H-Bridge with level shift - MOSFET gate voltage issues?

Addendum:

And if software proves troublesome - I'll leave a bit of space on the veroboard so we can add a bit of logic and try some other approaches which are less sensitive to software timing and make more use of hardware PWM on the Pi - like I said, this is allowed to "fail" for the education value :)

We might later switch to using an Atmel to do the dual PWM and hang that off the Pi via SPI or something.

With any luck, well have a practical unit by Haloween :->>>

So varying the drive duty cycle will make half of the string brighter (and maybe overload it) and make the other half dimmer.

Why do that? You could use an AC source and an SSR for on/off.

Not quite: 2 PWMs, one to each half of the bridge

Left Right LED-subset-L LED-Subset-R

The idea is to hold Right at 0, and PWM left for LED-Subset-R brightness, or hold Left at 0 and PWM Right for LED-Subset-R brightness.

Obviously you cannot do both at once :)

OK, you can do tricks if you drive the half-bridges indepently.

he 100R returned to that rail, but it would be cuter to use a bipolar NPN w ith its base tied to the +15V rail, return R1 to the emitter of that device (giving you a 0V to 14.4V swing on the gate of Tr3) while a second 100R re sistor from the collector to +30V would give you a +30V to +15V swing on th e gate of Tr2.

bipolar cascode out of saturation. A 13V swing would be quite enough to tu rn the part on hard.

y want resistive pull-ups there, so it gets messier.

r out of) a gate to guarantee rapid turn-on and turn-off - gate charge ver sus gate-source voltage is documented on MOSFET data sheets - while relying a resistor to keep the gate on or off. There are quite a few other options .

and it has two full bridges ..

have you looked at cross conduction?

as the gate voltage moves between its upper and lower values, is there a time, even a brief time, when both the upper and lower fets are both on.?

If so, you need to include current limiting resistors.

or add level shifters such that there is a time when both upper and lower FETs are off and there is no cross conduction..

I prefer belts and suspenders and would do both.

The FDC5614P is a logic level P. 60V, 3A, Vgs -1.6 NTF3055L108 - logic level N channel 60V 3 A Drive them from gpio pins on the Pi. If you decide to not roll your own, look at the DRV8871 from TI

As, I think JL mentioned, use AC supply and a couple of SCRs with a classic dimmer control. You would need zero crossing detector for the pi.

Ha - sorry, I never occurred to me to actually point that out :-|

I do apologise for any confusion there...

Yes - I saw that in CircuitLab, the simulator. That's why I added a 1 Ohm resistor to the 30V PSU to make it vaguely realistic (no PSU will produce infinite current, though arguably a fat capacitor and a low impedance bit of wire might get well on the way).

That was when I found the power dissipation in the power stage was vastly reduced by adding a driver stage in the middle (to shorten this transient).

I *was* taking some random example H-Bridges as working examples :-o

Indeed - I might add the 1 Ohm (or something suitable low) in so that at least the design transients were calculable and consistent. Me being me, I'd probably fuse it too.

Or that - I'd probably still do the above just in case either FET failed with a short.

Me too :)

These are low risk - they'll be outside on the ground, so even if they blew up, it would not be a huge problem. The PSU in the shed that powers everything will be a quality brand name thing.

I can generate my own confusion without help from anyone.

AC and a triac, with some clever gate timing, could do the same lighting tricks as two half-bridges.

fredag den 4. januar 2019 kl. 00.03.40 UTC+1 skrev John Larkin:

but then you have to find a real transformer instead of a cheap switching supply

My evil high voltage driver had lumped constant delay line so you didn't start turning on the upper fet until the lower fet was off, and vice versa.

The current surge is brief - and since the devices take a while to turn fully on - not all that big. But big enough and fast enough to be an interference problem.

It's a timing problem. If your level shifters provided the right sort of delays, they could work.

The absolute maximum gate-to-source voltage ratings mean that you do need level shifters. You also have to get the timing right, so you are probably stuck with doing both.

no, you'd need digital pots and a phase-shift network, Pi (running windows or linux) is not realtime enough to chase zero crossings.

Check out Tim Williams site, he has different gatedriver designs if you really want to go down that troublesome road

He is a regular here

Cheers

Klaus

Thanks - I will look him up on Google :)

Tims site , direct link to gatedriver:

You're building a neighborhood EMI generator. Long line lengths make good radiaors. If you can't fix that, don't go any further.

Switching at KHz, nobody would notice.

I have an Amazon stick-on dimmable LED strip for by workbench. It PWMs at about 4 KHz.