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Posted on 15 April 2019

Driver Units for Thyristors

 

 

 

 

 

 

 

The link between the electronic control components of a converter and the thyristors is the driver (driver circuit). The purpose of the driver is to generate suitable current pulses in order to drive the thyristors; the frequency, phase length, sequence, etc., of these pulses are affected by the signals delivered by the control electronics. As the thyristors in a converter circuit normally have different potentials (with differences of several hundred volts), the outputs of the driver device have to be isolated from one another. This is achieved with pulse transformers. A circuit diagram that includes a transformer is shown in Figure 1a; Figure 1b shows an equivalent circuit, while Figure 1c shows a typical drive signal.

Thyristor driver circuit

Figure 1. a) Drive circuit, b) Equivalent circuit; c) Time characteristic of drive signal

What is important is that positive trigger pulses are ruled out while the thyristor is poled in reverse direction (cathode potential is more positive than the anode potential). Such pulses will increase leakage current iR and consequently the off-state power losses in the thyristor and may lead to overheating of the component.

Drive pulse shape

To ensure that the thyristor is triggered reliably and safely if the main current increases steeply, a trigger pulse with sufficient current amplitude (≥ 5 · IGT) and rate of rise (≥ 1 A/μs) is needed. Even if the current in the commutation circuit rises relatively slowly, the RC element connected in parallel for overvoltage protection will often mean that a fast-rising discharge current is driven through the thyristor at every trigger. Thus, it is recommended that you always use sufficiently strong and steep drive pulses. This is particularly important for thyristors connected in parallel or series, since strong and steep drive pulses will improve synchronous triggering significantly. The exponential current rise is determined by the stray inductance of the pulse transformer LS:
 

\frac{di}{dt_0}=\frac{V_B}{L_{S1}+L_{S2}}

where VB is the driver supply voltage.

To determine the resultant peak current for a driver with known values for short-circuit current (IK ~ VB/R) and no-load voltage (V0 ~VB), the output characteristic determined from these values is entered into the trigger current/trigger voltage diagram for the relevant thyristor. In Figure 2, this diagram is shown in a linear scheme for better understanding.

Thyristor gate current/gate voltage characteristic

Figure 2. Gate current (IG) – gate voltage (VG) characteristic of a thyristor in linear scheme; dotted output characteristic, V0 – no-load voltage and IK – short-circuit current of the driver device; dash-dotted output characteristic of the control terminal – cathode of a typical thyristor

The actual input characteristics for the individual thyristors of the given type lie between the limiting characteristics in the diagram (dash-dotted line). Accordingly, the possible points of intersection with the output characteristic of the driver device lie between points A and B. The point of intersection of S with the output characteristic of the driver device results from the resulting drive pulse data, e.g.: 2.3 A; 10.7 V. The minimum duration of the drive pulses is 10 μs. In most cases, the latching current given in the datasheets applies to this pulse duration, too. The minimum trigger current and the latching current decrease for longer drive pulses.

For rectifiers with negative voltage, each thyristor cannot trigger until the instantaneous terminal voltage is higher than the negative voltage instantaneous value. Therefore, in order to achieve safe and reliable commutation, relatively long-lasting drive pulses are needed. An extreme cases is the AC converter under inductive load. Owing to phase displacement between current and voltage, a pulse duration of 180°-α is necessary, i.e. at 50 Hz for up to 10 ms. It goes without saying that the duration of the drive pulse should not be made unnecessarily long, since in combination with the necessary amplitude, this will lead to substantial control losses that then have to be factored in to the total thyristor losses. In addition, the maximum gate power dissipation PGM contained may not be exceeded under any circumstances. Otherwise the thyristor may be destroyed. Having said that, power dissipation values that lie well below these maximum values still have to be taken into account in thyristor dimensioning. For the example above, Pv = 2.3 A · 10.7 V = 24.6 W.

What is more, the more power is needed, the more complex the driver unit becomes. For the driver transformer, long pulse duration means larger voltage/time area, i.e. a larger (and hence more expensive) driver unit. The pulse length of the usable driver signal is determined by the main inductance of the transformer.

i=\hat{i}_G\cdot e^{\frac{-t}{L_H/R}}

The main inductance of the transformer is determined by the permeability of the core material and is temperature dependent. Very often the voltage/time area Vdt [μVs] is also given; this can then be used to calculate the maximum pulse duration. Figure 3 shows a typical characteristic for transformer voltage. The voltage/time area up to the saturation point is Vdt = tp · Vp(av). For the example shown, 16 V · 20 μs = 320 μVs.

Output voltage characteristic for a pulse transformer

Figure 3. Typical output voltage characteristic for a pulse transformer (24 V supply voltage)

In application, drive pulses > 1 ms are barely achievable. For these reasons, in such cases a chain of short pulses is often used instead of a single long pulse (frequency 5…10 kHz). If the gaps between pulses cause interference, a second pulse chain can be superposed to achieve a gap-free long pulse. In any case, the transformer only needs to be dimensioned for the short duration of one of the pulses in the chain (e.g. for around 70 μs at 7 kHz).

Figure 4 shows several output-stage circuits for driver devices with the relevant pulse shapes. As you can see, a diode is connected between transformer and thyristor. This is intended to suppress negative drive pulses generated by back-swing that the thyristor cannot cope with.

Output stage circuits and pulse shapes for drivers

Figure 4. Output-stage circuits for driver units and typical pulse shapes.

Driving six-pulse bridge circuits

The thyristors in (fully controlled) six-pulse bridge circuits have a current conduction angle of 120° for continuous current. In the case of discontinuous current or indirect commutation through a freewheeling diode, however, each current block disintegrates into two blocks with 60° distance to the starting times. In any case, when the device is turned on, two legs must be triggered at the same time. In six-pulse fully controllable bridge circuits, the driver devices therefore have to be able to deliver double pulses in a sequence of 60°.

 

For further information, please read the following articles:

 

Driver Circuits

Pulse Transformer Requirements

Drive Pulse Generation

Safe Firing of Thyristors

Reliable Thyristor Triggering

Criteria for Successful selection of Diodes and Thyristors

 

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2 Responses

  1. avatar LaoNiu says:

    usefull

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  2. avatar Reza says:

    How much is the price of Driver Units for Thyristors?

     

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