High side vs Low side PWM

Electric Motors and Controllers
billvon
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Re: High side vs Low side PWM

Post by billvon » Fri Oct 27, 2017 1:26 pm

trialspower2 wrote:Which parameter is the G-S capacitance on the datasheet? (FDH055N15A)
Ciss, input capacitance.
If the FET already has quite alot of G-S capacitance is there any different between adding more gate resistance compared to adding a G-S capacitor?
Well, again, if you want a given rise time then increase resistance until you achieve the desired rise time. Rise time 10 to 90% is given by 2.2RC where R is gate resistor and C is gate capacitance. However, since you really only care about time to gate threshold (about 3V in your case) then that also depends somewhat on gate drive voltage. Increasing capacitance just increases loss.
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trialspower2
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Re: High side vs Low side PWM

Post by trialspower2 » Fri Oct 27, 2017 1:35 pm

So in an ideal world I would want to slow the transition up to 3V, but once 3v is reached get to 15V as fast as possible? As once 3V is reached the switch on of the FET has essentially completed?

billvon
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Re: High side vs Low side PWM

Post by billvon » Fri Oct 27, 2017 2:06 pm

trialspower2 wrote:So in an ideal world I would want to slow the transition up to 3V, but once 3v is reached get to 15V as fast as possible?
Not really. Above about 5V the FET is, for all intents and purposes, fully on. (Data sheet goes into detail on this.)
As once 3V is reached the switch on of the FET has essentially completed?
Basically. Beyond that you get a very slight decrease in Rds_on. Again the data sheet shows how this happens.
--bill von

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Re: High side vs Low side PWM

Post by trialspower2 » Mon Oct 30, 2017 4:44 am

I have realised that 75% of my turn on spike is gone when measuring at the high side transistor compared to the low side. It turns out that the different is over the M3 bolts which connect the high side to low side transistors. I used brass as its a better conductor than steel and all the aluminium bolts I could find were anodized. I have however found a source for 3mm copper bar, so I will get some of this and tread the ends to make studs. This will give more cross section area and be out of a superior material for conductivity. Hopefully this will drop the 48v spikes from 60v to under 20v which I will be happy with for now.

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Re: High side vs Low side PWM

Post by Njay » Mon Oct 30, 2017 7:05 am

I have my doubts that changing to copper will reduce your spike... because it's the inductance that makes the spike, not the resistance.

Here's a diagram from another topic, just for reference as a half-bridge diagram with the parasitic inductors; the top FET (M2) is being used as a diode but it's functionally the same as it being independently driven. Motor phase is L1. When the top FET is ON, current goes through the parasitic inductors L5 and L7. When you turn it OFF in order to turn ON the bottom FET (M1), these inductors will create the voltage spikes as they are being starved of current. And similarly for the bottom FET, but with parasitic L4 and L6+L9.

Image

trialspower2
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Re: High side vs Low side PWM

Post by trialspower2 » Tue Oct 31, 2017 7:27 am

I was thinking that something with lower resistance would have lower inductance? Also if I use a stud rather than something threaded it will work out slightly larger in diameter. However if you dont think it will make a difference, I will leave it as it is.

The project that we are undertaking is to build an electric trials bike, so there is plenty of work to do else where. The controller is at a point where we can try it on a bike, this is enough for the time being as we need to get a bike built, then refine everything later.

I have a new design for the controller with the high side transistor standing upwards and the low side sticking down. This will ensure that the souce and drain transistor pins are connected directly together while keeping DC bus extremely close and the circular shape for power sharing. It will also allow me to water cool the heat sinks easily. I will also drop down to 5 FET's rather than 6 as I think this is overkill for a 100v, 400a controller. thank you for your help, I will be back when the next controller is done!

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