Flyback and forward converter design
Hello everyone,
here is my story!
My team constructed a universal DC/DC converter for our project in Power Electronics course. It has buck, boost and buck-boost functions (like Mohan's power pole board).
Our next assignment is to construct flyback and forward magnetics board. Professor told us to use PI Expert to make calculations (number of turns, core, inductance...).
The problem is, that we have already constructed a converter, and we need to construct flyback and forward daughterboards specifically for that converter.
In PI expert I can choose only some completed models, but I don't see a option to make your own model.
Anyway, our model is similar to Mohan's Power Pole Board. As input voltage we have laboratory 50V DC source (48V-52V). Output voltage can't exceed 100V and 100W power in any case. We can even go lower with output voltage if needed.
We're using IRF640 mosfet (200V, 18A) and MUR2020 diode (200V, 20A).
We can adjust frequency (30-100 kHz), and duty cycle (not to exceed maximum output voltage for flyback converter (which is actually buck-boost topology)).
Using PI expert we should calculate core size, number of turns (same for primary and secondary windings) and choose from the product list for flyback and forward converters, and additional diodes and inductance for forward converter. As I already mentioned maximum output voltage and power are 100v and 100W. When choosing components, maximum current ripple should be taken in consideration too.
I'm still pretty inexperienced in DC/DC converters design and I have never used PI Expert before (as our professor too unfortunately). So please if you think you can help me somehow I would appreciate it. Some tutorials would be great too. If there are more models libraries to download (maybe I will find more suitable models there) you can post me a link!
If I forgot to specify something important don't hesitate to ask :)
Thank you in advance,
Lovro
p.s. sorry for my english (if it is bad)
Comments
Traveler,
Thank you for your reply :)
I will try to be more specific this time. I must admit that my knowledge in power electronics is pretty poor. I understand it completely in theory, but when it comes to design, I'm completely inexperienced. My professor is also pretty uninterested in helping so, so we are on our own.
As I mentioned, we designed a universal DC/DC converter using Mohan's power pole board. Our board is actually a Mohan's copy, without any greater knowledge in all the circuitry. Our board consists of 2 power diodes and 2 power MOSFETs, which helps us to easily change from one configuration to another (buck to boost, boost to buck-boost...). The board has 2 heat sinks, one for each diode-MOSFET pair.
Using attachable magnetic daughterboard and suitable wiring we can achieve buck, boost and buck-boost topologies. For that reason we are using simple magnetic board with coil. (which determines current ripple). We have tested our board using scope and it's working properly.
Our next assignment is to design attachable flyback and forward daughterboards.
Flyback daughterboard should consist of transformer and diode that we have to design. For controllable switch we are using MOSFET (IRF640), attached on the main board (which was used in non-isolated topologies along with the diode pair).
Forward magnetic board consists of transformer, 3 diodes and a coil we have to design. We are using mainboard's IRF640 here to.
I will now try to describe circuitry and components we used.
- input and output capacitors: 680µF,100V electrolytic capacitor in parallel with 10µF block capacitor
- IRF640 (200V, 18A) power MOSFET
- IR2127 gate drive IC (drive and overcurrent protection)
- UC3823 PWM controller
- overvoltage protection cicuitry grounds PWM signal when input and output voltage reaches 110V
- 2 LEM LA25-NP Hall probes for input and output current measurement
- Power supply: we have 50V laboratory constant DC source (48-52V) which will be used for input voltage
- +/- 12V Power supply for ICs
- 20 ohm output resistance (As I can remember), but it can easily be modified.
Flyback and Forward converters should be designed in OPEN LOOP MODE!
Turn ratio should be 1 (equal number of turns of primary and secondary windings)
Maximum output power should be 100W (but we could go even lower). We can vary our switch frequency between 30 and 100 kHz.
Our goal is to create only a educational model with 2 basic isolated topologies. Any kind of optimization is an optional thing. The mainboard is not adjustable (except output resistance). We can only combine elements for flyback and forward boards (transformers, diodes, coil...).
We don't have to use snubbers and bias windings also.
It would be great if I could manually create my own circuitry (with the MOSFET and other specific components), and PI expert show me solutions and suggestions for my core size and number of turns (same for both sides) for transformers, diodes and coil inductance (for specific maximum current ripple (forward)).
I'm sending you Mohan's lab manual. Wiring schematics for flyback and forward converter can be found on pages 29 and 33.
Sorry if I'm too long, or forgot to mention something important. Also sorry if I'm not understood in some points of my post (I don't mean the language, but my descriptions :) )
Thank you very much for your help and your time,
Lovro
Lovro -
I'm afraid I don't have the bandwidth to help you a great deal with your school project. From the manual you sent me as well as the the general description you've outlined, it sounds like a lot of the design decisions have already been made for you.
When it comes to designing your magnetics, there are many different factors at play. The simplest rule and guideline can be summarized as: Don't saturate your magnetic components. If you have a power electronics text book available for your class, you should be able to use fairly general inductor equations to help you with this.
Our PI Expert software might be of help for specifications of different types of magnetic components. PI expert has a fairly thorough component library and has all the different core dimensions, geometries and specs that you can use to help with your design.
However, our software is ultimately intended for use with our products and it would be difficult to walk you through the design of your school project using our software as this isn't what it is designed to do.
In general though, once you have information on the different types of magnetic components as well as the handful of relevant design equations...most of what you need to do could be done in Excel or on paper.
-The Traveler
Thank you for your time. Meanwhile I solved all my problems with professor :)
If I will have any more questions regarding this topic, you will be asked :)
Lovro
Lovro -
First off...your English is great, stop worrying about that. :-)
It's not entirely clear to me what you're trying to accomplish. Are you just wanting to create a transformer design and then manually provide the MOSFET gate drive so that you have a flyback converter running entirely open loop? Are you using a power supply controller IC at all? How are you handling your MOSFET current limit circuitry?
So a couple of key Power Integration concepts might be helpful:
The first is the concept if our devices (I^2)*fsw parameter. This is a tightly controlled datasheet parameter on almost all of our products. The idea being that the theoretical maximum amount of power your can process through an inductor is going to be Lp*(Ilim^2)*fsw. fsw and Ilim are hard set in our controllers (you can't change them). So when it comes to deciding what your primary inductance value needs to be, you have to look at how much power you want to process through your inductor.
Another key Power Integrations concept is the use of the term VOR. This stands for Voltage-Output-Reflected (it would make more sense if we called it ROV). What this means is that when your MOSFET turns OFF and the output diode begins conducting, your output voltage will appear across the secondary winding (remember that your diode is ON). This output voltage across your secondary winding will then be reflected through your transformer, multiplied by your turns ratio and will be superimposed on your DC input voltage. Your output voltage and your turns ratio will ultimately decide what voltage rating you need on your MOSFET. Or alternatively, if you have a fixed choice of MOSFETs, you need to choose a output voltage and turns ratio that when reflected will not exceed the rating of your MOSFET. Don't forget you'll also need to leave some room for ringing due to leakage inductance which can easily fry your MOSFET.
If you can make some decisions on the specs of your supply, I can probably help further. As it is right now though, your specs are pretty vague and it's difficult to create a design with vague specs ;-)
-The Traveler