Regulator step response and stability testing

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.

LTSpice sim of voltage controlled load current.
LTSpice sim of voltage controlled load current.
Signal-generator controlled power supply voltage

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

One of two power supply and load switch test PCBs. This one will does not have the extra output filtering or text fixtures I talked about above.
One of two power supply and load switch test PCBs. This one will does not have the extra output filtering or text fixtures I talked about above — that will be in the next rev.