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LM5069 reset on input voltage step

JUAN VALENCIA
Prodigy 50 points
Other Parts Discussed in Thread: LM5069

Hello

 

I had design based on LM5069 working properly up to now.

Basicallly is a DCDC converter from 48 to 24 (75W)

 

First stage of the power supply is the inrush controller based on LM5069-2

Current Limit 5A

Power Limit 25W.

 

My problem appears when input voltage switches from 48vdc to 56vdc once the power supply is working. The basically the LM5069 restarts and the power supply also does it during the insertion time (aprox 1 second)

 

The power supply input comes from 2 sources:

A 48VDC power supply (through a diode)

A 48V battery (which is around 56 volts on some circumstances.) through a relay.

 

The switching from one to another input is made by a relay (kind of UPS), so the step from 48 to 56 is practically instatntaneous and with no ramp (few microseconds).

 

 

I checked the 5 protections of the IC in order to find where the fails comes from

Current Limit

Power Limit

Circuit Breaker

UVLO

OVLO

 

The issue is that measuring the timer pin I can see that just inmediatly (2-3 microseconds) after 56 volts is present the insertion ramp starts and when reaches the 4 volts the circuit starts again and correctly.

(Also gate swithces off and the output of the mosfet and power good signal)

 

So if I am not wrong in any failure mode the fail ramp (which is much faster than the insertion one) should start and then a cycle of restart in case of fails remains. But I definetley see the insertion ramp.

 

I had no clear if OVLO and UVLO worked in the same way (fail ramp) so I disabled them by grounding OVLO and pulling up UVLO.

Same result.

 

I solved the problem with a serial inductance (330microH) in the input, but I would like to understand the behaviour.

 

Even more if the inductance is lower (100u) the behaviour is slightly different.

Just inmediatly after the 56 volts is present the gate switches off but timer pin remains low.

Quickly gate starts again and inrush works again limitng the I and rising Vo up to 56volts (30-40 miliseconds). No activity at the timer pin, but power good signal cycles (as Vds is higher than threshold) this time switching off my second stage.

 

 

Could you give me any idea why this restarting cycles could be activated at the LM5069?

 

Any advice would be welcomed.

 

Regards

 

  • 0
    TI__Mastermind 41800 points

    Hi Juan,

    Here are my initial thoughts. The fast transient from 48V to 56V as you describe is creating a very large inrush current that is causing the LM5069 to shutdown (circuit breaker). Adding some inductance,as you have seen, would help limit the surge current.

    Regards,

    Darwin

  • 0 in reply to Darwin Fernandez
    Prodigy 50 points

    Hello Darwin,

     

    Yes it was a first idea, but I checked the timer pin, and as described, it starts a insertion cycle instead of a fault cycle. If I am not wrong the datasheet describes a fault cycle when circuit breaker protection acts.

    So I have no idea why this restart is happening.

    The inductance solve the problem partially, as I measured the insrush current waveform when inductance is applied. It is all but "nice". It is plenty of oscillations, no high peak current, so inrush limit is "working" but nothing similar to the waveform that is produced when no inductance is present.

    I will continue testing.