Saturday, January 7, 2012

Hysteresis Control Simulations

Teravolt pointed out the existence of an excellent chip on 4HV - the LTC1041 is a complete bang-bang controller in a cute little 8-pin DIP package. It can only sample at 10KHz, but 10KHz should be plenty given we're trying to pass a 100Hz waveform through the modulator.
And happily, the chip is from Linear, which means LTSpice has a built-in model for it.

1/2 ohm load, near-zero deadband:
1/2 ohm load, 100 mV deadband. Notice the decreased switching frequency:
5 ohm load, small deadband. Now the change in the integrator formed by the output filter and load decreases the frequency:
The switching frequency depends on the load - the more we load the output, the faster the converter switches and the smaller the ripple becomes.

Inductor and capacitor currents (green is the capacitor current):

Same, but with smaller L. The output (red) is still acceptable, but now the capacitor ripple current is horrendous. The tradeoff between L and C essentially boils down to that - how much ripple can your capacitor tolerate? Smaller inductors are less lossy, but capacitors that can handle 100+ amps of ripple are bulky and expensive.

UPDATE: 4HV worked its magic as usual, and told me to put a feed-forward capacitor in parallel with the upper resistor of the divider. This worked very well, as it forces the controller to switch at higher frequencies.

LTSpice file is here if you want to play with it; Eagle symbol for the LTC1041 here.

Good morning! And, good night!