Engineer It - How to test power supplies - Measuring Stability

hi my name is Bob Hanrahan application engineering at Texas Instruments and this is a series on measuring performance of power supplies here we'll be measuring stability of a power supply now a power supply is an amplifier obviously a DC amplifier but it's also an AC amplifier if you didn't have any AC component it wouldn't be able to react to load changes and like any control system we have to maintain a proper margin between the feedback of the output into the error amplifier to make sure that the output doesn't go in phase with the input and that amount of phase is the actual phase margin the today what will be utilizing is a traditional stability measurement tool called a frequency response analyzer or a network analyzer a low frequency Network analyzer designed for this measurement and that requires that there be some method of injecting an error signal into the feedback path of the control loop so I've shown on the graphic here we provide that point by adding a resistor in the feedback loop is above the topside Network in almost every regulator every power supply and this is in the order of 10 to 50 ohms for the testing the equipment will have an output that will be driven into an isolation transformer the isolation transformer is there so that there's no DC bias point that could either disturb the measurement itself or cause damage and then two points that come back up receiver points that come back back up into the equipment okay let's come over to the equipment itself in this case we'll be using an evaluation board it's the Ti TPS 56 to 21 now this is a 25 amp buck regulator with built in past devices we're feeding it with 12 volts from power supply here and it's delivering one vault up into a load box now when measuring stability it's important that you make measurements at all different operating conditions that you can expect what I mean by that is at minimum current output current and maximum output current in this case we'll run all the way down to zero output current and all the way up to 25 amps you also want to change your input voltage to from nominal to your maximum that you expect and the minimal that you expect and after you run with a load box highly recommend you connect your system because its output it's going to be different and run the stability one more time to get good results okay so let's now show you a couple of things first off calibration of a network analyzer is very important the technique for calibrating varies from system to system so I'm not going to get into that here now but we're using an AP 200 which is up here again probes coming down to the receiver point and there's a connection to the isolation transformer which is being injected into two connections that go to go down to a resistor that's on our evaluation board again we put all of our evaluation boards have that resisted just for this reason by the way good idea to put that resistor maybe even onto your PC board otherwise you'll have to wire it up too to get access to that feedback loop now we already calibrated and one method of verifying calibration with any system is to take your receivers connect them together to the output and verify that your bode plot becomes a straight line now I'll bring your attention to the GUI that patrol is drawing a bode plot right now and as you can see our phasing gain measurement is exactly horizontal so that's that's good news you know that everything is calibrated properly so now we go back and now we'll run some measurements I'll be running the plot at minimum current as well as as 25 amps so we'll turn on a power supply and now we see the bode plot being drawn now the blue line is your magnitude the actual gain of the feedback loop the red line is your phase and what we're interested in looking at here is the point where your magnitude pass is zero where my pointer is now move up to the phase and that Delta is the actual phase margin and this tool measures it for us and you can see down the lower left the phase margin in this case with no load is 51 degrees the gain margin is where the phase pass is zero here it's a negative gain you can see it's there and you could it measured minus fifteen point nine dB so you log that capture the screen for your log and then now I will turn the load box on full 25 amps you'll see another plot coming across it'll be very similar you can see there it's modifying the plot but because of the stability of this circuit it's almost identical as a matter of fact it's within a few degrees it's now 48 degree phase margin and minus 16 DB gain margin so in summary measuring stability gain and phase margin not complicated doesn't take long and it is important for almost every system so for more information on this or other videos on other power supply tests you can visit visit the following websites you

Channel: Texas Instruments

Views: 26,790

Rating: 4.9080458 out of 5

Keywords: webench, tps56221, power supplies, power supply, dc-dc, ac-dc, power management, testing, engineerit, fm_661, gp_39933, eq_361, eq_365, eq_362, eq_735, eq_737, eq_736, eq_738, bob hanrahan, robert hanrahan, how to test power supplies, testing power supplies, measuring stability, bode plots, switching power, switch mode power, regulators, power regulation, designing power, power design

Id: HJSaIqWzM9w

Length: 7min 7sec (427 seconds)

Published: Tue Mar 26 2013