Design of inverter snubber circuit using measured GTO anode current waveform

As shown in Figure 5, the reverse recovery characteristic curve of the buffer diode, the current on the buffer diode after t>t5 is approximately considered to be a quadratic curve, which can better illustrate the problem. The curve equation is: (1)(2)

Where trr is the buffer diode recovery time;

t5—time when ids=Ism;

Ido—the current value of the buffer diode when t=t7.

Figure 4 Ids, Irs waveforms after the snubber diode recovers its reverse blocking capability

3 Anode voltage waveform simulation

By using the mathematical model between the GTO anode voltage and anode current and computer simulation using MATLAB language, the simulated waveform of the anode voltage can be obtained from the measured anode current waveform and buffer circuit parameters. The simulated waveform has a very small error compared to the measured waveform. As shown in Figure 6, the curves in the figure are the actual measured anode voltage waveform and the corresponding simulated waveform under the conditions of CS=2μF and 5μF. It can be seen that the simulation accuracy can meet the optimization requirements.

4 Buffer circuit parameter optimization design scheme

4.1 Determination of the objective function

The following specifically discusses the indicators that can be used to determine whether the buffer circuit parameter settings are reasonable.

Figure 5 Reverse recovery characteristics of snubber diode

(1) There are several extremely important dynamic parameters in the GTO turn-off process, including peak voltage Up, peak power consumption Pfm, anode voltage rise rate dua/dt, and anode voltage peak UDM. If these dynamic parameters are too high, the GTO will fail, that is, the GTO's tolerance to these dynamic parameters is limited. Assume that the limit values ​​of these dynamic parameters are (Up)m, (Pfm)m, (UDM)m, (dua/dt)m, and (Urm-E)m. It can be seen that the smaller the ratio of the dynamic parameter value to its limit value during the GTO turn-off process, the better the working performance of the GTO device. Since the dynamic parameter value in practical use is closely related to the buffer circuit parameters, it can be said that once the GTO and gate drive circuit are determined, the dynamic parameter value of the GTO when it is turned off will depend on the buffer circuit parameters. Therefore, the ratio of the dynamic parameter value during actual operation to the limit value it can withstand, including Up/(Up)m, UDM/(UDM)m, (dua/dt)/(dua/dt)m, Pfm/(Pfm)m, (Urm-E)m, can be used as an indicator 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 the GTO operation process are important parameters for measuring 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 to a specific value (Eoff)m, (Esb)m as an indicator to measure the performance of the GTO device. Because 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 these 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 size of the operating frequency limit value of the entire GTO device. The smaller ton and toff are, the higher the operating frequency of the GTO device can be. 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 certain 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 defined as the switching time when CS and RS take the upper limit of the optimization space.

(4) Considering that the improvement of the dynamic characteristics of the buffer diode will lead to the deterioration of its power characteristics. 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 as indicators to measure the power characteristics of the GTO device, and are also indicators reflecting the working performance of the GTO device. The smaller the two ratios, the better the buffer circuit parameters. Among them: (Qr)m, (trr)m can be defined as the upper limit of the actual optimization space.

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