Comparative Analysis of Inverter Three-Level "1" Type and "T" Type Circuits

Abstract: This paper mainly analyzes the waveforms of "1"-shaped and "T"-shaped circuits based on the existing three-level research. Based on the waveform analysis, the specification selection and loss of the switch tube are analyzed and compared, and finally a suitable three-level circuit is selected.

1. Schematic diagram of three-level circuit

As shown in Figures 1 and 2, in order to distinguish the two circuits, according to the arrangement of the four switching tubes in the circuit diagram, we call the former a I type and the latter a T type.

Compared with the ordinary half-bridge circuit, the three-level circuit has the ability of midpoint freewheeling, so it has a good effect on improving output ripple and reducing loss.

2. Waveform analysis of two circuits

In order to compare the losses and specifications of the two circuits, this paper depicts the waveforms of the two circuits, as shown in Figure 3 and Figure 4.

The following is a partial description of the waveform diagram.

The waveform diagram assumes that the positive and negative buses are equal and that all components are ideal.

For the two circuits, the same driving signal waveform is selected for the convenience of analysis. The specific control method of the driving signal comes from references [1][2]. As shown in the figure, Q1 and Q3 are a group of PWM (with dead time), Q2 and Q4 are a group of PWM (with dead time), and there is also a dead time between Q1 and Q4.

The waveforms assume that the inductance and current iL are the same and cover several cases of all current states (see below, also marked in the figure).

VL represents the voltage at the point where the inductor is connected to the switch tube . As can be seen from the waveform diagram, the voltage waveforms at this point in the two circuits are the same.

The high level value of VL is 1 times Vbus, and other level values ​​are based on this as a reference according to the height ratio.

3. Comparison of the two circuits

1. Comparison of switch tube withstand voltage specifications:

It can be seen from the two waveforms in FIG3 and FIG4 that, in theory, the maximum voltage of the switch tube in the I-type circuit is 1Vbus; the maximum reverse voltage of the switch tubes Q1 and Q4 in the T-type circuit is 2Vbus.

Therefore, it seems that from the perspective of the voltage resistance of the switching tube, the I-type is better than the T-type.

However, in reality, for a 1-shaped circuit, when the voltage of two switches is connected in series and bears 2 times the bus voltage, due to the differences in the components themselves, the voltages borne by the two switches cannot be exactly the same. Therefore, in order to ensure the safe operation of the switches, the switches in the 1-shaped circuit should also be designed to bear 2 times the BUS voltage.

Therefore, from a practical point of view, the 1-type circuit does not have much advantage in the choice of switch withstand voltage.

2. Comparison of losses

The losses here mainly refer to the switching and conduction losses of the four switching tubes and the diodes .

Because the loss is closely related to the flow path of the current , it is divided into six states according to the flow path of the current. They are represented in Figures 3 and 4 according to different colors. Please refer to them .

A, In the 1-shaped circuit, the current is supplied by +BUS through Q1 and Q2. Its losses include Loss_Q1_turnon&turnoff Loss_Q1_On Loss_Q2_On

B, T-shaped circuit In which the current is supplied by +BUS through Q1, its losses include Loss_Q1_turnon&turnoff Loss_Q1_On

Comparison: In this state, it can be seen from the waveform that Loss_Q1_turnon&turnoff are almost the same, but the 1-shaped circuit has one more conduction loss of Q2 than the T-shaped circuit. In the negative bus power supply state
A, in the 1-shaped circuit, the current direction is from the inductor, through Q3, Q4 to the negative bus. Its losses include Loss_Q4_turnon&turnoff Loss_Q3_On Loss_Q4_On

B, In the T-shaped circuit, the current direction is from the inductor, through Q4 to the negative bus. Its losses include Loss_Q4_turnon&turnoff Loss_Q4_On

Comparison: In this state, the I-shaped circuit has one more conduction loss of Q3 than the T-shaped circuit.

A, In the 1-shaped circuit, the current direction is from the inductor through Q1 diode, Q2 diode to the positive bus

Its loss includes Loss_Q1diode_turnon&turnoff&on

Loss_Q2diode__turnon&turnoff&on

B, In the T-shaped circuit, the current direction is from the inductor through the Q1 diode to the positive bus.

Its loss includes Loss_Q1diode_turnon&turnoff&on

Comparison: In this state, the I-type circuit has one more conduction loss of Q2 than the T-type circuit.

Negative bus freewheeling state

A, In the 1-shaped circuit, the current direction is from the negative bus through the Q1 diode, Q2 diode to the inductor.

Its losses include Loss_Q3diode_turnon&turnoff&on Loss_Q4diode__turnon&turnoff&on

B, T-shaped circuit , the current direction is from negative bus through Q1diode to inductor. Its loss includes Loss_Q4diode_turnon&turnoff&onComparison: In this state, the one-shaped circuit has one more conduction loss of Q3 than the T-shaped circuit. Midpoint freewheeling iL>0 state

A, in the 1-shaped circuit, the current flows from GND, through D1, Q2 to the inductor. Its losses include Loss_D1 Loss_Q2 In the T-shaped circuit, the current flows from GND, through Q2, Q3 diode to the inductor.
Its losses include Loss_Q2 Loss_Q3 diode

Comparison: In this state, the losses of the two circuits are close. In the midpoint freewheeling iL<0 state A, in the 1-shaped circuit, the current flows from the inductor, through Q3, D2 to GND.
Its losses include Loss_Q3 Loss_D2

B. In the T-shaped circuit, the current flows from the inductor through Q3, Q2 diode to GND.

Its losses include Loss_Q2 Loss_Q3 diode

Comparison: In this state, the losses of the two circuits are close

Conclusion: From the above comparison, it can be seen that except for the midpoint freewheeling state, the loss of the T-type circuit in other states is better than that of the I-type circuit.

3. Comparison of the number of components

From the topology diagram, it is easy to see that the T-type circuit has two fewer diodes than the I-type circuit, which is good for reducing space.

IV. Conclusion:

Through the analysis in this article, it can be seen that compared with the T-shaped and 1-shaped three-level circuits, the 1-shaped circuit is theoretically better than the T-shaped circuit in terms of withstand voltage. However, from the perspective of practical application, the difference between the two is not much; in terms of loss, the T-shaped circuit is better than the 1-shaped circuit; in terms of the number of components, the T-shaped circuit has two fewer diodes. Therefore, according to the analysis in this article, the T-shaped circuit will be more advantageous in terms of lower loss and reduced space.