New 24-Fet Voltage Regulator Circuit
The new 24-fet controller board has a nicely improved voltage regulator circuit that appears superior to the VR circuit on the older 18-fet XieChang board.
Looking at the new VR circuit it is clear that it is a simple
"buck" step down converter tailored to the specific requirements of the controller board 5 volt and 15 volt circuitry. It should be very efficient (generating very little waste heat) at dropping the battery voltage from 100V (max) down to the input of the 15V linear regulator. The voltage “window†for safe controller operation should be very large easily accepting any battery from 24 to 72 volts with no modifications needed.
This is my best guess (as of today) of the functional parts of the new switching regulator circuit.
View attachment 2 Higher res pic
How I Think It Works
I don’t physically have this board (yet) so I am guessing how the VR circuit might work (based on the pics from Keywin). Comments and/or corrections are welcome of course.
BTW, I like to call the “thin red wire†that connects the VR circuit to the battery (+) the “ignition wire†as it is meant to be used with a key switch to turn the controller on and off. Also, there is space for a single “power resistor†(R115) that may well allow for voltages in excess of 100V max. For now, let’s assume R115 is zero ohms (a piece of wire) when analyzing the VR circuit. I’ll try guessing at values for R115 later on.
FYI, the “7550-1†is a 5V linear regulator and the “78L15†is a 15V linear regulator with a rated max input voltage of 30V. The “100V diode†is a SS110 (100V) schottky diode that (I believe) conducts current from GND during the “buck†switching cycles. I think that Q13 is the “buck†switch (I’m guessing it’s a mosfet) and that Q17 and Q18 are transistors that create a hysteresis feed back loop that controls Q13 switching frequency and keeps the input to the 15V regulator below the 30V max value. Also, my experience with 12-fet controllers tells me that the minimum current to the 15V regulator is about 30ma (motor, halls and throttle not connected) AND maybe 70ma (everything hooked up with motor cruising under load at 50% wide open throttle). The 15V bus powers the gate drivers (24 mosfets) and the 5V bus powers the “116†chip and the throttle/motor hall sensors.
I am guessing that this “buck†converter is switching at a very high but varying harmonic frequency depending on the loads and voltages applied to the circuit. Again, I am just guessing here. So let’s slow down time into microseconds and try to see what’s going on.
Idea 1: Ignition off. All voltages and currents in the circuit are ZERO. Q13 (switch) is OPEN. Transistors Q17 and Q18 are open (not conducting any current).
Idea 2: Ignition just turned on (say 100V bat). Cap C2 starts to charge up and the voltage increases. R117(15k) and R118(3.1k) start to conduct (this is a voltage divider). At 100V bat this divider (midpoint) cannot exceed 17V. The current thru this divider cannot exceed 5.5ma.
Idea 3: The divider is connected to the base of Q17. Q17 is placed between 2 resistors (R119 and R120, both 2.1k). Before Q17 starts to conduct, the voltage across it is full bat voltage 100V.
Idea 4: As the base voltage of Q17 begins to rise, Q17 starts to conduct. AND this circuit “pulls down†the gate voltage of Q13 (and current starts to conduct thru R116 (1.2k)). Q13 is now CLOSED and current runs to the “buck†inductor. The voltage across the inductor “jumps†from zero to 100V and current starts flowing thru the inductor and the inductor magnetic field grows (storing energy).
Idea 5: The more Q17 conducts, the lower the voltage drop across it. If Q17 was completely closed the voltage across it would be close to zero. Now we would have three resistors in series (R116, R119 and R120). But Q17 emitter voltage would be about 39V way above the base voltage max of 17V (Q17 would stop conducting). A-Ha! But we do have (a very small) cap C129 in series with R121. This provides some hysteresis causing a momentary voltage boost above 17V. This is a funky circuit. :lol:
Idea 6: As the voltage to the 15V regulator climbs to 30V (max allowable), ANOTHER voltage divider (R124-3.3k and R123-2.2k) is conducting. Eventually the voltage (in) to the 15V regulator climbs to maybe 30V (max allowable). When this happens (I guess) the Zener diode (ZD1) starts to conduct. AND this causes the base of Q18 to rise thus causing Q18 to conduct. When Q18 conducts the base of Q17 plummets to zero volts. And this OPENS switch Q13.
Idea 7: Well the inductor is “energized†but Q13 is open so the voltage before the inductor MUST drop BELOW zero volts as it “pumps†current (releases energy) into the 30V cap that feeds the 15V regulator. The schottky diode completes the circuit and conducts current from ground to the inductor coil.
Idea 8: Eventually the voltage to the 15V regulator drops under the 30V (max allowable), and Zener diode (ZD1) stops conducting. The base of Q18 goes to zero and the Q17 - Q19 “dance†starts up again (charging) the inductor and maintaining the 30V “bus†to the 15V regulator.
Cool. 8)
