VemsFrontier/ArmEfi/FetDrive

HW Developer page for driving FETs on VemsFrontier/ArmEfi

Issues

Decisions:


Why Schmitt inputs?

PGND=GND5 below (power ground), different (but close) than the MCU, which is on GND.

The reason for going with Schmitt inputs is that the logic will be on GND5. When turning on a FET GND5 _will_ increase somewhat compared to GND. I just want to take every precaution to avoid oscillation which would make the FETs very hot. The peace of mind is worth a few additional chips. Especially with the other advantage: we also douple the gate driving power.


When GND5 rises

When fitting the NAND chips as FET and IGBT drivers I found that they will not be possible to turn off if GND5 rises. The GND5 alarm will tell us that we are about to start a fire but we will be unable to turn the switches off!

Requirements:

With 2 NOR gates per channel: (3 external inputs, 1 of them is clock)

U2_1U1_1U1_2(clock)U1_out (U2_2) U2out(FETgate)
1 x x x 0
0 1 x 0 1
0 0 c -c c

Note that only 2 gates per channel would also work with NAND gate logically. But when PGND rises, NAND gate would switch the FET on. The NOR gate switches the FET off, as expected.

Nice solution, but it seem like NOR is a phantom chip, expensive as hell too => NAND is a much safer choice. NAND are much (5x) cheaper at digikey than NOR, that is at least alarming.

With 3 NAND gates (solution currently in eagle): (3 external inputs, 1 of them is clock). I don't write U2out column, you can calc it from -U2out=U3out=(U2_1 AND U2_2)

U2_1U1_1U1_2(clock)U1_out (U2_2) U3out(FETgate)
0 x x x 0
1 0 x 1 1
1 1 c -c -c

Hunt begins.

From the catalog of the backup distributor (price is in 1/180 USD units):

CD4093BCN (FAI) QUAD 2-INPUT NAND SCHMITT TRIGG.D	28,40
HEF4093BP (PSH) QUAD 2-INPUT NAND SCHMITT TRIGG.D	35,20
HCF4093BEY (STM) * QUAD 2-INPUT NAND SCHMITT TRIG	29,90
MC14093BCP (ONS) * QUAD 2-INPUT NAND SCHMITT TRIG	29,90
CD4093BE (TI) * QUAD 2-INPUT NAND SCHMITT TRIGG.D	19,40
...
N74F132N (PSH) QUAD 2-INPUT NAND SCHMITT TRIGGERS	41,20
M74HC132B1R (STM) QUAD 2-INPUT NAND SCHMITT TRIGG	26,90
MC74HC132AN (ONS) * QUAD 2-INPUT NAND SCHMITT TRI	30,80
SN74HC132N (TI) * QUAD 2-INPUT NAND SCHMITT TRIGG	23,40
CD74HCT132E (TI) QUAD 2-INPUT NAND SCHMITT TRIGGE	30,60
T74LS132B1 (STM) QUAD 2-INPUT NAND SCHMITT TRIGGE	18,00
54LS132/RBDQ (MOT) QUAD 2-INPUT NAND SCHMITT TRIG	118,00

Main distributor:

142839    74HCT132D,652  Philips   QUAD 2-INPUT NAND...          
440641    74HCT132D,653  Philips   QUAD 2-INPUT NAND...         
159194    74HCT132DB,118  Philips   QUAD 2-INPUT NAND...         
159195    74HCT132PW,112  Philips   QUAD 2-INPUT NAND...   
+ AHCT ...

74AHCT132PW is reasonable spec,price and shipping time (5 weeks)in MOQ=2000 => our choice. +25% price compared to 74HCT132PW and 5 weeks instead of 1 (SOIC chips), but nice choice.


Brainstorming - other options?

Is there any small/cheap but powerful output (20mA outputs, at least 50mA total) CPLD that operates from 5..6V (we would put it to PGND of course). The main processor would send CRC protected messages to it, and it could act as a full TPU. Since we have a lot of communication between the GND and PGND sides, some smarties would be justified. A smaller AVR or other u-controller could do it as well. Cost is more programming, benefit might be PCB real estate.

[XC9536] (44 pin package) is close, but still a bit low spec for pullup (pulldown 24mA is nice spec). It would work, but 74AHCT132 is probably safer design choice.

I would stay away from pal's and such. they have to be programed before they are soldered on (they will be soldered on blank and with the wrong code and the systems will have to be reworked)

they will lose there memory from temp, voltage, time

so you will have deaths over time and the parts will have to be replaced.

make sure: