Tuesday, September 23, 2014

Thermal Reistance in Rectifier Assemblies - Part III



In Part II we discussed the different characteristics of commonly used materials in rectifier assemblies.  In this post, we'll look at using an alumina substrate to support a rectifier assembly using surface mount components.

 

Alumina Substrates

Alumina substrates work great with surface mount or formed lead diodes. 

Advantages

Alumina substrates offer the advantage of higher thermal conductivity than typical FR-4 fiberglass board, the kind commonly found in computers, which helps keep the components running cool.   

The rigidity of alumina means it can provide more support for components without flexing or breaking them.  Wire bonds made to an alumina substrate are less subjected to thermal expansions and contractions compared to a printed circuit board.  When the wire is less than half a mil in diameter, board flexing can be a concern.   

Alumina’s low CTE also means it is compatible with many types of encapsulation materials, AND it provides greater voltage isolation over regular printed circuit boards.  A properly selected encapsulation material will adhere well to the substrate, which improves isolation voltages between terminals, nodes, components, and the outside world.    
Integrating passive components can be a cost saver.  Capacitors and resistors can be screen-printed onto the substrate, thereby reducing real estate and interconnections where space is at a premium.   

Besides wire bonds and soldering, conductive epoxy works well with alumina too.

Alumina can attached to a metal base plate by solder or conductive epoxy.  This will help get the heat out of the assembly even faster, and because alumina’s CTE more closely matches that of aluminum, there is less chance for thermally induced mechanical stresses. 

If that isn’t enough, direct-bond copper alumina substrate is a product that bonds copper directly to alumina, which increases the thermal conductity even more than regular alumina.  The trade-off is cost (once again).  And weight.  Direct-bond copper (DBC) weighs more, thanks to the amount of copper traces present.

Trade-offs

So why not use alumina for everything?  I’ll give you a hint.  It’s a four-letter word.  C-O-S-T.
The disadvantage of using alumina over FR-4 board is that it costs more.  Direct comparisons are hard to come by, but for about the same size, an alumina substrate can be four or more times the cost of a double-sided pcb.  Cost driving factors in a substrate include thickness, length, and width.  Drilling, plating, thick or thin films, and printing resistors or capacitors, are all cost adders.

When it comes down to it, one must weigh the cost-benefit of using alumina for each application.  It works very well in high-rel applications, but is less suited for high volume consumer applications that are cost sensitive.

Up next are cleaning processes in rectifier designs, and what to look for.

Thursday, September 11, 2014

Thermal Resistance in Rectifier Assemblies – Part 2

In Part One
Metal Heat Sinks
the focus was on adding heat sinking capabilities to high voltage discrete diodes assembled in a rectifier package.

When it comes to high voltage, higher power applications, additional cooling strategies, above and beyond heat sinks attached to diode leads, may be needed.  

Axial-leaded Diodes and Copper Heat Sinks

In the case of axial-leaded diodes with attached copper heat sinks, cooling can be enhanced by suspending the diode assembly over an aluminum mounting plate and encapsulating the entire assembly.

Pros and Cons

This approach provides an increased area with which to dissipate heat. The disadvantage is, one must take care to avoid positioning the assembly too close to the base plate, especially if the base plate is grounded. If the assembly is too close to the base plate, voltage isolation through the encapsulation material will be over-stressed. When encapsulation material is over-stressed, arcs can occur between high voltage components and the (usually) grounded base plate.

High Voltage SMD Diodes and Alumina Substrates

A second approach uses surface mount or formed lead diodes soldered directly to an alumina substrate.  Sometimes direct-bond copper is used, other times, a pattern of component pads and conductors are printed on a substrate, which is then fired at a high temperature.

Once the substrate is populated, it is attached to a mounting plate, which is usually metal.  The connection is made using solder or non-electrically-conductive thermally conductive epoxy.  Next, the whole assembly is encapsulated. All that’s visible from the outside is the base plate.
Direct-Bond-Copper Substrate


Pros and Cons

Copper or aluminum are common base plate materials. Aluminum more closely matches the thermal conductivity of alumina, but neither conducts heat as efficiently as copper.  Aluminum has the advantages of being less expensive than copper, and weighs considerably less.

Properties of Aluminum, Alumina, and Copper


Below is a table listing properties for alumina, aluminum, and copper.   

Alumina 96%
  • CTE  =  6.4ppm/°C
  • Thermal Conductivity (σ) = 0.89W/In°C
Aluminum 6061T6
  • CTE = 23.5 ppm/C
  • Thermal Conductivity (σ) =  3.960W/In°C
Copper OFC

  • CTE = 17 ppm/°C
  • Thermal Conductivity (σ) = 10.00W/In°C

In Part III we’ll discuss electrical and mechanical issues when using alumina substrates and aluminum base plates.

Thursday, September 4, 2014

High Voltage Rectifier Assemblies - Thermal Characteristics and Design Tips - Part 1

Thermal Resistance in Rectifier Assemblies


Are you interested in thermal resistance or conductance in high voltage rectifiers or assemblies? Over the next several posts we'll present design information, trade-offs, and process tips on using discrete high voltage diodes in assemblies.

High Voltage Diodes - The Building Blocks for Rectifier Assemblies


High voltage rectifiers, stacks, single- and three-phase bridges have one thing in common. They all use multiple diodes assembled together in one package.  Really high voltages may require several multi-junction diodes per leg which can impact heat transfer in the assembly, as a whole. 

Kandinsky
Kandinsky - On White II 

Heat Dissipation in a High Voltage Diode

In a multi-junction glass-body diode most of the heat is dissipated through the leads. Glass is used to prevent arcing between the closely spaced silicon junctions that are approximately 10 mils thick.  This is called 'passivation'. While the Coefficient of Thermal Expansion, CTE, for glass closely matches that of silicon, glass is not a good thermal conductor. Its Thermal Conductivity, σ, is 0.031 /In°C .  Consequently, a negligible amount of heat will be transferred through the body of the diode.  The only other available heat path is through the leads.

VMI diode leads are 99.99% silver. The Thermal Conductivity of silver is 10.500 W/In°C - more than 338 times that of glass.

For that reason, most of the heat generated by impurities in the silicon junctions is transferred via the leads. That is why heat spreaders and heat sinks are added to the leads of the diodes. Since thermal transfer is proportional to the length of the heat path and inversely proportional to the cross-sectional area of the heat path, it makes sense to increase the heat dispersing area.  This is expressed by the following equation -

Thermal Impedance for Conduction = [L / (σ x A)] in units of W/In°C

Where: L is the length of the thermal path
            σ is the thermal conductivity of the material
            A is the cross-sectional area of the heath path in inches-squared

 

More than One Diode

Things start to get interesting when more diodes are added.  For instance, if you have two diodes in series and the cathode of one connects to the anode of the next via a shared copper heat sink, calculating how hot the diode junctions get can get confusing.

To simplify the calculations, we assume that the heat sink area is shared equally.  In reality, the side connected to the cathode lead will tend to run hotter than the end connected to the anode lead.  The reasons are 1)  In the forward conducting mode, junctions get progressively hotter as current enters the diode and flows through it and 2) Junctions in the center may be a little hotter than the ends because the only thermal path available to them is through the adjacent junctions.  Regardless, the differences will be small since silicon is a good thermal conductor, so average values are used. 

Lead thickness and lead materials become important, and we'll tackle that next time.     



Tuesday, September 2, 2014

3 Reasons You Should Be Thinking About Rectifier Assemblies



Slim Pack High voltage Rectifier Assembly
SP Slim Pack High Voltage Rectifier Assembly
Voltage Multipliers Inc. manufactures high voltage rectifier assemblies.  We use our own diodes in our high voltage stacks, single-phase bridges, three phase bridges, and custom assemblies.

Rectifier assemblies come in a huge array of current and voltage combinations, but most all of them have a few things in common.  First, they use VMI diodes.  Second, if there is more than one diode in the assembly, they are welded or soldered together.  Third, most all rectifier assemblies are encapsulated.  Fourth, 100% of them are tested before leaving the plant.

Reason 1  - Electrical Specification Exceed That of Discrete Diodes

Rectifier assemblies are used when discrete diodes do not offer enough isolation voltage between leads, when complex circuits are needed, or when higher reverse voltages or currents are necessary.
Encapsulation offers increased resistance to arcing between leads or to external heat sinks and ground planes.   

Encapsulation materials are chosen based on isolation voltage and thermal conductivity needed in the application.  As is often the case, selecting an encapsulation material is a matter of balancing trade-offs between thermal conductivity and isolation voltage, or thermal coefficient of expansion with cost.   

Environmental concerns must be considered too, since some potting materials (like some silicones) expand quite a bit at low temperatures, inducing mechanical stresses that can cause breakage in confined space.  

Reason 2 - Improved Thermal Conductivity

Encapsulation provides better isolation voltage, but can make thermal conductivity worse.  In situations where more thermal conductivity is needed, internal heat sinks can be added.  The downside to adding heat sinks is that most of the time the package size goes up.  To improve thermal characteristics even more, sometimes diodes are soldered directly to an alumina substrate mounted to a heat sink, and then encapsulated.  In extreme cases, rectifier assemblies use individual heat sinks attached to diodes, an alumina substrate, and an external heat sink.

How much heat sinking is right for you?  For starters, the answer to that question depends on how much space is available for your design, how much isolation voltage you need, and how hot things will run.  Let’s not forget budgets.  Cost drivers for rectifier assemblies include voltage, current, and Trr.  Higher costs are generally associated with higher voltage, higher current, and fastest Trr.

Reason 3 - Design Experts

VMI has over 90 years of combined rectifier assembly design.  We have a long history of design, while staying receptive to new materials, new methods, and our customers.  When you speak, we listen.  

The next time you have a need for a rectifier, be it a high voltage stack, 1P- or 3P bridge, or a custom assembly, give us a call.