MAXREFDES1013: 12W offline flyback converter

The MAX17690 is a peak current mode, fixed-frequency switching controller specifically designed for the isolated flyback topology operating in Discontinuous Conduction Mode (DCM). The device senses the isolated output voltage directly from the primary-side flyback waveform during the off-time of the primary switch. No auxiliary winding or optocoupler is required for output voltage regulation. 

The MAX17690 is designed to operate over a wide supply range from 4.5V to 60V. The switching frequency is programmable from 50kHz to 250kHz. The MAX17690 provides an input overvoltage protection through the OVI pin. The 7V internal LDO output of the MAX17690 makes it suitable for switching both logic-level and standard MOSFETs used in flyback converters. With 2A/4A source/ sink currents, the MAX17690 is ideal for driving low RDS(ON) power MOSFETs with fast gate transition times. The MAX17690 provides an adjustable soft-start feature to limit the inrush current during startup. 

The MAX17690 provides temperature compensation for the output diode forward-voltage drop. The MAX17690 has robust hiccup-protection and thermal protection schemes, and is available in a space-saving 3mm x 3mm 16-pin TQFN package with a temperature range from -40℃ to +125℃. 

Key Features

• Programmable Switching Frequency Allows Optimization of the Magnetic and Filter Components, Resulting in Compact, Cost-Effective, Efficient Isolated/ Nonisolated Power Supplies 
• 100kHz to 1MHz Programmable Switching Frequency with Optional Synchronization 
• Peak Current Mode Control Provides Excellent Transient Response—Offline (Universal Input AC) and Telecom (36V to 72V) Flyback Controller 
• Programmable Frequency Dithering Enables Low EMI Spread-Spectrum Operation 
• Integrated Protection Features Enhance System Reliability 
• Adjustable Current Limit with External Current Sense Resistor 
• Fast Cycle-By-Cycle Peak Current Limiting Hiccup-Mode Short-Circuit Protection 
• Overtemperature Protection 
• Programmable Soft-Start and Slope Compensation 
• Input Overvoltage Protection

This board is not available for purchase.

An offline DCM flyback converter using the MAX17595 is demonstrated for a 24V DC output application. The power supply delivers up to 0.5A at 24V.

Designed, Built, Tested

This document describes the hardware shown in Figure 1. It provides a detailed systematic technical guide to designing a synchronous buck using Maxim’s MAX15112 current-mode controller. The power supply has been built and tested, details of which follow later in this document.

Design Resources