Microchip Mcp1661 Isolated Flyback Converter Instructions

MCP1661 Isolated Flyback Converter
MCP1661
Isolated Flyback Converter
Reference Design
Instructions

MCP1661 Isolated Flyback Converter

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MCP1661 Isolated Flyback Converter Reference Design
NOTES:

NOTICE TO CUSTOMERS

All documentation becomes dated, and this manual is no exception. Microchip tools and documentation are constantly evolving to meet customer needs, so some actual  dialogs and/or tool descriptions may differ from those in this document. Please refer to our website (www.microchip.com) to obtain the latest documentation available.
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Select the Help menu, and then Topics to open a list of available online help files.

INTRODUCTION

This chapter contains general information that will be useful to know before using the MCP1661 Isolated Flyback Converter Reference Design. Items discussed in this
chapter include:

• Document Layout
• Conventions Used in this Guide
• Recommended Reading
• The Microchip WebSite
• Customer Support
• Document Revision History

DOCUMENT LAYOUT
This document describes how to use the MCP1661 Isolated Flyback Converter Reference Design as a development tool. The manual layout is as follows:

• Chapter 1. “Product Overview” – Important information about the MCP1661 Isolated Flyback Converter Reference Design.
• Chapter 2. “Installation and Operation” – Includes instructions on how to configure the board and important information about MCP1661 Isolated Flyback Converter and a description of the Reference Design.
• Appendix A. “Schematic and Layouts”– Shows the schematic and layout diagrams for MCP1661 Isolated Flyback Converter Reference Design.
• Appendix B. “Bill of Materials” – Lists the parts used to build the MCP1661 Isolated Flyback Converter Reference Design.

CONVENTIONS USED IN THIS GUIDE
This manual uses the following documentation conventions:
DOCUMENTATION CONVENTIONS

RECOMMENDED READING
This user’s guide describes how to use MCP1661 Isolated Flyback Converter Reference Design. Other useful documents are listed below. The following Microchip documents are available and recommended as supplemental reference resources.

• MCP1661 – “High-Voltage Integrated Switch PWM Boost Regulator with UVLO” (DS20005315)
• MCP1662 – “High-Voltage Step-Up LED Driver with UVLO and Open Load Protection” (DS20005316)

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Microchip provides online support via our web site at www.microchip.com. This web site is used as a means to make files and information easily available to customers.
Accessible by using your favorite Internet browser, the web site contains the following information:

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• Technical Support

Customers should contact their distributor, representative or field application engineer (FAE) for support. Local sales offices are also available to help customers. A listing of
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Technical support is available through the web site at: https://www.microchip.com/support
DOCUMENT REVISION HISTORY
Revision B (September 2022)

• Updated Figure 2-1 and Figure 2-2 in Chapter 2. “Installation and Operation”.
• Updated Schematic in Appendix A. “Schematic and Layouts”.
• Updated Appendix B. “Bill of Materials”
• Minor text and format changes throughout. Revision A (November 2014)
• Initial Release of this Document.

Chapter 1. Product Overview

1.1 INTRODUCTION
This chapter provides an overview of the MCP1661 Isolated Flyback Converter Reference Design and covers the following topics:

• MCP1661 Device Short Overview
• Flyback Converter Topology Overview
• What is The MCP1661 Isolated Flyback Converter Reference Design?
• What does The MCP1661 Isolated Flyback Converter Reference Design Kit include?

1.2 MCP1661 DEVICE SHORT OVERVIEW
MCP1661 is a constant Pulse-Width Modulation (PWM) frequency boost (step-up) converter (see Figure 1-1), based on a Peak Current mode architecture which delivers high efficiency over a wide load range from two-cell and three-cell Alkaline, Energizer ®  Ultimate Lithium, NiMH, NiCd and single-cell Li-Ion battery inputs. A high level of
integration lowers total system cost, eases implementation and reduces board area.
1.2.1 MCP1661 Key Features

• 36V, 800mΩ Integrated Switch
• Up to 92% Efficiency
• High Output Voltage Range: up to 32V
• 1.3A Peak Input Current Limit:
  -IOUT > 200 mA @ 5V VIN, 12V VOUT
  -IOUT > 125 mA @ 3.3V VIN, 12V VOUT
  -IOUT > 100 mA @ 4.2V VIN, 24V VOUT
• Input Voltage Range: 2.4V to 5.5V
• Undervoltage Lockout (UVLO):
  -UVLO@VIN Rising: 2.3V, typical
  -UVLO@VIN Falling: 1.85V, typical
• No Load Input Current: 250 μA, typical
• Sleep Mode with 200 nA Typical Quiescent Current
• PWM Operation with Skip Mode: 500 kHz
• Cycle-by-Cycle Current Limiting
• Internal Compensation
• Inrush Current Limiting and Internal Soft-Start
• Output Overvoltage Protection (OVP) in the event of:
  – Feedback pin shorted to GND
  – Disconnected feedback divider
• Overtemperature Protection
• Easy Configurable for SEPIC or Flyback Topologies
• Available Packages:
  – 5-Lead SOT-23
  – 2 mm x 3 mm 8-Lead TDFN

1.3 FLYBACK CONVERTER TOPOLOGY OVERVIEW
The flyback converter is used in both AC/DC and DC/DC conversion having galvanic isolation between the input and one or more outputs. This type of converter is a derivation from a buck-boost converter with a transformer replacing the inductor, so that the voltage ratios are multiplied.
Being an isolated power converter, the control circuit needs to be isolated as well. There are two control types used for this converter: Voltage mode control and Current mode control; both require a signal related to the output voltage. This can be achieved either by using an optocoupler on the secondary circuitry to send a signal to the controller or by using a separate winding on the coil and rely on the cross regulation of the design.
The first approach involving an optocoupler is used to obtain very good voltage and current regulation, whereas the second was developed for cost-sensitive applications  where the output does not need to be as precisely controlled, but simplifies the overall design considerably. In applications where reliability is critical, optocouplers should be
avoided.
In this application, a simpler technique (explained in the following chapters) was used, but its main drawback is that the voltage regulation is poor. Therefore, in order to overcome this and provide smooth regulation, an LDO was added at the isolated output of the flyback converter.
1.3.1 Flyback Converter Working Principle
The schematic of a flyback converter is depicted in Figure 2-1; it derives from the buck-boost topology, but utilizes a transformer instead of the inductor. A very important
aspect is that flyback transformers have an air gap which allows energy storing without risking the occurrence of core saturation. Therefore, the operating principle of both
converters is very close:

• When the switch is closed (Figure 1-2, a), the primary winding of the transformer is connected to the input voltage source. The primary current and magnetic flux in the transformer increases, storing energy in the transformer’s core. The voltage induced in the secondary winding is negative, so the diode is reverse-biased. In this phase, the output capacitor supplies energy to the output load (LDO’s input, in this application).
• When the switch is opened (Figure 1-2, b), the primary current and magnetic flux drops. The secondary voltage is positive, forward-biasing the diode, allowing current to flow from the transformer to the capacitor and to the load.

1.4 WHAT IS THE MCP1661 ISOLATED FLYBACK CONVERTER REFERENCE DESIGN?
The MCP1661 Isolated Flyback Converter Reference Design is used to evaluate and demonstrate Microchip Technology’s MCP1661 in the following topology:

• 5V output Isolated Flyback Converter application supplied from 5V typical input voltage.

It is used to evaluate the 5-Lead SOT-23 package.
By changing the LDO, a lower/higher output voltage than 5V will be obtained, but with different capabilities regarding maximum output current and efficiency.
1.5 WHAT DOES THE MCP1661 ISOLATED FLYBACK CONVERTER REFERENCE DESIGN KIT INCLUDE?
This MCP1661 Isolated Flyback Converter Reference Design kit includes:

• MCP1661 Isolated Flyback Converter Reference Design (ARD00598)
• Important Information Sheet

Chapter 2. Installation and Operation

2.1 INTRODUCTION
MCP1661 device is a non-synchronous, fixed-frequency step-up DC-DC converter which has been developed for applications that require higher output voltage capabilities. MCP1661 can regulate the output voltage up to 32V and can deliver 125 mA typical load at 3.3V input and 12V output. At light loads, MCP1661 skips pulses in order to keep the output voltage in regulation, but the voltage ripple is maintained low. The regulated output voltage should be greater than the input voltage.
2.1.1 Board Features
The MCP1661 Flyback Converter has the following features:

• Input Voltage: 4.25V – 5.25V, Typical
  – USB standard input voltage range
• Output Capability:
  – Over 200 mA (at VOUT = 5V)
  – Galvanic isolation
  – Short-circuit protection
• Efficiency: up to 75%
• PWM Operation at 500 kHz

This application uses MCP1661 as an open-loop flyback converter, the primary winding of the transformer being used as inductor for the boost converter that clamps the primary output voltage (VOUTP) at around 13.5V. It is very important (for a normal operation of the entire circuitry and to avoid damaging some electronic components)
not to connect any additional load between VOUTP and GND. The output voltage of the flyback converter (VOUTS) drops with the increasing of output current, due to the fact that the feedback is taken from the primary side.
In order to achieve a very good output voltage regulation in the secondary side (VOUT), a 5V LDO is placed after the rectifying diode of the flyback converter, therefore the decrease of VOUTS when increasing the load is not critical.
The MCP1661 Isolated Flyback Converter Reference Design can be used for USB-powered applications, where a positive, regulated 5V output voltage is needed from an isolated input voltage that varies from 4.75V to 5.25V.
2.1.2 How Does the MCP1661 Isolated Flyback Converter Reference Design Work?
The converter is configured as non-synchronous; an external diode (D2) is connected between the inductor (primary winding of the transformer) and the high-voltage output (VOUTP). The chosen transformation ratio was 1:1, because the difference between the input voltage range (VIN) and the output voltage (VOUT) is small.
The output voltage of the flyback converter (VOUTS) decreases by increasing the load current, due to the lack of feedback from the secondary side of the transformer. The
amount of voltage drop (VOUTS) on the entire range of loads can be controlled by changing the load resistor RL. A higher dummy load for the primary side of the flyback
converter corresponds to a lower voltage drop in the secondary side (VOUTS) over the entire ourput current range, but the overall efficiency of the converter will decrease.
There is a compromise between the maximum output current capabilities, input voltage range and efficiency, by varying the values of the load resistor (RL) and feedback resistors (RT and RB). In this case, the values of the aforementioned electronic components were chosen in order to achieve good efficiency at 200 mA load current, up to 5.25V input voltage. The two sense resistors (RT and RB) set the output (VOUTP) at 13.5V, according to the following equation:
EQUATION 2-1: FEEDBACK RESISTORS RELATIONSHIPWhen designing such an application, special attention should be given to the values of the feedback resistors. When testing the board for a different output voltage, a potential issue related to the usage of higher value resistors might be caused by the environmental contamination, which can create a leakage current path on the Printed Circuit Board (PCB); this will affect the feedback voltage and the output voltage regulation. Engineers should utilize with precaution resistors that are larger than 1 MΩ.
In normal humidity conditions, the VFB pin input leakage current is very low and the resistors’ values will not affect the stability of the system.
All compensation and protection circuitry is integrated to minimize the number of external components. Ceramic input and output capacitors should be used.
Good efficiency is obtained at high load currents due to the decrease of the output voltage before the LDO (VOUTS).
2.2 GETTING STARTED
The MCP1661 Flyback Converter Reference Design is fully assembled and tested to evaluate and demonstrate the MCP1661 family of products.
2.2.1 Powering the MCP1661 Isolated Flyback Converter Reference Design
Input power connectors are placed on the left side of the board:

• VIN for positive power
• GND for negative power

The maximum input voltage should not exceed 5.5V; this can cause damage to the MCP1661.
The output connector is called VOUT; it is referenced to SGND and isolated from GND.
2.2.2 Board Testing
The variable power supply for testing requires output capability of at least 1A and a voltage range between 4V and 6V.
To test the board, follow next steps:

1. Connect the power supply at VIN and GND terminals of the board.
2. Set the power supply to 5V DC.
3. Connect a voltmeter and a 100Ω/1W resistor between VOUT and SGND connectors, as shown in Figure 2-2. Check the voltmeter to make sure it indicates approximately 5V.
4. Set the power supply to 4.75V and verify with the voltmeter if the output of the converter remains regulated (VOUT = 5V).
5. Set the power supply to 5.25V and verify with the voltmeter if the output of the converter remains regulated (VOUT = 5V).

The board has several test points that help engineers analyze the switch node’s waveforms or MCP1661’s output:

• The test point of the MCP1661 device’s switch node (SW).
• VOUTP test point shows the MCP1661 boost’s output voltage (this output is regulated).
• VOUTS test point shows the MCP1661 flyback’s output voltage (this output is unregulated and is referenced to SGND).

The regulated output voltage of the boost converter (VOUTP) is about 13.5V and is referenced to GND.
2.2.3 Results
MCP1661 Isolated Flyback Converter uses an uncommon design, because the feedback voltage is taken from the primary side, so the output voltage in the secondary side (VOUTS) drops down as long as the load current increases (see Figure 2-3). However, the overall efficiency is still high, even if the LDO wastes some energy in order to  keep the output voltage (VOUT) stable at 5V.

Refer to Figure 2-4 for the efficiency that can be obtained for different input voltages.
Figures 2-5 and 2-6 show the Discontinuous (at no load, 5V VIN) and Continuous Conduction Mode waveforms (50 mA load at 5V input voltage).Figure 2-7 shows the start-up waveforms for MCP1661 Isolated Flyback Converter at 150 mA load current.

Appendix A. Schematic and Layouts

A.1 INTRODUCTION
This appendix contains the following schematics and layouts for the MCP1661 Isolated Flyback Converter Reference Design:

• Board – Schematic
• Board – Top Silk
• Board – Top Copper and Silk
• Board – Top Copper
• Board – Bottom Copper

Schematic and Layouts

A.2BOARD – SCHEMATIC

A.3 BOARD – TOP SILKA.4 BOARD – TOP COPPER AND SILKA.5 BOARD – TOP COPPERA.6 BOARD – BOTTOM COPPER

Appendix B. Bill of Materials

TABLE B-1: BILL OF MATERIALS (BOM)

Note 1: The components listed in this Bill of Materials are representative of the PCB assembly. The released BOM used in manufacturing uses all RoHS-compliant components.
TABLE B-2: BILL OF MATERIALS (BOM) – MICROCHIP PARTS

Note 1: The components listed in this Bill of Materials are representative of the PCB assembly. The released BOM used in manufacturing uses all RoHS-compliant components.
TABLE B-3: BILL OF MATERIALS (BOM) – DO NOT POPULATE PARTS

Note 1: The components listed in this Bill of Materials are representative of the PCB assembly. The released BOM used in manufacturing uses all RoHS-compliant components.

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