Queries regarding component selection in PI expert tool

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Dear Sir,

I am making a new power supply with following configurations:

1. Input: Universal
2. Output: 12V,2.5A (30W) for industrial application

Device used is Inn3678C, 750V MOSFET.

Certain queries are there as highlighted in the schematic generated.

1. Point 1 (highlighted in image attached). How the thermistor is selected. (value and wattage) can you share the mathematics involved behind this.
2. Point 2 (highlighted in image attached). How the Primary Snubber component values is selected. (value and wattage) can you share the mathematics involved behind this. Lots of articles are present on this topic, but the values coming are different.
3. Point 3 (highlighted in image attached). Can this design be made without BIAS circuit. If yes, then how to do it in the tool.
4. Point 4 (highlighted in image attached). How this value is computed along with the wattage. In the datasheet it is recommended to use 47 ohm part. but how that part is selected. Please share the mathematics behind it.
5. Point 5 (highlighted in image attached). How the Secondary Snubber component values is selected. (value and wattage) can you share the mathematics involved behind this. Lots of articles are present on this topic, but the values coming are different.
6. Point 6 (highlighted in image attached). What factors are taken into consideration while selecting this capacitor.

Since lot of literature is available in the calculation of primary and secondary snubber and with those, the values coming are different, so share the mathematics used in PI expert tool. As i think lot of mathematics is taken into account to compute these values by PI tool.

Thanks
Manish Sharma

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Hi Manish,

Thank you for reaching out to us and using our product as well. You are asking for confidential information and the assigned engineer is currently verifying the most appropriate way to help you on your inquiry. Please give us some time and we'll get back to you shortly.

Thank you for your understanding.

Best Regards,

PI-Lupin the Third

Hello Manish,

thank you for your patience in waiting for the answers.

1. The thermistor value is computed by setting a limit on the inrush current appropriate for these designs and then accounting for the voltage drops across the capacitor and the input rectifier. The thermistor is designed so that both the bridge and the input bulk capacitor don't exceed their maximum current ratings during start up. There is a warning message in PI Expert if the thermistor is too small and the inrush current is exceeded.

2. The Clamp circuit is computed based on estimate of the energy of the voltage spike on the drain node of the PI device in worst case conditions (at the maximum input voltage and highest leakage inductance). Some of the parameters of the circuit parameters are based on bench measurements of similar designs. The exact magnitudes of the spikes are difficult to predict, but we focus on ensuring the safe operation of the design in all normal conditions.
I am not sure which calculations you refer to. Perhaps you could provide a link to them and I can compare the results? If the clamp circuit seems too aggressive (low resistor R1), please make sure that the leakage inductance is set appropriately. We often saw this issue if the leakage was set too large.

3. Primary side of the InnoSwitch requires a power source to operate. Power Integrations recommends a Primary Bias Circuit for all designs that use InnoSwitch.

4. The design team at Power Integrations has performed extensive simulations and bench testing and selected 47 Ohms on the FWD pin for correct operation and timing of the Synchronous Rectifier drive. Choosing a lower or higher value may affect device operation and is not crecommended.

5. The secondary snubber values depend on the voltage spike on the secondary side. They are designed to limit the voltage spike to a certain magnitude and to dissipate it within a particular time frame.

6. Please see AN-15: https://ac-dc.power.com/design-support/product-documents/application-notes/an-15-topswitch-power-supply-design/ page 8 on the factors used by PI to select the Y-capacitor.

Calculations of the primary and secondary snubber in PI Expert come down to estimating the magnitude of the worst-case voltage spikes on the windings and sizing the components to restrict them to the appropriate limits.