DC-DC 50W Isolator (or Transformer ) 1

[Dromroe]

Hi

I want to design a DC-DC isolator with the following spoiec

Input voltage 24-48V DC
Input Current : 0-2A
Power : Approx 50W

Output voltage : 24-48V
Output Current 0-2A
Output Power :Approx 50W
Efficiency : about 90%

Isolation : Input isolated from Output
Control : No control of the duty Cycle required- Fixed Duty Cycle


Application : This module will be used to isolate the Input DC supply from the Output DC supply . Deos anyone know where I could find a suitable reference design or can suggest a suitable DC-DC converter design i could use ? Basically is it is a 1:1 DC-DC Isolation Transformer .

 

[IRstuff]

Why reinvent the wheel:

http://www.vicorpower.com/cms/home/products/vi-chip/vichip_PRMVTM

TTFN
faq731-376

 

[Dromroe]

I don't want to regulate the input or output voltage . I want to isolate input voltage only

 

[IRstuff]

Output voltage : 24-48V
Output Current 0-2A
Output Power :Approx 50W
Efficiency : about 90%

That defines regulation; not good regulation, but regulation nonetheless.

TTFN
faq731-376

 

[OperaHouse]

So do you want it to track the input voltage. A TL494 could drive a couple of fets and a push push pull transformer. Rectify that and sense through an opto isolator. Compare that with a second opto isolator sensing the input voltage. That should track pretty well.

 

[Dromroe]

Thanks OperaHouse . Good suggestion !!

 

[Dromroe]

Hi OperaHouse

You proposed Push/Pull . Why did you choose push/pull ? Would a Two transistor forward coverter be easier to control and have simpler magnetics ?

Does the push pull have advantages over the forward converter for trhis application ?

 

[Skogsgurra]

Re Push-Pull
Yes it has advantages. Doesn't DC saturate the magnetics for one. Better efficiency, lots of other things going for it, too.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[DHambley]

Dromroe, I agree with skogsgura and operahouse. I did a similar isolated push pull once. 15V to 15V at 30W.
Here's a few tips to make it work:
* turns-ratio slightly higher that 1:1 to make up for the diode drops.
* 50% duty on each half. Use a dual-output switching regulator IC and ramp up the soft start enough to avoid over current at start up, but set the feedback pin low so it goes to full duty.
* small inductor at the center tap to avoid high cross coupling spikes and to keep the currents from being too trapezoidal.
* Do a quick model in spice and you'll see the waveforms of current I'm referring to above.

 

[Dromroe]

The power specification has been changed from 50 to 500W. The input of the DC-DC Isolator vvoltage will be from a capacitor bank that will be charged by a current source . The output of the DC-DC Isolator will be connected to a constant voltage load . Therefore we should not to provide closed loop input or output voltage regulation . A converter operating with a fixed duty cycle should be ok . The input to the converter will act like a constant voltage load . The output will be a fixed voltage . The input output voltage ratio will be defibned by the Transformer and the duty cycle 50% . Is a push /pull still the best topology condidering a 500W power level?

 

[OperaHouse]

"The output will be a fixed voltage . The input output voltage ratio will be defibned by the Transformer and the duty cycle 50% ."

Make up your mind. Either it tracks or the duty cycle varies. The reasons for push pull are even more so.

 

[Dromroe]

Hi Operahouse

Thank you for your comments

The input will be suppleid from a current source . The output is connected to a constant voltage load . The input /output voltage ratio of the isolator will be fixed ( 50% duty cycle + transformer ratio) .Therefore the input to the islator module will look like a constant voltage load to the circuit driving the input . If the input /output voltage ratio of the Isolator Module is 1 then the input voltage will adjust to be the same as the output voltage .Do you see any issues with this ?

 

[OperaHouse]

Obviously, I do.