In many CO2 HVPS the laser tube return is shunted directly to ground; often thru a front panel ammeter so the user can monitor the current. The HVPS neither directly monitors nor regulates the laser current. However, in some HVPS the laser tube return is one of several components that the SMPS incorporates into its regulation algorithm.
The tube return current is shaped by a component network before being passed to the SMPS control circuitry. This network has the effect of expanding the range for currents less than 10mA, which may come in handy for pre-ionization regulation.
This network has the effect of expanding the range for currents less than 10mA, which may come in handy for pre-ionization regulation.
Analysis:
- For ILaserRet <10mA, only R20 and R5 are important (along with minor contribution from the series combination of R1 and R2).
- R5 (2KΩ) represents the 2KΩ potentiometer which is used to calibrate the ILaserRet current.
- Taken together (390Ω) || (2K) || (10K + 2K) gives 317Ω, which nicely matches the measured value of 314Ω.
- Taken together (390Ω) ∥ (2K) ∥ (10K + 2K) gives 317Ω, which nicely matches the measured value of 314Ω.
- Above 10ma the U1 (TL431) is active and introduces an additional 100Ω (R21) in parallel with the above combination.
- This gives 76Ω (which is in the same ballpark of 88Ω (the slope measured off the graph).
- ZD2 (9.1V Zener diode) provides a hard limit for currents above 80mA and is likely for protection.
This is a plot of actual data collected from the HY031-D HVPS.
It nicely matched the simulation.
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Enrollment in local colleges, 2005
Source: Fictitious data, for illustration purposes only ⊥ ∥
Nate Note 