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Hi,
In general, isolated DC-DC converter is either flyback converter or forward converter.
flyback
forward
But why not just have switching, then 4-diodes rectifier bridge? Is there anything bad about such design?
The “dots” of the transformer are as below
Hope to hear from you soon
Thanks and best regards,
KH
Hi Ulrich,
Can i check with you on some circuit design and hope that you can share me some thoughts,
This is a real product (4-20mA) that is already being sold in the market. I left out many parts of the circuit as I can’t reveal too much.
1. Vin is fixed. There is voltage regulation circuit before feeding voltage to primary winding.
2. There is a controller, U2.
3. Here comes the unconventional part, in stead of Vout feedback via optocoupler/transformer, they used a fixed primary side design. This is ok as long as you know your load does not vary.
4. Output filtering has only RC, no L, unlike forward topology. This may look strange to you à 866 ohms looks like it will waste a lot of power. But in our industry, this is common, product used in oil & gas industry. Our current is very low. The design do have explosion proof --- intrinsic safety. R23, D7 to D10 are there for intrinsic safety.
What I’m trying to do is to understand what they voltage waveform would be like.
Am I right to say the following?
The wave form of primary is DC while on secondary side, it is AC. The turns ratio is 1:1.
Hope to hear from you soon!
Thanks and best regards,
KH
Hello KH,
Your 6.5V secondary is configured as a;
1. full wave bridge rectifier
2. zener clamp
3. another rectifier
4. RC filter, current limited output 866 ohm
I can not tell what would be driving the 3.4V output at PWRXFRMT
If you can send a better picture of the transformer connections it would help.
For the 6.5V I have drawn what I expect the voltages to look like.
After the full wave bridge you should see small dips corresponding to the AC input transitions at the bridge input.
Then it gets further filtered out to the output.
Here is a picture of what I expect the 6.5V signals to look like.
Hello KH, both primary and secondary must both have an average voltage of 0V.
Transformers work from a Volt x Seconds applied basis.
T1: Primary will apply Vpri for Tsec (VxS), building up flux in the core.
T2: The transformer core must reset itself, so after T1 the primary voltage will reverse and bring the flux back to where it started.
If the transformer has a DC component then eventually the core will saturate.
This picture illustrates the voltage action on a transformer.
Looking at the primary voltage, V+ is applied for +VxS and then the flux forces the exact same reverse voltage -VxS
The net result on the primary is 0V average.
There is deadtime after the core resets until the next cycle.
The secondary will look exactly the same, except the voltages are scaled by the turns ratio.
Vpri/Vsec = Npri/Nsec where N is the turns ratio.