How can we increase soft start time for VIPER319XDTR?

[Kshah.21]

Dear sir,

We have followed STEVAL-VP319X1B development board schematic for converting 110V/230V AC to 5V DC. In one of board we found that inrush current limiting resistor is burn out. We have followed same part number for resistor which is in development kit BOM. Can you suggest how we can increase soft start time.

Thanks

Kadam

[Peter BENSCH]

The soft start time of the VIPER31x is fixed to typically 8ms, as mentioned in its datasheet, table 7.

What about the parameters of the other components, like input filter capacitors and buck inductor (saturation current)?

Regards

/Peter

[Kshah.21]

Dear sir,

We have followed same schematic and BOM as per STEVAL-VP319X1B development board. Input capacitors are 10uF, inrush current limit resistor 10E,1W , input inductor saturation current is 370mA, output inductor saturation current is 1.38A.

Thanks

Kadam

[Kshah.21]

Dear sir,

We have followed same schematic as per development kit for Viper 319. We are facing issue for inrush current. In our prototype inrush current limit resistor is burn out due to higher inrush current. As we have simulate development kit schematic in Orcad Pspice. We found that pick inrush current is up to 20A. Please suggest how we can reduce inrush current?

[Kshah.21]

Hi Peter Bensch,

In our proto, we sometimes get issue of burning resistor R1 during power ON-OFF and we are getting inrush current of 5A. Can you provide some suggestion for controlling input inrush current?

Thanks

Kadam

[Peter BENSCH]

As mentioned before the VIPer319 limits the inrush current with the built-in soft start function, which is internally set to 8ms.

The large inrush current may therefore come from the two input capacitors. To be on the safe side, please check again whether you actually only have 10µF for both of them, as well as 1mH for L1?

Some possibilities to limit the inrush current:

• reduce C1
• increase L1
• substitute R1 by a e.g. PTC

Regards

/Peter

[Kshah.21]

Hi Peter Bensch,

Below is snapshot of actual schematic.

If we keep R24 10E,1W we have observation that with AC input 230V,50Hz, sometimes R24 gets damaged.

When we replace R24 with 10E,3W. It works fine and observation is that inrush is still 2 to 6A.

Later on analysis we have found that Max inrush across R24 is observed around 2 to 6A with pulse time period of 5-6mS.

As preliminary solution we are planning to increase wattage of R24 to 5W.

Please share your view regarding this.

On the other end we planning to reduce input capacitance by non populating C23.

What is minimum recommended input capacitance value for faithful operation of VIPER319XDTR?

Thanks

Kadam

[Peter BENSCH]

You forgot to insert the circuit diagram.

However, have you ever considered using a PTC instead of your current limiting resistor (R24)?

Regards

/Peter

[Kshah.21]

Hi Peter Bensch,

Please find attached schematic snap shot. However we have pasted the same in mail body earlier.

So far we have not consider PTC yet.

Thanks

Kadam

[Peter BENSCH · Accepted solution]

OK, now the schematic is visible.

C22, L2 and C23 form a low-pass filter to remove high frequency switching signals that would otherwise have an impact on the network. Instead of removing the capacitors or reducing their values, it may be better to use a PTC instead of R24.

Regards

/Peter