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.

**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.


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