KIT ATION EVALU E L B AVAILA 19-2082; Rev 0; 7/01 Current-Mode PWM Controllers with Integrated Startup Circuit for Isolated Power Supplies 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. Features ♦ Wide Input Range: (18V to 110V) or (13V to 36V) ♦ Current-Mode Control ♦ Leading-Edge Blanking ♦ Internally Trimmed 275kHz ±10% Oscillator ♦ Low External Component Count ♦ Soft-Start ♦ High-Voltage Startup Circuit ♦ Pulse-by-Pulse Current Limiting ♦ Thermal Shutdown ♦ SO-8 Package Ordering Information PART TEMP. RANGE PIN-PACKAGE MAX5014CSA* 0°C to +70°C 8-SO MAX5014ESA* -40°C to +85°C 8-SO MAX5015CSA* 0°C to +70°C MAX5015ESA* -40°C to +85°C *See Selector Guide at end of data sheet. 8-SO 8-SO Typical Operating Circuit Applications Telecom Power Supplies VIN Industrial Power Supplies Networking Power Supplies VOUT V+ VDD Isolated Power Supplies NDRV Pin Configuration CS MAX5015 GND TOP VIEW SS_SHDN V+ 1 8 VCC VDD 2 7 NDRV OPTO 3 6 GND 5 CS MAX5014/ MAX5015 SS_SHDN 4 VCC OPTO OPTOCOUPLER 8-SO ________________________________________________________________ Maxim Integrated Products For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com. 1 MAX5014/MAX5015 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. MAX5014/MAX5015 Current-Mode PWM Controllers with Integrated Startup Circuit for Isolated Power Supplies 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 1µs..………….…………….……±1A Continuous Power Dissipation (TA = +70°C) 8-Pin SO (derate 5.88mW/°C above +70°C) .………....471mW Operating Temperature Range…………..……...-40°C to +85°C Storage Temperature Range……………..…….-65°C to +150°C Lead Temperature (soldering, 10s) ……………… ………+300°C 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 10µF capacitor connects VCC to GND, VCS = 0, V+ = 48V, 0.1µF capacitor connected to SS_SHDN, NDRV = open circuit, OPTO = GND, TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS VDD = 0, V+ = 110V, driver not switching V+ = 110V, VDD = 0, VOPTO = 4V, driver switching V+ = 110V, VDD = 13V, VOPTO = 4V 0.85 1.3 1.4 2.6 VDD = 36V, driver not switching 0.9 1.3 VDD = 36V, driver switching, VOPTO = 4V 1.9 2.7 V+ Shutdown Current VSS_SHDN = 0, V+ = 110V 190 290 µA VDD Shutdown Current VSS_SHDN = 0 8 20 µA SUPPLY CURRENT IV+(NS) V+ Supply Current IV+(S) V+ Supply Current After Startup VDD Supply Current IVDD(NS) IVDD(S) 11 mA µA mA PREREGULATOR/STARTUP V+ Input Voltage 18 110 V VDD Supply Voltage 13 36 V V INTERNAL REGULATORS (VCC) VCC Output Voltage VCC Undervoltage Lockout VCC_UVLO Powered from V+, ICC = 7.5mA, VDD = 0 7.5 9.8 12 Powered from VDD, ICC = 7.5mA 9.0 10.0 11.0 VCC falling V 6.6 V OUTPUT DRIVER Peak Source Current VCC = 11V, (externally forced) 570 mA Peak Sink Current VCC = 11V, (externally forced) 1000 mA NRDV High-Side Driver Resistance ROH VCC = 11V, externally forced, NDRV sourcing 50mA 4 12 Ω NDRV Low Side Driver Resistance ROL VCC = 11V, externally forced, NDRV sinking 50mA 1.6 4 Ω -1.00 1.00 µA 2 3 V PWM COMPARATOR OPTO Input Bias Current VOPTO = VSS_SHDN OPTO Control Range Slope Compensation 2 VSCOMP MAX5014 26 _______________________________________________________________________________________ mV/µs Current-Mode PWM Controllers with Integrated Startup Circuit for Isolated Power Supplies (VDD = 13V, a 10µF capacitor connects VCC to GND, VCS = 0, V+ = 48V, 0.1µF capacitor connected to SS_SHDN, NDRV = open circuit, OPTO = GND, TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS THERMAL SHUTDOWN Thermal Shutdown Temperature 150 °C Thermal Hysteresis 25 °C CURRENT LIMIT CS Threshold Voltage VILIM VOPTO = 4V 419 465 510 mV 1 µA CS Input Bias Current 0 ≤ VCS ≤ 2V, VOPTO = 4V Current Limit Comparator Propagation Delay 25mV overdrive on CS, VOPTO = 4V 180 ns CS Blanking Time VOPTO = 4V 70 ns -1 OSCILLATOR Clock Frequency Range Max Duty Cycle VOPTO = 4V 247 275 302 MAX5014, VOPTO = 4V 75 85 MAX5015, VOPTO = 4V 44 50 kHz % SOFT-START SS Source Current ISSO VSS_SHDN = 0 2.0 SS Sink Current 4.6 6.5 µA 1.0 Peak Soft-Start Voltage Clamp No external load Shutdown Threshold mA 2.331 2.420 2.500 VSS_SHDN falling 0.25 0.37 0.41 VSS_SHDN rising 0.53 0.59 0.65 V V Typical Operating Characteristics (V+ = 48V, VDD = 13V, NRDV is open circuit, TA = +25°C, unless otherwise noted.) 277 NDRV FREQUENCY (kHz) 1.002 1.001 1.000 VOPTO = 4V, CS = GND 276 275 274 0.999 -20 0 20 40 TEMPERATURE (°C) 60 80 VOPTO = 4V, CS = GND 80.9 80.8 80.7 80.6 80.5 80.4 273 -40 81.0 MAXIMUM DUTY CYCLE (%) OPTO = CS = GND MAX5014 toc02 278 MAX5014 toc01 1.003 VSS_SHDN (V) (NORMALIZED TO VREF = 2.4V) MAX5014 MAXIMUM DUTY CYCLE vs. TEMPERATURE NDRV FREQUENCY vs. TEMPERATURE MAX5014 toc03 VSS_SHDN vs. TEMPERATURE (AT THE END OF SOFT-START) -40 -20 0 20 40 TEMPERATURE (°C) 60 80 -40 -20 0 20 40 60 80 TEMPERATURE (°C) _______________________________________________________________________________________ 3 MAX5014/MAX5015 ELECTRICAL CHARACTERISTICS (continued) Typical Operating Characteristics (continued) (V+ = 48V, VDD = 13V, NRDV is open circuit, TA = +25°C, unless otherwise noted.) V+ SUPPLY CURRENT vs. TEMPERATURE 47.4 47.2 47.0 1.43 1.42 1.41 1.40 1.39 0 20 60 V+ INPUT CURRENT vs. TEMPERATURE (AFTER STARTUP) V+ SHUTDOWN CURRENT vs. TEMPERATURE 40 60 80 MAX5014 toc07 11.25 11.20 11.15 V+ = 110V, VOPTO = 4V, CS = GND, VDD = 13V 11.10 11.05 4.80 4.75 VDD = 0, V+ = 110V, OPTO = CS = SS_SHDN = GND 4.70 4.65 4.60 80 0 20 40 60 80 CS THRESHOLD VOLTAGE vs. TEMPERATURE V+ = 110V, OPTO = SS_SHDN = CS = GND, VDD = 13V 193 -20 TEMPERATURE (°C) 195 194 -40 MAX5014 toc08 20 192 191 190 189 188 0.488 CS THRESHOLD VOLTAGE (V) -20 TEMPERATURE (°C) 0 4.85 4.50 -40 40 TEMPERATURE (°C) -20 V+ SHUTDOWN CURRENT (µA) -40 4.90 4.55 1.38 46.8 V+ INPUT CURRENT (µA) VOPTO = 4V, VDD = CS = GND 4.95 187 MAX5014 toc09 47.6 1.44 5.00 MAX5014 toc06 1.45 V+ INPUT CURRENT (mA) VOPTO = 4V, CS = GND MAX5014 toc05 47.8 MAX DUTY CYCLE (%) 1.46 MAX5014 toc04 48.0 SOFT-START SOURCE CURRENT vs. TEMPERATURE SOFT-START SOURCE CURRENT (µA) MAX5015 MAXIMUM DUTY CYCLE vs. TEMPERATURE 0.487 VOPTO = 4V, V+ = 110V 0.486 0.485 0.484 186 20 40 60 80 -40 -20 0 20 40 60 TEMPERATURE (°C) NDRV RESISTANCE vs. TEMPERATURE CURRENT-LIMIT DELAY vs. TEMPERATURE HIGH-SIDE DRIVER 3.5 3.0 2.5 2.0 188 CURRENT LIMIT DELAY (ns) 4.0 LOW-SIDE DRIVER 186 20 40 60 80 2.410 2.408 184 182 180 178 2.406 2.404 176 VOPTO = 4V, 100mV OVERDRIVE ON CS 174 1.5 0 VSS_SHDN vs. VDD 190 MAX5014 toc10 4.5 -20 TEMPERATURE (°C) TEMPERATURE (°C) 5.0 -40 80 MAX5014 toc12 0 VSS_SHDN (V) -20 MAX5014 toc11 -40 2.402 172 1.0 170 -40 4 0.483 185 11.00 NDRV RESISTANCE (Ω) MAX5014/MAX5015 Current-Mode PWM Controllers with Integrated Startup Circuit for Isolated Power Supplies -20 0 20 40 TEMPERATURE (°C) 60 80 2.400 -40 -20 0 20 40 TEMPERATURE (°C) 60 80 0 5 10 15 20 VDD (V) _______________________________________________________________________________________ 25 30 35 40 Current-Mode PWM Controllers with Integrated Startup Circuit for Isolated Power Supplies MAX5015 MAXIMUM DUTY CYCLE vs. VDD 269.0 268.5 VOPTO = 4V, CS = GND 268.0 VOPTO = 4V, CS = GND 47.6 47.5 DRIVER POWERED FROM VDD 47.4 47.3 267.5 47.1 267.0 47.0 10 15 20 25 30 35 9.8 9.6 DEVICE POWERED FROM V+ 9.5 0 40 9.9 9.7 DRIVER POWERED FROM V+ 5 10 15 20 25 30 35 40 0 5 10 15 VDD (V) VDD (V) 25 30 35 40 V+ INPUT CURRENT vs. VOLTAGE (AFTER STARTUP) 1.38 10 V+ INPUT CURRENT (µA) 1.39 1.37 1.36 1.35 VOPTO = 4V, CS = GND, VDD = 0 MAX5014 toc17 12 MAX5014 toc16 1.40 1.34 20 VDD (V) V+ SUPPLY CURRENT vs. V+ VOLTAGE V+ SUPPLY CURRENT (mA) VOPTO = 4V, CS = GND, VDD = 13V 8 6 4 1.33 2 1.32 0 1.31 20 40 60 80 0 10 20 30 40 50 60 70 80 90 100 110 100 V+ VOLTAGE (V) V+ VOLTAGE (V) VCC VOLTAGE vs. VCC CURRENT VCC VOLTAGE vs. VCC CURRENT 10.4 V+ = 110V, OPTO = CS = GND 10.2 VDD = 36V 10.0 9.8 VDD = 13V 9.6 VDD = OPTO = CS = GND 9.9 V+ = 110V V+ = 90V V+ = 72V V+ = 48V 9.8 VCC VOLTAGE (V) 10.0 MAX5014 toc19 0 MAX5014 toc18 5 DEVICE POWERED FROM VDD 10.0 47.7 47.2 0 10.1 VCC (V) 269.5 47.8 10.2 MAX5014 toc14 MAXIMUM DUTY CYCLE (%) 47.9 270.0 VCC VOLTAGE (V) NDRV FREQUENCY (kHz) 270.5 VCC vs. VDD 48.0 MAX5014 toc13 271.0 MAX5014 toc15 NDRV FREQUENCY vs. VDD 9.7 9.6 9.5 V+ = 36V 9.4 V+ = 24V 9.3 9.4 9.2 9.2 9.1 9.0 9.0 0 5.0 10.0 15.0 VCC CURRENT (mA) 20.0 0 5.0 10.0 15.0 20.0 VCC CURRENT (mA) _______________________________________________________________________________________ 5 MAX5014/MAX5015 Typical Operating Characteristics (continued) (V+ = 48V, VDD = 13V, NRDV is open circuit, TA = +25°C, unless otherwise noted.) Current-Mode PWM Controllers with Integrated Startup Circuit for Isolated Power Supplies MAX5014/MAX5015 Pin Description PIN NAME FUNCTION V+ 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. 2 VDD 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 4.7µF capacitor. 3 OPTO 4 SS_SHDN 5 CS 1 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. 6 GND Ground 7 NDRV Gate Drive. Drives a high-voltage external N-channel power MOSFET. 8 VCC 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 10µF tantalum capacitor in parallel with 0.1µF ceramic capacitor to GND. 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. 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 6 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 PWM Controllers with Integrated Startup Circuit for Isolated Power Supplies MAX5014/MAX5015 VDD VDD-OK V+ IN IN HIGHVOLTAGE REGULATOR GND EN BIAS WINDING REGULATOR OUT EN OUT 0.7V VCC UVLO MAX5014 ONLY 6.6V 275kHz OSCILLATOR SLOPE COMPENSATION 26mV/µs R NDRV Q 80%/50% DUTY CYCLE CLAMP 26mV/µs S ∑ ILIM PWM 125mV CS OPTO 5kΩ Vb SS_SHDN 70ns BLANKING 4µA 3R 2.4V BUF R 0.4V Figure 1. Functional Diagram _______________________________________________________________________________________ 7 MAX5014/MAX5015 Current-Mode PWM Controllers with Integrated Startup Circuit for Isolated Power Supplies 4.7nF 250VAC 1N4148 CDD 47µF VOUT CIN 3 × 0.47µF V+ VDD SBL204OCT 14 NR CMHD2003 6 NT VIN (36V TO 72V) NP 14 L1 4.7µH COUT 3 × 560µF 20Ω NS 5 M1 IRF640N 5V/10A 0.1µF 1nF NDRV 100Ω CS RSENSE 100mΩ MAX5015 GND SS_SHDN VCC 220Ω CCC 10µF CSS 0.1µF 4.75kΩ OPTO OPTOCOUPLER R1 25.5kΩ 3kΩ 0.1µF 240kΩ TLV431 R2 8.25kΩ 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. _______________________________________________________________________________________ Current-Mode PWM Controllers with Integrated Startup Circuit for Isolated Power Supplies MAX5014/MAX5015 4.7nF 250VAC NT VIN VOUT CDD CIN V+ VDD COUT NP NS M1 NDRV 100Ω CS RSENSE MAX5014 GND SS_SHDN VCC CCC CSS 220Ω 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: _______________________________________________________________________________________ 9 MAX5014/MAX5015 Current-Mode PWM Controllers with Integrated Startup Circuit for Isolated Power Supplies tstartup = 0.45 ms × Css nF 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 100Ω 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. 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/µs. 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 × RSENSE > VOPTO - VREF - VSCOMP VSCOMP is a ramp function starting at 0 and slewing at 26mV/µs (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 × RSENSE × VOUT × = 26mV / µs 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. 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: where IPRIMARY is the current through the N-channel MOSFET, V REF is the 2.4V internal reference and 10 ______________________________________________________________________________________ Current-Mode PWM Controllers with Integrated Startup Circuit for Isolated Power Supplies 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 × DMAX ) ≥ NP DMAX × 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 × 0.44) ≥ = 0.330 0.44 × 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 200µH. 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 1µH. 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 × 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 × 1-DMAX ′ DMAX ′ where: NR/NP = Reset winding turns ratio. DMAX’ = Maximum value of Maximum Duty Cycle. NR ≤ 14 × 1- 0.5 = 14 0.5 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 × 1 + P NR VDSMAX = Maximum MOSFET drain-source voltage. VIN_MAX = Maximum input voltage. 14 VDSMAX ≥ 72V × 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 follow- ______________________________________________________________________________________ 11 MAX5014/MAX5015 VREF R2 = VOUT R1 + R2 Current-Mode PWM Controllers with Integrated Startup Circuit for Isolated Power Supplies MAX5014/MAX5015 ing equation to calculate the tertiary winding turns ratio: 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. VDDMIN + 0.7 × NP ≤ NT ≤ VIN_MIN VDDMAX + 0.7 × 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 × 14 ≤ NT ≤ × 14 36 72 5.33 ≤ NT ≤ 7.14 Choose NT = 6. 6) Choose RSENSE according to the following equation: RSENSE ≤ VILIM NS × 1.2 × IOUTMAX NP 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). L≥ (5.5) × (1- 0.198) 0.4 × 275kHz × 10A 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 + VRIPPLE ,ESR ,C where: VRIPPLE is the combined RMS output ripple due to V RIPPLE,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 π 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. IOUTMAX is the maximum DC output current (10A in this example). RSENSE ≤ 0.465V = 109mΩ 5 × 1.2 × 10 14 = 4.01µH 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≥ 12 (VOUT + VD ) × (1- DMIN ) 2 × LIR × 275kHz × IOUTMAX ______________________________________________________________________________________ Current-Mode PWM Controllers with Integrated Startup Circuit for Isolated Power Supplies MAX5014/MAX5015 Table 1. Component Manufacturers International Rectifier Power FETS Current-Sense Resistors Diodes Capacitors Magnetics www.irf.com Fairchild www.fairchildsemi.com Vishay-Siliconix www.vishay.com/brands/siliconix/main.html Dale-Vishay www.vishay.com/brands/dale/main.html IRC www.irctt.com/pages/index.cfm On Semi www.onsemi.com General Semiconductor www.gensemi.com Central Semiconductor www.centralsemi.com Sanyo www.sanyo.com Taiyo Yuden www.t-yuden.com AVX www.avxcorp.com Coiltronics www.cooperet.com Coilcraft www.coilcraft.com Pulse Engineering www.pulseeng.com Selector Guide MAXIMUM DUTY CYCLE SLOPE COMPENSATION MAX5014CSA 85% Yes MAX5014ESA 85% Yes MAX5015CSA 50% No MAX5015ESA 50% No PART ______________________________________________________________________________________ 13 Current-Mode PWM Controllers with Integrated Startup Circuit for Isolated Power Supplies SOICN.EPS 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 © 2001 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.