Modern electronic systems require an ever increasing number of different system Dc voltage rails to operate. These complex power requirements combined with a drive towards more energy efficient products and ever shorter product design cycles can become a burden to companies who would rather focus resource on developing the key differentiating technology of their new product.
This article discusses some of the high level power architecture choices which are useful to consider at an early stage in the product design cycle. Choice of high level architecture has a signifi cant impact on the cost, efficiency and type of power topologies used to provide each of the required power rails.
System Approach
The power supply system for any product generally has a single energy source and requires many regulated DC outputs. The energy source depends on the product type but could be mains power, battery supply, telecoms DC input or a renewable energy source.
The power subsystem must normally provide a number of key functions:-
• Protect safely against abnormal conditions (faults in the product electronics or problems with the primary energy source)
• Provided regulated voltages to the product electronics independent of changes in the primary energy source or output load demand
• In many cases, provide galvanic isolation between the primary energy source and the product electronics
• Control the power on/off sequence of the DC power rails as required by the product electronics
• Provide all the above functionality whilst wasting the minimum amount of energy
In general the most energy efficient way to provide each DC output is with a single power converter stage for each power rail. In this case, the energy provided to each output passes through only a single power stage and suffers only one stage of conversion loss. In the case of mains power systems, often this is not commercially viable since isolated power stages are generally more expensive than non-isolated converters. One alternative option is to use an intermediate bus architecture (IBA) whereby a front end high efficiency isolated converter steps the input power down to a safe low voltage which then powers secondary side power converters. The IBA approach allows the majority of the system power converters to use non-isolated topologies which saves significant cost. The only downside of this approach is that the energy delivered on most of the output rails will experience power loss in two converter stages rather than one. In the case of the IBA shown in Figure 2, all the energy delivered on rails #2, #3 and #4 experiences loss in the front end converter PSU#1 as well as each respective secondary converter.
Since the IBA approach is often chosen to minimise cost, there are a number of guidelines which can be followed to maximise the IBA overall system efficiency:-
1. Choose an intermediate bus voltage equal to the DC output rail with the highest power requirement. This ensures that the largest possible share of the total system power flows through a single converter stage.
2. Where possible try to keep the intermediate bus voltage just slightly higher than the majority of the required DC output voltages. This allows for the most efficient conversion of the intermediate bus voltage down to the DC output voltages.
3. DC outputs with a very low load compared to the total system load will have minimal impact on the overall system efficiency. Rails with higher power requirements should be used to justify a particular intermediate voltage.
When considering the commercial implications of intermediate bus voltage choice, it is good to remember that the secondary side converters with a high input voltage capability are likely to be more expensive and have poorer efficiency than those where input voltage range is low. Conversely, the conversion efficiency of the front end mains power converter tends to improve with higher intermediate voltages. The best intermediate voltage choice is often driven by a combination of commercial and technical factors which are unique to each product. For many products, a good choice for system DC voltage rails of around +5V or lower is an intermediate bus voltage of around 12V. The +12V output mains power converter often represents a good compromise between commercial and technical constraints for output power levels up to around 60W.
Once the intermediate bus voltage has been chosen, the power topologies for the main isolated converter and secondary converters can be chosen. For a 60W total power capability with 12V intermediate bus voltage, a flyback converter can give good front end performance with conversion efficiency of 85 to 90%. The topologies used for the downstream converters depend on the output voltages and power levels required.
Conclusions
The complex power requirements of modern electronic products can present a significant challenge to designers who wish to maximise their new product energy efficiency. Commercially, the IBA architecture is often the best system level approach but the choice of bus voltage and secondary side power converter topologies then has a significant impact on product energy use. Key to minimising development risk is to use a flexible modular approach to system power design whereby the power system can be easily modified during the design process to optimise system power use. Often, power consumption measurements are just performed on the primary energy source (e.g. mains input power) but this can hide which parts of a product subsystem are really dominating power consumption.
Wouldn’t it be great at design time if we could understand the product power use at a much deeper level?
Article by Dr. Iain Mosely, Technical Director of Zonetech.
www.zonetech.com
Zonetech products will shortly be introduced to the RS stocked product range. Find them first at rswww.com/electronics
Article from eTech issue 5
