How about a bidirectional TVS like this one? would it do the job just as well?

I think they work okay, it's just that they really slow down the turn on/off time of the FET because of the high capacitance.

TVS


Zener

 

Thanks, I see what you mean now.
So, would it be ok if I used two back-to-back zeners with a breakdown voltage of 15V each instead?

 

Alright, my friend, I'm back, and ready to rumble again.

And I'm ready to learn. From what's been said so far, I guess I had another misunderstanding about these things. I thought they were used on the "logic" side of a circuit. To protect that side of the circuit from spikes and other bad things, not on the "power" side of the circuit.

 

Here is a fairly comprehensive pdf from Onsemi on both.
http://www.onsemi.com/pub_link/Collateral/HBD854-D.PDF
Max.

 

Thanks, I see what you mean now.
So, would it be ok if I used two back-to-back zeners with a breakdown voltage of 15V each instead?

Yes, I think back to back is the typical way. But as this points out noise in one way turns it on.
http://www.irf.com/technical-info/appnotes/an-936.pdf

 

And I'm ready to learn. From what's been said so far, I guess I had another misunderstanding about these things. I thought they were used on the "logic" side of a circuit. To protect that side of the circuit from spikes and other bad things, not on the "power" side of the circuit.

They're used to protect the whole thing! Mosfets, powerful as they are, are very delicate components. And their price will wake up the fraternal/maternal instinct of any designer.

 

Here is a fairly comprehensive pdf from Onsemi on both.
http://www.onsemi.com/pub_link/Collateral/HBD854-D.PDF
Max.

That looks like very interesting reading, Max. I'll definitely go through it next week. Thanks!

 

So, I'm already building the circuit, and will be testing it in the following days.

​

Question: Why do I feel a little uneasy about connecting the ground from the output of the 120VAC rectifier to the same ground of the rest of my circuit? This includes the MCU, oscillator, RS232 interface and a computer...

 

It sounds similar to a DC motor drive I am working on, RS232 Modbus control using a Picmicro, I have isolated the AC powered DC supply from the Pic with MOC8106 opto.
Using a slot opto for rpm monitor/control.
If your AC supply to the rectifier is galvanically isolated from the supply then you should be able to common up the -ve side of the HV supply.
Max.

 

Believe it or not, I'm about to finally start building this thing. I've already got all the components and I'm getting started on drawing the PCB.
I'm going to completely isolate the circuit using a DC-DC converter, and interface it to the MCU using an opto copuler, as Max has suggested. If all goes well, I might even add a simple external 555 controller instead of the MCU and post the whole thing in the completed projects collection.

Question: I'd like to make the motor come to a full stop real quick. For this, I'm planning on applying PWM reverse voltage through it while it's still spinning. Would the back EMF being generated by the motor destroy the MOSFETS? Or is the answer a matter of how much power the MOSFETS are capable of handling?

 

Normally braking would be achieved by turning on M2 & M3 but using a IR2110.
Max.

 

Normally braking would be achieved by turning on M2 & M3 but using a IR2110.
Max.

After looking at the datasheets, the main difference I could find was that the ir2110 lets you control the high and low side mosfets independently, thus allowing you to control the dead time between switching. But the ir2104 already includes a dead time of about 1/2 us.
Why are you suggesting a ir2110 instead? Do you think an ir2101 would be a better choice than the 2104? 'cause I have a few of those laying around.

 

I prefer the IR2110 because I can turn on fully the bottom mosfet while PWM'ing the top Mosfet this also conforms to the Picmicro PWM module standard output method.
And also gives the option of braking by switching on the bottom mosfets.
This would be the preferred way instead of reversing the voltage.
Max.

 

After looking at the datasheets, the main difference I could find was that the ir2110 lets you control the high and low side mosfets independently, thus allowing you to control the dead time between switching. But the ir2104 already includes a dead time of about 1/2 us.
Why are you suggesting a ir2110 instead? Do you think an ir2101 would be a better choice than the 2104? 'cause I have a few of those laying around.

I learned the hard way (twice) that you don't want to reverse it to stop it if you are using a power supply. When you reverse it the voltage across the motor gets added pumping up the supply voltage. So shorting it with the bottom FET's is the way to go.

 

Also of course, remember to switch the opposite Upper and lower sides, as per link in post #4.
Max.

 

Thanks Max. The IR2104 is supposed to work like that. With the lower side always on when the upper side is not active. So just by bringing both IR2104 inputs low, both lower fets remain active, allowing for the motor to dissipate its energy in its own winding (dynamic braking?)... I think... If I've said something stupid, please let me know.

Thanks for the warning, Ron. I'll try to avoid reversing the voltage to rapidly stop the motor... see if I can come up with a different technique.

Anyway, here's the motor. In a few days I'll be building the circuit and hope for the best.

​

Gonna give this a careful read, btw... see if I understand the problem thoroughly...

 

Question. Considering the diagram shown in post #88, how hard would it be to include back EMF into the simulation?
I'm guessing it would be real hard...

 

Another question. I'm considering replacing the FDA33N25 mosfet with an STGW30H60DFB IGBT, because I'm under the impression that IGBTs are sturdier for motor control applications than mosfets.

Can that IGBT directly replace the mosfet, using the same driver chip?

 

Have you seen the IR sheet on the comparison?
Max.

 

Have you seen the IR sheet on the comparison?
Max.

Thanks for posting. Yes, I have. And although this is a relatively low voltage application (90VDC) it's also highly inductive (although the transistors are being protected by TVSs) and low frequency (I don't think switching above 10 kHz will be necessary)