Hi Terry On 03/10/2018 10:25, Terry Coles wrote:
With that in mind we decided to use Power MOSFETs (specifically the IRLZ34NPBF (https://docs-emea.rs-online.com/webdocs/0791/0900766b807913d5.pdf)) because their ON Resistance is very low (about 0.05 Ohms in this case). To drive it we chose the HIP4082IPZ H-Bridge FET Driver (https://docs-emea.rs-online.com/ webdocs/14f5/0900766b814f52f3.pdf).
Don't know the background to this but that driver is a bootstrap device for use with high-side NFETs in an H bridge motor driver. It must be driven by PWM and with that type of device it's important that the PWM is kept below 100% otherwise the high side gate drive voltage falls and the high side FET is destroyed (for starters). Some good background info at https://www.fairchildsemi.com/application-notes/an/an-6076.pdf Detailed analysis on the relationship between maximum PWM duty cycle and bootstrap components at: https://www.fairchildsemi.com/application-notes/AN/AN-9052.pdf
That's the background, now the problem. I built a circuit based on the Application Block Diagram on Page 3, but I couldn't get it to work. To bring the 12 V / 3 V thing into the discussion; the chip can be powered by any voltage between 8.5 V and 15 V and the logic levels have to be less than approx 1 V for a Low and greater than approx 2.7 V for a High. In theory the Pi can do that, although we are aware that we may have to buffer the inputs to get the high level well above the minimum. What I found was that when the Pi put out a high (measured on the LHS of a 100 k limiting resistor), I got 3.3 V (standard Pi High level) and when the Pi put out a low, I got close to 0 V. However, the actual chip pins (RHS of the 100 k resistors) never went below 1 V.
From the data sheet, Iil is 100uA, so through a 100k resistor that's a 10V drop. Ends up settling at 1V as the bias current reaches equilibrium.
In desperation, I laid out a brand new chip on a breadboard, applied 12 V between Vdd and Vss (and nothing else) and measured the inputs. What I got was around 2 V or more. At first I thought the inputs were floating, so I connected the Pi directly, (without the 100 k limiting resistors) and I now have an ex-Pi, so there was current behind the volts.
This is not the type of circuit which lends itself to breadboarding. Lots of high peak currents so need really good ground and power planes and careful decoupling. Cheers Tim -- Next meeting at *new* venue: Bournemouth, Tuesday, 2018-11-06 20:00 Check if you're replying to the list or the author Meetings, mailing list, IRC, ... http://dorset.lug.org.uk/ New thread, don't hijack: mailto:dorset@mailman.lug.org.uk
