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Dealing with SMPS efficiency, thermal behaviour

Posted: 20 Aug 2013 ?? ?Print Version ?Bookmark and Share

Keywords:datasheets? SPICE? MOSFETs? SMPS? Multisim?

When defining the specification of their circuit design, analogue designers depend on datasheets provided by component manufacturers. Although datasheets include the specifications of individual components and remain an indispensable resource for the designer, yet they lack some information of how parts will behave within different design configurations. This is why circuit simulation is a good complementing design tool to datasheets that provides such insight.

Semiconductor manufacturers work on providing SPICE models within different simulation environments to facilitate the simulation task for their customers. SPICE technology is commonly used to model discrete components such as BJTs, Op-Amps, and MOSFETs. However, integrated systems that require a higher level of sophistication are getting more and more accurately modelled. Integrated power converters and switching controllers are examples of these parts where the capabilities of SPICE simulation could be leveraged for the evaluation of system performance parameters such as power efficiency, conduction and switching losses, and overall thermal behaviour.

The example presented here addresses Switch Mode Power Supplies (SMPS) modelling and simulation, an area where manufacturers have been focusing a lot of their modelling efforts and where SPICE is capable of yielding accurate results. Simulation in the Multisim SPICE environment of the new low VCEsat BJTs by NXP, used for medium power switching converter applications, is chosen as a use-case highlighting different performance attributes. The design of the BJT-based converter shows how to properly drive it and meet ripple and efficiency requirements, while determining power dissipation of the switching elements helped better understand the contributors to the losses and the implications on the thermal design.

This suggests an overall effective approach of designing SMPS by relying on advanced modelling and simulation capabilities. The resulting outcome is getting to an optimised design before leaving the desktop simulation stage of the design flow.

Medium power DC-DC buck converter
This example demonstrates how designers could benefit from the NXP simulation models in the Multisim environment designing the power stage of a DC-to-DC buck converter. The simulation helps in taking accurate design decisions that directly impact the circuit performance such as:
???Choosing capacitor and inductor values to meet ripple requirements
???Choosing gate driver components to ensure that the BJT is driven optimally
???Calculating the efficiency of the converter
???Calculating power dissipation to gain insight about the thermal behaviour
???Design specifications

Design specifications and constraints are defined to provide explicit information about the requirements for the circuit and how the circuit topology is to be put together. The design needs to meet the following conditions and design requirements:

Such an application is suitable for the low VCEsat products by NXP because it is optimised for high-speed switching. The high and constant current gain, the low saturation voltage and the good switching performance of NXP BISS-4 transistors allow the use of a bipolar transistor for the described circuit of medium-power DC-to-DC converter instead of a P-channel MOSFET. The BISS transistor proposed in the example is optimised for minimised switching times. The storage and switching times are much reduced compared to types optimised for an ultra-low VCEsat.

Circuit set-up
As a first pass, based solely on the operating voltage and current (10V, 1.65A), we chose the PBSS4032PD PNP transistor (30V, 2.7A) as the main switch and the PMEG3020BEP Schottky diode (30V, 2A) as the freewheeling diode.

Figure 1: Global design parameters.

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