Red Snubber

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Application Note AN-3008
RC Snubber Networks for Thyristor Power Control and Transient Suppression
Introduction
RC networks are used to control voltage transients that could falsely turn-on a thyristor. These networks are called snubbers.
I1 A IBP CJP CJN IJ IBN IK K IA = IJ IA PNP ICP I2 G CJ dV dt dV CJ dt 1– (αN + αp) K INTEGRATED STRUCTURE G t NE PB V PE NB C AThe simple snubber consists of a series resistor and capacitor placed around the thyristor. These components along with the load inductance form a series CRL circuit. Snubber theory follows from the solution of the circuit’s differential equation. Many RC combinations are capable of providing acceptable performance. However, improperly used snubbers can cause unreliable circuit operation anddamage to the semiconductor device. Both turn-on and turn-off protection may be necessary for reliability. Sometimes the thyristor must function with a range of load values. The type of thyristors used, circuit configuration, and load characteristics are influential. Snubber design involves compromises. They include cost, voltage rate, peak voltage, and turn-on stress. Practical solutions depend ondevice and circuit physics.

ICN NPN

CJ TWO TRANSISTOR MODEL C EFF = 1– (αN + αp) OF SCR

Figure 1.

( dV )s Model dt

Conditions Influencing

( dV )s dt

Transients occurring at line crossing or when there is no initial voltage across the thyristor are worst case. The collector junction capacitance is greatest then because the depletion layer widens at higher voltage. Small transientsare incapable of charging the self-capacitance of the gate layer to its forward biased threshold voltage (Figure 2). Capacitance voltage divider action between the collector and gate-cathode junctions and built-in resistors that shunt current away from the cathode emitter are responsible for this effect.
180 160 140 MAC 228-10 TRIAC TJ = 110°C

dV Static dt
What is Static
dt

dV ? dt

dVStatic ------- is a measure of the ability of a thyristor to retain a

blocking state under the influence of a voltage transient.
dV (V/µs) dt STATIC

( dV )s Device Physics dt
dV Static ------- turn-on is a consequence of the Miller effect and dt

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regeneration (Figure 1). A change in voltage across the junction capacitance induces current through it. This currentdV is proportional to the rate of voltage change  ------- . It triggers  dt 

the device on when it becomes large enough to raise the sum of the NPN and PNP transistor alphas to unity.

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PEAK MAIN TERMINAL VOLTAGE (VOLTS)

dV Figure 2. Exponential dt versus Peak Voltage s REV. 4.01 6/24/02

( )

AN-3008

APPLICATION NOTEdV Static ------- does not depend strongly on voltage for operation dt

Improving
dt

below the maximum voltage and temperature rating. Avalanche multiplication will increase leakage current and
dV reduce ------- capability if a transient is within roughly 50 volts dt

( dV )s dt
dt

dV Static ------- can be improved by adding an external resistor

from the gate to MT1 (Figure 4). Theresistor provides a path
dV for leakage and ------- induced currents that originate in the

of the actual device breakover voltage.
dV A higher rated voltage device guarantees increased ------- at dt

drive circuit or the thyristor itself.
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lower voltage. This is a consequence of the exponential rating method where a 400 V device rated at 50 V/µs has a rating. However, the samediffusion recipe usually applies for all voltages. So actual capabilities of the product are not much different.
dV Heat increases current gain and leakage, lowering  ------- ,  dt  s the gate trigger voltage and noise immunity (Figure 3).
180 160 140 MAC 228-10 VPK = 800 V

dV (V/µs) dt

dV higher ------- to 200 V than a 200 V device with an identical dt

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