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Basic Knowledge of High-Frequency High-Voltage Diodes

High-frequency high-voltage diodes generally function to stabilize the voltage in a circuit and can also provide a reference voltage value for the circuit. Manufactured using special processes, these diodes can operate stably for a long period of time and will not be damaged during reverse breakdown. As long as the operating current of the diode working in the reverse breakdown region is maintained within a certain range, the voltage fluctuation across the voltage regulator diode remains very small. A voltage regulator diode utilizes this characteristic to achieve voltage stabilization.

The ideal regulation value of a high-frequency high-voltage diode voltage regulator is a fixed voltage value. However, the actual stabilized voltage of a real voltage regulator fluctuates within a certain range. Some voltage regulator datasheets provide the nominal regulated voltage, minimum regulated voltage, and maximum regulated voltage, while others only specify the nominal regulated voltage. When selecting an axial lead diode, engineers should also verify its reverse-voltage rating, forward current, recovery time, and thermal requirements. The rated operating current refers to the current value at which the voltage regulator diode can operate stably for a long time while maintaining ideal voltage regulation performance.

The minimum stable operating current refers to the minimum current required for a voltage regulator diode to achieve voltage regulation. If the diode current is lower than this value, it will enter the reverse cut-off region, where it no longer provides voltage regulation. The maximum stable operating current refers to the highest current at which the voltage regulator diode can maintain voltage regulation. If this current is exceeded, the diode may be damaged. Some voltage regulator datasheets do not specify the maximum stable operating current because this parameter can be calculated from the nominal regulated voltage and maximum power dissipation.

The dynamic resistance of a high-frequency high-voltage diode voltage regulator is equal to the ratio of voltage variation to current variation. The smaller the dynamic resistance, the better the voltage regulation performance. This is because a smaller dynamic resistance means that the same amount of current variation will cause a smaller voltage change, resulting in a more stable output voltage. According to the reverse voltage-current characteristics of the voltage regulator diode, the closer the operating current is to the maximum stable operating current, the smaller the dynamic resistance becomes. Conversely, when the operating current approaches the minimum operating current, the dynamic resistance increases. Therefore, the voltage regulation performance is optimal when the operating current of the voltage regulator diode is close to its rated stable operating current.

In practical circuit applications, a high-frequency high-voltage diode voltage regulator is usually connected in series with a current-limiting resistor to protect the diode from damage caused by excessive operating current. In addition, by selecting an appropriate resistance value, the current-limiting resistor can prevent the diode from entering the reverse cut-off region due to insufficient operating current, which would result in the loss of voltage regulation capability. Therefore, the current-limiting resistor R not only provides protection but also helps establish an appropriate operating current for the voltage regulator diode.

As the load resistance RL decreases, the current IR in the load path increases directly. As a result, the current flowing through the current-limiting resistor also increases, causing a larger voltage drop across the resistor. Therefore, the voltage across the voltage regulator diode (i.e., the output voltage UO across the load resistance) decreases, causing the current through the diode IDZ to decrease. This reduction in diode current then causes the current through the current-limiting resistor to decrease, which reduces the voltage drop across the resistor and consequently increases the voltage across the voltage regulator diode (i.e., the voltage across the load).

The key principle of this process is that when the voltage across the voltage regulator diode decreases, the diode current also decreases, which then causes the voltage across the diode to rise again, maintaining a relatively stable output voltage.

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