Electronic – BJT driver, high peaks in power

bjtdriverltspice

Inspired by this post

Designing a *linear* MOSFET driver stage

I used this kind of driver for my controlled current source.

I use LTSpice for simulating and everything works fine until today 🙂 when it happens that I put a long cable as (highly inductive) load. Digging the mess, I realized that if low frequency waveform (1KHz) with high slew rate (1uS rise/fall) will pass to my driver, very high peaks of current (and power) will appear on 2n3904 (3906) which are Q1, Q2 in my schematic.

Of course, I can change Q's to something bugger (~5W) but I'm just curiouse if this is the answer. I cannot slow down the wave form since it's comming from a 1uS settling DAC.

Even the peak pulses are narrow (1uS) I wonder how will perform this in real life (see attached image). As can be observed, very high peaks appears in simulation. Will be the drivers overloaded and burn?

I tried to limit the current by inserting R16 but this will limit the dynamic range and so, performances.

I forgot to mention, average power is only 14.693mW so appears to be fine.
In real life would be needed bigger transistors?

Appreciate comments, thank you.

enter image description here

enter image description here

Best Answer

The power rating on transistors is typically for the continuous power. What you have here is 5W for 1μs, once every two milliseconds. This comes to 1μJ of energy going into your transistor every two milliseconds, which is an average power of half a milliwatt on top of whatever else is going on. So, based on energy alone you're probably fine--I'd be surprised if a 2N3904 burned out due to such low-energy peaks.

However, I can't seem to find any data on maximum pulsed current ratings. The 2N3904 is rated for 300mA continuous collector current, but it seems pulsed current isn't a standard figure of merit. I suspect that your transistors will be fine, unless that current pulse goes up to something ridiculous like 50A.

That's all assuming an unmodified circuit, though, and an accurate simulation. It's entirely possible, likely in fact, that parasitic elements that aren't simulated would stop this from even happening in the first place. And as @jonk says in the comments, if it is a problem, you can always slap a few ferrite beads on to spread the energy out over time, reducing the peaks.

Related Topic