InnoSwitch3-EP Start-Up Ton Time

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According to InnoSwitch3-EP datasheet, at start-up, the primary MOSFET current is limited to 75% of the programmed "ILIMIT" at 100 kHz. Apart from that, I noticed the primary MOSFET "Ton" during start-up is limited to a maximum value of ~7 μs...

I was surprised by that since this information is not in the datasheet and I hadn't expected the maximum MOSFET ON time should be so short at start-up, mainly because the switching frequency during it is only 25 kHz. In my case, this limits too much the initial power at low input voltages, which, by its turn, doesn't allow the power supply to start at full load.

Please confirm the information about the referred "Ton" limit at star-up and tell me if it will be changed in future silicon revisions.


Hi Christos,
As you said in your first statement, the frequency and Ilimit at soft-start is in the datasheet of InnoSwitch3-EP. It is publicly posted.
Do you encounter any problem during full-load start-up at low input voltages? Start-up frequency of 25 kHz and current limit of 75% only happened at a very short time (in ms time frame) when the primary is still in control. After BPS was charge to its VBPS level (see datasheet), secondary will take control. This is called handshake portion in the datasheet. The frequency and current limit will exponentially ramp up especially at low input voltages to deliver power. All this cycle happened in millisecond time frame.
We haven't encountered full load start-up issue related to 25kHz and 75% current limit (before handshake) in all existing projects. If you happen to have one, can you share with us your data/waveforms so that we could check?

Thank you for the explanation.

I had to study thoroughly the datasheet and, so, I'm reasonably familiarized with it.

Yes, I have an issue related to "25kHz and 75% current limit (before handshake)" at full load and low input voltage. My case is a little different from the majority: input required DC voltage range is 38 to 370 V (yes: Vmin is 38 V).

The only doubt I have right now is if the "Ton" time during start-up, or "25kHz and 75% current limit (before handshake)", is in fact limited to a maximum value of ~ 7 μs, as I observe in my prototype (but I can't find in the datasheet).


Please let me give you two additional pieces of information:

1) I'm aware of the 50 V minimal InnoSwitch drain voltage for its guaranteed operation. Due to that, I use a start-up regulator which supplies the required voltage to BPP pin at low input voltages, as related in "" discussion.

2) The reported 7 μs limit value of "Ton" also occurs at input voltages higher than 50 V.

Hi Christos,
Aside from your DC voltage range of 38V to 370V, can you also share your desired output power? (Volts and current).
This tool will you analyze what parameters is being violated in a very wide range design.
For very low input voltage, InnoSwitch has current limit and and on-time will depend on your transformer inductance (V=L di/dt).
ton(max) in the datasheet will only be reached if your inductance was high enough at 38VDC input.