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LT3752/LT8311 48Vin, 24Vout Active Clamp Forward Converter with High Light Load Efficiency

Fei Guo - Apps Engineer
Gabino Alonso - Strategic Marketing Engineer
Feb 11th 2015

The LT3752 includes a number of programmable features that make it suitable for this application. For instance, programmable delays between various gate signals is used to prevent cross-conduction and to optimize efficiency, and programmable turn-on current spike blanking (adaptive leading edge blanking plus programmable extended blanking) of the main MOSFET greatly improves the converter’s noise immunity. The LT3752 also includes an internal constant frequency flyback controller for generating housekeeping supply that efficiently provides bias for both primary and secondary ICs, eliminating the need to generate bias supplies from auxiliary windings in the main forward transformer.

The LT8311 is used on the secondary side to provide synchronous MOSFET control and output voltage feedback through an opto-coupler. The LT8311 offers a full featured opto-coupler controller, incorporating a 1.5% reference, a transconductance error amplifier and a 10mA opto-driver. Power good monitoring and output soft-start/overshoot control are also included. The LT8311 can also operate in discontinuous mode (DCM) at light load to get better efficiency than forced continuous mode (FCM).

 

 48Vin, 24Vout Active Clamp Forward Converter with High Light Load Efficiency48Vin, 24Vout Active Clamp Forward Converter with High Light Load Efficiency

 

This application is one example of the use of LT3752 & LT8311 in a synchronous closed loop discontinuous conduction mode (DCM) design. It is optimized to get 90% efficiency at Vin = 48V Iout = 0.34A. At 2.5A load the efficiency is 93% and the board temperature is below 60ºC at room temperature.

 

Efficiency vs Load Current @48VinEfficiency vs Load Current  at 48Vin

 

Thermal Image at 48Vin, 24V @ 2.5A Output, Without Forced Air (Bottom Side)Thermal Image at 48Vin, 24V @ 2.5A Output, Without Forced Air (Bottom Side)

 

Thermal Image at 48Vin, 24V @ 2.5A Output, Without Forced Air (Top Side)Thermal Image at 48Vin, 24V @ 2.5A Output, Without Forced Air (Top Side)

 

Startup at 48Vin, Light Load 150mAStartup at 48Vin, Light Load 150mA

 

Startup at 48Vin, Full Load (2.5A)Startup at 48Vin, Full Load (2.5A)

 

Load Transient Response at 48Vin (1.25A-2.5A-1.25A)Load Transient Response at 48Vin (1.25A-2.5A-1.25A)

 

Output Voltage Ripple at 48Vin, Full LoadOutput Voltage Ripple at 48Vin, Full Load

 

The LT3752 are current mode PWM controllers optimized for an active clamp forward converter topology. A DC/DC housekeeping controller is included for improved efficiency and performance. The LT3752 allows operation up to 100V input and the LT3752-1 is optimized for applications with input voltages greater than 100V. A programmable volt-second clamp allows primary switch duty cycles above 50% for high switch, transformer and rectifier utilization. Active clamp control reduces switch voltage stress and increases efficiency. A synchronous output is available for controlling secondary side synchronous rectification.

The LT8311 is used on the secondary side of a forward converter to provide synchronous MOSFET control and output voltage feedback through an opto-coupler. The LT8311’s unique preactive mode allows control of the secondary-side MOSFETs without requiring a traditional pulse transformer for primary- to secondary-side communication. In preactive mode, the output inductor current operates in DCM at light load. If forced continuous mode (FCM) operation is desired at light load, the LT8311 can, alternatively, be used in SYNC mode, where a pulse transformer is required to send synchronous control signals from the primary-side IC to the LT8311.