Right now we’re evaluating the switching power supplies for the new product and we’re going to have to use a higher-order passive filter on the output than the switchers are designed for (compensated for).
The switcher’s we’ll be using have a COMP pin, which allows you to add a network to move some poles/zeros around to adjust the stability. It was fairly clear that I couldn’t calculate directly the required values, or simulate it, so we’re going to have to build it and tweak it by hand.
Signal-generator controlled dynamic load
In order to properly tweak the stability (as well as test the stability overall) I really need to step the input supply voltage, and ideally also control the load. In an ideal world both of these could be controlled with a signal generator at high bandwidth.
Below is the little circuit I came up with today to control the load current from the signal generator. It has a bandwidth of about 100KHz (full 4A swing). The mosfet is IRLR8729PbF. The current sense resistor (also part load resistor, part current-limit resistor) will be lots of surface mount resistors in parallel to assure low inductance.
The lower op-amp just gains the current measurement slightly so that there’s a 1:1 correspondence between the signal generator (Volts) and the current (Amps) – i.e. 1V = 1A.
The top op-amp subtracts the real current from the desired current to create an error signal, then multiples this by 100. I tweaked the bandwidth-limiting capacitors to give the best step response.
For actual power supply test I’m thinking of using a high-current linear regulator and tie the low end of the resistor divider to an opamp, which will just adjust the signal generator voltage to provide a 1:1 correspondence between signal generator voltage and power supply voltage (within the operating range).
Also – thanks to everyone who came out Thursday night for our open house, it was a great time! I’m sure Nate will have pics later this week.
Joe
