Need Best Topology For Full Wave Rectifier

Thread Starter

reynolds087

Joined Jul 22, 2020
3
Hi, I am new here, and hoping someone can help.
I am designing a motor controller for a 110v DC motor. The motor is 800 watts, and going to be used as a spindle in my custom-built CNC mill.
I have got it working with a PWM controller I built, so I feel confident that I have the switching side dialed in very nicely, but I'd like to improve the power supply.

I have a massive 2000 watt, 70 VAC transformer that rectifies to about 109 volts DC. From there, I'd like to filter the output and get a nice flat DC output. I am familiar with a basic full wave rectifier circuit, but I am not skilled enough to design a circuit that will minimize ripple, provide about 12 amps of max output (allowing some headroom), roughly 800 watts continuous, with low ripple that will supply a switching motor control circuit with clean power. Simplicity of the design and low-cost components are also a major concern, and since I already have the big transformer I'd like to use that rather than SMPS. I've looked at capacitance multiplier circuits to reduce ripple, but I can't figure out what transistors to use, or how to put them in parallel. I have some really powerful mosfets capable of 500V at 100A.

I looked at possibly using a low-current LC or RC filter to remove ripple on the rectified output, and then use the mosfets to amplify the current, but I can't figure out how to do it. Max threshold voltage of the mosfets is only 30V, so that's why I thought maybe I am stuck using transistors? Really hoping someone can suggest the most efficient path forward with both cost and complexity considered.
 

WBahn

Joined Mar 31, 2012
26,068
What is your requirement for how low the ripple needs to be? If you can accept 5 V of ripple the solution looks very different than if you can't accept 5 mV of ripple.

What's the lowest DC voltage that you could use?

You might consider using a voltage regulator circuit to get down to a well-regulated 105 V (or whatever will work) and let the regulator eat the ripple.
 

Thread Starter

reynolds087

Joined Jul 22, 2020
3
What is your requirement for how low the ripple needs to be? If you can accept 5 V of ripple the solution looks very different than if you can't accept 5 mV of ripple.

What's the lowest DC voltage that you could use?

You might consider using a voltage regulator circuit to get down to a well-regulated 105 V (or whatever will work) and let the regulator eat the ripple.
To be honest, I don't know enough about brushed DC motors to say what the ripple generally should be, and there was no datasheet because it was just something I found on Ebay. I think 5 volts would probably be fine as long as it doesn't cause the motor to run inefficiently, create a lot of EMI, or reduce it's life. Are you suggesting to buy a high voltage linear regulator, and then drive the mosfets from that? I think I would still have the same problem with the max threshold of the mosfets being 30V but not sure?
 

AlbertHall

Joined Jun 4, 2014
10,243
Universal motors use brushes and they will happily run on AC - maximum ripple possible.
If you are using PWM then that also is making a maximum amount of ripple.
I don't think you need to worry at all about the ripple.
 

WBahn

Joined Mar 31, 2012
26,068
I had in my mind that this was for position control and so I thought perhaps the varying supply voltage might affect the accuracy/precision of the positioning. But I see that it is for the spindle, so I would agree that it should be quite happy running with the unregulated power from the rectifier even if there's a lot of ripple.
 

KeithWalker

Joined Jul 10, 2017
1,147
I would only smooth the supply to the speed controller circuit. You can tap it off the motor DC supply through a series diode on the positive side. It will be much simpler to just smooth that low current supply.
Regards,
Keith
 
Top