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Power & Energy
Blog GaN Point of Load converter 48V to 1V 50A - part 3: Dynamic Test Load
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  • Author Author: Jan Cumps
  • Date Created: 10 Mar 2019 4:06 PM Date Created
  • Views 113 views
  • Likes 6 likes
  • Comments 3 comments
  • gallium_nitride
  • dc_dc_converter
  • smps
  • lmg5200
  • gan
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Recommended

GaN Point of Load converter 48V to 1V 50A - part 3: Dynamic Test Load

Jan Cumps
Jan Cumps
10 Mar 2019

I'm reviewing a Gallium Nitrate step-down converter for Point of Load (PoL) high power conversion.

In this post, I'm checking the sub-circuit that can inject an 8 A load pulse into the converter output. The injected load is steep and allows you to evaluate the response of the design to fast current demand rise and fall. For an explanation of the main DC converter, check the links at the end of this post.

 

The converter is designed to deliver high current, up to 50 A, at point of load. You can bring the power to the PoL via high voltages (this design supports 36 to 75 V).

This takes care that you can use smaller copper traces because the current will never get above 2 A before conversion.

At the point of load, this voltage is converted to 1V (programmable between 0.5 and 1.5 V).

By putting the converter directly near the load, the part of your PCB that has to transport the high current can be kept short.

 

Dynamic Behaviour

 

The loads that this converter is targeting (processors, ASICs, FPGAs) are very dynamic by nature.

They can suddenly request several Amps extra. And the 1 V output level doesn't leave much room for voltage drop and recovery artifacts.

 

The design that we have here is resilient to these big changes in demand.

To proof this, the evaluation board has the option to inject a fast rise/fall 8 A extra current draw (the slew rate is a fast 10 A/μs).

This is done by switching a 120 mΩ resistor in parallel with the output.

It adds and removes that 8.3 A to the current that you are already drawing from the converter.

 

555 Timer

 

One of the reasons why I wanted to show this part of the DC converter, even if it's not part of the core design, is because it uses a 555 timer.

Every time I spot one in a modern design, I feel the urge to blog.

Here, it's the CMOS variant LMC555 that can clock up to 3 MHz. It's configured for 1.3 kHz.

 

The timer is controlling a gate driver IC. That one drives a power MOSFET. When this FET is driven, it switches that extra load in parallel with your test load.

When it's not driven, only your test load is active.

This means that the current changes dynamically two times per cycle, giving a 2.6 kHz dynamic change frequency (the documentation says 2.5 kHz).

The measured duty cycle is 34% (33% in the documentation).

 

The 120 mΩ resistor is in reality a set of twelve 3W 40 mΩ resistors - two strings of six in parallel.

In a next post I'll show the behaviour of the converter when this sudden change of demand happens at its output.

 

Blog Posts
part 1: Design Overview
part 2: Current Doubler
part 3: Dynamic Test Load
Related Blog
Checking Out GaN Half-Bridge Power Stage: Texas Instruments LMG5200 - Part 1: Preview
Anonymous

Top Comments

  • DAB
    DAB over 3 years ago +1

    Nice post Jan.

     

    I wonder if this circuit could drive an inductive load?

     

    DAB

  • Jan Cumps
    Jan Cumps over 3 years ago in reply to DAB +1

    The design document doesn't mention it - but in essence there's nothing special in this design that makes it more or less suited for inductive loads.

    It is not a typical use case though. Not a lot…

  • DAB
    DAB over 3 years ago in reply to Jan Cumps +1

    You know me, I am not looking at typical implementations.

     

    DAB

  • DAB
    DAB over 3 years ago in reply to Jan Cumps

    You know me, I am not looking at typical implementations.

     

    DAB

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  • Jan Cumps
    Jan Cumps over 3 years ago in reply to DAB

    The design document doesn't mention it - but in essence there's nothing special in this design that makes it more or less suited for inductive loads.

    It is not a typical use case though. Not a lot of inductive loads need 1 V and 50 A.

    The typical load would be a semiconductor of some sort:

     

    1.1 Typical Applications

    The 48-V to 1-V solution described in this document is applicable in numerous down-conversion applications from 48 V, including:

    • Processor supply for computing environments with a 48-V bus

    • Telecom and Datacom applications processor supply

    • Industrial and aerospace FPGA and ASIC applications

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  • DAB
    DAB over 3 years ago

    Nice post Jan.

     

    I wonder if this circuit could drive an inductive load?

     

    DAB

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