How Can I Obtain the Required Thyristor Triggering Pulse Timing Sequense for Syn

It would appear that each of the six phase circuits drives three modules. I assume that a set of three modules is not wired and driven in parallel, exactly, but that each driver circuit has three independent outputs? Alternately, a wired parallel connection of the power-driving signal is made to the three module-station inputs, but each station has its own isolating transformer and associated thyristor gate-drive circuitry?

Whew, if I were trouble-shooting and testing such a thing, I'd want to start with a reduced voltage and a light-load resistor bank to simulate the motor, or some such, rather than full voltage and power, full load, cross my fingers, look the other way, arm shielding my face, and throw the power switch! But then I'm not an expert, broadly experienced with these things.

(Pictured, a typical SCR module with two SCRs to drive one phase. Sanrex PK55F-120, rated a puny 1.2kV, 55A, 1.6kA surge. On sale at eBay by seller "fictron", $145 for a set of five pieces, new unused, more than 10 available. What, why five, why not three or six?)

Mustafa, we'd be interested to know the part number of your thyristor modules.

> rated a puny 1.2kV, 55A, 1.6kA surge

Sorry, that's 1.6kV rated V_RRM, Repetitive Peak Reverse Voltage.

Thanks for the answer, but i am looking for a driver circuit for those large SCRs.

Best Regards

Mustafa

Most thyristors are relatively easy to trigger, e.g., the PK55F-160 part I illustrated is guaranteed to trigger with 70mA of gate current, so the designer might pick a 200mA drive current, etc. (one wants to keep the average gate power down, say under 1 watt). The really big modules could need up to an amp or two, still quite easy to deal with.

What p/n are you using?

Generally these SCR gates are driven with small transformers, with fairly simple circuitry. The real complexity lies in the controller and the surrounding circuits. Didn't you say that that's working correctly now?

Thanks Win, i know i need small transformer for driving SCR, but the problem is how to select or design such transformer ?

Best Regards

Mustafa

Is that really what you need to know?

Usually one buys them, because they're commodity parts, and because the insulation specs are non-trivial. But to help advise, we need to know the part number of your thyristor modules. The really big ones may require special parts.

Pictured: a Pulse P0584 1:1:1 transformer with 3kV insulation, 0.5μH leakage inductance, and 80mΩ DCR (winding resistance), in stock at Mouser.

Let us take Crydom F1827CCD44 25A SCR Module

How i can select the proper transformer for that SCR.

Mustafa

According to my Crydom series F18 datasheet, that particular part number is two diodes, rather than two SCRs. They do give specs for the SCRs in that series on the datasheet; the maximum-required current to trigger the SCR as 150mA, which is a lot, especially for a 25A part.

You would benefit from learning more about Thyristor Theory and Design Considerations, right? That's the name of a free 240 page handbook by ON Semiconductor, document number HBD855/D (link to a 2.5MB Acrobat file, rev 1, Nov-2006). Sections 5 and 6 apply to SCRs and Triacs. It's an old book, but it's free! They include some newer application notes after section 6, which promote the use of optical couplers to turn on the triacs.

International Rectifier also has a free 486-page SCR Applications Handbook, dating from 1974, but they seem to have removed it from their website. Other websites have copies.

Dozens of manufacturers offer hundreds of relevant application notes, which are generally more up-to-date (sadly, Crydom does not appear to have much offer). On Semi, IRF, ST, NXP, Teccor, EUPEC, ABB, Semikron and Fairchild have good app notes. (Most of the companies have moved on to IGBTs.)

Powerex is a biggie in SCRs and triacs, but unfortunately their huge Rectifier and Thyristor Data Book is not available online as a single-piece document.

Here's more: Come back after you've absorbed some of this and we can talk.

ST's note AN436, Triac control by pulse transformer is a start, with a typical driving circuit (fig 8). It also applies to SCRs. Their AN437, AN439 and AN440 are good, but they're about triacs rather than SCRs.

Thanks Win for your help.

Here is the Data sheet for Crydom F1827CCD400 25A SCR Module

http://datasheet.octopart.com/F1827CCD400-Crydom-datasheet-118697.pdf

Sorry Win ,you are right this module is for two diodes not two SCR,s.

OK,let us go with the SCR module you showed before: Sanrex PK55F-120

How i can select the transformer for this SCR Module ?

The question has been raised several times for more details about designing transformer-drive circuits for thyristors. Here's an answer.

OK, let's take the transformer that I suggested and pictured before, a Pulse P0584, with 1:1:1 windings, 3kV insulation, 0.5μH leakage inductance, and 80mΩ DCR (winding resistance), in stock at Mouser (link, with datasheet avail).

A good understanding of transformers and pulse transformers takes time to obtain, but I'll just present a brief look.

The drawing is a sample application circuit, driving the gate of an SCR or triac. We're using a small MOSFET to make a nominal 5-volt pulse into the transformer's primary, and since it's 1:1 we'll get about 5 volts out, at no load. The voltage drop across the SCR's gate will be 1.5 to 2V, so we have a series resistor Rs to limit the primary current, in this case to say 300mA, to pick a value.

The P0584 is a fairly serious pulse transformer, as you can guess from the photo (small number of large windings), and the DCR spec (80mΩ), and the low leakage inductance. This means it can easily deliver several amps to the destination if we desire. For example in our simplified model, a current of 2A would only cause 0.32V drop across two 80mΩ windings (we'll ignore skin effect and proximity effect that raise the ac resistance).

Let's consider the transformer's electrical model. Of course we have the perfect transformer portion, with turns ratio N, which transform voltage and current by the turns ratio N, and impedance by N², and gives us our output pulse. And we have the series copper winding resistances, with their optimistic low DC values.

But, very important, we also have the L_M inductor portion, which grabs a current to magnetize the core. In a pulse transformer the magnetizing current I_Lm rises proportional to time and primary voltage, dI/dt=V/L. Pulse Engineering (their old name, whatever happened to branding?) doesn't tell us the value of L_M for the P0584, but we can guess it'll be no more than 100x the leakage inductance L_ls, or 50uH. They do give us an ET spec, with units in V-μsec, we'll come back to that later. Perhaps this is a good point to look at some waveforms.

At the top we have the MOSFET's gate drive, and after that we see its switched drain voltage, and then the L_M magnetizing current waveform. When the pulse is finished, we know the L_M inductor has to be discharged, with a flyback, as shown in the V_D waveform, and the I_Lm current drops to zero. The circuit includes a diode and zener for this purpose. This is often referred to as resetting the transformer's core magnetization.

The next waveform shows the transformer's output voltage, V_S, and the SCR gate-drive current I_G, which is set by our resistor R_S. The primary current, I_D is the sum of the output drive and magnetizing currents.

As you can see, the magnetizing current I_Lm is rising with time. As mentioned, Pulse Engineering gives us an ET spec, or more accurately, an E•T spec, which tells us how long a pulse we are allowed, in volts • time units. In this case it's 95 V-μs, or 16 μs for a 5-volt input. (They also give a 2100 Gauss flux-density limit, that will occur when the 95 V-μs limit is exceeded, and the transformer saturates - at that point the winding inductance L_M would collapse and the transformer would look like an 80mΩ short; that's why we have an input res and bypass cap, to limit fault currents).

We'll pick a 3us pulse ON time. This means the sum of the two MOSFET drain / primary current portions will rise to 0.3 + 0.3 = 0.6A peak. The flyback reset current will be 0.3A. If in future we need a higher SCR gate-drive current, that may exceed the magnetizing current, which will make our application look more like a perfect transformer.

Finally, let's turn to the leakage inductance, what's its effect? This affects the pulse transformer's rise and fall time (note the V_S slopes). We can write dI/dt = V/L, and calculate that with 5 volts across the transformer, and L_ls = 50uH, we have a 10A/us current slew rate. This predicts the transformer can support a 30ns risetime for a 0.3A output gate-drive current, if the MOSFET is fast enough.

If we were transformer-shopping for this application, we'd likely pick a part with higher inductances, and lower current ratings (and more loss), but which would allow for longer pulses. This transformer is meant for short fast high-current pulses, such as large MOSFET and IGBT-module gates.

Engineers who design with transformers learn that it's all about making compromises. That, and for one crowd, finding suitable parts - and for another smaller crowd, selecting a core and bobbin and designing your own transformer. I'm a big fan of the latter approach, but that's another much longer story.

Oops, I'm sorry about the V_S waveform, its quiescent-voltage line should go across the middle at ground.

Hello,

mybe I'm in bad thread - but when I need to drive 2kW with MCU do I need firing transformer or it will be ok with some logic friendly triac (BTA...SW)?

Thanks for answer!
Mike

i think you have to have a zero crossing detector for all the phases of the rotor you have(i dont know how much). the excitation will b dc i hope, which is that normally. to control the output voltage you have to control the firing of the individual thyristors with respect to the zero crossing to get a variable voltage just as it is done in dc drives. also you must use thyristor protection fuses (ultra rapid) to protect the thyristors.