AN1009
9
Miscellaneous Design Tips and Facts
Introduction
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)
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
Exponential dv/dt = 0.63 • [V ] = (t – t )
PK
2
1
Resistor Capacitor circuit t = RC = (t – t )
2
1
Resistor Capacitor circuit 4 • RC = (t – t )
3
2
Characteristics Formulas for Phase Control Circuits
(Peak Value)
100%
63%
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:
Numerical dv/dt
10%
0%
t
t
Time
t
0
2
t
3
1
I
I
I
I
I
I
PK = π IAV = 3.14 IAV
AV = (1/π) IPK = 0.32 IPK
PK = 2 IRMS
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)
RMS = 0.5 IPK
AV = (2/π) IRMS = 0.64 IRMS
RMS = (π/2) IAV = 1.57 IAV
Linear dv/dt = 0.8 • [V ] = (t – t )
PK
2
1
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.
Linear dv/dt = [0.9 • V – 0.1 • V ] = (t – t )
PK
PK
2
1
90%
10%
0%
t
Time
t
t
0
2
1
Figure AN1009.1
SCR Anti-parallel Circuit
Figure AN1009.3
Linear dv/dt Waveform
©2002 Teccor Electronics
Thyristor Product Catalog
AN1009 - 1
http://www.teccor.com
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