Voltage rating and current rating of thyristor

Voltage rating and current rating of thyristor

Voltage rating

(1) Off-state repetitive peak voltage UDRM

The off-state repetitive peak voltage is the positive peak voltage that is allowed to be repeatedly applied to the device when the gate is open and the junction temperature is at the rated value (see Figure 1). Internationally stipulated that the repetition frequency is 50Hz, and the duration of each time does not exceed 100ms. It is stipulated that the off-state repetitive peak voltage UDRM is 90% of the off-state non-repetitive peak voltage (ie, the maximum instantaneous voltage of the off-state) UDRM. The off-state non-repetitive peak voltage should be lower than the forward transition voltage UBO.

Voltage rating and current rating of thyristor
Figure 1 – Volt-ampere characteristics of thyristor

(2) Repetitive peak reverse voltage URRM

The reverse repetitive peak voltage is the reverse peak voltage that is allowed to be repeatedly applied to the device when the gate is open and the junction temperature is at the rated value (see Figure 1). The reverse repetitive peak voltage URRM is specified to be 90% of the reverse peak voltage (ie, the maximum reverse transient voltage) URSM. The reverse non-repetitive peak voltage should be lower than the reverse breakdown voltage.

The smaller standard value of the UDRM and URRM of the thyristor is usually taken as the rated voltage of the device. When selecting, the rated voltage should be left with a certain margin. Generally, the rated voltage is 2-3 times the peak voltage of the thyristor during normal operation.

(3) On-state voltage UTM

The on-state voltage is the maximum on-state voltage drop of the thyristor under the specified junction temperature and forward current conditions. On-state voltage UTM will affect the loss and heating of the device, so a device with a small on-state voltage drop should be selected.

Current rating

(1) On-state average current IT (AV)

The international standard on-state average current is the average value of the maximum power frequency sine half-wave current allowed when the stable junction temperature of the thyristor exceeds the rated junction temperature when the ambient temperature is 40°C and the specified cooling conditions. This current is nominally called the rated current parameter of the thyristor.

As can be seen from the above definition, the thyristor, like other electrical equipment, limits the maximum current to temperature. The rated current of the thyristor is calibrated by the average on-state current, and the heat generation is related to the effective value of the device’s current. For this reason, it is necessary to convert the effective value current I under the rated state according to the on-state average current IT (AV) of the device [see formula (1-1)] [see formula (1-2)]

Voltage rating and current rating of thyristor
(1-1),(1-2)

Then the relationship between the effective value current in the rated state and the on-state average current IT (AV) is

                     I=1.57 IT (AV)                     (1-3)

In actual selection, calculate the effective value of the current according to the actual waveform, and select the rated current of the thyristor based on the principle that the heating effect caused by the maximum sine half-wave current (the average value is the average on-state current IT (AV)) allowed by the thyristor is equal (that is, the effective value is equal), and a certain margin should be left. For example, if the effective value of a certain waveform current that a certain thyristor actually bears is 500A, the rated current IT(AV) (average on-state current) of the thyristor can be selected as

Voltage rating and current rating of thyristor
(1-4)

If 1.5 to 2 times the margin is considered, a thyristor with a rated current of 600A or 700A can be selected.

(2) Maintain current IH

The sustain current refers to the minimum current necessary to keep the thyristor turned on, and is generally tens to hundreds of milliamps. IH is related to the junction temperature. The higher the junction temperature, the smaller the IH.

(3) Holding current IL

The holding current refers to the minimum current at which the thyristor can maintain the conduction of the thyristor just after the thyristor is turned from the off state to the on state and the trigger signal is removed. For the same thyristor, IL is usually 2~4 times of IH.

(4) Surge current ITSM

Inrush current refers to the non-repetitive maximum forward overload current that is caused by abnormal current conditions and causes the junction temperature to exceed the rated junction temperature. The surge current has two levels, the upper and the lower. This parameter can be used as the basis for designing the protection circuit.