DESIGN METHODOLOGY OF A HIGH POWER DENSITY

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Design of a 320W Telecoms DC/DC converter.

Introduction
This report describes the design of a 320W Telecoms DC/DC converter. A push-pull forward
converter running with current mode control using the LM5030 from National Semiconductor
delivers high performance and small solution size. Custom planar magnetics from Payton
have been used throughout to reduce component build height, increase efficiency and aid
cooling.
This report contains target specification, schematic, bill of materials, magnetics design
information as well as a detailed design analysis. A full set of performance measurements is
also included taken from the prototype unit shown in Figure 1. Measurements include
conversion efficiency, power stage device temperature rise, line/load regulation, start-up
behaviour, transient load response and loop gain/phase characteristic.

IR Thermography

The design of reliable power electronic converter systems depends
partly on an accurate knowledge of the power stage component
operating temperatures under extremes of load and input voltage.
This article demonstrates how the use of modern infra-red (IR)
imaging techniques can significantly enhance the design qualification
process and show up potential problems at a very early stage in
the product development cycle. The techniques highlighted are
applicable to any power converter system or electronic product
where knowledge of component operating temperatures is
important for reliability modelling and lifetime prediction.
By Dr. Iain Mosely, Technical Director, Converter Technology Ltd

One of the main limiting factors affecting the power capability and
reliability of any electronic power converter system is the operating
temperature of key power stage components. Excessive component
temperature will reduce product operating lifetimes and could result
in early field returns. Traditionally, thermocouples are used to
measure the operating temperature of components.
Whilst thermocouples can give very accurate temperature
measurements, they do have a few potential drawbacks:
• Thermocouples can pick up noise if they are placed near to power
components with high dv/dt switching waveforms present and this
can give misleading measurement results.
• Thermocouples will sink a small amount of heat away from the device
they are attached to. For physically small components, this can lead
to measurement inaccuracy.
• Thermocouples are often only placed on components which are
expected to show a reasonable temperature rise. Other components
may not be monitored at all and this could lead to problems if a
design error leads to a high operating temperature on a component
which hasn’t been monitored.

IR thermography is a non-contact measurement technique which
uses a calibrated infrared camera to form athermal image of the
system under test.
As the measurement technique is noncontact, the noise susceptibility
and heatsinking effects sometimes encounteredwith thermocouples
are no longer an issue. More importantly, an entire PCB can be imaged
which immediately shows up any hotspots or problem components that
may have otherwise been overlooked.

An example thermographic image ofa power converter is shown
in Figure 1.The power converter used has an issue with a snubber
TVS diode which can be seen to be running at >150°. Using the
thermal imaging camera is this example would immediately alert
the designer to a potential problem with the PSU before it reaches
the pre-production or production stage. The real value of thermography
in power electronic design is this ability to rapidly flag potential
design issues at a very early stage.

Infrared Imaging of Power Electronic Converters

Thermal management of electronic devices and packaging is important when dealing with high-voltage high-power systems as excessive device heating can have a catastrophic affect on the performance of a system and can lead to premature device and system failure. Therefore, it is necessary to accurately assess and characterize the thermal performance of power electronic components.
The use of thermal imaging equipment (infrared cameras) allows us to see beyond the visible into the invisible infrared. Our eyes are capable of detecting visible radiation but not infrared radiation. Infrared radiation is a form of electromagnetic radiation as are: visible light, radio waves, ultraviolet, and X-rays. Infrared radiation is longer in wavelength than visible light and is classified in a different waveband. Although few objects emit in the visible waveband, in the infrared waveband all objects emit. It is only that it cannot be seen with the physical eye. An infrared camera, therefore, becomes the "infrared eyes" into the infrared world. Infrared radiation is absorbed and emitted by objects. Absorptivity is the measure of how well an object or material absorbs radiation. Emissivity is the factor that correlates to the ability of an object to radiate infrared energy. In the way that visible light reflects off a mirror, infrared radiation reflects off many objects. For example, infrared radiation reflects clearly off metals such as aluminum. The fact that metals are good reflectors makes them poor emitters.

Manufacturing and process engineers boost efficiency and cut costs by turning to machine-vision systems like automated infrared (IR) imaging. The technique is being used in a host of industrial production applications, including process monitoring and control, quality assurance, asset management, and machine-condition monitoring.