[Guide]Power diode thyristors are widely used in AC/DC converters, UPS, AC static switches, SVC and electrolytic hydrogen, etc. However, most engineers do not understand such bipolar devices as well as IGBTs. For this reason, we Organized 6 serials, including forward characteristics, dynamic characteristics, control characteristics, protection, and loss and thermal characteristics. The content is extracted from Infineon’s “Bipolar semiconductor Technology Information”.
7. Protection
Thyristors and diodes must be reliably protected to avoid excessive current and voltage and pulse interference in the control circuit.
7.1 Overvoltage protection
In general, the causes of overpressure in the device are as follows:
Internal overvoltage-due to the carrier storage effect of power semiconductors
External overvoltage-due to the switching process on the line and atmospheric influences, such as:
● Switch when the transformer is no-load
● Switch for inductive load
● The moment the fuse blows
● Suffered from lightning
Thyristors and diodes may be damaged due to overvoltage of a few microseconds, so special attention should be paid to their overvoltage protection. When designing a suitable snubber circuit, the blocking capability (VDRM,VRRM) And the critical rate of rise of voltage (dv/dt)cr.
7.1.1 Separate buffer (RC buffer circuit)
When the load current of the thyristor or diode is turned off, due to the carrier storage effect, the load current will not stop flowing at zero crossing, but will continue to flow in the reverse direction as a reverse recovery current (Figure 23). Once the reverse peak recovery current is reached, the reverse delay current will drop sharply to a certain extent, which will cause a spike voltage to be generated on the inductance in the load loop and superimposed on the voltage across the device, which may damage the semiconductor overvoltage.
Separately buffering the semiconductor through the RC buffer circuit can effectively reduce this overvoltage. In order to ensure that the snubber circuit is suitable, it is necessary to understand the most important influencing factors, such as the current intensity of the on-state current iTM or iFM and the current rate of change -diT/dt or -diF/dt, the reverse repetitive peak blocking voltage of the semiconductor VRRM, The critical voltage change rate of the thyristor (dv/dt)cr. In the grid commutation converter, the thyristor and diode RC snubber circuit can be used under the following conditions and under the normal working conditions shown in Table 2:
● The short-circuit voltage uK>4% of the converter power supply transformer. When directly connected to the grid, the size of the choke must be adjusted accordingly.
● The safety margin of the ratio between the repetitive peak off-state voltage and the peak value of the power supply voltage>2.2
Table 2. RC snubber circuit for separate buffering in grid applications
Especially when the on-state current change rate is high or the blocking capability safety margin is low, check whether the above-mentioned recommended RC snubber circuit is suitable. In this case, a capacitor with a larger capacitance value and a resistor with an appropriately re-adjusted resistance value are usually required. According to the following formula, calculate the most favorable non-periodical optimal equivalent resistance for suppressing the overvoltage process:
Among them, R’and C’are the equivalent values of the RC series snubber circuit, and L’is the equivalent value of the converter inductance.
Table 3. Equivalent value of converter circuit
R, C= RC snubber circuit value
LS = stray inductance of the converter transformer (one phase)
Lg = the inductance of the smoothing choke
For the thyristor, it must be noted that the resistance value of the RC snubber circuit must be
Its purpose is to prevent the thyristor from withstanding excessive discharge current from the snubber circuit during the turn-on process (see also 3.4.1.2.3).
Calculate the power dissipation of the resistor according to the following formula:
k=2*10-6 Suitable for uncontrollable rectifier
k=4*10-6 Suitable for controllable single pulse and double pulse circuits and AC controllers
k=6*10-6 Suitable for controllable three-pulse and six-pulse circuits and three-phase controllers
Make sure to use values with the following units in the formula:
PR[W]
Vr[V]
C[µF]
f[Hz]
If required, the snubber circuit can be modified according to Figure 41 to reduce the overvoltage and thereby reduce the stress that the thyristor bears during the turn-on process.
Figure 41. Example of extended RC snubber circuit for thyristor
a-Using bipolar voltage surge suppressor
b-Use RCD combination to suppress the turn-on current
c-Use RCD combination to suppress dv/dt and forward off-state voltage
Note: Do = fast diode, especially in the case of turn-on
When there is a transformer buffer, as long as the highest voltage rise rate of the thyristor used reaches (dv/dt)cr>500V/µs, the RC snubber circuit may not be used in the rectifier working circuit (see 7.13),
7.1.2 Input buffer of AC controller
In AC controllers and three-phase controllers, the thyristors used in the anti-parallel configuration can be used for phase control and for full-wave operation in soft starters, for example. Figure 42
Figure 42. Snubber circuit of AC controller
Shown is the buffer circuit.
The recommended values for the RC series snubber circuit in Table 2 are applicable to the thyristor buffer under normal operating conditions and the following conditions:
● The induced phase angle between the power supply voltage and current is <30°el (cos9> 0.866). This can ensure that the oscillation caused by the series connection of the capacitor and the inductance that may appear in the snubber circuit is suppressed.
● The safety margin between the repetitive peak blocking voltage of the thyristor and the peak value of the power supply voltage>2.2 (see 3.1.2.1).
● The critical voltage rise rate of the thyristor (dv/dt)cr>500V/µs.
Note: The on-state current ITAV specified in Table 2 is accurate enough to be regarded as the average value of the thyristor in a unidirectional configuration. In order to determine the load current, the ITRMS RMS value of a single thyristor in the anti-parallel configuration and the IRMS RMS value of the total circuit can be calculated by the following formula:
For high-power semiconductors and light-triggered thyristors implemented in large-scale devices, the buffer circuit is often optimized according to the circuit parameters and the type of semiconductor used. In this case, the voltage rise rate can be ignored, because the critical voltage rise rate of these thyristors is significantly higher than the above-mentioned standard.
Therefore, it is not necessary to make general recommendations on the design of the snubber circuit.
Figure 43. AC controller current calculation
7.1.3 Power supply buffer circuit for grid-commutated converter
It is best to use a combined snubber circuit to suppress high-energy overvoltages from the grid or caused by converter transformers or choke switches. For converters with thyristors or diodes, the snubber circuit is located on the AC side and consists of an auxiliary rectifier with a diode and a protective capacitor with a discharge resistor. The diode bridge will hinder the discharge of the snubber circuit capacitance, so these discharge resistors are essential. The discharge resistance must be designed so that the capacitor can be discharged in one cycle. (See Figure 44 and Table 4).
Figure 44. Combined snubber circuit on the AC side of the controllable rectifier
Table 4. components in the combined snubber circuit on the AC side of a controllable three-phase bridge
All the thyristors and diodes in the converter and auxiliary rectifier usually do not need to use a separate snubber circuit, because the combined snubber circuit can also function as an RC network. Unless it is some dual-converter circuit, such as two three-phase inverted parallel bridges. Pay attention to the following components when designing a combined snubber circuit:
Series resistance R1
This component can prevent the possible oscillation of the converter transformer during switching. At the same time, it can limit the discharge spikes generated when the protective capacitor is turned on and subjected to overvoltage through the diode in the auxiliary rectifier.
Protection capacitor C1
When the converter transformer or choke is turned off, this component must absorb the accumulated energy to prevent the voltage from exceeding the maximum allowable repetitive peak off-state voltage of the thyristor or diode to be protected; switching arc losses are excluded.
Discharge resistance R2
When continuous overvoltage energy discharge time constant R2·When C=80ms, select the resistance of the component based on actual experience.
Auxiliary rectifier diode
When selecting an auxiliary rectifier diode, in addition to the required blocking capability, the allowable surge current of the device must also be considered. The allowable surge current depends on the charge surge current of the protection capacitor. The occurrence time of overvoltage is shorter and the interval time is longer, so the utilization rate of the auxiliary rectifier tube is lower, and the power dissipation is also lower. Usually no heat sink is needed.
7.1.4 Other options for high-energy overvoltage protection
RLC filter
It consists of the stray inductance of the converter transformer or the inductance of the commutation choke and an RC network grounded at the star point. They are suitable for suppressing short-term low-energy overvoltage, because considering the discharge current of the capacitor, resistors with too low resistance may not be selected. In addition, the size of the capacitor will be limited due to losses (see Figure 45).
Spark gap arrester
It can be used when high energy consumption and overvoltage are expected in the line. Since it will delay turn-on after reaching the trigger voltage, it is usually necessary to adopt additional overvoltage protection measures (see Figure 45).
DC buffer
A DC buffer can be used to suppress overvoltage on the load side (see Figure 45).
Voltage sensitive resistors such as metal oxide varistors can be used to replace the RC network. On the one hand, it should be remembered that varistors are usually not suitable for limiting repeated overvoltages, because they will cause poor thermal stability and severe aging under repeated overvoltages. The other party should be careful not to use a varistor with an inappropriate specification, otherwise it will prevent the high-energy overvoltage protection device (usually a spark gap arrester) from functioning.
Figure 45. Other options for high-energy overvoltage protection
7.2 Overcurrent protection
Thyristors and diodes can carry large operating currents, but they may also be damaged due to overcurrent, so appropriate protective measures need to be taken. Select the appropriate protection device according to the type of overcurrent. Usually divided into short-term protection and long-term protection.
7.2.1 Realize short-term protection with ultra-fast semiconductor fuses
A special semiconductor protection fuse with ultra-fast open circuit characteristics is used to achieve short-term protection. The overcurrent generated by the short circuit is limited to a certain value through short-term protection. Will face the risk of damage. In the worst case, they can reach the ∫i²dt value specified in the data sheet for the specific type when they are turned off.
When the semiconductor is subjected to the ∫i²dt value, it completely or partially loses its off-state and blocking ability until the junction temperature drops to the value allowed for continuous operation. After a few seconds, this stress may reappear, and this stress will only occur with a limited number of pulses during the entire working time of the converter (see also 3.1.16).
7.2.1.1 Fuse selection
The fuse can be placed in a phase or branch (bridge arm). The branch circuit fuse can realize the safest short-term protection, and allows the maximum current load of the thyristor or diode. The use of phase fuses can reduce the complexity of the structure.
However, for the possible feedback of the load with back electromotive force, a fuse must be additionally used at the output of the converter, because the short-circuit current fed back from the load to the DC bus does not necessarily flow through the phase fuse.
For some thyristors or diodes with high current carrying capacity, it is necessary to connect two fuses in parallel. The following values need to be considered when selecting a fuse:
Fuse rated voltage
The rated voltage of the fuse must be higher than the voltage that drives the short-circuit current.
Voltage to drive short-circuit current
This voltage is usually equal to the power supply voltage; only when the AC converter is working, this voltage is 1.8 times the power supply voltage.
Repeat voltage VRMS
This voltage is equal to the voltage V used to drive the short-circuit currentKRMSDivide by the number N of series fuses in the short-circuit path and multiply by the safety factor Fs=1.3 The result obtained. The following formula applies:
For example, in the B2 and B6 circuits, VRMS=1/2*1.3*VKRMS= 0.65*VKRMS
Fuse arcing voltage
In the arc extinguishing process, the fuse generates an arcing voltage (this voltage is related to the structure of the fuse) and a repetitive voltage. The peak of these voltages must not exceed the peak voltage of the semiconductor surge to prevent damage to any reverse-biased components in the circuit.
Fuse nominal current rating
This value usually refers to a sine wave AC current, and will be higher or lower than the rated value due to deviation from the current waveform. The nominal current of the fuse should be slightly higher than the expected phase or branch current.
∫i²t cut-off value
This value is the sum of fusing integral and arc integral, so it must be lower than the ∫i²dt value of the thyristor.
Figure 46. Turn-off characteristics of ultra-fast fuses
Table 5. Calculation of branch (arm) current and phase current
When the short-circuit current increases, the fuse is blown first. Then cover the filler-usually quartz sand-to extinguish the resulting arc. These fuses blow within 3 to 5 ms (see Figure 46)
You can use the formula shown in Table 5 to calculate the RMS value of the branch current or phase current with the output current of various converter circuits.
These factors apply to resistive loads and zero delay output.
7.2.2 More protection design: short-term protection of high-power semiconductors
7.2.2.1 High-speed DC circuit breaker
Electric trigger can be realized within a few milliseconds when short-circuited. Because of the high cost, such devices are rarely used.
7.2.2.2 Crowbar circuit (Electronic short circuit)
This kind of circuit is most commonly used in voltage source inverters with turn-off components (IGBT, GTO, IGCT). Once the DC bus voltage exceeds the specified protection level, the crowbar circuit is triggered and the DC bus capacitor is discharged. When the pulse current reverses the polarity, it is fed through a special diode or a freewheeling diode in the inverter circuit.
7.2.2.3 Grid-side circuit breaker
The semiconductor must carry the short-circuit current until the circuit breaker disconnects the grid. In large installations, this happens after three to five half waves.
7.2.2.4 Block trigger pulse
When the specified level is exceeded, the trigger pulse of the thyristor is suppressed. Then the thyristor bears the current half-wave and the reverse off-state voltage and the forward off-state voltage successively. This requires the semiconductor to have a sufficiently strong blocking ability.
7.2.3 Long-term protection
Long-term protection can be achieved by suitable thermal and magnetic overcurrent protection methods or fuses. The shut-off characteristics of these protection devices should be lower than the overvoltage during short-term operation. The blocking capability of the thyristor or diode will remain unchanged. Therefore, long-term protection of the thyristor can also be achieved by blocking the trigger pulse. If the maximum blocking capability is not required, the interruption characteristics can be determined according to the maximum overload on-state current characteristics described in section 3.1.14.
7.2.4 Full load rated protection
This kind of protection consists of long-term protection and short-term protection. In fact, it can be achieved only by combining several protection measures.
7.3 Limit the dynamic current by the Inductor in the load circuit
If the inductance in the load circuit is low, the current rise rate may be too high when the thyristor is turned on. In order to avoid damage, it is necessary to insert an additional inductor LZ, which can reduce the rate of rise of the on-current (see Figure 47). This method can also reduce turn-on losses.
For linear inductors, the current density in the area of the silicon chip triggered by diffusion decreases as the current rises.
In a saturated choke, when a larger part of the silicon chip is already in the conducting state, the step time tst(See Figure 47) A higher rate of current rise will then appear. At the beginning of the step time, the step current iTSt(See Figure 47) It should be roughly equal to the repetitive turn-on current IT(RC)M(See 3.4.1.2.3).
If the step current is lower, you can pass the resistor R in parallel with the chokepIncrease the step current. If the voltage V0 is applied at time 0, the current i is calculated according to the following formulaRSt:
Figure 47. Schematic diagram of thyristor turn-on current changes with different series inductances
a: Maximum allowable area
b: work is not allowed, and the current rise rate is not limited
c: Allow to work, with linear series inductance in the load circuit
d: Allow to work, with series saturated choke in the load circuit
7.4 Reduce the interference pulse in the gate circuit
The converter will cause the load circuit to produce sudden changes in current and voltage. At this time, due to the inductive or capacitive coupling between the gate pin and the electronic components of the trigger, interference pulses may appear at the gate terminal of the thyristor. As a result, the thyristor may be accidentally triggered and cause an operating failure in the device.
Common measures to reduce coupling to avoid interference pulses include winding or shortening the gate pin, and even improving the shielding of the trigger transformer or trigger electronics. In addition, the gate circuit can be protected (see Figure 48).
Figure 8. Example of gate protection of thyristor
For standard phase-controlled thyristors, the recommendations are as follows:
Cx=10…47nF
Rx is determined according to tX=RxCx=10…20µs
Dx fast diode
The discharge resistance Rx must be used, otherwise the critical voltage rise rate (dv/dt)crWait for the thyristor data may attenuate. If the buffer circuit adversely affects the control circuit, this must be taken into consideration when designing the flip-flop circuit (see also 3.3.1.8).
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