current limit changing with load for tinyswitch

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We are using TNY280GN. It is skipping pulse due to light load and switching at 32 to 48 KHz. I expected the current limit to reduce as per the datasheet which says current limit reduces when load is medium to light. But when I look at the secondary winding current for 5 V I can back calculate the primary current to be 850 mA which is full rating. My load is 2W (estimate).
Datasheet does not have numerical values/graph to tell user how much does the current limit reduce as load comes down.
I would like to have current limit reduced so that I will get less ripple current on the filter capacitor.

Hi,

Thank you for using power integrations product.
To help you with your concern,
Is it possible for you to provide the schematic?
The current limit usually depends on the BP capacitor value.
Is it possible for you to provide switching waveform of the primary current.

Is it possible for you to provide the schematic?
attached.
The current limit usually depends on the BP capacitor value.
I understand that. Datasheet says on page 6
"At medium loads, cycles will be skipped and the current limit will be reduced (Figure 8)."
I want to know what is medium load and how much current reduction happens ?
Is it possible for you to provide switching waveform of the primary current.
I don't have primary but I have secondary current(attached-blue color channel voltage across 0.1 ohm resistor) on 5 V output and turns ratio is 40 to 6.

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power schematic92.96 KB
Secondary_current.jpg266.79 KB

Hi,

can you please send again the schematic, the image file resolution is too low that I cant see
the values of the components. The Current Limit for mid load is around 70% of the device current limit.

https://www.power.com/sites/default/files/PDFFiles/seminars/TinySwitch-III_07052006.pdf

Please refer to the current limit state machine operation and the Device current limit table

Hope this helps.

Schematic attached.
Can you tell what is "light load" and medium load" for TNY280 with 10 uF capacitor for current limit ?
How does the tinyswitch determine load is medium or light ? Is it step change from 100 % to 70% to 40 % or it is continuous change ?
In the PPT slides you shared there is example circuit for DVD player power supply. Filter capacitor is missing on 5 V winding before the choke L3. Is that intentional or typo?

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schematic.jpg212.76 KB

Hi,

Can you also send the Transformer design? if you have the PI expert version the better.
What is your maximum operating current?
Light load means that the operating current ( connected to Electronic load) is minimum. Like wise Full load means the converter is operating on maximum current, eg. for 5V 2A design, 2A is the full load.

The switching frequency will be proportional to the load.

You can expect that in light load conditions the switching freq is low and the current limit will also be at minimum around (40%). When the operating load is increased the current limit change will be a step change.

The output voltage regulation will be maintained through the feedback circuit on the Enable Pin.
During normal operation, switching of the power MOSFET is controlled by the Enable Pin.

Normaly in different load conditions, current will flow in the optocoupler LED, This will cause the transistor of the optocoupler to sink current.When this current exceeds the ENABLE pin threshold current the next switching cycle is inhibited or skipped. When the output voltage falls below the feedback threshold, a conduction cycle is allowed to occur and, by adjusting the number of enabled cycles, output regulation is maintained. As the load reduces, the number of enabled cycles decreases

In the DVD power supply example, Filter capacitor in 5V winding is C8 (10V 1000uF). L3 and C11 is the post filter for High frequency switching noise.

While I gather information for you can you answer my question ?

"How does the tinyswitch determine load is medium or light ?"

What is the sensing mechanism ?

Hi

The sensing mechanism is through the feedback circuit connected on the enable pin.
Whenever a load is applied, we expect that the output voltage will drop, so the controller will decide to skip switching cycles or not.

The internal clock of the TinySwitch-III runs all the time. (132khz). At the beginning of each clock cycle, it samples the EN/UV pin to decide whether or not to implement a switch cycle, and based on the sequence of samples over multiple cycles, it determines the appropriate current limit

For example, the load is high and the current is operating at full load (100% iLim) and then suddenly we decreased the load, we expect that the switching cycles will be skipped, the no of switching cycles skipped are sampled and based on the sequence of samples, the controller desides to lower the current limit (70% or 40% iLim)

Normaly in different load conditions, current will flow in the optocoupler LED, This will cause the transistor of the optocoupler to sink current.When this current exceeds the ENABLE pin threshold current, the next switching cycle is inhibited or skipped.