this has always bothered me a little. PWM sends a pulsetrain at 500Hz which, in components with a slower reaction time, generates a current proportional to the duty cycle.
From the Sparkfun documentation on PWM:
So as I understand it, mosfets connected to PWM turn on and off every cycle 2ms. Is this correct?
Then doesn't this create timing issues where one mosfet goes into conduction while another is still leaving its conducting state?
I've never seen this mentioned. What measures can be taken to prevent this issue (if there is any).
Here the stripped down version of the H bridge like we've all seen in school to provide some clarity.
Supposed they are labeled as
1 2
3 4
B is fixed high, so 4 is conducting permanently and the B side is connected to mass.
A is receiving the PW signal from the arduino. On A = low, 1 conducts and 3 is not, so the motor runs. When A turns high, 3 might turn into conducting before 1 goes out of it (for the sake of this example, let's take the cut off is 1V). Doesn't create this very briefly a short circuit?
Obviously H bridges work fine, but this keeps bugging me... Feel free to correct me, school has been a while.
PS: I tried adding this to the electronics forum, where I feel it is more suited, but my account kept logging off whenever I went onto that page. So apologies for posting it here.
This is a simplified diagram. The easiest way to prevent a short is adding 2 diodes to the gates, so the on-values differ a bit and there's a deadband in the raising/falling flank where both fets isolate.
Well, I assumed so. The above diagram is how I was thaught in school and it has bugged me since.
I assumed it was just a simplified diagram, but whenever I see a tutorial I never see this issue mentioned while other issues with this simplified version (like floating ground, single input control, etc...) are pointed out. So it made me worry if I was seeing something that wasn't there.
AndereSpeler:
Then doesn't this create timing issues where one mosfet goes into conduction while another is still leaving its conducting state?
This is called shoot-through and its extremely bad news.
I've never seen this mentioned. What measures can be taken to prevent this issue (if there is any).
Dead-time is used to prevent this. If you look at the datasheets for high-low MOSFET driver chips you'll
frequently see the dead-time specification, or even a programmable amount of dead-time.
Here the stripped down version of the H bridge like we've all seen in school to provide some clarity.
Supposed they are labeled as
1 2
3 4
B is fixed high, so 4 is conducting permanently and the B side is connected to mass.
A is receiving the PW signal from the arduino. On A = low, 1 conducts and 3 is not, so the motor runs. When A turns high, 3 might turn into conducting before 1 goes out of it (for the sake of this example, let's take the cut off is 1V). Doesn't create this very briefly a short circuit?
Absolutely, that's a bad circuit - it is likely to fail rapidly, and for high power the typical failure mode is
the devices explode their packages!
That's definitely not how to build an H-bridge. You see this poor design far too often though. If you want
the real deal I'd suggest looking at the datasheet for a full blown MOSFET H-bridge driver chip, the HIP4081A: https://www.renesas.com/eu/en/document/dst/hip4081a-datasheet
This explains all the protection circuitry and dead-time management and shows how all n-channel
MOSFETs are used in bridges these days (n-channel have 1/3rd of the on-resistance to p-channel for
the same voltage and die-size, so it really matters).
A massive advantage of this style of H-bridge is that the power supply for the motor doesn't have to be 12V - it can be anything within the voltage rating of the MOSFETs and driver (very low voltages too if you want). And most importantly you can power-down the driver or the bridge independent of each other without phantom powering.
