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Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
bwang wrote ...
The datasheet says this can be worked around with a bypass cap, but I'm afraid the integrating effect of the capacitor will give me poor transient response.
Nope it's actually quite the opposite, if selected correctly the capacitor will actually improve your transient response.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Just be careful that the feedforward capacitor doesn't "feed forward" big enough voltage spikes to blow the controller chip. You won't see these spikes in your simulation because they come from filter capacitor ESR/ESL and wiring inductance. It's a wise move to have some series resistance.
Registered Member #2922
Joined: Sun Jun 13 2010, 12:08AM
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Posts: 226
Just be careful that the feedforward capacitor doesn't "feed forward" big enough voltage spikes to blow the controller chip. You won't see these spikes in your simulation because they come from filter capacitor ESR/ESL and wiring inductance. It's a wise move to have some series resistance.
Something between 470ohm and 1k will work just fine
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
bwang wrote ...
Goodchild wrote ...
Well yes and no; I do sample from the bus voltage, but not the bus voltage of the buck, rather the output of the buck which also happens to be the bus of the inverter.
Oops, that's what I meant by "bus voltage".
Gregory wrote ...
For some reason the LTC1041 doesn't like it when the divider resistors are too high (at least the simulation says that).
I think that with a higher resistor values you get less high frequency gain. Try increase the resistor and use a feed-foward capacitor at your divider, it will "derivate" and will increase the gain for the high frequency (ripple). Some 270p capacitor in parallel with the upper resistor of the divider will make the job.
And what do you know, the feed-forward capacitor works magic Though not in the way I expected it too. Turns out I missed the bit on the datasheet about charging the input capacitor in the LTC1041 Mmm I'm liking that waveform...hopefully it works in real life!
something I just noticed also you should use antiparallel diodes with your IGBTs in real life as well, most IGBT half bridge modules have them built in already so it probably won't be a big deal. I also added in the diode in my simulations as well, because you do get some switching action with the diodes even though it is a sync buck. The low side diode turns on at the same time as the low side switch.
On the note of a series resistor with the feedforword cap, also note that it will change the switching windows as well, so change the cap or resistor value accordingly.
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QCW coil
Though not in the way I expected it too.
