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

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

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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

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9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

Selection of Switch Mode Power Supply (SMPS) for Reliability

By Ron Wunderlich, PhD

DfR Solutions Open House March 16, 2015

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9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

History

  • The following is a history of Switch Mode Power Supplies:
  • SMPS were developed back in the late 70’s and early 80’s
  • This technology helped to significantly reduce the size of

power supplies by switching at high frequency compared to line frequency

  • Previous designs were linear regulators, ferro-resonant

transformers and even motor-generators with phase control (IBM Mainframes)

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History

  • Back in the late 80’s there was a shift in the industry
  • SMPS were becoming a commodity no different than a

resistor or capacitor that you buy

  • So, costs and profit margins were driven down over the

years

  • Profit margins for a power supply is less than a few percent

while IC chips are 30% to 70%

  • Almost all power supply design and manufacturing is now
  • verseas
  • However, SMPS are still required
  • They have become a “Necessary Evil”
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Complexity

  • Even though SMPS are treated like a commodity, they are

in some cases more complex than the final product they go in

  • Let’s look at some of the technology that is involved in a

SMPS

  • 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

  • Need high voltage components like MOV’s to handle up to

4KV surges

  • Heavy copper boards (2oz or higher) to handle high current
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Complexity

  • Need large amount of capacitance but with high ripple

current or else you can dry out aluminum electrolytics

  • Need fuses that can open easily under faults yet survive

shock/vibe

  • Inductors and transformer’s themselves are difficult

handmade sub-assemblies with ferrite material that have complex behavior

  • Semiconductors that must handle both high voltage and high

current and yet act as ideal as possible

  • To cool devices need thermal cooling like heatsinks
  • Need to operate at high temperature
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Complexity

  • Then there are regulations that need to be met
  • For AC/DC’s there’s UL 60950 and others for safety specs
  • To meet safety you need safety approved components

and the required spacing's between primary-secondary

  • If there are safety issues in the field, this will result in a

recall

  • Worse if someone gets shocked
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Complexity

  • Conducted and Radiated EMC
  • Used to be just conducted noise was an issue
  • Now with faster switching converters, radiated noise has

become an issue

  • DfR has seen cases where the switching power supply

generated more noise than the high end processor and FPGAs

  • Layout and switching speeds are critical
  • Require ferrite beads, Toshiba spike killers, common

mode chokes

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Complexity

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

Complexity

  • We are just scratching the surface
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Selection

  • Unless you do a full detail analysis, you won’t know

exactly how good the power supply is

  • However, there are some guidelines that can help you in

your selection process

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9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

Selection

  • Load current
  • You know exactly what voltage you need but the load

current is not as well identified

  • The common practice is to add up all the worst case currents

from all the devices in the circuit

  • This will give you the worst case load current
  • Do you select a power supply that is rated for the worst

case load current or 1.5X or even 2X ?

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Selection

  • My suggestion is to pick a power supply that is designed for

2X the worst case load

  • Just like components, power supplies must be de-rated
  • Why choose 2X ?
  • In the OEM power supply market there is a dirty little secret
  • They assume that customers typically run the power supply

at ½ load so some OEM’s will base their reliability on 50% load

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Selection

  • Standards
  • As discussed before, there are standards (for AC/DC’s) and

each standard has a level

  • The higher the level, the more robust the product will be
  • IEC61000-4-2 ESD
  • IEC61000-4-3 RF Immunity
  • IEC61000-4-4 Electrical Fast Transient
  • IEC61000-4-5 Surge
  • IEC61000-4-6 Conducted Immunity
  • IEC61000-4-8 Power Frequency Magnetic Field Immunity
  • IEC61000-4-11 Dips and Sags
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Selection

  • For example, IEC61000-4-5 Surge
  • A product that meets Level/Class 4 will be more robust than

a product that meets Level/Class 3

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Selection

  • Fans for cooling
  • For high power converters, a fan is required for cooling
  • Fans are mechanical systems that wear out due to the

bearings

  • They are limited by the number of revolutions
  • There are many bearing types but all have their pluses and

minuses

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Selection

  • There are tricks for increasing fan life
  • Use a large fan but run it at a lower speed (less

revolutions)

  • Run the fan only when needed for cooling
  • Variable speed fan based on cooling requirements
  • N+1 fan cooling system
  • Sometimes how the cooling system works will be specified in

the datasheet or else you have to ask

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Selection

  • Aluminum Electrolytics
  • These type of caps have a liquid which can dry up over time
  • The life of an aluminum electrolytic is determine by
  • Lr = rated lifetime of the capacitor at rated ripple

current

  • Tr = rated temperature of the capacitor
  • TE = actual temperature of the electrolyte
  • This formula is based on the evaporation of a liquid

      

 

10

2

E r T

T r

L L

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9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

Selection

  • Here’s some interesting things
  • The life decreases exponentially with temperature
  • The rated life is performed at rated ripple current
  • For SMT caps and some PTH, the rated ripple current is 0A
  • That means as soon as you apply any ripple current, the life

decreases

  • If the caps have rated ripple current > 0A and if you run

them at a lower ripple current, the life increases

  • Capacitors with 85C rated temperature are usually made

by 3rd Tier vendors

  • First Tier vendors have outsourced these caps but still have

their names on them

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Selection

  • The larger the capacitor, the higher the rated lifetime is
  • Watch for local hotspots
  • The transformer below is heating up the capacitor
  • DfR has seen in one product a resistor running at 200C next

to the cap

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Selection

  • When looking over a power supply, you would like to have

a 105C/1000Hr minimum capacitor rating

  • If possible, ask the vendor for the temperature profile of the

power supply and the ripple current

  • This way you can calculate the life of the capacitors your

self

  • 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

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9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

Selection

  • Parts Position on the PCB
  • If the parts are too close to the board edge, they can crack

because of the depanelization process

  • If they are placed along the same axis as the bend in the

card, they can also crack

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Selection

  • Below shows the preferred axis of bending
  • There are some components that are close to the edge but

the orientation is such that it minimizes the cracking

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Selection

  • In this case, parts marked in red are in wrong orientation
  • Bend the board and they can crack
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Selection

  • Board Cleanliness
  • You can tell a lot about the company from the construction of

the power supply

  • A clean board shows the company is meticulous and has

good processes in place

  • A dirty board can put you at risk with abnormal circuit

behavior and arcing

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Selection

  • This is a nice clean board
  • No flux
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Selection

  • Hand solder flux left on board
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Selection

  • No clean solder flux left on board
  • This is a problem with high impedance analog circuits
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Selection

  • More solder flux left on the board
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Selection

  • Solder Workmanship
  • Because of mixed technologies (SMT and PTH), certain

components have to be hand soldered

  • If you are going to hand solder, make sure its soldered

properly

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Selection

  • Poor solder workmanship
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Selection

  • Glue
  • For high density converters glue will be required
  • Its used to hold components together for shock/vibe
  • It's also used as an insulating material to meet primary-

secondary isolation

  • However, this needs to be put in by hand which can result in

mistakes

  • Safety spacing is most critical
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Selection

  • Here’s an example with minimum amount of glue and not

used for critical safety areas

  • This is a good design
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Selection

  • Poor glue operation
  • Not only is this sloppy but they had to coat high voltage

wires to meet safety requirements

  • This cannot be done consistently
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Selection

  • Tape
  • Another method for decreasing the safety spacing's for high

density power supplies is to use tape

  • Again, like glue, tape has to be put on manually and

mistakes can happen

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Selection

  • Here’s an example where only the transformer has tape for

safety requirements

  • This is a good design
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Selection

  • In this example there is a significant amount of tape used
  • Hopefully they don’t make a mistake

C01 Heatsink C01 Transformer

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Selection

  • Assembly
  • Overall look at how the power supply is constructed
  • Ask yourself questions
  • Are components being force fit ?
  • Can you get a screw driver to mount a device to a

heatsink without slipping and causing other damage ?

  • If there are problems in the assembly, most likely you will

see issues later on

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Selection

  • C4, Q6 and Q2 are pressed against each other
  • The parts can crack over time with thermal cycling

C4 Q6 Q2

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Final Word

  • Final Word
  • This presentation will give you enough information to choose

a power supply

  • Also give an overview on checking the quality of the power

supply

  • At DfR Solutions, we take it one step further where a

detailed analysis consisting of a paper/hardware review of the circuit is performed next

  • Also if there are issues with the power supply, we can help

diagnose the problem and suggest corrective actions

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9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

  • The electrical review process entails the following
  • Review of specifications
  • This gives an understanding of the product but also if

there is sufficient information and are levels and ranges correct

  • Such as
  • Is the operating temperature range sufficient
  • Are there appropriate standards and high enough levels

Review Process

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  • The electrical review process entails the following (cont)
  • Review of the circuit
  • Understand and review the functionality of the circuit
  • Make sure the circuit functions as is
  • Sometimes suggest improvements in functionality or cost
  • Check the robustness of the circuit
  • Look at EMC and ESD aspect of the circuit
  • Check the de-rating of components
  • Power dissipation, voltage rating (even on resistors)
  • dV/dt rating of FETs for LLC
  • VGS breakdown of FETs
  • Temperature rating

Review Process

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  • The electrical review process entails the following (cont)
  • Stability
  • Circuits with feedback loops like power supplies
  • Even LDOs (like using low ESR ceramics where high ESR

aluminum electrolytics are needed to be stable)

  • Life of aluminum electrolytics
  • Half of all design reviews run into issues
  • Use temperature variation over seasons to be as accurate

as possible

  • Magnetics
  • Transformer documentation and strcture
  • Saturation current of magnetics
  • Core loss

Review Process

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9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

  • The electrical review process entails the following (cont)
  • Component Risk
  • Flag components that may have known issues or possibly be

discontinued

  • Layout
  • Placement of components
  • Power flow
  • Check for sufficient spacing's
  • Software
  • Averaging routines for noise signals

Review Process

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Thanks Any Questions?

Contact Ron Wunderlich rwunderlich@dfrsolutions.com The End