Development of a "reference" class D starting point

[Kenshin]

classD4sure:
In the robocon race we use the IRF540 N-FET and IRF9540 P-FET(without oversize) full bridge to drive our motors, two FET have almost the same cost. And the gate driver is also simple and reliable.So I consider modify the motor driver board - it has been tested by many users - into an audio one.

[classd4sure]

Originally posted by Kenshin
classD4sure:
In the robocon race we use the IRF540 N-FET and IRF9540 P-FET(without oversize) full bridge to drive our motors, two FET have almost the same cost. And the gate driver is also simple and reliable.So I consider modify the motor driver board - it has been tested by many users - into an audio one.

Hi,

Discrete methods for driving both high side P and N channel fets...
has the same part count.

Sounds like a good idea I'd go for it, it can work of course.

What kind of robot did you build anyway? I made a maze crawler before, I stole parts off it to make my amp too.

At least with a P channel you probably wont' get that regenerative affect either.

[Kenshin]

ClassD4sure:

The robots we build are for USTC school robot race "robogame" and for Asia Broadcasting Union's Asia-wide race "Robocon". The biggest motors we used have a power ratings around 24V 3A, and the starting current is probably up to 10A.

what's the chip appearing in your schematic ?

[classd4sure]

Hi,

Yeah I've got motors like that that do 50K rpm!

According to the picture, it's a UC3625, brushless DC motor controller.

I copied that out of it's data sheet

So,

I tried a little modification to the gate drive.
What do you guys think of these drive signals??


Here is the before:

[phase_accurate]

Since things like PSU toroidal transformers and large can electrolytics of high quality are big ticket expensive items, this would be a substantial savings!

A quick shot from the hip lets one think that. But if you have a closer look then you see that you need about the same amount of PSU capacitance and the same voltage rating for your electrolytics when using a half-bridge topology. The only disadvantages of half-bridge are larger Vds and supply pumping IMO.

Regards

Charles

[classd4sure]

deleted

[Kenshin]

Originally posted by phase_accurate


A quick shot from the hip lets one think that. But if you have a closer look then you see that you need about the same amount of PSU capacitance and the same voltage rating for your electrolytics when using a half-bridge topology. The only disadvantages of half-bridge are larger Vds and supply pumping IMO.

Regards

Charles

pumping effect means bigger electrolytics is needed.

full bridge have a PSU current frequency doubled than the audio signal, so the big electrolytics could be half the size.

also,half bridge is more sensitive on PSU unsymmetry and PSU flucations.If full bridge is robust enough, we could use a SMPS--it's more cheaper.

overcurrent / overvoltage protection may also be simpler.

[classd4sure]

aaaaaaand the after~!

Charles, thanks for correcting me, you are quite right.

I'm not sure if you'd save on the transformer either, cost goes up by VA rating but independant of voltage I think.... so it would likely cost more.

I still think it would be worth it to get away with a lower Vds.

[Kenshin]

Maxim/Dallas has produced some full bridge chip class-D amplifiers which can be powered directly by a "dirty" SMPS for logic circuits and send out a voice signal with 0.03%(typical) THD. that's nice.

the protection problem is another major reason I favoured the full bridge : +/- PSU means doubleing the whole protect circuit.

power diode to prevent foolish PSU connect

Current-sensing resistor

Reuseable electronic fuse--some users often replace the blown fuse with iron screw, so general fuse isn't safe enough for a commercial module (if massive producted).

TVR devices(or SCR) to short the rails and blow the fuse during serious overvoltage

These additional complexity and cost may be able to buy another pair of FETs

[subwo1]

Originally posted by Kenshin
Maxim/Dallas has produced some full bridge chip class-D amplifiers which can be powered directly by a "dirty" SMPS for logic circuits and send out a voice signal with 0.03%(typical) THD. that's nice.

the protection problem is another major reason I favoured the full bridge : +/- PSU means doubleing the whole protect circuit.

power diode to prevent foolish PSU connect

Current-sensing resistor

Reuseable electronic fuse--some users often replace the blown fuse with iron screw, so general fuse isn't safe enough for a commercial module (if massive producted).

TVR devices(or SCR) to short the rails and blow the fuse during serious overvoltage

These additional complexity and cost may be able to buy another pair of FETs

I am glad you mentioned these things about SMPS design characteristics. The problem of DIYing these types of supplies so as to lessen radiated and conducted noise is why I am working on the design of a couple of zero voltage switching (ZVS) approaches. I am still in the simulation stage of the process as of now.

Several posts back, I displayed an image of a phase shift modulator to be used in a bridge version of one of the circuits that I am working on, but I learned that the input protection diodes of the CD40106B ( 74C14) were clipping off some of the charge on the phase shift capacitor, throwing off balance in the duty cycle.

Now, it finally came to me, correctly this time I think, that removing some of the voltage from the phase shift resistor using a voltage divider could even out the mark/space timing by compensating for the charge lost from the capacitor. Possibly, a single voltage divider resistor could work in series with a capacitor, but the two resistors placed across the power supply should be better in most cases.