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August 2000 PRELIMINARY
ML4802 PFC/PWM Controller Combo with Green Mode
GENERAL DESCRIPTION
The ML 4802 is a controller for power factor corrected, switched mode power supplies that offers Green Mode operation and reduced start-up and operating currents. Green Mode is an efficiency improving circuit feature which operates automatically in low power situations. This feature helps meet the demands of Energy StarTM programs. Power Factor Correction (PFC) offers the use of lower cost bulk capacitors, reduces power line loading and stress on the switching FETs. The ML4802 includes circuits for the implementation of a leading edge, average current, "boost" type power factor corrector and a trailing edge Pulse Width Modulator (PWM). The PFC frequency of the ML4802 is automatically synchronized to be one half that of the PWM. This technique allows the user to design with smaller PWM components while maintaining the optimum operating frequency for the PFC. An over-voltage comparator shuts down the PFC section in the event of a sudden decrease in load. The PFC section also includes peak current limiting and brown-out protection.
s s
FEATURES
Internally synchronized PFC and PWM in one IC Green Mode maximizes efficiency during low power standby operation Low supply current (Start-up 200A typ., operating 5.5mA typ.) Average current continuous boost leading edge PFC High efficiency trailing edge PWM can be configured for current mode operation Reduced ripple current in the storage capacitor between the PFC and PWM sections PFC overvoltage comparator eliminates output "runaway" due to load removal Current fed gain modulator for improved noise immunity Overvoltage protection, UVLO, and soft start
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s s
s
s
s
s
BLOCK DIAGRAM
VEAO 16 IEAO 1 POWER FACTOR CORRECTOR
+
VCC 13
VEA VFB 15 2.5V IAC VRMS ISENSE RAMP 1 2 4 3 8 GAIN MODULATOR 1.8k
+ -
1.8k
+
IEA PFC CONTROLLER 8V
2.75V
-
OVP
7.5V REFERENCE
14 VREF
-1V
+ -
PFC ILIMIT
PFC OUTPUT DRIVER
12 PFC OUT
/2 VFB OSCILLATOR DC LIMIT VDC GREEN MODE CONTROLLER
RT/CT 7
RAMP 2
9 8V VLS 1.25V
-
GREEN MODE
VDC
6
VCC 25A
+
VIN OK
+
DC ILIMIT
+
VFB 2.43V
+
SS
5 8V
1.5V
-
PWM OUTPUT DRIVER
11 PWM OUT
PULSE WIDTH MODULATOR
1
ML4802
PIN CONFIGURATION
ML4802 16-Pin PDIP (P16) 16-Pin Narrow SOIC (S16N)
IEAO IAC ISENSE VRMS SS VDC RT/CT RAMP 1 1 2 3 4 5 6 7 8 16 15 14 13 12 11 10 9 VEAO VFB VREF VCC PFC OUT PWM OUT GND RAMP 2
TOP VIEW
PIN DESCRIPTION
PIN NAME FUNCTION PIN NAME FUNCTION
1 2 3 4 5 6 7 8
IEAO IAC ISENSE VRMS SS VDC RT/CT RAMP 1
PFC current error amplifier output PFC gain control reference input
9 10
RAMP 2 GND
PWM current feedback/overcurrent limit input Ground
Current sense input to the PFC current limit comparator Input for PFC RMS line voltage compensation Connection point for the PWM soft start capacitor PWM feedback voltage input
11 12 13 14 15
PWM OUT PWM driver output PFC OUT VCC VREF VFB VEAO PFC driver output Positive supply input Buffered output for the internal 7.5V reference PFC voltage error amplifier input PFC voltage error amplifier output
Connection for master (PWM) oscillator frequency setting components PFC ramp input
16
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Datasheet
August 2000
ML4802
ABSOLUTE MAXIMUM RATINGS
Absolute maximum ratings are those values beyond which the device could be permanently damaged. Absolute maximum ratings are stress ratings only and functional device operation is not implied. VCC .......................................................................... 18V ISENSE Voltage ................................................-3V to 5V Voltage on Any Other Pin .... GND - 0.3V to VCC + 0.3V IREF ....................................................................... 20mA IAC Input Current ................................................... 10mA PeakCurrent, Source or Sink PFC OUT, PWM OUT ....................................... 500mA PFC OUT, PWM OUT Energy Per Cycle .................. 1.5J Junction Temperature .............................................. 150C Storage Temperature Range ..................... -65C to 150C Lead Temperature (Soldering, 10 sec) ..................... 260C Thermal Resistance (qja) Plastic DIP ....................................................... 80C/W Plastic SOIC ................................................... 105C/W
OPERATING CONDITIONS
Temperature Range ML4802CX ................................................. 0C to 70C ML4802IX .............................................. -40C to 85C
ELECTRICAL CHARACTERISTICS
Unless otherwise specified, VCC = 15V, RT = 29.4kW, RRAMP1 = 15.4kW, CT = 270pF, CRAMP1 = 620pF, TA = Operating Temperature Range (Note 1)
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS VOLTAGE ERROR AMPLIFIER Input Voltage Range Transconductance Feedback Reference Voltage Input Bias Current Output High Voltage Output Low Voltage Source Current Sink Current Open Loop Gain PSRR CURRENT ERROR AMPLIFIER Input Voltage Range Transconductance Input Offset Voltage Input Bias Current Output High Voltage Output Low Voltage Source Current Sink Current Open Loop Gain PSRR 11V < VCC < 16.5V DVIN = 0.5V, VOUT = 6V DVIN = 0.5V, VOUT = 1.5V -40 40 60 60 6.0 VNON INV = VINV, VEAO = 3.75V -1.5 50 -5 100 3 -0.5 6.7 0.65 -70 70 75 75 1.0 -150 150 2 150 10 -1.0 V 11V < VCC < 16.5V DVIN = 0.5V, VOUT = 6V DVIN = 0.5V, VOUT = 1.5V -1 -1 50 50 Note 2 6.0 VNON INV = VINV, VEAO = 3.75V 0 20 2.4 35 2.5 -0.5 6.7 0.2 -2.5 -2.5 60 60 0.5 -4 -4 5 50 2.6 -1.0 V
V A V V A A dB dB
mV A V V A A dB dB
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Datasheet
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ML4802
ELECTRICAL CHARACTERISTICS (Continued)
SYMBOL OVP COMPARATOR Threshold Voltage Hysteresis PFC ILIMIT COMPARATOR Threshold Voltage
DPFC ILIMIT Threshold - Gain Modulator Output
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
2.65 175
2.75 250
2.85 325
V mV
-0.9 120
-1 220 150
-1.1
V mV
Delay to Output DC ILIMIT COMPARATOR Threshold Voltage Input Bias Current Delay to Output VIN OK COMPARATOR Threshold Voltage Hysteresis GAIN MODULATOR Gain (Note 3) IAC = 100A, VRMS = VFB = 0V IAC = 50A, VRMS = 1.2V, VFB = 0V IAC = 50A, VRMS = 1.8V, VFB = 0V IAC = 100A, VRMS = 3.3V, VFB = 0V Bandwidth Output Voltage OSCILLATOR Initial Accuracy Voltage Stability Temperature Stability Total Variation Ramp Valley to Peak Voltage Dead Time CT Discharge Current GREEN MODE VGMT-L Green Mode Threshold Low VGMT = VDC -VLS VGMT = VDC -VLS 70 360 PFC Only VRAMP 2 = 0V, VRAMP 1 = 2.5V 260 3.5 Line, Temp 182 TA = 25C 11V < VCC < 16.5V 188 IAC = 100A IAC = 250A, VRMS = 1.15V, VFB = 0V 0.6 0.6 1.8 0.8 0.2 2.33 0.8 1.4
300
ns
1.5 0.3 150
1.6 1 300
V A ns
2.43 1.0
2.55 1.2
V V
0.8 2 1 0.3 10 0.75
1.05 2.80 1.25 0.4 MHz 0.9 V
200 1 2
212
kHz % %
218 2.5 400 5.5 7.5
kHz V ns mA
275 580 1.25
mV mV V
VGMT-H Green Mode Threshold High VLS Level Shift
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Datasheet
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ML4802
ELECTRICAL CHARACTERISTICS (Continued)
SYMBOL REFERENCE Output Voltage Line Regulation Load Regulation Temperature Stability Total Variation Long Term Stability PFC Minimum Duty Cycle Maximum Duty Cycle Output Low Voltage VIEAO > 4.0V VIEAO < 1.2V IOUT = -20mA IOUT = -100mA IOUT = -10mA, VCC = 11V Output High Voltage IOUT = 20mA IOUT = 100mA Rise/Fall Time PWM DC VOL Duty Cycle Range Output Low Voltage IOUT = -20mA IOUT = -100mA IOUT = -10mA, VCC = 11V VOH Output High Voltage IOUT = 20mA IOUT = 100mA Rise/Fall Time SUPPLY Start-up Current Operating Current Undervoltage Lockout Threshold Undervoltage Lockout Hysteresis
Note 1: Limits are guaranteed by 100% testing, sampling, or correlation with worst-case test conditions. Note 2: Includes all bias currents to other circuits connected to the VFB pin. Note 3: Gain = K x 5.3V; K = (IMULO - IOFFSET) x IAC x (VEAO - 1.5V)-1.
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
TA = 25C, I(VREF) = 1mA 11V < VCC < 16.5V 1mA < I(VREF) < 10mA
7.4
7.5 2 2 0.4
7.6 30 20
V mV mV %
Line, Load, Temp TJ = 125C, 1000 Hours
7.35 5
7.65 25
V mV
0 85 90 0.4 0.7 0.8 VCC - 0.8 VCC - 2.0 50 0.8 2.0 1.5
% % V V V V V ns
CL = 1000pF
0-44
0-47 0.4 0.7 0.8
0-50 0.8 2.0 1.5
% V V V V V
VCC - 0. 8 VCC - 2.0 50
CL = 1000pF
ns
VCC = 12V, CL = 0 VCC = 14V, CL = 0 12 2.5
200 5 13 2.8
350 7 14 3.1
A mA V V
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Datasheet
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ML4802
FUNCTIONAL DESCRIPTION
The ML4802 consists of a combined average-currentcontrolled, continuous boost Power Factor Corrector (PFC) front end and a synchronized Pulse Width Modulator (PWM) back end. It is distinguished from earlier combo controllers by its unique Green Mode operation and dramatically reduced start-up and operating currents. The PWM section has been optimized for use in current mode topologies. The PWM stage uses conventional trailingedge duty cycle modulation, while the PFC uses leadingedge modulation. This patented leading/trailing edge modulation technique results in a higher useable PFC error amplifier bandwidth, and can significantly reduce the size of the PFC DC bus capacitor. The synchronization of the PWM with the PFC simplifies the PWM compensation due to the reduced ripple on the PFC output capacitor (the PWM input capacitor). The PWM section of the ML4802 runs at twice the frequency of the PFC, which allows the use of smaller PWM output magnetics and filter capacitors while holding down the losses in the PFC stage power components. In addition to power factor correction, a number of protection features have been built into the ML4802. These include soft-start, PFC over-voltage protection, peak current limiting, brown-out protection, duty cycle limit, and under-voltage lockout. GREEN MODE OPERATION Green Mode automatically improves efficiency by up to 20% or more during low power operation. This feature is particularly helpful in meeting the demands of Energy StarTM programs. When the PWM's output falls to nominally 17% of its design maximum power, Green Mode operation is initiated. The upper Green Mode threshold corresponds roughly to 1/3 of rated full power level. In Green Mode, the PWM operates in a pulseSTART
skipping mode. This significantly reduces the frequency of operation, and therefore the dissipation in the PWM output driver and switch. Since the pulse-skipping is synchronous to the PWM's master clock, the noise spectrum of the PWM retains a strong relationship to its spectrum during continuous-mode operation, which eases input and output filter design. PWM pulse frequency reductions in excess of 10:1 are common, with no increase in peak-to-peak output ripple. During Green Mode, the PFC also cycles on and off, running only as often as necessary to maintain its feedback voltage (VFB) between 2.25V and 2.5V (corresponding typical values of VBUSS are 382V and 425V). The PFC uses a built-in softstart to minimize line current peaks and component stress when turning on. See Figure 1 for a flow chart detailing Green Mode and Normal Mode operation.
2.25V
- +
S R
Q Q
PFC ON
VFB 2.5V VDC VTH1 VTH2
+ -
- +
PWM OFF
Figure 2. Green Mode Section Block Diagram Entering Green Mode The Green Mode Controller is detailed in Figure 2. Key to the ML4802's operation in Green-Mode is the fact that the PWM's output power is related to the voltage on the VDC (PWM Duty Cycle Control Voltage) pin by a known transfer characteristic. Therefore, the output power POUT drawn from an ML4802 supply can be inferred by monitoring the value of VDC fed back to the ML4802 from the (external) reference/error amplifier combination. When the output power taken from the PWM is reduced, the voltage on VDC will decrease. When VDC falls below VTH1 (1.30V typical), the part enters Green Mode operation. Once this happens, the threshold to which VDC is compared for further PWM operation is set to a higher value VTH2 (1.58V typical). This causes the PWM to enter a pulse-skipping mode while maintaining the desired output voltage. Pulse-skipping occurs because VTH2 is a higher voltage than VTH1, and because the PWM drive (PWMOUT) is disabled until VDC VTH2. Since the primary current of the PWM output transformer is determined by VTH2 in Green Mode, and VTH2 > VTH1, each PWM output pulse will carry slightly more energy during Green Mode operation than during all but the highest duty cycle regimes of continuous-mode operation. In Green Mode, the power in each PWM output pulse is: PPULSE IPRIMARY(PWM) x VBUSS
NORMAL MODE OPERATION NO VFB 2.50V? VDC < 1.30V? YES
NO
YES
TURN PFC OFF
VFB < 2.25V? YES
NO
VDC 2.25V? YES ISSUE PWM POWER PULSE
NO
TURN PWM ON
TURN PFC ON
Figure 1. ML4802 Operational Flow Chart
PPULSE (VGMT/RSENSE[PWM]) x 380V
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Datasheet
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ML4802
FUNCTIONAL DESCRIPTION (Continued)
On an instantaneous basis, an increase in VOUT above its programmed value will cause the error voltage presented to VDC to decrease. This will shut off PWMOUT to keep the loop in regulation. If the output voltage goes below its intended level, VDC will increase. When the feedback voltage VDC rises above VTH2, PWMOUT is re-enabled causing the output voltage to increase. This series of actions will repeat, maintaining the average VOUT at its design value. Since the PWM error amplifier gain is quite high in the average configuration, this action introduces no appreciable ripple on the PWM's DC output(s). One item to note here is that, to keep the pulse skipping action as clean as possible (that is, to prevent pulse grouping), a relatively fast error amplifier with an electrically quiet feedback path to VDC is desirable. When the PWM enters its pulse-skipping mode, the PFC is shut off completely. The PWM then runs off of the energy stored in the PFC buss capacitor. During this period, the voltage on the buss capacitor will decay. When VBUSS falls below a user-set threshold VPFC1 (typically 382V), the PFC is turned on again, charging its output capacitor back to a higher voltage VPFC2 (typically 425V). Simultaneously, the threshold to which VDC is compared is switched back to VTH1. As soon as the output voltage of the PFC exceeds VPFC2, the PFC shuts off and VDC is again compared to VTH2. This cycle repeats as long as the power consumption from the PWM remains below the Green Mode threshold. Exiting Green Mode The ML4802 enters Green Mode at any time that VDC < VTH1. In order to reliably exit Green Mode, VTH1 must be used as the exit criterion as well (using VTH2 as a comparison voltage to exit Green Mode would eliminate the part's ability to skip pulses throughout the Green Mode power range). Therefore, once the voltage on VDC has set the part into Green Mode operation, the ML4802 can only exit Green Mode when the PFC is recharging the buss capacitor. As noted above, VDC is compared against VTH1 during the PFC recharge time. Another way of viewing this is as follows: every time the PFC turns on, the ML4802 exits Green Mode, and will either return to Green Mode or remain in continuous-mode operation depending upon whether the voltage on VDC exceeds VTH1. Note that this means that there will be brief periods of continuous PWM operation even while the output power drawn from the PWM is within the Green Mode range. This is a normal and harmless consequence of the ML4802's Green Mode logic. GREEN MODE THRESHOLD To a first approximation, the Green Mode Threshold as a percentage of the PWM's maximum rated power output is given by: PGMT = (VGMT/VCURRENT LIMIT(PWM)) x POUT(MAX) PGMT @ (0.25V/1.5V) = 0.167 x POUT(MAX) For example, a flyback supply designed for 100W maximum output will nominally enter and exit Green Mode operation at 17W. Similarly, a 200W forward converter would have a Green Mode threshold of about 34W. In actual designs, the Green Mode threshold will often be at a slightly lower power level than is given by this simplified equation. This is principally due to the fact that VFB is an average-responding voltage, while POUT is inferred from the instantaneous peak current through RSENSE(PWM). On a short-term basis, the output current demand as sensed by VFB is essentially a DC level. This is not true of V(RAMP1), however: V(RAMP2) is given by (RSENSE(PWM) x IPRIMARY(PWM)), which for most designs is a combination of DC (pedestal) and peak (ramp) currents. It is the ramp current portion of IPRIMARY(PWM) which causes real-world designs to typically enter Green Mode at several percentage points lower output power than would otherwise occur. POWER FACTOR CORRECTION Power factor correction makes a non-linear load look like a resistive load to the AC line. For a resistor, the current drawn from the line is in phase with, and proportional to, the line voltage, so the power factor is unity (one). A common class of non-linear load is the input of a most power supplies, which use a bridge rectifier and capacitive input filter fed from the line. The peakcharging effect which occurs on the input filter capacitor in such a supply causes brief high-amplitude pulses of current to flow from the power line, rather than a sinusoidal current in phase with the line voltage. Such a supply presents a power factor to the line of less than one (another way to state this is that it causes significant current harmonics to appear at its input). If the input current drawn by such a supply (or any other non-linear load) can be made to follow the input voltage in instantaneous amplitude, it will appear resistive to the AC line and a unity power factor will be achieved. To hold the input current draw of a device drawing power from the AC line in phase with, and proportional to, the input voltage, a way must be found to prevent that device from loading the line except in proportion to the instantaneous line voltage. The PFC section of the ML4802 uses a boost-mode DC-DC converter to accomplish this. The input to the converter is the full wave rectified AC line voltage. No filtering is applied following the bridge rectifier, so the input voltage to the boost converter ranges, at twice line frequency, from zero volts to the peak value of the AC input and back to zero. By forcing the boost converter to meet two simultaneous conditions, it is possible to ensure that the current which the converter draws from the power line matches the instantaneous line voltage. One of these conditions is that the output voltage of the boost converter must be set higher than the peak value of the line voltage. For the ML4802, a good value to use is 425V DC out, to allow for a high line of 270V AC while in Green Mode. The other condition is that the current which the converter is allowed to draw from the line at any given instant must be proportional to the line voltage. The first of these requirements is satisfied by establishing a suitable voltage
August 2000
Datasheet
7
ML4802
FUNCTIONAL DESCRIPTION (Continued)
control loop for the converter, which in turn drives a current error amplifier and switching output driver. The second requirement is met by using the rectified AC line voltage to modulate the output of the voltage control loop. Such modulation causes the current error amplifier to command a power stage current which varies directly with the input voltage. In order to prevent ripple which will necessarily appear at the output of the boost circuit (typically about 10VAC on a 385V DC level, or about 40VAC during Green Mode operation) from introducing distortion back through the voltage error amplifier, the bandwidth of the voltage loop is deliberately kept low. A final refinement is to adjust the overall gain of the PFC such to be proportional to 1/VIN2, which linearizes the transfer function of the system as the AC input voltage varies. Since the boost converter topology in the ML4802 PFC is of the current-averaging type, no slope compensation is required. PFC SECTION Gain Modulator Figure 3 shows a block diagram of the PFC section of the ML4802. The gain modulator is the heart of the PFC, as it is this circuit block which controls the response of the current loop to line voltage waveform and frequency, rms line voltage, and PFC output voltage. There are three inputs to the gain modulator. These are: 1) A current representing the instantaneous input voltage (amplitude and waveshape) to the PFC. The rectified AC input sine wave is converted to a proportional current via a resistor and is then fed into the gain modulator at IAC. Sampling current in this way minimizes ground noise, as is required in high power switching power conversion environments. The gain modulator responds linearly to this current. 2) A voltage proportional to the long-term rms AC line voltage, derived from the rectified line voltage after scaling and filtering. This signal is presented to the gain modulator at VRMS. The gain modulator's output is inversely proportional to VRMS2 (except at unusually low values of VRMS where special gain contouring takes over to limit power dissipation of the circuit components under heavy brownout conditions). The relationship between VRMS and gain is designated as K, and is illustrated in the Typical Performance Characteristics. 3) The output of the voltage error amplifier, VEAO. The gain modulator responds linearly to variations in this voltage.
VEAO 16
IEAO 1 POWER FACTOR CORRECTOR
+
VCC 13
VEA VFB 15 2.5V IAC VRMS ISENSE RAMP 1 RT/CT 2 4 3 8 7 GAIN MODULATOR 1.6k
+ -
1.6k
+
IEA PFC CONTROLLER 8V
2.75V
-
OVP
7.5V REFERENCE
14
VREF
-1V
+ -
PFC ILIMIT
PFC OUTPUT DRIVER
12 PFC OUT
/2 OSCILLATOR DUTY CYCLE LIMIT
FROM GREEN MODE CONTROLLER
Figure 3. PFC Section Block Diagram
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Datasheet
August 2000
ML4802
FUNCTIONAL DESCRIPTION (Continued)
The output of the gain modulator is a current signal, in the form of a full wave rectified sinusoid at twice the line frequency. This current is applied to the virtual-ground (negative) input of the current error amplifier. In this way the gain modulator forms the reference for the current error loop, and ultimately controls the instantaneous current draw of the PFC from the power line. The general form for the output of the gain modulator is:
IGAINMOD = IAC VEAO 1V VRMS 2
Overvoltage Protection The OVP comparator serves to protect the power circuit from being subjected to excessive voltages if the load should suddenly change. A resistor divider from the high voltage DC output of the PFC is fed to VFB. When the voltage on VFB exceeds 2.75V, the PFC output driver is shut down. The PWM section will continue to operate. The OVP comparator has 250mV of hysteresis, and the PFC will not restart until the voltage at VFB drops below 2.5V. The VFB should be set at a level where the active and passive external power components and the ML4802 are within their safe operating voltages, but not so low as to interfere with the boost voltage regulation loop. Error Amplifier Compensation The PWM loading of the PFC can be modeled as a negative resistor; an increase in input voltage to the PWM causes a decrease in the input current. This response dictates the proper compensation of the two transconductance error amplifiers. Figure 4 shows the types of compensation networks most commonly used for the voltage and current error amplifiers, along with their respective return points. The current loop compensation is returned to VREF to produce a soft-start characteristic on the PFC: as the reference voltage comes up from zero volts, it creates a differentiated voltage on IEAO which prevents the PFC from immediately demanding a full duty cycle on its boost converter. This then works in conjunction with the low output current of the VEA to ensure low component stress at PFC startup.
More exactly, the output current of the gain modulator is given by:
IGAINMOD = K VEAO - 0.625V IAC
where K is in units of V-1. Note that the output current of the gain modulator is limited to @ 500A. Current Error Amplifier The current error amplifier's output controls the PFC duty cycle to keep the current through the boost inductor a linear function of the line voltage. At the inverting input to the current error amplifier, the output current of the gain modulator is summed with a current which results from a negative voltage being impressed upon the ISENSE pin (current into ISENSE @ VSENSE/1.8kW). The negative voltage on ISENSE represents the sum of all currents flowing in the PFC circuit, and is typically derived from a current sense resistor in series with the negative terminal of the input bridge rectifier. As stated above, the ground. Given this fact, and the arrangement of the duty cycle modulator polarities internal to the PFC, an increase in positive current from the gain modulator will cause the output stage to increase its duty cycle until the voltage on ISENSE is adequately negative to cancel this increased current. Similarly, if the gain modulator's output decreases, the output duty cycle will decrease to achieve a less negative voltage on the ISENSE pin. There is a modest degree of gain contouring applied to the transfer characteristic of the current error amplifier, to increase its speed of response to current-loop perturbations. Cycle-By-Cycle Current Limiter The ISENSE pin, as well as being a part of the current feedback loop, is a direct input to the cycle-by-cycle current limiter for the PFC section. Should the input voltage at this pin ever be more negative than -1.5V, the output of the PFC will be disabled until the protection flip-flop is reset by the clock pulse at the start of the next PFC power cycle.
0
5
VREF
PFC OUTPUT VEAO VFB 15 2.5V IAC 2 VRMS 4 ISENSE 3
+
16 IEAO
1
VEA 1.6k
+ -
IEA
+ -
GAIN MODULATOR 1.6k
Figure 4. Compensation Network Connections for the Voltage and Current Error Amplifiers
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Datasheet
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ML4802
FUNCTIONAL DESCRIPTION (Continued)
The major concern when compensating the ML4802's voltage loop error amplifier is that the current amplifier compensation is chosen to optimize frequency response while maintaining good stability. This leads to the following rules of thumb: the crossover frequency of the current amplifier should be at least 10 times that of the voltage amplifier to prevent interaction with the voltage loop. It should also be limited to less than 1/6th that of the switching frequency, e.g. 16.7kHz for a 100kHz switching frequency. For more information on compensating the current and voltage control loops, see Application Notes 33, 34, and 55. Application Note 16 also contains valuable information for the design of this class of PFC. Oscillator The oscillator frequency is determined by the values of RT and CT, which determine the ramp and off-time of the oscillator output clock:
fOSC = 1 tRAMP + DEADTIME
RAMP 1 The ramp voltage on this pin serves as a reference to which the PFC control signal is compared in order to set the duty cycle of the PFC switch. The external ramp voltage is derived from an RC network similar to the oscillator's. The PWM's oscillator sends a synchronous pulse every other cycle to reset this ramp. The ramp voltage should be limited to no more than the output high voltage (6V) of the current error amplifier. The timing resistor values should be selected such that the capacitor will not charge past this point before being reset. In order to ensure the linearity of the PFC loop's transfer function and improve noise immunity, the charging resistor should be connected to the 13.5V VCC rather than the 7.5V reference. This will keep the charging voltage across the timing capacitor in the "linear" region of the charging curve. The component value selection is similar to oscillator RC component selection.
fOSC = 1 tCHARGETIME + tDISCHARGETIME
The deadtime of the oscillator is derived from the following equation:
tRAMP = CT RT IN
VREF - 1.25 VREF - 3.75
The charge time of RAMP 1 is derived from the following equations:
tCHARGE = 2 fOSC
at VREF = 7.5V:
tRAMP = CT RT 0.51
The ramp of the oscillator may be determined using:
tDEADTIME = 2.5V CT = 455 CT 5.5mA
tCHARGE = C R 1n
VCC - RampValley VCC - RampPeak
At VCC = 13.5V and assuming RampPeak = 5V to allow for component tolerances:
tCHARGE = 0.463 R C
The deadtime is so small (tRAMP >> tDEADTIME) that the operating frequency can typically be approximated by:
fOSC = 1 tRAMP
The capacitor value should remain small to keep the discharge energy and the resulting discharge current through the part small. A good value to use is the same value used in the pwm timing circuit (CT). For the application circuit shown in Figure 7, using a 200kHz PWM and a 100pF timing capacitor yeilds RT:
EXAMPLE: For the application circuit shown in the data sheet, with the oscillator running at:
fOSC = 200kHz = 1 tRAMP
Rt =
1 10 -5 0.463 100 10 -12
2
7
tRAMP = 0.51 RT CT = 1 10 -5
Solving for RT x CT yields 1 x 10-4. Selecting standard components values, CT = 100pF, and RT = 100kW.
Rt = 215kW
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Datasheet
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ML4802
FUNCTIONAL DESCRIPTION (Continued)
PWM SECTION Pulse Width Modulator The PWM section of the ML4802 is straightforward, but there are several points which should be noted. Foremost among these is its inherent synchronization to the PFC section of the device, from which it also derives its basic timing (at twice the PFC frequency in the ML4802). The PWM is primarily intended for current-mode operation. In current-mode applications, the PWM ramp (RAMP 2) is usually derived directly from a current sensing resistor in the primary of the output stage, and is thereby representative of the current flowing in the converter's output stage. DC ILIMIT, which provides cycle-by-cycle current limiting, is internally connected to RAMP 2. No voltage error amplifier is included in the PWM stage of the ML4802, as this function is generally performed on the output side of the PWM's isolation boundary. To facilitate the design of optocoupler feedback circuitry, an offset has been built into the PWM's RAMP 2 input which allows VDC to command a zero percent duty cycle for input voltages below 1.25V. VIN OK Comparator The VIN OK comparator monitors the DC output of the PFC and inhibits the PWM if this voltage on VFB is less than its nominal 2.5V. Once this voltage reaches 2.5V, which corresponds to the PFC output capacitor being charged to its rated boost voltage, the soft-start commences. PWM Control (RAMP 2) RAMP 2 is the sampling point for a voltage representing the current in the primary of the PWM's output transformer, derived from a current sensing resistor. PWM Current Limit The DC ILIMIT pin is a cycle-by-cycle current limiter for the PWM section. It is connected internally to the PWM control pin. Should the input voltage at this pin ever exceed 1.5V, the output of the PWM will be disabled until the output flip-flop is reset by the clock pulse at the start of the next PWM power cycle. Soft Start Start-up of the PWM is controlled by the selection of the external capacitor at SS. A current source of 25A supplies the charging current for the capacitor, and startup of the PWM begins at 1.25V. Start-up delay can be programmed by the following equation:
CSS = tDELAY 25mA 1.25V
L1 + I1 VIN SW1 C1 RL RAMP SW2 I2 I3 I4
It is important that the time constant of the PWM soft-start allow the PFC time to generate sufficient output power for the PWM section. The PWM start-up delay should be at least 5ms. Solving for the minimum value of CSS:
CSS = 5ms 25mA @ 200nF 1.25V
Generating VCC The ML4802 is a voltage-fed part. It requires an external 15V10% (or better) Zener shunt voltage regulator, or other controlled supply, to maintain the voltage supplied to the part at 15V nominal. This allows a low power dissipation while at the same time delivering 13V nominal of gate drive at the PWM OUT and PFC OUT outputs.
LEADING/TRAILING MODULATION
Conventional Pulse Width Modulation (PWM) techniques employ trailing edge modulation in which the switch will turn on right after the trailing edge of the system clock. The error amplifier output voltage is then compared with the modulating ramp. When the modulating ramp reaches the level of the error amplifier output voltage, the switch will be turned OFF. When the switch is ON, the inductor current will ramp up. The effective duty cycle of the trailing edge modulation is determined during the ON of the switch. Figure 5 shows a typical trailing edge control scheme.
DC
VEAO REF U3 + -EA DFF RAMP OSC U4 CLK + - U1 R Q D U2 Q CLK VSW1
TIME
TIME
Figure 5. Typical Trailing Edge Control Scheme
where CSS is the required soft start capacitance, and tDELAY is the desired start-up delay.
August 2000
Datasheet
11
ML4802
FUNCTIONAL DESCRIPTION (Continued)
In the case of leading edge modulation, the switch is turned OFF right at the leading edge of the system clock. When the modulating ramp reaches the level of the error amplifier output voltage, the switch will be turned ON. The effective duty-cycle of the leading edge modulation is determined during the OFF time of the switch. Figure 6 shows a leading edge control scheme. One of the advantages of this control technique is that it requires only one system clock. Switch 1 (SW1) turns off and switch 2 (SW2) turns on at the same instant to minimize the momentary "no-load" period, thus lowering ripple voltage generated by the switching action. With such synchronized switching, the ripple voltage of the first stage is reduced. Calculation and evaluation have shown that the 120Hz component of the PFC's output ripple voltage can be reduced by as much as 30% using this method.
TYPICAL APPLICATIONS
Figure 7 is the application circuit for a complete 100W power factor corrected power supply, designed using the methods and general topology detailed in Application Note 33.
L1 + I1 VIN
SW2
I2
I3 I4
DC
SW1 C1
RL RAMP
VEAO U3 + -EA
REF
VEAO + - CMP U1
DFF R Q D U2 Q CLK
TIME VSW1
RAMP OSC U4 CLK
TIME
Figure 6. Leading/Trailing Edge Control Scheme
12
Datasheet
August 2000
D12 VBUSS
F1 3.15A Q1G Q2G R19 33 T2C Q2 C25 0.1F R24 10k D5 600V 12V D11A L2 L3 12V, 100W Q3G D11B Q3 R10 249k R17 3 R21 3.3 R22 3.3 R30 1.5k C22 10F R32 8.66k T1A R23 220 R37 1k Q4 D4 5.1V ML4802 1 IEAO VDC 16 U1 VFB 15 VREF 14 VCC 13 PFC OUT 12 PWM OUT 11 GND 10 RAMP 2 9 D8 C31 330pF R15 2.94k D10 C15 0.22F C13 0.22F C8 .047F C9 6.8nF J8 C10 10F R31 10k R43 1.5M VFB VCC REF R11 R26 1.3M 10k R40 470 2 IAC 3 ISENSE 4 VRMS 5 SS 6 VDC 7 RT/CT 8 RAMP 1 R36 200k C18 100pF R25 10k U2 R44 10k C6 820pF PWM ILIMIT C20 0.47F D6 600V C24 0.47F C21 1500F D7 16V T1B R13 249k R27 82k Q1 R20 22 R14 249k D3 600V C4 4.7nF C5 100F
L1
D1 8A HFA08TB60
AC INPUT 85 TO 260V R1 357k R9 249k R2 357k C26 47F D2 15V C12 220F 50V R18 33 R28 1k
C1 0.47F
BR1 4A, 600V KBL06
ISENSE
R5 1.2
R6 1.2
R7 1.2
R8 1.2
C32 0.47F
C30 680F
C3 R3 0.22F 100k
R38 100k R16 10k R4 13.2k R12 124k
Figure 7. 100W Power Factor Corrected Power Supply
R29 1.2k VDC U3 TL431C C11 220pF C28 220pF C19 0.22F C33 10nF PRI GND
August 2000
R34 240
C2 0.47F
R39 33
Datasheet
RT/CT
R35 221k
RAMP1
D14 1N914
C23 10nF
C29 10nF
D13 1N914
D15 1N914
C27 100pF
R33 2.26k
12V RET 12V RETURN
NOTE:
D8, D10; IN5818 D3, D5, D6, D12; BYV26C D11; MBR2545CT L1; PREMIER MAGNETICS TSD-1047 L2; PREMIER MAGNETICS VTP-02007 L3; PREMIER MAGNETICS TSD-904 T1; PREMIER MAGNETICS PMGD-03 T2; PREMIER MAGNETICS TSD-1218 UNUSED DESIGNATORS; C7, C14, C16, C17, D9, R42
ML4802
13
ML4802
PHYSICAL DIMENSIONS inches (millimeters)
Package: P16 16-Pin PDIP
0.740 - 0.760 (18.79 - 19.31) 16
PIN 1 ID
0.240 - 0.260 0.295 - 0.325 (6.09 - 6.61) (7.49 - 8.26)
0.02 MIN (0.50 MIN) (4 PLACES)
1 0.055 - 0.065 (1.40 - 1.65) 0.100 BSC (2.54 BSC) 0.015 MIN (0.38 MIN)
0.170 MAX (4.32 MAX)
0.125 MIN (3.18 MIN)
0.016 - 0.022 (0.40 - 0.56)
SEATING PLANE
0 - 15
0.008 - 0.012 (0.20 - 0.31)
Package: S16N 16-Pin Narrow SOIC
0.386 - 0.396 (9.80 - 10.06) 16
PIN 1 ID
0.148 - 0.158 0.228 - 0.244 (3.76 - 4.01) (5.79 - 6.20)
1 0.017 - 0.027 (0.43 - 0.69) (4 PLACES) 0.050 BSC (1.27 BSC) 0.059 - 0.069 (1.49 - 1.75) 0 - 8
0.055 - 0.061 (1.40 - 1.55)
0.012 - 0.020 (0.30 - 0.51)
SEATING PLANE
0.004 - 0.010 (0.10 - 0.26)
0.015 - 0.035 (0.38 - 0.89)
0.006 - 0.010 (0.15 - 0.26)
14
Datasheet
August 2000
ML4802
ORDERING INFORMATION
PART NUMBER ML4802CP ML4802CS ML4802IP ML4802IS
TEMPERATURE RANGE 0C to 70C 0C to 70C -40C to 85C -40C to 85C
PACKAGE 16-Pin Plastic DIP (P16) 16-Pin Narrow SOIC (S16N) 16-Pin Plastic DIP (P16) 16-Pin Narrow SOIC (S16N)
Micro Linear Corporation 2092 Concourse Drive San Jose, CA 95131 Tel: (408) 433-5200 Fax: (408) 432-0295 www.microlinear.com
Micro Linear makes no representations or warranties with respect to the accuracy, utility, or completeness of the contents of this publication and reserves the right to make changes to specifications and product descriptions at any time without notice. No license, express or implied, by estoppel or otherwise, to any patents or other intellectual property rights is granted by this document. The circuits contained in this document are offered as possible applications only. Particular uses or applications may invalidate some of the specifications and/or product descriptions contained herein. The customer is urged to perform its own engineering review before deciding on a particular application. Micro Linear assumes no liability whatsoever, and disclaims any express or implied warranty, relating to sale and/or use of Micro Linear products including liability or warranties relating to merchantability, fitness for a particular purpose, or infringement of any intellectual property right. Micro Linear products are not designed for use in medical, life saving, or life sustaining applications.
(c) Micro Linear 2000. is a registered trademark of Micro Linear Corporation. All other trademarks are the property of their respective owners. Products described herein may be covered by one or more of the following U.S. patents: 4,897,611; 4,964,026; 5,027,116; 5,281,862; 5,283,483; 5,418,502; 5,508,570; 5,510,727; 5,523,940; 5,546,017; 5,559,470; 5,565,761; 5,592,128; 5,594,376; 5,652,479; 5,661,427; 5,663,874; 5,672,959; 5,689,167; 5,714,897; 5,717,798; 5,742,151; 5,747,977; 5,754,012; 5,757,174; 5,767,653; 5,777,514; 5,793,168; 5,798,635; 5,804,950; 5,808,455; 5,811,999; 5,818,207; 5,818,669; 5,825,165; 5,825,223; 5,838,723; 5.844,378; 5,844,941. Japan: 2,598,946; 2,619,299; 2,704,176; 2,821,714. Other patents are pending.
DS4802-02
August 2000
Datasheet
15

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