An improved soft-switching inverter to eliminate pre-charge current

To address the current stress problem brought on by the auxiliary resonant pole inverter’s pre-charge state, this paper gives an improved auxiliary circuit based on the original inverter. The improved circuit avoids a pre-charge state by adding auxiliary capacitors to form a resonant branch. In contrast to the original inverter, this inverter lowers the current stress on both the auxiliary and primary switches by resolving the pre-charge state’s current stress issue. As a result, the inverter’s losses are decreased, which raises the system’s efficiency. The working principle of this inverter is given in the paper. After a comparison and analysis of the inverter’s pre-charge state, an experiment is used to confirm the device’s viability.


Introduction
With the increasing application of inverters in the fields of photovoltaic grid connection, new energy generation, and electric vehicles [1][2], the switching frequency and the system efficiency are important indexes to measure the performance of inverters.Increasing the inverter's switching frequency can reduce the volume of the filtering inductor but will raise the switches' switching loss, which will cause their efficiency to drop.Soft-switching technology is an efficient way to solve this problem, which can ensure the switching frequency of the system while cutting the inverter's switching loss, improving the system's efficiency [3][4][5].
Currently, soft-switching inverters are known as resonant DC link (RDCL) inverters or auxiliary resonant pole (ARP) inverters.ARP inverters are preferred because RDCL inverters not only lower the voltage usage of the inverter but also affect the quality of the output waveform.
In [6], an ARP inverter with a simple structure is proposed, but its auxiliary circuit (AC) requires the pre-charge state, which raises the primary switch's current stress in addition to the AC's losses, leading to an increase in the cost of the system.The improved AC presented in [7] cuts the device's pre-charge current, which minimizes the primary switch's current stress.It is important to note that the pre-charge limit issue with this inverter still exists.That pre-charge current increases the switch's stress due to the current, and a pre-charge state cannot be completed with a narrow pulse.The precharge current cannot reach the current limits, which leads to the soft-switching failure problem and affects the system's stability.
To avoid the inverter's pre-charge state and completely solve the primary switch's current stress issue, this paper proposes an improved AC based on [7].

Improved topology
The ARP soft-switching inverter proposed in this paper is shown in Figure 1, and the single-phase circuit of this inverter as well as the positive orientation of each element are shown in Figure 2. In Figure 2, iL is the positive direction of the inductor current and io is the positive direction of the output current.The AC of an ARP inverter proposed in this paper consists of two switches (S3-S4), four diodes (D3-D6), four resonant capacitors (C1-C4), and a resonant inductor (L).

A C
A C

Working principle of the inverter
The switching state of Phase A is used as an example to show in detail the working principle of the inverter.In Figure 3, the conceptual waveforms of the inverter's elements are displayed, and in Figure 4, the equivalent circuits for the various modes are provided.In Figure 3, tdead is the dead time.In the derivation of the mathematical equation, we set the resonant capacitors C1 = C2 = Ca and C3 = C4 = Cb, while the load current io is maintained steady during a single pulse width modulation (PWM) cycle.
The working principle of this inverter is as follows: 1) State 0 [~, t0]: This state is the steady state.Before t0, AC does not action, the primary switch S1 is ON, and the flow path of io is S1.The resonant elements in this state are all in the initial state: uC1 (t0) = uC3 (t0) =0, uC2 (t0) = uC4 (t0) =E, and iL (t0) =0.
2) State 1 [t0, t1]: This state is resonant state.At the moment of t0, S1 is turned off, S4 is turned on, D6 conducts, and Inductor L resonates with Capacitors C2, C3, and C4.Since Capacitor C3 restricts how quickly the voltage changes when S1 is OFF, S1 achieves quasi-ZVS turn-off.Since Inductor L restricts how quickly the current changes when S4 is ON, the turn-on of quasi-zero current switching (quasi-ZCS) is accomplished by S4.The voltage and current of each element in this state are: where This state is the circulating current state.At the instant t1, the voltage of C4 is 0, and diode D2 turns on.The inductor current circulates in L-S4-D2, the primary switch S2 is turned on within this state, and S2 can realize the ZVS turn-on.
4) State 3 [t2, t3]: This state is the resonant state.At the instant t2, S4 is turned off and Capacitor C1 resonates with Inductor L. During the resonance process, C1 absorbs energy, the voltage of the capacitor rises, the inductor releases energy, and the current of the inductor decreases.When the voltage of C1 is E, the resonant state ends.Capacitor C1 limits the voltage change rate of S4, so S4 achieves quasi-ZVS off.The values of C1 in this state are: where   ( ) This state is the resonant state.At the moment of t6, the current of D2 drops to 0, D3 turns on, and the inductor L resonates with the capacitors C1, C3, and C4.In this state, Capacitors C1 and C3 release energy, the voltage of C1 and C3 drops, Capacitor C4 absorbs energy, the voltage of C4 rises, and energy is absorbed by Inductor L, causing its current to increase.The values of capacitors and inductors in this state are: This state is the circulating current state.At the instant t7, the voltage of Capacitor C3 drops to 0, the anti-parallel diode D1 turns on, the inductor current circulates in S3-L-D1, the primary switch S1 is activated within the state, and S1 can realize the ZVS turn-on.10) State 9 [t8, t9]: This state is the resonant state.At t8, S3 is off, D5 turns on, and Capacitor C2 resonates with Inductor L. During the resonance process, the capacitor absorbs energy, the voltage of C2 rises, the inductor releases energy, and the current of L falls.When the voltage of C2 is E, the resonant state ends.Capacitor C2 limits the voltage change rate of S3, so S3 achieves quasi-ZVS off.The values of C2 in this state are: (15) 11) State 10 [t9, t10]: This state is the energy feedback state.At t9, the voltage of C2 is E, the diodes D5 and D6 conduct, and the DC power source receives the inductor's remaining energy.The energy feedback state is ended when iL drops to -io.In this state, the current of Inductor L is: (16) 12) State 11 [t10, t11]: This state is an energy reutilization state.At t10, the inductor current is -io.In this state, the load receives the remaining energy from the inductor.iL in this state is:

Current stress in a pre-charge state
According to Equation (3), the primary switch current iS1max in the state of pre-charge is: where ib is the pre-charge current of [7].According to Equation (7), the current of the auxiliary switch after the resonant mode is completed is: The resonant state of this paper is given in Figure 4 (b).Combined with State 1, it is evident that this inverter's primary switch current iS1max is always the load current.
1max So ii = (20) From Equation (1), the current of the auxiliary switch after the resonant state is completed is: According to Equations ( 18) and (20), the primary switch of this present inverter's current stress comparison with [7] is shown in Figure 5.As seen in Figure 5, this improved AC provides a new resonant route for the resonant circuit by adding two resonant capacitors, so that the inductor still has enough energy to participate in the resonance, which ensures the success of soft-switching.Compared to [7], this inverter avoids the pre-charge state, which reduces the stress caused by the current on the primary switch as well as the auxiliary switch.

Experimental analysis
This proposed ARP inverter is experimentally verified on a 2 kW experimental prototype.The control system of this inverter uses FPGA-EP4CE10E22C8.The experimental platform is depicted in Figure 6 and the parameters of the system are given in Table 1.Furthermore, to avoid soft-switching failure, the dead time tdead must not be less than the resonance time t0-1 of State The waveforms of the primary switch S1 are shown in Figure 7. Based on the foregoing theoretical analysis, it is evident that S1 can achieve ZVS turn-on in State 8 and attain quasi-ZVS turn-off when Capacitor C3 is present.The experimental waveforms given in Figure 7 (a) and Figure 7 (b) confirm the accuracy of the foregoing theory.
Figure 8 exhibits the switching waveform of S2.According to the theoretical analysis, the current goes via anti-parallel diode D2 of S2 rather than via S2, allowing S2 to perform ZVZCS turn-on-off.
The hard-switching inverter identified in [7] and the current inverter's simulation efficiency chart are contrasted in Figure 9. From Figure 9, it can be observed that the present inverter has the highest output efficiency of 98.8% at the rated output power, which is an improvement of about 1.31%.Compared to the hard-switching inverter, an efficiency improvement is about 0.34%.The precharge limit issue is resolved by this paper's inverter, which cuts the primary switch's current stress and decreases the losses in both primary and auxiliary circuits.Compared with [7], the efficiency benefit of this inverter is clearer, and the efficiency of this inverter is better in the full load range.It should be noted that the soft-switching inverter's efficiency is slightly less than the hard-switching inverter's with light loads, because the inherent losses of the AC account for a higher percentage of losses, and the primary switches' switching losses are smaller in light load conditions.The advantage of the soft-switching is not obvious, which is a common disadvantage of the current ARP softswitching inverters.However, with the increase in power, the advantage of the soft-switching is becoming more and more obvious.

Conclusion
In this paper, two resonant capacitors are added to the original ARP inverter to change the resonance routes during its soft-switching state, which avoids the pre-charge state.The following conclusions have been confirmed from both theory and simulation experiments: 1) Compared with [7], this paper's AC avoids the pre-charge state, hence resolving the problem with stress caused by current on both the primary and auxiliary switches.
2) Compared with [7], this inverter prevents the pre-charge state, which lowers the loss of both the primary and auxiliary switches and increases system efficiency even more.

Figure 3 .
Figure 3.The key theoretical waveforms of this AC.

Figure 4 .
Figure 4. Equivalent circuits of different states.

7 ) 6 )
State 5 [t4, t5]: This state is the steady state.At the instant t4, current flows via the anti-parallel diode D2 onto the load because the diodes D3 and D4 are turned off.7)State 6 [t5, t6]: This state is the resonant energy storage state.At the instant t5, S2 is turned off and S3 is turned on.The current of D2 starts to decrease and the current of L starts to increase.When iD2 decreases to zero, this state ends.S2 achieves zero-voltage-zero-current switching (ZVZCS) turn-off because current flows via the anti-parallel diode of S2.The inductor L sets a restriction on the current change speed when S3 is turned on, so S3 achieves quasi-ZCS turn-on.The value of each element in this state are:

Figure 5 .
Figure 5.Comparison of primary switch current stress.

Table 1 .
Devices and parameters of the system The waveforms when S1 is ON; (b) The waveforms when S1 is OFF.

Figure 7 .
Figure 7. Waveforms of the primary switch S1 in experimentation.

Figure 8 .
Figure 8. Waveforms of the primary switch S2 in experimentation.
11This state is the energy feedback state.The state ends when the inductor's energy is zero.At t3, the voltage of C1 is E, Diode D3 activates, and the remaining energy of the inductor is sent back to the DC power source.iL in this state is: 1.