In air conditioning compressors (especially in variable-frequency air conditioning compressors), the rectifier bridge plays a crucial role in converting the AC input power into DC electricity, providing a stable DC bus voltage (DC Bus) for the subsequent inverter drive circuit. This is one of the core steps in achieving variable-frequency speed control of the compressor motor.
The following are the specific applications and key technical details compiled by SHYSEMI (for more information, you can send a message privately):
System Position:
Located at the front end of the power input of the outdoor electronic control board of the air conditioner, immediately after the AC input (utility power or three-phase power).
Typical Path:
AC input → EMI filter → Rectifier bridge → DC bus capacitor filtering → Inverter bridge (IGBT/MOSFET) → Compressor motor
Core Function:
- AC-DC Conversion: Convert the input AC power (single-phase 220V or three-phase 380V) into pulsating DC power.
- Build DC Bus: Cooperate with the large-capacity electrolytic capacitor in the subsequent stage to form a stable high-voltage DC bus (DC Bus) (e.g., approximately 310V DC when single-phase 220V input, approximately 540V DC when three-phase 380V input).
- Power Supply for Inverter: The DC bus voltage supplies the inverter (composed of IGBT or MOSFET), and the inverter then converts the DC power into adjustable three-phase AC power to drive the compressor motor to operate at variable speeds.
Rectifier Bridge Type and Topology
Single-phase air conditioner (common in household use)
- Topology: Standard single-phase full-bridge rectification (4 diodes)
- Input: 220V AC (50Hz/60Hz)
- Output: Pulsating DC → Filtered to obtain ≈310V DC bus voltage.
- Rectifier Bridge Example: GBU/J series (e.g., GBU1506: 600V/15A).
Three-phase air conditioner (commercial/ high-power)
- Topology: Three-phase full-bridge rectification (6 diodes)
- Input: 380V AC (three-phase three-wire or four-wire system)
- Output: Pulsating DC → Filtered to obtain ≈540V DC bus voltage.
- Rectifier Bridge Example: Modular three-phase rectifier bridge (e.g., D30JCB80: 800V/30A).
Key Technical Challenges and Design Points
1.High Surge Current (Critical)
Problem: During the air conditioner startup, a large-capacity DC bus capacitor (often up to several hundred to several thousand μF) needs to be charged, while the compressor rotor is in a stationary state (with a large starting torque), resulting in extremely high instantaneous surge current, up to 5-10 times the rated current.
Solution:
Rectifier Bridge Selection: Must select a model with IFSMb> (peak surge current) value high (such as 50A-100A or above).
Pre-Charging Circuit:
Connect a current-limiting resistor (+ relay) or NTC thermistor in series at the output of the rectifier bridge.
During startup, first charge the capacitor slowly through the resistor/NTC, and after the bus voltage is established, short-circuit the resistor (or the resistance value of the NTC decreases) to avoid long-term loss.
If not handled: The rectifier bridge may be burned out due to the surge current!
2.High Voltage Rating
- Selection Rules:V_RRM ≥ √2 × Peak Input Voltage × Safety Factor (usually ≥ 1.5)
Example:
- Single-phase 220V input: Peak voltage = 220 × √2 ≈ 311V → Select 600V or above (such as 600V/800V).
- Three-phase 380V input: Line voltage peak = 380 × √2 ≈ 537V → Select 800V/1000V or above.
3.Heat Dissipation Design (High Temperature Environment)
Power Consumption Source:
Conduction Loss: P_loss = 2 × V_F × I_rms (single-phase bridge) or P_loss ≈ 3 × V_F × I_rms (three-phase bridge, simplified calculation)
V_F: Forward Voltage Drop of Diode (approximately 1.0V-1.5V, higher for large current).
Heat Dissipation Measures: Force air cooling: The rectifier bridge is closely attached to the heat sink, and it cools down using the outdoor fan of the air conditioner.
Heat transfer material: Apply high-performance thermal conductive silicone grease to ensure a tight contact between the metal baseplate of the rectifier bridge and the heat sink.
Temperature monitoring: Some high-end models install temperature sensors on the heat sink to protect the machine from abnormal high temperatures.
4. Suppression of high-frequency interference
Problem: The rectifier bridge switch generates high-frequency noise instantaneously, especially during reverse recovery current, which interferes with the control system.
Solution:Parallel X capacitors and common-mode inductors (located in the EMI filter) are connected to the AC input end of the rectifier bridge.
High-frequency ceramic capacitors (0.1μF - 1μF) are connected in parallel to the DC output end to absorb high-frequency ripple.
Common faults and maintenance concerns
Breakdown short circuit:
- Symptom: The air conditioner trips the circuit breaker or the fuse melts.
- Cause: Excessive surge current, poor heat dissipation, voltage spikes (such as lightning strike).
- Detection: Measure the resistance between the pins of the rectifier bridge using a multimeter's diode setting. If the resistance between any two pins is close to 0Ω, it indicates a breakdown.
Open circuit failure:
- Symptom: The DC bus voltage is 0 or very low, and the compressor does not start.
- Cause: Internal diodes are burned out.
- Detection: The AC input is normal, but there is no voltage at the DC output.
Performance degradation due to poor heat dissipation:
- Symptom: The air conditioner stops running after a period of operation, and the heat sink is hot to the touch.
- Treatment: Clean the heat sink dust; check if the thermal conductive silicone grease is dry; replace the heat sink fan.
Differences from fixed-frequency air conditioners
Fixed-frequency air conditioner compressor:
Directly controlled by AC contactors for on-off, no need for frequency conversion drive.
Usually, no rectifier bridge (unless the control board requires a low-voltage DC power supply, in which case a small-power rectifier can be used).
Fixed-frequency air conditioner compressor:
Must rely on the rectifier bridge + inverter bridge to achieve motor speed regulation. The rectifier bridge is the core hub for energy conversion.
Summary
The core value of the rectifier bridge in the air conditioner compressor
Final goal: Provide a stable, low-ripple DC bus voltage for the subsequent inverter, ensuring the compressor motor achieves precise frequency conversion control, thereby improving the energy efficiency ratio (APF), reducing noise and enhancing comfort. During maintenance, focus on checking for surge and heat dissipation issues!