Capacitor discharge question

[Mook Johnson]

I think that you're asking for trouble using tantalum caps in pulsed
power circuits. I'm not sure what options there are at 200C. Placing
the capacitive energy storage in the hostile environment, if there are
alternatives, could be asking for trouble.

You don't mention bleeder equalization parts, or other frequency
compensating components that I would expect to see in a
series-parallel connected module of this sort. This argues against
predictable pulse performance.

RL

The short circuiting is a infrequent but possible fault condition. Im not
concerned about the capacitor being damaged. I'm more concerned if this
capacitor discharged into .1ohms what peak energy rating on the resistor is
required to withstand this without blowing open. If that .1ohm resistor
opens up, there will be hell to pay. I have as big of a resistor I can
comfortably fit there now, I just need to know if it is up to the task
without actually doing an arch test. I will do one eventially but not right
away.

 

[Winfield]

On Jan 12, 2:29 pm, "Mook Johnson" <m...@mook.net> wrote:

I think that you're asking for trouble using tantalum caps in pulsed
power circuits. I'm not sure what options there are at 200C. Placing
the capacitive energy storage in the hostile environment, if there are
alternatives, could be asking for trouble.

You don't mention bleeder equalization parts, or other frequency
compensating components that I would expect to see in a
series-parallel connected module of this sort. This argues against
predictable pulse performance.

RL

The short circuiting is a infrequent but possible fault condition. Im not
concerned about the capacitor being damaged. I'm more concerned if this
capacitor discharged into .1ohms what peak energy rating on the resistor is
required to withstand this without blowing open. If that .1ohm resistor
opens up, there will be hell to pay. I have as big of a resistor I can
comfortably fit there now, I just need to know if it is up to the task
without actually doing an arch test. I will do one eventially but not
right away.

Your 60uF 600V capacitor bank stores 11J of energy.
Most small power resistors I've looked at have a
transient-power maximum spec of 5x their rated power
for 5 seconds. That means, for example, a 1-watt
power resistor would be able to safely absorb 25J
into its thermal mass over 5 seconds. Presumably
this energy could be absorbed into the thermal mass
much faster than 5 seconds. A de-rating can be used,
to take into account that very rapid events, say
faster than 10 to 50us, might be absorbed entirely
into the resistance wire, which has less thermal
mass than the entire resistor with its leads.

If your capacitor bank has the 3 ohms = maximum esr,
then a 3.1-ohm discharge will have a 186us time
constant. But if in fact it's much lower, say 0.5
ohms, then the 0.6-ohm discharge time constant will
be 36us. And more of the bank's energy will go into
the 0.1-ohm resistor. However, it still looks good,
even with a small 1-watt power resistor.

 

[Chuck]

On Fri, 11 Jan 2008 19:08:40 -0800 (PST), Winfield
<winfieldhill@yahoo.com> wrote:

Chuck wrote:
Winfield Hill wrote:
Winfield Hill wrote:
and we can estimate that ordinary esr loss exceeds
the dielectric loss at about 1/100 of that frequency,
or 90Hz (the factor of 100 is for D = 0.01, etc.).
^^^^
9 Hz

Thanks for the additional information, Winfield.

My point was only that because ESR is frequency-dependent and the
frequency at which 3 ohms was measured is unstated, 3 ohms may not
be valid for the analysis.

No, that's not exactly right.

It is still not clear to me how we know the ESR is not less than
3 for the OP's time constant. None of the posts seemed to address
this. If I understand your analysis, you have taken 3 ohms as the
actual ESR at the frequency of interest, just as the 0.1 ohm
resistor and the 90 uF capacitor values were taken as actual.

I'm open to "recalibration".

OK, I'll explain. This is what we see if we measure and
analyze many electrolytic caps. ** First, if you examine
datasheets, you'll see esr is usually specified at 100kHz.
In fact, there's a broad region where the ESR changes very
little, e.g., from 0.48 to 0.40 ohms from 1kHz to 200kHz,
for a 68uF 350V electrolytic I measured this afternoon.
From 5kHz and up, the nearly-constant ESR is well below
1/Xc, and this shows a that single value at 100kHz is a
genuinely-useful parameter. (I apologize for not posting
a graph to show this better - we'll do that in AoE 3rd-ed.)

I can tell you, a 1-to-200kHz relatively-flat esr frequency
range is what we generally what we see when measuring small
electrolytics. We have to take the OP's 3 ohms for his part.

** Second, as we go down in frequency, where does Xc take
over from esr? f = 1 / 2pi C Resr = 1 / 2pi 63uF 0.44-ohms
= 5.7kHz for the "68uF" 350V capacitor I measured. Now, to
get into the dielectric series-resistance loss region, shown
in the QuadTech document you referenced, we have to go down
another factor of 50 to 100 in frequency from there, e.g.,
to below 60Hz. In fact, my "68uF" cap has a loss resistance
of 1/69 Xc at 60Hz, and I have to go all the way down to 5Hz
to reach the 1/100 dielectric loss that we expect to see for
an electrolytic. So, clearly there are dramatically-different
regions for electrolytic capacitors, and we can generalize
about them, and most of the time the dielectric losses are
really not much of an issue, being at very low frequencies.

** For example, consider my 68uF cap at 120Hz, the operating
frequency for a bridge-rectifier storage cap. Here the esr
measures about 0.7 ohms, not a whole lot higher than its 0.45
ohms in the 5 to 20kHz region. But consider, in a rectifier
storage capacitor situation, with a short charging-conduction
time, say 1/5 of a cycle's peak, we're really talking about
5*120 = 600Hz. Here I measured an esr of about 0.5 ohms, or
nearly as low as a datasheet-frequency 100kHz esr = 0.4-ohms.

In conclusion, we can safely rely on a single reported value
for capacitor esr, and not worry about whatever dielectric
losses might be at frequencies far below f = 1 / 2pi C Resr.

Many thanks, Winfield, for the badly needed calibration! -

Now maybe some kind soul will offer to reconcile that analysis with
charts like these:

http://www.google.com/search?q=equivalent+series+resistance+electrolytic+frequency&hl=en&safe=off&start=10&sa=N


Chuck

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[Chuck]

Oops!

The charts are here:

http://download.siliconexpert.com/pdfs/Caps/EPCO/00730077.pdf

Sorry about that.

Chuck

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[Winfield]

On Jan 12, 5:37 pm, Chuck wrote:

Oops!

The charts are here:
http://download.siliconexpert.com/pdfs/Caps/EPCO/00730077.pdf

Sorry about that.

esr vs temperature is all I see...
Can you repost the link?

 

[Phil Allison]

"Mook Johnson"

Here are some specifics on the capacitor. The 60uF capacitor is actually
a module consisting of 60 indvidual capacitors. These are the capacitors
used in the module
http://www.vishay.com/docs/40072/134d.pdf The one we used is the 100uF
125VDC @25C in the T case.

The capacitor module is 6 parallel strings of 10 series connected
capacitors to make a module that is 1250V @ 25c and 750C @ 200C capacitor
bank that is 60uF total.

This application will see 200C temperature so that severely limits the
type of capacitor used to this large monstrosity.

** The ESR figure in the table ( 1.8 ohms) is clearly stated to be a
maximum value and at 120Hz.

You did not read this ??

The actual ESR figure will be lower and will fall at higher frequencies than
120 Hz - by a factor of several times.

With an ESR of say 1 ohm total for the bank, the peak surge current into
0.1 ohms will be in the order of 600 amps or 100 amps per series string.

The tantalum caps I know about will not survive that abuse, even once.

Ordinary electros are well able to survive it.



........ Phil

 

[Tom Bruhns]

On Jan 12, 3:16 pm, Winfield <winfieldh...@yahoo.com> wrote:

On Jan 12, 5:37 pm, Chuck wrote:

Oops!

The charts are here:
http://download.siliconexpert.com/pdfs/Caps/EPCO/00730077.pdf

Sorry about that.

esr vs temperature is all I see...
Can you repost the link?

?? The first page of the linked PDF (pg 73) is vs temperature; the
following several pages are all ESR and Z vs frequency. Perhaps the
file got munged when you downloaded it??

Cheers,
Tom

 

[Winfield]

Mook Johnson wrote:

I think that you're asking for trouble using tantalum caps in pulsed
power circuits. I'm not sure what options there are at 200C. Placing
the capacitive energy storage in the hostile environment, if there are
alternatives, could be asking for trouble.

You don't mention bleeder equalization parts, or other frequency
compensating components that I would expect to see in a
series-parallel connected module of this sort. This argues against
predictable pulse performance.

RL

The short circuiting is a infrequent but possible fault condition.
I'm not concerned about the capacitor being damaged. I'm more
concerned if this capacitor discharged into 0.1 ohms what peak
energy rating on the resistor is required to withstand this without
blowing open. If that .1ohm resistor opens up, there will be hell
to pay.

Phil says, "The tantalum caps I know about will not survive
that abuse, even once," referring to the higher current you
are likely to see with the lower actual esr for real parts,
and the lower esr at the higher actual frequency of a short-
discharge event. Can you tell us more about your problem,
and the event that's got you worried. E.g., how is it that
you can place 0.1 ohms in series with the cap(s)?

 

[Mook Johnson]

Phil says, "The tantalum caps I know about will not survive
that abuse, even once," referring to the higher current you
are likely to see with the lower actual esr for real parts,
and the lower esr at the higher actual frequency of a short-
discharge event. Can you tell us more about your problem,
and the event that's got you worried. E.g., how is it that
you can place 0.1 ohms in series with the cap(s)?

Its a motor drive application where the phase wiresmay be shorted phase ot
phase or phase to shield (ground).

I know this puts a IGBT or two in series but I'd like to be able to backup
my claim that even a direct discharge into the .1ohm resistor would not be
sufficient to blow it open. Another design (not mine) had this happen and
everythign connected across that resistor (diff amp, +12V rail, etc) got
exposed to 500V with the obvious results (BIG cloud of smooke and red faced
engineer).

The PCB will also have very stout traces throught the entire current path
between the 0.1 ohm resistor and ground.

 

[Phil Allison]

"Mook Johnson"

Its a motor drive application where the phase wiresmay be shorted phase ot
phase or phase to shield (ground).

I know this puts a IGBT or two in series but I'd like to be able to backup
my claim that even a direct discharge into the .1ohm resistor would not be
sufficient to blow it open.

** GOD YOU ARE STUPID !!!!!!!!!!

Far as that 60 uF tantalum cap bank is concerned, a 0.1 ohm resistor IS and
damn short.

So, eliminate you worries about the welfare of the resistor by eliminating
the fucking resister !!!!!!!!!


Dickhead.




........ Phil

 

[Tom Bruhns]

Winfield wrote:

Mook Johnson wrote:
I think that you're asking for trouble using tantalum caps in pulsed
power circuits. I'm not sure what options there are at 200C. Placing
the capacitive energy storage in the hostile environment, if there are
alternatives, could be asking for trouble.

You don't mention bleeder equalization parts, or other frequency
compensating components that I would expect to see in a
series-parallel connected module of this sort. This argues against
predictable pulse performance.

RL

The short circuiting is a infrequent but possible fault condition.
I'm not concerned about the capacitor being damaged. I'm more
concerned if this capacitor discharged into 0.1 ohms what peak
energy rating on the resistor is required to withstand this without
blowing open. If that .1ohm resistor opens up, there will be hell
to pay.

Phil says, "The tantalum caps I know about will not survive
that abuse, even once," referring to the higher current you
are likely to see with the lower actual esr for real parts,
and the lower esr at the higher actual frequency of a short-
discharge event. Can you tell us more about your problem,
and the event that's got you worried. E.g., how is it that
you can place 0.1 ohms in series with the cap(s)?

?? I have some wet-slug tantalums in my collection of parts. I just
connected an SCR directly across one, a 68uF 60V part. I connected a
10k R from anode to a 58V power source, return to cathode. An HP3312
function generator set to generate pulses feeds the SCR gate through a
3.3nF cap (to keep gate drive short), with a 56 ohm termination gate
to cathode. Terminal voltage on the cap decays with about 0.8usec
time constant; the SCR current is low enough that it turns off within
about 1.5msec after the trigger. It's been pulsing away at about 2
seconds per pulse for several minutes now, and no change in
performance that would indicate the cap "not surviving." I haven't
measured the ESR of the cap I'm testing; suppose I could do that
without too much trouble, but not sure I can be bothered.

What I know of solid tants with normal ESR tells me that they survive
this sort of thing just fine if you don't do it too often, too many
times. YMMV; I suppose it depends on the quality of part you use.
OTOH, it's not a good idea to do it with low ESR solid tants.

Cheers,
Tom

 

[Mook Johnson]

"Tom Bruhns" <k7itm@msn.com> wrote in message
news:b98af056-e11a-4a28-b574-0512265c7548@p69g2000hsa.googlegroups.com...

?? I have some wet-slug tantalums in my collection of parts. I just
connected an SCR directly across one, a 68uF 60V part. I connected a
10k R from anode to a 58V power source, return to cathode. An HP3312
function generator set to generate pulses feeds the SCR gate through a
3.3nF cap (to keep gate drive short), with a 56 ohm termination gate
to cathode. Terminal voltage on the cap decays with about 0.8usec
time constant; the SCR current is low enough that it turns off within
about 1.5msec after the trigger. It's been pulsing away at about 2
seconds per pulse for several minutes now, and no change in
performance that would indicate the cap "not surviving." I haven't
measured the ESR of the cap I'm testing; suppose I could do that
without too much trouble, but not sure I can be bothered.

What I know of solid tants with normal ESR tells me that they survive
this sort of thing just fine if you don't do it too often, too many
times. YMMV; I suppose it depends on the quality of part you use.
OTOH, it's not a good idea to do it with low ESR solid tants.

Cheers,
Tom

Thanks for the testing, I'll be doing some spark testing in the near future
but the guys at Vishay/Tansitor are not afraid of shorting their capacitors
every blue moon.

Should be some interedting times in the lab.

Just out of curiosity, why use an SCR for this test? wouldn't an IGBT or
MOSFET be easier? I guess its all the same but I haven't used SCRs in a
design yet so I'm not comfortable with them. Any advantages to SCRs in this
application?

 

[Phil Allison]

"Mook Johnson"

Just out of curiosity, why use an SCR for this test? wouldn't an IGBT or
MOSFET be easier? I guess its all the same but I haven't used SCRs in a
design yet so I'm not comfortable with them. Any advantages to SCRs in
this application?

** SCRs require only a brief ( few uS ) pulse to be triggered into a low
resistance state.

When the voltage across the SCR falls to circa 1.5 volts - it turns off
by itself.

So, they are widely used in strobe lights and camera flashes.



......... Phil

 

[Rich Grise]

On Sun, 13 Jan 2008 17:29:23 -0600, Mook Johnson wrote:

Just out of curiosity, why use an SCR for this test? wouldn't an IGBT
or MOSFET be easier? I guess its all the same but I haven't used SCRs
in a design yet so I'm not comfortable with them. Any advantages to
SCRs in this application?

I've used SCRs before and never had a problem - they're really simple
to fire - just put the trigger current through the gate. The "advantage"
is you don't have to continue to drive the gate - it latches on, like
a crowbar circuit, and stays on until the forward current drops to
zero. I don't know if that's an advantage in your app, but I'm
guessing it's some kind of crowbar so a fault doesn't blow up the
whole unit; in that case an SCR would be just the ticket.

Cheers!
Rich

 

[Tom Bruhns]

On Jan 13, 3:29 pm, "Mook Johnson" <m...@mook.net> wrote:

"Tom Bruhns" <k7...@msn.com> wrote in message

news:b98af056-e11a-4a28-b574-0512265c7548@p69g2000hsa.googlegroups.com...





?? I have some wet-slug tantalums in my collection of parts. I just
connected an SCR directly across one, a 68uF 60V part. I connected a
10k R from anode to a 58V power source, return to cathode. An HP3312
function generator set to generate pulses feeds the SCR gate through a
3.3nF cap (to keep gate drive short), with a 56 ohm termination gate
to cathode. Terminal voltage on the cap decays with about 0.8usec
time constant; the SCR current is low enough that it turns off within
about 1.5msec after the trigger. It's been pulsing away at about 2
seconds per pulse for several minutes now, and no change in
performance that would indicate the cap "not surviving." I haven't
measured the ESR of the cap I'm testing; suppose I could do that
without too much trouble, but not sure I can be bothered.

What I know of solid tants with normal ESR tells me that they survive
this sort of thing just fine if you don't do it too often, too many
times. YMMV; I suppose it depends on the quality of part you use.
OTOH, it's not a good idea to do it with low ESR solid tants.

Cheers,
Tom

Thanks for the testing, I'll be doing some spark testing in the near future
but the guys at Vishay/Tansitor are not afraid of shorting their capacitors
every blue moon.

Should be some interedting times in the lab.

Just out of curiosity, why use an SCR for this test? wouldn't an IGBT or
MOSFET be easier? I guess its all the same but I haven't used SCRs in a
design yet so I'm not comfortable with them. Any advantages to SCRs in this
application?

An SCR is easy to trigger, and once triggered it turns on and stays on
till the current falls below the part's holding current. I used one
that I have previous experience with, and know it ramps from very low
current to essentially full conduction in under 100 nanoseconds.
Though a power MOSFET could be a bit faster, this should be adequately
fast for the 'speriment at hand. I'm probably exceeding the rated max
pulse current of this particular part in this test, but my experience
has been that it can take it OK, at least for short pulses. It was
easier for me to set up the test with an SCR than with a power mosfet
(though I certainly could have used an IRFP4229 I have handy; rated at
180A max pulse current). With the mosfet, I'd have had to arrange a
low-impedance gate drive to get high speed. With the SCR, I could use
a 50 ohm output impedance generator and a simple coupling cap (3.3nF)
to get short turn-on pulses.

Cheers,
Tom

 

[MooseFET]

On Jan 14, 10:23 am, Rich Grise <r...@example.net> wrote:

On Sun, 13 Jan 2008 17:29:23 -0600, Mook Johnson wrote:
Just out of curiosity, why use an SCR for this test? wouldn't an IGBT
or MOSFET be easier? I guess its all the same but I haven't used SCRs
in a design yet so I'm not comfortable with them. Any advantages to
SCRs in this application?

I've used SCRs before and never had a problem - they're really simple
to fire - just put the trigger current through the gate.

Check the datasheet for how much trigger current to use. Too much is
obviously bad. Too little is bad too if the anode current rises
quickly. What happens when you use a low trigger current is that you
only trigger part of the device and then the conduction spreads out
until the whole device is on. If the anode current rises quickly, a
small portion of the whole device tries to take the whole current.

The "advantage"
is you don't have to continue to drive the gate - it latches on, like
a crowbar circuit, and stays on until the forward current drops to
zero.

There is a small "holding current" number that the current must fall
below before the device will turn off. The current doesn't have to
really stop. The turning off process is slow. If the anode current
only briefly stops (lets say 1uS) the device may not turn off.

When fired an SCR looks very much like a normal Si diode that is
conducting. The forward drop follows the same sort of rule as a
rectifier of about the same size would.


I don't know if that's an advantage in your app, but I'm

guessing it's some kind of crowbar so a fault doesn't blow up the
whole unit; in that case an SCR would be just the ticket.

Cheers!
Rich

 

[Vladimir Vassilevsky]

MooseFET wrote:

Check the datasheet for how much trigger current to use. Too much is
obviously bad. Too little is bad too if the anode current rises
quickly. What happens when you use a low trigger current is that you
only trigger part of the device and then the conduction spreads out
until the whole device is on. If the anode current rises quickly, a
small portion of the whole device tries to take the whole current.

They usually recommend a trigger of the initial pulse of high current
followed by a pedestal of lower current. It is even better to trigger
the SCRs by a burst of short pulses of high current to the gate. This
allows for the quickest transition of the whole structure into the
conductive state without risk of damaging the gate. The PWMs on some of
the motor control MCUs are supporting for this mode of operation.

There is a small "holding current" number that the current must fall
below before the device will turn off. The current doesn't have to
really stop. The turning off process is slow. If the anode current
only briefly stops (lets say 1uS) the device may not turn off.

I'd say 1uS would be a good time for the ordinary SCR to turn on. The
relaxation time is at the order of hundreds of microseconds.

When fired an SCR looks very much like a normal Si diode that is
conducting. The forward drop follows the same sort of rule as a
rectifier of about the same size would.

There are two forward biased junctions hence the drop is twice as big as
that of a diode. Usually around 1.5..2 Volts.



Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

 

[MooseFET]

On Jan 14, 4:47 pm, Vladimir Vassilevsky <antispam_bo...@hotmail.com>
wrote:

MooseFET wrote:
[....]
There is a small "holding current" number that the current must fall
below before the device will turn off. The current doesn't have to
really stop. The turning off process is slow. If the anode current
only briefly stops (lets say 1uS) the device may not turn off.

I'd say 1uS would be a good time for the ordinary SCR to turn on. The
relaxation time is at the order of hundreds of microseconds.

That depends a lot on the type of SCR and how it gets down to the
holding current. If the decrease is rapid, there is a huge stored
charge and it will be a longer time than if the decrease is slow.

When fired an SCR looks very much like a normal Si diode that is
conducting. The forward drop follows the same sort of rule as a
rectifier of about the same size would.

There are two forward biased junctions hence the drop is twice as big as
that of a diode. Usually around 1.5..2 Volts.

This isn't true. Observe Figure 9 in
www.st.com/stonline/products/literature/ds/10670.pdf

and figure E6.19 in
http://www.teccor.com/data/en/Data_Sheets/E6SCR.pdf

Compare with Figure 1 in:
http://www.fairchildsemi.com/ds/FF/FFPF20UP60DN.pdf

The simple model of the SCR with two transistors is not accurate but
lets consider it:


-------------+------- Anode
!
e\!
PNP !-----
/! !
! !
! !
! !/
Gate----+-----! NPN
!\e
!
-------------------+--- Cathode

Notice that the EB junction drop is in series with the collector of
the other transistor in each case. There are two diodes in there but
they are more like in parallel than series. This makes the on voltage
far less than two diode drops and a lot more like the forward drop of
just one diode.

In actual fact the SCR when it is on is not really like the two
transistors any more. Internally there are 4 layers like this:


PPPPPPPPPPP < heavy doping
PPPPPPPPPPP
nnnnnnnnnnn < light doping
nnnnnnnnnnn
ppppppppppp < light doping
ppppppppppp
NNNNNNNNNNN < heavy doping
NNNNNNNNNNN

Unlike a real transistor each on in the model has a lightly doped
collector. When the SCR in in heavy conduction, there are many more
carriers in the lightly doped parts than would normally be the case.

 

[Vladimir Vassilevsky]

"MooseFET" <kensmith@rahul.net> wrote in message
news:65b93c64-7128-4981-8580-a97bef08ae54@s8g2000prg.googlegroups.com...

When fired an SCR looks very much like a normal Si diode that is
conducting. The forward drop follows the same sort of rule as a
rectifier of about the same size would.

There are two forward biased junctions hence the drop is twice as big as
that of a diode. Usually around 1.5..2 Volts.

This isn't true.

You are right.

Observe Figure 9 in
www.st.com/stonline/products/literature/ds/10670.pdf

and figure E6.19 in
http://www.teccor.com/data/en/Data_Sheets/E6SCR.pdf

Compare with Figure 1 in:
http://www.fairchildsemi.com/ds/FF/FFPF20UP60DN.pdf

I am aware of the voltage drop of thyristors; what I did not expect is that
there are some diodes with the voltage drop that high. Thank you for the
references. I checked some other similar diodes and they are rated at Vf =
1...1.2V at max. current.
Would it be the correct assumption that the diode drop is considerably less
than that of thyristor?

The simple model of the SCR with two transistors is not accurate but
lets consider it:
Notice that the EB junction drop is in series with the collector of
the other transistor in each case. There are two diodes in there but
they are more like in parallel than series. This makes the on voltage
far less than two diode drops and a lot more like the forward drop of
just one diode.

V = Vbe + Vce. Not like one diode, but neither like two diodes.

In actual fact the SCR when it is on is not really like the two
transistors any more. Internally there are 4 layers like this:

[...]

I see. Thank you for the detailed explanation.

VLV

 

[MooseFET]

On Jan 15, 5:13 am, "Vladimir Vassilevsky"
<antispam_bo...@hotmail.com> wrote:

"MooseFET" <kensm...@rahul.net> wrote in message

news:65b93c64-7128-4981-8580-a97bef08ae54@s8g2000prg.googlegroups.com...

When fired an SCR looks very much like a normal Si diode that is
conducting. The forward drop follows the same sort of rule as a
rectifier of about the same size would.

There are two forward biased junctions hence the drop is twice as big as
that of a diode. Usually around 1.5..2 Volts.

This isn't true.

You are right.

Observe Figure 9 in
www.st.com/stonline/products/literature/ds/10670.pdf

and figure E6.19 in
http://www.teccor.com/data/en/Data_Sheets/E6SCR.pdf

Compare with Figure 1 in:
http://www.fairchildsemi.com/ds/FF/FFPF20UP60DN.pdf

I am aware of the voltage drop of thyristors; what I did not expect is that
there are some diodes with the voltage drop that high. Thank you for the
references. I checked some other similar diodes and they are rated at Vf =
1...1.2V at max. current.
Would it be the correct assumption that the diode drop is considerably less
than that of thyristor?

The package size and ability to get rid of heat are what sets the
maximum power of semiconductor parts. As a result, the rated maximum
current of a schottky will tend to be higher than for a normal
rectifier but the voltage at the rated curent not as much lower as you
would expect.

In a given package size, the power loss in a SCR will be about the
same as the rectifier's. This makes the curves end up being scaled to
me more alike.