Bidirectional voltage source high frequency link inverter

Bidirectional voltage source high frequency link inverter

The bidirectional voltage source high frequency link inverter topology family is shown in Figure 4. From the perspective of the input side inverter stage, the push-pull circuit is suitable for low voltage input conversion; the half-bridge and full-bridge circuits are suitable for high voltage input. From the output side frequency conversion stage, the full-wave circuit has high power switch voltage stress, a small number of power switches, and low transformer winding utilization, which is suitable for low voltage output conversion; the full-bridge circuit has low power switch voltage stress, a large number of power switches, and high transformer winding utilization, which is suitable for high voltage output.

The bidirectional voltage source high frequency link inverter has the advantages of bidirectional power flow and reduced power conversion stages, but it has an inherent disadvantage, that is, the output cycloconverter using traditional PWM technology blocks the continuous energy in the leakage inductance of the high frequency transformer during commutation, thus causing voltage overshoot between the high frequency transformer and the output cycloconverter. Therefore, this type of inverter usually needs to use a buffer circuit or an active voltage clamp circuit to absorb the energy stored in the leakage inductance, thereby increasing the number of power devices and the complexity of the control circuit
. At the same time, it is also necessary to ensure that the high frequency transformer avoids transformer core saturation during the positive and negative half-cycle unipolar reciprocating operation of the low frequency AC signal, and ensure that the low frequency AC signal is transmitted linearly.
In response to the voltage overshoot problem, experts and scholars are constantly seeking better methods and have proposed some new control strategies and technologies, such as unipolar and bipolar phase shift control technology with commutation overlap, which adjusts the output voltage and power flow by controlling the phase shift of the high frequency inverter and the cycloconverter, realizes the natural commutation of the cycloconverter power tube, and eliminates the voltage spike; there is also a technology that combines series resonance technology with bidirectional voltage source high frequency link inverter.
In order to solve the core saturation problem, some new circuit topologies are proposed. Two improved circuits are briefly introduced as shown in Figure 5. In Figure 5 (a), the two forward converters on the primary side of the transformer divide the high-frequency unipolar SPWM pulse sequence into two groups of drive pulses. The two forward converters are controlled by the two groups of SPWM drive pulses respectively, so the maximum working duty cycle can be greater than 0.5, there is no core saturation problem, and the voltage stress is lower. The two main switch tubes on the secondary side are controlled by a low-frequency square wave with the same output frequency
, so the control is simple and it is easy to realize soft switching, which can reduce switching loss and noise. At the same time, two energy feedback circuits are added to the secondary side, so a path is provided for the inductive current to avoid the voltage overshoot. Figure 5(b) is actually two single-ended flyback converters that share a transformer core and secondary side, which complete the tasks of transforming, isolating, and transmitting low-frequency electric power. However, when the switch tube receives the control signal pulse train and turns on, the voltage applied to the transformer winding is in the same direction during the positive and negative half cycles of the low-frequency modulation signal. The magnetic flux in the transformer core may increase gradually in a step-by-step manner, leading to core saturation, magnetic bias or unidirectional magnetization, resulting in low-frequency electric signal amplification distortion or failure to work normally due to a large magnetizing current. Therefore, a pulse-by-pulse magnetic reset technology is proposed, which is to take timely measures after each high-frequency pulse to restore the magnetic flux increase caused by each high-frequency pulse to zero, thereby avoiding core saturation.