My experience with the 12-fet Infineon tells me that there is a 48V “mentality†when it comes to the “off-the-shelf†voltage regulator design. I worked around this limitation using the “transistor mod†to allow safe operation at 36V or 72V or higher.
The 12-fet uses an LM317 to regulate the 12V bus while the 18-fet uses the comparator circuit. These are two completely different voltage regulator designs.
It’s not just the voltage to consider but also the current in the regulator.
The 12-fet needs a minimum of about 30ma (motor/throttle not connected) and peaks at about 70ma (motor running at about ½ throttle). These values where obtained from direct measurement.
I would guess that the 18-fet pcb would require more current than the 12-fet. Maybe even as high as 100ma.
If we use the 92V bat as an example and we have 27ma from the "base series resistor(s)" the remaining current required passes thru the “primary†transistor and the “switched series resistorsâ€Â. In the example the “switched series resistors†= 500 ohm.
So passing 10ma thru the 500 ohm creates a V drop of 5V.
Passing 20ma thru the 500 ohm creates a V drop of 10V.
Passing 40ma thru the 500 ohm creates a V drop of 20V.
Passing 60ma thru the 500 ohm creates a V drop of 30V.
But the overall V drop is 92 – 12 = 80V. This means there must some voltage drop across the “primary†transistor. I suspect that the “primary†transistor is really acting as a “variable†resistor as controlled by the secondary transistor and the comparator feedback circuit.
In any event … the “primary†transistor (TO92) might get very hot during 92V operation.
Passing 10ma thru the “primary†transistor creates a V drop of (80-5) = 75V.
W = V x A = 75 x 0.01 = 0.75 Watts
Passing 20ma thru the “primary†transistor creates a V drop of (80-10) = 70V.
W = V x A = 70 x 0.02 = 1.4 Watts
Passing 40ma thru the “primary†transistor creates a V drop of (80-20) = 60V.
W = V x A = 60 x 0.04 = 2.4 Watts
Passing 60ma thru the “primary†transistor creates a V drop of (80-30) = 50V.
W = V x A = 50 x 0.06 = 3 Watts
Also this 18-fet pic from Geoff57 looks different … no circuits to short out on the controller case.
