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19-2082; Rev 0; 7/01
KIT ATION EVALU BLE AVAILA
Current-Mode PWM Controllers with Integrated Startup Circuit for Isolated Power Supplies
General Description
The MAX5014/MAX5015 integrate all the building blocks necessary for implementing DC-DC fixed-frequency isolated power supplies. These devices are current-mode controllers with an integrated high-voltage startup circuit suitable for isolated telecom/industrial voltage range power supplies. Current-mode control with leading-edge blanking simplifies control-loop design and internal ramp compensation circuitry stabilizes the current loop when operating at duty cycles above 50% (MAX5014). The MAX5014 allows 85% operating duty cycle and could be used to implement flyback converters, whereas the MAX5015 limits the operating duty cycle to less than 50% and can be used in single-ended forward converters. A high-voltage startup circuit allows these devices to draw power directly from the 18V to 110V input supply during startup. The switching frequency is internally trimmed to 275kHz 10%, thus reducing magnetics and filter component costs. The MAX5014/MAX5015 are available in 8-pin SO packages. An evaluation kit (MAX5015EVKIT) is also available. Warning: The MAX5014/MAX5015 are designed to operate with high voltages. Exercise caution. o Current-Mode Control o Leading-Edge Blanking o Internally Trimmed 275kHz 10% Oscillator o Low External Component Count o Soft-Start o High-Voltage Startup Circuit o Pulse-by-Pulse Current Limiting o Thermal Shutdown o SO-8 Package
Features
o Wide Input Range: (18V to 110V) or (13V to 36V)
MAX5014/MAX5015
Ordering Information
PART MAX5014CSA* MAX5014ESA* MAX5015CSA* TEMP. RANGE 0C to +70C -40C to +85C 0C to +70C PIN-PACKAGE 8-SO 8-SO 8-SO 8-SO
MAX5015ESA* -40C to +85C *See Selector Guide at end of data sheet.
Applications
Telecom Power Supplies Industrial Power Supplies Networking Power Supplies Isolated Power Supplies
VIN
Typical Operating Circuit
VOUT V+
VDD
NDRV
Pin Configuration
TOP VIEW
CS
MAX5015
GND SS_SHDN VCC
V+ 1 VDD OPTO 2 3
8 7
VCC NDRV GND CS
OPTO OPTOCOUPLER
MAX5014/ MAX5015
6 5
SS_SHDN 4
8-SO
________________________________________________________________ Maxim Integrated Products
1
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Current-Mode PWM Controllers with Integrated Startup Circuit for Isolated Power Supplies MAX5014/MAX5015
ABSOLUTE MAXIMUM RATINGS
V+ to GND ...................................................-0.3V to +120V VDD to GND...................................................-0.3V to +40V VCC to GND...............................................-0.3V to +12.5V OPTO, NDRV, SS_SHDN, CS to GND .......-0.3V to VCC + 0.3V VDD and VCC Current ...............................................20mA NDRV Current Continuous...........................................25mA NDRV Current for Less than 1s.....................................1A Continuous Power Dissipation (TA = +70C) 8-Pin SO (derate 5.88mW/C above +70C) ..............471mW Operating Temperature Range.......................-40C to +85C Storage Temperature Range........................-65C to +150C Lead Temperature (soldering, 10s) .................. .........+300C
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(VDD = 13V, a 10F capacitor connects VCC to GND, VCS = 0, V+ = 48V, 0.1F capacitor connected to SS_SHDN, NDRV = open circuit, OPTO = GND, TA = -40C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER SUPPLY CURRENT IV+(NS) V+ Supply Current V+ Supply Current After Startup VDD Supply Current V+ Shutdown Current VDD Shutdown Current PREREGULATOR/STARTUP V+ Input Voltage VDD Supply Voltage INTERNAL REGULATORS (VCC) VCC Output Voltage VCC Undervoltage Lockout OUTPUT DRIVER Peak Source Current Peak Sink Current NRDV High-Side Driver Resistance NDRV Low Side Driver Resistance PWM COMPARATOR OPTO Input Bias Current OPTO Control Range Slope Compensation VSCOMP MAX5014 VOPTO = VSS_SHDN -1.00 2 26 1.00 3 A V mV/s ROH ROL VCC = 11V, (externally forced) VCC = 11V, (externally forced) VCC = 11V, externally forced, NDRV sourcing 50mA VCC = 11V, externally forced, NDRV sinking 50mA 570 1000 4 1.6 12 4 mA mA VCC_UVLO Powered from V+, ICC = 7.5mA, VDD = 0 Powered from VDD, ICC = 7.5mA VCC falling 7.5 9.0 9.8 10.0 6.6 12 11.0 V V V 18 13 110 36 V V IVDD(NS) IVDD(S) IV+(S) VDD = 0, V+ = 110V, driver not switching V+ = 110V, VDD = 0, VOPTO = 4V, driver switching V+ = 110V, VDD = 13V, VOPTO = 4V VDD = 36V, driver not switching VDD = 36V, driver switching, VOPTO = 4V VSS_SHDN = 0, V+ = 110V VSS_SHDN = 0 0.85 1.4 11 0.9 1.9 190 8 1.3 2.7 290 20 1.3 2.6 mA A mA A A SYMBOL CONDITIONS MIN TYP MAX UNITS
2
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Current-Mode PWM Controllers with Integrated Startup Circuit for Isolated Power Supplies MAX5014/MAX5015
ELECTRICAL CHARACTERISTICS (continued)
(VDD = 13V, a 10F capacitor connects VCC to GND, VCS = 0, V+ = 48V, 0.1F capacitor connected to SS_SHDN, NDRV = open circuit, OPTO = GND, TA = -40C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER THERMAL SHUTDOWN Thermal Shutdown Temperature Thermal Hysteresis CURRENT LIMIT CS Threshold Voltage CS Input Bias Current Current Limit Comparator Propagation Delay CS Blanking Time OSCILLATOR Clock Frequency Range Max Duty Cycle SOFT-START SS Source Current SS Sink Current Peak Soft-Start Voltage Clamp Shutdown Threshold No external load VSS_SHDN falling VSS_SHDN rising ISSO VSS_SHDN = 0 2.0 1.0 2.331 0.25 0.53 2.420 0.37 0.59 2.500 0.41 0.65 4.6 6.5 A mA V V VOPTO = 4V MAX5014, VOPTO = 4V MAX5015, VOPTO = 4V 247 75 44 275 302 85 50 kHz % VILIM VOPTO = 4V 0 VCS 2V, VOPTO = 4V 25mV overdrive on CS, VOPTO = 4V VOPTO = 4V 419 -1 180 70 465 510 1 mV A ns ns 150 25 C C SYMBOL CONDITIONS MIN TYP MAX UNITS
Typical Operating Characteristics
(V+ = 48V, VDD = 13V, NRDV is open circuit, TA = +25C, unless otherwise noted.)
VSS_SHDN vs. TEMPERATURE (AT THE END OF SOFT-START)
MAX5014 toc01
NDRV FREQUENCY vs. TEMPERATURE
MAX5014 toc02
MAX5014 MAXIMUM DUTY CYCLE vs. TEMPERATURE
VOPTO = 4V, CS = GND MAXIMUM DUTY CYCLE (%) 80.9 80.8 80.7 80.6 80.5 80.4
MAX5014 toc03
1.003 VSS_SHDN (V) (NORMALIZED TO VREF = 2.4V) OPTO = CS = GND 1.002
278
81.0
277 NDRV FREQUENCY (kHz) VOPTO = 4V, CS = GND 276
1.001
275
1.000
274 0.999 -40 -20 0 20 40 60 80 TEMPERATURE (C)
273 -40 -20 0 20 40 60 80 TEMPERATURE (C)
-40
-20
0
20
40
60
80
TEMPERATURE (C)
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Current-Mode PWM Controllers with Integrated Startup Circuit for Isolated Power Supplies MAX5014/MAX5015
Typical Operating Characteristics (continued)
(V+ = 48V, VDD = 13V, NRDV is open circuit, TA = +25C, unless otherwise noted.)
MAX5015 MAXIMUM DUTY CYCLE vs. TEMPERATURE
MAX5014 toc04
V+ SUPPLY CURRENT vs. TEMPERATURE
MAX5014 toc05
SOFT-START SOURCE CURRENT vs. TEMPERATURE
SOFT-START SOURCE CURRENT (A) 4.95 4.90 4.85 4.80 4.75 4.70 4.65 4.60 4.55 4.50 VDD = 0, V+ = 110V, OPTO = CS = SS_SHDN = GND
MAX5014 toc06
48.0 47.8 MAX DUTY CYCLE (%) VOPTO = 4V, CS = GND 47.6 47.4 47.2 47.0 46.8 -40 -20 0 20 40 60 80 TEMPERATURE (C)
1.46 1.45 V+ INPUT CURRENT (mA) 1.44 VOPTO = 4V, VDD = CS = GND 1.43 1.42 1.41 1.40 1.39 1.38 -40 -20 40 TEMPERATURE (C) 0 20 60 80
5.00
-40
-20
0
20
40
60
80
TEMPERATURE (C)
V+ INPUT CURRENT vs. TEMPERATURE (AFTER STARTUP)
MAX5014 toc07
V+ SHUTDOWN CURRENT vs. TEMPERATURE
194 V+ SHUTDOWN CURRENT (A) 193 192 191 190 189 188 187 186
MAX5014 toc08
CS THRESHOLD VOLTAGE vs. TEMPERATURE
MAX5014 toc09
11.25
195 V+ = 110V, OPTO = SS_SHDN = CS = GND, VDD = 13V
0.488
11.20 V+ INPUT CURRENT (A)
CS THRESHOLD VOLTAGE (V)
0.487 VOPTO = 4V, V+ = 110V 0.486
11.15 V+ = 110V, VOPTO = 4V, CS = GND, VDD = 13V
11.10
0.485
11.05
0.484
11.00 -40 -20 0 20 40 60 80 TEMPERATURE (C)
185 -40 -20 0 20 40 60 80 TEMPERATURE (C)
0.483 -40 -20 0 20 40 60 80 TEMPERATURE (C)
NDRV RESISTANCE vs. TEMPERATURE
MAX5014 toc10
CURRENT-LIMIT DELAY vs. TEMPERATURE
MAX5014 toc11
VSS_SHDN vs. VDD
MAX5014 toc12
5.0 4.5 NDRV RESISTANCE () 4.0 HIGH-SIDE DRIVER 3.5 3.0 2.5 2.0 1.5 1.0 -40 -20 40 TEMPERATURE (C) 0 20 60 80 LOW-SIDE DRIVER
190 188 CURRENT LIMIT DELAY (ns) 186 184 182 180 178 176 174 172 170 -40 -20 0 20 40 60 80 TEMPERATURE (C) VOPTO = 4V, 100mV OVERDRIVE ON CS
2.410
2.408 VSS_SHDN (V)
2.406
2.404
2.402
2.400 0 5 10 15 20 VDD (V) 25 30 35 40
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Current-Mode PWM Controllers with Integrated Startup Circuit for Isolated Power Supplies
Typical Operating Characteristics (continued)
(V+ = 48V, VDD = 13V, NRDV is open circuit, TA = +25C, unless otherwise noted.)
MAX5015 MAXIMUM DUTY CYCLE vs. VDD
MAX5014 toc13 MAX5014 toc14
MAX5014/MAX5015
NDRV FREQUENCY vs. VDD
271.0 270.5 NDRV FREQUENCY (kHz) 270.0 269.5 269.0 268.5 268.0 267.5 267.0 0 5 10 15 20 VDD (V) 25 30 35 40 VOPTO = 4V, CS = GND 48.0 47.9 MAXIMUM DUTY CYCLE (%) 47.8 47.7
VCC vs. VDD
MAX5014 toc15
10.2 10.1 10.0 VCC (V) 9.9 9.8 9.7
VOPTO = 4V, CS = GND
DEVICE POWERED FROM VDD
47.6 47.5 47.4 47.3 47.2 47.1 47.0 0 5 10 15 20 VDD (V) 25 30 35 40 DRIVER POWERED FROM V+ DRIVER POWERED FROM VDD
9.6 9.5 0 5 10
DEVICE POWERED FROM V+ 15 20 VDD (V) 25 30 35 40
V+ SUPPLY CURRENT vs. V+ VOLTAGE
1.39 V+ SUPPLY CURRENT (mA) 1.38 1.37 1.36 1.35 1.34 1.33 2 1.32 1.31 0 20 40 60 80 100 V+ VOLTAGE (V) 0 VOPTO = 4V, CS = GND, VDD = 0
MAX5014 toc16
V+ INPUT CURRENT vs. VOLTAGE (AFTER STARTUP)
MAX5014 toc17
1.40
12 10 V+ INPUT CURRENT (A) VOPTO = 4V, CS = GND, VDD = 13V 8 6 4
0 10 20 30 40 50 60 70 80 90 100 110 V+ VOLTAGE (V)
VCC VOLTAGE vs. VCC CURRENT
MAX5014 toc18
VCC VOLTAGE vs. VCC CURRENT
9.9 9.8 VCC VOLTAGE (V) 9.7 9.6 9.5 9.4 9.3 9.2 V+ = 36V V+ = 24V VDD = OPTO = CS = GND V+ = 110V V+ = 90V V+ = 72V V+ = 48V
MAX5014 toc19
10.4 V+ = 110V, OPTO = CS = GND 10.2 VDD = 36V VCC VOLTAGE (V) 10.0 9.8 VDD = 13V 9.6 9.4 9.2 9.0 0 5.0 10.0 15.0 VCC CURRENT (mA)
10.0
9.1 9.0 20.0 0 5.0 10.0 VCC CURRENT (mA) 15.0 20.0
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Current-Mode PWM Controllers with Integrated Startup Circuit for Isolated Power Supplies MAX5014/MAX5015
Pin Description
PIN 1 NAME V+ FUNCTION High-Voltage Startup Input. Connect directly to an input voltage between 18V to 110V. Connects internally to a high-voltage linear regulator that generates VCC during startup. VDD is the Input of the Linear Regulator that Generates VCC. For supply voltages less than 36V, VDD and V+ can both be connected to the supply. For supply voltages greater than 36V, VDD receives its power from the tertiary winding of the transformer and accepts voltages from 13V to 36V. Bypass to GND with a 4.7F capacitor. Optocoupler Input. The control voltage range on this input is 2V to 3V. Soft-Start Timing Capacitor Connection. Ramp time to full current limit is approximately 0.45ms/nF. This pin is also the reference voltage output. Bypass with a minimum 10nF capacitor to GND. The device goes into shutdown when VSS_SHDN is pulled below 0.25V. Current Sense Input. Turns power switch off if VCS rises above 465mV for cycle-by-cycle current limiting. CS is also the feedback for the current-mode controller. CS is connected to the PWM comparator through a leading edge blanking circuit. Ground Gate Drive. Drives a high-voltage external N-channel power MOSFET. Regulated IC Supply. Provides power for the entire IC. VCC is regulated from VDD during normal operation and from V+ during startup. Bypass VCC with a 10F tantalum capacitor in parallel with 0.1F ceramic capacitor to GND.
2
VDD
3 4
OPTO SS_SHDN
5 6 7 8
CS GND NDRV VCC
Detailed Description
Use the MAX5014/MAX5015 PWM current-mode controllers to design flyback- or forward-mode power supplies. Current-mode operation simplifies control-loop design while enhancing loop stability. An internal highvoltage startup regulator allows the device to connect directly to the input supply without an external startup resistor. Current from the internal regulator starts the controller. Once the tertiary winding voltage is established the internal regulator is switched off and bias current for running the IC is derived from the tertiary winding. The internal oscillator is set to 275kHz and trimmed to 10%. This permits the use of small magnetic components to minimize board space. Both the MAX5014 and MAX5015 can be used in power supplies providing multiple output voltages. A functional diagram of the IC is shown in Figure 1. Typical applications circuits for forward and flyback topologies are shown in Figure 2 and Figure 3, respectively.
limit without being blanked. The leading-edge blanking of the CS signal prevents the PWM comparator from prematurely terminating the on cycle. The CS signal contains a leading-edge spike that is the result of the MOSFET gate charge current, capacitive and diode reverse recovery current of the power circuit. Since this leading-edge spike is normally lower than the current limit comparator threshold, current limiting is not blanked and cycle-by-cycle current limiting is provided under all conditions. Use the MAX5014 in discontinuous flyback applications where wide line voltage and load current variation is expected. Use the MAX5015 for single transistor forward converters where the maximum duty cycle must be limited to less than 50%. Under certain conditions it may be advantageous to use a forward converter with greater than 50% duty cycle. For those cases use the MAX5014. The large duty cycle results in much lower operating primary RMS currents through the MOSFET switch and in most cases a smaller output filter inductor. The major disadvantage to this is that the MOSFET voltage rating must be higher and that slope compensation must be provided to stabilize the inner current loop. The MAX5014 provides internal slope compensation.
Current-Mode Control
The MAX5014/MAX5015 offer current-mode control operation with added features such as leading-edge blanking with dual internal path that only blanks the sensed current signal applied to the input of the PWM comparator. The current limit comparator monitors the CS pin at all times and provides cycle-by-cycle current
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Current-Mode PWM Controllers with Integrated Startup Circuit for Isolated Power Supplies MAX5014/MAX5015
VDD VDD-OK V+ IN HIGHVOLTAGE REGULATOR EN OUT EN BIAS WINDING REGULATOR OUT 0.7V VCC IN
GND
MAX5014 ONLY 6.6V SLOPE COMPENSATION 26mV/s 26mV/s 275kHz OSCILLATOR
UVLO
R Q
NDRV
80%/50% DUTY CYCLE CLAMP PWM ILIM
S
125mV CS
OPTO 5k Vb SS_SHDN 4A 70ns BLANKING
2.4V BUF
3R
R
0.4V
Figure 1. Functional Diagram
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Current-Mode PWM Controllers with Integrated Startup Circuit for Isolated Power Supplies MAX5014/MAX5015
4.7nF 250VAC
1N4148 VIN (36V TO 72V) 6 NT 14 NR CMHD2003 CIN 3 x 0.47F NP 14 M1 IRF640N NDRV 100 CS MAX5015 GND SS_SHDN CSS 0.1F VCC CCC 10F 4.75k OPTO OPTOCOUPLER 3k 0.1F R1 25.5k RSENSE 100m 20 NS 5 SBL204OCT VOUT L1 4.7H COUT 3 x 560F 1nF 5V/10A
CDD 47F
VDD
V+
0.1F
220
240k TLV431
R2 8.25k
Figure 2. Forward Converter
Optocoupled Feedback
Isolated voltage feedback is achieved by using an optocoupler and a shunt regulator as shown in Figure 2. The output voltage set point accuracy is a function of the accuracy of the shunt regulator and feedback resistordivider tolerance.
Internal Regulators
The internal regulators of the MAX5014/MAX5015 enable initial startup without a lossy startup resistor and regulate the voltage at the output of a tertiary (bias) winding to provide power for the IC. At startup V+ is
8
regulated down to VCC to provide bias for the device. The VDD regulator then regulates from the output of the tertiary winding to VCC. This architecture allows the tertiary winding to only have a small filter capacitor at its output thus eliminating the additional cost of a filter inductor. When designing the tertiary winding calculate the number of turns so the minimum reflected voltage is always higher than 12.7V. The maximum reflected voltage must be less than 36V.
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Current-Mode PWM Controllers with Integrated Startup Circuit for Isolated Power Supplies MAX5014/MAX5015
4.7nF 250VAC
VIN
NT VOUT V+ CIN COUT NP M1 NDRV 100 CS MAX5014 GND SS_SHDN VCC CCC 220 RSENSE NS
CDD
VDD
CSS
OPTO OPTOCOUPLER R1
TLV431
R2
Figure 3. Flyback Converter
To reduce power dissipation the high-voltage regulator is disabled when the VDD voltage reaches 12.7V. This greatly reduces power dissipation and improves efficiency. If V CC falls below the undervoltage lockout threshold (VCC = 6.6V), the low-voltage regulator is disabled, and soft-start is reinitiated. In undervoltage lockout the MOSFET driver output (NDRV) is held low. If the input voltage range is between 13V and 36V, V+ and VDD may be connected to the line voltage provided that the maximum power dissipation is not exceeded. This eliminates the need for a tertiary winding.
Undervoltage Lockout (UVLO), Soft-Start, and Shutdown
The soft-start feature of the MAX5014/MAX5015 allows the load voltage to ramp up in a controlled manner, thus eliminating output voltage overshoot. While the part is in UVLO, the capacitor connected to the SS_SHDN pin is discharged. Upon coming out of UVLO an internal current source starts charging the capacitor to initiate the soft-start cycle. Use the following equation to calculate total soft-start time:
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9
Current-Mode PWM Controllers with Integrated Startup Circuit for Isolated Power Supplies MAX5014/MAX5015
tstartup = 0.45 ms x Css nF VSCOMP is a ramp function starting at 0 and slewing at 26mV/s (MAX5014 only). When using the MAX5014 in a forward-converter configuration the following condition must be met to avoid control-loop subharmonic oscillations: NS k x RSENSE x VOUT x = 26mV / s L NP where k = 0.75 to 1, and NS and NP are the number of turns on the secondary and primary side of the transformer, respectively. L is the output filter inductor. This makes the output inductor current downslope as referenced across RSENSE equal to the slope compensation. The controller responds to transients within one cycle when this condition is met.
where CSS is the soft-start capacitor as shown in Figure 2. Operation begins when VSS_SHDN ramps above 0.6V. When soft-start has completed, VSS_SHDN is regulated to 2.4V, the internal voltage reference. Pull VSS_SHDN below 0.25V to disable the controller. Undervoltage lockout shuts down the controller when VCC is less than 6.6V. The regulators for V+ and the reference remain on during shutdown.
Current-Sense Comparator
The current-sense (CS) comparator and its associated logic limit the peak current through the MOSFET. Current is sensed at CS as a voltage across a sense resistor between the source of the MOSFET and GND. To reduce switching noise, connect CS to the external MOSFET source through a 100 resistor or an RC lowpass filter (Figures 2, 3). Select the current-sense resistor, RSENSE according to the following equation: RSENSE = 0.465V / ILimPrimary where ILimPrimary is the maximum peak primary-side current. When VCS > 465mV, the power MOSFET switches off. The propagation delay from the time the switch current reaches the trip level to the driver turn-off time is 170ns.
N-Channel MOSFET Gate Driver
NDRV drives an N-channel MOSFET. NDRV sources and sinks large transient currents to charge and discharge the MOSFET gate. To support such switching transients, bypass VCC with a ceramic capacitor. The average current as a result of switching the MOSFET is the product of the total gate charge and the operating frequency. It is this current plus the DC quiescent current that determines the total operating current.
Applications Information
Design Example
The following is a general procedure for designing a forward converter (Figure 2) using the MAX5015. 1) Determine the requirements. 2) Set the output voltage. 3) Calculate the transformer primary to secondary winding turns ratio. 4) Calculate the reset to primary winding turns ratio. 5) Calculate the tertiary to primary winding turns ratio. 6) Calculate the current-sense resistor value. 7) Calculate the output inductor value. 8) Select the output capacitor. The circuit in Figure 2 was designed as follows: 1) 36V VIN 72V, VOUT = 5V, IOUT = 10A, VRIPPLE 50mV 2) To set the output voltage calculate the values of resistors R1 and R2 according to the following equation:
PWM Comparator and Slope Compensation
An internal 275kHz oscillator determines the switching frequency of the controller. At the beginning of each cycle, NDRV switches the N-channel MOSFET on. NDRV switches the external MOSFET off after the maximum duty cycle has been reached, regardless of the feedback. The MAX5014 uses an internal ramp generator for slope compensation. The internal ramp signal is reset at the beginning of each cycle and slews at 26mV/s. The PWM comparator uses the instantaneous current, the error voltage, the internal reference, and the slope compensation (MAX5014 only) to determine when to switch the N-channel MOSFET off. In normal operation the N-channel MOSFET turns off when: IPRIMARY x RSENSE > VOPTO - VREF - VSCOMP where IPRIMARY is the current through the N-channel MOSFET, V REF is the 2.4V internal reference and
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Current-Mode PWM Controllers with Integrated Startup Circuit for Isolated Power Supplies
VREF R2 = VOUT R1 + R2 where VREF is the reference voltage of the shunt regulator, and R1 and R2 are the resistors shown in Figures 2 and 3. 3) The turns ratio of the transformer is calculated based on the minimum input voltage and the lower limit of the maximum duty cycle for the MAX5015 (44%). To enable the use of MOSFETs with drain-source breakdown voltages of less than 200V use the MAX5015 with the 50% maximum duty cycle. Calculate the turns ratio according to the following equation: NS VOUT + (VD1 x DMAX ) NP DMAX x VIN_MIN where: NS/NP = Turns ratio (NS is the number of secondary turns and NP is the number of primary turns). VOUT = Output voltage (5V). VD1 = Voltage drop across D1 (typically 0.5V for power Schottky diodes). DMAX = Minimum value of maximum operating duty cycle (44%). VIN_MIN = Minimum Input voltage (36V). In this example: NS 5V + (0.5V x 0.44) = 0.330 0.44 x 36V NP Choose N P based on core losses and DC resistance. Use the turns ratio to calculate NS, rounding up to the nearest integer. In this example NP = 14 and NS = 5. For a forward converter choose a transformer with a magnetizing inductance in the neighborhood of 200H. Energy stored in the magnetizing inductance of a forward converter is not delivered to the load and must be returned back to the input; this is accomplished with the reset winding. The transformer primary to secondary leakage inductance should be less than 1H. Note that all leakage energy will be dissipated across the MOSFET. Snubber circuits may be used to direct some or all of the leakage energy to be dissipated across a resistor. To calculate the minimum duty cycle (DMIN) use the following equation: VOUT = DMIN = = 19.8 N VIN_MAX x S - VD1 NP where VIN_MAX is the maximum input voltage (72V). 4) The reset winding turns ratio (NR/NP) needs to be low enough to guarantee that the entire energy in the transformer is returned to V+ within the off cycle at the maximum duty cycle. Use the following equation to determine the reset winding turns ratio: NR NP x 1-DMAX DMAX
MAX5014/MAX5015
where: NR/NP = Reset winding turns ratio. DMAX' = Maximum value of Maximum Duty Cycle. 1- 0.5 = 14 0.5
NR 14 x
Round NR to the nearest smallest integer. The turns ratio of the reset winding (N R /N P ) will determine the peak voltage across the N-channel MOSFET. Use the following equation to determine the maximum drain-source voltage across the N-channel MOSFET: N VDSMAX VIN_MAX x 1 + P NR VDSMAX = Maximum MOSFET drain-source voltage. VIN_MAX = Maximum input voltage. 14 VDSMAX 72V x 1 + = 144V 14 Choose MOSFETs with appropriate avalanche power ratings to absorb any leakage energy. 5) Choose the tertiary winding turns ratio (NT/NP) so that the minimum input voltage provides the minimum operating voltage at VDD (13V). Use the follow11
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Current-Mode PWM Controllers with Integrated Startup Circuit for Isolated Power Supplies MAX5014/MAX5015
ing equation to calculate the tertiary winding turns ratio: VDDMIN + 0.7 x NP NT VIN_MIN VDDMAX + 0.7 x NP VIN_MAX where: VDDMIN is the minimum VDD supply voltage (13V). VDDMAX is the maximum VDD supply voltage (36V). VIN_MIN is the minimum input voltage (36V). VIN_MAX is the maximum input voltage (72V in this design example). NP is the number of turns of the primary winding. NT is the number of turns of the tertiary winding. 13.7 36.7 x 14 NT x 14 36 72 5.33 NT 7.14 Choose NT = 6. 6) Choose RSENSE according to the following equation: RSENSE VILIM NS x 1.2 x IOUTMAX NP where VD is the output Schottky diode forward voltage drop (0.5V) and LIR is the ratio of inductor ripple current to DC output current. L
0.4 x 275kHz x 10A
(5.5) x (1- 0.198)
= 4.01H
8) The size and ESR of the output filter capacitor determine the output ripple. Choose a capacitor with a low ESR to yield the required ripple voltage. Use the following equations to calculate the peak-topeak output ripple:
2 2 VRIPPLE = VRIPPLE,ESR + VRIPPLE,C
where: VRIPPLE is the combined RMS output ripple due to VRIPPLE,ESR, the ESR ripple, and V RIPPLE,C , the capacitive ripple. Calculate the ESR ripple and capacitive ripple as follows: VRIPPLE,ESR = IRIPPLE x ESR VRIPPLE,C = IRIPPLE/(2 x x 275kHz x COUT)
Layout Recommendations
All connections carrying pulsed currents must be very short, be as wide as possible, and have a ground plane as a return path. The inductance of these connections must be kept to a minimum due to the high di/dt of the currents in high-frequency switching power converters. Current loops must be analyzed in any layout proposed, and the internal area kept to a minimum to reduce radiated EMI. Ground planes must be kept as intact as possible.
where: VILim is the current-sense comparator trip threshold voltage (0.465V). NS/NP is the secondary side turns ratio (5/14 in this example). IOUTMAX is the maximum DC output current (10A in this example). RSENSE 0.465V = 109m 5 x 1.2 x 10 14
Chip Information
TRANSISTOR COUNT: 589 PROCESS: BiCMOS
7) Choose the inductor value so that the peak ripple current (LIR) in the inductor is between 10% and 20% of the maximum output current. L
2 x LIR x 275kHz x IOUTMAX
(VOUT + VD ) x (1- DMIN )
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Current-Mode PWM Controllers with Integrated Startup Circuit for Isolated Power Supplies MAX5014/MAX5015
Table 1. Component Manufacturers
International Rectifier Power FETS Fairchild Vishay-Siliconix Current-Sense Resistors Dale-Vishay IRC On Semi Diodes General Semiconductor Central Semiconductor Sanyo Capacitors Taiyo Yuden AVX Coiltronics Magnetics Coilcraft Pulse Engineering www.irf.com www.fairchildsemi.com www.vishay.com/brands/siliconix/main.html www.vishay.com/brands/dale/main.html www.irctt.com/pages/index.cfm www.onsemi.com www.gensemi.com www.centralsemi.com www.sanyo.com www.t-yuden.com www.avxcorp.com www.cooperet.com www.coilcraft.com www.pulseeng.com
Selector Guide
PART MAX5014CSA MAX5014ESA MAX5015CSA MAX5015ESA MAXIMUM DUTY CYCLE 85% 85% 50% 50% SLOPE COMPENSATION Yes Yes No No
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Current-Mode PWM Controllers with Integrated Startup Circuit for Isolated Power Supplies MAX5014/MAX5015
Package Information
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
14 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 2001 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
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