Misunderstanding of Voltage Sharing Problem of Series Diode

Since the reverse withstand voltage of the  step-recovery diode is limited, when used in a high- voltage environment, it is often necessary to use plurality of step-recovery rectifier diodes in series to meet the need of reverse withstand voltage. Due to the inconsistency in the volt-ampere characteristics, turn-on time, and recovery charges of the diode during the production process, the voltage sharing problem occurs when used in series, which causes the reverse voltage of a certain diode to be too high and damaged, disturbing the normal operation of other diodes, which ultimately affects the reliability and stability of the entire device.

Diodes are very commonly-used power electronic devices. Generally, we are easier to miss the key parameters of familiar devices, and only care about macroscopic parameters on the macro level, such as reverse withstand voltage, on-state current, and reverse leakage current. While the junction capacitance, turn-off and turn-on characteristics of the diode are easily overlooked.

The main reason for the voltage sharing problem may be caused by either internal reasons or external ones. The internal reasons is related to the processing technology, and the external cause is about the external circuit.

The volt-ampere characteristics of the diodes produced in the same batch are inconsistent, resulting in static uneven voltage of the diode; the inconsistency between the reverse recovery time and the on state state causes the dynamic uneven voltage of the diode.

External circuit design can cause stray inductance and capacitance, which can cause uneven voltage problems in high-voltage high-frequency environments.

At the macro level, the diodes in series does not need to have voltage sharing, the main consideration is the influence of the diode itself. As shown in Figure 1, the two diodes are connected in series with an external reverse DC voltage.

The diode with a small reverse saturation current can withstand a large voltage as the two diodes are connected in series, and the reverse saturation current is constant under an externally applied voltage rating. As shown in Figure 2, assumed that there is difference in the reverse saturation current between the two diodes, and the reverse saturation current of D2 is smaller, the above conclusion can be clearly drawn.

In actual operation, macroscopically, diodes have different voltage drops due to their own differences as shown in Figure 2. When the the voltage drop on D2 from the external voltage U increases reaches the critical point, the voltage drop of D1 is relatively small and still in the safe and stable region due to its large reverse saturation currents. 

According to the above analysis, when U increases continuously. , the voltage drop on D2 will break through the critical transition voltage, causing the current to increase sharply. As D1 and D2 are connected in series in the main circuit, the current of D1 certainly increase with D2.

However, the volt-ampere curve of D1 shows that the reverse voltage drop of D1 should reach the breakover voltage when a large current pass through it, so the sum of u1 and u2 is greater than U, which proves the speculation above is not true.

Therefore, when U increases, the voltage of D2 will not continue to increase, and the voltage of D1 will keep increasing until its value is more than the sum of the reverse withstand voltages of the two tubes, at which point diode breakdown occurs.

The pins and the layout of the diode on the board naturally evolve into stray capacitance and inductance in a high-voltage and high-frequency environment. The introduction of stray capacitance and inductance directly affects the turn-on and turn-off waveforms of the diode.

The introduction of capacitance prevents a sudden change in voltage and the introduction of an  inductance prevents a sudden change in current. Figure 3 is the equivalent circuit diagram of the diodes in series at high frequency. In the figure, C1 is the junction capacitance, R is the reverse resistance, C2 is the stray capacitance at the high voltage, C3 is the stray capacitance formed by the diode to the ground.

The high-frequency power supply of the electrostatic precipitator outputs high-frequency PWM waves, and the waves pass through the boosting transformer and the rectifier module to finally become an approximate linear voltage waveform. The rectifier module is integrated in the boosting transformer, and the diodes are in series.

Because the voltage level is relatively high, it is generally considered to reach 10KV or more. Since the requirement for the accuracy of the output voltage waveform is not high, the diode can be directly connected in series, and the high-frequency rectifier diode is selected. The rectified output simulation waveform is shown in FIG. 4, and the measured waveform is shown in FIG. 5.

As shown in the figures, the rectified output voltage has high and low peaks due to the influence of stray parameters. With the continuous development of science and technology, the manufacturing process of diodes is constantly improving, and the influence of its own factors is negligible.

By simply analyzing the voltage sharing problem of the series diode from both macroscopic and microscopic perspectives, it is concluded that when the diode is used in series in a high-voltage high-frequency environment, the diode itself will generate uneven voltage but not cause breakdown.

The uneven voltage generated by external factors is the result of breakdown of the diode, and it will cause even more serious problems.