small voltage drop under load

Hi guys,

I have a design that is very similar to the the one in the demo circuit as per below link

http://cds.linear.com/docs/en/demo-board-manual/dc1929afa.pdf

With my spec being

Vin-170V-350V
Vout24V @1.25A
No secondary sync used

For some reason when I put a load of 5W the voltage is 23.7V and it regulated out the whole input voltage (170-400V)

When I put the load to 10W the output drops to 23.3V and it regulated at that through the whole input range

What could be causing this output drop? Feedback?

is it in voltage mode?..and if it is do you have a type 3 compensator

I think you should investigate the series components after the voltage sensing. Starting from the output inductor ant then the transformer.

I was common for old SMPS designs to require 5% minimum preload to get 0.1% load regulation error. your design seems to be lacking the dynamic range on current & voltage error sense to maintain low errorwith no load.

For some reason when I put a load of 5W the voltage is 23.7V and it regulated out the whole input voltage (170-400V).
When I put the load to 10W the output drops to 23.3V and it regulated at that through the whole input range
What could be causing this output drop? Feedback?

Click to expand...

Depends on where you are measuring the o/p volts if you measure at the same point that the volt control loop picks off the o/p voltage the change will be smaller, if you measure at the load at the end of some wires you will see a droop similar to the one you are seeing...

Hi,

I'm a bit confused by the term "5W load". One could imagine it is an intelligent load to draw 5W independent of voltage.
I never heared of such a constant 5W load.

But I assume the OP..
* either uses an ohmic load (with a fix ohmic value, lets say 115 ohms)
* or a constant current as load (lets say 200mA)

Klaus

is it in voltage mode?..and if it is do you have a type 3 compensator

Click to expand...

It is in current mode and I have a type 2.

Depends on where you are measuring the o/p volts if you measure at the same point that the volt control loop picks off the o/p voltage the change will be smaller

Click to expand...

Would it be by a significant amount if they were on the same rail as picture below, with the voltage loop picking off from the electrolytic point, and the load being connected across the output?

I'm a bit confused by the term "5W load"

Click to expand...

What i meant is that I have used and ohmic load and measured the output voltage, then worked out roughly how much power it is drawing, not specifically put a "5W" on the output.

Would it be by a significant amount if they were on the same rail as picture below, with the voltage loop picking off from the electrolytic point, and the load being connected across the output?

Click to expand...

I presume you have a digital voltmeter. As far as it refers to possible voltage drops, why don't you answer the question yourself by measuring voltages in the circuit operating under load?

Quite generally speaking, there are different possible reasons why the circuit doesn't regulate correctly.

- The feedback doesn't sense the output voltage correctly (e.g. is affected by voltage drops)
- Your own measurement is flawed
- The regulator circuit doesn't work as expected (e.g. not regulate the average value due to inappropriate circuit design)

In latter regard, a statement like "I have a design that is very similar to the one in the demo circuit " is problematic. Nobody knows how the actual circuit is different from the original design and if the change may bring up the problem. Without exact schematics, the only serious answer is:

Find how the circuit works and what causes the problem!

Where exactly are you measuring the o/p volts on your DVM... ?

Where exactly are you measuring the o/p volts on your DVM... ?

Click to expand...

Is i measure across the ohmic load i get around 23.5V, it it is at the point where the feedback comess off it is around 23.6V

How much ground return drop?