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LT1508 Power Factor and PWM Controller (Voltage Mode)
FEATURES
s s s s s s s s s s s s
DESCRIPTION
The LT (R) 1508 is a complete solution for universal off-line switching power supplies utilizing active power factor correction. The PFC section is identical to the LT1248 PFC controller except the EN/SYNC pin is removed because PFC and PWM are synchronized internally. The voltage mode PWM section (LT1509 is the current mode counterpart) contains all the primary side functions to convert the PFC preregulated high voltage output to an isolated low voltage output. The PWM duty cycle is internally limited to 47% (maximum 50%) to prevent transformer saturation. PWM soft start begins when the PFC output reaches the preset voltage. In the event of brief line loss, PWM will be shut off when the PFC output voltage drops below 73% of the preset value.
, LTC and LT are registered trademarks of Linear Technology Corporation.
PFC and PWM Single Chip Solution Synchronized Operation up to 300kHz 99% Power Factor Over 20:1 Load Current Range Voltage Mode PWM Instantaneous Overvoltage Protection Dedicated Overvoltage Protection (OVP Pin) Minimal Line Current Dead Zone Typical 250A Start-Up Supply Current Line Switching Noise Filter Low Quiescent Current: 13mA Fast 1.5A Peak Current Gate Drivers Separate Soft Start Control
APPLICATIONS
s
Universal Power Factor Corrected Power Supplies and Preregulators
BLOCK DIAGRAM
VCC 16V TO 10V
VAOUT 10
VREF 7.5V VREF RUN 12
MOUT ISENSE 8 7
CAOUT 6
+ -
2.2V
7A
+
M1
VSENSE 14 7.5V
- -
EA
-
IM CA
IA 25k IB IA2IB IM = 200A2
- +
RUN
+
IAC 9
+
+
0.7V R 2R
7.9V OVP 11 SS1 16 14A
+ -
-
OSC 16V GND2 2
+
1V
-
CL
4 CSET
RSET 15 55% DELAY S
-
14A SS2 13 PWMOK
-
7V TO 4.7V
+ + -
+
NOTE: PWM PULSE IS DELAYED BY 55% DUTY CYCLE AFTER PFC PULSE
19 ILIM
VC 18
50A
U
W
U
GND1 3
PKLIM VCC 5 17
- +
R R S Q GTDR1 1
R R
Q
GTDR2 20
200ns BLANKING
16V
1508 BD
1
LT1508
DESCRIPTION
By using fixed high frequency PWM current averaging without the need for slope compensation, the LT1508 achieves far lower line current distortion with a smaller magnetic element than systems that use either peak current detection, or zero current switching approach, in both continuous and discontinuous modes of operation. The LT1508 also provides filtering capability to reject line switching noise which can cause instability when fed into the multiplier. Line current dead zone is minimized with low bias voltage at the current input to the multiplier. The LT1508 provides many protection features including peak current limiting and overvoltage protection. Implemented with a very high speed process, the LT1508 can be operated at frequencies as high as 300kHz.
ABSOLUTE MAXIMUM RATINGS
Supply Voltage ........................................................ 27V GTDR Current Continuous ...................................... 0.5A GTDR Output Energy ................................................ 5J IAC, RSET, PKLIM Input Current .............................. 20mA VSENSE, OVP Input Voltage .................................... VMAX ILIM, VC Input Voltage ................................................ 8V ISENSE, MOUT Input Current ................................... 5mA Operating Junction Temperature Range LT1508C ................................................ 0C to 100C LT1508I ............................................ - 40C to 125C Thermal Resistance (Junction-to-Ambient) N Package ................................................... 100C/W SW Package ................................................ 120C/W
PACKAGE/ORDER INFORMATION
TOP VIEW GTDR1 1 GND2 2 GND1 3 CSET 4 PKLIM 5 CAOUT 6 ISENSE 7 MOUT 8 IAC 9 VAOUT 10 N PACKAGE 20-LEAD PDIP 20 GTDR2 19 ILIM 18 VC 17 VCC 16 SS1 15 RSET 14 VSENSE 13 SS2 12 VREF 11 OVP SW PACKAGE 20-LEAD PLASTIC SO WIDE
ORDER PART NUMBER LT1508CN LT1508CSW LT1508IN LT1508ISW
TJMAX = 125C, JA = 100C/ W (N) TJMAX = 125C, JA = 120C/ W (SW)
Consult factory for Military grade parts.
ELECTRICAL CHARACTERISTICS
Maximum operating voltage (VMAX) = 25V, VCC = 18V, RSET = 15k to GND, CSET = 1nF to GND, IAC = 100A, ISENSE = 0V, CAOUT = 3.5V, VAOUT = 5V, OVP = VREF. No load on any outputs unless otherwise noted.
PARAMETER Overall Supply Current (VCC in Undervoltage Lockout) Supply Current On VCC Turn-On Threshold (Undervoltage Lockout) VCC Turn-Off Threshold Voltage Amplifier (PFC Section) Voltage Amp Offset Input Bias Current Voltage Gain Voltage Amp Unity-Gain Bandwidth Voltage Amp Output High (Internally Clamped) Voltage Amp Output Low Voltage Amp Short-Circuit Current CONDITIONS VCC = Lockout Voltage - 0.2V 11.5V VCC VMAX
q q q q
MIN
TYP 0.25 13 16.5 10.5
MAX 0.45 19 17.5 11.5 10 - 250
UNITS mA mA V V mV nA dB MHz V V mA
15.5 9.5 - 10 70
VAOUT = 3.5V VSENSE = 0V to 7V
q q
q q
11.3 3
VAOUT = 0V
q
- 25 100 3 13.3 1.1 8
2 17
2
U
W
U
U
WW
U
W
LT1508
ELECTRICAL CHARACTERISTICS
Maximum operating voltage (VMAX) = 25V, VCC = 18V, RSET = 15k to GND, CSET = 1nF to GND, IAC = 100A, ISENSE = 0V, CAOUT = 3.5V, VAOUT = 5V, OVP = VREF. No load on any outputs unless otherwise noted.
PARAMETER Current Amplifier (PFC Section) Current Amp Offset Voltage ISENSE Bias Current Current Amp Voltage Gain Current Amp Unity-Gain Bandwidth Current Amp Output High Current Amp Output Low Current Amp Short-Circuit Current Input Range, ISENSE, MOUT (Linear Operation) Reference Reference Output Voltage VREF Load Regulation VREF Line Regulation VREF Short-Circuit Current VREF Worst Case Current Limit PKLIM Offset Voltage PKLIM Input Current PKLIM to GTDR Propagation Delay Multiplier Multiplier Output Current Multiplier Output Current Offset Multiplier Maximum Output Current Multiplier Gain Constant (Note 1) IAC Input Resistance Oscillator Oscillator Frequency CSET Ramp Peak-to-Peak Amplitude CSET Ramp Valley Voltage Overvoltage Comparator (PFC Section) Comparator Trip Voltage Ratio (VTRIP / VREF) Hysteresis OVP Bias Current OVP Propagation Delay Gate Drivers (GTDR1 and GTDR2) Max Output Voltage Output High Output Low (Device Unpowered) Output Low (Device Active) Peak Output Current Rise and Fall Time Max Duty Cycle (PFC) Max Duty Cycle (PWM) (Note 2) CONDITIONS
q q
MIN
TYP 1 - 25 110 3 8.5 1.1 8
MAX 4 - 250
UNITS mV nA dB MHz V V mA V V mV mV mA V mV A ns A A A V -2 k kHz kHz V V
80
q q
7.2 3 - 0.3 7.39
CAOUT = 0V
q q
2 17 1 7.60 20 50 7.68 25 - 100
IREF = 0mA, TA = 25C - 5mA < IREF < 0mA 11.5V < VCC < VMAX VREF = 0V Load, Line, Temperature
q q q q
- 20 12 7.32 - 25
7.50 5 5 28 7.5
PKLIM = - 0.1V PKLIM Falling from 50mV to - 50mV IAC = 100A, RSET = 15k RAC = 1M from IAC to GND IAC = 450A, RSET = 15k, VAOUT = 7V, MOUT = 0V IAC from 50A to 1mA RSET = 15k, CSET = 1000pF RSET = 15k, CSET = 1500pF
q
- 50 400 35 - 0.05 - 260 0.035 25 100 68 4.7 1.3 1.05 0.35 0.2 100 15 0.9 0.5 0.2 2 25 96
q q
- 286 15
- 0.5 - 235 35 115 78 5.0 1.55 1.06 1
q q
85 58 4.35 1.15 1.04
q
OVP = 7.5V
q
V A ns V V V V V A ns % %
0mA Load, 18V < VCC - 200mA Load, 11.5V VCC 15V VCC = 0V, 50mA Load (Sinking) 200mA Load (Sinking) 10mA Load 10nF from GTDR to GND 1nF from GTDR to GND
q q q q q
12 VCC - 3.0
17.5 1.5 1 0.4
90 44
50
3
LT1508
ELECTRICAL CHARACTERISTICS
VCC = 18V, RSET = 15k to GND, CSET = 1nF to GND, IAC = 100A, ISENSE = 0V, CAOUT = 3.5V, VAOUT = 5V, OVP = VREF. No load on any outputs, unless otherwise noted.
PARAMETER Soft Start Current SS1 Current (PFC) SS2 Current (PWM) Comparators in PWM Section ILIM Input Current Current Limit Comparator (CL) Threshold GTDR2 Switching Off Threshold at VC or at SS2 VC Input Current PWMOK Comparator Low Threshold (in Terms of VREF) VC Pin High Voltage GTDR2 Turn-On Blanking Time CONDITIONS SS1 = 2.5V SS2 = 1V ILIM = 0V, VC = 1.6V VC > 2.6V ILIM = 0V VC = 0V 1mA into VC Pin
q q q q q q q q
MIN 5 5
TYP 12 12 - 0.3 1.1
MAX 30 30 -2 1.20 - 80 0.70 7.5
UNITS A A A V V A V ns
0.95 1 - 20 0.57 6.2
0.63 6.9 180
The q denotes specifications which apply over the full operating temperature range. IM Note 1: Multiplier Gain Constant: K = IAC (VAOUT - 2)2
Note 2: GTDR2 (PWM) pulse is delayed by 53% duty cycle after GTDR1 (PFC) is set. See PFC/PWM Synchronization graph in the Typical Performance Characteristics section.
TYPICAL PERFORMANCE CHARACTERISTICS
PFC Voltage Amplifier Open-Loop Gain and Phase
100 80 GAIN 60
GAIN (dB)
GAIN (dB)
40 20 0 -20
PHASE
10
100
1k 10k 100k FREQUENCY (Hz)
4
UW
1M
1508 G01
PFC Current Amplifier Open-Loop Gain and Phase
0 -20 -40 -60 -80 -100 100 80 GAIN 60 40 20 0 -20 -40 -60 -80
PHASE (DEG) PHASE (DEG)
PFC/PWM Synchronization
0 -20
PFC (GTDR1) 53%
PHASE
PWM (GTDR2)
-100 -120 10M
1508 G02
-120 10M
10
100
1k 10k 100k FREQUENCY (Hz)
1M
TIME
1508 G03
LT1508 TYPICAL PERFORMANCE CHARACTERISTICS
Reference Voltage vs Temperature
7.536 7.524
REFERENCE VOLTAGE (V)
7.512 7.500 7.488 7.476 7.464 7.452 7.440 7.428 -75 -50 -25 0 25 50 75 100 125 150 JUNCTION TEMPERATURE (C)
1508 G04
IM (A)
Supply Current vs Supply Voltage
16 15 14 TJ = -55C
SUPPLY CURRENT (mA)
13
TIME (ns)
SUPPLY CURRENT (A)
12 11 10 9 8 7 6 5 10
TJ = 125C
TJ = 25C
21 SUPPLY VOLTAGE (V)
Frequency vs RSET and CSET
500 450 400 RSET = 10k RSET = 15k RSET = 20k RSET = 30k 1.00 0.99 0.98
MAXIMUM DUTY CYCLE
FREQUENCY (kHz)
350 300 250 200 150 100 50 0 200 600
0.97 0.96 0.95 0.94 0.93 0.92 0.91 RSET = 10k RSET = 15k RSET = 20k RSET = 30k 600 1800 1400 CSET CAPACITANCE (pF) 1000 2200
1508 G10
MOUT CURRENT (mA)
1800 1400 CSET CAPACITANCE (pF)
1000
UW
1508 G06 1508 G09
Multiplier Current
300 VAOUT = 7V VAOUT = 6.5V VAOUT = 6V 150 VAOUT = 5V VAOUT = 4.5V VAOUT = 4V VAOUT = 3.5V 0 VAOUT = 3V VAOUT = 2.5V 500
1508 G05
VAOUT = 5.5V
0
250 IAC (A)
GTDR Rise and Fall Time
400
550 500 450
Start-Up Supply Current vs Supply Voltage
300
400 350 300 250 200 150 100 50 0 -55C 25C 125C
FALL TIME 200 RISE TIME 100 NOTE: GTDR SLEWS BETWEEN 1V AND 16V
32
0
0
10
20 30 40 LOAD CAPACITANCE (nF)
50
1508 G07
0
2
4
6 8 10 12 14 16 18 20 SUPPLY VOLTAGE (V)
1508 G08
GTDR1 Maximum Duty Cycle vs RSET and CSET
1.5 1.0 0.5 0 -0.5 -1.0 -1.5 -2.0 -2.5 -3.0 -3.5 -4.0
MOUT Pin Characteristics
TJ = 125C TJ = 25C TJ = -55C
2200
0.90 200
-2.4
-1.2 1.2 0 MOUT VOLTAGE (V)
2.4
1508 G11
5
LT1508 TYPICAL PERFORMANCE CHARACTERISTICS
RSET Voltage vs Current
120 100 80 60 40 20 0 -20 -40 -60 -80 -100 0 -0.2 -0.4 -0.8 -0.6 RSET CURRENT (mA) -1.0
1508 G12
PKLIM CURRENT (A)
VRSET - VREF (mV)
PIN FUNCTIONS
PFC SECTION
GTDR1 (Pin 1): The PFC MOSFET gate driver is a fast totem pole output which is clamped at 15V. Capacitive loads like the MOSFET gates may cause overshoot. A gate series resistor of at least 5 will prevent the overshoot. GND2 (Pin 2): Power Ground. High current spikes occur in this line when either GTDR1 or GTDR2 switches low. GND1 (Pin 3): Analog Ground. CSET (Pin 4): The capacitor from this pin to GND and RSET determines oscillator frequency. The oscillator ramp is 5V and the frequency = 1.5/(RSET CSET). PKLIM (Pin 5): The threshold of the peak current limit comparator is GND. To set current limit, a resistor divider can be connected from VREF to the current sense resistor. CAOUT (Pin 6): This is the output of the current amplifier that senses and forces the line current to follow the reference signal that comes from the multiplier by commanding the pulse width modulator. When CAOUT is low, the modulator has zero duty cycle. ISENSE (Pin 7): This is the inverting input of the current amplifier. This pin is clamped at - 0.6V by an ESD protection diode. MOUT (Pin 8): This is the multiplier high impedance current output and the noninverting input of the current amplifier. This pin is clamped at - 0.6V and 3V.
6
UW
PKLIM Pin Characteristics
-360 TJ = 125C TJ = 25C TJ = -55C -300 -240 -180 -120 -60 0 60 120 180 240 300 -0.8 -0.4 0.4 0 PKLIM VOLTAGE (V) 0.8
1508 G13
TJ = 125C TJ = 25C TJ = -55C
U
U
U
(For application help with the PFC portion of this chip, see the LT1248 data sheet)
IAC (Pin 9): This is the AC line voltage sensing input to the multiplier. It is a current input that is biased at 2V to minimize the crossover dead zone caused by low line voltage. At the pin, a 25k resistor is in series with the current input, so that a lowpass RC can be used to filter out the switching noise coming down from the line with a high line impedance environment. VAOUT (Pin 10): This is the output of the voltage error amplifier. The output is clamped at 13.5V. When the output goes below 2.5V, the multiplier output current is zero. OVP (Pin 11): This is the input to the overvoltage comparator. The threshold is 1.05 times the reference voltage. When the comparator trips, the multiplier, which is quickly inhibited, blanks PFC switching to prevent further overshoot. This pin is also the input to the PWMOK comparator that releases the PWM soft start (SS2) after the PFC output gets close to the final voltage and has a hysteresis of approximately 150V for 382V PFC output. VREF (Pin 12): This is the 7.5V reference. When VCC goes low, VREF will stay at 0V. VREF biases most of the internal circuitry and can source up to 5mA externally. VSENSE (Pin 14): This is the inverting input to the voltage amplifier.
LT1508
PIN FUNCTIONS
RSET (Pin 15): A resistor from RSET to GND sets the oscillator charging current and the maximum multiplier output current which is used to limit the maximum line current. IM(MAX) = 3.75V/RSET SS1 (Pin 16): Soft Start. SS1 is reset to zero for low VCC. When VCC rises above the lockout threshold, SS1 is released to ramp up at a rate set by the internal 12A current source and an external capacitor. During this ramp up, PFC reference voltage is equal to SS1 voltage. After SS1 rises past 7.5V, reference voltage remains at 7.5V. VCC (Pin 17): This is the supply for the chip. The LT1508 has two fast gate drivers required to fast charge high power MOSFET gate capacitances. Good supply bypassing is required consisting of a 0.1F ceramic capacitor in parallel with a low ESR electrolytic capacitor (56F or higher) in close proximity to IC GND.
APPLICATIONS INFORMATION
Voltage Error Amplifier (PFC Section) The voltage error amplifier has a 100dB DC gain and 3MHz unity-gain frequency. The output is internally clamped at 13.3V with VCC = 18V. Maximum error amp output voltage decreases to VCC - 1.5V for VCC less than 12V. The noninverting input is tied to the 7.5VREF through a diode and can be pulled down with the SS1 pin. Referring to Figure 1, VOUT = VREF [(R1 + R2)/R2]. With R1 = 1M and R2 = 20k, VOUT = 382V. R1 through R4, C1 and C2 form the compensation for the voltage loop. Gain of the voltage error amp with the values shown is given by:
VAOUT =- VOUT 1+j
C2 0.047F C1 0.47F
(j)(f)(6.6) 1 + j f 11
))
(RREF)(PIN) RS(RIAC + 25k)
f 1
The small-signal gain for the remaining portion of the voltage loop for frequencies below the current loop bandwidth is (see Figure 2):
VOUT VIN = VAOUT (5)(j)(f)(COUT)(VOUT)
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U
U
U
U
U
(For application help with the PFC portion of this chip, see the LT1248 data sheet)
PWM SECTION SS2 (Pin 13): PWM Soft Start. The comparator PWMOK monitors the OVP pin and releases the SS2 after the PFC output gets close to the final voltage. VC (Pin 18): PWM voltage mode control voltage. Normally connects to the optocoupler amplifier output. A pull-up current of 50A flows out of the pin. ILIM (Pin 19): PWM current sense input with limit set to 1.1V. GTDR2 (Pin 20): The PWM MOSFET gate driver is a 1.5A fast totem pole output. It is clamped at 15V. Capacitive loads like the MOSFET gates may cause overshoot. A gate series resistor of at least 5 will prevent the overshoot.
REGULATOR OUTPUT VOUT = 382V R3 20k VSENSE
R4 330k
R1 1M
- + 7.5VREF ERROR AMP
VAOUT
R2 20k
OVP
LT1508 - + 1.05VREF OVERVOLTAGE COMPARATOR
1508 * F01
Figure 1
With VIN = 120VAC, PIN = 150W, RS = 0.15, RREF = 4k, RIAC = 1M, VOUT = 382V and COUT = 470F, VOUT/VAOUT = 85/(j)(f). At very low frequencies, the loop has a - 40dB/ decade slope. Additional zero-pole compensation is added at 1Hz and 11Hz. The resulting loop gain and phase margin is shown in Figure 3. The unity-gain bandwidth is low compared to 120Hz, which results in low distortion and a high power factor.
7
LT1508
APPLICATIONS INFORMATION
VIN
+
VIN
L RS 0.15 RREF 4k R5 4k C4 300pF
D1
VOUT COUT 470F
-
IIN
R6 20k VIN RIAC 1M
C3 0.001F
IM IAC
Figure 2
80 60
LOOP GAIN (dB)
40 20 0 -20 -40 0.1 1 10 FREQUENCY (Hz)
1508 * F03
100
CURRENT LOOP GAIN (dB)
Figure 3
Current Amplifier (PFC Section) The current amplifier has a 110dB DC gain, 3MHz unitygain frequency and a 2V/s slew rate. It is internally clamped at 8.5V. Note that in the current averaging operation, high gain at twice the line frequency is necessary to minimize line current distortion. Because CAOUT may need to swing 5V over one line cycle at high line condition, 20mV AC will be needed at the inputs of the current amplifier for a gain of 260 at 120Hz. Especially at light load when the current loop reference signal is small, lower gain will distort the reference signal and line current. But, if signal gain at switching frequency is too high, the system behaves more like a current mode system and can cause subharmonic oscillation.
8
+
-
MOUT
ISENSE
CA LT1508
1508 * F02
60
45
PHASE MARGIN (DEG)
30 15 0 -15
-30 1000
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To avoid subharmonic oscillations, the amplified downslope of the inductor current must be less than the slope of the oscillator ramp.
VCA(OUT) (VOSC)(L)(fSW) VRS (VOUT)(RS) = (5V)(500H)(100k) = 4.4 (382V)(0.15)
CAOUT
If the current amplifier gain at 100kHz is less than 4.4, there will be no subharmonic oscillation. The open-loop gain of the current loop is given by:
VRS VCA(OUT) = = (VOUT)(RS) (j)(2f)(L)(VOSC)
(382V)(0.15) 3648 = (j)(2f)(500H)(5V) (j)(f)
The current error amp, with R5 = 4k, R6 = 20k, C3 = 0.001F and C4 = 300pF, provides zero pole compensation resulting in 16kHz loop crossover frequency. The current amp gain at 100kHz is 1.7. The resulting current loop gain and phase margin is shown in Figure 4.
80 60 40 20 0 -20 -40 0.1 1 10 FREQUENCY (kHz)
1508 * F04
60 45
PHASE MARGIN (DEG)
30 15 0 -15 -30 1000
100
Figure 4
Multiplier The multiplier has high noise immunity and superior linearity over its full operating range. The current gain is IM = (IACIEA2)/(200A2) with IEA = (VAOUT - 2V)/ 25k. The error amplifier output voltage required at the input to the multiplier is:
LT1508
APPLICATIONS INFORMATION
VAOUT = 2 + (PIN)(RS)(25)(RIAC + 25k) (VIN2)(RREF) The multiplier output acts as the command signal to the current loop error amplifier. During steady-state operation the voltage across RREF = (IM)(RREF) = (IIN)(RS). Based on this the value for RS is determined by: RS (IM(MAX))(RREF)(VIN)(eff) POUT 2
See Figure 2 for RREF. VAOUT is squared in the multiplier, resulting in excellent performance over a wide range of output power and input voltage without the addition of feedforward line frequency ripple. Care must be taken to avoid feeding switching frequency noise into the multiplier from the IAC pin. An internal 25k is provided in series with the low impedance multiplier input so that only a capacitor from the IAC pin to GND1 is required to filter noise. The maximum multiplier output current, which ultimately limits the input line current, is set by a resistor from the RSET pin to GND1 according to the formula: IM(MAX) = 3.75V/RSET. Figure 5 shows IM versus IAC for various values of VAOUT. Note that Figure 5 data was taken with RSET = 15k.
300 VAOUT = 7V VAOUT = 6.5V VAOUT = 6V VAOUT = 5V VAOUT = 4.5V VAOUT = 4V VAOUT = 3.5V 0 VAOUT = 3V VAOUT = 2.5V 500
1508 * F05
VAOUT = 5.5V
IM (A)
150
0
250 IAC (A)
Figure 5. Multiplier Current IM vs IAC and VAOUT
Oscillator Frequency and Maximum Line Current Setting The oscillator frequency is set by RSET and CSET. RSET is the resistor from the RSET pin to GND1 and CSET is the capacitor from the CSET pin to GND1. RSET should be determined first. The oscillator frequency, which is equal to the switching frequency for both the PFC and PWM section, is determined by:
1.5 fOSC = (RSET)(CSET)
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with RSET = 15k, IM(MAX) = 3.75/15k = 250A. For a 300W converter with an efficiency (eff) of 0.8 at low line (90VRMS) and RREF set to 4k, RS should be less than:
(250A)(4k)(90VAC)(0.8) 300W2
= 0.169
A 0.15 resistor will yield a maximum peak input current of (IM(MAX))(RREF/RS) = (250A)(4k)/0.15 = 6.67A. For a 100kHz switching frequency with RSET = 15k, CSET = 1.5/ (100kHz)(15k) = 1nF. For added protection the LT1508 provides a second independent current limit comparator. When the input voltage to the comparator (PKLIM pin) dips below 0V, GTDR1 pin quickly goes low turning off the PFC power switch. A resistor divider from VREF to RS (Figure 6) senses the voltage across the line current sense resistor (RS) and limits the peak input line current to [(7.5V/R1) + 50A] (R2/RS). The 50A represents the PKLIM input current which flows out of the PKLIM pin. With R1 = 10k and R2 = 1.8k, IIN = 9.6A peak above the 6.67A peak average plus the input inductor peak ripple current. Always use RSET to set the primary line current limit. The PKLIM comparator is only for secondary protection. When the line current reaches the primary limit, VOUT can no longer be supported with the given input current and begins to fall. System stability is maintained by the current loop which is controlled by the current amplifier. When the
R2 1.8k R1 10k 7.5V VREF LT1508
+
RS IPKLIM C1 1nF
-
0.15
PKLIM
- +
ILINE
C1 IS TO REJECT NOISE, CURRENT LIMIT DELAY IS ABOUT 2s
1508 * F06
Figure 6
9
LT1508
APPLICATIONS INFORMATION
line current reaches the secondary limit, the comparator takes over control and hysteresis may occur causing audible noise. Overvoltage Protection (PFC Section) Because of the slow loop response necessary for power factor correction, output overshoot can occur following a sudden load reduction or removal. To protect downstream components, the LT1508 provides an overvoltage comparator which senses the output voltage and quickly reduces the line current demand. Referring back to Figure 1, VOUT is 382V and during normal operation, since no current flows in R3, 7.5V appears at both the VSENSE and OVP pins. When VOUT overshoots its preset value, the overcurrent from R1 will flow through R2 as well as R3. The voltage amplifier feedback will keep VSENSE at 7.5V. Therefore, the equivalent AC resistance seen by the OVP pin is R2 in parallel with R3 or 10k. With these values and the overvoltage comparator trip level internally set at 1.05VREF, the comparator trips when VOUT overshoots 10%. Overvoltage trip level is given by:
R4 1.05M 0.047F 0.47F VOUT R1 1M R2 20k 330k
1.05VREF VOUT = 382V OVERVOLTAGE = 420V
1508 * F07
Figure 7
(%)VOUT = 5%
(
R2 + R3 R3
)
this condition, the amplifier M1 (see Block Diagram) becomes active. When VAOUT reduces to 2.2V, M1 supplies up to 7A of current to the resistor at the ISENSE pin in order to cancel a negative VOS and keep VOUT error to within 2V. Undervoltage Lockouts and Soft Start The LT1508 turns on when VCC reaches 16V and remains on until VCC falls below 10V, whereupon the chip enters the lockout state. In the lockout state, the oscillator is off and the VREF and gate driver pins remain low. A capacitor from SS1 to GND1 determines the ramp-up time of the PFC section. SS1 is released from a zero when VCC rises above the lockout threshold. Once released, an internal 14A current source ramps the voltage error amplifier's reference voltage to 7.5V. SS1 voltage then continues beyond 7.5V. A second capacitor from SS2 to GND1 determines the start-up time from the PWM section. A PWMOK comparator (see Block Diagram) holds SS2 low until the OVP pin reaches 7V. This corresponds to the PFC output voltage reaching approximately 93% of its preset voltage. SS2 is diode coupled to the PMW comparator which is connected to the VC pin by a second diode. Holding SS2 low at any time will disable PWM output. Once released, the 14A current source ramps the PWM comparator
For additional protection, the OVP pin can be connected to VOUT through an independent resistor divider (see Figure 7). This ensures overvoltage protection during safety agency abnormal testing conditions, such as opening R1 or shorting R2. The output of the multiplier looks like a high impedance current source. In the current loop, offset line current is determined by multiplier offset current and input offset voltage of the current error amplifier. A - 4mV current amplifier VOS translates to 27mA line current and 6.7W input power for 250VAC line if a 0.15 sense resistor is used. Under a no-load condition or when the load power is less than the offset output power, the offset line current could slowly charge the output to an overvoltage level. This is because the best the overvoltage comparator can do is to reduce the multiplier output current to zero. Unfortunately, this does not guarantee zero output current if the current amplifier has offset. To regulate VOUT under
10
+
R5 20k
-
OVP
+
-
U
W
U
U
VSENSE
VAOUT
ERROR AMP LT1508
OVERVOLTAGE COMPARATOR
LT1508
APPLICATIONS INFORMATION
input up to VC and then the SS2 voltage continues beyond VC. The PWMOK comparator contains hysteresis and will pull SS2 low disabling the PWM section if the PFC output voltage falls below approximately 62% of its preset value (240V with nominal 382V output). Start-Up and Supply Voltage The LT1508 draws only 250A before the chip starts at 16V on VCC. To trickle start, a 91k resistor from the power line to VCC supplies trickle current, and C4 holds VCC up while switching starts (see Figure 8); then the auxiliary winding takes over and supplies the operating current. Note that D3 and the larger values of C3 are only necessary for systems that have sudden large load variations down to minimum load and/or very light load conditions. Under these conditions the loop may exhibit a start/restart mode because switching remains off long enough for C4 to discharge below 10V. Large values for C3 will hold VCC up until switching resumes. For less severe load variations D3 is replaced with a short and C3 is omitted. The turns ratio between the primary winding determines VCC according to : Output Capacitor (PFC Section) GTDR2 (PWM) pulse is synchronized to GTDR1 (PFC) pulse with 53% duty cycle delay to reduce RMS ripple current in the output capacitor. See PFC/PWM Synchronization graph in the Typical Performance Characteristics section. The peak-to-peak 120Hz PFC output ripple is determined by:
VOUT N =P VCC - 2V NS
for 382V VOUT and 18V VCC, Np/Ns 19.
LINE MAIN INDUCTOR NP NS D1 R1 91k 1W D3
+
D2
C1 2F C2 2F
+
+
C3 390F
+
Figure 8
U
W
U
U
VP-P = 2ILOAD(DC)(Z)
where ILOAD(DC) is the DC load current of the PWM stage and Z is the capacitor impedance at 120Hz. For 470F, impedance is 2.8 at 120Hz. At 335W load, ILOAD(DC) = 335V/382V = 0.88A, VP-P = (2)(0.88)(2.8) = 5V. If less ripple is desired higher capacitance should be used. The selection of the output capacitor is based on voltage ripple, hold-up time and ripple current. Assuming the DC converter (PWM section) is designed to operate with 240V to 382VIN , the minimum hold-up time is a function of the energy storage capacity of the capacitor:
tHOLD =
(0.5)COUT (382V - 0.5VP-P)2 - 240V2 POUT
with COUT = 470F, VP-P = 11.5V, and POUT = 335W, tHOLD = 60ms which is 3.6 line cycles at 60Hz. The ripple current can be divided into two major components. The first is the 120Hz component which is related to the DC load current as follows:
I120HZ ILOAD(DC) 2
The second component is made up of switching frequency components due to the PFC stage charging the capacitor and the PWM stage discharging the capacitor. For a 300W output PFC forward converter running from an input voltage of 100VRMS, the total high frequency ripple current was measured to be 1.79ARMS. For the United Chemicon KMH 450V capacitor series, ripple current at 100kHz is specified 1.43 times higher than the 120Hz limit.
VCC C4 100F
1508 * F08
11
LT1508
APPLICATIONS INFORMATION
The total equivalent 120Hz ripple in the output capacitor can be calculated by:
2 2 + IHF
Table 1. PFC Capacitor RMS Ripple Current
100W VINRMS 100 120 230 I120HZ 0.2 0.2 0.2 IHF 0.6 0.5 0.53 0.41 0.41 0.41 200W I120HZ IHF 1.18 0.97 0.87 0.62 0.62 0.62 300W I120HZ IHF 1.79 1.45 1.26
IRMS =
I120HZ
()
1.43
IHF = 100kHz Ripple Current. For ILOAD(DC) = 0.88A, 1120Hz = 0.62A and the equivalent 120Hz ripple current is:
IRMS =
1.79 2 0.622 + = 1.4ARMS 1.43
()
Table 1 lists the ripple current components from lab measurements for various output powers and line voltages. The 120Hz ripple current rating at 105C ambient is 1.72A for the 470F KMH 35mm x 50mm capacitor. The expected life of the output capacitor may be calculated from thermal stress analysis:
(105C + TK) - (TA + TO) 10 L = (LO)2
where L = Expected life time LO = Hours of load life at rated ripple current and rated ambient temperature TK = Capacitor internal temperature rise at rated condition. TK = (I2R)/(KA), where I is the rated current, R is capacitor ESR and KA is a volume constant. TA = Operating ambient temperature TO = Capacitor internal temperature rise at operating condition
12
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W
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In our example, LO = 2000 hours assuming TK = 5C at rated 1.72A. TO can then be calculated from:
TO = TK
()()
2
IRMS 1.4A = 5C = 3.3C 1.72A 1.72A
2
Assuming the operating ambient temperature is 60C, the approximate lifetime is:
L = (2000)(2)
(105C + 5C) - (60 + 3.3C) 10
= 50,870 Hours
For longer life a capacitor with a higher ripple current rating or parallel capacitors should be used. PWM Comparators The LT1508 includes two comparators in the PWM section which implement voltage mode PWM control. The VC or control voltage pin sets duty factor. An additional current limit comparator turns GTDR2 off in the event the ILIM pin voltage exceeds 1.1V. On-chip blanking avoids reset due to leading edge noise. Typical Application Figure 9 shows a 24VDC, 300W power factor corrected, universal input supply. The 2-transistor forward converter offers many benefits including low peak currents, nondissipative snubber, 500VDC switches and automatic core reset guaranteed by the LT1508's 50% maximum duty cycle.
15V R14 3.3 D1 ERA82-004 D4 MURH860CT VOUT 382VBUS
T1
*
T3 D2 ERA82-004
R8 10
90VAC TO 264VAC
+
C1 1F 400V C7 2.2F 50V R39 20k R10 20 Q1 IRFP450 D8 MUR160 Q3 IRF840 R13 20k R4 0.51 R5 0.51 R9 10
C6 2.2F 50V
*
*
R41 20k D7 MUR160
Q1 IRF840
+
C14 330F 450V
C10 1F 63V
E1
LINE 3 VREF C3 0.1F C25 1nF C24 1nF R11 20 6 1 CAOUT GTDR1 GTDR2 ILIM LT1508 19 C28 100pF 20 R24 10k 5 PKLIM R36 R25 4.02k 20k 1% 8 7 MOUT ISENSE D9 1N5818 D10 1N5818 R1 0.15 5W
~
+
LINE
2
R2 1M 1/2W
C9 1F "X"
4
U1 KPBC606
C8 330F 35V
*
*
~
R37 4.02k C29 1% 300pF
-
APPLICATIONS INFORMATION
VIN
382VBUS
C2 0.47F R22 330k
C5 4.7nF "Y" R40 470 D11 MUR3020PT C16 1nF
VREF
14
10 12 VAOUT VREF VSENSE
R32 499k 1% R33 499k 1%
R34 499k 1% R35 499k 1%
R27 20k 1% 11
OVP SS1 16 15V C20 47nF 17 18
9 SS2 13 C21 1nF C33 1F RSET VCC VC
IAC C4 4.7nF "Y" 6 5 4 1 2 U3 3 CNY17-3 U4 LT1431 R20 2.2k
C22 4.7nF 15 R16 15k 1%
GND2 GND1 CSET 2 3 4
R7 10 1W E2 24V
R26 20k 1%
+ +
C30 120F 35V C32 1F C19 10nF C26 120F 35V C31 1F
R21 2K
+
33 C18 0.068F R19 0.1F 1k R31 30.1k 1% C11 1F 63V R30 3.4k 1%
C12 1000F 35V
+
C13 1000F 35V
R6 10 2W
NOTE: UNLESS OTHERWISE SPECIFIED 1. ALL RESISTORS 1/4W, 5% 2. ALL CAPACITANCE VALUES IN MICROFARADS
C1: ELECTRONIC CONCEPTS 5MP12J105K L1: COILTRONICS CTX02-13140-X3 R1: JW MILLER/FUKUSHIMA MPC71 RT1: KETEMA SG57 SURGE GARD
T1: COILTRONICS CTX02-13141-X2 (407) 241-7876 T2: COILTRONICS CTX02-13063-X3 T3: BI TECHNOLOGY HM41-11510 (714) 447-2345
L1 67F E3 COM
1508 F09
Danger!! Lethal Voltages Present
LT1508
13
Figure 9. 24V, 300W Off-Line PFC Supply
U
GND
3
C23 47nF
W
T2
1
F1 6.3F
2
1
RT1 SG57 D3 ERA82-004
+ +
D5 1N965B 15V D6 1N965B 15V
U
*
U
Danger!! Lethal Voltages Present In This Section
VIN
R3 91k 1W
LT1508
APPLICATIONS INFORMATION
An LT1431 reference/amplifier coupled to a low cost optoisolator closes the loop from secondary side to primary side. Efficiency versus power and line voltage is shown in Figure 10. The PFC preregulator alone has efficiency numbers between 90% and 97% over line and load. A 3-turn secondary added to the 70-turn primary of T1 bootstraps VCC to about 15V supplying the chip's 13mA requirement as well as about 39mA to cover the gate current of the three FETs and high side transformer. A 0.15 sense resistor is used to sense input current and servo to the command created by the outer voltage and multiplier. Thus the input current follows the input line voltage, and changes as necessary, in order to maintain constant bank voltage. The forward converter sees a voltage input of 382VDC unless the line voltage drops out, in which case the 330F main capacitor discharges to 240VDC before the PWM stage is shut down. Compared to a typical off-line converter, the effective input voltage range of the forward converter is much smaller, simplifying the design. Additionally, the higher bus voltage provides greater hold-up times for given capacitor size.
90 200W/300W 85 100W 80
EFFICIENCY (%)
75 30W
70
90
14
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W
U
U
132
180 VRMS
250
1508 * F1O
Figure 10
LT1508
PACKAGE DESCRIPTION
0.300 - 0.325 (7.620 - 8.255)
0.009 - 0.015 (0.229 - 0.381)
0.015 (0.381) MIN
0.005 (0.127) MIN 0.100 0.010 (2.540 0.254) *THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.010 INCH (0.254mm)
(
+0.025 0.325 -0.015 +0.635 8.255 -0.381
)
Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
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Dimensions in inches (millimeters) unless otherwise noted. N Package 20-Lead PDIP (Narrow 0.300)
(LTC DWG # 05-08-1510)
1.040* (26.416) MAX 20 19 18 17 16 15 14 13 12 11
0.255 0.015* (6.477 0.381)
1 0.130 0.005 (3.302 0.127)
2
3
4
5
6
7
8
9
10
0.045 - 0.065 (1.143 - 1.651)
0.065 (1.651) TYP 0.125 (3.175) MIN 0.018 0.003 (0.457 0.076)
N20 0695
15
LT1508
PACKAGE DESCRIPTION
0.291 - 0.299** (7.391 - 7.595) 0.010 - 0.029 x 45 (0.254 - 0.737)
0.009 - 0.013 (0.229 - 0.330)
NOTE 1 0.016 - 0.050 (0.406 - 1.270)
NOTE: 1. PIN 1 IDENT, NOTCH ON TOP AND CAVITIES ON THE BOTTOM OF PACKAGES ARE THE MANUFACTURING OPTIONS. THE PART MAY BE SUPPLIED WITH OR WITHOUT ANY OF THE OPTIONS *DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE **DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
RELATED PARTS
PART NUMBER LT1084 LT1105 LT1241-5 LT1247 LT1248 LT1249 LT1509 DESCRIPTION 5A Low Dropout Linear Regulator Simplified Off-Line Controller High Frequency Current Mode PWM Controller High Frequency Current Mode PWM Controller Full-Feature Average Current Mode Power Factor Controller Minimal Parts Count Power Factor Controller Power Factor and PWM Controller COMMENTS Good for Post Regulation of Switching Power Supplies Solution for Universal Off-Line Inputs with Output to 100W Operates at Oscillator Frequencies up to 500kHz Operates at Oscillator Frequencies up to 1MHz Provides All Features in 16-Lead Package Simplified PFC Design Current Mode PWM
16
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417 q (408) 432-1900 FAX: (408) 434-0507q TELEX: 499-3977 q www.linear-tech.com
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Dimensions in inches (millimeters) unless otherwise noted.
SW Package 20-Lead Plastic Small Outline (Wide 0.300)
(LTC DWG # 05-08-1620)
0.496 - 0.512* (12.598 - 13.005) 20 19 18 17 16 15 14 13 12 11
NOTE 1
0.394 - 0.419 (10.007 - 10.643)
1 0.093 - 0.104 (2.362 - 2.642)
2
3
4
5
6
7
8
9
10 0.037 - 0.045 (0.940 - 1.143)
0 - 8 TYP
0.050 (1.270) TYP 0.014 - 0.019 (0.356 - 0.482) TYP
0.004 - 0.012 (0.102 - 0.305)
S20 (WIDE) 0396
1508f LT/TP 0697 7K * PRINTED IN USA
(c) LINEAR TECHNOLOGY CORPORATION 1995

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