how to increase load regulation

kemred -


Just from the standpoint of component tolerances, reaching a 0.05% load regulation is going to be pretty much impossible. If your application actually does call for that precise of an output voltage, I don't know if a SMPS will be the answer. Just from a cabling perspective, 2.5mV @ 3A means you will need less than 0.8mOhm of cabling resistance.


If there are only a couple parts of your application that require such a precise voltage, you could use a linear regulator placed physically very close to the components. This would re-process the output of your SMPS and provide a low-noise tightly regulated voltage at the device(s) that need it.


I'm not sure why you'd be seeing a lot of 20kHz noise at the output (our devices switch primarily at 60-120kHz) unless you have some kind of control loop problem or possibly some oscillation/ringing from your post filter. I would definitely evaluate your control loop transfer function as well as probe around in your design and see if you can locate any components in the circuit that are causing this noise.

-The Traveler

Dear Sir,

First of all there is not any cable in the circuit, our power supply is mounted on the PCB of the overall device, so it can be seen as an internal supply...

I meaned by 20 kHZ is the load variation, this is not related with PI device, I thing microprocessor or some other device behaves like a variable load and it pulls current periodicaly, it is not a constant load.

I will investigate high precise linear voltage regulators,

best regards,

Kemred -


In regards to my comment about cabling, I was trying to illustrate that at 3-5 amps of output current, your load regulation spec of +/- 2.5 mV is going to be difficult due to trace/cabling resistance alone. When you add in switching noise, other loads on the output, temperature variation, production variation and component tolerances, a load regulation requirement of +/- 2.5 mV isn't very practical.


If there are only a few components in your design that require this tight of a spec, you might be better off doing post-regulation of your SMPS output at the device that requires the +/- 2.5 mV spec. This is a very common solution in power supply designs with strict load regulation and noise specifications.


If you have devices like microprocessors connected to the output of your power supply, it could definitely explain the 20kHz noise you're seeing. Increasing the size/qty of your decoupling caps could help with this noise.


A commonly overlooked area of designs with microprocessors is PCB layout. PCB layout can have a big impact on how high-frequency noise gets spread through your design. Creating separate digital and analog ground areas with a star topology can sometimes be helpful in these situations. A quick google search for "high speed digital PCB layout" should bring up a lot of resources to look at.

-The Traveler

I have also 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?

Hi Kemred,
Please send your schematic so we can suggest more.
For example, if your flyback is primary sense regulation then this is definitely bad.

Also, usually as the load increases, the ripple also increases(high freq) and this may affect your regulation as your feedback may be tricked by the output ripple. So it is very important to keep output ripple at a minimum.

As for the transient load of 20KHz, you can improve your feedback system thru the frequency compensation if you are using reulators such as TL431.

Please let me know if it helped.

Regards.

Hi pewdsbheem,

Obviously you have a different problem.
You should be debugging your board for errors. Keep your circuit at a minimum and removed unnecessary circuit for the meantime. 48V to 32 V regulation suggest that the flyback circuit not working properly. You have to check the switching waveforms as you increase the output load.

You can also check your transformer start and finish in every winding as it is usually the culprit when you have this kind of problem.

Let us know if you have any update.

Regards.