PCB热设计电子工程师必备的基础知识

Thermal Design Guideline 1 Purpose 1 2 Scope 2 3 Part Properties 2 3.1 Power Devices 2 3.2 Relays 5 3.3 Fuses 7 Fuses 7 3.4 Connector Systems 33 Connector Systems 33 3.5 Substrate 34 3.5.1 Heat Spreader widthü 34 Conduction heat transfer in a printed circuit board 37 3.6 Housing 46 3.6.1 Thermal simulationü 46 4 Thermal Design 47 4.1 Heat expansion 48 4.2 Cooling Systems 49 4.3 Power Calculation 49 4.3.1 Single switching and PWM usageü 49 First definition (general) 52 Some typical thermal conductivities (k values) 53 External links 54 References 54 Some typical thermal conductivities (k values) 56 4.4 Critical Operation 58 4.5 Life Time Cycles 58 4.6 PCB track properties 61 4.6.1 Continuous current capability 61 4.6.2 Via’s 62 4.7 Component Placement 63 5 Risk Assessment 64 1 Purpose This document is a guide line for thermal system design. It supports the various aspects of right selection of components as well as showing the inter dependencies between material, parts and environmental conditions. Thermal system design aspects are shown in the picture below: A m b I e n t H O U S I N G H E A T S I N k C O N N E C T O R Parts Runners M A L E F E M A L E Harness Load Substrate Wire Solder joints 2 Scope This document applies to all C&S sites in Europe. 3 Part Properties 3.1 Power Devices Dependency between cooling area and heating of the power device: Thermal modeling of power semiconductor: See 3.2 Relays 3.3 Fuses Fuses A thermic fuse is a component that opens circuit automatically and stops application if temperature exceeds a certain level. To put circuit back on, fuse must be replaced. Parameters to be considered when selecting a fuse: 1. voltage rating The fuse must open quickly, extinguish the arc after the fuse element has melted and prevent the system´s open-circuit voltage from restriking across the open fuse element 2. current rating The current rating of a fuse identifies its current carrying capacity based on a controlled set of test conditions An operating current of 80% or less of the rated current is recommended for operation @25°C to avoid nuisance openings 3. ambient temperature The ambient temperature will affect the fuse´s opening and current carrying characteristic ( temperature re-rating curves ) See example for the Minifuse: picture 1a and b: limiting factor is the element temperature picture 2a and b: limiting factor is the blade temperature 4. overload conditions and opening times see time vs current curves in the data sheet . See picture 3 example for Minifuse: 5. melting integral ( I²t ) The melting integral is the energy required to melt a fuse. 6. pulse and inrush characteristics Transient surge or pulse currents result from any start up , inrush ( e.g bulbs ), surge and overload event in the circuit. Size the fuse properly to allow these pulses to pass without nuisance opening or degradation of the fuse element. The melting I²t value of the fuse must be greater than the Pulse I²t multiplied by a pulse factor Fp. The pulse factor is dependant on the construction of the fuse I²t_fuse > I²t_pulse * Fp 7. characteristics of components to be protected check requirements of the application 8. physical size and available board space Picture 1a: element limits continuous 225°C, pulsed 300°C @25°C ambient Picture 1b: element limits continuous 225°C, pulsed 300°C @85°C ambient Picture 2a: blade limits for plating 120°C @25°C ambient Picture 2b: blade limits for plating 120°C @85°C ambient Picture 3: time rating curve Reference documents: IEC 60127-10 (2001-11) Ed. 1.0 3D Modelling and Simulation of Fast Fuses for Power Semiconductors Comment: Fuses with a nylon isolation are much more temperature stable than with PVC isolation Recommended load conditions: For load currents more than 0,5 x I_nominal the heat generated by the MINI fuse has to be considered crefully: Recommended load conditions of MINI fuses 5A 7.5A 10A 15A 20A Working area for: Fuses, PCB, solder resist 3.4 Connector Systems Connector Systems Current carrying capability of a connector system The current capability of a connector system is described by the derating curve 1. Test set up for the derating curve according IEC 512 2. „Grenztemperatur“ is