Rought Draft #2 of schematic, and a draft PCB layout

I have posted into the Schematic link above a .pdf of the latest schematic.  Not a whole bunch has changed from the 1st rough-cut, Refining the FETs - settled on an asymmetrical pair:  Faster switching top FET, and lower Rdon lower FET.  Also have gone with DPAK devices for this 1st PCB FAB, may upgrade to the better PQFP ones later, but the DPAK ones will be simpler to replace if need be :-)    Replaced the LDO with a 2nd switcher in the power supply sub-system, saving that 125mW overhead.  And did a little SPICE modeling on the FETs and drivers and hence added a spot for some additional gate drive options (specifically the fast pull-down diodes on the High FETs) that might help with some of the cross-over losses.

Click for larger view <BR/> Also see the .pdf under the Schematics link above.

And here are a couple of views of the PCB layout Top and bottom:

Top view

The PCB comes in at 10x7cm, and a rough BOM cost of around $75-80, including an off-shore PCB fab.  I tried to select higher temperature 125c rated parts; that did increased BOM cost perhaps 10%.  If one wanted to back off and use an external USB <--> TTL adapter, and drop the CAN ports, might save $15-20???  Dropping one bank might save another $10??  (The PCBs come in around $2-3 using off-shore fab, so not sure it is worth doing more than one PCB design - just do stuffing options)

In the top 3D rendering above you can see some odd parts, notably the USB connector.  This is the result of stuffing options in the PCB design - I have in there the ability to use horizontal or vertical components for the USB, DIP switch, and the RJ-45 ports.  Or, one can populate headers and use external cables (e.g., to place this into a waterproof box and use waterproof external connectors vs. wiring glands).  Because all the stuffing options are in the schematic, they are rendered in the 3D view - each on top of the others.


Still to do:  I need to go through and confirm all the switching power supply calculations, inductors, caps, etc. (and there are three of them on the board!)  The caps used on the solar buck are rather costly (around $2-3 each), and want to make sure I have in what is both needed, but not more than what is.  I also need to finish the PCB layout, just a couple more routs to do, and confirm some thermal dissipation around the FETs - as I am relying on the PCBs as the heat-sinks.  Look for a physical interference between the caps and the large solar and battery connectors - the 3D rendering shows an overlap, but the connector 3D modes are mocked up and may not be accurate.  And I want to put some time into the xxM1 uCs and Arduino IDE - there are a few postings out there about using the uC with the Arduino IDE, want to make sure it will actually work before getting much further along.

There is a lot going on family wise these days, the above may take me a bit of time to complete.  My intention is once I complete some of the tasks above to order a set of 3x OSH-PARK PCBs and a set of components to build one up.  If someone is interested in building up one of the other PCBs, drop me a line.  At that time will also post up the CAD files to the links above.   I figure once things are proven a bit, will revise the PCB and get a set of them made - perhaps look into have some of the SMT parts machine assembled.  Might also revise the FETs to the 'better', but harder to hand assemble, QPFP packages.

Fine tuning the FETs

Have done some more modeling and think I am about at the end of incremental improvements using this tool.  I posted the .xls sheet with the additional drivers, FETs, and inductors I have been using in the reference tab above.

Below as the detailed results for three potential FETs.  All SMT, one the new PQFP package and two DPAK ones.  Costs are 1x qty from Mouser.com  Parameters use for each run was:

                Vpanel = 32.0v

                Vbat      = 13.8v

                22uF inductor

                100Khz PWM frequency

Early on I had to look to increase the operating frequency from 50Khz to 100Khz else the output capacity ESR losses became very large.  Unfortunately, increasing the frequency also increased switching losses in the FETs…  And at this point the switching losses for the high side FET are in the 350mW range.   Here are the detailed tables:

1)  IRFH7185 – PQFP - $3.92

IOUT (A)	HS Conduction Losses	LS Conduction Losses	HS Switching Losses	Diode Conduction Losses	Reverse Recovery Losses	MOSFET Output Capacitance Losses	High Side Gate Drive Losses	Low Side Gate Drive Losses	Inductor Winding Losses	Output Power (W)	Input Power (W)	Efficiency	HS Die Temperature (˚C)	LS Die Temperature (˚C)
.00	.0021	.0025	.0000	.0000	.3520	.1096	.0360	.0360	.0000	.00	.5382	0.00%	41.23	25.09
1.00	.0042	.0049	.0386	.0032	.3520	.1096	.0360	.0360	.0029	13.80	14.3873	95.92%	42.65	25.28
2.00	.0104	.0121	.0771	.0064	.3520	.1096	.0360	.0360	.0114	27.60	28.2511	97.70%	44.22	25.65
3.00	.0209	.0242	.1157	.0096	.3520	.1096	.0360	.0360	.0257	41.40	42.1297	98.27%	45.94	26.18
4.00	.0358	.0413	.1543	.0128	.3520	.1096	.0360	.0360	.0458	55.20	56.0235	98.53%	47.81	26.89
5.00	.0554	.0635	.1929	.0160	.3520	.1096	.0360	.0360	.0715	69.00	69.9328	98.67%	49.84	27.78
6.00	.0798	.0909	.2314	.0192	.3520	.1096	.0360	.0360	.1030	82.80	83.8579	98.74%	52.05	28.85
7.00	.1093	.1239	.2700	.0224	.3520	.1096	.0360	.0360	.1401	96.60	97.7993	98.77%	54.43	30.12
8.00	.1442	.1626	.3086	.0256	.3520	.1096	.0360	.0360	.1830	110.40	111.7576	98.79%	57.00	31.59
9.00	.1848	.2073	.3471	.0288	.3520	.1096	.0360	.0360	.2317	124.20	125.7333	98.78%	59.77	33.26
10.00	.2314	.2585	.3857	.0320	.3520	.1096	.0360	.0360	.2860	138.00	139.7272	98.76%	62.75	35.17
11.00	.2845	.3167	.4243	.0352	.3520	.1096	.0360	.0360	.3461	151.80	153.7403	98.74%	65.96	37.32
12.00	.3444	.3822	.4629	.0384	.3520	.1096	.0360	.0360	.4118	165.60	167.7733	98.70%	69.41	39.72
13.00	.4117	.4557	.5014	.0416	.3520	.1096	.0360	.0360	.4833	179.40	181.8273	98.67%	73.12	42.40

2)  IPD096N08N3 – DPAK - $1.78

IOUT (A)	HS Conduction Losses	LS Conduction Losses	HS Switching Losses	Diode Conduction Losses	Reverse Recovery Losses	MOSFET Output Capacitance Losses	High Side Gate Drive Losses	Low Side Gate Drive Losses	Inductor Winding Losses	Output Power (W)	Input Power (W)	Efficiency	HS Die Temperature (˚C)	LS Die Temperature (˚C)
.00	.0054	.0063	.0000	.0000	.2912	.0502	.0260	.0260	.0000	.00	.4051	0.00%	42.34	25.32
1.00	.0106	.0124	.0321	.0040	.2912	.0502	.0260	.0260	.0029	13.80	14.2553	96.81%	44.20	25.82
2.00	.0264	.0306	.0641	.0080	.2912	.0502	.0260	.0260	.0114	27.60	28.1339	98.10%	46.59	26.93
3.00	.0533	.0616	.0962	.0120	.2912	.0502	.0260	.0260	.0257	41.40	42.0423	98.47%	49.55	28.68
4.00	.0923	.1062	.1283	.0160	.2912	.0502	.0260	.0260	.0458	55.20	55.9820	98.60%	53.10	31.11
5.00	.1445	.1656	.1604	.0200	.2912	.0502	.0260	.0260	.0715	69.00	69.9553	98.63%	57.31	34.28
6.00	.2113	.2418	.1924	.0240	.2912	.0502	.0260	.0260	.1030	82.80	83.9658	98.61%	62.25	38.29
7.00	.2944	.3368	.2245	.0280	.2912	.0502	.0260	.0260	.1401	96.60	98.0173	98.55%	68.02	43.24
8.00	.3963	.4542	.2566	.0320	.2912	.0502	.0260	.0260	.1830	110.40	112.1155	98.47%	74.71	49.31
9.00	.5201	.5980	.2886	.0360	.2912	.0502	.0260	.0260	.2317	124.20	126.2678	98.36%	82.51	56.70
10.00	.6693	.7740	.3207	.0400	.2912	.0502	.0260	.0260	.2860	138.00	140.4834	98.23%	91.57	65.70
11.00	.8488	.9910	.3528	.0440	.2912	.0502	.0260	.0260	.3461	151.80	154.7760	98.08%	102.15	76.75
12.00	1.0658	1.2600	.3848	.0480	.2912	.0502	.0260	.0260	.4118	165.60	169.1639	97.89%	114.60	90.40
13.00	1.3280	1.5970	.4169	.0520	.2912	.0502	.0260	.0260	.4833	179.40	183.6706	97.67%	129.31	107.45

3) IPB090N06N3 – DPAK - $1.73,  60V max part

IOUT (A)	HS Conduction Losses	LS Conduction Losses	HS Switching Losses	Diode Conduction Losses	Reverse Recovery Losses	MOSFET Output Capacitance Losses	High Side Gate Drive Losses	Low Side Gate Drive Losses	Inductor Winding Losses	Output Power (W)	Input Power (W)	Efficiency	HS Die Temperature (˚C)	LS Die Temperature (˚C)
.00	.0037	.0046	.0000	.0000	.1280	.0655	.0360	.0360	.0000	.00	.2739	0.00%	32.89	25.19
1.00	.0072	.0091	.0302	.0040	.1280	.0655	.0360	.0360	.0029	13.80	14.1188	97.74%	34.24	25.52
2.00	.0178	.0223	.0603	.0080	.1280	.0655	.0360	.0360	.0114	27.60	27.9855	98.62%	35.87	26.21
3.00	.0358	.0446	.0905	.0120	.1280	.0655	.0360	.0360	.0257	41.40	41.8742	98.87%	37.79	27.26
4.00	.0614	.0763	.1207	.0160	.1280	.0655	.0360	.0360	.0458	55.20	55.7856	98.95%	40.03	28.69
5.00	.0950	.1176	.1509	.0200	.1280	.0655	.0360	.0360	.0715	69.00	69.7206	98.97%	42.58	30.51
6.00	.1371	.1693	.1810	.0240	.1280	.0655	.0360	.0360	.1030	82.80	83.6799	98.95%	45.47	32.73
7.00	.1882	.2319	.2112	.0280	.1280	.0655	.0360	.0360	.1401	96.60	97.6650	98.91%	48.72	35.40
8.00	.2490	.3065	.2414	.0320	.1280	.0655	.0360	.0360	.1830	110.40	111.6775	98.86%	52.36	38.54
9.00	.3203	.3939	.2715	.0360	.1280	.0655	.0360	.0360	.2317	124.20	125.7189	98.79%	56.41	42.19
10.00	.4029	.4955	.3017	.0400	.1280	.0655	.0360	.0360	.2860	138.00	139.7916	98.72%	60.93	46.42
11.00	.4979	.6129	.3319	.0440	.1280	.0655	.0360	.0360	.3461	151.80	153.8983	98.64%	65.93	51.27
12.00	.6066	.7482	.3621	.0480	.1280	.0655	.0360	.0360	.4118	165.60	168.0423	98.55%	71.49	56.85
13.00	.7307	.9036	.3922	.0520	.1280	.0655	.0360	.0360	.4833	179.40	182.2275	98.45%	77.66	63.23

System overhead:

         o    Output capacitor losses – est:   26mW (using 3.57A inductor ripple current)
         o    Input capacitor losses    - est:  162mW using 9A load.

·                O       +5v rail:  25mA -->   125mW

o   uC consumption:       14mA @5v (per datasheet typical)

o   OpAmps:                     100uA @ 5v (per datasheet typical)

o   CAN consumption:     6mA @ 5v (assuming a 1% dominate/recessive ratio)

o   Various pull-ups:         5mA @ 5v (WAG)

·    ·           O       uC power supply loss:  134mW

o   Switcher loss:                    9mW (Per datasheet, 25mA load, 36v Vin)

o   Linear Loss:                   125mW  (10v à 5v = 5v drop at 25mA)  -- Clearly need to look at this closer…

·    ·           O       Vbat & Vpan resistor network:  1.2mW

o   Vbat:                     190uW

o   Vpanel:                1mW

Total estimated overhead:  26mW + 162 + 125mW + 134mW + 1.2mW =   449mW  --> 450mW

Based on 250w input this would be a 0.18% reduction in the above efficiency numbers.  So, the IRFH7185 selection, instead of being 98.78% efficiency would come in at 98.60% overall.

If one did a depopulated (one bank only) configuration, and eliminated the CAN subsystem – that would save 60mW in losses between the CAN transceiver and uC power supplies.

The straw-man has a liner LDO regulator taking the 10v from the LTC3639 SPW down to 5v.  I had selected this as a cost trade-off in support of very low +5 current consumption if the uC is placed into sleep.  Most low cost small switching power supplies need a min of 5-10mA to remain stable, and in sleep the uC will be in the low uA range.  Perhaps need to consider another switching power supply, but a more costly one as the 125mW loss associated with the LDO is very significant.

How does this look, does it seem like I have missed something?

-al-