IGBT-controlled flashes vs. Voltage-controlled flashes
|Next recommended reading: Understanding HyperSync and High Speed Sync|
Variable Voltage Control
Many studio strobes use variable voltage to control the output of the flashtube. This means that the amount of energy emitted from the flashtube is varied to generate different power levels. Full power emits a flash curve with a tall “peak” of flash energy output, and a diminishing “tail.”
Lower power levels still have a tall peak energy output, and a diminishing tail, but the tail may diminish over a longer or shorter period of time, and the peak may not be so abrupt (instantaneous) as compared to a higher power level.
When HyperSync is used with a voltage controlled flash, any power level can be used. Best results will happen at the longest duration, and we generally recommend full power for best results. You can see what effect voltage controlled flash power can have on HyperSync by seeing our Power Level section of our Understanding HyperSync and High Speed Sync page.
IGBT Power Control
Some studio strobes (and almost all speedlights) use a device called an IGBT to control their power output. IGBT stands for “Isolated-gate bipolar transistor.” This is a high speed switch that is designed to turn power on and off rapidly.
At full power, the IGBT is not engaged, and the flash fully discharges its capacitors. Notice how this graph looks similar to the ones above.
At power levels lower than full power, the IGBT shuts off the flashtube before the capacitors in the flash have emitted all their power. The voltage sent to the flash tube before the IGBT engages is the same as at full power until the IGBT is engaged. When shown in graph form, an IGBT-controlled pulse looks like a full power pulse with the tail portion cut off.
The same thing happens with the Einstein E-640 and other IGBT controlled studio strobes.
Effects on HyperSync
This IGBT method of power control allows for very short flash durations. At X-sync, motion blur from flash is reduced with shorter flash durations. However, very short flash durations are not beneficial to HyperSync. Because there is no flash “tail,” the camera cannot expose the entire sensor across some portion of the flash duration. When using HyperSync, only a small band of light will be illuminated in the middle of the frame. This band becomes smaller the lower the power level used on the flash or Speedlight.
Because of the effect the IGBT has on HyperSync results, we recommend that Speedlight users who need to adjust power above X-sync to use High Speed Sync or FP-Mode instead.