Difference Between Unipolar SPWM & Bipolar SPWM

1. Basic Definition

SPWM (Sinusoidal Pulse Width Modulation) is widely used for BLDC sine drive, single-phase / three-phase inverters, UPS and power converters. According to the level of output voltage during switching, it is divided into unipolar SPWM and bipolar SPWM.

2. Core Working Principle

Bipolar SPWM

The carrier triangular wave swings between +Vcc and -Vcc.

Reference sine wave compares with triangle wave
The output terminal only has two states: +DC bus voltage / -DC bus voltage
No zero voltage level during operation

Unipolar SPWM

Triangular carrier fluctuates between 0 ~ Vcc. Each bridge arm operates independently:

Upper arm: switch according to sine positive half cycle
Lower arm: switch according to sine negative half cycle Output contains three levels: +Vbus / 0V / -Vbus

3. Key Differences Comparison Table

Item	Bipolar SPWM	Unipolar SPWM
Output voltage levels	2 levels (+Vbus, -Vbus)	3 levels (+Vbus, 0, -Vbus)
Switching action	Both upper & lower tubes of H-bridge switch simultaneously	Only one side switches at any moment, opposite arm keeps OFF
Voltage swing amplitude	Full DC bus voltage jump (+Vbus ↔ -Vbus)	Voltage jumps between 0 and Vbus, smaller voltage step
dv/dt value	Large	Small
EMI noise	Higher	Lower
Harmonic distribution	Main harmonic near 2×carrier frequency	Main harmonic near carrier frequency; easier filter
Filter requirement	Larger inductor / capacitor needed	Smaller filter parameters, lower loss
Loss characteristic	Higher switching loss	Lower switching loss
MCU algorithm difficulty	Simple, easy programming	More complex sampling & logic
Typical application	Low-cost small inverter, simple BLDC driver prototype	High-performance sine BLDC, audio amplifier, high-quality UPS

4. Advantages & Disadvantages

Bipolar SPWM

✅ Advantages

Algorithm simple, low requirement for MCU computing power
Less logic code, stable debugging
Suitable for low-cost control scheme

❌ Disadvantages

Large voltage jump, high dv/dt, serious EMI interference
High switching loss, MOS generates more heat
Output current ripple bigger

Unipolar SPWM

✅ Advantages

Small voltage transition amplitude, greatly suppress EMI
Lower switching loss, reduce MOS heating
Current ripple smaller, motor vibration & noise lower (critical for BLDC)
Filter volume can be reduced

❌ Disadvantages

Complicated program implementation
Need higher performance MCU (such as China Micro MCU)
Must strictly prevent upper and lower arms simultaneous shoot-through

5. Practical Engineering Suggestion for BLDC Motor

Cost-sensitive low-end fan/pump: choose Bipolar SPWM
High-end silent BLDC equipment (vacuum cleaner, garden tools, precision water pump): adopt Unipolar SPWM
When using unipolar SPWM: match Winsok low Qg MOSFET to further cut switching loss
No matter which mode, dead time setting is necessary to avoid H-bridge shoot-through damage MOS tubes

6. Summary

The most essential difference: Bipolar SPWM switches both bridge sides, voltage flips between positive and negative bus voltage. Unipolar SPWM only switches single arm, introduces zero voltage level, brings lower ripple, less EMI and less heating.