CW arc lamp PSU design, now the rat has moved out.
Hi PL,
Some years ago, we bought an industrial YAG laser from another member here, and now the time has come to install it. It is a CW arc lamp pumped design. Electrically and optically it looks similar to a Laserscope in terms of design and scale. It is a prototype design made by IE Optomech (UK) almost 20 years ago now, and I can not find any information whatsoever about any of the electrical or optical specifications. Though I do have the lamp specs, 198 V * 21 A = 4 kW!
Unfortunately we had a small rodent infestation *inside* the arc lamp PSU. This guy had stockpiled about 500g of dog biscuits inside the power supply, he dragged them in through a fan with no finger guard on it. Sadly he didn't respect the don't **** where you live rule, and his **** is very corrosive. The cleanup was prolonged and very nasty. The power electronics look OK, but most of the logic electronics are badly corroded and probably useless.
So, I set about to redesign the logic electronics, reusing the same power electronics. I have drawn the power electronics schematic below. I have a few questions for anybody who is familiar with this common "series triggering" design.
1) It looks like the left hand side IGBT is unused (the gate is connected to the emitter, so it will always be turned off). This is a prototype design, so this configuration is possible (maybe they used a dual IGBT brick when they only needed one IGBT). I just want to be sure, is there any possible purpose for it?
2) The gate drive signal for the right hand side IGBT (which controls the lamp current) originates from a 555 timer, so I suspect it is a PWM signal. Since PWM modulated current is not good for the lamp, I suppose it must be smoothed by the series inductance, is that possible? Is the PWM frequency critical in this design (does it have some relationship to the inductance, or can it just be nice and fast e.g. 10 kHz)?
I will visit the triggering electronics later, I will leave it disconnected for now. Since the triggering electronics is disabled, I am considering to use a huge resistor in place of the lamp for testing the power supply. Aside from finding a 4 kW resistor, do you foresee any problem with this?
I'd propose going towards 3525, adding a LEM for current control loop and You're almost there ! For IGBT keep the freq low, like below 30kHz.
Oh, the shorted IGBT is a vital part of the setup - it's a hefty diode, a must for buck converter. The other one (i.e. diode) is just a protection for a start pulse not to go to deep into PS.
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I'm in a similar boat right now, do you have any idea what the values for the inductors are? When I get home tonight I will send you some schematics. I need inductor values, you need schematics.... See a common theme here?
For inductor value You just need the pulse frequency, in & out voltage, load amps, and current ripple in %
Assuming:
ripple=10%
Vin=300V
Vout=150V
Iout=30A
freq=25kHz
You, OK I get 1000uH.
A long time ago I put a program into my Casio calc. and it's working 'til now.
A practical attitude to the subject, however would be:
-finding / deciding a switch that matches Your needs (IGBT / MOSFET)
-finding a C or E or a pot core big enough to accept the winding ( I like copper flat, 2x8mm in epoxy-glass insulation
- winding the inductor to fill the core
- testing the coil for saturation using Your almost done PWM supply
- tuning the gap for nice triangle cuurent throgh the coil at max current.
- measuring what induction You just built
For 25kHz and IGBT I use 350-600uH as I allow up to 20% ripple. If You find a ready-made inductor You must test it for saturation in Your almost-done supply. Then You can modify the gap.
Careful with PS output capacitor. If above 1uF these like to induce oscillations in current loop.
So far I was walking slightly harder way (than LEM) by using a shunt for current and a high side switch. Shunt has less bandwidth limitations so I was mainly using compensation on pin 9, then fine tuned with cap 1nF to gnd from pin 1 (inv. input). Actually I currently use 3524.
Testing ? I never throw out the electrical kettles, I keep them, then fill them up with water, plug into common extension cord then to PS output. This gives about 10 minutes for testing before replacing the water
Creaky Old Award Winning Bastard Technologist Infinitus Excellentia Ion Laser Dominatus
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I have a great deal of experience with mouse damage. It is invariably fatal. I lost a PTS160 RF synthesizer to a mouse crawling in a open DB9 hole and having her young. Bye bye 0-160 mhz in .1 hz steps. No matter what I did I could not stop the corrosion.
Also had several cases at a former employer when we would pull stuff out of the storage warehouse and open it up and find mice. . A mouse gets in it, you might as well scrap it.
CW arc lamp PSU design, now the rat has moved out.
The power electronics look OK, but most of the logic electronics are badly corroded and probably useless.
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