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 todays ADuM3190 2.5 kV rms Isolated Error Amplifier analog, mixed-signal, and RF design challenges. For more Constant-Frequency, Current-Mode information and/or support, visit www.analog.com/CN0342. ADP1621 Step-Up DC-to-DC Controller Flyback Power Supply Using a High Stability Isolated Error Amplifier The entire circuit operates from 5 V to 24 V, allowing it to be EVALUATION AND DESIGN SUPPORT used with standard industrial and automotive power supplies. Circuit Evaluation Board The output capability of the circuit is up to 1 A with a 5 V input CN-0342 Circuit Evaluation Board (EVAL-CN0342-EB1Z) and 5 V output configuration. 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 CIRCUIT FUNCTION AND BENEFITS with good efficiency and a small form factor. Examples include The circuit shown in Figure 1 is an isolated, flyback power 10 W to 20 W telecommunication and server power supplies, supply that uses a linear isolated error amplifier to supply the where power efficiency and printed circuit board (PCB) density feedback signal from the secondary side to the primary side. are important, and 48 V supplies are common. 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. V V IN 5V TO 24V OUT D1 T1 5V C26 C25 C14 C23 C24 C3 R4 R12 C21 R19 820 220nF 390 100k 47F 47F 100F 47F 47F 100F Q2 R3 51k R5 D2 SDSN IN 82 R2 GND CS 47k COMP PIN FB GATE Q3 R1 R18 FREQ PGND R16 2k 0 0 R20 ADP1621 0 R6 100k C17 V V DD1 DD2 0.1F REF OUT ADuM3190 REF +IN EA OUT IN C10 1nF COMP GND GND 1 2 R19 C9 15k 2.2nF 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 One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. determining its suitability and applicability for your use and application. Accordingly, in no event shall Tel: 781.329.4700 www.analog.com Analog Devices be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) Fax: 781.461.3113 2014 Analog Devices, Inc. All rights reserved. 11763-001CN-0342 Circuit Note The average transformer primary side current, I , is given by CIRCUIT DESCRIPTION LAVG The isolation amplifier is the ADuM3190, which includes a I N LOAD S I = LAVG 1.225 V voltage reference and an error amplifier with 10 MHz 1 D N P unity-gain bandwidth product. An external resistive divider where: (R1, R2, R3, and R4) and compensation network (R9, C9, and I is the load current. LOAD C10) complete the analog feedback loop. D is the duty cycle under maximum load current. The input supply range of the ADuM3190 is 3.0 V to 20 V on NS/NP is the turn ratio of the transformer. both sides, and internal low dropout regulators provide stable The output voltage is supplies for the voltage reference, error amplifier, and analog isolator. The ADuM3190 is compatible with the Distributed- N P V (1 D) = V D OUT IN power Open Standards Alliance (DOSA) output voltage trim N S method. The duty cycle (D) is 50% because the output and input are both The ADP1621 provides pulse-width modulation (PWM) 5 V, and the transformer turn ratio is 1:1. control for the flyback power supply. The internal 5.5 V shunt The peak-to-peak primary inductor ripple current is inversely regulator provides the capability of high supply input voltage proportional to the inductor value. with the addition of an external NPN transistor (Q2). The ADP1621 also provides lossless current sensing for current V D IN I = L mode operation, providing excellent line and load transient f L SW response. where: Setting the Output Voltage f is the switching frequency. SW The output voltage is set through a voltage divider from VOUT to L is the primary inductor value. the IN pin of ADuM3190. The feedback resistor ratio sets the Assuming continuous conduction mode (CCM) operation, the output voltage of the system. Using the internal voltage reference in primary inductor current is given by the ADuM3190, the regulation voltage at the IN pin is 1.225 V. V D I N IN LOAD S I = + R4 + R3 LPK 1 D N 2 f L P SW V =1.225 V 1 + OUT R1 + R2 Assuming the primary ripple current is 50% of the average current For a 5 V output configuration, the resistor divider values are on the transformer primary side, a reasonable choice for the R1 = 2 k , R2 = 47 k , R3 = 51 k, and R4 = 100 k. inductor value is Voltage Reference V D (1 D) 5 V 0.5 (1 0.5) N IN P L = = = 12.5 H 0.5 f I N 0.5 200 kHz 1 A The ADuM3190 provides an internal 1.225 V voltage reference SW LOAD S specified for 1% accuracy over the 40C to +125C temperature The transformer used in this design is a 1:1 turn ratio transformer range. The reference voltage output pin (REF ) can be connected OUT with 16 H primary side inductance (Halo Electronics, Inc., to the +IN pin of the error amplifier to set the output voltage. TGB01-P099EP13LF). When higher accuracy or a special output voltage is required, and Compensation Network a different reference voltage must be used, the +IN pin can also be connected to an external reference In a flyback topology power supply, the output load resistance, the output capacitor, and its effective series resistance (ESR) add a zero Transformer Selection and a pole at frequencies that are dependent on the component The transformer choice is important because it dictates the type and values. There is also a right half plane (RHP) zero in primary inductor current ripple. the control-to-output transfer function. Because the RHP zero For this design example, the following parameters were used: reduces the phase by 90, the frequency of 0 dB gain (crossover frequency) is lower than the RHP zero. VIN = 5 V With the ADuM3190 providing the error amplifier, a Type II VOUT = 5 V compensation network can be provided from the IN pin to the IOUTMAX = 1 A COMP pin to compensate for the control loop for stability. The fSW = 200 kHz compensation network values depend on the selected components. Transformer turn ratio = 1:1 Rev. 0 Page 2 of 5