Modern Design Techniques in Switch Mode Power Supplies

by Kevin Ackerley, Future Electronics Vancouver, BC

After having spent a number of years as a Power Systems Designer in New Zealand and Canada, I caught up with a colleague in Vancouver, British Columbia to examine the design processes that are used today in large power systems (>1kW). In particular, we focused on the rectifier (AC-DC) SMPS (Switch Mode Power Supply) for the telecom industry.

Gueorgui Anguelov, Senior Design Engineer from Argus Technologies, sat down with me to discuss improvements in design methodology over the last 10 years. The actual chronological order of the design process may vary, as this type of power supply includes a multi-talented team of Research, Power, Hardware, Software, CAD, Systems and Mechanical Engineers.

 

Design Steps

The cost pressures in today’s designs come from dramatically falling $/W and are compounded by the four fold increase in energy densities from 5W per cubic inch to more than 22W/cin in today’s best designs.

The design for manufacture concepts are critical in specifying the mechanical form factors and short list of components to design with. As with any good Product Marketing department, their diligent research will determine the target market, feature set and price point that will support a particular market. Cost is the one overriding concern that must be focused on at all steps of the design process. The design engineer’s task is now to determine the cost of the power supply from their best design for manufacture techniques. Often, very innovative and creative approaches are developed in the design processes in the pursuit of greater efficiencies. There should be an award given for the lowest cost heat sink with the highest performance designed from the most original materials.

Given a mechanical form factor and power density requirement, the design engineer can determine what the optimal topology will be. Often full resonant converters are the preferred topology because of the cost/efficiency benefits in high power designs. However, resonant converters have the disadvantage of being impossible to electrically simulate as their transfer functions contain periodic functions that cannot be completely resolved to the component level and optimized.

Series resonant converters operated above the resonant frequency have inherent short circuit protection, zero-voltage commutations, limited harmonics in the resonant current and maximum power transfer at minimum switching frequency.

The first electrical design phase will be a mathematical simulation, testing stability of the power supplies’ transfer function. Digital code verification of the DSP (Digital Signal Processor) or microcontroller can also be carried out using imported compiled .dll files into the simulator. The electrical simulation will also provide and optimize the currents and voltages which can then be used to design and electrically model the magnetics. Parasitic voltages and currents can be added at this stage to improve the electrical model.

For magnetic design, 3D field distribution FEA (Finite Element Analysis) software is used. Ansoft’s PExprt is one example.

 

 

A basic proof of concept is then built to prove system stability over load and other environmental inputs. A full product prototype is subsequently built and tested on a network analyzer for stability.

The next electrical design process in design for manufacture is to thermally model the magnetics and all the other major components such as fans, heat sinks, diodes, transistors and other ICs. Solid Works is one tool that can be used to accurately model the mechanical form factor and perform thermal modeling and mechanical stress tests. Electrical component dimensions and thermal models can be imported from PCB design software such as Protel. These accurate thermal models within the actual power supply form factor will then determine if the design will meet the initial specifications and what design challenges need to be solved.

The PCB design software should also follow Creepage and Clearance rules, set up as several Net Classes, to ensure basic safety compliance. Avago Technologies provides opto-isolators with industry leading reliability to help meet isolation and safety compliance requirements.

 

 

Design Challenges

Some of the design challenges come from not being able to accurately thermal model every component such as capacitors. This is essential for reliability and operating in the correct voltage and frequency region after the capacitors have been thermally derated. Sometimes design engineers have to resort to modeling these devices themselves. A small resistor can be placed inside the capacitor by drilling a hole in the capacitor. Now, by applying a fixed wattage to the resistor and measuring the temperature difference across the capacitor with thermocouplers, the thermal resistance can then be calculated and the capacitor accurately thermally modeled over temperature.

If the design can support an E core transformer design with standard 3FE ferrite with a triple isolated Litz wire winding, then we will be approaching optimal cost vs. efficiency transformer design. If higher efficiency is required, then non-standard magnetics may be used such as core material with a higher permeability or a toroi core for higher coupling efficiency. However, production costs may increase.

With the constraints of fixed form factor and topology, some magnetics will inevitably be closely located to other magnetics on the PCB and cause significant inductive coupling and common mode noise. These parasitic voltages can swing from 0 to 400V in 20-30ns, which can generate 100’s fF across a resistor and form a voltage divider generating several volts! This common mode voltage noise can play havoc with control loops. High impedance inputs to ICs should be capacitive decoupled to ground and 1K resistors liberally used in line to reduce noise in the circuit.

EMI (Electromagnetic Interference) is another design challenge. More of a black art, EMI is difficult to design for, but inevitable when switching large currents at high frequencies in the presence of ground loops. Often when solving an EMI problem another one is created. Some engineers call this “Batting the Mole.” NEC-Tokin has EMI suppression and absorbent materials highly suitable for temporary or permanent production fixes.

 

Component Selection

In large power supplies, design engineers generally prefer to use discrete components when designing the control loops for their ruggedness, survivability and ability to multi-source these components even when integrated solutions may be more cost effective. However, improvements in PWM (Pulse Width Modulation) and PFC (Power Factor Controller) controllers’ quiescent current, increased ruggedness and EMI performance have made them an excellent choice for lower power designs. Vendors such as ST, IR, Fairchild Semiconductor, Infineon and ON Semiconductor, all have excellent PWM and PFC controller ICs.

Areas where design engineers review product improvements continuously are the rapidly changing fields of packaging technology, new improved materials and the design processes of transistors and diodes. An example of one of the improvements in packaging technology is the removal of internal wire bonds, enabling a more compact internal structure and thereby vastly improving the thermal performance. IR’s direct FET (Field Effect Transistor) is a good example. An example of material innovation is using Silicon Carbide in diodes. Silicon Carbide is a material that provides the benefit of having a very small reverse recovery characteristic when switched, thus improving efficiency with fewer switching losses but more importantly vastly simplifying the design of EMI filters in PFC applications. An example of process improvements are Infineon, IXYS (CoolMOS) and Fairchild Semiconductors (SuperFET) Trench technology that improves on-resistance and efficiency at voltages up to 800V.

 

 

Future Developments

We are seeing an industry change to move to fully digital control of feedback control loops, which can utilize both a DSP for digital control and a micro-controller for the supervisory and communication functions or a single DSP. Freescale and Microchip both have reference designs using DSPs with full digital control. Often an FPGA (Field Programmable Gate Array) is also used to buffer data between the microcontroller and DSP. Altera has excellent design resources offered through Future Electronics’ dedicated FPGA Advanced Engineers. A fully digitally controlled SMPS has many advantages over a mixed analog and microcontroller design, as this allows improved control algorithms in real time, improving efficiency and performance as well as enhanced protection of intellectual property.

The design challenges continue to be driven through cost constraints, through the continuously improving specifications from the silicon vendors, but also from the increasing cost of production. In the future we will start to see fully digital control systems with fewer discrete devices and higher levels of manufacturability focusing on high levels of automation.

 

Kevin Ackerley BE (Elec.) is a New Zealand born Canadian citizen with 6 years design of experience in Power Systems Engineering for Swichtec Power Systems Ltd. and Alpha Technologies Ltd. Kevin has also worked as a design engineer for dBA Telecom Ltd. and MacDonald Millar & Associates as a Consulting Engineer. For the past 7 years, Kevin has worked for Future Electronics as an Advanced Engineer (Application Engineer) specializing in Power and RF.