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Control loop stability Innoswitch3-EP

Posted by: Ralf Negele on 11/01/2021

Hello

We testing an Flyback 24V/60W with high line input. We saw that the device always change the operation point.

In the first part of the measument, the current could be higher, in the second part the current should be lower. The

change of the operation point is relatifly large. I couldn't find much how could we adjust the control loop. The voltage

overshoot is higher as normal (additional inductance of the current measurement). We saw a D-part parallel to the high

side resistor of the voltage divider, but no informations about it. Do you have an idea how could we optimize the control 

loop.

Thank you

Ralf

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Submitted by Ralf Negele on 11/01/2021

I saw if I change the device from INN3678C to INNN3670C to reduce the conduction losses, this have a large influence to the design with PI Expert, why?

Thank you

Submitted by PI-Jedidiah on 11/09/2021

Hi Ralf Negele,

->InnoSwitch3 start-up frequency is approximately 25kHz, after that, it will ramp-up its FSW dependent on the output power.

Figure 6 in datasheet is the graph between Ilim (primary current) vs FSW of operation. The higher your FSW of operation, the higher Ilim will be.

-> INN3678C has higher RDSon vs INN3670C. INN3678C has lower Ilim vs INN3670C so it will affect that computation generated by PIexpert.

Regards,

Jedidiah

 

Submitted by Ralf Negele on 11/09/2021

Dear Jedidiah

Thank you for your answer. At startup we have a "normal" operation without changing from continuous to discontinuous mode. I couldn't find out why the device change the mode. The test with the INN3678 was only a test, but this insn't the problem. What could influcene the operation mode at DC-Input and constant load? For the control loop we have not much informations.

Thank you and best regards
Ralf 

Submitted by PI-Jedidiah on 11/09/2021

Hi Ralf Negele,

If you have low input voltage, your flyback would most probably be in continuous mode (this depends on your KP, KP<1 can make the flyback be in CCM mode). But for high input voltage, your operation will change to discontinuous since you don't need CCM to deliver the same amount of output power at high input voltage.

Regards,

Jedidiah