RCD clamp circuit in forward converter 150 watt

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--BawA--

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I am designing a forward converter , which has following configurations

Input voltage - 85Vac to 265Vac
Output voltage - 12V
power 150 watt.

i want to use a RCD clamp circuit to reset the transformer , i don't want to use RESET winding technique or active clamp technique.
what is the procedure for determining the values of R, C , D ?
 

Is air gap is necessary in the forward converter's core?
How do we decide if air gap is needed in core of a particular topology?
 

Is air gap is necessary in the forward converter's core?
Click to expand...

No.

How do we decide if air gap is needed in core of a particular topology?
Click to expand...

Air gap in core is required for inductors. A transformer core does not need air gap. The multi-winding magnetic component in forward converter is a transformer, so it does not require air gap. But the multi-winding magnetic component in flyback converter is a multi-winding inductor, so it requires air gap.

NOTE: On residual flux issue, a very small air gap in forward converter's transformer some times maybe useful.
 
Thanks ahsan
Could you please tell me what are the equations for calculating the value of R and C for RCD clamp forward converter.?
 

There is a guide line in the topic titled "RCD type forward converter" on page 3-2 of application note "slup108" ( **broken link removed** ).
 

--BawA-- said:
Thanks ahsan
Could you please tell me what are the equations for calculating the value of R and C for RCD clamp forward converter.?
Click to expand...

The capacitor must absorb the brief spike generated by the primary.

The resistor must discharge the capacitor before the next cycle begins.

Values are not critical. It's a tradeoff. You can charge a small uF to a high V. Or else charge a large uF to low V.

Ohm value should be low enough so that you can be certain it will drain most of the capacitor charge. Its watt rating must be adequate.
 

Bradtherad , i know that the R should be capable of disscipating the energy to reset the flux before next ON cycle. but is there any formula to determine the value of R and C?
 

--BawA-- said:
Bradtherad , i know that the R should be capable of disscipating the energy to reset the flux before next ON cycle. but is there any formula to determine the value of R and C?
Click to expand...

The RC time constant is a guideline.

I think it's a good idea to make the capacitor rated at least as high as the supply voltage. Choose its Farad value so it charges up to 50 to 80% of its rating. It charges almost instantly. (You can try to simulate it but the real behavior can only be known with a real transformer, etc.)

Then choose the resistor so it is mostly discharged before the next cycle begins.

The capacitor voltage will create a sawtooth waveform. It should drop deep enough, that you can be sure the charge will never increase above the capacitor's rating.
 
Ohk Bradtherad , I got you,
but still having a question , How to calculate the magnetizing inductance of the transformer for forward converter?
 

--BawA-- said:
Ohk Bradtherad , I got you,
but still having a question , How to calculate the magnetizing inductance of the transformer for forward converter?
Click to expand...

I'm afraid I don't know enough about that part of the forward converter. I understand a third winding is used to demagnetize (reset) the core at the end of each cycle.

Perhaps someone more knowledgeable will answer your question.
 

How to calculate the magnetizing inductance of the transformer for forward converter?

You need a good book on SMPS design since your background is weak then you can rely on reliable tools. I dont know if these are best but they are free. https://www.poweresim.com/ It is important to understand how designs fail before you rely on tools.

Lm =0.4π(Np)2Ae × 10−9/(la + li/u)
li = length of iron path, cm
la = length of air gap, cm
Im = magnetizing current, A
Forward converters cannot use any energy stored in the inductor, as the magnetization current is just used to transform the power. When gapped, the core losses in forward converters are less but copper losses are more but more tolerant to overloads with a transfer impedance of ~10.

Flyback use the energy stored in the primary inductance to transfer to the output, so useful for high voltage but prone to higher eddy current losses.
 

So if you don't use a reset winding or an active clamp, what do you suppose will happen to the magnetizing energy in the transformer every cycle? Do you plan on just burning it as heat? You might consider using a two switch (or diagonal) forward converter, which can recover magnetizing energy without a reset winding or snubber (but is limited to 50% duty cycle, unlike the active clamp).

Do you need to operate with duty cycles over %50?

And forward converter transformers often do need a small air gap, in order to prevent saturation.
 

I am planning to use RCD clamp circuit to reset the transformer because of some cost constraints. Do i still need to use air gap core?
Duty cycle will be from 25% to 75%.
 

I am planning to use RCD clamp circuit to reset the transformer because of some cost constraints. Do i still need to use air gap core?
Click to expand...
The stored energy and respectively snubber loss is lowest without an air gap. The disadvantage is that saturation starts at a relative low magnetizing current. But it's worth a try, I think. You can still add an air gap to your transformer.

Duty cycle will be from 25% to 75%.
Click to expand...
75% duty ccyle means that the reset voltage is 3 times the forward voltage. Would need > 1500 V switch transistor rating, not actually a good idea.

- - - Updated - - -

Looking at the specification in post #1, I assume that 75% duty cycle won't be used with high input voltage, so a 600 - 800 V switch might work. But you'll have a hard time tuning the reset voltage with a RCD snubber.
 
FvM said:
Looking at the specification in post #1, I assume that 75% duty cycle won't be used with high input voltage, so a 600 - 800 V switch might work. But you'll have a hard time tuning the reset voltage with a RCD snubber.
Click to expand...
This might be true under steady state conditions, but unless the max duty cycle limit on the pwm controller is automatically adjusted based on line voltage, there is still a risk of destruction if a transient load occurs at high line voltage.
 

This might be true under steady state conditions, but unless the max duty cycle limit on the pwm controller is automatically adjusted based on line voltage, there is still a risk of destruction if a transient load occurs at high line voltage.
Click to expand...

Yes, the risk can't be denied. All in all, it's probably no good idea to implement a forward converter for wide input voltage range, or if you do, it should use a safe voltage limiting method like active clamp.
 

Intended dutycycle will be in the range of 30% to 65% with an AC voltage range of 125VAC to 265VAC,
currently I am not using the gapped core.
 

--BawA-- said:
Intended dutycycle will be in the range of 30% to 65% with an AC voltage range of 125VAC to 265VAC,
currently I am not using the gapped core.
Click to expand...
Again, those are probably steady state duty cycles. Under transient conditions you should be able to swing higher, or suffer very poor response.

What controller do you plan on using? Unless it has an adaptable duty cycle limit I wouldn't consider using a simple forward converter.
 

mtwieg said:
Again, those are probably steady state duty cycles. Under transient conditions you should be able to swing higher, or suffer very poor response.
Click to expand...
I didn't get it , Kindly explain a bit more , I've been learning and practicing SMPS since last two years, so i am not expert.

mtwieg said:
What controller do you plan on using? Unless it has an adaptable duty cycle limit I wouldn't consider using a simple forward converter.
Click to expand...
I'll use this Forward converter for my Inverter battery Charger, so I'll follow the simple alogorithm, i.e Duty cycle will be increased or decreased by a fix amount , so that constant current could be maintained . Intended current is 10 ampere .
 

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