Transistor an1048d

AN1048/D

RC Snubber Networks For Thyristor Power Control and Transient Suppression
By George Templeton Thyristor Applications Engineer http://onsemi.com

APPLICATION NOTE

INTRODUCTION
Edited and Updated

RC networks are used to control voltage transients that could falsely turn-on a thyristor. These networks are called snubbers. The simple snubber consists of a series resistor andcapacitor 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 and damage to the semiconductor device. Both turn-on and turn-offprotection 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 on device and circuit physics. STATIC dV dt
WHAT IS STATIC dV? dt dV is a measure of the ability of a thyristor to Static dt

dV DEVICE PHYSICS dt s

Static dV turn-on is a consequence of the Miller effect
dt

and regeneration (Figure 1). A change in voltage across the junction capacitance induces a current through it. This current is proportional to the rate of voltage change dV . It
dt

triggers the device on when it becomes large enough toraise the sum of the NPN and PNP transistor alphas to unity.
A A IB P I1 IC N NPN CJ P CJ N IJ IB N IK K TWO TRANSISTOR MODEL OF SCR IA IA PNP IC IJ P I2 G V PE NB dv dt CJ G t NE PB C

+ 1*

CJ

CEFF

+ 1*(aN)ap)

dV dt (aN ap) CJ

)

K INTEGRATED STRUCTURE

retain a blocking state under the influence of a voltage transient.

Figure 6.1.

dV Model dt s

© SemiconductorComponents Industries, LLC, 1999

1

August, 1999 – Rev. 2

Publication Order Number: AN1048/D

AN1048/D
CONDITIONS INFLUENCING dV dt s
170 150 130 STATIC dV (V/ µs) dt 110 90 70 50 30 10 25 40 55 70 85 100 115 130 145 MAC 228A10 VPK = 800 V

Transients occurring at line crossing or when there is no initial voltage across the thyristor are worst case. The collector junction capacitance isgreatest then because the depletion layer widens at higher voltage. Small transients are incapable of charging the selfcapacitance of the gate layer to its forward biased threshold voltage (Figure 2). Capacitance voltage divider action between the collector and gate-cathode junctions and builtin resistors that shunt current away from the cathode emitter are responsible for this effect.
180 160 140STATIC dV (V/ µs) dt 120 100 80 60 40 20 0 200 300 400 500 600 100 PEAK MAIN TERMINAL VOLTAGE (VOLTS) 700 800 MAC 228A10 TRIAC TJ = 110°C

TJ, JUNCTION TEMPERATURE (°C)

Figure 6.3. Exponential

dV versus Temperature dt s

dV FAILURE MODE dt s

Occasional unwanted turn-on by a transient may be acceptable in a heater circuit but isn’t in a fire prevention sprinkler system or for thecontrol of a large motor. Turn-on is destructive when the follow-on current amplitude or rate is excessive. If the thyristor shorts the power line or a charged capacitor, it will be damaged. Static dV turn-on is non-destructive when series impeddt

ance limits the surge. The thyristor turns off after a halfcycle of conduction. High dV aids current spreading in the
dt

Figure 6.2. Exponential

dVversus Peak Voltage dt s

thyristor, improving its ability to withstand dI. Breakdown
dt

Static dV does not depend strongly on voltage for operadt

turn-on does not have this benefit and should be prevented.
140 120 100 MAC 228A10 800 V 110°C

tion below the maximum voltage and temperature rating. Avalanche multiplication will increase leakage current and reduce dV capability if a...