Hello Jayashree,

Good day and thank you for continuously using Power Integrations products.

Can you confirm what input voltage have you measured the 85°C thermal data of the IC. I assume that this is at 65 VAC input. Please correct me if my assumption is incorrect.

It would also be great if you could share the full schematic, PI Expert file (or link), and the transformer datasheet of this design so we can cross-check what can be improved further in the parameters.

For the time being, one thing I can suggest is to implement a bias winding to externally supply the InnoSwitch-EP IC. This prevents the BPP capacitor from being charged internally to Vbpp (5V) by drawing current from the voltage on the drain pin whenever the power MOSFET is off. The result might not be that significant compared to increasing further the heatsink copper area, but it still helps with the thermal signature of the IC and, especially, the overall no-load power consumption of the design.

I will be waiting for the PI Expert file and the transformer datasheet. Thank you and have a great day ahead.

Regards,

Tommy

Hi Tommy,

Thanks for responding. 

The input voltage during the temperature rise test was 230 VAC.

I don't have the PI expert file for this design, but I have attached the pdf of the full schematic made on Allegro.

I have attached the datasheet of the core used, and the transformer specifications that is present with me.

Also, this board is a 2-layer PCB. Would increasing the number of layers help in mitigating the heating issue.

Kindly let me know if you need any further information from my end.  Have a good day!
 

Regards

Jayashree

Hello Jayashree,

Good day. Thank you for providing the documents.

Since you don't have a PI Expert file for this design, I have tried to replicate it in the PI Expert online tool, and it shows warnings on the total input capacitance, maximum flux density, and exceeds the maximum duty cycle of the device. We don't want this kind of warnings as it will affect the overall performance of the device and the whole circuit. Higher thermal signature is one example that might happen when the design is not in line with the PI Expert and our design guidelines.

Here are the following points that you need to consider, especially with a very low input voltage (65 VAC).

1. Since the input voltage is as low as 65 VAC, increasing the total input capacitance of the design is recommended. A minimum of 112 uF is suggested to be able to deliver the required power at 65 VAC input.
2. Removed capacitor C16 in the design.
3. The connection of IS resistors is incorrect. The IS pin should be directly connected to the negative output terminal (TP3) like in the attached image1.
4. Connect C6 directly to the negative output terminal.
5. I've tried using an EF20 in the PI Expert, however, it is too small to fit all windings. Using a bigger core like EE22 or EF25 is recommended. Kindly refer to the attached file (InnoSwitch3EP900V Flyback Design2) for the PI Xls that corresponds to your input and output specifications. Here's the link to it if you want to modify the design yourself and change core that is available in your side: https://piexpertonline.power.com/piexpert/design/select?share=9528e59956593528f745ed38d27a0050777529e875ab41adf78e0b9784518163
6. Kindly ensure the that all the voltage and current ratings of the components have sufficient margin.
7. You might want to switch to the latest device of the INN2904K, which is the InnoSwitch3-EP family. In the InnoSwitch3-EP family, we can set the switching frequency that we want to use, which helps optimize the design easily.  Kindly refer to the attached file named "InnoSwitch3-EP_24V_0.75A" for your reference to an initial design utilizing InnoSwitch3-EP device. Here's the link as well if you wish to modify it yourself: https://piexpertonline.power.com/piexpert/design/select?share=d0b5eaa1d2a3cc128e00c2a2d6c8eadb83ad156391e682427f537a553c40e8d3. Please note that this initial design is still subject to testing to verify its actual performance.

I hope this helps. Thank you and let me know if you have any questions or clarifications.

Best Regards,

Tommy

Hi Tommy,

Thank you so much for responding.

I have the following questions for each of your inputs.

1. You suggested to increase the input capacitance, could you please specify (with the legends) which exact capacitors you are referring to in the schematic?

2. Should the other end of C16 be pulled to ground or completely removed from the schematic?

   I shall look into the rest of your inputs.

   Thanks once again.

   Regards
   Jayashree
    

Hi Jayashree,

Please refer below for my answers to your questions.

1. You suggested to increase the input capacitance, could you please specify (with the legends) which exact capacitors you are referring to in the schematic?
   1. I'm referring to the input capacitor C3 (please refer to the attached file).
2. Should the other end of C16 be pulled to ground or completely removed from the schematic?
   1. It should be completely removed from the schematic and layout.

Let me know if you have any more questions or clarifications. Thank you and I wish you a great day.

Regards,

Tommy

Hi Tommy,

For the input capacitance C3, what is the maximum value of capacitance I can go ahead with? Also kindly mention the minimum voltage and current ratings that would be suitable for the said application.

Thanks once again for responding.

Regards

Jayashree

Hello Jayashree,

Let's target the value of input capacitance C3 in the range from 112 uF to 136 uF.

Regarding the minimum voltage and current ratings, are you still referring to the voltage and current ratings of input capacitor C3?

Looking forward to your response.

Regards,

Tommy

Hi Tommy,

Could you please explain how you arrived at the range of the input capacitance? 

And yes, I'm referring to the voltage and current ratings of the input capacitor C3.

Regards

Jayashree

Hello Jayashree,

When designing with PI devices, we highly suggest our customer to use PI Expert Online especially with special requirements like very low input voltage. After entering the input and output specifications, the tool automatically calculates and estimates the parameters of the design and shows if there's any warnings that needs to be configured. The tool also includes required input capacitance for the design (line 12) and the goal is to have a warning free design. Please refer to the attached PI Expert file that I generated for your reference. If you want to modify the PI Expert attached, here's the link to it: https://piexpertonline.power.com/piexpert/design/select?share=9528e59956593528f745ed38d27a0050777529e875ab41adf78e0b9784518163

The main reason why input capacitance value is higher at very low input voltage (like 65 VAC) is because it needs to support the required output power of the PSU during startup. Using lower capacitances might result in unwanted triggering of auto-restart function of the device. For the maximum value, there's no hard rule for it. The more important thing to keep an eye on is the minimum input capacitance if it's able to support the output power during startup.

For the ratings of the input capacitor, the following parameters are the ones you need to consider.

1. Voltage rating: The voltage rating should be higher than the peak voltage of the highest input. For your case, the peak voltage is approximately 375 VDC (265 VAC *1.414 V), so the voltage rating of the input capacitor should be 400 VDC.
2.  Temperature rating: In some cases, the required ambient temperature is from -25°C to 105°C. The designer should check the datasheet of the actual input capacitor and verify if the temperature rating of the input capacitor is sufficient and has an acceptable margin.
3. Lifecycle: If the design will operate in a high ambient temperature, the designer should also consider the lifecycle of the input capacitor that you're going to use.
4. Ripple Current: As for the ripple current rating of the input capacitor, it's not that much of a concern in most cases. Any general-purpose or low-ESR electrolytic capacitors will do fine. However, using a low ESR type in the design will result in a lower ripple in the rectified DC voltage across the input capacitor.
    

I hope that this clarifies your question. Let me know if you have further questions. Thank you.

Regards,

Tommy
 

Hi Tommy,

Should I also increase the voltage ratings of capacitor C6? It's currently 2.2nF/300V, I have another part which has 750V with the same capacitance.

Thank you so much for solving all my queries.

Regards

Jayashree

Hello Jayashree,

No need to increase the voltage rating of the capacitor C6. A 300 Vac rating should be fine based on your highest input voltage.

You're welcome and I'm glad to help. If you require further assistance, just let me know.

Thank you and have a great day ahead.

Regards,

Tommy

Hi Tommy,

Thank you so much for the prompt and elaborate responses.

Wishing you a great day ahead too!

Regards

Jayashree

Hi Tommy,

I have made some changes based on your suggestions. To add the bias winding in the primary section of the transformer, kindly verify the connections that I have made. And also suggest what RC values and diode has to be used.

The pdf of the schematic has been attached for your reference.

Regards

Jayashree

Hello Jayashree,

Good day.

Please note that the bias winding should be out-of-phase to the primary winding like the secondary windings in order to be flyback configuration and the recommended bias winding voltage is 12 V. I want to mention as well that the return of the bias winding should not be connected to secondary GND and should be connected to primary GND (specifically, to input capacitor GND).

For the bias diode D10, a standard recovery diode is suggested with a voltage rating of 100 V (typical, and should be adjusted accordingly depending on the actual PIV) and current rating of 1 A. For C21, a 22 uF with a voltage rating of 50 V is recommended. For R25, let's say that the DC voltage across capacitor C21 at no-load condition is 12 V, so by calculating the Rbp (R25) resistor value, it should have a resistance of around 18.8 k ohms.

For full details, kindly refer to page 13-14 of the InnoSwitch-EP datasheet and look for the "Bias Winding and External Bias Circuit".

I hope this helps. Thank you and have a great day ahead.

Regards,

Tommy

Hi Tommy,

Thanks for the response. I have made an updated schematic based on your suggestions. Kindly let me know if it has any issues.

Regards

Jayashree

Hello Jayashree,

Here are some of the findings in your latest schematic that you need to consider:

1. As mentioned previously and according to the replicated PI XLs file I've shared, kindly increase the value of the input capacitance to approximately 120 uF if your input specifications are still 65 - 265 VAC. This is to ensure that the design can startup at very low input voltage.
2. The drain pin (pin 1) of InnoSwitch-EP device should be directly connected to pin 3 of the transformer winding.
3. The value of capacitor C18 is too high. This will result in a slower ramp time of the secondary bias winding voltage. Reducing it to 22 uF is recommended.
4. The IS pin connection is directly connected to the GND. This means that the CC operation is disabled. Please refer to the attached image that I've edited for your reference connection of the 24V_NEG going to the IS pin.
5. Please ensure that the correct dot winding convention is followed in order to operate the design in flyback. Kindly refer to the other attached file for your reference.

I hope that the information above helps. Let me know if you require further assistance or have any more questions and concerns.

Regards,

Tommy

Hi Tommy,

Thank you so much for the response.

Furthermore, I require your help in designing the same circuit, but with INN3694C.

Regards

Jayashree