In your mcp14700 data sheet, pg 13 covers your decoupling and boot capacitors recommendations
I spoke about. But I haven't found my other International Rectifier source, other than DT98-2, pg4 speaks
about ceramic low esr for the decoupling, or the use of a ceramic or tantalum boot cap, with
no decoupling cap. If I find the source, In will pass it along.
I have my own BLDC project that has been moving slowly for the last year or so, and I have
been anxious to learn about anything that pertains to BLDC esc that may help me out. I had
previously read about mosfets, and their drivers grenading because of the Boot Capacitors, so
your thread was interesting. But unfortunately my enthusiasm gets the best of me.
Not offended. I just get kind of direct sometimes. All help so far has been constructive, I feel it has helped me move this forward.
My thought is that 'low esr ceramic' is a little vague. I downloaded a datasheet for a ceramic that mouser speced as low esr, there was no info in the datasheet with any specs to back it up.
It is possible that that is the issue I suppose, that the boot cap lags enough at the wrong point.
Based on some more trial and error, here is what I have learned, maybe this will help someone else.
The mcp14700 is only capable of driving the source to gate to vdd, or 5v, so pick mosfets accordingly.
I tried two in the series, one with 610pf, the other with 1700pf input capacitance. The boot cap must be sized according to mosfet input capacitance. If the boot cap is too small, it starts misfiring at greater pwm times. The 14700 apparently is unable to fully drive the larger mosfet with a 1uf boot cap. The symptoms are misfires, which you can see on the scope as missed pwm spikes, typically spaced every other one.
These fets run about 70C at 1.5 amp approximately.
Those mosfets have a metal plane on the underneath side, I think they can do better than 0.3w. Tell me where my calcs are off, but at a max of 2 amps, that is 2a * .05 ohm * 2a = .2w * 2 = .4watts + switching. The datasheet specs these at 65w, which I think is rubbish.
The supply can give 4 amps or so, but peak pulse amps are much higher, I estimate closer to 10, based on mclv testing.
I don't know if I have it figured out yet, but one major source I have identified is fairly severe ground bounce, enough to reset the uP. 100 ohms resistance on the gates has made a huge difference. I think the ground bounce was also affecting the driver outputs. These mosfets switching speed is specked at 5ns rise time, 15ns fall, compared to the mclv board at 250 rise, 140 fall.
I also changed the .1uf bypass caps on the driver for 1 uf, now the switching and driving waveforms look the same as the mclv board.
Yet another problem, I have forward and reverse constants, I had accidentally added a 0 to the end of one, thus one direction was causing issues.
That motor's windings are 0.215 ohm, driven from 24V supply they will take over 100 amps worst case from the motor supply decoupling caps... You need beefier MOSFETs, 0.01 ohm or lower Rds(on) or better.
Have you been testing at 24V? Always start with a low motor supply voltage and checkout the waveforms before ramping up the power - doing this (and limiting the current with a power resistor) can eliminate expensive blown chips (Stick to the rated voltage, these motors are already at their thermal and mechanical limits).
Make sure your PWM frequency is sensible for the inductance of the windings (BLDC RC motors have low inductance)
The bootstrap caps should be around 10 times the effective gate capacitance of the MOSFETs (which is the total gate charge divided by gate drive voltage). Too large and the bootstrap diode may blow, too small and the high-side gate drive will droop.
Keep the traces short and wide for the bootstrap components, place lots of ceramic decoupling right next to the drivers (much larger than the bootstrap caps). Don't run groundplane under the bootstrap or output traces, do run groundplane under the logic signals (and keep them away from the bootstrap and output traces). Basically minimize stray inductance and capacitance for the high voltage side.
You might want to consider a 3-phase driver like the the HIP4086 rather than 3 separate drivers.
Are you sure about 100 amps? I think when you take into account inductance that that may be different. I did some extensive testing using the microchip mclv board, which has current sensing built in. I found around 8 amp peaks, depending on pwm period. The mosfets the mclv uses have an rds on of .026, in d2pak size.
They still run hot, but workable. I may need to tune my gate resistors. I also suspect that adding schottkeys to take the field collapse load off the mosfets, and lower the voltage drop would dramatically lower power dissipation.
Comments
I spoke about. But I haven't found my other International Rectifier source, other than DT98-2, pg4 speaks
about ceramic low esr for the decoupling, or the use of a ceramic or tantalum boot cap, with
no decoupling cap. If I find the source, In will pass it along.
I have my own BLDC project that has been moving slowly for the last year or so, and I have
been anxious to learn about anything that pertains to BLDC esc that may help me out. I had
previously read about mosfets, and their drivers grenading because of the Boot Capacitors, so
your thread was interesting. But unfortunately my enthusiasm gets the best of me.
Didn't mean to offend
Bill M.
My thought is that 'low esr ceramic' is a little vague. I downloaded a datasheet for a ceramic that mouser speced as low esr, there was no info in the datasheet with any specs to back it up.
It is possible that that is the issue I suppose, that the boot cap lags enough at the wrong point.
The mcp14700 is only capable of driving the source to gate to vdd, or 5v, so pick mosfets accordingly.
I tried two in the series, one with 610pf, the other with 1700pf input capacitance. The boot cap must be sized according to mosfet input capacitance. If the boot cap is too small, it starts misfiring at greater pwm times. The 14700 apparently is unable to fully drive the larger mosfet with a 1uf boot cap. The symptoms are misfires, which you can see on the scope as missed pwm spikes, typically spaced every other one.
These fets run about 70C at 1.5 amp approximately.
That motor's windings are 0.215 ohm, driven from 24V supply they will take over 100 amps worst case from the motor supply decoupling caps... You need beefier MOSFETs, 0.01 ohm or lower Rds(on) or better.
Have you been testing at 24V? Always start with a low motor supply voltage and checkout the waveforms before ramping up the power - doing this (and limiting the current with a power resistor) can eliminate expensive blown chips
Make sure your PWM frequency is sensible for the inductance of the windings (BLDC RC motors have low inductance)
The bootstrap caps should be around 10 times the effective gate capacitance of the MOSFETs (which is the total gate charge divided by gate drive voltage). Too large and the bootstrap diode may blow, too small and the high-side gate drive will droop.
Keep the traces short and wide for the bootstrap components, place lots of ceramic decoupling right next to the drivers (much larger than the bootstrap caps). Don't run groundplane under the bootstrap or output traces, do run groundplane under the logic signals (and keep them away from the bootstrap and output traces). Basically minimize stray inductance and capacitance for the high voltage side.
You might want to consider a 3-phase driver like the the HIP4086 rather than 3 separate drivers.
I am currently at 20k pwm.
My new design uses the NTD5865NLT4G http://www.mouser.com/ProductDetail/ON-Semiconductor/NTD5865NLT4G/?qs=sGAEpiMZZMvECErq9cesgAUNEE%2fUZZuqxMuje5ZW5os%3d and the L6388 http://www.mouser.com/ProductDetail/STMicroelectronics/L6388ED013TR/?qs=6hSH1YZbOIgtbL%2fVTQr83Q%3d%3d and two ceramic 1uf bootcaps.
So 29nc/24V = 1.2?? uf??
They still run hot, but workable. I may need to tune my gate resistors. I also suspect that adding schottkeys to take the field collapse load off the mosfets, and lower the voltage drop would dramatically lower power dissipation.