MP9488 in Buck boost Mode

DC-DC Power Converters
1

Hello ,
I am trying to use MP9488 ic in buck-boost mode as mentioned in the datasheet of the same .
Vin range is 50 to 143 volts and output required is 74 volts with 150mA continous current . i have few doubts reagrding this .

  1. can MP9488 be used in buck boost mode as mentioned in the datasheet of the same for above Vin range requirements ?
  2. what switching frequnecy MP9488 has for nominal load ( 150mA) ?

Best Regards
Rohan

2

No, in buck-boost mode the output is inverted compared to the input. So it for sure can convert 50 - 143 Vin to -74V but not to 74V. If you need 74V you need an isolated flyback, or might I suggest a nice SEPIC converter. You will be very near the max current capability of the part

3

Inverting output can aslo work for me .

  1. Do you have any idea about the switching frequency of converter when its operated for nominal load like 150 mA in my case ? that frequency is necessary to caculate the inductor value .

  2. This part has peak current limit set to 780mA . so for 150mA , if i assume 40 % peak inductor current ( 210 mA) still it does not cross 780 mA limit . then how the part will be very near to max current capacity. ?
    please provide your feedback .

Thanks Rohan

4

Not really, but the first thing you have to understand,is that in the buck-boost topology the current in the switch is Iin+Iout so for 74V in and -74Vout with 150mA of load your Switch current would be 300mA or so plus the ripple.

Reading the datasheet it looks like the part turns on for the lesser of 10usec or until the current limit is reached. Furthermore there is a minimum off time of 13usec. So that sets an fmax of about 77khz. The minimum frequency? I dunno as the load gets lighter and lighter the pulses space out and the frequency drops through some hidden MPS magic the peak current is also reduced in an attempt to prevent frequency dropping into the audio band and to improve efficiency. It looks like it may drop to about 150mA peak. Which drops the energy per pulse by about 20X.

At a guess, at 40V in at full load on the op 11.25W You get an Iin of 280mA . The inductor current will be Iin + IOout 430mA . With a peak turn off off 620mA the inductor is going to have about 380mA of ripple. To get a delta I of 380ma in less than 10usec with 40V the inductor needs to be about 950uH. Pick maybe 1.2mH with an Isat of above 750mA and you should be good. Once the inductor falls beneath about 1mH you run the risk of not being able to support the load at Vmin.

1 Like
5

Hello Sir ,

I have following doubts regarding inductor design

  1. calculation of inductor done at minimum input voltage will result in DCM ( ripple current increases ) when input voltage is at its higher value of 143V ?

  2. when inductor current ( Iin + Iout) decreases for maximum input voltage 143 volts , that condition IC will reduce the switching frequency ?

  3. calculation of inductor at minimum input voltage works for when input voltage is maximum ?

Best Regards
Rohan

6

Hi, yes likely the inductor will go DCM under light load and high vin. Nothing should be harmed by that.

The chip appears to control its output power by a combination of peak current reduction and switching frequency reduction. So yes switching frequency will change.

The problem with getting enough power out of this circuit is at minimum Vin. If we can get the power we need at Vin min then Vin max is no problem.

but I am just some guy on the internet so there is that.

7

Thank you sir for the insight and providing your useful comment’s. I will build the proto and check the actual performance .