lets say a worst case scenario of 200W with a TJ = 125C and a heatsink = 60C.
Hotspots on the die and local thermal runaway are definitely a problem with BJTs, particularly at higher voltages, known better as second breakdown. this is mostly a problem at higher continuous current above about 15v across the transistor.
There is a time element involved with this, as its a localised thermal issue.
An IGBT is really a high voltage PNP BJT in disguise, and all the above issues occur with these as well. They make superb switches, but have serious limitations for continuous high power operation in the linear region.
Its why you never see IGBTs normally used as linear regulators.
Mosfets look much more promising, they suffer none of these evils. you can definitely run them continuously in the linear region at high power with safety, and they will even current share reasonably well.
But they have considerable high frequency current gain, and can be difficult to tame if run in common source configuration.
Used as source followers, there can be enough degeneration to calm things down, especially if you can damp the gate circuit with some resistance (or ferrite beads) to kill any high Q resonances with parasitic inductance and the gate capacitance.
None of that is likely to show up in circuit simulation, its more a case of build it and then try to figure out why it is unstable. Layout is critical, another thing circuit simulations will not tell you.
With a bit of patience and some serious thought it should be possible to get something working.
Try running it open loop first.
That will tell you if its basically stable without any deliberate feedback.
Only then will you have any hope of successfully closing the loop.
SCRs and triacs are more ideal for battery chargers and similar applications where fast response to load changes are not required.
They can be used for a bench power supply, but the transient regulation will be very poor and well below most requirements as a general purpose supply.
They could be used as a pre regulator for a linear supply, but it gets pretty complicated to set it all up. Tap switching on the transformer is a lot simpler and works almost as well.
SCRs will be much better for this than a triac, because the commutation is forced each half cycle.Yes the aim is to use the controlled rectifier as a pre regulator, to keep the voltage across the transistor minimized to say 5-10V. As long as the supply operated at a constant voltage the output should be fairly stable.
SCRs will be much better for this than a triac, because the commutation is forced each half cycle.
Mosfets are good because the bit that gets hot increases in resistance.
BJTs and IGBTs are poor because the bit that gets hottest reduces in resistance and hogs the current. Its an accelerating self destruct mechanism.
Mosfets tend to be thermal equalisers, both between devices, and across a single die, so the whole thing is a lot more rugged.
SCRs will be much better for this than a triac, because the commutation is forced each half cycle.
Mosfets are good because the bit that gets hot increases in resistance.
BJTs and IGBTs are poor because the bit that gets hottest reduces in resistance and hogs the current. Its an accelerating self destruct mechanism.
Mosfets tend to be thermal equalisers, both between devices, and across a single die, so the whole thing is a lot more rugged.
There will always be a need for some ballast resistance in the sources to balance the current between individual mosfets, just as emitter ballast resistors are always required for individual BJTs when run in parallel.
At very low output current, balance is not important. But flat out it becomes of far more significance.
You cannot run your scr or triac straight into a reservoir capacitor, the current spike at turn on would be horrendous. There absolutely must be a choke after the rectifier to even out the current flow over the conduction cycle.
The problem then is that the current keeps flowing during and throughout the voltage zero crossing, and triacs in the transformer primary tend not to always reliably turn off.
Back to back scrs in the transformer primary, or used as rectifiers in the secondary are a far more robust and reliable solution to follow through conduction problem.
A choke input filter will remove very high di/dt that the scr/triac would otherwise create. The reservoir capacitor should create a fairly smooth dc with only slight ripple at twice mains frequency.
If your linear regulator feedback loop is worth half a damn, it should very easily cope with that.
I would very strongly recommend you stick with SCRs rather than a triac.
The second gate is a nuisance, but a pulse transformer with two secondaries takes care of that problem.
All these words of caution come from several years of designing commercial high power phase controlled battery charging equipment.
Triacs are an instrument of the devil..........!
Thanks guys.
Question : Has anyone thought or seen a PS using a controlled triac rectifier? I'm pondering this to reduce overshoot after the load is removed and reduction in power dissipation.
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