A Conversation with Bill Mellema, Director of Engineering at Morningstar Corp

In Uncategorized by hannah

I sat down with Bill at the recent Solar Power International exhibition to understand more about what Morningstar means by “reliability”.  Having distributed solar products in Africa for more than 15 years, I know which manufacturers tend to last and which ones fail.  Morningstar being very much in the lasting category, I was interested to hear Bill’s thoughts on why his products last so long.

“From an engineering perspective a lot of it comes down to having power electronics expertise,” said Bill.  “We are a power electronics company first.  Our conversations around Morningstar are about transients, stability, system response….we think about how the product will behave in the real world when it is subject to all sorts of inputs and split second changes in voltages and current. Our passion is elegant power electronics design.”

Selection of components is important, but reliability is mostly about how well the software controls the hardware.  It really comes down to how fast the device can respond to inputs.  Battery and PV voltage and current can change in split seconds and the power electronics have to be able to respond fast enough.  Most of us do not see these dramatic swings going on within our controllers and inverters because we don’t have the scopes and sophisticated monitors that could show us what is actually happening.

“Lots of quirky things can happen” says Bill.  “When we talk about control, we are talking about orchestrating the power control devices in the system and responding instantaneously.”

Processor utilization is another important priority. The Morningstar software team develops software architectures that extract the maximum performance possible from the chosen processor. This optimizes the cost to performance ratio and allows snappy power electronics control; even in modest microprocessors commonly used in smaller controllers and inverters.

So how does this software and design affect Morningstar products in real life?  For example, the Tristar PWM controllers became instant successes partly because they could survive where their common competitor could not. This is because the Tristar is so much faster at reacting to changes in inputs than the competitor.  And compared to another famous competitor, the Tristar MPPT provides more power per day, because the competitor can’t keep a constant output and must drop and then recalibrate.  A well-known inverter swings wildly when connected to a generator because it can’t control the incoming power fast enough.  These problems are mostly down to software control and design, according to Bill.

He points out that hardware damage is very prevalent in our industry.  But it doesn’t need to be that way.  At Morningstar about 2 out of every 1,000 returned items actually have hardware failures.  This is because of protections in the software and due to the quality and ratings of the hardware selected.  Morningstar typically over-sizes components to leave a margin of error for users.

“Speaking of design, thermal management is huge,” says Bill.  “It seems to be an afterthought with some competing designs – heat-producing components are often buried deep in the case where the thermal path to ambient air is long or circuitous.”

The Morningstar design team strives to perfectly balance thermal, power electronic, mechanical, and DFM/DFA constraints to yield a highly functional and efficient product. Short thermal paths coupled with generous heatsinking eliminate the need for forced convection solutions and improve product reliability. I asked whether being fan-less is part of the Morningstar doctrine.  “We may use a fan someday” says Bill, “But we will never blow air across our boards.”  He points out that blowing air brings dust, insects, corrosion and early hardware failure. Anyone who has had a PC in their home or office knows that after a few years the power supply fan gets clogged with dust, even in a relatively clean environment. Imagine how quickly things can go wrong in an off-grid environment.

Finally, Morningstar works hard to make their designs simple and elegant. This keeps the component count low and fewer components means lower overall MTBF (Mean Time Between Failure).  Bill notes that board layout in power electronics is also key – one must think about inductance, creepage & clearance, power and signal routing, trace lengths, etc.  Poor layout can lead to unpredictable glitches, excessive heating, and poor signal integrity. In short, there are thousands of design decisions and trade-offs that must be considered during development. The Morningstar team is able to leverage years of industry experience to market highly reliable products with peak performance.