I am making a step up DC-DC converter using a full bridge configuration and bridge rectifier output, using high speed rectifier diodes.
I see that in many designs on the net, they use an additional inductor on the output side between the rectifying diodes and smoothing capacitor. I have tried this, but it causes horrific oscillations in the voltage at the rectifier and they very easily go over their voltage limit. A snubber circuit would make this more useable but I am then disipating a lot of power in it. What is the purpose of this inductor and is it really necessary?

 

The inductor plus capacitor form a low-pass filter to smoothe the output.

 

It is absolutely necessary. Without it, the MOSFETs driving the transformer see a the capacitor as a dead short when they turn on, and then you need to replace the MOSFETs.
What you should see on the transformer secondary is the same as the primary multiplied by the transformer turns ratio.
If that's not what you see, then perhaps there is a problem with the transformer.
Then, as Alec said, the inductor and capacitor form a low-pass filter, so that the DC output is the average of the voltage on the secondary.
Do you have a scope that can save the waveform. If so, please post it.

 

Inductors are great for storing energy, but when you put an Inductor and a Capacitor together
you get a "Resonant" Oscillator.
The sizes of the components in relation to each other, and in relation to the driven Load,
must be chosen by a calculator so that they do not oscillate with each other.
I routinely use a Speaker Crossover Design Calculator for the initial selection of component values
and then double check it with various other R-L-C Calculators to insure stability.

Inductors are a real "Bucket of Worms" to design, if you get into very high frequencies you
can move into "Air-Core" Inductors, which are much more predictable, but much larger physically.

One way around the fluctuating output from an R-L-C Filter is to design the Power Supply to
keep its output ~1 to ~5 Volts above the desired Voltage,
then do the final regulation with a Linear Voltage Regulator.
This reduces the power dissipation associated with Linear Regulators and can produce
an extremely quiet, and stable, output voltage,
with most of the heavy lifting work done by a far more efficient Switching Regulator.

High Current Linear Regulators are available .......
Check out ....
.

 

But this isn't anything like designing speaker crossovers. This is the output of a buck regulator, and the value of the inductor isn't critical, but you are absolutely correct about avoiding the resonance. If it corresponds with the switching frequency, then it will blow up, but it's generally operated so far from resonance that it almost isn't worth considering.
Design it from the usual equation V/L=dI/dt. The secret is knowing which V, which I and which t.
Use t as the off time (when no MOSFET is switched on) and I as about 10% of the output current. You can go up to 30% if you really want to cheap out on the inductor, but it will increase its core losses. That gives V/L = 0.1I/ toff. V is the final output voltage. So L= V.toff/0.1I
It can be also calculated from the ON time of the MOSFETs and will get a similar answer.
The inductance of an inductor carrying DC falls as the current increases, so it is not a very accurate parameter.
The value of C will be determined more by finding one with adequate ripple current, but make sure the resonant frequency 1/(2.pi.root(LC)) is somewhere below the square root of the switching frequency.
If you are designing your own inductor, you probably want to use an iron powder core, and there is a handy design tool on the Micrometals website. You certainly don't want an air-cored inductor as there is no difference between an air cored inductor and a low-frequency radio transmitter. You will get interference all over the rest of your circuit and you will fail EMC tests. A toroid is the best solution, although you will get sore thumbs winding it yourself!

You might still need a snubber, but a good transformer design, good layout and a good choice of output diodes will keep its power losses to a minimum.

And Yes, High current linear regulators are available, and so are heatsinks, but they don't do "step up"!
And they are not the solution if efficiency is important.

Just wondering . . .if this is a step-up design and the input is mains, then what is the output? Or perhaps the input isn't mains, in which case it has already been stepped down???

 

@Ian0 Its a design to step up a low voltage DC battery source. I have actually shorted the inductor, and I get much better results, the system is more efficient and of course there is no resonance, but it is still rather lossy - about 68% which cannot be fully accounted for in the diode and FET. I did actually use the TI power stage designer to calculate the values of the inductances, turns ratio etc. but running at 100kHz I get some very ugly high voltage 1Mhz ringing at the rectifier diodes, I am testing at much lower than I calculated for but I am a bit scheptical if the design cannot cope with light load situations.

 

Yes, that type of design should easily exceed 90% efficiency. Is it a push-pull circuit with two N-channel FETs both with their sources connected to 0V? What type of diodes did you use in the bridge? What core did you use for the inductor, or did you buy an off-the-shelf inductor? Did you make your own transformer? How did you calculate/measure the loss? What is the output voltage?

 

Hi @Ian0 It is a full bridge with 4 N-fets these are driven by 4 FET drive opto isolators with 2x 15V DC- isolated supplies for the high sides. (A design I saw on a brushless motor design, which i kind of like as the Fet drive ICs tend to be rather delicate and very susceptible to negative voltage swings cause by ringing).
I am using 4 fast recovery diodes for rectifying. The transformer I have wound myself on a 2" power torrid core, and checked the inductance using an LRC meter, and these fit the calculated number of turns, and TI power stage calculations. Likewise for the inductor. I have actually just made a larger one to fit the calculations of the lower test power, but the results are very similar with numerous ringing pulses on the secondary side per pulse on the primary. I am measuring the input voltage and current and output voltage and current though a resistive load. The efficiency is still much the same.
I am controlling the PWM with a microcontroller, and was going to use a current sense system to limit the current pulse by pulse as I have read about the problems of "flux walking". However the current ringing problem really makes a mess of the pulse by pulse current reading so I am unable to use this at present.

 

Apologies if I'm telling you something you already know, but. . .
Your transformer core should be ferrite, but your inductor core should be iron powder. If your inductor core is ferrite, it will saturate, and then you will have a bit of a torrid time! (sorry)
Both primary and secondary windings should be nicely spread out to cover the entire circumference of the toroid.

 

@Ian0 Ahhh... I am currently just using a small toroid of the same sort for the inductor as for the transformer (both ferrite)... I will change this and see what happens.. many thanks!

 

Micrometals is your friend!

 

@Ian0 Alas not so friendly. I have tried a very big torroid for inductor But was not sure if it is iron or ferrite).. but now just a coil of wire, but the results are very much the same.. the same ringing in the secondary and same crappy efficiency.

 

The correct core will be yellow on one side and white on the other (type 26); or blue on one side and green on the other (type 52). You will need many more turns on an iron powder core than you would on a ferrite core.

 

@Ian0 So far changing the inductor, size, material etc..etc.. has not actually made any noticeable difference. I have however just removed half of the diode bridge, and I see that the ringing has got much smaller. I am beginning to wonder if there is something a bit screwy with the diodes.

 

How can it still work with only half the bridge? A normal 4-diode bridge would not give any output at all with two diodes missing, unless you have both + and - outputs, in which case you would need two inductors. By the way, you haven't mentioned the input and output voltages and the power.

 

@Ian0 It becomes a half bridge.. only rectifying on half the cycles. The input voltage should be about 30 volts in the end and the output is ultimately supposed to be for driving a sinewave inverter. For starters I'm looking at something like 50W of power ideally and testing with a lower voltage while I iron out the kinks! From what i have seen on the scope and with changing diodes and inductors in and out, it seems like rectifying part of the circuit is causing the ringing.
I've enclosed a scope image so you can see the problem.

The yellow is one of the primary coil connections, the Blue is the "corresponding" secondary connection before the rectifiers. Its actually with a x10 probe so its some serious ringing! (The second half of the wave starts at about 2 large squares from the right of the image - the opposite primary connection is then pulled high.
Changing the inductor after the rectification, doesn't actually seem to make all that much difference with this ringing, unless I do away with it completely!
I have over engineered the components, the diodes Im using are these: https://no.mouser.com/datasheet/2/427/vs-5ewl06fn-m3-1768862.pdf I have tired with a few other diodes I have about the place, but the effect is much the same.

 

Looks like poor coupling in the power transformer.

 

@Ian0 I have wound a new transformer using a P Type core - an enclosed circular bobbin type, so there should be very could coupling. (Its a 43mm diameter one so should be good for much higher power.)
Much the same really though Maybe better grounding on the scope! And similar crappy efficiency

 

The first one looked better - at least it started at the same time as the secondary. Now you seem to have a 1us delay. Does anything get warm?
Have you tried a capacitor in series with the primary? It will have to be a film capacitor and possibly quite large (don't want it to go into resonance) but it would eliminate any possibility of saturation.
Or possibly a small gap (thickness of a sheet of paper or two) - that would also reduce the possibility of saturation?
How about a scope screenshot of both sides of the primary, on 2us/div?

 

@Ian0 Thanks for the input, much appreciated! Not tired doing much with the primary, to be honest that part actually seems to be doing what its supposed to! Not sure what is creating the delay. The fets and the diodes are getting warm but no more than expected for all the power loss! The transformer is VERY over engineered and if it was saturating I would expect it to get hot.
I have put the scope across both connections of one of the diodes this time.Magenta is the input to the inductor - Diode(s) output, and the blue is the input to one of the diodes from the secondary. I cant quite get my head around it. The other side of the inductor that goes to the storage capacitors is a steady 80-90V DC.. hmmm
But I think I need to have a break for the day now, my mind is getting a bit scrambled!
In case I am doing something really silly, here is the schematic (sorry its rather a "work in progress" (mess)).
(I corrected the protection diode that was in backwards!)