INN4177C - Bugs in PI Expert?

Posted by: christoph.friedrich on 03/09/2026

I am developing a power supply for converting 80V...410V DC to 25V / 7.5A using the INN4177C.

I designed it with PI Expert, had the transformer wound according to PI Expert's specifications, and built a prototype. The PI Expert design is attached.

During commissioning, I noticed three issues that PI Expert appears to have calculated incorrectly:

I specified 470V as the line overvoltage threshold, undervoltage threshold is disabled. PI Expert calculates 1.50M + 1.47M + 40.2k = 3.01MΩ. According to the datasheet, the overvoltage threshold is 115µA into the V pin. However, that corresponds to 3.01MΩ × 115µA = 346V — which is what I measured on my prototype. Is this a bug in PI Expert?
For the resistor on the FW pin, PI Expert specifies 47Ω. However, the datasheet explicitly states that 300Ω must always be used — no more, no less. Why does PI Expert specify 47Ω here?
To reduce output voltage ripple, an RC series circuit is to be connected between the FB pin and the output voltage. PI Expert calculates 1nF and 10kΩ for this. However, this results in very loud audible noise in the frequency range of 1kHz...3kHz, depending on input voltage and output current. Changing the capacitor to 330pF leads to less noise. When I remove this RC network, the noise disappears. Could it be that these values are also calculated incorrectly? Are there rules for how to calculate them? I have not found anything in the datasheet, and there is no application note for the InnoSwitch4-CZ.

I would appreciate your support with these issues.

Best regards

Christoph Friedrich

Files

PI Expert Design of the described Prototype (2.3 MB)

Hi Cristoph,

Thank you for using PI Expert. Please see the response to your inquiries:

This is a bug in both Schematic and GUI displays. The tool should display 1.5 MΩ, 1.5 MΩ, 1.47 MΩ, and 40.2 kΩ in both schematic and GUI. Total V-Pin resistance should be 4.51 MΩ. In the Design Results tab under InnoSwitch4-CZ section, the correct resistances (V-Pin Resistors) are suggested. Using these resistances, the OV range should be from 471 V to 535 V (as displayed in the GUI). The team will fix this bug on the next patch. In the meantime, please use the correct V-Pin resistors in your circuit. Ensure that you use 1206 resistors, especially on the large resistance components, for safety.
PI Expert recommends 47 Ω to ensure sufficient IC supply current especially at start-up condition. Additionally, this ensures optimal synchronous rectifier gate drive duty cycle for better efficiency. A higher resistance value up to 300 Ω can be used to adjust synchronous rectifier gate drive duty and to limit negative current injection. While this contradicts what the InnoSwitch4-CZ datasheet recommends, several Design Example Reports (https://www.power.com/products/innoswitch/innoswitch4-cz#product_resources) using InnoSwitch4-CZ use 47 Ω. I would like to get back to you on this one as I clarify this design rule within the team.
The RC series circuit connected between the FB pin and the output voltage helps reduce output voltage ripple and prevent pulse bunching, especially when the ripple caused by the output capacitance is greater than the ripple caused by the capacitor ESR (i.e. ESR used is very low). This is because the RC network ensures the InnoSwitch4-CZ FB pin signal sees a healthy amount of ripple for proper switching across all load conditions by providing an alternate path for the high frequency components of the output ripple to reach the FB pin. The RC circuit suggested by PI Expert acts as an starting point, and should be tuned accordingly to optimize the output ripple. Audible noise you were observing may be caused some presence of low frequency ripple within audible noise range. Reducing the value of capacitor reduces the bandwidth of the ripple, which potentially eliminates audible noise. If your output voltage ripple requirement is met without the RC series circuit, you can remove that from your circuit provided that your dynamic loading and startup requirements are also satisfied.

Best Regards

INN4177C - Bugs in PI Expert?

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