Electronics > Projects, Designs, and Technical Stuff
problems with a class B amplifier (high voltage)
SiliconWizard:
Hopefully the OP is not designing an electromagnetic brainwashing machine though.
Marco:
--- Quote from: OM222O on July 11, 2019, 07:01:39 am ---please provide a schematic on how to properly bias the fets rather than words if possible. thanks.
--- End quote ---
You can probably do it simpler, but this should work. You can use your wallet killer opamp to drive the midpoint too.
OM222O:
--- Quote from: MagicSmoker on July 11, 2019, 10:21:38 am ---
--- 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.
--- End quote ---
Those parts are exactly what I was looking for, not sure why they don't show up in parametric search and I haven't done anything with audio designs before, so I didn't knew they existed!
--- Quote from: duak on July 11, 2019, 05:17:46 pm ---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.
--- End quote ---
I'm going with the MJE3x0 pair for the time being. I will include some 110V zener diodes from the output to each rail, but I'm not sure if I should also add a resistor in series with it and if not, what should the power rating be on the diodes?
maybe a MOV is better in these situations?
the actuator certainly won't be moving back and forth at 10KHz! the 1 to 2ms is just a requirement for the PID control loop to be stable, otherwise it will be reacting to old data and losing stability when the actuator actually moves.
I also realized that my resistor values for the base resistor and the base emitter resistors were quite low and the op amp was still doing the majority of the work, so I increased them. here is the schematic as it stands:
also here is the power dissipated by each component (blue is the op amp, red is the PNP transistor and green is the NPN transistor):
MagicSmoker:
--- Quote from: OM222O on July 12, 2019, 09:11:42 am ---
--- Quote from: MagicSmoker on July 11, 2019, 10:21:38 am ---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.
--- End quote ---
Those parts are exactly what I was looking for, not sure why they don't show up in parametric search and I haven't done anything with audio designs before, so I didn't knew they existed!
--- End quote ---
FWIW, my specific search methodology to find a matched BJT pair is to first filter on PNP parts that are in stock because there are far fewer PNP parts, then select the range of Vceo, then the range of Ic, and, optionally the package(s). This usually results in a far more manageable list of candidates that I sort on ascending price to then start checking the datasheets for the recommended matched NPN device and application, though generally speaking, if a PNP device has a matching NPN device it is almost certainly going to be optimized for linear/amplifier applications rather than switching.
duak:
OM2220, the power dissipation graphs are quite interesting. I can see the slight ringing in the load current expanded in the transistor power dissipation curves. I think this circuit is underdamped. I would increase the value of R2 to at least 1K0 and adjust C1 until I get a clean pulse response. BTW, I think a value of 100R for R2 is too low and loading down the opamp output more than it needs to be.
Please show us want you want to do with the zener diodes. MOVs are probably not the best choice for repetitive pulse voltage limiting.
Please try different output currents to confirm where the maximum power is dissipated.
I have no idea of what the payload of the actuator is. It could be an optical element on an air bearing with an interferometric position sensor that would indeed be sensitive to high frequencies.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version