Inductors in Parallel
When two or more uncoupled inductors are connected in parallel, each inductor experiences the same voltage while the total current is shared between them. The equivalent inductance is always less than the smallest individual inductance. Parallel-connected inductors are used in power electronics, filters, high-current converters and experimental circuits where lower inductance or greater current capability is required.
Parallel Connection
In a parallel connection, both terminals of each inductor are connected together so that all inductors have the same voltage across them.
+------L1------+
------+ +------
+------L2------+
The total current divides between the parallel branches according to their impedance.
Equivalent Inductance (Uncoupled Inductors)
For uncoupled inductors connected in parallel, the reciprocal of the total inductance equals the sum of the reciprocals of the individual inductances.
1/LT = 1/L1 + 1/L2 + 1/L3 + ...
For two inductors only:
LT = (L1 × L2) / (L1 + L2)
Example:
L1 = 100 µH L2 = 100 µH LT = (100 × 100) / (100 + 100) LT = 50 µH
Current Sharing
The total current supplied to the circuit is divided between the inductors. Ideally, identical inductors share the current equally.
| Inductors | Current Sharing |
|---|---|
| Identical inductors | Approximately equal current. |
| Different inductances | Current distribution depends on inductive reactance and circuit conditions. |
Voltage Across Parallel Inductors
Every inductor connected in parallel has the same voltage across its terminals.
V1 = V2 = V3
This is the same rule that applies to all parallel-connected components.
Magnetic Coupling
If parallel inductors are mounted close together, their magnetic fields may interact through mutual inductance. This changes the equivalent inductance and current distribution.
In most practical circuits, inductors are positioned so that unwanted magnetic coupling is minimised.
Advantages
- Reduces the overall inductance.
- Can increase the total current-handling capability when current sharing is appropriate.
- Provides flexibility when standard inductance values are unavailable.
- May reduce heating by sharing current between inductors.
- Useful in prototype and experimental circuits.
Limitations
- Equivalent inductance is lower than the smallest individual inductance.
- Unequal inductors may not share current evenly.
- Magnetic coupling may affect circuit performance.
- Additional PCB space is required.
- Differences in DCR can cause uneven current distribution.
Typical Applications
| Application | Purpose |
|---|---|
| High-Current DC-DC Converters | Share load current. |
| Power Supplies | Reduce effective inductance. |
| Laboratory Prototypes | Create custom inductance values. |
| High-Power Electronics | Increase current capability. |
| Filter Networks | Adjust inductance to meet design requirements. |
Worked Examples
| Parallel Combination | Equivalent Inductance |
|---|---|
| 10 µH || 10 µH | 5 µH |
| 22 µH || 22 µH | 11 µH |
| 100 µH || 100 µH | 50 µH |
| 1 mH || 1 mH | 0.5 mH |
Testing Parallel Inductors
- Measure the equivalent inductance using an LCR meter.
- Verify continuity of each branch.
- Measure the DC resistance of each inductor.
- Inspect for overheating or damaged windings.
- Check for unwanted magnetic coupling if measurements differ from calculations.
Common Mistakes
- Assuming inductances add directly in parallel.
- Ignoring differences in current rating.
- Using inductors with significantly different DCR values.
- Overlooking magnetic coupling between closely spaced inductors.
- Selecting replacement inductors based only on inductance.
Interesting Facts
- Two identical inductors connected in parallel produce half the original inductance.
- Parallel inductors can improve current-handling capability when properly matched.
- Unequal winding resistance may cause one inductor to carry more current than the other.
- High-current power converters sometimes use multiple parallel inductors or multiphase designs to distribute current.
- PCB layout can influence current sharing in parallel inductors.
Key Points
- The equivalent inductance of uncoupled parallel inductors is always less than the smallest individual inductance.
- All parallel inductors have the same voltage across them.
- The total current is shared between the branches.
- Magnetic coupling and DCR differences can affect current distribution.
- Current rating, saturation current and DCR should all be considered when connecting inductors in parallel.