Engineering Guide
Parallel MOSFET Current Sharing: Static Balance, Gate Drive, Layout, and Thermal Design
Parallel MOSFETs are one electrothermal switching system, not interchangeable resistors. Reliable sharing requires branch-by-branch resistance, gate-loop, inductance, cooling, stress, and measurement checks.
Parallel-Bank Design Workflow
| Step | Action | Engineering Check |
|---|---|---|
| Define operating states | Separate steady conduction, switching edges, dead time, startup, current limiting, fault interruption, and reverse-current operation. | |
| Build branch models | Include hot RDS(on), package resistance, source and drain copper, connectors, fuses, shunts, and intentional ballast for every device. | |
| Solve static sharing | Divide current by branch conductance and evaluate production spread at the relevant gate voltage and junction temperature. | |
| Check electrothermal feedback | Iterate branch loss, temperature, and hot resistance instead of assuming every device has the same junction temperature. | |
| Analyze dynamic sharing | Compare gate charge, threshold, driver impedance, common-source inductance, loop inductance, and switching delay. | |
| Design symmetric geometry | Use matched power paths, individual gate resistors, Kelvin-source returns, and comparable cooling conditions. | |
| Check device stress | Verify the most heavily loaded device against loss, junction temperature, SOA, and transient limits. | |
| Measure each branch | Use low-inductance current sensing and simultaneous VGS, VDS, and branch-current waveforms over temperature and operating corners. |
Static Current Sharing
Formula reference
Conductance-based branch model
Gi = 1 / RiIi = ItotalGi / ΣGPi = Ii,rms²Ri,hotPbank = ΣPiVariable definitions
- Ri includes hot RDS(on) and every series branch resistance
- Use the relevant VGS, TJ, duty cycle, and production limits
- Solve every branch before calculating total loss
- The equal-share result Itotal/N is only the matched limit
The MOSFET Parallel Current Sharing Calculator evaluates a bounded static case with one minimum-resistance device and the remaining devices at maximum resistance. It does not predict switching-edge sharing.
Why Static and Dynamic Sharing Differ
| Mechanism | Effect | Design Response |
|---|---|---|
| RDS(on) spread | Lower total branch resistance carries more steady current | Use hot guaranteed data, symmetric copper, and bounded mismatch analysis |
| Temperature | RDS(on) usually rises with temperature in strong inversion, providing some steady negative feedback | Do not assume this corrects dynamic mismatch or every bias region |
| Common-source inductance | LCSI converts di/dt into source voltage that subtracts from effective VGS | Use Kelvin source and minimize unequal shared-source inductance |
| Gate-loop mismatch | Unequal resistance, inductance, charge, and threshold shift turn-on and turn-off timing | Give each MOSFET a local gate resistor and symmetric driver return |
| Power-loop inductance | Unequal overshoot and commutation impedance redistribute transient current | Keep drain and source paths compact and geometrically balanced |
| Cooling mismatch | Different case-to-sink or junction-to-board paths change hot resistance and margin | Match interface pressure, copper, vias, airflow, and nearby heat sources |
| Source ballast | Small source resistance strengthens local current feedback | Account for I²R loss, voltage drop, tolerance, and parasitic inductance |
Formula reference
Parasitic source feedback
VLS = LS × di/dtVGS,effective = Vdriver − VLS − other path dropsVariable definitions
- Common-source inductance participates in both the power and gate-drive loops
- Unequal LS can create substantial effective gate-voltage mismatch
- Sign and magnitude depend on topology and switching direction
Gate Drive and Layout
Place an individual damping resistor close to each gate, route matched gate and Kelvin-source paths, keep the commutation loop compact, and avoid daisy-chained source returns. The MOSFET Gate Resistor Calculator provides a first-pass drive estimate, but final values require branch waveform verification because gate charge and parasitics vary.
Thermal Iteration and Worst-Device Stress
Use each predicted branch current with its own hot resistance and thermal path. Recalculate junction temperature, update resistance, and repeat until the result converges. Check the worst device, not only average bank loss. The Conduction Loss Calculator and Junction Temperature Calculator support these bounded steps.
Worked Examples
Ideal sharing
A 60 A bank with three identical branches carries 20 A per MOSFET before mismatch.
Static conductance split
For branch resistances of 4, 5, and 5 mΩ at 60 A, the 4 mΩ branch carries 23.08 A and each 5 mΩ branch carries 18.46 A.
Loss concentration
The 4 mΩ branch dissipates about 2.13 W while each 5 mΩ branch dissipates about 1.70 W; current and loss are not divided equally.
Calculator worst case
With one minimum-resistance device and the remaining devices at maximum resistance, use the parallel-sharing calculator to expose the most heavily loaded branch.
Source ballast
Adding 1 mΩ to each 4 and 5 mΩ branch changes the ratio from 1.25:1 to 1.20:1, improving balance while adding bank loss.
Common-source inductance
A 2 nH mismatch at 500 A/µs creates a 1 V difference in effective source feedback, large enough to alter a fast switching edge.
Gate timing
A 10 ns turn-on mismatch at high di/dt can force one MOSFET to carry most of the transition current even when steady sharing is acceptable.
Thermal iteration
A branch that starts with lower resistance may heat differently; recompute each hot resistance from its own loss and thermal path.
Parallel conduction loss
Calculate loss from individual branch RMS current, not simply total current divided by device count when imbalance is material.
Measurement
Matched current shunts or calibrated current probes can reveal both steady offset and transient current hogging; probe loading and deskew must be controlled.
Common Mistakes
Related Calculators
Support reference
FAQ
Can power MOSFETs be connected in parallel?
Yes, but the bank must be designed for both static and dynamic current balance, thermal conditions, gate drive, layout parasitics, and worst-device stress.
How is static current divided between parallel MOSFETs?
For resistive on-state branches, current divides in proportion to branch conductance: Ii = Itotal(1/Ri) divided by the sum of all branch conductances.
Does a positive RDS(on) temperature coefficient guarantee current sharing?
No. It can provide useful steady-state feedback in strong inversion, but it does not remove initial resistance spread, unequal cooling, threshold effects, or dynamic timing mismatch.
Why use individual gate resistors?
A resistor placed at each gate isolates local gate loops, damps ringing, and allows switching behavior to be balanced without one common resistor coupling all gates.
What is common-source inductance?
It is source-path inductance shared by drain current and the gate-drive return. Its L di/dt voltage changes effective VGS and can create unequal switching behavior.
When are source-balancing resistors useful?
Small source resistors can improve static feedback when resistance spread is important, but their loss, voltage drop, tolerance, inductance, and thermal rating must be acceptable.
Should parallel MOSFETs share one heatsink?
A common thermal structure can improve temperature tracking, but each interface and local heat path must still be comparable and electrically isolated where required.
How should conduction loss be calculated for an imbalanced bank?
Calculate Irms squared times hot branch resistance for each MOSFET, then sum the branch losses. Do not assume equal current when the mismatch model predicts otherwise.
How can dynamic current sharing be verified?
Measure branch currents together with VGS and VDS using suitable bandwidth, low-inductance sensing, probe deskew, and realistic switching, temperature, and fault conditions.
Do matched part numbers ensure equal sharing?
No. Parts from the same type still vary, and PCB impedance, driver routing, thermal contact, and operating temperature can dominate the result.
Can a static current-sharing calculator predict switching transients?
No. It estimates resistive steady sharing and associated loss. Dynamic sharing requires gate-charge, threshold, driver, and parasitic-inductance analysis or measurement.
What limits the number of MOSFETs that can be paralleled?
Driver current, aggregate gate charge, layout symmetry, parasitic oscillation, cooling, protection, cost, and diminishing resistance reduction all impose practical limits.
Summary
Solve static current from hot branch conductance, then separately control dynamic mismatch through local gate damping, Kelvin-source routing, low and symmetric inductance, comparable cooling, and branch-level measurement. Validate the worst MOSFET against loss, junction temperature, and SOA over all operating states.