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| Need helping figuring out noise and issues with TPS40210 AND QX9920 + LED |
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| MatCat:
I amm trying to drive a CREE LED rated for 36v / 50 watts. I used TI Power Bench app to come up with the circuit using TPS40210, and someone else had pointed me to QX9920 for current control. The power supply for this project is a 360W 12V power supply. The TPS40210 is set to put out 42V, and the QX9920 is set to draw 1.36A of current. However it is only drawing about 170mA, the voltage supply circuit is supplying 42V, however its extremely noisy. This is actually the second version of this board I did, I respun in to add a bunch of caps and a common mode choke on the 3.3V and 5V rails. Attached are schematics for the QX9920 and TPS40210 circuits, as well as various measurements of the 5V rail, 12V rail, QX9920 current sense, and mosfet drive pin for the QX9920. Any help possible in getting this circuit to work is GREATLY appreciated. Any additional info needed and I will gladly post. |
| MagicSmoker:
I've come to the conclusion that the TPS40210 is terminally stupid, especially in how it deals with (the inevitable) noise on the current sense signal. Try increasing the filter capacitor C23 to achieve a time constant of around 200-300ns; reduce the value of R6 if necessary*. The bursts of ringing are from the lumped stray capacitance at the drain of the switch (including from the Schottky) resonating with the boost inductance. An RC damper across the Schottky or the switch (preferred) can help here. * - EDIT - this sounds backwards - and it is - but the key here is that you don't want to arbitrarily increase the impedance of the current sense signal line. For example, you may need to triple C23 but cut R6 in half to get a good combination of time constant and node impedance. |
| MatCat:
I put a snubber across Q1 consisting of a 820pF cap and 33 ohm resistor, attached is the images of current sense for QX9920 and mosfet gate of QX9920, it's better, but not good enough, still got plenty of ringing noise that is higher then the 250mv threshhold for the QX9920 current sense. I also changed the QX9920 filter cap to a 220pF from 150pF. |
| MagicSmoker:
What is the switching frequency supposed to be* and what kind of resistor are you using for current sense? Note that any stray inductance in the resistor adds a leading edge spike to the current sense signal, and it doesn't take much inductance to equal - or exceed - 9mR. You can also try increasing the gate resistor to slow down the transitions - and you want to change it, anyway, because no one uses values from the E192 series for gate resistors!?! Maybe try 15R to 22R. More/better decoupling for the TPS40210 couldn't hurt, either. Last but not least, the layout of switchers is crucial to good performance. You need to minimize the area and trace length of the connections between inductor, switch and diode (aka, the "switching node") and minimize the loop area of the diode - output capacitor - ground return. * - I'm too lazy to figure it out from the RC values. |
| MatCat:
Switching frequency of the TPS40210 is set to be 565KHz, the current sense resistor is a 2512 PE2512FKE7W0r009L I tried to lay everything out according to the datasheet recommended layouts and to keep the circuitry outside of the primary current loop, if I have to though I can completely re-lay it out. I attached an image of the layout, it has a few notes on it. I went big on certain copper areas because I wanted to help minimize heat, though with a heatsink on the giant overkill diode I am using for the TPS40210 there is no heat, so next spin I will drop that down to TO-220 with heatsink. |
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