Wire the MPPTs in series instead of having high current traces on solar sense.
Reduces parasitic impedance
Reduces the number of high current rated connectors and wires required.
Cheaper isolators
Current SPI isolators have a built-in power isolator as well, which is convenient but it’s more expensive than an external voltage isolator.
Ideally, the MPPT would power the isolated side directly from the array.
Use ADCs with SPI or an MPPT with I2C?
Only 1 type of isolator would be required.
Non-automotive relay?
Current relay is labelled on Digikey as “automotive”, but the datasheet doesn’t mention anything about automotive certifications.
There are much cheaper “non-automotive“ relays on Digikey that consume less power (as far as I know, the relay driver circuit on solar sense has not worked).
If the PCB is smaller, it might be cheaper to have a 4 layer PCB, rather than a 2 layer PCB with 2 oz copper.
Increase the resistance values for I2C pull-ups? Can choose to sacrifice on the I2C bus speed in favour of lower power consumption with higher resistance. Will need to confirm that pull-ups are compatible with all I2C devices on the bus (to be done after all parts are finalized). Power savings may be negligible.
Adjust current-limiting resistor values for LED2 and LED3? A lot more current flows through LED2 even though it is the same part as LED3. Possibly more/less current is necessary.
As per note on Thermal Management sheet: Need to replace caps with more appropriate size for noise filtration on long temperature sense lines.
Possibly redesign relay circuit as it has been problematic in the previous iteration.
Make power rail test points in more convenient locations on the PCB (away from LEDs and other test points)
List of components that need to be re-examined and possibly replaced. Numberings correspond to BOM
14. D2: DIODE ZENER 6.2V 5W DO214AA (min qty is 3000)
17. D6: DIODE ZENER 15V 500MW SOD523 (not in stock)
18. F1: FUSE 12 A 600VAC/DC KLM-12 (expensive)
19. K1: RELAY AUTOMOTIVE SPST 15A 12V (expensive)
31. Q1: MOSFET N-CH 250V 64A TO263-3 (weird vendor, unknown min qty)
52. U1, U4_1-6: IC ADC MCP3427 10MSOP (not in stock)
56. U6_1-6: IC DGTL ISOL 2500VRMS 2CH I2C 8SOIC (not in stock)
57. U7_1-6: IC DGTL ISO 5KV 4CH GE (not in stock)
58. U8: ACS722LLCTR-10AB-T (not in stock)
61. U11: IC OP AMP DUAL GP RR 10MHZ 8-VSSOP (not in stock)
63. U13: IC GATE NAND 1CH-2-IN (sus vendors?)
Solar Array
Mech has come up with a solar array design for MSXV. I have labeled the stacks below:
...
In order to meet competition standards, stack 5(b) has been left out. Below is a table of the number of cells per stack
Stack | Length | Width | Total cells |
|---|---|---|---|
1 | 7 | 3 | 21 |
2 | 7 | 3 | 21 |
3 | 2 | 6 | 12 |
4(1) | 5 | 2 | 10 |
4(2) | 5 | 2 | 10 |
7 | 7 | 4 | 28 |
8 | 7 | 4 | 28 |
9 | 7 | 4 | 28 |
10 | 7 | 4 | 28 |
11 | 4 | 4 | 16 |
13 | 7 | 3 | 21 |
15 | 7 | 3 | 18 |
16 | 7 | 3 | 18 |
Total | 259 | ||
The nominal battery voltage is 129.6 V with a maximum of 151.2V. Each Nomura MPPT has a maximum boost voltage of 33V. 151.2/33 = 4.58. Taking ceil(4.58) we get 5 which means we need at least 5 MPPT’s in one string.
According to the Maxeon Gen 3 Datasheet, under Standard Test Conditions, we should get (0.632 V * 5.89 A) 3.722 W of power from each cell. We assume (because manufacturer’s lie and connection impedances) that the actual output of each cell is 75% of that which is given in the datasheet which means we are getting 2.79 W of power per cell.
(To be continued)