Low Voltage Step-Down Converter TPS54A20 - First Check

RE: Low Voltage Step-Down Converter TPS54A20 - First Check

jc2048 — Fri, 05 Aug 2016 11:34:55 GMT

This is fun. Couldn't resist looking at the compensation on the feedback.

Here's what it looks like in a simulator.

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The response [of the converter] is rolled off from a couple of 100kHz upwards. Might be interesting to hit the output with a heavy load at a few hundred kHz and see what happens (as that's the area where they think the compensation is becoming necessary but it's only just starting to have effect). With a power FET, load resistor and a waveform generator you could do a slowish sweep from 100kHz up to 2MHz and quickly see if there were any problem frequencies where the output was getting a bit skittish.

Sorry if I'm hijacking your thread a bit. Tell me to go away and do blogs of my own, if you want.

RE: Low Voltage Step-Down Converter TPS54A20 - First Check

jc2048 — Thu, 04 Aug 2016 21:31:48 GMT

"Leave your better explanation of the design in the comments."

Not sure this is a better explanation, but this is the way I'd look at it.

When Q1a is on, current flows through the capacitor and coil La from Vin - charge accumulates on the capacitor, the coil establishes a magnetic field, and the load is powered. When Q1a turns off, and Q2a turns on, the coil La keeps the current to the load going, gradually depleting its magnetic field (because that's what coils do). At the same time, Q1b turns on and the energy in the capacitor (notice that the negative of the capacitor is now connected to ground by Q2a) is used as the power source for the other phase, with Lb establishing its field and driving current into the load. After a time, Q1b is turned off and Q2b is turned on, and Lb keeps the current going by retrieving energy from its field.

The steady state average (there has to be some ripple) voltage on the cap is half the supply because during one phase it hangs from the supply and for the other it sits on ground. If the supply voltage to the coils is to be the same for both phases, that can only happen if the capacitor voltage splits the supply in two (the voltage driving La is the supply minus the capacitor voltage, the voltage driving Lb is the capacitor voltage). I'm not clear whether that happens naturally by virtue of the circuit or whether they have to force it through monitoring the currents and controlling the switching. I think it self-adjusts - if the capacitor voltage increases, the current in La goes down, the current in Lb goes up, that drains the capacitor more than it charges resulting in the voltage coming down. Does that sound reasonable?

RE: Low Voltage Step-Down Converter TPS54A20 - First Check

Jan Cumps — Thu, 04 Aug 2016 19:59:56 GMT

Object under test.

1.2 V, 1 A flowing trough the load. I'm trying to capture how the series capacitor on its own steps down the source voltage to half value.

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(also posting this because I think it's a sweet photo)

(power cables courtesy [mention:f80b53cee57c44bc9d7c577d07d7c791:e9ed411860ed4f2ba0265705b8793d05])

RE: Low Voltage Step-Down Converter TPS54A20 - First Check

jw0752 — Wed, 03 Aug 2016 00:02:37 GMT

Hi Jan,

This looks like a fascinating circuit. I will look forward to learning more in your subsequent posts.

John