# Storage for High Power High Current Pulse

I am currently developing a high voltage, high current amplifier for an EMI test pulse generator. I have been able to generate the pulse in 5V level so far and I am now trying to boost the pulse (in voltage, power and current) with an amplifier circuit using discrete components. I've used the search function and got a direction pointed towards the amplifier circuit here: High power pulse generator

But I still have a remaining question. I've done the Math and I am pretty sure I have to buffer the energy for such a pulse. The pulse is:

t: maximum 300ms V: 0-90V R: 2 Ohms

What do you guys recommend? A large battery? Any ideas on that? Could someone point me in the right direction? Thank you very much for your help and looking forward to your opinions!

• You don't simply "amplify" a 5V pulse into HV/HC levels. – PlasmaHH Mar 7 '16 at 14:02
• Do you mean that the term amplify is wrong? It's a student's project my team is working on and we have advisors that will certainly look over our schematics before plugging it in (if you are trying to warn me because of the dangerous levels). But I somehow have to generate the pulse. – Havefun Mar 7 '16 at 14:14
• No, the whole idea is wrong. You don't take a 5V pulse and "make that somehow 90V". You create that 90V pulse by somehow switching 90V around and shaping that pulse. If to do that you have a 5V pulse that is controlling something then that is kinda coincidental since the goal is to shape that 90V pulse, not some intermediate control voltages. Especially when you want high current, the current you put out on your 90V is not that much related to currents in your control circuits. – PlasmaHH Mar 7 '16 at 14:22
• You do understand that you're asking to create a ~4050W-max pulse, right? I hope you also understand the dangers associated with that. I'm positive you'd have to have access to a three phase power source just to get that type of power unless you're storing it in some kind of physically and electrically huge capacitor. That's 1215 joules a pulse (max). And I don't know how much of a voltage drop you can take before your "pulse" isn't square enough. But, even if you can take a 5V drop, you'd still need about 97 Farads. – Dave Mar 7 '16 at 14:40
• How often do you need to generate the pulses? – Icy Mar 7 '16 at 14:56

This is an extremely challenging idea. First, let's say that the output doesn't have to be exactly 90 volts, but should be more or less constant during the pulse. Then, if you're willing to buy some car batteries, it's not too hard:

simulate this circuit – Schematic created using CircuitLab

Automotive batteries have a rating called CCA, Cold Cranking Amps. This is the current a new battery will provide at zero F for 30 seconds. If you take a look here, for instance you'll see CCAs in the range of 600 to 800 amps, so your 45 amps is pretty easy. In other words, you can get the cheapest batteries you can find.

High-current Li-Pos would work, but you'd need about 25 of them.

The p-type MOSFET needs to be chosen for low Rds(on). At 90 volts and 2 ohms (45 amps) for 0.3 seconds the total pulse energy will be about 1200 J. Assuming an Rds(on) of 1% of load (0.2 ohms) the MOSFET will only dissipate 40 watts and 12 J total. The MOSFET needs to be rated at 100 V plus, and 50 amps drain current, but that's not hard to find.

I've assumed that, for EMI purposes, you need one end of the load grounded, and this drives the choice of p-type. If the load can float, using grounded n-types will make your life much easier.

What gets hard is trying to produce a non-rectangular pulse. I suggest a two-step pulse is achievable, with a period at 45 amps (or a bit more) and then a period at a lower current which is some multiple of 45/8 amps. You can do this by

simulate this circuit

In this case, when M1 is turned on the voltage at the load is about 96 volts, or whatever the batteries put out at that current, which you need to determine. When M1 is off and M2 is on, the load voltage will be about 48 volts. This approach only allows discrete current levels but is pretty forgiving of power dissipation in the pulse generator. The MOSFETs are run as switches and dissipate relatively little power as long as they are properly driven. And don't forget that the diodes need to be sized correctly, both for voltage and current.

If you want something less "digital" you may have a difficulty, depending on exactly which waveform you need. Since this is EMI testing, you may need an inverse exponential, which can be done straightforwardly, but not necessarily cheaply.

Let's say you want an exponential with 90 amps peak and a time constant of 10 msec. Then you can do something like

simulate this circuit

I've switched to an n-type here, as it makes the gate drive a lot easier.

A low-power 90 volt supply charges the capacitor to 90 volts. How long this takes depends on the current capability of the supply and the value of the charge resistor.

When the FET is driven ON, the capacitor will discharge with an approximate response of $$i(t) = \frac{V_{supply}}{R}(1-{e^{-\frac{t}{RC}}})$$ where RC is the time constant in seconds. So if you want a 10 msec time constant, $$\tau = .01 = RC$$ and $$C = \frac{\tau}{R} = \frac{.01}{2} = .005 = 5,000 \text{\mu F}$$ and you'll find that getting hold of 5,000 to 10,000 uF of capacitor at 90 volts which will handle short-circuit discharges gracefully (in the long run) is not as easy as you might think. You can get the capacity and voltage fairly cheap, but be careful of failures after you've done a few pulses.

You'll note that I've not specified the gate drive mechanics, and that's because there are any number of ways it can be done. Start doing some thinking.

Also, keep in mind that, for diodes and transistors, you MUST add in safety factors. Switching large currents fast will, by its nature, produce large voltage spikes in places you won't expect the first time around.

I remember doing a test with a similar power requirements. We had a dedicated battery for that purpose, rated at 120V, 150A. If you don't get any dedicated equipment, consider getting car batteries as jms suggested. I have seen test setups involving car batteries in practice. Note that charging 8 batteries before each test is quite a challenge: if you have to do tests every day, you'll need at least 4 chargers and a dedicated person who will switch the batteries at night. If you need to do your tests non-stop, you'll need 2 complete sets of batteries.

In any case, don't forget a fuse or a circuit breaker: they will pay off if something goes wrong and your pulse gets a bit longer than 300 ms. Look for a fuse with about 15-20A nominal current which allows for 40A overload for a couple of seconds.

You'll probably going to use a MOSFET or an IGBT as a switch to generate your pulses. Don't neglect a flyback diode: any long wires will act as inductance provided sufficient current, especially if they are longer than necessary and someone decides to arrange them in loops to make the test site look cleaner.

Check out ultra capacitors, AKA supercapacitors or electric double-layer caps. They have higher capacity in a smaller package and are better suited to short pulses than batteries. You can use multiple caps in parallel instead of choosing just one.

Also heed the advice of the other answers and comments on things such as fly back diodes and safety of this high power project.