AC08FSM datasheet(185/223 Pages) TECCOR

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AN1009 - 1

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Miscellaneous Design Tips and Facts

Introduction

This application note presents design tips and facts on the follow-

ing topics:

•

Relationship of IAV, IRMS, and IPK

•

dv/dt Definitions

•

Examples of gate terminations

•

Curves for Average Current at Various Conduction Angles

•

Double-exponential Impulse Waveform

•

Failure Modes of Thyristor

•

Characteristics Formulas for Phase Control Circuits

Relationship of IAV, IRMS, and IPK

Since a single rectifier or SCR passes current in one direction

only, it conducts for only half of each cycle of an AC sinewave.

The average current (IAV) then becomes half of the value deter-

mined for full-cycle conduction, and the RMS current (IRMS) is

equal to the square root of half the mean-square value for full-

cycle conduction or half the peak current (IPK). In terms of half-

cycle sinewave conduction (as in a single-phase half-wave cir-

cuit), the relationships of the rectifier currents can be shown as

follows:

IPK = π IAV = 3.14 IAV

IAV = (1/π) IPK = 0.32 IPK

IPK = 2 IRMS

IRMS = 0.5 IPK

IAV = (2/π) IRMS = 0.64 IRMS

IRMS = (π/2) IAV = 1.57 IAV

When two identically rated SCRs are connected inverse parallel

for full-wave operation, as shown in Figure AN1009.1, they can

handle 1.41 times the RMS current rating of either single SCR.

Therefore, the RMS value of two half sinewave current pulses in

one cycle is

√2 times the RMS value of one such pulse per cycle.

Figure AN1009.1

SCR Anti-parallel Circuit

dv/dt Definitions

The rate-of-rise of voltage (dv/dt) of an exponential waveform is

63% of peak voltage (excluding any overshoots) divided by the

time at 63% minus 10% peak voltage. (Figure AN1009.2)

Exponential dv/dt =

Resistor Capacitor circuit t = RC =

Resistor Capacitor circuit

Figure AN1009.2

Exponential dv/dt Waveform

The rate-of-rise of voltage (dv/dt) of a linear waveform is 80% of

peak voltage (excluding any overshoots) divided by the time at

90% minus 10% peak voltage. (Figure AN1009.3)

Linear dv/dt =

Figure AN1009.3

Linear dv/dt Waveform

0.63

VPK

[]

•

t2 t1

–

()

t2 t1

–

()

4RC

•

t3 t2

–

()

(Peak Value)

100%

63%

Time

Numerical dv/dt

10%

0.8

VPK

[]

•

t2 t1

–

()

0.9 VPK

•

0.1 VPK

•

–

[]

t2 t1

–

()

90%

10%

Time

AN1009

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