Paralleling Three Full Wave Rectifier Bridges

[Anand P. Paralkar]

Hi everyone,

I am trying to build a relatively high voltage-high current DC source.
The scheme is simple and uses no regulation (therefore no feedback
control). The scheme is as follows:

Utility mains supply => Variac => Three full wave rectifier bridges in
parallel => Huge capacitor bank => Load.

Variac: something similar to this one:
http://orgchem.colorado.edu/Technique/Equipment/Communityequip/Variac.html

Bridge: KBPC3510

Capacitor bank: 6800uF, 400V

I could not find a full wave bridge rectifier with a sufficiently high
current rating and therefore, I thought of paralleling three that were
readily available.

I start (slowly) increasing the output AC voltage of variac so as to
increase the DC supply to the load. However, the variac fuse blows-up
at around 10V AC output!

Paralleling three bridges may not be the most elegant way to build a
high-current DC source, but I do not understand what could cause the
fuse in the variac to blow-up. (Please note, everything works fine with
a single bridge rectifier. This ofcourse limits the amount of load
current I can draw out from the source.)

Thank you for your help and greetings for festive season. Wish everyone
a new year full of good health and prosperity!

Regards,
Anand

 

[Spehro Pefhany]

Hi everyone,

I am trying to build a relatively high voltage-high current DC source.
The scheme is simple and uses no regulation (therefore no feedback
control). The scheme is as follows:

Utility mains supply => Variac => Three full wave rectifier bridges in
parallel => Huge capacitor bank => Load.

Variac: something similar to this one:
http://orgchem.colorado.edu/Technique/Equipment/Communityequip/Variac.html

Bridge: KBPC3510

Capacitor bank: 6800uF, 400V

I could not find a full wave bridge rectifier with a sufficiently high
current rating and therefore, I thought of paralleling three that were
readily available.

I start (slowly) increasing the output AC voltage of variac so as to
increase the DC supply to the load. However, the variac fuse blows-up
at around 10V AC output!

Paralleling three bridges may not be the most elegant way to build a
high-current DC source, but I do not understand what could cause the
fuse in the variac to blow-up. (Please note, everything works fine with
a single bridge rectifier. This ofcourse limits the amount of load
current I can draw out from the source.)

Thank you for your help and greetings for festive season. Wish everyone
a new year full of good health and prosperity!

Regards,
Anand

Maybe one of your rectifiers is bad. Check them with a meter on the
diode scale and mayb try them one at a time.


Best regards,
Spehro Pefhany

 

[miso]

On 12/26/2013 9:39 PM, Anand P. Paralkar wrote:


I have no idea on the variac fuse issue, but you really can't parallel
diodes due to current hogging.

Maybe there is a scheme that isn't too complicated to use power mos
devices instead of diodes, i.e. active rectification.

 

[[email protected]]

Utility mains supply => Variac => Three full wave rectifier bridges
in parallel => Huge capacitor bank => Load.

If your mains supply is US style 120 V, it's not common to have more
than about 20 A available. You *can* get up to 50 A on the right
circuit, but not from a "normal" outlet. For Europe-style 230 V or
240 V, you probably have 16 A or less available.

Variac: something similar to this one:
http://orgchem.colorado.edu/Technique/Equipment/Communityequip/Variac.html

That size Variac is only going to be good for something like 10 A.
Bigger Variacs do exist.

Both the mains supply and the Variac will limit how much current you can
get. Neither one has much to do with your fuse-blowing problem, though.

Bridge: KBPC3510

35 A, 700 V bridge rectifier. OK.

Capacitor bank: 6800uF, 400V

Probably OK. If this is made out of multiple capacitors, you might try
using just one at a time - maybe you have a bad one.

I could not find a full wave bridge rectifier with a sufficiently high
current rating and therefore, I thought of paralleling three that were
readily available.

This may or may not work. The diodes in each bridge will turn on at
very slightly different points in the AC cycle. When an AC cycle
begins, one bridge rectifier may temporarily be carrying all the
current, until the AC voltage rises a little more.

Do you have the bridge rectifier(s) on a heat sink?

I start (slowly) increasing the output AC voltage of variac so as to
increase the DC supply to the load. However, the variac fuse blows-up
at around 10V AC output!

Like Sphero said, test each bridge individually. Maybe you have a bad
one, or maybe the terminals are incorrectly labeled.

You also have a lot of capacitors to charge up; they will appear as a
dead short at the beginning of the first AC cycle and draw a large
current for a brief time. (On the other hand, it does work if you use
just one bridge - but does it work with each of the three bridges used
individually?)

If the Variac has a fast-acting fuse in it, this may be part of the
problem; try a "slow blow" fuse of the same current rating and physical
size. Be aware that you might blow the next fuse in line (like the fuse
or circuit breaker for the building mains) instead when you do this.

I know this *can* work, as I've helped do something similar to charge
a 288 V battery pack: 240 V mains, isolation transformer (1:1), Variac,
bridge rectifier, small capacitor (around 1000 uF), battery pack. I
only used one bridge, though. The control system was a student that
watched a voltmeter and an ammeter and turned the Variac knob.

Matt Roberds

 

[Jasen Betts]

I could not find a full wave bridge rectifier with a sufficiently high
current rating and therefore, I thought of paralleling three that were
readily available.

Are they identical? parallelling disparate rectifiers is likely to
fail due to current hogging,

I start (slowly) increasing the output AC voltage of variac so as to
increase the DC supply to the load. However, the variac fuse blows-up
at around 10V AC output!

have you got a clamp meter to probe for fault currents with?

Paralleling three bridges may not be the most elegant way to build a
high-current DC source, but I do not understand what could cause the
fuse in the variac to blow-up.

only one thing: too much current.

 

[Artem]

increase the DC supply to the load. However, the variac fuse blows-up

at around 10V AC output!

1. Check Capacitor polarity.
2. Connect any tungsten lamp between variac and rectifier. It will prevent future explosions.
3. Use only one rectifier. Google "diode parallel".
4. Do not forget discharge you capacitors after.

 

[Artem]

On Friday, December 27, 2013 8:49:47 AM UTC+2, Artem wrote:
tungsten lamp
220 Volt!!!

 

[Chris Jones]

Hi everyone,

I am trying to build a relatively high voltage-high current DC source.
The scheme is simple and uses no regulation (therefore no feedback
control). The scheme is as follows:

Utility mains supply => Variac => Three full wave rectifier bridges in
parallel => Huge capacitor bank => Load.

Variac: something similar to this one:
http://orgchem.colorado.edu/Technique/Equipment/Communityequip/Variac.html

Bridge: KBPC3510

Capacitor bank: 6800uF, 400V

I could not find a full wave bridge rectifier with a sufficiently high
current rating and therefore, I thought of paralleling three that were
readily available.

I start (slowly) increasing the output AC voltage of variac so as to
increase the DC supply to the load. However, the variac fuse blows-up
at around 10V AC output!

Paralleling three bridges may not be the most elegant way to build a
high-current DC source, but I do not understand what could cause the
fuse in the variac to blow-up. (Please note, everything works fine with
a single bridge rectifier. This ofcourse limits the amount of load
current I can draw out from the source.)

Thank you for your help and greetings for festive season. Wish everyone
a new year full of good health and prosperity!

Regards,
Anand

I agree with Spehro. Perhaps one rectifier is bad or wrongly connected.

Also, regarding putting rectifiers in parallel:

Whilst it is not the cause of the present problem, it may be worth
keeping in mind that the forward voltages of diodes tends to decrease
when they get hotter. This means that whichever of the three rectifiers
is hottest will tend to hog more than its fair share of the current, and
may get hotter still, possibly leading to damage. The usual way to fix
this problem is to mount the semiconductor devices in good thermal
contact with each other, and then add individual resistors in series
with each device so that if there were any current imbalance, it would
tend to self-correct due to the voltage dropped in the resistance being
large enough to swamp any thermal mismatch. These balancing resistors
are most often seen when multiple bipolar transistors are used in
parallel, in which case each transistor has the resistor in its emitter.
In the case of the rectifiers, it might turn out to be the case that
just using individual long wires for each rectifier, rather than joining
them together with short thick wires, would be enough balancing
resistance. It might even be the case that the rectifiers themselves
have enough parasitic internal ohmic resistance to prevent thermal
runaway. Anyway, checking the current sharing (e.g. with a hall effect
clamp meter) under full load, especially after you artificially heat or
cool one device, might be a test worth doing once you get the circuit
working.

Chris

 

[Artem]

I know this *can* work, as I've helped do something similar to charge
a 288 V battery pack: 240 V mains, isolation transformer (1:1), Variac,
bridge rectifier, small capacitor (around 1000 uF), battery pack. I

I'm not sure that capacitor is necessary. DC Choke will be more better solution.

only used one bridge, though. The control system was a student that
watched a voltmeter and an ammeter and turned the Variac knob.

I prefer use capacitor in series of primary coil for current stabilisation.

 

[[email protected]]

Why do you use multiple bridge rectifiers instead of four discrete
sufficiently big rectifiers ?

Anyway, the specifications for most rectifier bridges seem to be quite
"optimistic". In a single phase bridge rectifier two diodes connect
all the time, causing two Vf voltage drops and the power dissipated
P = 2 x Vf x I, thus at 35 A, one could expect about 70 W heat
generation.

One data sheet for the KBPC3510 claimed Iav at Ta=55 C with 3/8"
leads, which would suggest that the bridge was suspended in air
without heatsink. Of course, this is ridiculous.

The derate curve makes more sense, in which the derate should start at
55 C _case_temperature, which is believable. With Tj(max)=150 C and
Ta=20 C, the thermal resistance from junction to case is 1.4 C/W and
from case to ambient air 0.5 C/W. To achieve such low heat sink
thermal resistance, you would need huge fans to circulate cold air or
have a constant supply of (ice)water . And as a reality check, who
would design a product to operate at the maximum junction temperature
anyway ?

Assuming some more realistic design parameters, say Tj=120C, Ta=40 C
(inside an equipment) and Rth=3 C/W (from junction to air) a 27 W
dissipation would be tolerated thus the load current should be kept
below 15-20 A.

Perhaps a diode has been failed short in your previous experiment.

If you do not have sufficient balancing resistance (or inductance) in
_each_ feed to the rectifiers, if one rectifier gets hotter
(insufficient cooling in the middle unit etc.), the Vf drops, more
current flows through this bridge, while the current in the other
drops. Finally the hot diode overheats and breaks.

Use small separate resistors from the AC input point to each rectifier
AC input to balance the current even if there is a temperature
imbalance. Of course you cold use such resistors also between each
bridge (+)-side and storage capacitors or if you have three storage
capacitors, feed each with a dedicated bridge and combine the output
from each capacitor to the final load.

Of course, balancing series resistor dissipate some power and cause
voltage sag. One way to reduce the dissipation is to use series
inductors from the main AC input to the individual AC inputs of the
bridges. This is also gentler to the rectifiers and electric network,
since it reduces the peak current during startup and during each mains
cycle.

 

[Jasen Betts]

On Friday, December 27, 2013 8:49:47 AM UTC+2, Artem wrote:
tungsten lamp
220 Volt!!!

variac

the lamp only sees the difference.

a 12V 100W lamp might be a good start.

 

[[email protected]]

35 A, 700 V bridge rectifier. OK.

Do you have the bridge rectifier(s) on a heat sink?

I have not seen any specification for the actual thermal resistance
from juction to ambient air without heat sink, but for packages of
similar sizes, this is somewhere between 10-30 C/W, take 10 C/W,
Tj=150, Ta=25 C, the maximum power dissipation would be 12.5 W.
assuming Vf=0.9 V/diode, which would allow 7 A continuous current, For
Rth(ja)= 30 C/W, we are down to 2.5-3.0 A.

Looking at data sheets from various manufacturers, the Vf=1.2 A at 35
A rather than 1.0 V that I had assumed in a previous post. The Rth(jc)
is 2 C/W, while I had assumed 1.4 C/W based on the derate curve. Thus,
even my calculations gave a too optimistic view of the situation.

 

[Fred Abse]

Hi everyone,

I am trying to build a relatively high voltage-high current DC source.
The scheme is simple and uses no regulation (therefore no feedback
control). The scheme is as follows:

Utility mains supply => Variac => Three full wave rectifier bridges in
parallel => Huge capacitor bank => Load.

Variac: something similar to this one:
http://orgchem.colorado.edu/Technique/Equipment/Communityequip/Variac.html

Bridge: KBPC3510

Capacitor bank: 6800uF, 400V

I could not find a full wave bridge rectifier with a sufficiently high
current rating and therefore, I thought of paralleling three that were
readily available.

I start (slowly) increasing the output AC voltage of variac so as to
increase the DC supply to the load. However, the variac fuse blows-up
at around 10V AC output!

Paralleling three bridges may not be the most elegant way to build a
high-current DC source, but I do not understand what could cause the
fuse in the variac to blow-up. (Please note, everything works fine with
a single bridge rectifier. This ofcourse limits the amount of load
current I can draw out from the source.)

Thank you for your help and greetings for festive season. Wish everyone
a new year full of good health and prosperity!

What current? The pictured variac you referred to looks like less than 5
amps rating. What fuse does it have? It certainly doesn't look capable of
the 35 amps that one KBPC3510 is rated at.

Did you test it under load, or unloaded?

Bear in mind that, with the rectifier-capacitor configuration you
describe, the peak current drawn can be much higher than the average
current, dependent on the impedance of the supply. Variacs have low
resistance windings, and also, being autotransformers, the currents in
each half, either side of the wiper are very unequal. Variacs are not
really suitable for reservoir capacitor rectifier operation.

A 5 amp variac will only *deliver* 5 amps at whatever voltage. You can't
use them for current step-up, I've seen melted windings where people have
tried.

3*35 amps = 105 amps, which is a BIG variac, which would weigh hundreds of
pounds, and cost thousands of bux. The only ones that big that I've seen were
motor driven, three phase, and needed a crane.

Paralleling block bridge rectifiers is never a good idea, unless you can
guarantee a close match on forward voltage and temperature coefficient thereof,
and ensure they're all at the same temperature, under all conditions. Individual
diodes can be paralleled with a suitable resistor in series with each.

If you can't find a suitable bridge, you'll just have to make one out of
individual diodes. They're available up to hundreds of amps.

Check your rectifier connection (easy to get wrong), and your capacitor
polarity.

 

[[email protected]]

On Friday, December 27, 2013 12:39:11 AM UTC-5, Anand P. Paralkar wrote:

The variac in your link does not supply enough current to stress that KBPC3510 bridge you're using, not even close. Get real!

 

[Fred Abse]

I know that RF power transistors are often built so that they're a bunch
of small transistors with emitter loading resistors, all put in parallel
on one chip.

Most RF power transistors I know of are a single transistor, with multiple
emitters.

From memory, the humble 2N3866 has 9 emitters.

 

[[email protected]]

Normally, you will need a way to equalize current
For real high power the way would be an inductor _before_ the capacitor.
This limits the peak current, but lowers the output voltage,
With 3 bridges and 3 inductors it could work.
From this in your case it could be simpler to use 4 high power diodes,
optionally with the inductor.
But that is old design, those inductors are huge,
Look here:
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/rectbr.html
Scroll down to: Bridge Rectifier, LC Filter

The nasty thing with that configuration is that the inductor will also
carry DC, requiring a quite big choke with an air gap (size comparable
to the output transformer in audio applications). Putting the
inductance prior to the rectifier and you do not have to handle the DC
current and no air gap needed.

 

[mike]

The nasty thing with that configuration is that the inductor will also
carry DC, requiring a quite big choke with an air gap (size comparable
to the output transformer in audio applications). Putting the
inductance prior to the rectifier and you do not have to handle the DC
current and no air gap needed.

If the inductor is sized to carry the peak current without saturating,
why does it matter which side of the rectifier?
What am I missing?

 

[[email protected]]

Connecting power diodes in parallel or series is standard practice. It

may or may not require resistors. Check with datasheet for particular

diodes.

It definitely is NOT standard practice to parallel semiconductor diodes.

 

[Kevin McMurtrie]

Hi everyone,

I am trying to build a relatively high voltage-high current DC source.
The scheme is simple and uses no regulation (therefore no feedback
control). The scheme is as follows:

Utility mains supply => Variac => Three full wave rectifier bridges in
parallel => Huge capacitor bank => Load.

Variac: something similar to this one:
http://orgchem.colorado.edu/Technique/Equipment/Communityequip/Variac.html

Bridge: KBPC3510

Capacitor bank: 6800uF, 400V

I could not find a full wave bridge rectifier with a sufficiently high
current rating and therefore, I thought of paralleling three that were
readily available.

I start (slowly) increasing the output AC voltage of variac so as to
increase the DC supply to the load. However, the variac fuse blows-up
at around 10V AC output!

Paralleling three bridges may not be the most elegant way to build a
high-current DC source, but I do not understand what could cause the
fuse in the variac to blow-up. (Please note, everything works fine with
a single bridge rectifier. This ofcourse limits the amount of load
current I can draw out from the source.)

Thank you for your help and greetings for festive season. Wish everyone
a new year full of good health and prosperity!

Regards,
Anand

You've wired something up wrong.

Bridge rectifiers can be purchased in enormous sizes meant for powering
heavy machinery. There's no need to gang them up. Check any online
parts catalog.

Micro Commercial MB354W-BP
Fairchild GBPC3504W
400V 35A continuous at 55C. The surge current charts shows they can
survive a 60Hz 4 cycle inrush current of 200A. That inrush current
rating is at least as high as normal-grade electrolytic caps allow.

 

[Jasen Betts]

If the inductor is sized to carry the peak current without saturating,
why does it matter which side of the rectifier?
What am I missing?

wind-up