AN-3008 RC Snubber Networks for Thyristor Power Control and ...

AN-3008
APPLICATION NOTE
TRIAC Design Procedure
dV
(
dt
) c
dV
1. Refer to Figure 18 and select a particular damping factor
(ρ)giving a suitable trade-off between V PK and ------ .
dt
dV
Determine the normalized ------ corresponding to the
dt
chosen damping factor.
The volage E depends on the load phase angle:
E 2 V RMS Sin (φ) where φ = tan – 1 X =
⎛------
L ⎞ where
⎝R L
⎠
φ = measured phase angle between line V and load I
R L = measured dc resistance of the load.
dV
( Safe Area Curve
dt
) c
dV
Figure 26 shows a MAC16 TRIAC turn-off safe operating
area curve. Turn-off occurs without problem under the curve.
The region is bounded by static ------ at low values of ⎛dI
----⎞
dt
⎝dt⎠c
and delay time at high currents. Reduction of the peak current
permits operation at higher line frequency. This TRIAC
operated at f = 400 Hz, T J = 125 °C, and I TM = 6.0 amperes
using a 30 ohm and 0.068 µF snubber. Low damping factors
extend operation to higher ⎛dI
----⎞
⎝
, but capacitor sizes
dt⎠c
increase. The addition of a small, saturable commutation
inductor extends the allowed current rate by introducing
recovery delay time.
Then
V RMS 2 2
Z -------------- R
I L + X L X L Z 2 2
= = – R L and
RMS
X
L = --------------------- L
2 π f Line
If only the load current is known, assume a pure
inductance. This gives a conservative design. Then:
V RMS
L = --------------------------------------- where E = 2 V
2 π f LINE I RMS
RMS
For Example:
120
E = 2 120 = 170 V; L = ----------------------------------- = 39.8 mH
( 8 A) ( 377 rps)
(V/µs)
100
10
c
dV
dt
1
0.1
10
dl
dt
-ITM = 15A
c = 6 f ITM x 10-3 A/ms
WITH COMMUTATION L
14 18 22 26 30 34 38 42 46 50
dV
AMPERES/MILLISECOND
dt c
MAC 16-8, COMMUTATIONAL L = 33 TURNS #18,
52000-1A TAPE WOUND CORE 3/4 INCH OD
Read from the graph at ρ = 0.6, V PK = (1.25) 170 = 213V.
dV
Use 400V TRIAC. Read ------ = 1.0
dt ( ρ = 0.6)
2. Apply the resonance criterion:
ω ⎛ 0 spec dV ------ ⎞ dV
= ⁄ ⎛------ E⎞
⎝ dt ⎠ ⎝ dt ( P)
⎠
510 × 6 V/S
ω 0 = ---------------------------- =
( 1) ( 170 V)
29.4 × 10 3 rps
1
C = ------------- = 0.029 µF
2
ω 0 L
3. Apply the damping criterion:
R S 2ρ L 39.8 × 10 – 3
= --- = 2( 0.6)
C ------------------------------ 0.029 × 10 – 6 = 1400 ohms
Static
dl
( ) c
( ) c
dV
Figure 26.
dt
versus
dt
T J = 125 °C
dV
dt
Design
There is usually some inductance in the ac main and power
wiring. The inductance may be more than 100 µH if there is
a transformer in the circuit or nearly zero when a shunt
power factor correction capacitor is present. Usually the line
inductance is roughly several µH. The minimum inductance
must be known or defined by adding a series inductor to
insure reliable operation (Figure 27).
One hundred µH is a suggested value for starting the design.
Plug the assumed inductance into the equation for C. Larger
values of inductance result in higher snubber resistance and
dI
reduced ---- . For example:
dt
12 REV. 4.01 6/24/02