Power Supply (SMPS) for Reliability By Ron Wunderlich, PhD DfR - PowerPoint PPT Presentation

Selection of Switch Mode Power Supply (SMPS) for Reliability By Ron Wunderlich, PhD DfR Solutions Open House March 16, 2015 1 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

History o The following is a history of Switch Mode Power Supplies: o SMPS were developed back in the late 70’s and early 80’s o This technology helped to significantly reduce the size of power supplies by switching at high frequency compared to line frequency o Previous designs were linear regulators, ferro-resonant transformers and even motor-generators with phase control (IBM Mainframes) 2 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

History o Back in the late 80’s there was a shift in the industry o SMPS were becoming a commodity no different than a resistor or capacitor that you buy o So, costs and profit margins were driven down over the years o Profit margins for a power supply is less than a few percent while IC chips are 30% to 70% o Almost all power supply design and manufacturing is now overseas o However, SMPS are still required o They have become a “Necessary Evil” 3 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

Complexity o Even though SMPS are treated like a commodity, they are in some cases more complex than the final product they go in o Let’s look at some of the technology that is involved in a SMPS o Industry has shifted to making SMT parts but many power components have to be PTH so there is a need to deal with a mix of SMT/PTH components o Need high voltage components like MOV’s to handle up to 4KV surges o Heavy copper boards (2oz or higher) to handle high current 4 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

Complexity o Need large amount of capacitance but with high ripple current or else you can dry out aluminum electrolytics o Need fuses that can open easily under faults yet survive shock/vibe o Inductors and transformer’s themselves are difficult handmade sub-assemblies with ferrite material that have complex behavior o Semiconductors that must handle both high voltage and high current and yet act as ideal as possible o To cool devices need thermal cooling like heatsinks o Need to operate at high temperature 5 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

Complexity o Then there are regulations that need to be met o For AC/DC’s there’s UL 60950 and others for safety specs o To meet safety you need safety approved components and the required spacing's between primary-secondary o If there are safety issues in the field, this will result in a recall o Worse if someone gets shocked 6 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

Complexity o Conducted and Radiated EMC o Used to be just conducted noise was an issue o Now with faster switching converters, radiated noise has become an issue o DfR has seen cases where the switching power supply generated more noise than the high end processor and FPGAs o Layout and switching speeds are critical o Require ferrite beads, Toshiba spike killers, common mode chokes 7 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

Complexity o Then there are standards (for AC/DC’s) o IEC61000-4-2 ESD o IEC61000-4-3 RF Immunity o IEC61000-4-4 Electrical Fast Transient o IEC61000-4-5 Surge o IEC61000-4-6 Conducted Immunity o IEC61000-4-8 Power Frequency Magnetic Field Immunity o IEC61000-4-11 Dips and Sags 8 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

Complexity o We are just scratching the surface 9 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

Selection o Unless you do a full detail analysis, you won’t know exactly how good the power supply is o However, there are some guidelines that can help you in your selection process 10 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

Selection o Load current o You know exactly what voltage you need but the load current is not as well identified o The common practice is to add up all the worst case currents from all the devices in the circuit o This will give you the worst case load current o Do you select a power supply that is rated for the worst case load current or 1.5X or even 2X ? 11 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

Selection o My suggestion is to pick a power supply that is designed for 2X the worst case load o Just like components, power supplies must be de-rated o Why choose 2X ? o In the OEM power supply market there is a dirty little secret o They assume that customers typically run the power supply at ½ load so some OEM’s will base their reliability on 50% load 12 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

Selection o Standards o As discussed before, there are standards (for AC/DC’s) and each standard has a level o The higher the level, the more robust the product will be o IEC61000-4-2 ESD o IEC61000-4-3 RF Immunity o IEC61000-4-4 Electrical Fast Transient o IEC61000-4-5 Surge o IEC61000-4-6 Conducted Immunity o IEC61000-4-8 Power Frequency Magnetic Field Immunity o IEC61000-4-11 Dips and Sags 13 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

Selection o For example, IEC61000-4-5 Surge o A product that meets Level/Class 4 will be more robust than a product that meets Level/Class 3 14 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

Selection o Fans for cooling o For high power converters, a fan is required for cooling o Fans are mechanical systems that wear out due to the bearings o They are limited by the number of revolutions o There are many bearing types but all have their pluses and minuses 15 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

Selection o There are tricks for increasing fan life o Use a large fan but run it at a lower speed (less revolutions) o Run the fan only when needed for cooling o Variable speed fan based on cooling requirements o N+1 fan cooling system o Sometimes how the cooling system works will be specified in the datasheet or else you have to ask 16 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

Selection o Aluminum Electrolytics o These type of caps have a liquid which can dry up over time o The life of an aluminum electrolytic is determine by o L r = rated lifetime of the capacitor at rated ripple current o T r = rated temperature of the capacitor o T E = actual temperature of the electrolyte    T r T   E     10 L L 2 r o This formula is based on the evaporation of a liquid 17 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

Selection o Here’s some interesting things o The life decreases exponentially with temperature o The rated life is performed at rated ripple current o For SMT caps and some PTH, the rated ripple current is 0A o That means as soon as you apply any ripple current, the life decreases o If the caps have rated ripple current > 0A and if you run them at a lower ripple current, the life increases o Capacitors with 85C rated temperature are usually made by 3 rd Tier vendors o First Tier vendors have outsourced these caps but still have their names on them 18 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

Selection o The larger the capacitor, the higher the rated lifetime is o Watch for local hotspots o The transformer below is heating up the capacitor o DfR has seen in one product a resistor running at 200C next to the cap o 19 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

Selection o When looking over a power supply, you would like to have a 105C/1000Hr minimum capacitor rating o If possible, ask the vendor for the temperature profile of the power supply and the ripple current o This way you can calculate the life of the capacitors your self o Instead of life testing to see when the capacitor fails, DfR has developed a method of using weight loss due to evaporation as a method of determining life time 20 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

Selection o Parts Position on the PCB o If the parts are too close to the board edge, they can crack because of the depanelization process o If they are placed along the same axis as the bend in the card, they can also crack 21 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

Selection o Below shows the preferred axis of bending o There are some components that are close to the edge but the orientation is such that it minimizes the cracking 22 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

Selection o In this case, parts marked in red are in wrong orientation o Bend the board and they can crack 23 9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com