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[SOLVED] Please help to improve load regulation of flyback SMPS power supply circuit

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pewdsbheem

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I am trying to make a power supply. I have a 48V 3.5A output.
Load Regulation is very bad and some values are-
At no load, output voltage = 48V
At full load(3.5A), output voltage=32V.
I am attaching the FLYBUCK SMPS POWER SUPPLY CIRCUIT. Please suggest me some solutions to improve load regulation.
I have used LTV-817A, Transistor Output Optocouplers in my circuit to improve load regulation using feedback but I does'nt helps or improves it.


I have need %0.05 (2.5mV) regulation in all load cases from 0 A load to 3 A... how can I fix the problem. I increase the value of capacitors, and decrease ESR but it doesn't make any help, ripple decrease 70mV to 50mV.. When I decrease the output filter inductance by half value ripple decrease from 50 mV to 35mV..Is there any feasible solution?
 

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  • Flybuck SMPS circuit.PNG
    Flybuck SMPS circuit.PNG
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Re: Please help to improve load regulation of flybuck SMPS power supply circuit

1. That schematic is a multi-output flyback, not a flybuck.
2. 48V 3.5A is pretty high power for a flyback, you may want to consider using another topology (forward, two switch flyback, etc).
3. Dropping from 48V to 32V at full load implies that some stage is saturating or hitting a hard limit. For example a peak current limit, a max duty cycle, transformer saturation, etc. Without more info it's impossible to say.
4. 0.05% is very tight, especially for a flyback. It will depend mostly on the DC gain of your feedback loop. First you should solve problem #3...
5. Load regulation and output ripple are two entirely different matters.
 

Re: Please help to improve load regulation of flybuck SMPS power supply circuit

you are possibly hitting the current limit in the device - TOP261, or the Tx is not quite designed for the full power you are seeking...it may be starting to saturate - giving the same I limit issue,

Power in a Tx in a fully DCM design = 0.5L x (Ipk)^2 x freq x eff. To get higher power you need higher peak I in the Tx, but do not exceed the Bmax in the core for the freq chosen...
 

Re: Please help to improve load regulation of flybuck SMPS power supply circuit

Hi all,
Voltage drop that usually occurs when there is a less resistance in the circuit.
We can describe it using the Ohm's law
V=IR,
Voltage to the circuit is directly proportional to the resistance in the circuit. So, when you try to increase the
resistance in the circuit. The voltage also gets increases.

Thanks & Regards
Dinesh M
 

Re: Please help to improve load regulation of flybuck SMPS power supply circuit

It's completely unclear if the voltage drop happens due to circuit overload or due to insufficient tracking of the regulated 5V and "half"-regulated 48V output. Please clarify! What happens to the 5V output when loading the 48V output? Most multiple output SMPS have minimal load specifications for each output. What's the designed load range of this circuit?
 

Re: Please help to improve load regulation of flybuck SMPS power supply circuit

Now
It is not a multiple output smps. Only 48V output is taken.
 

Re: Please help to improve load regulation of flybuck SMPS power supply circuit

Hi,

Maybe you don't use the 5V path, but the schematic shows it.

Measuring the 5V as recommended gives information about the error cause.

Klaus
 

It is not a multiple output smps. Only 48V output is taken.
Either the schematic in post #1 is incorrect, then please come with corrected one, or you are taking the apparently unused 5V output for voltage regulation, which would be a really bad idea. The requested 0.05 % regulation can be only achieved by sensing the actual output voltage for feedback.
 

To get the ripple down to very small levels - you have 2 options

i) either a linear regulator after the main o/p

ii) a 2 stage L - C filter ...
 

1.
Schematics and design attached.
View attachment SMPS PCB.pdfView attachment SMPS.pdf
2.
Test Results
test result.PNG
3.1
The load regulation, was around 3-4V now. However, on 3A load, the Transformer, Topswitch, Output Diode (D3), and D1 were getting too hot (schematic attached for reference). When we went for ripple testing, the supply got faulty and the output voltage was now just 4-5V.
3.2
This supply was working perfectly with exactly 48V output at nominal 2A load (50V set at no load). However, after 2-3 minutes, the same components were getting hot again, so we switched off the supply.
4.
New Test Results
new test results.PNG
5.
This issue looking related to transformer design..

Please help in this Load Regulation Problem
 

The schematic doesn't tell what's possibly wrong with transformer or capacitor dimensioning. According to datasheet, TOP261 should work well 150W.

I already mentioned that you can't achieve the intended load regulation in half-regulated 48V output. The feedback loop need to be redesigned for 48V sense.
 

You need to re-design the Tx with lower leakage, more de-coupling on the HVDC, e.g. 220nF right by the Tx, tighter layout on the pri & sec sides...

- - - Updated - - -

Also your creepage & clearance appears to be way too low in places, esp around U2, try to design for 5mm min between any ckt on the mains side to the output side.
 

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