A
Adam. Seychell
- Jan 1, 1970
- 0
I'm building an electronic fluorescent lamp circuit with the basic lamp
circuitry shown below. The input frequency is typically 100kHz during
normal operation. The starting sequence begins by turning on the TRIAC ,
and setting the frequency so about 200mA flows through the filaments.
Once heated the circuit is powered off for about 1ms, and the TRIAC is
then allowed to turn off. Power is turned back on at the LC resonant
frequency thereby producing high voltage across the lamp to start the
arc. Once started the lamp reduces impedance and the circuit behaves
like conventional inductor ballast.
o---2.0mH--. ,-----.--------.
| | | |
|---| | |
| | | |
| | | |
180V | | | |
square wave | | | 1.0nF |
input | | --- |
| | --- |
| | | T2|
| | | ----- TRIAC
| | | /\ \/
| | | -----
|---| | T1|\___G
| | | |
o----------' '-----o--------`
|
GND
I'm concerned about reaching maximum off state dV/dT ratings of the
TRIAC and causing it false trigger during lamp operation or starting.
The device I have tested is a BT134 with a rated off state dV/dT of
250V/uS (type). Strangely the TRIAC does not seem to trigger when
exceeding the rated dV/dT. For example, if I temporary remove the lamp
so the circuit effectively becomes an unloaded series LC then the
voltage across the TRIAC was measured to be 700Vpk at 140kHz. This
corresponds to a dV/dT of over 600V/uS across the TRIAC, yet still did
not turn on. However, the TRIAC will sometimes trigger when the circuit
is powered instantly, but if the 140kHz voltage is ramped to 700Vpk over
a few milliseconds, then the TRIAC never triggers. Is there more to the
story than just an off state dV/dT rating ?
circuitry shown below. The input frequency is typically 100kHz during
normal operation. The starting sequence begins by turning on the TRIAC ,
and setting the frequency so about 200mA flows through the filaments.
Once heated the circuit is powered off for about 1ms, and the TRIAC is
then allowed to turn off. Power is turned back on at the LC resonant
frequency thereby producing high voltage across the lamp to start the
arc. Once started the lamp reduces impedance and the circuit behaves
like conventional inductor ballast.
o---2.0mH--. ,-----.--------.
| | | |
|---| | |
| | | |
| | | |
180V | | | |
square wave | | | 1.0nF |
input | | --- |
| | --- |
| | | T2|
| | | ----- TRIAC
| | | /\ \/
| | | -----
|---| | T1|\___G
| | | |
o----------' '-----o--------`
|
GND
I'm concerned about reaching maximum off state dV/dT ratings of the
TRIAC and causing it false trigger during lamp operation or starting.
The device I have tested is a BT134 with a rated off state dV/dT of
250V/uS (type). Strangely the TRIAC does not seem to trigger when
exceeding the rated dV/dT. For example, if I temporary remove the lamp
so the circuit effectively becomes an unloaded series LC then the
voltage across the TRIAC was measured to be 700Vpk at 140kHz. This
corresponds to a dV/dT of over 600V/uS across the TRIAC, yet still did
not turn on. However, the TRIAC will sometimes trigger when the circuit
is powered instantly, but if the 140kHz voltage is ramped to 700Vpk over
a few milliseconds, then the TRIAC never triggers. Is there more to the
story than just an off state dV/dT rating ?