Most ESC briefs begin with a simple requirement: “4S to 6S.” That gives a useful starting point, but cell count alone does not settle the MOSFET choice. A practical first pass is to translate the pack into its full-charge voltage, allow for switching transients, then work through current, conduction loss and heat. The same sequence helps whether you are reviewing a new design or narrowing a shortlist for an existing board.
This guide uses standard LiPo packs as the reference and focuses on the checks that matter in a compact multirotor ESC. Final selection still depends on the switching topology, layout, gate drive, package and cooling available in the actual design.
1. Translate 4S–6S into battery voltage
“S” is the number of cells connected in series. A standard LiPo cell is about 3.7 V nominal and reaches 4.2 V when fully charged. For voltage-rating checks, use the full-charge value—not the nominal voltage printed on a pack label.
| Pack | Nominal voltage | Full-charge voltage |
|---|---|---|
| 4S LiPo | 14.8 V | 16.8 V |
| 6S LiPo | 22.2 V | 25.2 V |
Some pilots use LiHV packs, which can be charged to 4.35 V per cell. That puts a 4S LiHV pack at 17.4 V and a 6S LiHV pack at 26.1 V. Confirm the cell chemistry and charge limit in the customer’s specification before setting the bus-voltage requirement.
2. Choose VDS with transient margin
The MOSFET’s drain-to-source rating (VDS) is an absolute limit, not a normal operating target. Start with the highest pack voltage, then leave headroom for the voltage spikes created by switching-loop inductance, commutation and motor energy returning to the DC bus. A tight PCB layout and good gate-drive design help, but they do not replace measurement.
| Design scope | Pack at full charge | Initial VDS screening direction |
|---|---|---|
| Standard 4S LiPo | 16.8 V | 40 V class is a common starting point |
| Standard 6S LiPo | 25.2 V | 60 V class provides more voltage headroom |
| One ESC for 4S–6S | Up to 25.2 V | Screen around the 60 V class, then validate the actual peak |
These are shortlist guidelines, not a substitute for checking the waveform. Measure VDS at the MOSFET terminals under the most demanding operating conditions, using a suitable probe and enough bandwidth to capture ringing. Keep the measured peak below the device rating with margin for tolerances, temperature and operating variation. If the design uses LiHV, repeat the check at the higher pack voltage.
3. Size current for the real motor and propeller
There is no single ESC current value that can be inferred from “4S–6S.” Current depends on the motor, propeller, throttle profile, battery, airflow, PCB copper and thermal path. Treat continuous current and short-duration peak current separately, and test with the motor and propeller combination the customer intends to use.
Also distinguish battery current from phase current. Battery current is measured on the DC input; the three motor phases carry switching waveforms, and the current through each MOSFET varies with PWM duty and commutation. The MOSFET’s headline current rating is usually specified under stated case-temperature and mounting conditions. It does not automatically equal the current a finished ESC can deliver continuously.
For a first estimate, use the system’s expected power and bus voltage to check battery current, then use the ESC topology, modulation and measured phase waveforms to estimate each device’s RMS current and conduction interval. Validate the board at hot ambient temperature and realistic airflow, checking junction-temperature margin, copper temperature and connector limits as well as the MOSFET data sheet.
4. Relate current to RDS(on)
For a MOSFET carrying current while it is on, a useful first-order estimate of conduction loss is:
Pcond ≈ IRMS2 × RDS(on)
Current is squared, so even a modest change in RDS(on) can matter at high current. For example, at 20 A RMS while conducting, one device with 5 mΩ dissipates about 2 W; at 2.5 mΩ, the estimate is about 1 W. This is a simplified per-device comparison. In an ESC, actual dissipation depends on the current waveform, PWM duty, commutation and time spent conducting. Use hot RDS(on) at the intended gate voltage for a realistic estimate.
Choosing the lowest RDS(on) is not always the best answer. A lower-resistance device may have higher gate charge or capacitance, which can increase switching loss and place more demand on the driver. Compare RDS(on) at the actual gate-drive voltage and temperature alongside gate charge, switching behaviour, package, thermal impedance and safe operating area.
5. Where dtPower Semi MOSFETs fit in the shortlist
The following parts illustrate how voltage rating and on-resistance shape an initial screen. They are not final design approvals; check each current data sheet for package, test conditions, gate-charge data, thermal limits and transient performance against the ESC design.
| Device | VDS | RDS(on) | Initial screening use |
|---|---|---|---|
| DTPMN30R0014LA | 30 V | 1.4 mΩ | Lower-voltage designs where measured VDS peaks remain comfortably within the derated limit; not a default 6S choice. |
| DTPMN40R0055LA | 40 V | 5.5 mΩ | Candidate to evaluate for standard 4S designs when its conduction and switching losses fit the thermal budget. |
| DTPMN60R0025LA | 60 V | 2.5 mΩ | Natural starting point for 6S or a shared 4S–6S platform that needs additional voltage headroom. |
A 60 V device can simplify the voltage-margin discussion on a 6S platform, while a lower-resistance 30 V part can look attractive on paper. The measured peak voltage, required current and total loss decide whether either is appropriate in a particular ESC. Include switching loss and heat removal before finalizing the choice.
Engineering checklist
- Confirm standard LiPo or LiHV chemistry and the maximum charge voltage.
- Choose a VDS class with margin, then measure switching peaks on the actual board.
- Define continuous and peak current from the motor, propeller and operating profile.
- Separate DC battery current from phase current and per-device RMS current.
- Estimate conduction loss using hot RDS(on); add switching and dead-time losses.
- Check gate drive, layout, package, cooling and temperature in the real ESC.
FAQ
What is the voltage of a 4S or 6S LiPo battery?
A standard LiPo is 14.8 V nominal and 16.8 V fully charged for 4S. For 6S, it is 22.2 V nominal and 25.2 V fully charged. Use the full-charge value for voltage-rating checks.
Is a 40 V MOSFET enough for a 4S ESC?
It is a common starting point for standard 4S designs, but it is not automatically safe. Measure VDS overshoot at the device terminals under worst-case conditions and keep the peak below the derated rating with margin.
Should I choose a 60 V MOSFET for a 6S ESC?
A 60 V device is a sensible initial screen for a standard 6S platform because the pack reaches 25.2 V at full charge. The final rating still depends on measured switching transients, the device’s derating policy and whether the system uses LiHV cells.
How do I choose the MOSFET current rating?
Start with the ESC’s real continuous and peak requirements for its motor and propeller, then estimate or measure phase current and per-device RMS current. Check the data-sheet conditions behind the current rating and verify the complete board thermally; do not treat the MOSFET’s headline current number as the ESC rating.
Does lower RDS(on) always mean a better MOSFET?
No. Lower RDS(on) reduces conduction loss, but gate charge, switching loss, package, voltage margin and cooling also matter. Compare the whole power stage at the intended gate drive and operating temperature.