MOSFET Parallel Current Sharing Calculator
Estimate static current distribution in a parallel MOSFET bank using nominal RDS(on), tolerance, hot-resistance multiplier, device count, and duty cycle.
The worst-case model assigns one device the minimum resistance and all remaining devices the maximum resistance to expose current and thermal imbalance before PCB verification.
Engineering tool
MOSFET Parallel Current Sharing Calculator
Estimate static current imbalance, worst-case device current, RDS(on) mismatch, and conduction loss for parallel MOSFETs.
Total on-state current delivered by the parallel MOSFET bank.
Enter a whole number of at least two devices.
Reference on-resistance at the selected VGS and temperature.
Worst-case resistance spread around the nominal value.
Temperature multiplier applied equally to minimum and maximum resistance.
Fraction of time the MOSFET bank conducts current.
Current sharing status
High Current Imbalance
The estimated lowest-resistance MOSFET carries substantially more than ideal current. Improve device matching, use symmetrical copper and source paths, consider source ballast, and verify individual junction temperatures.
Result console
- Ideal current per MOSFET
- 10A
- Worst-case high-current MOSFET
- 13.333333A
- Other MOSFET current
- 8.888889A
- Lowest hot RDS(on)
- 6mΩ
- Highest hot RDS(on)
- 9mΩ
- Worst-case device power
- 1.066667W
- Total conduction loss
- 3.2W
- Current sharing imbalance
- 33.333333%
Formula reference
Parallel MOSFET Current Sharing Formulas
Current divides according to hot branch conductance. This static model does not include switching delay, common-source inductance, or transient gate mismatch.
Ideal current: Iideal = Itotal / NResistance limits: Rmin = RDS(on) × (1 − tolerance), Rmax = RDS(on) × (1 + tolerance)Hot values: Rmin,hot = Rmin × multiplier, Rmax,hot = Rmax × multiplierWorst-case current: Iworst = Itotal × (1/Rmin,hot) / [(1/Rmin,hot) + (N−1)(1/Rmax,hot)]Other-device current: Iother = (Itotal − Iworst) / (N−1)Worst-device loss: Pworst = Iworst² × Rmin,hot × DutyTotal loss: Ptotal = Pworst + (N−1) × Iother² × Rmax,hot × DutyVariable definitions
- Itotal
- total bank drain current
- N
- number of parallel MOSFETs
- Tolerance and duty are converted from percent to decimal
- All internal current, resistance, and power calculations use SI units
Worked Example
Itotal = 40 A, N = 4, nominal RDS(on) = 5 mΩ, tolerance = 20%, hot multiplier = 1.5, and duty = 100%.
Iideal = 40 / 4 = 10 A
Rmin,hot = 5 mΩ × 0.8 × 1.5 = 6 mΩ; Rmax,hot = 5 mΩ × 1.2 × 1.5 = 9 mΩ
Iworst ≈ 13.333 A; each other device carries ≈ 8.889 A
Pworst ≈ 13.333² × 0.006 = 1.067 W
Ptotal ≈ 1.067 + 3 × (8.889² × 0.009) = 3.2 W
Current imbalance above ideal is approximately 33.33%.
Engineering Notes
Positive temperature behavior
MOSFETs can share current better than BJTs because RDS(on) often increases with temperature in normal conduction.
Static mismatch
Static sharing still depends on RDS(on) tolerance, package and source resistance, and local temperature.
PCB symmetry
Symmetrical drain, source, and Kelvin measurement paths are important for parallel MOSFET current balance.
Dynamic sharing
Individual gate resistors and small source resistors may improve dynamic and static sharing in demanding designs.
Thermal environment
Thermal coupling, heatsink contact, airflow, and nearby heat sources affect current balance and device stress.
Support reference
FAQ
Can MOSFETs be connected in parallel?
Yes. Parallel MOSFETs are commonly used to increase current capacity and reduce conduction loss, but their gate drive, PCB layout, thermal environment, and electrical parameters must support balanced operation.
How do parallel MOSFETs share current?
Static current divides approximately in proportion to each branch conductance, so a MOSFET with lower total drain-source path resistance carries more current than a higher-resistance device.
Why does RDS(on) tolerance matter?
RDS(on) variation changes branch conductance. The lowest-resistance MOSFET can carry more than the ideal current, increasing its local loss and thermal stress.
Do parallel MOSFETs need separate gate resistors?
Separate gate resistors are often useful for damping individual gate loops and improving dynamic sharing. Their values and placement should be verified with switching waveforms and layout parasitics.
How do you reduce current imbalance?
Use matched devices, symmetrical drain and source copper, equal gate paths, Kelvin source connections, thermal coupling, and where necessary small source-balancing resistors.
Documentation
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