High-power diode thyristor knowledge series - control characteristics

i G is the current flowing through the control channel (terminal G-HK).

Thyristors can only be triggered by pulses in the forward off-state phase.

Due to the transistor effect, a forward trigger pulse during the reverse off-state phase will lead to a significant increase in off-state losses. Such losses have a negative impact on functionality and may lead to component damage.

Exception: For light-triggered transistors, a control pulse during the reverse off-state phase is permitted.

VG is the forward voltage applied between the gate terminal (G) and the cathode (K) or auxiliary cathode (HK) .

I GT is the minimum gate current value required to trigger the thyristor. This value depends on the voltage between the main terminals and the junction temperature. At the specified gate trigger current value, all thyristors of the specified type will be triggered. The gate trigger current increases as the junction temperature decreases, therefore, the value is specified at 25°C.

The trigger pulse generator must safely exceed the data sheet value I GTmax (see also 3.3.1.8).

Exception: For light-triggered thyristors, the minimum optical power required to trigger all specified types of thyristors is specified.

V GT is the voltage generated between the gate terminal and the cathode when the gate trigger current I GT flows. This value depends on the voltage between the main terminals and the junction temperature. At the specified gate trigger voltage value, all specified types of thyristors will be triggered. The gate trigger voltage decreases as the junction temperature increases, so this value is specified at 25°C. V GT is measured when the specified load current flows .

I GD is the value of the gate current that just does not trigger the thyristor. This value depends on the voltage between the main terminals and the junction temperature. At the specified maximum value, a thyristor of a specified type does not trigger. The gate non-triggering current decreases with increasing junction temperature, therefore, this value is specified at T vj max .

V GD is the value of the gate voltage that just does not trigger the thyristor. This value depends on the voltage between the main terminals and the junction temperature. At the specified maximum value, a thyristor of a specified type does not trigger. The gate non-triggering voltage decreases with increasing junction temperature, therefore, this value is specified at T vj max .

Figure 14. Example of the trigger region of the control characteristic v G =f(i G ) when V D =12 V

The control characteristics show the statistical distribution limits of the input characteristics of a certain type of thyristor. The statistical distribution diagram of the input characteristics shows in detail the temperature-dependent triggering area and the maximum permissible gate power dissipation curve P GM (a-20W/10ms, b-40W/1ms, c-60W/0.5ms).

In conventional applications, the control circuit should be designed according to the control data. This article describes in detail the relationship between the control data and the critical rise time of the on-state current, the gate control delay time and the holding current (see Figure 15).

The minimum control data provided in 3.3.1.3 and 3.3.1.4 are only valid for applications with low requirements in terms of current critical rise time and gate control time. In practice, overdriving I GT specified in the data sheet by 4 to 5 times ensures safe operation even in cases with high requirements for current rise time and gate control delay time. The meanings of the relevant terms are as follows:

As the slew rate of the on-state current di T /dt and the repetitive turn-on current IT (RC)M from the snubber circuit increases, attention should be paid to the effect of the load circuit on the gate current iG (see 3.4.1.2 and Figure 21).

Figure 15. Thyristor trigger circuit design

During the turn-on of a thyristor, initially only a small area near the gate region of the die conducts, resulting in a high current density and a voltage rise. Due to internal coupling, this voltage also appears at the control terminal, which results in a moderate drop in the gate trigger current. To avoid possible damage to the thyristor, i G must not drop below the gate trigger current I GT . To prevent an excessive drop in the gate pulse, it may be necessary to compensate by increasing the open-circuit voltage V C of the trigger circuit . For thyristors connected in parallel or in series, a sharp synchronous high pulse is necessary to achieve the same turn-on effect. See also the distribution of gate control delay time values ​​(3.4.1.2.1).

Exception: For controlling light-triggered thyristors, a laser diode emitting in the 900 to 1000 nm region is required. The specified minimum optical power PL and the specified turn-on voltage ensure safe triggering of the thyristors. The optical power is determined at the output of the optical cable. Even for turn-on, overdriving is recommended, especially for series or parallel connections with high di/dt requirements.

Infineon recommends aligning the laser diode SPL PL90 with suitable accessories (see Figure 16). Infineon provides the laser diode, alignment accessories and optical cable as auxiliary components.

Figure 16. LTT with optical cable

The laser diode SPL PL 90 complies with the following laser classes: If the laser diode is terminated in a fiber optic cable, the control system complies with laser class 1. Not hazardous to operate.

If the laser diode is open for operation or the optical cable is broken, the control system is a class 3b laser according to IEC 60825-1. In this case, there is an operating hazard due to invisible radiation. Direct or indirect contact with eyes or skin must be avoided.

Figure 17. Typical relationship between optical power and control current of laser diode SPL PL 90

To control the light-triggered thyristor, we recommend applying current pulses to the laser diode SPL PL90 as shown in Figure 18. The diode SPL PL90 is not suitable for long-term control, so we recommend controlling the laser diode with a frequency of around 6kHz and the pulses shown in Figure 18.

Figure 18. Recommended current pulse for laser diode SPL PL 90

The trigger pulse should be applied at least after the thyristor's holding current (3.1.6) is exceeded, otherwise the thyristor will return to the off state. Before the trigger pulse ends, the thyristor's gate trigger current must be maintained at least at the rated value.

For applications with very short current rise times or low load currents, trigger profiles with multiple pulses (e.g. a repetition rate of 6kHz) are often used.

For light-triggered thyristors, ensure that the temperature of the laser diode is within the permissible range when using multiple pulses. The optical power of a current-controlled laser diode decreases as the temperature increases.

For applications with a high rate of rise, the current i GT may be overdriven to a higher degree than described in 3.3.1.8. In this case, the gate current should be increased to 8 to 10 times I GT within t G = 10 to 20 μs and then reduced for a sufficient time t G . To ensure a high inert gate current, the open-circuit voltage of the trigger circuit should be at least 30 V.

Figure 19. Safe overdrive of gate trigger current