Vishay SiC967 microBRICK Buck Converter on the bench

Table of contents

RoadTest: Become a Tester of the Vishay microBRICK® Synchronous Buck Regulator EVB

Author: Jan Cumps

Creation date:

Evaluation Type: Power Supplies

Did you receive all parts the manufacturer stated would be included in the package?: True

What other parts do you consider comparable to this product?: small footprint integrated designs: Power Integrations LinkSwitch and Texas Instruments Swift range

What were the biggest problems encountered?: none

Detailed Review:

Review of a Vishay microBRICK's SiC967 switch-mode dc converter reference design.
This regulator can convert 4.5 V to 60 V to a 0.8 V - 15 V out, max 6A.
The evaluation kit allows to test the IC in the 4.5 V to 50 V input range, at full load. At the same time it serves as the PCB layout example.

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In my road test, I put the converter on the test bench, and will look at Vishay's design support for the regulator.

SiC967 microBRICK Buck Regulator

The IC is a switch mode DC converter with these specifications:

  • input 4.5 V to 60 V
  • output 0.8V  to 15 V, 1% accuracy
  • output 6A max
  • switch frequency 100 kHz to 2 MHz

Functions:

  • soft start
  • current limit
  • power save mode
  • over- and under voltage protection
  • over-current and shortcut protection with retry
  • over-temperature protection
  • enable function
  • optional external BIAS

Claims specific to the design:

  • highly integrated: the package contains the regulator, power transistors and the required magnetics.
  • low ripple output and small output bulk capacitance needs 
  • up to 95% efficiency
  • internal compensation1

1 The datasheet doesn't specify what Vishay means with internal compensation. ChatGPT gives the following answer:

> what is Vishay MicroBrick internal compensation?

Internal compensation refers to the module's ability to stabilize the control loop of a DC-DC converter internally, without requiring external compensation components (e.g., resistors or capacitors) that would typically be used to tune the feedback loop.

Reference Board for 4.5 V to 60 V Input, 6 A, DC/DC Synchronous Buck Modules

The road test kit is an evaluation board for the SiC967. It doubles a s the reference design. These are the features that Vishay made available:

  • test the IC up to max output, with input between 4.5 V and 50 V
  • 3 preset output voltage / switch frequency combinations, option to set your own
  • enable / disable power save mode, with or without external BIAS supply
  • Limit valley current2 
  • test points for Vin, Vout and Vpowergood. Because of the integrated design, there is no access to internal switching signals.

2 I asked ChatGPT what the definition and meaning of Valley Current is:

> what is "valley current" in a buck converter?

In a buck converter, valley current refers to the lowest point of the inductor current during each switching cycle. It is an important parameter in converters operating in continuous conduction mode (CCM) or discontinuous conduction mode (DCM). Understanding valley current is crucial for analyzing performance, designing control schemes, and ensuring stable operation.

Context of Valley Current in Buck Converters:

  1. Inductor Current Behavior:

    • The inductor current in a buck converter is triangular (or trapezoidal in some cases) due to the switching action of the MOSFET and diode (or synchronous rectifier).
    • The current rises when the high-side switch (MOSFET) is on and the inductor is charged.
    • The current decreases when the switch is off and the inductor discharges into the load.

  2. Peak and Valley Current:

    • Peak current is the highest point of the inductor current in a switching cycle.
    • Valley current is the lowest point of the inductor current, which occurs just before the next charging phase starts.
  3. [...]

  4. Significance of Valley Current:

    • Efficiency Analysis: Knowing the valley current helps evaluate conduction losses in the inductor and MOSFETs.
    • Discontinuous Conduction Mode (DCM): In DCM, the valley current can reach zero, indicating the inductor is fully discharged during part of the cycle.
    • Control Techniques: In valley current-mode control, the converter monitors and regulates the valley current as part of its control scheme, instead of the peak or average current.

  5. Impact on Design:

    • The valley current affects the selection of the inductor and the sizing of the components.
    • Ensuring the valley current does not drop below zero in CCM is essential to avoid unintentional mode transitions to DCM.

If you are working on a specific buck converter design or application, understanding valley current dynamics is vital for optimizing performance and ensuring reliable operation.

The schematics are available, and an image of each of the 4 PCB layers. This info, together with the PCB LAYOUT RECOMMENDATIONS section of the datasheet, will help designers to integrate the IC in a design.

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Evaluation board with the core area highlighted. Image source: edited figure 1 of Vishay SiC967 datasheet

The test points have probe points that make it easy to use a DMM or oscilloscope. For a more permanent, it's easy to solder test wires directly to the PCB 

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What allows for a small footprint and little need for additional components?

The microBRICK is a hybrid IC. It contains control and power silicon, and the inductor.

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image source: excerpt from Markt&Technik Stromversorgung & Power-Management 2024 www.weka-fachmedien.de - fair use

The only external components required are in- and output capacity. And a few function selector resistors. It provides its own regulator working power from input (if you want that).

 

On the test bench

The kit has been on the lab bench for a full month. I looked at efficiency, protection and behaviour.

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Efficiency:

The IC's maximum efficiency is 95% This was confirmed by a test in the optional range of the design. The exercise below checks a scenario on the lower voltage input range: 10 V, and medium load: 0.2 to 3.1 A. In continuous conduction mode. 

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(sample 22)

I’ve ran many more tests, with input voltages between 12 V and 48 V, in mode 2 and 4. With low to maximum load. Here are the charts for Mode 4.
note: you need to populate R14 with a 0R 0402 resistor to use the bias voltage.

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(sample 34)

Check out LabView process to measure DC:DC converter efficiency for details on the bench setup, instruments used, Labview process and the additional measurements. The post includes a spreadsheet with the data.

startup behaviour with external bias

The soft-start ramp is fixed. The datasheet specifies 6 ms. 
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The capture below confirms this. I used the enable signal to trigger a startup. The yellow trace is the input voltage, the blue one is the output signal during soft start.
The time between 10% and 90% is 6 ms. From 0 to 100%: 6.72 ms.

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under voltage with auto retry

This little exersice checks the protection behaviour, and shows the Power Good signal during an error situation:

Settings:

  • chan 1: Vout
  • chan 2: Vpgood
  • Vin: 8 V
  • Vout: 5 V
  • OCP: 50%

Iout: 5.030 A

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Iout: 4.390 A

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Over current / short circuit protection

The image below shows shutdown in an over-current situation. The Power Good signal follows along. The IC will auto retry, and re-soft-start.

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Input undervoltage mechanism

The IC has to be configured for the minimum voltage it will operate on.  has elaborated on the calculations in his review. I've checked if the evaluation board behaves as configured.
note: the values of R5 and R6 on the evaluation board are different than what the board's user manual says: R5 = 10K, R6 = 43K.

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Calculations indicate that the swicher should enable at 7.42V. and switch off when input drops below 6.36V. I used these test conditions: Vout  = 5V, Iout = 0.317 A.

Ven measured: 7.48441 V

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Vunder measured: 6.16347 V

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Design resources

In this section: the documentation and design tools. How does Vishay support the engineer?

PowerCAD designer

This is Vishay's online application that helps you design DC converters with their products. I asked it to design a DC converter, manipulating its in-and out criteria in the hope that it would generate a SiC967 design:

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This was the schematic it made:

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Not surprisingly, it closely resembles the reference design's schematic. With configuration resistor values based on my requirements. There's a rich set of simulations of your design available. Here's a subset.

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If you are planning to use the SiC967, this is a good application to suggest good values for the external components. And to get a feel for its operational behaviour.

Documentation

The datasheet and collateral are good. They contain a good mix of raw specs and charts. And good designer information. It is a fairly enjoyable read. This snippet shows some of the charts of the datasheet. I selected these because they roughly match the measurements I did in the efficiency section of this review.

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There is clear info on how to design a circuit, how to configure the protections and operating area. And good guidance to properly implement a PCB. This section, together with the evaluation kit's images of each PCB layer, will get you on track. These are the resources I used:

microBrick datasheet https://www.vishay.com/docs/76444/sic967.pdf
board user manual https://www.vishay.com/docs/77501/sic967.pdf
microBuck/Brick product sheet https://www.vishay.com/docs/48652/2309-microbuck-brick.pdf
sic976 product landing page https://www.vishay.com/en/product/76444/
sic976 infographic https://www.vishay.com/docs/48924/ig32989088-23xx-sic967.pdf
DG2034 multiplexer https://www.vishay.com/docs/73172/dg2034e.pdf
Exploded package https://passive-components.eu/wp-content/uploads/2020/08/microbrick-3D-package-integrated-power-module.jpg.webp
PowerCAD video: https://youtu.be/cNhndMu8QKw

AI use in this road test:

  • blue background: AI
  • no blue background: no AI

text in tables with light blue background is generated by ChatGPT. I provide the query and unaltered reply (except for removal of some content, indicated by [...]). 
It is only used to explain terms used by Vishay in their datasheet. The tests itself, and the writeup, is done without AI input.

Thank You for reading. And thank you e14 and Vishay for the road test material.

Anonymous
  • Case temperature, with:

    • 48 V Vin
    • 5 V Vout
    • 6 A Iout
    • Mode = 2

    For 4 minutes, until the temperature stopped increasing.

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    (sample 39)

    This is measured using the same LabVIEW flow as the other measurements. I set it to a stable ever sampling condition

    • 48 V start
    • DAC = 33000.(+- 6 A)
    • take 0 steps per iteration
    • run until stopped

    I put a little piece of Kapton tape just where the thermocouple was sitting, to keep heat local. 

  • I have monitored the temperature at the top of the housing during my tests. A thermocouple firm in contact - but no adhesives or tape, just mechanically held down by the springy-ness of the thermocouple's wire.

    image

    I got maxima in the mid-to-high 60° C. 45° - 50° above ambient.


  • Nice review. It looks like an interesting part! I guess it uses SiC MOSFETs although they don't seem to explicitly mention that anywhere.

    Neat idea to combine it all into a 'micro brick' since there'd be no simple way to have the SiC MOSFETs and the controller in a single IC.. and then why stop there, may as well stick the inductor on top inside the brick, if there's not going to be so much heat dissipation! : )