Isolated discrete forward DC/DC design: Page 2 of 5

June 24, 2016 // By Bruce Haug
Isolated discrete forward DC/DC design
High density isolated DC/DC converters have significantly changed during the course of the past 25 years. The introduction of the full and half-brick form factors back then created a rush to use them in distributed power architectures in telecom, datacom, industrial and medical systems where a bus voltage is routed to every board within a system and each board had its own isolated DC/DC converter.

Isolated outputs are required for a broad range of DC/DC converter applications - not just the telecom and datacom mandated 48V isolation requirements. Isolation can be necessary for noise sensitive devices needing ground separation from a noisy input voltage, such as a car battery, intermediate bus and industrial inputs. Displays, programmable logic controllers, GPS systems and some medical monitoring devices can all be negatively affected by a noisy bus voltage. Examination cameras, dental instruments, sleep and vital sign monitors all use displays that can be adversely affected by a noisy source voltage. Nevertheless, an isolated power supply provides ground separation which can eliminate the noise causing display irregularities.

Linear Technology has a full line of topology specific controllers that can be used in isolated high-density DC/DC converters for flyback, forward, push-pull, full-bridge topologies. Some versions exist with or without synchronous rectification, most without optocouplers, some with a signal transformer to close the feedback loop and others sensing the output voltage from the primary side transformer winding.  They consist of teo two fundamental input voltage ranges, one for the Industrial market (9-32V) and the other for the Telecom/Datacom (36-75V) market, along with some that operate over an 18V to 75V input voltage range. Linear Technology has pre-fabricated designs for all of these topologies and input voltage and outputs voltages ranging from 1.2V to 48V. Users can have access to demonstration boards, electrical schematics, and bill of materials and Gerbers files for each design. A quick start guide provides performance curves, including load/line regulation, ripple & noise, efficiency and transient response.

Isolated forward converters

One of the most popular topology’s used in isolated high-density DC/DC converters is the forward converter. Linear Technology offers several types of forward controllers, some of which operate with a single primary ICs for control. Others have both a primary and secondary IC’s for control, timing and the driving of synchronous MOSFETs. Buck converter designers have long benefited from their simplicity, high efficiency and fast transient response; made possible by the latest controller ICs featuring synchronous rectification and multiphase interleaved power stages. However, these same features are now available in forward converters and the recently released LT8310 is an example of a device that can be used in a low parts count application. In fact, the LT8310 schematic below in figure 1, uses only 20 parts to complete an isolated forward converter with an output power of up to 78 watts.

Fig. 1: LT8310 Isolated forward delivers 78W output with only 20 parts.

This circuit produces a 12V output at up to 6.5A from a nominal 48V input with efficiencies as high as 92%. This primary-side forward controller, with resonant reset operates, over an input voltage range of 6V to 100V and is targeted for power levels up to 200W.

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