
150pF, VRMS = 1.5V, IAC = 100µA, ISENSE = 0V, CAOUT = 4V, VAOUT= 3.5V, VSENSE = 3V. TA = TJ. TA = –55°C to 125°C
(UC1855A/B), –40°C to 85°C (UC2855A/B), 0°C to 70°C (UC3855A/B).
PARAMETER
Current Amplifier
Input Offset Voltage
Input Bias Current (Sense)
Open Loop Gain
VOUT High
VOUT Low
Output Short Circuit Current
Common Mode Range
Gain Bandwidth Product
Reference
Output Voltage
Load Regulation
Line Regulation
Short Circuit Current
Oscillator
Initial Accuracy
Voltage Stability
Total Variation
Ramp Amplitude (P–P)
Ramp Valley Voltage
Enable/OVP/Current Limit
Enable Threshold
OVP Threshold
OVP Hysteresis
OVP Propagation Delay
OVP Input Bias Current
PKLIMIT Threshold
PKLIMIT Input Current
PKLIMIT Prop. Delay
Multiplier
Output Current - IAC Limited
Output Current - Zero
Output Current - Power Limited
Output Current
Gain Constant
Gate Driver Output
Output High Voltage
Output Low Voltage
Output Low Voltage
Output Low (UVLO)
Output RISE/FALL Time
Output Peak Current
TEST CONDITIONS
VCM = − 2.5V
VCM = 2.5V
VCM = 2.5V, VOUT = 2 to 6V
ILOAD = −500µA
ILOAD = 500µA
VOUT = 0V
MIN
−4
−500
80
TYP
4
500
110
6
0.3
1
FIN = 100kHz, 10mV, P–P, TA = 25°C
−0.3
2.5
IREF = 0mA, TA = 25°C
IREF = 0mA
IREF = 1 to 10 mA
VCC = 15 to 35V
REF = 0V
7.388
7.313
−15
−10
20
7.5
7.5
170
200
1
TA = 25°C
VCC = 12 to 18V
Line, Temp.
V= 7.5V
1.25
VPKLIMIT = 1.5V
IAC = 100µA, VRMS = 1V
IAC = 0µA
VRMS = 1.5V, VAOUT = 5.5V
VRMS = 1.5V, VAOUT = 2V
VRMS = 1.5V VAOUT = 5V
VRMS = 5V, VAOUT = 2V
VRMS = 5V, VAOUT = 5V
Refer to Note 1
lOUT = −200mA, VCC = 15V
lOUT = 200mA
lOUT = 10mA
lOUT = 50mA, VCC = 0V
CLOAD = 1nF
CLOAD = 10nF
3
−235
−2
−250
−0.95
12
0.5
3
5
5
45
160
4.9
1.1
200
MAX UNITS
mV
nA
dB
V
V
mA
V
MHz
7.613
7.688
15
10
65
V
V
mV
mV
mA
230
kHz
%
kHz
V
V
240
5.9
1.6
1.8
7.5
400
200
1
1.5
100
100
2.2
7.66
600
−205
−0.2
−209
−26
−190
−3
−17
−0.85
−175
2
−160
−0.75
µA
µA
µA
µA
µA
µA
µA
1/V
12.8
1
300
2.2
500
V
V
mV
0.9
35
1.5
10
1.75
1.5
V
V
mV
ns
µA
V
µA
ns
V
ns
A
UC1855A/B
UC2855A/B
UC3855A/B
ELECTRICAL CHARACTERISTICS: Unless otherwise specified: VCC = 18V, RT = 15k, RVS = 23k, CT = 470pF, CI =
150pF, VRMS = 1.5V, IAC = 100µA, ISENSE = 0V, CAOUT = 4V, VAOUT= 3.5V, VSENSE = 3V. TA = TJ. TA = –55°C to 125°C
(UC1855A/B), –40°C to 85°C (UC2855A/B), 0°C to 70°C (UC3855A/B).
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX UNITS
ZVT
Reset Threshold
2.3
2.6
2.9
V
6
20
µA
Input Bias Current
V = 2.5V, VCT = 0
Propagation Delay
Measured at ZVTOUT
100
ns
Maximum Pulse Width
400
ns
12
12.8
V
Output High Voltage
lOUT = −100mA, VCC = 15V
1
2.2
V
Output Low Voltage
lOUT = 100mA
300
900
mV
lOUT = 10mA
Output Low (UVLO)
lOUT = 50mA, VCC = 0V
0.9
1.5
V
35
ns
Output RISE/FALL Time
CLOAD = 1nF
0.75
A
Output Peak Current
CLOAD = 10nF
Current Synthesizer
VION = 0V
30
50
mV
ION to CS Offset
Cl Discharge Current
IAC = 50µA
105
118
140
µA
IAC = 500µA
5
µA
IAC Offset Voltage
0.3
0.65
1.1
V
ION Buffer Slew Rate
10
V/µs
2
15
µA
ION Input Bias Current
VION = 2V
RVS Output Voltage
23k from RVS to GND
2.87
3
3.13
V
Note 1: Gain constant (K) =
IAC •(VAOUT – 1. 5V ) at V = 1.5V, VA = 5.5V.
RMS
OUT
2
(VRMS • IMO )
PIN DESCRIPTIONS
CS: The reconstructed inductor current waveform generated on the CI pin is level shifted down a diode drop to
this pin. Connect the current amplifier input resistor between CS and the inverting input of the current amplifier.
The waveform on this pin is compared to the multiplier
output waveform through the average current sensing
current amplifier. The input to the peak current limiting
comparator is also connected to this pin. A voltage level
greater than 1.5 volts on this pin will trip the comparator
and disable the gate driver output.
CA−: This is the inverting input to the current amplifier.
Connect the required compensation components between this pin and CAOUT. The common mode operating
range for this input is between −0.3V and 5V.
CAO: This is the output of the wide bandwidth current
amplifier and one of the inputs to the PWM duty cycle
comparator. The output signal generated by this amplifier
commands the PWM to force the correct input current.
The output can swing from 0.1V to 7.5V.
CT: A capacitor from CT to GND sets the PWM oscillator
frequency according to the following equation:
CI: The level shifted current sense signal is impressed
upon a capacitor connected between this pin and GND.
The buffered current sense transformer signal charges
the capacitor when the boost switch is on. When the
switch is off, the current synthesizer discharges the capacitor at a rate proportional to the dI/dt of the boost inductor current. In this way, the discharge current is
approximately equal to
f≈
1
.
11200 • CT
Use a high quality ceramic capacitor with low ESL and
ESR for best results. A minimum CT value of 200pF insures good accuracy and less susceptibility to circuit layout parasitics. The oscillator and PWM are designed to
provide practical operation to 500kHz.
3V
IAC
.
–
RRVS
4
Discharging the CI capacitor in this fashion, a “reconstructed” version of the inductor current is generated using only one current sense transformer.
GND: All voltages are measured with respect to this pin.
All bypass and timing capacitors connected to GND
should have leads as short and direct as possible.
4
UC1855A/B
UC2855A/B
UC3855A/B
PIN DESCRIPTIONS (cont.)
REF: REF is the output of the precision reference. The
output is capable of supplying 25mA to peripheral circuitry and is internally short circuit current limited. REF is
disabled and low whenever VCC is below the UVLO
threshold, and when OVP is below 1.8V. A REF “GOOD”
comparator senses REF and disables the stage until
REF has attained approximately 90% of its nominal
value. Bypass REF to GND with a 0.1µF or larger ceramic capacitor for best stability.
GTOUT: The output of the PWM is a 1.5A peak totem
pole MOSFET gate driver on GTOUT. A series resistor
between GTOUT and the MOSFET gate of at least 10
ohms should be used to limit the overshoot on GTOUT.
In addition, a low VF Schottky diode should be connected
between GTOUT and GND to limit undershoot and possible erratic operation.
IAC: This is a current input to the multiplier. The current
into this pin should correspond to the instantaneous
value of the rectified AC input line voltage. This is accomplished by connecting a resistor directly between IAC and
the rectified input line voltage. The nominal 650mV level
present on IAC negates the need for any additional compensating resistors to accommodate for the zero crossings of the line. A current equal to one fourth of the IAC
current forms one of the inductor current synthesizer inputs.
RVS: The nominal 3V signal present on the VSENSE pin
is buffered and brought out to the RVS pin. A current proportional to the output voltage is generated by connecting a resistor between this pin and GND. This current
forms the second input to the current synthesizer.
VAO: This is the output of the voltage amplifier. At a
given input RMS voltage, the voltage on this pin will vary
directly with the output load. The output swing is limited
from approximately 100mV to 6V. Voltage levels below
1.5V on this pin will inhibit the multiplier output.
IMO: This is the output of the multiplier, and the noninverting input of the current amplifier. Since this output is
a current, connect a resistor between this pin and ground
equal in value to the input resistor of the current amplifier.
The common mode operating range for this pin is −0.3V
to 5V.
VCC: Positive supply rail for the IC. Bypass this pin to
GND with a 1µF low ESL, ESR ceramic capacitor. This
pin is internally clamped to 20V. Current into this clamp
should be limited to less than 10mA. The UC1855A has a
15.5V (nominal) turn on threshold with 6 volts of hysteresis while the UC1855B turns on at 10.5V with 500mV of
hysteresis.
ION: This pin is the current sensing input. It should be
connected to the secondary side output of a current
sensing transformer whose primary winding is in series
with the boost switch. The resultant signal applied to this
input is buffered and level shifted up a diode to the CI capacitor on the CI pin. The ION buffer has a source only
output. Discharge of the CI cap is enabled through the
current synthesizer circuitry. The current sense transformer termination resistor should be designed to obtain
a 1V input signal amplitude at peak switch current.
VRMS: This pin is the feedforward line voltage compensation input to the multiplier. A voltage on VRMS proportional to the AC input RMS voltage commands the
multiplier to alter the current command signal by
2
1/VRMS to maintain a constant power balance. The input to VRMS is generally derived from a two pole low
pass filter/voltage divider connected to the rectified AC
input voltage. This feature allows universal input supply
voltage operation and faster response to input line fluctuations for the PFC boost preregulator. For most designs, a voltage level of 1.5V on this pin should
correspond to low line, and 4.7V for high line. The input
range for this pin extends from 0 to 5.5V.
OVP: This pin senses the boost output voltage through a
voltage divider. The enable comparator input is TTL compatible and can be used as a remote shutdown port. A
voltage level below 1.8V, disables VREF, oscillator, and
the PWM circuitry via the enable comparator. Between
1.8V and VREF (7.5V) the UC1855 is enabled. Voltage
levels above 7.5V will set the PWM latch via the hysteretic OVP comparator and disable both ZVTOUT and
GTOUT until the OVP level has decayed by the nominal
hysteresis of 400mV. If the voltage divider is designed to
initiate an OVP fault at 5% of OV, the internal hysteresis
enables normal operation again when the output voltage
has reached its nominal regulation level. Both the OVP
and enable comparators have direct logical connections
to the PWM output and exhibit typical propagation delays
of 200ns.
VSENSE: This pin is the inverting input of the voltage
amplifier and serves as the output voltage feedback point
for the PFC boost converter. It senses the output voltage
through a voltage divider which produces a nominal 3V.
The voltage loop compensation is normally connected
between this pin and VAO. The VSENSE pin must be
above 1.5V at 25°C, (1.9V at –55°C) for the current synthesizer to work properly.
5
UC1855A/B
UC2855A/B
UC3855A/B
PIN DESCRIPTIONS (cont.)
ZVS: This pin senses when the drain voltage of the main
MOSFET switch has reached approximately zero volts,
and resets the ZVT latch via the ZVT comparator. A minimum and maximum ZVTOUT pulse width are programmable from this pin. To directly sense the ≈400V drain
voltage of the main switch, a blocking diode is connected
between ZVS and the high voltage drain. When the drain
reaches 0V, the level on ZVS is ≈0.7V which is below the
2.6V ZVT comparator threshold. The maximum ZVTOUT
pulse width is approximately equal to the oscillator blanking period time.
ZVTOUT: The output of the ZVT block is a 750mA peak
totem pole MOSFET gate driver on ZVTOUT. Since the
ZVT MOSFET switch is typically 3X smaller than the
main switch, less peak current is required from this output. Like GTOUT, a series gate resistor and Schottky diode to GND are recommended. This pin may also be
used as a high current synchronization output driver.
For more information see Unitrode Applications Note U-153.
5.992 496 516 MHz
120
Gain
-90
100
120
Phase
Phase
Margin
degrees
-45 Phase
80
Gain (dB)
Degrees
0
60
100
80
60
40
Open-Loop 40
20
Gain
dB
0
20
0
-20
0.1
-20
1
10
100
1000
10000
Frequency
kHz
-40
-60
10kHz
1MHz
100kHz
10MHz
log f
Figure 1. Current Amplifier Frequency Response
Figure 2. Voltage Amplifier Gain Phase vs Frequency
24
3.10
3.08
22
3.06
20
3.04
3.02
18
mA
VOLTS
3.00
2.98
2.96
16
14
2.94
12
2.92
2.90
10
-60
-40
-20
0
20
40
60
80
100 120 140
-60
TEMPERATURE °C
-40
-20
0
20
40
60
TEMPERATURE °C
Figure 3. Voltage Amplifier Input Threshold
Figure 4. Supply Current ON
6
80
100
120
140
UC1855A/B
UC2855A/B
UC3855A/B
230
-0.75
225
-0.77
220
-0.79
215
210
-0.83
205
-0.85
200
kHz
GAIN CONSTANT (K)
-0.81
-0.87
195
190
-0.89
185
-0.91
180
-0.93
175
-0.95
170
-60
-40
-20
0
20
40
60
80
100 120 140
-60
TEMPERATURE °C
-40
-20
0
20
40
60
TEMPERATURE °C
Figure 5. Multiplier Current Gain Constant
Figure 6. Oscillator Initial Accuracy
7
80
100 120 140
UC1855A/B
UC2855A/B
UC3855A/B
TYPICAL APPLICATION
UDG-95165-1
Figure 7. Typical Application
UNITRODE CORPORATION
7 CONTINENTAL BLVD. • MERRIMACK, NH 03054
TEL. (603) 424-2410 FAX (603) 424-3460
8
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