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TNY290 and PFC circuit

Posted by: thebremar on

Hi guys, 

I have designed an SMPS power supply using TNY290. The PS has 2x20V, each rail with 1A current capability, so in theory up to 40W, but in reality 36W. 

The power factor is quite poor here, believe it or not, it goes from 0.3 when it is in an idle state, and up to 0.7 when the motor is at full speed. The circuit has 270uF/400VDC right after the rectifier so probably there goes the low PF.

Now I want to add the PFC circuit to sort this thing out but I have never done that before. What I also want to achieve is to have PF from 0.8 to 1 in the full range of load. Btw, the motor is powered with the rectified voltage so that means that I have not only 40W but also an additional 80W that goes directly on the motor. 

Is it OK to expect that high PF below 5W of power load? Which IC would you use for this?

 

Regards

Marko

回應

Submitted by PI-TOMMY BRADFORD on 03/22/2022

Hello Marko,

Thank you for using our device.

It is normal and expected that at low load or no load condition, the power factor is low because the amplitude of current and intersection of the voltage and current is small. For example in application note 53 (https://www.powerint.cn/downloads/product-docs/an53.pdf) figure 1, the current leads voltage and is out of phase of 90 degrees. The area where the voltage and current intersects with one another is the power delivered at the load. At no-load, the amplitude of current is smaller so the area of intersection between current and voltage is also smaller hence the power delivered is also smaller. Therefore the power factor is expected to be small since power factor is equal to power delivered to the load over the power demanded by the circuit.

Generally at light load or no-load condition, the power factor is approximately 0.1 to 0.3 and at full load condition, the power factor is approximately 0.8 to 0.9 depending on the circuit.

If you want to further pursue to add a PFC circuit, here are some application notes that might help you:

1. Active Power Factor Correction - Basics: https://www.powerint.cn/downloads/product-docs/an53.pdf

2. HiperPFS application note: https://www.power.com/ja/downloads/documents/an52.pdf

We hope that this will resolve your inquiry.

Thank you and have a great day ahead.

Submitted by PI-Wrench on 03/28/2022

Another option for higher power factor without use of an active PFC is the "valley fill"  PF correction circuit.  As with any approach to PF correction, the PF will deteriorate at light load. This is true for an active PFC or for a simpler option like the valley fill circuit. Also, the maximum PF will be somewhat lower for the valley fill circuit as compared to an active PFC solution. Here is the link for a report describing an LED driver with valley fill PF correction:

https://www.power.com/sites/default/files/documents/der-637_35watt_high-pf_isolated_flyback_switched_valley_fill_pfc_lytswitch-6_led_driver.pdf