Circuit Note CN-0342 Devices Connected/Referenced Circuits from the Lab® reference designs are engineered and tested for quick and easy system integration to help solve today’s analog, mixed-signal, and RF design challenges. For more information and/or support, visit www.analog.com/CN0342. ADuM3190 2.5 kV rms Isolated Error Amplifier ADP1621 Constant-Frequency, Current-Mode Step-Up DC-to-DC Controller Flyback Power Supply Using a High Stability Isolated Error Amplifier EVALUATION AND DESIGN SUPPORT The entire circuit operates from 5 V to 24 V, allowing it to be used with standard industrial and automotive power supplies. The output capability of the circuit is up to 1 A with a 5 V input and 5 V output configuration. Circuit Evaluation Board CN-0342 Circuit Evaluation Board (EVAL-CN0342-EB1Z) Design and Integration Files Schematics, Layout Files, Bill of Materials This solution can be adapted for use in applications where higher dc input voltages are used to create lower voltage isolated supplies with good efficiency and a small form factor. Examples include 10 W to 20 W telecommunication and server power supplies, where power efficiency and printed circuit board (PCB) density are important, and −48 V supplies are common. CIRCUIT FUNCTION AND BENEFITS The circuit shown in Figure 1 is an isolated, flyback power supply that uses a linear isolated error amplifier to supply the feedback signal from the secondary side to the primary side. Unlike optocoupler-based solutions, which have a nonlinear transfer function that changes over time and temperature, the linear transfer function of the isolated amplifier is stable and minimizes offset and gain errors when transferring the feedback signal across the isolation barrier. VOUT 5V TO 24V T1 C26 C25 C14 47µF 47µF 100µF R12 820Ω C21 220nF D1 R19 390Ω C23 C24 C3 47µF 47µF 100µF 5V R4 100kΩ Q2 R18 0Ω IN SDSN CS GND COMP PIN GATE FB FREQ PGND ADP1621 R3 51kΩ R5 82Ω D2 Q3 R16 0Ω R6 100kΩ C17 0.1µF R2 47kΩ R1 2kΩ R20 0Ω VDD1 VDD2 ADuM3190 REF REFOUT +IN EAOUT –IN C10 1nF COMP GND1 GND2 R19 15kΩ C9 2.2nF 11763-001 VIN Figure 1. Simplified Schematic of Flyback Power Supply Circuit Rev. 0 Circuits from the Lab reference designs from Analog Devices have been designed and built by Analog Devices engineers. Standard engineering practices have been employed in the design and construction of each circuit, and their function and performance have been tested and verified in a lab environment at room temperature. However, you are solely responsible for testing the circuit and determining its suitability and applicability for your use and application. Accordingly, in no event shall Analog Devices be liable for direct, indirect, special, incidental, consequential or punitive damages due toanycausewhatsoeverconnectedtotheuseofanyCircuitsfromtheLabcircuits. (Continuedonlastpage) One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 www.analog.com Fax: 781.461.3113 ©2014 Analog Devices, Inc. All rights reserved. CN-0342 Circuit Note CIRCUIT DESCRIPTION The average transformer primary side current, ILAVG, is given by The isolation amplifier is the ADuM3190, which includes a 1.225 V voltage reference and an error amplifier with 10 MHz unity-gain bandwidth product. An external resistive divider (R1, R2, R3, and R4) and compensation network (R9, C9, and C10) complete the analog feedback loop. The input supply range of the ADuM3190 is 3.0 V to 20 V on both sides, and internal low dropout regulators provide stable supplies for the voltage reference, error amplifier, and analog isolator. The ADuM3190 is compatible with the Distributedpower Open Standards Alliance (DOSA) output voltage trim method. The ADP1621 provides pulse-width modulation (PWM) control for the flyback power supply. The internal 5.5 V shunt regulator provides the capability of high supply input voltage with the addition of an external NPN transistor (Q2). The ADP1621 also provides lossless current sensing for current mode operation, providing excellent line and load transient response. Setting the Output Voltage The output voltage is set through a voltage divider from VOUT to the −IN pin of ADuM3190. The feedback resistor ratio sets the output voltage of the system. Using the internal voltage reference in the ADuM3190, the regulation voltage at the −IN pin is 1.225 V. VOUT I LAVG = where: ILOAD is the load current. D is the duty cycle under maximum load current. NS/NP is the turn ratio of the transformer. The output voltage is VOUT × (1 − D) × The peak-to-peak primary inductor ripple current is inversely proportional to the inductor value. ∆I L = The ADuM3190 provides an internal 1.225 V voltage reference specified for ±1% accuracy over the −40°C to +125°C temperature range. The reference voltage output pin (REFOUT) can be connected to the +IN pin of the error amplifier to set the output voltage. When higher accuracy or a special output voltage is required, and a different reference voltage must be used, the +IN pin can also be connected to an external reference For this design example, the following parameters were used: • • • • • VIN = 5 V VOUT = 5 V IOUTMAX = 1 A fSW = 200 kHz Transformer turn ratio = 1:1 f SW × L where: fSW is the switching frequency. L is the primary inductor value. Assuming continuous conduction mode (CCM) operation, the primary inductor current is given by Voltage Reference The transformer choice is important because it dictates the primary inductor current ripple. VIN × D VIN × D I LOAD N S × + 1 − D N P 2 × f SW × L Assuming the primary ripple current is 50% of the average current on the transformer primary side, a reasonable choice for the inductor value is For a 5 V output configuration, the resistor divider values are R1 = 2 kΩ, R2 = 47 kΩ, R3 = 51 kΩ, and R4 = 100 kΩ. Transformer Selection NP = VIN × D NS The duty cycle (D) is 50% because the output and input are both 5 V, and the transformer turn ratio is 1:1. I LPK = R4 + R3 = 1.225 V × 1 + R1 + R2 I LOAD N S × 1 − D NP L= VIN × D × (1 − D) 0.5 × f SW × I LOAD × 5 V × 0.5 (1 − 0.5) NP = = 12.5 μH N S 0.5 × 200 kHz × 1 A The transformer used in this design is a 1:1 turn ratio transformer with 16 µH primary side inductance (Halo Electronics, Inc., TGB01-P099EP13LF). Compensation Network In a flyback topology power supply, the output load resistance, the output capacitor, and its effective series resistance (ESR) add a zero and a pole at frequencies that are dependent on the component type and values. There is also a right half plane (RHP) zero in the control-to-output transfer function. Because the RHP zero reduces the phase by 90°, the frequency of 0 dB gain (crossover frequency) is lower than the RHP zero. With the ADuM3190 providing the error amplifier, a Type II compensation network can be provided from the −IN pin to the COMP pin to compensate for the control loop for stability. The compensation network values depend on the selected components. Rev. 0 | Page 2 of 5 Circuit Note CN-0342 Insulation and Safety The zeros and poles of a Type II compensation network are derived as follows: f ZERO The ADuM3190 is packaged in a small, 16-lead QSOP package for a 2.5 kV rms isolation voltage rating. 1 2 R9 C9 f POLE Safety specifications for the ADuM3190 are shown in Table 1. C10 C9 Table 1. Safety Specifications for the ADuM3190 2 R9 C10 C9 In this particular design, the compensation network is set by R9 = 15 kΩ, C9 = 2.2 nF, and C10 = 1 nF. The zero and the pole provided by this compensation network are fZERO = 4.8 kHz and fPOLE = 15.4 kHz. Increasing the zero and pole frequency improves the load transient response but decreases the phase margin of the feedback loop and can cause instability in the power supply. Parameter Rated Dielectric Insulation Voltage Minimum External Air Gap (Clearance) Minimum External Tracking (Creepage) Minimum Internal Gap (Internal Clearance) Tracking Resistance (Comparative Tracking Index) Isolation Material Group Value 2500 3.8 min 3.1 min 0.017 min >400 Unit V rms mm mm mm V II COMMON VARIATIONS Snubber Network When the power MOSFET (Q3) is turned off, there is a high voltage spike on the drain due to the transformer leakage inductance. This excessive voltage can overstress the power MOSFET and lead to a reliability issue or damage. Therefore, it is necessary to use an additional network to clamp the voltage. The resistor, capacitor, and diode (R19, C21, and D2) snubber network absorbs the current in the leakage inductance by turning on the snubber diode when the MOSFET drain voltage exceeds the cathode voltage of D2. The ADP1621 operates in lossless mode with the drain of the power MOSFET connected to the CS pin. In a practical design, limit the voltage at the CS pin to an absolute maximum of 33 V, and a practical maximum of 30 V to maintain accuracy. If the measured peak voltage exceeds 30 V, or if a more accurate current limit is desired, connect the CS pin to an external current sense resistor in the source of the MOSFET. Primary Side Power Supply The ADP1621 supply voltage range is 2.9 V to 5.5 V, and the ADuM3190 supply voltage range is 3.0 V to 20 V. To work with a 5 V to 12 V input voltage supply, a small-signal NPN transistor (Q2) can be used as a voltage regulator. For higher input voltages, the current sense resistor must be used with a current control loop. Change the R20 resistor from 0 Ω to the value required by the application. For the 1 A output configuration, choose a sense resistor value of 50 mΩ. According to the internal current feedback loop in the ADP1621, the maximum PWM duty cycle of switching is reduced when the current sensing voltage rises. If the sense resistor value is too high, the duty cycle of the switching is limited; the maximum output current is then also limited under the specific output voltage. The ADP1621 compensation pin (COMP) has a 0 V to 2 V effective input voltage range. It is recommended to limit the voltage on the CS pin to less than 0.1 V to guarantee an adequate switching duty cycle. When working with an input voltage less than 5 V, bypass the input regulation transistor by shorting the jumper between the collector and the emitter. For applications such as telecommunication or server power supplies with a −48 V input, regulate the supply voltage for the primary side controller to +5 V. The NPN transistor Q2 needs a higher VCE breakdown voltage, and the power MOSFET Q3 needs a higher VDS (100 V, VDSMAX). In addition, change the diode in the RCD snubber circuit, D2, to a 70 V reverse voltage rating. To reduce the current on the internal regulator in the ADP1621, increase R12 to 1.5 kΩ. The IN pin of ADP1621 has a 5.5 V internal voltage regulator and is connected to the base node of the NPN transistor Q2. This connection biases Q2 so that the emitter node can be regulated to 5.5 V − 0.7 V = 4.8 V, which can be used as the power supply voltage for the ADP1621 (PIN) and the ADuM3190 (VDD1). Table 2 summarizes the alternative components selected for different input voltage configurations. Table 2. Component Values for Different Configurations Secondary Side Power Supply The ADuM3190 has a 3.0 V to 20 V supply voltage range on the secondary side (VDD2), with an internal regulator providing a 3.0 V operating voltage. If VOUT is set higher than 20 V, add an external voltage regulator to provide the specified VDD2 voltage. Input Voltage Q2 Q3 D2 R12 5 V to 7 V PMST2369 NTD18N06L MBR0540T1 390 Ω 7 V to 24 V PMST2369 NTD18N06L MBR0540T1 820 Ω 24 V to 48 V BC846 NVD6824NL MMSD701T1 1.5 kΩ A complete design support package, including the schematics, layouts, and bill of materials, is available at www.analog.com/CN0342-DesignSupport. Rev. 0 | Page 3 of 5 CN-0342 Circuit Note Performance Results Figure 2 shows the measured efficiency with three different input voltages: 5 V, 12 V, and 24 V. Figure 4. 100 mA to 900 mA Load Transient Response Figure 2. Flyback Circuit Output Efficiency vs. Load Current for Input Voltages of 5 V, 12 V, and 24 V Figure 3 shows the output voltage over a temperature range from −40°C to +125°C. The total output voltage error over this range is less than ±20 mV (±0.4%). Figure 5. 900 mA to 100 mA Load Transient Response Figure 3. Flyback Circuit Output Voltage vs. Temperature Figure 4 and Figure 5 show the transient response for increasing and decreasing load current. The transient response time is 32 µs for a load current increase from 100 mA to 900 mA, and 45 µs for a load current decrease from 900 mA to 100 mA. A photograph of the EVAL-CN0342-EB1Z evaluation board is shown in Figure 6. Figure 6. EVAL-CN0342-EB1Z Evaluation Board Photo Rev. 0 | Page 4 of 5 Circuit Note CN-0342 CIRCUIT EVALUATION AND TEST LEARN MORE The circuit is tested using a dc power supply and a source/ measurement unit for efficiency and load regulation. Load transient response and output ripple are measured with an oscilloscope and current probe. CN0342 Design Support Package. Equipment Needed Brown, Marty. Practical Switching Power Supply Design. Academic Press, 1990. Gottlieb, Irving M. Power Supplies, Switching Regulators, Inverters, and Converters. Second Edition, McGraw Hill (TAB Books), 1994. The following equipment is required: A 30 V power supply with 3 A current output capability and current measurement function A source/measurement unit with 1 A load current capability An oscilloscope with >300 MHz bandwidth and a current probe with >1 A input range Getting Started The circuit does not require software support for evaluation. Connect the power supply, and the output is regulated based on the configuration. Brown, Marty. Power Supply Cookbook. ButterworthHeinemann, 1994. Lenk, John D. Simplified Design of Switching Power Supplies. Butterworth-Heinemann, 1995. Billings, Keith. Switchmode Power Supply Handbook. McGraw-Hill, 1989. Chryssis, George. High-Frequency Switching Power Supplies: Theory and Design. Second Edition, McGraw-Hill, 1989. Setup and Test Pressman, Abraham I. Switching Power Supply Design. McGraw-Hill, 1991. Connect the 5 V power supply to the primary side input connector (J4), and connect ground to J5. ADIsimPower Power Management Design Tool. Analog Devices, Inc. Connect the source meter to the secondary side, where J1 is the 5 V output, and J3 is the output ground. Data Sheets and Evaluation Boards Place a jumper on J15 to use the internal 1.225 V voltage reference of the ADuM3190. ADP1621 Data Sheet ADuM3190 Data Sheet REVISION HISTORY 11/14—Revision 0: Initial Version (Continued from first page) Circuits from the Lab reference designs are intended only for use with Analog Devices products and are the intellectual property of Analog Devices or its licensors. While you may use the Circuits from the Lab reference designs in the design of your product, no other license is granted by implication or otherwise under any patents or other intellectual property by application or use of the Circuits from the Lab reference designs. Information furnished by Analog Devices is believed to be accurate and reliable. However, Circuits from the Lab reference designs are supplied "as is" and without warranties of any kind, express, implied, or statutory including, but not limited to, any implied warranty of merchantability, noninfringement or fitness for a particular purpose and no responsibility is assumed by Analog Devices for their use, nor for any infringements of patents or other rights of third parties that may result from their use. Analog Devices reserves the right to change any Circuits from the Lab reference designs at any time without notice but is under no obligation to do so. ©2014 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. CN11763-0-11/14(0) Rev. 0 | Page 5 of 5