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Attribute

Single Buck

4 Bucks

Component Selection

  • Requires a relatively expensive buck controller IC

  • IC usually has more features than what is required for this application

  • more complex circuit layout due to using external FETs

  • Can use buck ICs with internal FETS

  • ICs easier to find since lower current requirements

Cost

  • Probably going to be more expensive due to more high performance components

Board Layout / form factor

  • Likely going to need to take up more space due to having 3 discrete bucks instead of one

Decision: We selected single buck option to save board space.

Component Selection

Buck ICs

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  • AAM mode improves the efficiency of the the Buck at light or no load, and since we are powered by a battery, it would make sense to improve the efficiency of the board where possible.

  • Choose 196K resistor, this keeps the V_AAM at 600mV which is above the 480mV (R_FREQ is also 200K so the resistors cancel out which leaves V_AAM at 600mV).

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Soft-Start Capacitor

  • Soft-start capacitor is used to determine the soft-start time, prevents converter output voltage from overshooting rated voltages

  • If we want a t_ss of ~100ms, I_ss = 4 uA, V_REF = 0.8V, C_ss = 500nF

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Purpose: generate bias to drive the gate of the High Side FET

We could also use a P-MOS for the switching FET but:

→ Extra board space & larger Rdson

→ Extra money

→ N-MOS + bootstrap circuit is cheaper

→ use N-MOS as switching FET

How it works:

  1. When SW node is GND, internal 5V VCC charges up Cboot through forward biased diode

  2. When SW node is Vin, Cboot still maintains that 5-Vf voltage since the charges on the capacitor has nowhere to go and V = Q/C. The schottky diode blocks current from flowing back to the voltage source.

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Why do we need this filter?

The thrust motors on the fixed wing plane draws significant current. This high current generates a huge magnetic field and we want to avoid inductive coupling to servo module’s power rails. In addition, if the motors are not grounded properly, the noisy ground will be passed over to servo module’s ground as both servo motor and DC motor share the same battery groundcommon mode choke serves as a protective measure against potential ESC and environmental EMI common mode noises. Typically, it functions as a short, connecting the battery and the buck converter input. The power FETs of the ESC constantly alternate between the battery's power and ground, causing ringing at both VBAT and ground, known as common mode noise. Since the servo module draws power from the same battery, the noise has the potential to be transferred to the buck converter's input. Ideally, the ESC addresses its FET ringing issue on the PCB. We added common mode choke for an extra layer of noise protection.

How does the common mode choke work?

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