Transient period over current problem.

[sonerA]

A 24 VDC, 10 A AC/DC converter design has been done using the eDesignSuite on the ST website.

The converter gives a very stable output at 24 VDC. We did not encounter any problems when we used the converter at normal loads below 10 A.

However, when we connect loads whose normal driving current is below 10 A but transient period current is above 10 A to the output of the converter, the converter protected itself by cutting off the output current. For example, when we tried to drive two LED bulbs together, with a normal driving current of 2.5 A and a startup transient period current of around 8 A, the converter cut off the output current and protected itself. At this point, our expectation is that the converter can tolerate short-term transient currents greater than 10 A.

Although it is an excellent feature that the converter we manufactured using ST eDesignSuite protects itself by cutting off the output current at 10 A, this situation becomes a problem for loads with transient period current greater than 10 A.

We have the following questions on the subject:

1. Are there any improvements that can be made to this converter design for loads with normal current below 10 A but transient period current greater than 10 A? For example, can a design change be made in this converter so that it can deliver up to 16 A output current for a 100 milliseconds transient period? If your answer is yes, which circuit element or elements' values ​​should be changed and what should their values ​​be?
2. If your answer is no to our first question, would you recommend us to make a converter with higher output current or change the converter topology from QR flyback to another topology?
3. When we reduce the value of the Rcs resistor (130 mOhm) used in current measurement, will the converter start to cut the output current at a higher current value? If so, does this negatively increase the self-protection threshold value of the converter while allowing us to drive loads higher than the 10 A transient current?

Thanks in advance.

[Patrizia BELLITTO]

Hi @sonerA ,

thanks a lot for your interest in eDesignSuite. I am forwarding your question to the application engineer responsible for this kind of design. I'll let you know the answer as soon as possible.

Best regards,

Patrizia

[Patrizia BELLITTO]

Hi @sonerA ,

I'm copying the answer from the application engineer:

From the controller's point of view it is possible:

1- lower the value of the RCS sense resistor to increase the maximum power at which the converter will work.

2- it is possible to temporarily manage the overload by appropriately resizing the CSS (soft start capacitance) between the SS and GND pins.

In the current scheme CSS=100nF is loaded by an internal current generator ISS1 that determines the soft start time upon reaching the 2V threshold. This same capacitance, during the overload, is instead loaded by an internal current ISS2 =(ISS1)/4 until reaching the 5V disable threshold. The time that ISS2 takes to charge the 100nF capacitance beyond the 5V threshold determines the overload management time. These details are however explained in the datasheet. (5.10, Soft-start and delayed latched shutdown upon overcurrent).