LM5030: Startup issue due to output capacitor inrush current

[Arbit1]

Hi, I was designing dc-dc converter using LM5030 , the output specifications are

Output 1: 13V, 4.9A

Output 2: 13V, 3.8 A

the primary input voltage is 74V and the estimated primary continuous current is 4.3A

so for overcurrent protection is was going to set the current sense resistor connected to current sense pin to stop the ic for current above 5 A on the primary side

The issue is as per the simulation the inrush current on the output capacitor was 22 A so the corresponding increase in current demand would be reflected on the primary side which would exceed the set current limit hence would stop the ic and hence the converter wouldn't work at all

I tried increasing soft start time by increasing soft start capacitor value but didn't affect the output inrush current

Any solution to this?

 

[KlausST]

Hi,

Show your complete schematic, with all part values, voltages...
You may also show your simulation results, if you have.

One common way to reduce power up current is to connect a resistor from V_out to FB. Don't know if this is a suitable approach for your circuit.

Klaus

 

[FvM]

Sounds like you basically misunderstanding current limiting behaviour of the device. I would also consider to set reasonable softstart time.

 

[cupoftea]

is your output cap too big?
Is your source current limit resistor too small?

 

[Arbit1]

The schematic:

simulation result:

 

[KlausST]

Hi,

Your schematic is a bit messy. Please correct placement and wiring of R1, R2, C1 ... and similar confusing wiring.
Mind: a schematic should be easy to read.
--> Usually one chooses meaningful component placement to get short wires and low wire crossing count.

Who designed this schematic? Show us the original schematic or post a link.
The schematic differs a lot from datasheet schematic. It surely can't work this way. There are a lot of issues.
Most of all: Where are your MOSFETs?

Klaus

 

[Arbit1]

Hi,

Your schematic is a bit messy. Please correct placement and wiring of R1, R2, C1 ... and similar confusing wiring.
Mind: a schematic should be easy to read.
--> Usually one chooses meaningful component placement to get short wires and low wire crossing count.

Who designed this schematic? Show us the original schematic or post a link.
The schematic differs a lot from datasheet schematic. It surely can't work this way. There are a lot of issues.
Most of all: Where are your MOSFETs?

Klaus

I will sort the schematic a bit to make it look less messy
And i haven't added the mosfet since there is some issue with ltspice, I am not getting the output when I attach mosfets , simulation runs when directly connected

 

[KlausST]

Hi

simulation runs when directly connected

Datasheet says they are "gate driver outputs" = not meant to drive transformers directly.

Did you draw the schematic on your own?
Why are there so many (I'd say more than 20) modifications with respect to the datasheet schematic.

Klaus

 

[FvM]

A real LM5030 would immediatey burn when connected in this circuit. I won't expect however that the simulation model gives realistic output for any possible misuse. Respectively the shown results are just meaningless.

 

[Easy peasy]

Why is there no SS, soft start cap in the schematic ?

 

[KlausST]

Hi,

Why is there no SS, soft start cap in the schematic ?

Hard to see: it's C1.

Klaus

 

[Easy peasy]

Ah, thank you - it's too small anyway, to be of much use ....

 

[mtwieg]

The schematic:
View attachment 171515

simulation result:
View attachment 171525

Schematic nitpicking aside, I don't see anything remarkable about these waveforms. You've set a 5A peak current limit on the primary side, and with Vin=74V that means peak power will be 370W. Your waveform shows secondary side current and voltage, and it seems that the peak current reaches 22A when Vout is around 6V, so peak output power is 132W. Totally within the limits you've set.

 

[KlausST]

Hi,

there are so many issues....(thus in my eyes it´s far from nitpicking)
One thing that the current limit circuit can not work is that the diode is connected the wrong way.

And the missing MOSFETS make the tranformer drive signal to be the wrong polarity. The outputs are LOW on overcurrent (Both MOSFETs OFF) but here they cause a short circuit.

It´s just a waste to time to discuss about a circuit with so many mistakes.
--> The schematic needs to be corrected first.

Klaus

 

[Easy peasy]

obviously you have to charge up the output cap at start up - the quicker you do this - the larger the peak currents in the system at start.