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problems with a class B amplifier (high voltage)

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Marco:
It's not a divider, the 1.5k was what the other thread said the coil resistance is.

OM222O:

--- Quote from: magic on July 11, 2019, 07:47:31 am ---You should be careful with MJE15034/MJE15035, their SOA is barely enough and appears to be specified at Tc=25°C. It seems like with BJTs you are limited to audio output transistors such as aforementioned 2SA1943/2SC5200. More choice would exist in a single rail design >:D

Before biasing anything, hack together a prototype and check if you really need it. If you do, you will also need another HV transistor for current source, unless you go for something unconventional like an isolated DC-DC converter or floating batteries.

I would discourage anything involving complementary common source FETs, like a BJT+FET Sziklai, on the grounds of crossconduction risk.

--- End quote ---

That is exactly my point! There are less than a total of 5 parts I found that barely fit the apllication! That's why I'm consideeing fets now. For the supply I will be using batteries (A few 8s lipo packs) so no issues there!
Can you please explain the issue with common source arrangement? I could mix and match but I'm not sure if that's a good idea. I.e: use a P channel fet and a NPN BJT. That makes biasing a bit weird too. So I'm still open to suggestions.

Edit: the issue with the recommended parts is there is almost no headroom whatsoever! Vceo and Vcbo are 230V and the application needs 220V! Maybe lowering the input voltages can add some safety margin without slowing the response too much, but I'd like to have some margin. In my own search I started at 300volts to have a decent headroom.

magic:

--- Quote from: OM222O on July 11, 2019, 09:11:59 am ---Can you please explain the issue with common source arrangement?
--- End quote ---
Look up what "common source" means and what a "Sziklai pair" is. Then don't use it :D

MagicSmoker:

--- Quote from: OM222O on July 11, 2019, 09:11:59 am ---Edit: the issue with the recommended parts is there is almost no headroom whatsoever! Vceo and Vcbo are 230V and the application needs 220V! Maybe lowering the input voltages can add some safety margin without slowing the response too much, but I'd like to have some margin. In my own search I started at 300volts to have a decent headroom.

--- End quote ---

MJx340/MJx350 are a commonly specified complementary pair for audio applications that have a 300V Vceo/Vcbo and a DC SOA of 50mA at 100V Vce and 125C Tj. NB - "x" specifies the package, and is E for TO-220, D for Dpak, etc. Dirt cheap, too.

duak:
Inductive loads can be difficult to drive and can exceed the SOA limits of the output devices unless the devices are saturated.  Work out the power dissipation in the output devices for various output voltages.  For a square wave, purely resistive loads have maximum dissipation at half voltage output.

If you go to BJT you'll have to provide clamping diodes from the output to the supply busses.

Back to back zener diodes are fine for protecting the gates of MOSFETs and IGBTs.  The zener or avalanche effect is quite fast - we're talking tens of ns if not faster.  What is slow is the diode shutting off in the forward direction ie., when it is operating as a normal diode.  It shouldn't be a problem here as the circuit is not operating at MHz rates.

I'm not sure that crossover distortion is a particularly big deal when the time constant of the load is L/R = 2H/ 1K0 = 2 ms.  There will be a bit of a deadband in the transfer function, but the overall negative feedback should bury it - at least at low frequencies.  However, if you're thinking of operating this at 10 KHz, and are expecting high accuracy, then it will be a problem.

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