What’s a push pull inverter and how to choose?

In modern life, push pull inverter plays an important role in a variety of application scenarios because of its simple structure and low cost as a device that converts direct current to alternating current. This article will give you a detailed introduction to the working principle of push pull inverter, advantages and disadvantages, the difference between them and other types of inverters, and how to choose the right push pull inverter according to your needs.

Introduction to push pull inverter

Push pull inverters convert direct current to alternating current. It belongs to the relatively simple structure of the inverter, using a simple push-pull circuit topology, by alternating between the two internal switching elements (such as MOSFETs or transistors) to generate alternating current.

How a push pull inverter generates ac?

The whole working process is actually the process of switching elements on and off, thus generating alternating current.

When Q1 is switched on, DC current flows through Q1 from the positive side of the power supply into half of the primary side winding of the transformer.
When the current flows from the center tap to the Q1 end, a magnetic field is formed and a voltage is induced in the secondary winding of the transformer. Due to the flow of current in the primary winding, the secondary winding generates an induced voltage, the direction is determined by Lenz's law, and a half cycle of alternating current is formed in the secondary winding.

When the switching element Q1 is off and Q2 is switched on, the DC current flows through Q2 from the positive terminal of the power supply and enters the other half of the primary side winding of the transformer. At this time, the current direction is the opposite of stage 1, from the center tap to the Q2 end, changing the magnetic field direction of the primary winding of the transformer.

Due to the change in the direction of the current in the primary winding, the direction of the induced voltage generated in the secondary winding also changes, forming another half cycle of alternating current. In fact, the whole process is that through the drive circuit, the switching components Q1 and Q2 are switched on in turn, and Q2 is turned off when Q1 is switched on, and vice versa. The frequency of the drive signal determines the output frequency of the inverter.

Because the switching action of push pull inverter is usually on or off, the output waveform is usually square wave or improved square wave. It is usually necessary to add a filter to the output to have a sine wave output.

The main advantages and disadvantages of push pull inverter

The advantage is that the circuit structure is simple and easy to maintain: the circuit structure of the push pull inverter is relatively simple, and it is generally two switching elements and a transformer with a center tap. Because the circuit is simple, design and maintenance are relatively easy. It is generally suitable for low and medium power applications and appliances with low waveform requirements, such as small household appliances, portable power supplies and emergency lighting.

Check applications of inverter for more details.

The disadvantage is that the output waveform quality is poor, and push pull inverter usually produce square wave or improved square wave output, which contains a large number of harmonic components. Sensitive equipment is not suitable, it is easy to interfere with it, and it can not be directly used for audio equipment and precision instruments with high waveform quality requirements.

Due to the high harmonic distortion of the output waveform, electromagnetic interference (EMI) may be caused to the power grid and load equipment. Also due to the high-speed switching of the switching elements of the push pull inverter, it is easy to produce high-frequency noise and electromagnetic interference, and additional filtering circuits and shielding measures may be required to reduce interference.

The difference between push pull inverters, half-bridge inverters and full-bridge inverters

Push pull inverters and half-bridge inverters are among the more basic inverter topologies, suitable for low to medium power applications.

Circuit complexity: A half-bridge inverter usually has a slightly more complex circuit structure than a push pull inverter because it has two capacitors to divide the voltage, but it provides a more stable output than a push pull inverter. Half-bridge inverters are more commonly used in some cases with slightly higher waveform requirements, and push pull inverter are more suitable for application scenarios that require simple and low cost. Although half-bridge inverters generally also output square waves or improved square waves, the waveform distortion is slightly lower than that of push pull inverter.

Both full-bridge inverters and push pull inverter can convert direct current to alternating current, and both use switching elements (such as MOSFETs) to control the direction of current. Circuit complexity: The circuit of the full-bridge inverter is more complex than the push pull inverter, because it uses four switching elements, while the push pull inverter usually requires only two, and this structural difference also causes the output waveform to be different.

Full-bridge inverters can provide better output waveforms, such as sine waves or improved sine waves, which are suitable for waveform sensitive equipment. And in terms of power, full-bridge inverters are generally more efficient and more suitable for high-power applications, while push pull inverters are more suitable for low - and medium-power scenarios.

How to choose the right push pull inverter?

1. Output power
First of all, the rated output power of the inverter must be determined to meet the load demand. To meet the load demand, the rated power of the inverter should be slightly higher than the total power of the required load to prevent overload and provide a certain safety margin. It is also important to take into account that some devices will generate instantaneous high current when starting (such as motors, compressors, etc.), and it is necessary to ensure that the peak power of the inverter is sufficient to cope with these instantaneous loads.

2. Input voltage
The input voltage of the push pull inverter should be selected according to the output voltage of the inverter battery, such as the common input voltage of 12V, 24V, 48V and so on. If you use a 12V lithium ion battery pack, you need to choose an inverter that supports 12V input. If the input voltage is inconsistent with the power supply, the inverter may fail, fail to work properly, or be damaged.

3. Output waveform
Because push pull inverter usually output square waves or modified square waves, they are not applicable if the appliance you are using requires high-quality pure sine waves. Push pull inverters are suitable for loads that do not require high waveform quality. So if you want to drive audio equipment, medical instruments and other waveform sensitive equipment, then the push pull inverter may not be suitable.

4. Protection function
Because the two switching components of the push pull inverter need to work strictly alternately, if there is a synchronization error, it may lead to short circuit, so the inverter should have a short circuit protection function to improve safety.

Some inverters with overload protection function can automatically power off in the case of excess load to prevent excessive load causing damage to the inverter.
Low voltage/overvoltage protection, for battery powered applications, the low voltage protection function can prevent the battery from excessive discharge and avoid damage to the battery. The overvoltage protection function can make the inverter work under the normal input voltage to avoid damage.

An application example of push pull inverter

1. Emergency lighting system
In emergency situations, such as power outages in office buildings, factories and hospitals, emergency lighting systems need to provide lighting quickly. Push pull inverter convert the battery's direct current to alternating current for emergency power. Since the waveform quality requirements of lighting equipment are not high, push pull inverters are a cost-effective option.

2. Power supply for small household appliances
In outdoor camping, people usually use some small household appliances, such as fans, portable heaters, lamps, etc., which can be powered by push pull inverters, especially in the absence of mains power supply, such as outdoor or camping.
For example, 12V direct current can be converted to 220V or 110V AC, which can supply power for portable fans or small desk lamps, which can meet the basic life scenarios.

3. Solar power supply system
In some off-grid solar power supply systems(off grid inverter), push pull inverters can convert direct current generated by solar panels into alternating current to power small appliances or lighting equipment.

In a solar lighting system in a remote area, solar panels charge the battery, and then a push pull inverter converts the battery's direct current to alternating current to power LED lights or small household appliances. These devices usually do not have high requirements for the power waveform, so push pull inverters become an economical and practical option.

4. Portable power supply equipment
Some portable power devices can also use push pull inverter, such as mobile power stations or small UPS systems, to provide emergency power for some devices that do not have high requirements for the inverter waveform. We can convert it into AC power for mobile phones, cameras, LED lights and other equipment, which can provide basic power support in the absence of mains power.

Conclusion

Push pull inverters, with their simple and reliable characteristics, have a place in low and medium power applications. Although the quality is not as good as full-bridge inverters in terms of output waveform, its convenience in cost and maintenance makes it the first choice in many scenarios.

Especially in some general occasions such as emergency lighting, small household appliance power supply, solar power supply systems, push pull inverter can provide effective power conversion solutions. With the advancement of technology, push pull inverter technology and structure are also continuously optimized to adapt to a wider range of application needs.

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