Protection of Power Semiconductor Devices

Reliable operation of a thyristor demands that its specified ratings are not exceeded. There are two types of protection required:

1. di/di Protection
• If the rate of rise of anode current, i.e. di/dt is large as compared to the spread velocity of carriers, local hot spots will be formed near the gate connection. This localized heating my destroy the thyristor.
• The value of di/dt can be maintained below acceptable limit by using a small inductor, called di/dt inductor in series with the anode circuit.
• Typical di/dt limit value of SCRs are 20-500 A/μ-sec.
2. dv/dt Proctection
• If the rate of rise of suddenly applied voltage across thyristor is high, the device may get turned on. It leads to false operation of the thyristor circuits.
• Typical values of dv/dt are 20-500 V/μ-sec.
• False turn-on of a thyristor by large dv/dt can be prevented by using a snubber circuit in parallel with the device.

Design of Snubber Circuits

• When switch S is closed, a sudden voltage appears across the circuit as given in Fig. 1. Capacitor Cs behaves like a short circuit, Therefore voltage across SCR is zero.
• With the passage of time, Voltage across Cs builds up at a slow rate such that dv/dt across Cs and therefore across SCR is less than the specified maximum dv/dt rating.
• Before SCR is fired by gate pulse, Cs charges to full voltage Vs. When the SCR is turned ON, capacitor discharges through the SCR and sends a current equal to Vs/(Resistance of local path forward by Cs and SCR).
• In order to limit the magnitude of discharge current ID, a resistance Rs is inserted in series with Cs.
• In actual practice, Rs, Cs, and the load circuit parameters should be such that dv/dt across Cs during its charging is less than the specified dv/dt rating of the SCR, and discharge current at the turn ON of SCR is within reasonable limits.

Over-voltage Protection

Transient over-voltages causes either maloperation of the circuit by unwanted turn-ON of a thyristor or permanent damage to the device due to reverse breakdown. A thyristor may be subjected to internal or external over-voltages.

• Large voltages may be generated internally during the commutation of a thyristor. External over-voltages are caused due to the interruption of current flow in an inductive circuit and also due to lightening strokes on the lines feeding the thyristor system.
• Suppression of over-voltages: Thyristors are chosen with their peak voltage ratings of 2.5 to 3 times their normal peak working voltage.
• The effect of over-voltages is usually minimized by using RC circuits and non-linear resistors called voltage clamping device.
• RC snubber is not enough for over-voltage protection of SCR, therefore a combined protection consisting of RC snubber and VC device is provided to thyristor.
• The VC device has falling resistance characteristics with increasing voltage. a) Under normal working conditions of voltage below the clamping level: The device has a high resistance and draws only a small leakage current. b) When a voltage surge appears: The varistor device operates in the low resistance region and produces a virtual short circuit across the SCR.
• Selenium thyrector diodes, metal oxide varistors or avalnche diodes suppressors are commonly employed for protecting the thyristor circuit against over-voltages.

Over-current Protection

Thyristors have small thermal time constants. Therefore, if a thyristor is subjected to over-current due to faults, short circuits or surge currents, it junction temperature may exceed the rated value and the device may be damaged.

• Over-current protection is achieved by the use of circuit breakers (CB) and fast acting fuse. A circuit breaker has long tripping time, Therefore generally it is used against surge currents of long duration. A fast acting current limiting fuse (FACLF) is used against a large surge currents of very short duration.
• The tripping time of CB and the fusing time of FACLF must be properly coordinated with the rating of thyristor.

As thyristor possesses high surge current capability, it can be used in an electronic crowbar circuit for over-current protection of power converters using SCRs as given in Fig. 4.

Gate Protection

Over-voltages across the gate circuit can cause false triggering of the SCR, while Over-current may raise junction temperature beyond specified limit leading to its damage.

• Protection against over-voltages is achieved by connecting a zener diode (ZD) across the gate circuit as given in Fig. 3. A resistor R2 connected in series with the gate circuit provides protection against over-currents.
• Protection against undesirable firing is obtained by using shielded cables or twisted gate leads.
• A capacitor and a resistor are also connected across gate to cathode to bypass the noise signals.  (C<0.1μF)

Reference

1. M. H. Rashid, “Power Electronics: Circuits, Devices and Applications,” Prentice Hall India, Second Edition, 2006.
2. P. S. Bimbhra, "Power Electronics," Khanna Publishers, fifth edition, 2012.