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How isolated forward DC/DC converters can be created using a reduced parts count

High density isolated DC/DC power converters have changed significantly over the past 25 years. When full brick and half brick devices were introduced, they created a huge amount of excitement; they integrated several hundred parts, making it much easier to use them than to design your own.

Used in telecom applications, where the –48V inputs need isolation from the input to output due to the high amount of power available from the bus voltage, these converters helped to popularise the distributed power architecture in industrial systems. Today, these devices are available in 1/4, 1/8 and 1/16 brick formats.

Nevertheless, application topology specific DC/DC controllers, along with off the shelf planar power transformers and inductors, have made it simpler to produce unique designs. In fact, an isolated flyback converter can be created using fewer than 15 parts and a forward converter with fewer than 20 parts. This new era has provided designers with a different way to develop isolated DC/DC converters.

Isolated outputs are required for many DC/DC converter applications; it can be necessary for noise sensitive devices needing ground separation from a noisy input voltage, such as an intermediate bus and industrial inputs. Displays and some medical monitoring devices can all be affected by a noisy bus voltage. An isolated power supply provides ground separation that can eliminate noise which can cause display irregularities.

Linear Technology has a range of topology specific controllers that can be used in isolated high density DC/DC converters for flyback, forward, push-pull, full-bridge topologies.
Versions exist with or without synchronous rectification, some have optocouplers, while others use a transformer to close the feedback loop.

There are two fundamental input voltage ranges: 9 to 32V for the industrial market; and 36 to 75V for telecom/datacom applications. Some parts operate from an input ranging from 18 to 75V. Designs have been prefabricated for these topologies and for output voltages ranging from 1.2 to 48V.

One of the most popular topologies for isolated high density DC/DC converters is the forward converter and Linear can offer single and dual switch forward controllers which operate with primary and secondary side ICs to control, time and drive synchronous MOSFETs.

Buck converter designers have long benefited from the high efficiency and fast transient response enabled by the latest controller ICs. These 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 (see fig 1).



The circuit in fig 1 produces a 12V output at up to 6.5A from a nominal 48V input with an efficiency as high as 92%. This primary side forward controller with resonant reset operates from an input ranging from 6 to 100V and is targeted for power levels up to 200W and in synchronous and non synchronous applications.

For synchronous operation, the LT8310 sends a control signal via a pulse transformer to a secondary side MOSFET driver for synchronous rectification timing. Synchronous designs are most advantageous for higher power or lower output voltage applications. An output voltage regulation of ±8% can be attained without the use of an optocoupler.

When an optocoupler is used, ±1.5% regulation can be attained. A programmable volt-second clamp provides a safeguard for transformer reset, preventing saturation and protecting the MOSFET. This allows for the transformer and MOSFET to be optimised for size and cost.

In applications with output voltage feedback (opto isolated or directly wired), the volt-second clamp is set higher than the natural duty cycle of the converter to provide a duty cycle guardrail that prevents transformer saturation during load transients. For non isolated applications, the LT8310 contains a voltage error amplifier and feedback pin with both positive and negative references. This enables simple fully regulated forward converters with positive or negative output voltages.

The LT8310 has a programmable switching frequency ranging from 100kHz to 500kHz and can be synchronised to an external clock.

Flyback design
For a simpler isolated DC/DC converter solution at lower power levels, the flyback topology can be used. Flyback converters have been used in isolated DC/DC applications for many years, but are not always the designer's first choice: flyback converters are usually selected out of necessity, rather than for ease of design.

The flyback converter has stability issues due to the right half plane zero in the control loop, which is complicated by the ageing and gain variation of an optocoupler. It requires a significant amount of time to be devoted to design of the transformer, complicated by the possibility that a custom transformer may be required.

However, recent advances in power conversion technology have made lower power isolated converters easier to design.

Linear's LT8302 isolated flyback converter eliminates the need for an optocoupler, a secondary side reference voltage and a third winding off the power transformer, whilst maintaining isolation between the primary and secondary side. Only one part, the power transformer, crosses the isolation barrier.

It uses a primary side sensing scheme, which can sense the output voltage through the flyback primary-side switching node waveform. During the switch off period, the output diode delivers the current to the output and the output voltage is reflected to the primary side of the flyback transformer. The magnitude of the switch node voltage is the summation of the input voltage and reflected output voltage, which the LT8302 can reconstruct. This feedback technique results in total regulation of better than ±5% over the full line, load and temperature range. Fig 2 shows a flyback converter schematic which uses only 14 external components.



Transformer selection and design
Transformer specification and design is probably the most critical part of applying the LT8302 successfully. In addition to the usual requirements for low leakage inductance and close coupling, the transformer turns ratio must be tightly controlled. Since the secondary side voltage is inferred by the voltage sampled on the primary side, the turns ratio must be tightly controlled to ensure a consistent output voltage.

Linear has worked with leading magnetic component manufacturers to produce predesigned flyback transformers for use with the LT8302. These transformers typically withstand a 1500V AC breakdown voltage for 1minute from the primary to secondary, but higher breakdown voltages and custom transformers can also be applied.

Conclusion
The widely available application specific control ICs have allowed companies to design their own high density DC/DC converters without the long development cycles typical of just a few years ago. Companies designing their own isolated power converters, allow them complete control over the design process and component selection, enabling them to add specific functions that may not be available in standard products and oftentimes allows them to realise a lower overall cost.

With the advent of topology specific controllers, off the shelf planar magnetics, application specific circuits, demonstration boards and associated Gerber files, users are provided with a complete line up of power conversion designs, making it easier to design and develop their own isolated DC/DC converters.

Bruce Haug is a senior product marketing engineer with Linear Technology.

Author
Bruce Haug

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