Switchable rectifier bridge made of BJT's?

[jmaf]

A friend approached me yesterday with a problem: he wants a full wave rectifier bridge that he can switch on and off.

I thought he could just wire up 4 BJT's collector - emitters as diodes and switch the whole bridge on and off by applying a positive voltage to the 4 bases tied together.

He wants to be able to switch the bridge on and off several thousand times as an experiment he's carrying out, so relays are likely out of the question.

What do you think?

[Wilder Amplification]

I don't think they make a BJT that can handle the reverse voltage on the negative swings of the AC to be honest. Even on low voltage circuits, you hook a BJT up backwards and you let the magic smoke out.

[mooreamps]

Then I would recommend he look at using FET's instead of BJT's.


-g

[Merlinb]

Surely silicon controlled rectifiers (SCRs) are the tool for this job?

[jmaf]

Thanks all who replied.

Merlin, the SCR's work ok with an inductive load? I think he's driving something related to a soldering machine, I believe it's a coil after the rectifier bridge. Do SCR's work ok for that?

[Steve Conner]

There are a lot of fine details that need attended to, to keep the magic smoke in.

First of all I assume it is line voltage, 110 or 220.

If I wanted to do this quickly, I'd use an off-the-shelf solid-state relay connected in the AC feed to an off-the-shelf bridge rectifier. This should be fine with an inductive load on the DC side: the bridge rectifier will allow the load current to freewheel down after the AC is removed. SSRs also work OK switching transformer primaries: the triac turns off at zero current, so there's no stored inductive energy to smoke it.

If I wanted to do it efficiently (the above circuit has about 1 more diode drop than needed) I'd replace some of the diodes in the bridge with SCRs. I've built a couple of controlled rectifiers before, but I'd still budget a week to get the drive issues sorted out, assess reliability, and so on.

For low voltage, high current, a MOSFET switch on the DC side might be more attractive. But this would require a freewheeling diode to handle an inductive load.

[mooreamps]

I'm not so sure this is the best forum to ask this question... Sure I do tend to agree with comments all the others have made here. But we don't know all the details of the "experiment", and I'm just wondering if this type of thread could be handled better on one of those solid state forums...

-g

[jmaf]

I'm not so sure this is the best forum to ask this question... Sure I do tend to agree with comments all the others have made here. But we don't know all the details of the "experiment", and I'm just wondering if this type of thread could be handled better on one of those solid state forums...

-g

I'll try at electro tech and others. Thanks.

[J M Fahey]

Basically yes, of course it will need some thought and maybe a couple extra diodes so currents flow the right way.
MosFets can be used too.
In fact they *are* used that way in some SMPS , they are called switched or syncronic rectifiers or something like that.
For really high speeds and currents they can be more efficient than diodes.
As of comments such as:

I'm not so sure this is the best forum to ask this question...
and I'm just wondering if this type of thread could be handled better on one of those solid state forums...

last time I checked *this one* could very well be
"one of those solid state forums."
I don't see the label "ultra exclusive Tube Theory and Design " anywhere on this page or anywhere else in the whole Site, for that matter.

[jmaf]

Thanks J M Fahey, adding series diodes with the BJT's emitters is a good idea. Thank you.

About mooreamps' comment, I think he meant it as "we're not the most qualified folks" for solid state stuff, not as "please don't post this kind of topic here". I appreciate all the feedback I got from the folks here, invaluable information.

[Steve Conner]

JM: Yes, they are called synchronous rectifiers. Used in SMPS for output voltages below 12V when you want high efficiency: above 12V they don't deliver enough benefit to be worth the extra cost and hassle. The main consumer application is core voltage supplies for PCs, which may be something like 1.2V at 100A.

As you can see, the optimum solution depends on what voltage and current level you'll be running. But for any one-off industrial or experimental type application I'd heavily recommend a solution based on off-the-shelf modules, like a SSR and bridge rectifier.

[J M Fahey]

Check smps synchronous rectifier - Google Search
There you will find a wealth of information.
The technique is so widely used that suppliers offer specific controllers for them, abbundant and for peanuts.
It's not exactly "an experiment".
You will find comments such as:

SYNCHRONOUS RECTIFICATION
.... Synchronous rectification utilizes power
MOSFETs to rectify the output voltage of the power transformer.
These MOSFETs are synchronized to the converter
frequency and perform more efficiently the rectification of
the output voltage than rectifying diodes due to the low I×R
drop through the channel. The N channel power MOSFET
offers the lowest ON resistance and is relatively inexpensive.

If he's trying to control an (arc/TIG/MIG) soldering machine , we are talking 20 to 60V .

Want to keep it in the nostalgic/vintage tube area?
No problem.
In *real* early times , when even a rectifier tube was something expensive , and you wanted to run your radio or amplifier out of batteries, vibrator power supplies were often used.
Some of these used a synchronous rectifier vibrator to save a tube:

If you find this facinating , as I did, check these guys restoring a 1935 Cord/Crosley (obviously tube) car radio.
Antique Radio Forums :: View topic - 6 Volt Positive Ground Synchronous Vibrator ?
OOOOPS !! Looks like we simul-posted.
I was doing a little research.
My "experiment" quote refers to post #7, not to #11 which obviously I had not read.

[Steve Conner]

Well, if the guy is trying to make a square wave inverter for an AC TIG welder, that is a completely different story. The industry standard way is an H-bridge of four dirty big IGBTs on the output of a DC welding power supply. 600V IGBTs are used to stand the huge amount of noise and spikes from the arc. This is why AC TIG machines are so much more expensive than the little DC ones.

A bidirectional synchronous rectifier would be more efficient, because it can replace the DC welding supply's own rectifier, but I don't know if it is done. For bidirectional blocking, each element needs to be two devices back to back in series, and from a voltage drop point of view the efficiency gain isn't much compared to rectifying with Schottky diodes and inverting again with IGBTs. (Which also works out as two devices in series.) And from a control and drive point of view it is a nightmare.

The bidirectional synchronous rectifier is pretty much the same as the cycloconverter, which is widely studied and also has a reputation for being a nightmare to control.

[jmaf]

Well, if the guy is trying to make a square wave inverter for an AC TIG welder, that is a completely different story. The industry standard way is an H-bridge of four dirty big IGBTs on the output of a DC welding power supply. 600V IGBTs are used to stand the huge amount of noise and spikes from the arc. This is why AC TIG machines are so much more expensive than the little DC ones.

A bidirectional synchronous rectifier would be more efficient, because it can replace the DC welding supply's own rectifier, but I don't know if it is done. For bidirectional blocking, each element needs to be two devices back to back in series, and from a voltage drop point of view the efficiency gain isn't much compared to rectifying with Schottky diodes and inverting again with IGBTs. (Which also works out as two devices in series.) And from a control and drive point of view it is a nightmare.

The bidirectional synchronous rectifier is pretty much the same as the cycloconverter, which is widely studied and also has a reputation for being a nightmare to control.

Thank you, Steve. Priceless information, appreciate it.

If you find this facinating , as I did, check these guys restoring a 1935 Cord/Crosley (obviously tube) car radio.

I'd never heard of that kind of rectifier, that is indeed fascinating. Thanks for sharing.

[mooreamps]

Thanks J M Fahey, adding series diodes with the BJT's emitters is a good idea. Thank you.

About mooreamps' comment, I think he meant it as "we're not the most qualified folks" for solid state stuff, not as "please don't post this kind of topic here". I appreciate all the feedback I got from the folks here, invaluable information.

That's not what I meant...