Actions
Engagement
  • Author Author: Jan Cumps
  • Date Created: Date Created
  • Last Updated Last Updated:
  • Views 6952 views
  • Likes 7 likes
  • Comments 68 comments
Related
Recommended

Programmable Electronic Load - Power Stage

This blog documents focuses on the power stage of the electronic load that , and are designing.

 

image

In this post we're laying out a PCB for the power stage - as much as possible with surface mount components. The FET is close to the one uses in the original design.

 

The BOM

 

ComponentHeader 2Header 3Header 4
P18 pin header, 2.54mm
P2 abinding post, redhirschmann 931714101hirschmann 931714101 -  SOCKET, 4MM, BLACK, PK5 , MLS
P2 bbinding post, blackhirschmann 931714100hirschmann 931714100 -  SOCKET, 4MM, BLACK, PK5 , MLS
P3 abinding post, blacktenma 2301tenma 2301 - Binding Post, 36 A, 500 V, Nickel Plated Contacts, Panel Mount, Black
P3 bbinding post, redtenma 2302tenma 2302 - Binding Post, 36 A, 500 V, Nickel Plated Contacts, Panel Mount, Red
TH1NTC Thermistor, 10KVishay NTCS0805E3103JLTVishay NTCS0805E3103JLT -  THERMISTOR, 10K, 5%, SMD, NTC
Q1N-Channel MosfetInfineon IRF3205SPBFInfineon IRF3205SPBF -  MOSFET Transistor, N Channel, 110 A, 55 V, 8 mohm, 10 V, 4 V
D1, D2DiodeDIODES SBR2A40P1-7DIODES SBR2A40P1-7 -  Standard Recovery Diode, PowerdiRegistered, 40 V, 2 A, Single, 500 mV, 50 A
R1100R1206 any brand
R20R05Vishay WSHP2818R0500FEBVishay WSHP2818R0500FEB -  SMD Current Sense Resistor, 0.05 ohm, 10 W, 2818 [7146 Metric], ± 1%, WSHP2818 Series
Cooler HeatsinkStartech.com FAN370PRO - Socket 7/370 CPU Cooler Heatsink and Fan

 

 

 

NTC

 

For a detailed description on the temperature protection mechanism, check Programmable Electronic Load - Temperature Protection.

 

The voltage sent to the ADC is very dependent on the NTC. I've selected a Vishay NTCS0805E3103JLTVishay NTCS0805E3103JLT -  THERMISTOR, 10K, 5%, SMD, NTC.

I'll program the key values. The behaviour is non-linear and it's easier to make a lookup table if the firmware has to be able to deal with different components.

This will require access to flash to permanently store tha values, and a SCPI function to alter the table if another component is used.

For the first version I'm going to be selfish and just program for the device that I've ordered.

image

 

PCB

 

Exposed copper

 

For good thermal relief, and to get the NTC as good termally coupled to the FET as possible,

I placed a copper pour (here on the front, I'll do the same on the back and stitch them for thermal transport with vias)

Then i drew a pour on the front mask. The area of pour will expose copper. That means that the NTC has physical contact with the copper that the FET is soldered on.

In the fine-tuning I will place that NTC closer to the FET so that I can put a tad of heat paste in between. Or I could put a tad of paste between the NTC and exposed copper ...

image

 

Attention when placing the binding posts. For the power input, RED is 1 and BLACK is 2.

For the sense input, BLACK is 1 and RED is 2.

This is the result of me labeling pin 7 and 8 of the connectors between the driver board and FET board wrong, on both boards image.

The documentation and KiCAD zips are now updated with corrected schematics.

 

I used these 2 Contextual Electronics videos to refresh how to expose copper layers and place VIA arrays:

 

Here's the top side of the completed design. I've drawn the FET in green to give perspective.

image

In red you see the copper layer, orange is where the solder mask is removed and copper exposed.

Pink are the drill holes. They are 0.9652 mm, in an array of 9 * 8, spaced 2 mm apart.

image

 

On the bottom, the copper pad (green) has the size of my heat sink + some. The removed mask (blue) has the exact size of the sink's bottom profile.

The pink lines are the mounting slots for the heat sink (see below).

image

 

Slots

 

My heat sink has brackets for mounting. I've cut out slots to allow the brackets to through the PCB and fix them on the top side.

 

image

 

I've put some exposed non-connected copper pour around the slots for strength.

The slot is drawn on the Edge.Cuts layer. I hope that the PCB fab interprets that as slots to be milled out ...

image

 

I've attached the KiCAD project, component libs and Gerbers in a single zip. Also the VIA lib that's used here as a separate file (because I share that one across projects).

Attachments:
vias.pretty.zip
eload_offboard_20171227.zip

  • I've attached the KiCad project and dependencies.

  • in reply to michaelkellett

    I've ordered these from Seeed Studio. OSH Park, attractive for small size boards, charges 72 € for 3, including standard shipping.

  • in reply to Jan Cumps

    I'll be interested to see how well it works.

    Where did you get the boards  - that's a much lower price than I pay.

     

    Thanks.

     

    MK

  • in reply to Jan Cumps

    This is the layout I submitted. Top left, bottom with pad for heat sink right.

    If everything is right, the two copper rectangular pads left and right of the FET should be routed out and become slots .

    These are for the mounting bracket of the heat sink ...

    image

  • in reply to michaelkellett

      wrote:

     

    ... there's an interesting document from Cree about thermal design of pcbs (focused on LED applications but generally useful).

     

    You can find it by Googling 'XLamp_PCB_Thermal'.

     

    It's good because it gives numbers for via conductivity and even compares solder filled with plain as well as other useful stuff.

     

    ...

    I've checked out that document. Very interesting.

    I have 61 VIAs, hole size 0.95mm. According to the document I could have done with a lower amount (for the Cree LED, they found that 14 0.25mm holes very close to each other under the core, were optimal).

    They advice to fill the holes with copper.

     

    Just for info, my PCB is FR-4,  1.6 mm thick, Copper Weight - 1oz.

    36 € including shipping for 10 boards.

  • in reply to Jan Cumps

    Good luck - there's an interesting document from Cree about thermal design of pcbs (focused on LED applications but generally useful).

     

    You can find it by Googling 'XLamp_PCB_Thermal'.

     

    It's good because it gives numbers for via conductivity and even compares solder filled with plain as well as other useful stuff.

     

    I know it's  a bit late now since you've designed your board image

     

    MK

  • in reply to michaelkellett

    Michael, I will be paying a hit for this design. But it has been used in other similar designs too.

    The best is to mount the heat sink on the iron of a through-hole transistor. Anything in between is a heat resistor and will degrade cooling.

     

    In my case, the transistor's heat sink is soldered to copper. That's a fairly good transfer.

    Then the heat is transferred to the underside copper pad of the PCB via an array of 60+ VIAs that will be filled up by solder. Another decent transfer.

    That copper pad on the backside has the same shape as my decent heat sink with fan. I can mount that straight on that bottom copper pad (the solder mask is removed, so it's copper on heatsink.

     

    It's not the ideal world but I expect it to be good enough. I will test this once it's built up. Components and PCB are ordered ...

  • Won't putting the power device on a pcb rather than directly on a heat sink rather limit the maximum power you can dump. I have a couple of commercial loads and they can both put away 250W+. (Which is  a lot but only yesterday I was putting 15W into one and I've often needed 50W or more).

     

    MK