- SCR Definition: An SCR (Silicon Controlled Rectifier) is a type of thyristor used to control high voltage and current in electrical circuits.
- Series Connection: Connecting SCRs in series is used to handle higher voltage demands, but requires resistors and capacitors for equal voltage division.
- Parallel Connection: SCRs are connected in parallel to meet higher current demands, with equal temperature maintenance crucial to avoid thermal run away.
- Equalizing Circuits: Static and dynamic equalizing circuits, including resistors and capacitors, help manage voltage and current distribution in series and parallel connections.
- Thermal Run Away Prevention: Mounting SCRs on the same heat sink and using magnetic coupling can prevent unequal current division and thermal run away.
When one device cannot meet a converter’s rating, designers may connect SCRs in series or parallel. A series string supports a higher blocking voltage, while parallel paths support a higher current. Device ratings cannot simply be added because manufacturing tolerances, temperature and circuit asymmetry affect how stress is shared. Each SCR must remain within its static and transient limits under the worst conditions.
Extra devices and design margin reduce the nominal stress assigned to each SCR, but they do not guarantee reliability. A series or parallel assembly also adds components and possible failure modes. The derating factor (DRF) expresses device-rating utilisation in the string; it is not a direct measure of reliability.
Series Operation of SCR
When the required blocking voltage exceeds one SCR’s rating, matched devices can be connected in series. The same current passes through every device, but differences in leakage current, recovery charge and gate delay produce unequal static and dynamic voltage sharing. Two 5 kV ratings therefore do not by themselves establish a safe 10 kV operating voltage.
The diagram illustrates static sharing at a common leakage current. The voltage across SCR1 is V1, while the voltage across SCR2 is V2. If V2 is lower than V1, the second device, SCR2, is underused when the first reaches its limit. In the example, the usable string voltage is V1 + V2 = 8 kV rather than the 10 kV sum of nameplate ratings, giving 80% utilisation.
A voltage-sharing resistor across each SCR can dominate the differences in off-state leakage. Equal-value, suitably rated resistances then force a more uniform steady-state voltage division. The resistor value must be calculated from the maximum leakage-current spread at operating temperature, the allowed imbalance and the continuous power loss.
In the relationship, n is the number of SCRs in the string
Vbm is the voltage across the SCR with the minimum leakage current
ΔIb is the difference between maximum and minimum SCR leakage current
Vs is the total string voltage
The parallel resistance forms the static equalising circuit. It does not correct voltage imbalance caused by different turn-on delays or reverse-recovery charge. An RC network adds a capacitor and resistor across each device for dynamic sharing and damping. Synchronous, steep gate pulses and matched devices also reduce transient imbalance. Some designs add diodes or other clamp elements, but the complete network must be calculated for the converter waveform.
Parallel Operation of SCR
Parallel SCRs can carry a load current above one device’s safe rating, provided both steady-state and turn-on current share acceptably. Differences in on-state voltage, junction temperature, gate delay and path impedance can make one device carry more current. Consider two parallel transistors rated at 1 kA in the illustration. At operating voltage V, SCR1 carries 1 kA while SCR2 carries 0.8 kA. SCR2 is underused, so the pair carries 1.8 kA rather than the theoretical 2 kA. Its utilisation is 90%.
If one SCR carries excess current, its losses and junction temperature rise. The resulting change in on-state resistance and voltage can worsen current imbalance for some operating ranges, causing thermal runaway. Use devices matched for on-state voltage, provide equal cooling and verify the worst-case junction temperature. Symmetrical busbars should give each path similar resistance, inductance and magnetic flux.
Equal conductor geometry helps make leakage and mutual inductance similar in every branch. This reduces dynamic current imbalance during turn-on. Where direct paralleling cannot provide sufficient sharing, coupled current-balancing reactors can add opposing impedance to the branch carrying excess current.
When I1 = I2, the anti-series windings produce no resultant core flux, so each branch has the same effective inductance. If I1 > I2, the net flux induces voltages that oppose current in branch 1 and aid current in branch 2. The coupled reactor therefore reduces the current difference. Its current and frequency ratings must cover the transient duty.
