LMR16030: LMR16030 issue with AC input source

Hoi Wa Ip

Part Number: LMR16030

Hi TI experts,

We leveraged LMR16030 to step down a 24VAC input to 12VDC@2A output in a thermostat project (see schematic diagram below), where the AC source input is full bridge rectified to DC before feeding to LM16030. We noticed that when LMR16030 is connected to a load and source 1.6A current, it somehow affects AC source input waveform(original Sine wave),and we observed corrupted Sine wave signal. This is not what we want, as we expected that the AC waveform remains Sine Wave while LMR16030 is functioning. Is there any way to fix this issue, please kindly help? Thanks.Peter

over 8 years ago

0 Luke Pan over 8 years ago

TI__Intellectual 2790 points

Hi,
Where did you measure the sine waveform? At input side of the full bridge or at the AC source input side?
There will be inrush current at the AC input side when using full bridge rectifier. If the AC source is weak, it may distort due to the large inrush current.

0 Luke Pan over 8 years ago in reply to Luke Pan

TI__Intellectual 2790 points

Minimizing C68 will decrease the distortion.

0 Hoi Wa Ip over 8 years ago in reply to Luke Pan

Intellectual 720 points

Hi Luke,
The distorted Sine wave was measured at AC source input side. What we really want to acheive is trying to minimize signal distortion as much as possible while allowing LMR16030 deliver 12V@2A... The same AC input signal source will be feeded to other subsystems in a Theromstat product. If it is servely distorted, it will defintely cause malfunction to other subsystems. By the way, do you have any good suggestion on AC/DC conversion, which will help improve in rush current effect? Peter

0 Luke Pan over 8 years ago in reply to Hoi Wa Ip

TI__Intellectual 2790 points

Below circuit can improve the PF. However it may result in the input voltage of LMR16030 drops to below 12V sometimes for 24VAC input. So may need a transformer to boost the AC voltage to 48VAC first, then apply below circuit to minize the distortion of the sine waveform.
www.infineon.com/.../an-1074.pdf
An other solution is APFC circuit which will minize the distortion most.
pdfs.semanticscholar.org/.../0e4dd963d029222ca77bbe6a3ee5e1c2d289.pdf

0 Alan Martin over 8 years ago

TI__Expert 4365 points

There is a different approach to Valley fill in the following Ap Note. Perhaps it can be considered.

www.ti.com/.../sluuba8.pdf

Alan Martin

0 Alan Martin over 8 years ago

TI__Expert 4365 points

Hello again,

Perhaps the better answer is to consider applying a Power Factor Controller approach. This will essentially drawn sine shaped current waveform and reduce distortion. The following link will lead to the PFC product lineup. There are numerous application notes and reference designs. Usually these designs are created for off-line applications with 100-240VAC input range. But many of the actual PFC ICs operate from fairly low Vcc so could probably be applied to a 24V secondary application. Look through the following guides and see if there is an appropriate application and affordable solution. I'm sure thermostat controls are very challenging when BOM cost is concerned but if low input distortion for the power level you are considering is of primary* importance, consider this approach.

http://www.ti.com/lsds/ti/power-management/power-factor-correction-products.page#p1694=Single-phase PFC

*(Sorry about the pun)

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LMR16030: LMR16030 issue with AC input source