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MOST space telescope turns 15, and a secret is told

posted Jun 30, 2018, 6:18 PM by Doug Sinclair
Today is the 15th anniversary of the launch of the MOST space telescope.  This little mission was ground breaking for both its science and its technology.  Scientifically, it has observed oscillations in stars in a way that no other instrument could.  Technically, it demonstrated sub-arcsecond pointing stability in a microsatellite platform for the first time.  And after all this time it is still working and producing useful science.

I built a rather unusual power system for this spacecraft.  It needed 12 switch-mode converters: 10 boosts for the solar array strings, and 2 Cuks for the shunts that bleed off excess power when the battery was full.  I couldn't afford the area and power consumption of 12 discrete controller ICs.  Instead, I built a 12-channel 4-phase controller out of a 4000-series ripple counter, 74-series CMOS flip-flops and LM339 quad comparators.  It is a byzantine gated-oscillator (not PWM!) system, but it runs in a bounded limit cycle and the total chip count is less than 12.

In the late 90's, when all of this was being designed, there was a global shortage of surface-mount tantalum capacitors.  They were all snapped up by manufacturers of cell phones and Internet routers, and a small project like ours just could not buy them.  So what was I to do for bulk capacitance in the power supply?  Aluminum electrolytics were too big, and unsuitable for vacuum.  Ceramic capacitors were my only option.

But I wasn't the only one thinking this.  Very quickly the good military-grade stacked ceramics also disappeared from inventories.  It was even difficult to find high-capacity parts in wide temperature range X7R dielectric.  I was forced to some very low performance dielectrics -- Y5V and Z5U.  These lose capacitance rapidly when hot or cold, so to compensate I needed a lot of physical volume.  I used massive surface-mount chips the size of thumbnails.

These low-grade parts have terrible piezoelectric properties.  As the voltage on the capacitors goes up and down they get physically larger and smaller.  The gated oscillator carrier frequency is 450 kHz, but the converter limit cycles could be down at 10 kHz or so and are a strong function of the battery voltage and load current.  Each capacitor acts like a little speaker, and the combined effect of 12 simultaneous channels of noise meant that everybody in my work area had to wear earplugs when I was testing.

Now MOST is fifteen.  The NiCd batteries lost all of their capacity long ago, but its orbit keeps it in sunlight ten months of each year and the shunts continue to keep the bus voltage regulated.  And I take comfort that the little spacecraft continues to scream outloud with the pure joy of being in space.