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Registered Member #882
Joined: Sat Jul 07 2007, 04:32AM
Location:
Posts: 103
One idea leads to another as always, and so my quest to make a laser o-scope for audio display led to motor control.
I wanted to drive a radio-shanty motor really slow. Without doing any math, i figured a voltage divider would not be able to bring the speed down enough, and i didn't even want to consider dissipation in a pot.
"Shouldnt that inverter i've been playing with on breadboard do the job?," i thought. I was almost right.
The typical half/full-bridge we consider on 4hv are for driving transformers, ala SMPSs. Be it a flyback, TC primary, induction heating, or a custom wound ferrite-power transformer for your CCPS, its all AC output (DC biases aside, and yeah, the CCPS would be rectified back to DC, but you get my point).
My lil DC-Brushfull motor wanted DC though.
Most DC motor controllers will use a H-Bridge. But i didn't need to reverse my motor, and the extra part count and control for 2 more fets was undesirable. Plus, i wanted to use my preexisting 1/2-bridge somehow.
I found mention of the half-controlled h-bridge somewhere on wikipedia (i forget what article, but somewhere in relation to motor control), and a ill google-ing gave me a schematic that demonstrated the topology.
Just replace two of the switches with diodes, as so, and the H-bridge becomes a unidirectional 1/2-bridge. I just adjusted the frequency to where i could reduce the duty cycle as low as possible without stalling the motor.
I forget who's tl494 layout i used, but it was from someone's TC.
Project Page, for the laser o-scope, not too much focus on the motor control, but pics are pics (vids at the bottom; excuse the domain, my playing-with-plone site turned into my website, and i haven't gotten my own hosting yet.)
This topology might also be useful for the magnetic suspension desk toy i'm planning....
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
The more common name for this is "Asymmetric Half Bridge" and it is indeed often used for motor control applications.
You also occasionally hear terms like "Asymmetric forward converter" or "Two-switch flyback converter". Both of these are just variants on the standard single-switch forward and flyback converters. However, they take advantage of the fact that the diodes clamp the back EMF of the inductive load to the DC bus. This has the consequence that each switch only needs to be rated to the DC bus voltage plus a small safety margin. This is typically a lot less than the DC bus + reflected voltage + leakage inductance spike + safety margin, particularly in the case of the flyback converter!
The downside is the added complexity of driving that high-side switch!
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
That is true, but there are applications for the Asymmetirc half-bridge where you don't necessarily want the drive to be... erm... Symmetric
The GDT is great for inverters but not so good for things like a forward converter or motor drive where you might not want equal + and - times in the gate drive signal.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
GDTs are completely useless for motor drive, because you regularly want duty cycles near 100%, which makes resetting the core impossible. Use the IR21xx high-side drivers instead.
If you don't want to reverse the motor, you only need one MOSFET, one diode and no high-side drive circuitry.
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half-controlled H-bridge
