The main reason I avoid components on the back side is because I want to use this for heatsink and connect a thermal pad between the PCB’s and the aluminium box – using the box itself + possible external heatsinks to cool down content inside. So far it is only the 8 x PWM module with a teoretical 40A in peak that will require cooling- components can deliver > 40A in peak, but the PCB and design is limited to a total of 10A. In fact I might consider reducing the module from 8 to 6 channels to support more juice as I could need more pins for power input.
This mechanical design will be a challenge, but I think it can be done. I also have a 8th position over the PSU that can be used for extra temperature sensors inside the module – I will support PT100 etc later.
The snip below show PWM/Power routing and I need to improve this a bit:
I have only used two layers for power out + I use bottom layer for PWM out while 24V in is on an internal layer – it will be more current on the 24V lane, so I need to swap those and (1) widen the 24V track + (2) add more internal planes to support. I am serious out of space on this module, but if I move 24V to bottom I can also add a bit of capacitance using the backside for 1206 size capacitors on the 24V itself. The result can be viewed below as it worked out quite well. I will wait a bit and test before I consider reducing to 6 channels – my concern is that a single pin is not sufficient for 24V PSU – in which case I remove 2 x PWM and have 3 pins for 24V, but I can patch that on the existing board if I need to as I am sure I will need a 2nd revision.
Using 1206 size capacitors above is not idea because you use 20 x 15uF capacitors to keep a low profile. It cost a bit extra and is greatr for subsea applications, but I would have preferred an ordinary capacitor on the front – no space. That said – this provides 300uF and worked out very well.
As for the functionality on this module:
- 8 x PWM independent Half Bridges.
- 8 x Current Sensors with independent programmable gain.
- Programmable rise times on PWM pulses.
- Programmable analogue current trip thresholds in HW.
- Overtemperature protection.
- Undervoltage detection – the module should work between 5 to 24V.
- Diagnostics in silicon.
- Open load detection.
The reason I consider reducing to 6 pins is that I currently use a single pin to input 24V and removing two channels will enable 3 pins for 24V – that said I have 9 pins for ground because I want a pair of PWM/Ground for each output channels and if you look at the connector you also see that is will be difficult to use the right side pins for PWM. This is the price for hole-through connectors as you pay a lot of extra for mechanical strength.
Another review comment is protection against wrong polarity. It is easy to add a diode that gurantee survival if someone get the 24V pins incorrect. I probably need to put that on the back as well. Polarity protection is one of those things you apreciate if you assemble a system under stress and get it wrong – it saves the module from a mistake that is easy to do.
Since this is a ECU designed to live on vehichles with extreme vibrarion/shock we also need to think about mechanical support for PCB’s – and thick thermal pads towards outside is more than heat dissipation. The box itself will need to be mounted on rubber feets, but I will be back on that – I am not sure about the inside between the PCB boards, but I am considering to fill that as I close a box. I have a few tricks up my sleve that is worth trying out.