Design the inverter buffer circuit using the measured GTO anode current waveform

   Abstract: The snubber circuit parameter values ​​play a vital role in the turn-off performance of GTO and the working performance of the entire GTO inverter. This paper analyzes the anode current and anode voltage waveforms during the GTO turn-off process, and proposes a snubber circuit parameter optimization scheme with "comprehensive indicators" as the objective function. The GTO snubber circuit components can be determined according to the specific requirements for the performance of the GTO device. the best parameters.

    Keywords: GTO snubber circuit design anode current

1 Introduction

    The parameter value of the snubber circuit directly affects the turn-off performance of the GTO and the working performance of the entire GTO inverter. Therefore, how to reasonably design the snubber circuit parameters when designing a GTO inverter has become an important issue.

    By analyzing the anode current and anode voltage waveforms during the GTO turn-off process, this article proposes and demonstrates the argument that the GTO anode current waveform has nothing to do with the snubber circuit parameters, and that the reverse recovery process of the snubber diode has nothing to do with the snubber circuit parameters. On this basis, a simple and practical buffer circuit parameter optimization design scheme is proposed. The optimal parameters of the GTO buffer circuit components can be determined based on the specific requirements for the performance of the GTO device. When simulating the anode voltage and turn-off power consumption waveform during the GTO turn-off process, in order to improve the simulation accuracy, the measured anode turn-off current waveform was used. And derive the shutdown power consumption waveform accordingly. Comparing the simulation results with the experimental waveforms, the error is extremely small. This paper proposes a buffer circuit parameter optimization scheme with "comprehensive index" as the objective function.

2 Prerequisites for simulating anode voltage waveform using anode current waveform

    The GTO buffer circuit can be equivalent to the circuit shown in Figure 1. If you want to use the measured anode current to simulate the anode voltage, you first need to prove that the following two conditions are true:

    (1) The GTO anode current waveform has nothing to do with the snubber circuit parameters;

    (2) The reverse recovery process of the snubber diode has nothing to do with the snubber circuit parameters.

2.1 GTO anode current waveform has nothing to do with buffer circuit parameters

    Figure 2 shows the anode current waveform when the GTO is turned off. The whole process can be divided into 3 stages: storage time period, decline time period and tailing time period.

    In the storage time period and fall time period, the storage time t s and fall time t f values ​​only depend on the gate extraction capability and the internal structure of the GTO, and have nothing to do with the buffer circuit parameters. The anode current waveforms of these two sections are also independent of the snubber circuit parameters.

    During the tailing period, the tailing current is basically determined by the anode current waveform and junction temperature during the falling period, and has nothing to do with the snubber circuit parameters.

    The eight curves in Figure 3 are the anode current and anode voltage waveforms when CS=2, 3, 4, and 5μF. It can be seen that after the buffer circuit parameters change, the anode voltage waveform changes greatly, and the four anode current curves basically overlap completely. This experiment can verify the correctness of the above analysis.

　　Curves (1), (2), (3), and (4) in the figure are the measured anode voltage waveforms after the buffer circuit parameters are changed;

    Curves (5), (6), (7), and (8) are the measured anode current waveforms after the buffer circuit parameters are changed.

2.2 The reverse recovery process of the snubber diode has nothing to do with the snubber circuit parameters

    The stored charge Qr and the recovery time trr are two important parameters in the reverse recovery process of the buffer diode. When analyzing the GTO turn-off process, Qr and trr can be approximately considered to be constants. This can be proven by Figure 4. Figure 4 shows the measured snubber resistor branch current and snubber diode branch current after changing the distributed inductance of the snubber resistor branch. It can be seen that after Lrs changes, irs changes greatly, while ids almost remains unchanged. It can be considered that trr is only related to the characteristics of the snubber diode itself.

    Curves (1), (2), and (3) in the figure are the measured snubber resistor branch current waveforms before and after Lrs is changed.

    Curves (4), (5), and (6) are the measured snubber diode branch current waveforms before and after Lrs is changed;

    As shown in the reverse recovery characteristic curve of the snubber diode shown in Figure 5, the current on the snubber diode after t>t 5 is approximately considered to be a quadratic curve, which can better illustrate the problem. The curve equation is:

where t rr —snubber diode recovery time;

    t 5 —i ds =I sm time;

    I do —The current value of the snubber diode when t=t 7 .

3 Anode voltage waveform simulation

    Using the mathematical model between GTO anode voltage and anode current, and using MATLAB language for computer simulation, the simulated waveform of the anode voltage can be obtained from the measured anode current waveform and buffer circuit parameters. Compared with the measured waveform, the error between the simulated waveform and the measured waveform is extremely small. As shown in Figure 6, the curves in the figure are the actually measured anode voltage waveforms and corresponding simulated waveforms under the conditions of CS=2μF and 5μF. It can be seen that the simulation accuracy can meet the optimization requirements.

4 Optimization design scheme of buffer circuit parameters

4.1 Determination of objective function

    The following discusses in detail the indicators that can determine whether the parameter settings of the buffer circuit are reasonable.

    (1) There are several extremely important dynamic parameters during the GTO turn-off process, including peak voltage Up, peak power consumption Pfm, anode voltage rise rate dua/dt, and anode voltage peak UDM. Too high of these dynamic parameters will lead to the failure of the GTO, that is, the GTO's ability to withstand these dynamic parameters is limited. Let the limit values ​​of these dynamic parameters be (Up)m, (Pfm)m, (UDM)m, (dua/dt)m, (Urm-E)m respectively. It can be seen that the smaller the ratio between the actual dynamic parameter value and its limit value during the GTO turn-off process, the better the working performance of the GTO device. Since the practical dynamic parameter values ​​are closely related to the snubber circuit parameters, it can be said that once the GTO and gate drive circuit are determined, the dynamic parameter values ​​when the GTO is turned off will depend on the snubber circuit parameters. Therefore, the ratio of the dynamic parameter value during actual operation to the limit value it can withstand includes Up/(Up)m, UDM/(UDM)m, (dua/dt)/(dua/dt)m, Pfm/( Pfm)m, (Urm-E)m can be used as indicators to measure whether the buffer circuit parameter settings are reasonable. The smaller these ratios are, the better the buffer circuit parameter settings are.

    (2) The GTO turn-off energy consumption Eoff and the buffer circuit energy consumption Esb during GTO operation are important parameters to measure the working performance of the GTO device. If these parameters are too large, although the GTO may not fail in a short time, it will increase the energy consumption of the entire device, thereby affecting the working stability and reliability of the device. Therefore, we can use the ratio of Eoff, Esb and a specific value (Eoff)m, (Esb)m as an indicator to measure the performance of the GTO device. Since Eoff and Esb are closely related to the buffer circuit parameters, the above two ratios Eoff/(Eoff)m and Esb/(Esb)m can also be used as indicators to measure whether the buffer circuit parameter settings are reasonable. The smaller the two ratios are, the better the buffer circuit parameter settings are.

    (3) The turn-on time ton and turn-off time toff of GTO are directly related to the working frequency limit of the entire GTO device. The smaller ton and toff are, the higher the operating frequency of the GTO device can be increased. Its limit value is fmax=1/(ton+toff). Therefore, the values ​​of ton and toff are related to the working performance of the entire device. The ratio of ton+toff to a specific value tm can be used as an indicator to measure the frequency performance of the GTO device. Similarly, the size of ton and toff is closely related to the parameters of the buffer circuit. For example, if the buffer circuit parameters are CS and RS, it is impossible to make the GTO turn-on time lower than 5RSCS. Therefore, (ton+toff)/tm can be used as an indicator to measure whether the buffer circuit parameter settings are reasonable. The smaller this ratio is, the better the buffer circuit parameter settings are. Among them, tm can be set as the switching time when CS and RS take the upper limit of the optimization space.

    (4) Consider that the improvement of the dynamic characteristics of the snubber diode will cause its power characteristics to deteriorate. The ratio of the stored charge Qr to its specific value, Qr/(Qr)m, and the ratio of the recovery time trr to a specific value, trr/(trr)m, can be used to measure the power characteristics of the GTO device and also reflect the operation of the GTO device. performance indicators. The smaller the two ratios are, the better the buffer circuit parameters are. Among them: (Qr)m, (trr)m can be set as the upper limit of the actual optimization space.

    From the above analysis, it can be seen that the objective function J of buffer circuit optimization can be defined as: where (Up)m, (UDM)m, (dUa/dt)m, (Pfm)m, (Urm-E)m are the GTO switches respectively. The limit values ​​of dynamic parameters during the breaking process;

　　U p , U DM , du a /dt, P fm are the GTO dynamic parameter values ​​under specific conditions;

    (E off )m, (E sb )m, tm are specific values ​​determined based on actual requirements;

    k 1 , k 2 , k 3 , k 4 are coefficients determined according to the importance of each indicator. Its value can be determined according to specific requirements, generally k 2 >k 1 >k 3 >k 4 .

4.2 Determination of constraints

    The limit values ​​of the dynamic parameters that the GTO can withstand during the turn-off process can be used as constraints for optimization. Specifically speaking, there are the following items:

①U p <(U p )m;②U DM <(U DM )m;③du a /dt<(du a /dt)m;④P fm <(P fm )m;⑤(E－U rm )<(E -U rm )m;⑥t on +t off <1/f, f is the operating frequency of the GTO device.

4.3 Optimization program block diagram

    As shown in Figure 7, in the block diagram (C s ) max , (C s ) min , (R s ) max , (R s ) min , (Q r ) max , (Q r ) min , (t rr ) max , ( t rr ) min is the upper and lower limits of the optimization space; N 1 , N 2 , N 3 , N 4 are the step coefficients.

4.4 Optimization program running results

    Optimization design purpose: KG-91-2-5GTO is used in 1000VGTO chopper with operating parameters of 600A. The rated parameters of GTO are 1000A, 2300V. Determine the optimal buffer circuit parameters.

    Determination of optimization program parameters:

    (1) Determination of the optimization space

    C S : From 1μF to 10μF, the step size is set to 1μF, that is, N 1 =9;

    R S : from 1Ω to 21Ω, the step size is set to 5Ω, that is, N 2 =4;

    Q r : From 100μC to 400μC, the step size is set to 100μC, that is, N 3 =3;

    t rr : from 1μs to 7μs, the step size is set to 2μs, that is, N 4 =3;

    (2) Determination of objective function

    Considering that the actual operating parameters of GTO are quite different from its rated parameters, a larger K2 is selected when determining the objective function to highlight the energy consumption index, so that the device has lower energy consumption when working under optimized parameter conditions.

    Actual selection:

    K 1 =1; K 2 =5; K 3 =2; K 4 =1.

    Program running results:

    Minimum value of objective function: J min =16.74;

    Optimal buffer circuit parameters: C S =3μF; R S =6Ω;

    Q r =200μC; t rr =3μs.