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  • Author Author: jc2048
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TPS54A20EVM evaluation - part 2

jc2048

In this next part I'm going to look at the output on a static load.

 

Here's my output load:

 

 

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For 10A at 1.2V I need a load of 120 mOhms (1.2V/10A) and it has to be able to dissipate 12W (1.2V x 10A).

I made this up from cheap 1 ohm 0.25W resistors. Three resistors in series is 3 ohms; twenty-five of those in

parallel is then 120 mOhms. Each resistor only has to dissipate 160mW, so they're reasonably comfortable. It

measured 123 mOhms, so that will be an output current of something like 9.7A [it will move a bit when it heats up].

 

Here's the output voltage running the load. [Being a bit contrary, I'm running against the excellence-in-

photography tide.] That's fine - it's just 0.3% high.

 

image

 

 

Next let's look at the output voltage ripple on load. The following two traces are of exactly the same signal - the first is using the probe's flying-lead ground and the second is using the spring clip ground. Both traces are ac coupled.

 

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Next is the ripple with the bandwidth limited to 20MHz and accumulated traces. This nicely shows the underlying form of

the ripple and we can immediately see that there are four phases which presumably correspond to the phases that the

converter goes through, the spikes coming at the transitions which we would kind of expect.

 

image

 

Finally here's one of the spikes. Unfortunately I'm right up against the limits of my scope here (200MHz,

1Gsps and passive probes). The rise time of the scope is going to be 1.7nS, or so, and that's what I'm seeing

here, so the spike itself is much faster and far taller. The peak on the trace is where the scope is scrabbling to catch up with the spike

when it's on its way down again. Essentially, I'm testing the response of the scope with an impulse waveform.

 

image

  • in reply to rachaelp

    No, it was quite fast. The resistors were on a bandolier, so I cropped one side and the band on the other then held them in place and gave the neat spacing. Went along and soldered them to a piece of tinned copper wire. Cropped the other side, repeated three times, final piece of wire along the other side and that was it. The advantage (other than spreading the heat, low-ish inductance, and the low cost - about 50p) is that I will be able to split it up and do transient testing between set currents (100% to 20%, and that kind of thing).

  • You must have some patience to solder all those individual resistors together, that must have taken you ages!

     

    Very nice write up, it's good to see you showing the difference between using the long probe flying lead and the spring clip for grounding, the excessive bouncing around or spikes caused by using 6" of wire for the ground catches a lot of people out and they chase their tails trying to work out why their signals look so bad when in fact it's just their scope probe configuration.