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73 lines
3.8 KiB
Markdown
73 lines
3.8 KiB
Markdown
# Solar Simulator
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# The Problem: AKA Oh dear god why is everything so expensive
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Because we wanted to test our solar panels in our thermal vacuum chamber, we needed a solar simulator that could simulate the AM0 light spectrum while also being compact, power efficient, and (most importantly) cheap!
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Normal incandescent solutions were too bulky to fit in our tiny vacuum chamber, they require a lot of power, and dissipate a whole lot of heat into the system. LED solutions do exist, but they are insanely expensive. So-being the loveable open source rogues that we are- decided to build our own!
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# The Design
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### LED Selection
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The wavelengths of light we care about ranges from about 376.5 nm - 899 nm (this is the approximate), meaning we can discard wavelengths in the AM0 spectrum outside of these bounds.
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We are only using three LED channels for our design (R, G, and B) most commercial simulators use way more channels than we do, but because our panels have a relatively flat response curve, and we don't care *that* much about super precision, we think we can get away with just 3. So we split the wavelength spectrum into 3 roughly equal bands, and sum up the total irradiance found within the band. This will give us the total irradiance needed for each band of LEDs.
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Blue Irradiance(376.5 nm - 504.5 nm) = 221.9 W/m^2
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Green Irradiance(505.5 nm - 637 nm) = 233.8 W/m^2
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Red Irradiance(639 - 899 nm) = 161.5 W/m^2
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Each channel is composed of CREE X-QE LEDs, because they are insanely small and power efficient and come in the wavelengths that work for us ( 440 nm, 520 nm, and 630 nm respectively). In future revs we may shift our bands to better fit these wavelengths.
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### Driver Selection
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Each channel is driven by a TPS92512 LED driver, since they can push stupid amounts of power at a very high frequency, and can be controlled with either a low frequency PWM from a microcontroller, or an analog input. Each channel is in parallel and calculations are done assuming 56V Vin, meaning that the Vpp should be capable of 56V 3A. (NOTE: We could get away with lower voltages, but using 56V allows us to use a higher switching frequency of 1 MHz)
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### Microcontroller
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We are using an Arduino Nano (Atmega328P) as a daughter board to control the 3 LED LM3404HVs via an I2C DAC (MCP4728), as well as reading the 4 thermistors and reporting it over USB. It is quick, dumb, and easy. In the future we may move to an embedded microcontroller but this is easy enough for now.
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# Component Calculations
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Vin = 56 V
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Vout = 48.2 V (worst case)
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switching frequency = fsw = 1 MHz
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* Output Inductor
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* NOTE: Iripple must be at least 75 mA for reliable operation. 400 mA chosen to decrease size of inductor.
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* L = (Vout * (Vin - Vout) ) / (Iripple * Vin * fsw)
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= (48.2 * (56-48.2) )/ (.4 * (56) * (10^6) )
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= 170 uH
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* Maximum Duty Cycle
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* D = Vout/Vin
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= 56/48.2
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= 86%
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* Input Capacitance
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* Note: ΔVin is assuming a 5% ripple on Vin
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* Cin = (Iled * D * (1-D) )/(ΔVin * fsw)
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= (1 * .86 * .14)/(2.8*10^6)
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= 43 nF
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* Dynamic resistance of LEDs
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* Rled = (ΔVled/ΔIled) * #LED
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= .5 * 15
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= 7.5 Ω
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* Output Capacitance
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* Zcout = (Rled * ΔI) / (Iripple - ΔI)
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= (7.5 * .05) / (.2 - .05)
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= 2.5 Ω
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* Cout = 1 / (2 * π * fsw * Zcout)
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= 1 / (2 * π * 10^6 * 2.5)
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= .15 uF
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* Power Dissipation
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* Psw = D * Rds * Iled^2
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= .86 * .28 * 1^2
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= .24 W
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* Piq = Vin * (400 * 10^-6 + (.002 * fsw) / 10^6)
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= 56 * (400 * 10^-6 + (.002*10^6) / 10^6)
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= .13 W
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* Pac = .73 * 10^-9 * fsw * Vin^2 * Iled
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= .73 * 10^-9 * 10^6 * 56^2 * 1
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= 2.29 W
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* Ptotal = (Psw + Piq + Pac) * #LEDDrivers
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= (.24 + .13 + 2.29) * 3
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= 2.66 * 3
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= 7.98 W
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