Dear Engineers,
The PI3526 Buck DCDC module looks like an Ace from Vicor.
We wish to use it with an external inductor we design ourselves instead of the recommended one by Eaton.
Please answer some questions about the choice of inductor value for the PI3526?
PI3526 Buck DCDC module datasheet
https://www.vicorpower.com/documents/datasheets/ds_PI352x-00.pdf
No Need to read the below if you already know info on inductor choice for PI3526, and may kindly share it here.
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Our Spec:
Vin = 14S (42V-59V), Vout = 12V, 11A.
Inductor choice for PI3526 Buck:
With “nominal” Vin=48V,Vout=12V,Iout=18A,Fsw=450kHz……and the datasheet recommended 480nH inductor, the Buck would be just inside DCM from 34-60Vin.
As such, I am guessing that the PI3526 Buck is meant to be operated in DCM, or rather, very close to the CCM/DCM boundary? Fig 79 of datasheet shows the PI3526 switching frequency being increased as load reduces…..thus keeping the Buck near the CCM/DCM border and preventing too high peak currents.
This seems to be saying that the internal diodes in the PI3526 are not Schottky type diodes?.... since if they were Schottky, then there wouldn’t be much need to operate in DCM or close to it.
Please could you confirm that the internal diodes in the PI3526 are not Schottky type? We ask because if they are Schottky type, then we will pick an inductor which is higher value than 480nH, and have the Buck operating well into CCM, so as to reduce the peak inductor current, and reduce FET RMS currents. However, as you know, operating in CCM can result in more “ground bounce” and general noise, which tends to need slightly more LEB time, and a bit stronger current sense filtering….if the PI3526 module wont be able to tolerate this extra noise, then it would be a poor choice for us to pick a higher value inductor and operate in deeper CCM.
Also, as you know, the dynamics change between CCM and DCM operation. The inductor value is also a feedback loop parameter. So we do not want to push it toward instability by selecting the wrong value inductor. Its not possible to adjust the feedback compensation components in PI3526, as they are all inside the module, "locked away".
Also, are we correct in assuming that if we select an inductor of say 3uH for operation at Vin=48V, Vout =12V 11A, then the switching frequency would be 450kHz (the lowest available)? (it would be in CCM at 450kHz with I(peak) = 14A and I(pedestal) = 7.5A. If the internal electronics in the PI3526 “waits” for the discharge of the inductor before turning the FET back ON, then the frequency would be even lower than 450kHz. Please could you advise on whether the PI3526 has such an “inductor discharge detector”?
Please could you also advise what is the lowest possible switching frequency that the PI3526 would operate at? ie, would it ever go below the lowest F(sw) shown on Fig 79, page 27?
So, we don’t want to be “tied in” to a particular Offtheshelf inductor, due to Lead times and availability. We would need to design our own inductor.
So to summarise, please may we request what is the freedom of choice in inductor value selection for PI3526?..... bearing in mind that our operating conditions are Vin=42-59V,Vout=12v, 0-11A?
The PI3526 Buck DCDC module looks like an Ace from Vicor.
We wish to use it with an external inductor we design ourselves instead of the recommended one by Eaton.
Please answer some questions about the choice of inductor value for the PI3526?
PI3526 Buck DCDC module datasheet
https://www.vicorpower.com/documents/datasheets/ds_PI352x-00.pdf
No Need to read the below if you already know info on inductor choice for PI3526, and may kindly share it here.
_____-------_____------______------_______-------______-------______
Our Spec:
Vin = 14S (42V-59V), Vout = 12V, 11A.
Inductor choice for PI3526 Buck:
With “nominal” Vin=48V,Vout=12V,Iout=18A,Fsw=450kHz……and the datasheet recommended 480nH inductor, the Buck would be just inside DCM from 34-60Vin.
As such, I am guessing that the PI3526 Buck is meant to be operated in DCM, or rather, very close to the CCM/DCM boundary? Fig 79 of datasheet shows the PI3526 switching frequency being increased as load reduces…..thus keeping the Buck near the CCM/DCM border and preventing too high peak currents.
This seems to be saying that the internal diodes in the PI3526 are not Schottky type diodes?.... since if they were Schottky, then there wouldn’t be much need to operate in DCM or close to it.
Please could you confirm that the internal diodes in the PI3526 are not Schottky type? We ask because if they are Schottky type, then we will pick an inductor which is higher value than 480nH, and have the Buck operating well into CCM, so as to reduce the peak inductor current, and reduce FET RMS currents. However, as you know, operating in CCM can result in more “ground bounce” and general noise, which tends to need slightly more LEB time, and a bit stronger current sense filtering….if the PI3526 module wont be able to tolerate this extra noise, then it would be a poor choice for us to pick a higher value inductor and operate in deeper CCM.
Also, as you know, the dynamics change between CCM and DCM operation. The inductor value is also a feedback loop parameter. So we do not want to push it toward instability by selecting the wrong value inductor. Its not possible to adjust the feedback compensation components in PI3526, as they are all inside the module, "locked away".
Also, are we correct in assuming that if we select an inductor of say 3uH for operation at Vin=48V, Vout =12V 11A, then the switching frequency would be 450kHz (the lowest available)? (it would be in CCM at 450kHz with I(peak) = 14A and I(pedestal) = 7.5A. If the internal electronics in the PI3526 “waits” for the discharge of the inductor before turning the FET back ON, then the frequency would be even lower than 450kHz. Please could you advise on whether the PI3526 has such an “inductor discharge detector”?
Please could you also advise what is the lowest possible switching frequency that the PI3526 would operate at? ie, would it ever go below the lowest F(sw) shown on Fig 79, page 27?
So, we don’t want to be “tied in” to a particular Offtheshelf inductor, due to Lead times and availability. We would need to design our own inductor.
So to summarise, please may we request what is the freedom of choice in inductor value selection for PI3526?..... bearing in mind that our operating conditions are Vin=42-59V,Vout=12v, 0-11A?
