INN) 3977 Fail open capacitors for BPP

Hello Sarvesh,

Good day and thank you for choosing Power Integrations.

Fail-open capacitors are recommended if two BPP capacitors in parallel are used to comply with functional safety requirements. This is because in the event that a capacitor cracks open—or fails open—there will still be another BPP (decoupling) capacitor in the circuit that will ensure that the response of the IC is still safe. If there will be no capacitor across the BPP pin, there will be no decoupling capacitor for the internal circuitry, and we cannot ensure its operation, which may lead to failure. That's why we highly recommend using two capacitors in parallel for automotive applications.

According to the InnoSwitch3-AQ design guide (AN-106), use a parallel combination of 100 nF with 470 nF for the STANDARD current limit and 100 nF in parallel with 4.7 mF for the INCREASED current limit. Note that if the 4.7 mF capacitor fails (assuming the capacitor cracks or fails open) in the latter combination, the current limit of the unit will revert to STANDARD during the next power cycle, resulting in reduced power delivery capacity.

I hope this answers your question. Let me know if you need anything else or have any more questions or clarifications.

Regards,

Tommy

Hi,

INN3977 is working across wide inout range and wide output power range.

So, for a particular Input-Output condition, the duty ratio is fixed ( say for 100-300VDC input & 15V 5W out put D@100VinDC=0.108 D@300VinDC=0.034 from the tool). So , Xer is designed accordingly. Am I correct?

Or can we increase the duty cycle to say, 0.45 @100VinDC somehow? If yes, please explain how to do that.

Because, in traditional design of flyback, we consider Dmax=0.45 (45%) (Vmin and Full load). Here, duty ratio is not going beyond 10.8% . Of course, as long as the converter is in within regulation, there is no issue. But, if there any way to increse the duty cycle from 10.8% given by tool, please let me know.

Attached the PIXLS file for reference.

Regards,"
Sarvesh.

Hello Sarvesh,

Unfortunately, you cannot modify the duty cycle of the device. The InnoSwitch3 family of devices (including the INN3977) doesn't use the traditional PWM control. Instead of varying the duty cycle, it varies both the current limit and the switching frequency of the InnoSwitch3 device depending on the design’s input and output condition. As load increases, both current limit and switching frequency increase as well. Therefore, you can expect that at minimum input voltage and full load condition, the switching frequency and current limit are at their maximum value. For more detailed information, you can refer to page 5, "Current Limit Operation", of the InnoSwitch3-AQ datasheet and to the attached image.

You're correct that the transformer should be designed accordingly depending on the input and output specifications of the design. That's why we highly recommend using our PI Expert Online tool when designing with our devices to see if the design will pass and is within our recommendations.

I hope the information above helps. Feel free to contact us again if you have any further questions or clarifications.

Thank you and have a great day ahead.

Regards,

Tommy

Hi,

Thanks for the clarification.

Duty cycle is decided by Input and output conditions ( As the device operates wide input range 30VDC=800VDC input and 10W to 30W output) only. Is my understanding correct?

Else, what are the parameters device consider to decide the duty cycle?

Thanks!

Sarvesh

Hello Sarvesh,

I understand that in a traditional PWM control, duty cycle is decided by input and output conditions. However, as I mentioned in my last reply, the InnoSwitch3 family doesn't use the traditional PWM control and is using a simple ON/OFF control (non-linear). This means that instead of changing the duty cycle, the InnoSwitch3 family varies both the current limit and the switching frequency of the IC to be able to deliver the required output power at a certain input voltage. For example, you can expect that at minimum input voltage (let's say 30 VDC) and full load condition, the switching frequency and current limit are at their maximum value.

Please refer to the attached image for more information regarding this one.

Thank you and I hope that this clarifies the confusion. Let me know if you have any further questions or clarifications.

Best Regards,

Tommy

Hi,

I may not express my query properly in the previous message.  What I am having doubt is 

For my input=100VDC-300VDC, VO=15V, 5W  Duty cycle from the tool is : D@100VinDC=0.108 D@300VinDC=0.034. But, this is not the duty cylce for other input/output conditions, I think. Can you explain What are the parameters decides the duty cycle for Inno Switch?

Thanks!

Hello Sarvesh,

Thank you very much for the clarifications.

If you want to know the duty cycle of the InnoSwitch3 inside the PI Expert tool at a specified input voltage like 100 VDC and 300 VDC, you should scroll down to the "Input voltage set-point analysis" section at the bottom of the tool. Please refer to the attached image.

So, the parameters that set the duty cycle of the InnoSwitch3-AQ device are as follows and can be found in the PI Expert Online tool.

1. VOR = the reflected voltage across the primary winding when the primary switch turns off.
2. VINDC = specific input voltage.
3. VDRAIN_ON_PRSW = the primary switch on-time voltage drop. 
4. KP = a measure of how discontinuous or continuous the primary current waveform is for a specific operating point.

The calculation of the duty cycle for the InnoSwitch3-AQ device is as follows and depends on which mode of operation it is operating in.

• If the design is in Continuous Conduction Mode (CCM), DUTY _CYCLE = VOR / (VOR + VINDC - VDRAIN_ON_PRSW)
• If the design is in Discontinuous Conduction Mode (DCM), DUTY_CYCLE = VOR / {VOR + [KP * (VINDC - VDRAIN_ON_PRSW)]}

I hope this answers your question. Feel free to reach out again if you have any further questions.

Thank you and have a great day ahead.

Best Regards,

Tommy