Output voltage sag at low temperature
We are using a TNY290 in a 7.2V @ 2A design. At low temperatures, the voltage sags under load and the TNY enters a cycling mode where it will generate 14 drain pulses then an off time of 120us. This occurs even at 1A load. This condition was improved by increasing the output capacitor values by a factor of 10x over the PI Expert values (330uF increase to 3300uf and 100uf increased to 1000uF). We are still seeing the voltage sag and periodic operation at -30C and below. What is the cause of this pulsing mode and do you have recommendations on correcting it?
Comments
Hi Terry,
Attached are scope plots and the design file. The design has been updated to use the TNY290 instead of the TNY288. In the scope plots, pink is the 7.2V output, green is the drain waveform, and yellow the drain current. All taken with a 40mA load stepped to 2A then removed (although the symptoms are present at 1A step load as well). With the nominal circuit values there is a transient in the output voltage when the step load is applied even at ambient temperature. Lowering the temperature aggravates the issue and ultimately causes regulation to be lost when in the "periodic" mode. Modifying the output capacitor C8 to 1000uf or higher improves the performance. We suspect that the issue with the output capacitors may not be the capacitance itself but the decrease in ESR with the larger capacitor values. One of our concerns also is that the leakage inductance of the transformer is fairly high and there may be an interaction between the leakage inductance and the output capacitors. Any insight you can shed would be appreciated.
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| Data for voltage sag at cold (970.47 KB) | 970.47 KB |
Hi Terry,
Do you have any input for us on the voltage sag issue? As a note, we ran additional transformers through your rapid prototype service. The first had modified gap and shield removed to lower leakage inductance. The second also had the 7.2V output wound with 4 32AWG wires instead of the single 28AWG to improve coupling between primary and secondary. Both of these versions are also exhibiting the voltage sag out of regulation as temperature is lowered and the periodic operation (bursts of 14 drain pulses). We could use some guidance on next steps. Thanks for your help.
Hi,
Thanks for sharing the information.
I have not really seen this behavior under cold temperature as what you just described in any of the TinySwitch designs before.Would you please share with me the waveform and the design documents that I can further assistant you? The comparison waveform with 330uF and 3300uF capacitor will be also helpful. If this happens, 14 continuous pulses and then off even at 1A load, does it mean the output voltage is out of regulation?
Thanks