Introduction
Inverters convert DC power into AC power to operate AC equipment and devices. They utilize power electronic switching at different frequencies to generate the AC output. This articles examines low frequency inverters operating near the AC line frequency versus high frequency inverters using much higher switching frequencies. The comparative advantages and disadvantages are analyzed to determine which approach is better suited for different applications.
Defining Low and High Frequency Inverters
Low Frequency Inverters
- Use switching frequencies close to 50/60Hz line frequency
- Typically 100 Hz to 5 kHz range
- Requires large inductors and capacitors
High Frequency Inverters
- Switch at much higher 10s of kHz to MHz frequencies
- Very high ratio between switching and output frequency
- Allows use of smaller filter components
Frequency Ratio
- Low frequency inverters have ratio of up to 1:100
- High frequency inverters reach 1:1000 to 1:50,000 ratios
Low Frequency Inverter Operation
[Simple circuit diagram]
- Full bridge topology commonly used
- MOSFETs or IGBTs switched at line frequency rates
- Large AC inductor filters output to smoothen waveform
- Produces nearly sinusoidal voltage at line frequency
High Frequency Inverter Operation
[Simple circuit diagram]
- Similar bridge topology used
- Operates at much higher switching frequency
- Smaller inductor and capacitor filters the output
- Generate clean sine wave at desired line frequency
Comparison of Characteristics
| Parameter | Low Frequency Inverter | High Frequency Inverter |
|---|---|---|
| Topology | Full bridge usually used | Most use full bridge |
| Switching Frequency | 100 Hz to 5 kHz | 20 kHz to 1 MHz |
| Frequency Ratio | Up to 1:100 | Up to 1:50,000 |
| Filter Size | Large inductors and capacitors | Small inductors and capacitors |
| Semiconductors | BJTs, MOSFETs, IGBTs | MOSFETs, IGBTs |
| Control Complexity | Simple | Complex high-speed control |
| Efficiency | Moderate | Very high |
| Power Density | Low | Very high |
| Distortion | Higher harmonics present | Low harmonic distortion |
| Response Speed | Slow | Very fast |
| Physical Size | Large | Compact |
| Weight | Heavy | Light |
| Applications | UPS, motor drives, PV inverters | Drives, UPS, aircraft, medical |
Low Frequency Inverter Benefits
Some benefits of low frequency inverters include:
Simplicity
- Requires only basic controls for switching
- Open loop voltage control is straightforward
- Does not need specialized driver circuits
Robustness
- High overload capacity due to larger components
- Less affected by noise and temperature variations
- High reliability and ruggedness
Low EMI
- Very little high frequency interference generated
- Minimal filtering needed for EMI compliance
Low Acoustic Noise
- Any switching noise present at inaudible frequencies
- Minimal acoustic noise issues
Lower Cost
- Cost effective magnetic components
- Simple semiconductor selection
- Minimal additional circuitry needed
High Frequency Inverter Advantages
Salient advantages of high frequency inverters:
Compact Size
- Extremely small due to high-frequency magnetics
- High power density up to 50W/cubic-inch
- Ideal for lightweight portable applications
Fast Response
- Very fast transient response speed
- Precise and agile control ability
- Well suited for dynamic speed or voltage regulation
High Efficiency
- Up to 97% efficiency achievable
- Lower conduction and switching losses
- Reduces running costs and heat sinking needs
Light Weight
- Massive reduction in magnetics and filters
- Superior power-to-weight ratio
- Easier mounting and portability
Quiet Operation
- Any audible noise pushed into ultrasonic range
- Nearly silent operation suitable for occupied spaces
Disadvantages of Low Frequency Inverters
Some drawbacks of low frequency inverters include:
Large Size
- Significant space needed for large inductors and capacitors
- Limits applications with space constraints
Slower Response
- Cannot quickly change output voltage or frequency
- Not suitable for dynamic speed or voltage control
Distortion
- May have considerable lower order harmonics
- Additional filtering required for sensitive loads
Acoustic Noise
- Audible noise around switching frequency
- Irritating hum or buzz if in human audible range
Lower Efficiency
- Hard-switched topology causes increased losses
- Heat sinking needs add cost and size
Drawbacks of High Frequency Inverters
Some limitations of high frequency inverters:
Complexity
- Requires sophisticated control methods and ICs
- Challenging gate drive design for reliable switching
- Overall system design complexity increased
EMI Issues
- High frequency switching generates interference
- Needs substantial EMI filtering
Reliability Concerns
- Hard switching and thermal cycling degrade reliability
- Device failure rates may be higher
Acoustic Noise
- Ultrasonic losses need to be attenuated
- Can cause issues with adjacent electronic systems
Higher Cost
- Specialized high-speed devices increase costs
- Additional circuits add expense
- May require modular construction
Choosing Between Low and High Frequency Inverters
Low frequency inverters are advantageous for:
- Simple low cost inverter designs
- High reliability requirements
- High overload capacity needed
- Situations where EMI is detrimental
- Applications with audible noise constraints
High frequency inverters are better for:
- Very compact portable systems
- Weight critical applications like aerospace
- High precision fast response speed needed
- High efficiency essential to reduce losses
- Meeting tight harmonic distortion requirements
Conclusion
- Low frequency inverters are simpler, more robust and easier to control.
- High frequency inverters enable miniaturization, fast response, efficiency and ultra-quiet operation.
- The choice depends on the specific size, performance, cost, reliability and noise criteria for the application.
- Hybrid inverters running at medium frequencies can balance the tradeoffs.
- Advances in topology, modulation, devices and materials continue to improve low and high frequency inverter performance.
Frequently Asked Questions
What is the typical frequency range considered as “high frequency” for inverters?
Inverters operating from around 10 kHz up to 1 MHz are considered high frequency, compared to low frequency inverters running under 5 kHz.
What are the main advantages of low frequency inverters?
Simplicity, ruggedness, low EMI, and low acoustic noise are some of the advantages of low frequency inverters. They also have higher overload capacity.
What semiconductor devices are commonly used in high frequency inverters?
MOSFETs, IGBTs, and new widebandgap devices like GaN transistors allow switching frequencies up to 1 MHz range for high frequency inverters.
What are some examples of applications suited for high frequency inverters?
Motor drives, uninterruptible power supplies, induction heating, avionics, EV charging, renewable energy integration favor high frequency inverters for their compact, lightweight and dynamic characteristics.
How does efficiency compare between low frequency and high frequency inverters?
Properly designed high frequency inverters can achieve over 97% efficiency by using advanced devices to minimize switching and conduction losses at higher frequencies.
