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Old 20th August 2009, 08:34 AM   #1
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Default Thyristor bridge rectifier

Dear Forum participants,

It has been quite a while now that I've been playing with the thoughts of making a new power supply,for
my ancient Quad405 amplifier.
I was thinking of replacing the bridge rectifier by a thyristor version.Ofcoarse one would require a controller,
to fire the thyristors,but that's not the issue I wanted to discuss here.
First of all,let me point out the benefits of a thing like this.

1) soft start : Firing the thyristors every period of the 50Hz a little earlier, the DC voltage will
ramp-up gently.This prevents the Thummmm at power up and avoids unnesesary stress for transformer and capacitors.
2) Auto on/off : since there is a microcontroller in control of it all,it will be easy to make a signal detector
and shut down the amp when there is no signal for a while,and power it up again when signal arrives.
3) emergency shut down : in case of a fault condition (DC on the speaker) the whole bridge can be shut down,limiting
the amount of destructive energy to the contents of the electrolytic capacitors.
4) a thyristor is hardly more expensive than a diode.

So that's for the benefits I personally see,but what about the downsides? what about EMI? I thougt of that too,
and it can indeed be a problem when cutting the sine, but that's only during the first seconds at startup.
In the "ON" phase i will have the thyristors fed with a continuous gate current,effectively turning them into
plain rectifier diodes.

The fact is, I see a lot buzzing around this forum about rectifier bridges,building them with stealth or
soft recovery diodes,but nothing on thyristors.Has anyone done this before?

I made a simulation of the results see attach.
Attached Files
File Type: pdf SOFTSTART.pdf (110.4 KB, 289 views)
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Old 21st August 2009, 09:23 PM   #2
TOINO is offline TOINO  Portugal
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http://bryston.com/BrystonSite05/pdf...-1C(Oct02).pdf

See 2™ page...
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Old 23rd August 2009, 08:54 AM   #3
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Well Toino, this is not what I had in mind,but this design uses a plain diode bridge for rectification and a triac for softstart,besides this it requires a small mains transformer (TX1) to power the softart circuit.

I was hoping to find anyone who had some experience with thyristor bridges for power amplifiers,Or else if it behaves badly I'd like to know why.
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Old 23rd August 2009, 09:32 PM   #4
TOINO is offline TOINO  Portugal
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Cowboy, it is possible to do the same in a bridge with thyristors. You must use a similar phase control scheme, and itís the reason I have posted the link.
I use that kind of bridge in a big Pa. amplifier and it works without any problems.
Before that I have tried all the conventional ways with lots of problems: exploded NTC or PTC, burned contact relays, etc.

Anyway donít forget that you have the power capacitors charged and turn-off or shut-down is never instantaneous.

See page 68/69 of this one: http://www.meyersound.com/pdf/produc...y/ms-1000a.pdf

Page 68 the thyristor here is only on-off switch but is possible to simplify, modifying the bridge with thyristors; then you have full phase controlÖ if you need or want.

On last page you have a circuit to force capacitor discharge.

More on soft-start here: Soft-start circuit with no relays and no aux. transformers
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Old 25th August 2009, 08:50 AM   #5
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Looking at the Meyersound schematics,they use a diode bridge AND thyristors.
So either they didn't bother OR they have a good reason to use a plain diode bridge instead of a thyristor bridge.Heat dissipation is worse, and the thyristors carry the double amount of current.
I think I'll just have to build it and see how it performs.At least in simulation it performs very well.
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Old 25th August 2009, 05:53 PM   #6
star882 is offline star882  United States
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A NTC inrush limiter would be much simpler. Parallel it with a relay if you're concerned about efficiency.
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Old 25th August 2009, 10:56 PM   #7
TechGuy is offline TechGuy  United States
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A simple approach is to place a saturatable induction between your output rectifier and your output diodes. During initial start up the induction will have a high impediance that reduces the sudden inrush current. Once its saturates the impediance will drop to near zero. it will also help reduce output noise.

Another option would be to use buck regular with a soft start option that is between your rectifier and your output caps:
http://cds.linear.com/docs/Datasheet/3724fc.pdf

The Buck regulator would also add voltage regulation to your output. I think the Buck Regulation option would be much simpler to implement than something using Thyristors.

Best of Luck to you!
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Old 26th August 2009, 06:30 PM   #8
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Yes star882, i am familiar with the NTC,but it lacks the feature of being able to turn the amp off.

and TechGuy neither of your suggestions is viable in my opinion.When starting up the current will be high,so the inductor saturates immediately. So the effect id the opposite of the desired.And the buck... will require an electrolyte at its input: will we fill it up with a regular bridge without softstart ?

Everybody gives me alternatives, but there is really no argument against the thyristor bridge.Complex ?At first I thought so too,but i was able to simplify a lot.It needs only 4 small transistors and 2 dual diodes for a single bridge and 6 transistors for a dual version.
I do not count the thyristors themselves for they are substituting a diode anyway.

In attachment is the schematic I used for simulation.Please note that the firing circuit is the same for either branche except for the polarity.
Attached Files
File Type: pdf softbridge_3.pdf (45.5 KB, 109 views)
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Old 26th August 2009, 10:53 PM   #9
TechGuy is offline TechGuy  United States
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Quote:
Originally Posted by cowboy_film_fan View Post
and TechGuy neither of your suggestions is viable in my opinion.When starting up the current will be high,so the inductor saturates immediately. So the effect id the opposite of the desired.And the buck... will require an electrolyte at its input: will we fill it up with a regular bridge without softstart ?
You've got it backwards. The inductor resists change in current, until it saturates, during which time, the inductor acts like a choke to prevent excessive current flow. The only issue would be if you have a huge cap that swamps the inductor so that after it saturates the cap still isn't charged enough to prevent excessive input current A inductor with a higher inductance can offset a larger cap. The only issue is that a really large cap bank would require a very large and expensive inductor.

For the buck regulator, you would need a relativily small capacitor to prevent excessive ripple on the output. I suppose during the initial charge up of your filter caps their would be an elevated ripple on the output if you use too small of a soft-start RC value. Once the filter cap is charged your using the caps to power the equipment and the buck is just keeping the caps topped off.

Another option is to use a one-shot PWM that starts off with a low duty cycle and finished off with a 100% duty cycle and remains at 100% until the power supply is switched off. You would need a PWM controller, and inductor, a MOSFET and a high-side mosfet driver unless you can find a PWM with an integrated mosfet controller. You would use a RC circuit to program the PWM duty cycle ramp up, an easier option might be to use small PIC microcontroller to avoid designing a RC circuit to program the PWM duty cycle.

The problem I see with a thyristor bridge is that it will difficult to turn them on after the peak voltage is reached. If you look at a typical thryristor bridge. You can design a simple circuit to turn on after the AC zero voltage crossing reaches a selected voltage and before peak voltage is reached (see gif image below). But you want to turn on the thyristor well after the peak voltage so that the input voltage remains low during startup. If you turn on at the peak or before the peak, your excerbated your problem because the input voltage at initial start up will be higher causing even higher current flow. You need a circuit that turns on with a low voltage, and turns off when the voltage rises to point it causes excessive current flow. Unfortunately you can't turn off thyristors, you need to wait until the voltage drops to zero before they can be turned off. Hopefully my writing is clear enough for you to grasp my point.

http://upload.wikimedia.org/wikipedi..._rectifier.gif
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Old 27th August 2009, 04:47 PM   #10
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My take is that each thyristor needs a floating power supply. Too, it might be advantageous to regulate the primary side of a transformer, especially in the case of a toroid.

I have decided to use MOSFETs instead. They can be switched off as soon as a current threshold is reached. I have found it simpler to set up a control circuit and feedback loop under that circumstance where there is no turn-off delay as with thyristors. MOSFETs also need only very low current gate control supplies.

The best way I have found is to use 4 MOSFETs total. I have set up a circuit that uses two pairs of two MOSFETs that are connected source to source to permit dual polarity operation of each pair.

One pair of MOSFETs operates at a time. Usually, the pair that connects the transformer primary winding to the mains hot wire is operating by itself unless current limiting is operating. When the main pair of MOSFETs is off, so as to cut off the AC input, the other pair turns on to short the transformer primary. It serves to reduce noise and spikes.

Just another possibility. I see how thyristors can be a good option as well. My project, as it presently is, uses them.
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