Why does DC-DC Buck Converter MC33167's Output voltage drops down on load increase?

[yourdreamz]

I'm using MC33167 from On-Semi as DC-DC buck converter (with 190uH, 5A Iron powdered core inductor, SB540 5A Schotkey Diode and 4700uF, 50V Electrolytic Cap as other elements). I've attached the schematics for you reference.



Now I noticed that when I'm feeding the MC33167 from a 24V battery (through Vbat) it allows upto 6A of output load current but when I supply it from a Transformer-Bridge-Filter power supply of 33V (through Vin) it allows only upto 4.6A of load current and then starts reducing the output voltage when tried to increase the load, to maintain the current at around 4.6A.

Any idea why this could be happening? Is it because of the feature 'Cycle-by-Cycle Current Limiting' that acts more for higher input voltages? Could anyone please help?

 

[dick_freebird]

The datasheet shows the current limit declining with
temperature, perhaps your increased VIN is raising the
power dissipation in the high side switch and pushing
the limit value lower?

The part advertises a 5A current limit so you're not
too far from that. Your operating load wants to be
well less than limit value.

 

[RCinFLA]

4700 uF filter cap on the rectifier output is going to give you over 10v ripple on input to switcher. You need to supply the peak coil current from the filter cap. Alternately, the coil may be marginal for the peak current depending on input to output voltage conversion. The greater the difference between input and output voltage the greater the peak coil current.

You did not mention what the output voltage is.

 

[yourdreamz]

The datasheet shows the current limit declining with
temperature, perhaps your increased VIN is raising the
power dissipation in the high side switch and pushing
the limit value lower?

The part advertises a 5A current limit so you're not
too far from that. Your operating load wants to be
well less than limit value.

Well the converter is attached with adequate heatsink and this thing happens from the beginning, when neither the device nor the heatsink is even warm. So doesn't look like a temperature play here, I think. Even after continuous use for few minutes on full load the device doesn't get very hot.

 

[yourdreamz]

4700 uF filter cap on the rectifier output is going to give you over 10v ripple on input to switcher. You need to supply the peak coil current from the filter cap. Alternately, the coil may be marginal for the peak current depending on input to output voltage conversion. The greater the difference between input and output voltage the greater the peak coil current.

You did not mention what the output voltage is.

Thanks for the reply RCinFLA. The Converter is designed to give 24V output.

Now I am little confused with your reply. Do you mean to say that because at 5A and 4700uF filter Cap, the 10V ripple would not allow the coil to charge to the peak thus this might be happening? Do you mean that if I increase the capacitance to say 9400 uF and so ripple would reduce to say 5.3V, would this allow more current for my load? Please clarify.