piątek, 29 marca 2013

Short pulse generator, part 1.

Some time ago I have started designing an short pulse generator for time-resolved spectroscopy of electroluminescence. At first the design requirements were high, but after a market query, in search for devices capable of meeting these requirements they were lowered down significantly.

The sample for which this device is planned is a thin sheet of material suspended on an alumina frame, that acts as electrodes. We have measured DC I/V curve for the sample to assess the working parameters:

Therefore what we need is voltage of at least 35 V with current of at least 250 mA. In order to have an reserve in the parameters I assume that we need at least 40 V and 400 mA. For sake of simplicity I assume that the load of the device is almost purely resistive. Assesing the time parameters was a harder thing to do, so I assumes that the lower the better (the pulse time). Where is the limitation? In the power stage.

Using a simple generator, shown below, we are capable of generating a ~1 ns wide 5 V pulse with ease, using standard, from-the-shelf components. This could be even improved with faster ICs (comparators and AND gate.

From this Linear application note.

But this circuit generates only 5 V pulses (or close to that, this is the power supply voltage of the output AND gate). Also, the current is very limited - to 10 mA, offered by a TTL gate. In order to improve these parameters we have to add an output power stage, that will have output current and maximal voltage on the level of the planned device and could be controlled by an TTL pulse. To meet these requirements I have chosen several routes to achieve that:

  • A set of parallel bipolar transistor put in the common base topology. This should allow to achieve high rise and fall time of the pulse, but a current of a single fast transistor is low (tens of mA) so I need to put lots of them in parallel and I am not convinced that this is totally safe and will not affect the working of the device.
  • A (most probably single) FET/MOSFET device. An easy scheme for achieving high currents, even of hundreds of amps, but the pulse time will be significantly longer. With a dedicated driver and chip-to-pcb mount some people achieved 25 ns pulse width, although with a current of 100 A (as soon as I find the paper I'll post a link here). A stand-alone mosfet usually will have a rise time of several ns and fall time of 100 ns or more.
  • A dedicted IC. A RF power amplifier or a MOSFET driver. The first type of ICs seems to be a good idea as it offers tens of GHz in bandwidth, although when I have looked closer in this matter it seems that such IC will only work good in a typical circuit, for example an WiFi amplifier or so. On the other hand, some MOSFET drivers, are capable of almost meeting my requirements. EL7158 from Intersil is capable of producing 12 A pulses with rise and fall time of 12 ns (with 2000 pF load). This gives a chance to produce a 25 or shorter ns pulse, although the voltage is limited up to 18 V. Although this is not the only such driver on the market...
So currently I'm on the stage of parts requirements stage. As soon as I get something new I will post it here for sure.

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