We have a product that we need to operate offline, 85 VAC to 265 VAC, with a possibility of 300 VAC as we have a customer who uses 277 VAC.
The product that has 4 voltage rails :
24 VDC , 500mA = 12 Watts
15 VDC, 500mA =7.5 Watts
5 VDC 200mA = 1 Watt
3.3 VDC 200mA = 660mW
Total Load Watts =21 Watts
Assuming 84% efficiency = 25 Watts drawn by power supply
The original design was 12 watts loads assuming 15 Watts.
The Bulk High Voltage Capacitor was sized at 47µF 450V.
Moving to the 25 Watt had to increase the Bulk Capacitor to 80µF.
The original design used a 3844 current mode controller with a flyback operating in Discontinuous Conduction Mode .
The 3.3V, 15V and 24V Windings were stacked to share current to help with regulation, the 5V winding was on its own but referenced to the stacked 3.3V, 15V, and 24V winding.
The switching frequency was set at 130Khz.
The primary was design to be 268µH
The Transformer was custom designed, and is in a EF20 bobbin with 5 pins per side.
The high voltage side of the transformer has a split primary, a boot winding, and a shield connection.
The secondary had 5v, 5V_GND, 24V, 15V, 3.3V and 24\15\3.3V_GND
The 3.3V supply was monitored and regulated on.
The power supply did its job, but at full load was running a bit warmer that we liked.
The Transformer had about 2.1 Watts loss.
The mosfet was a bit toasty since it was not valley switched DPAK package.
The primary Voltage clamp, Mosfet Protection had about 1 Watt loss
We have the product in a custom enclosure with very tight size constraints.
The power supply has 30mm ( w ) ,70mm ( L ), 20mm ( H ) of available space in the package.
Since we needed to move to 25 Watts I have no room for the larger high voltage bulk Capacitor.
To save space, and reduce some of the heating I thought it best to abandon the UC3844, and move to a part with Valley switching, and limiting features.
I came across the TPS92010 used for LED lighting. It’s a BCM converter.
I thought by ding a single stage PFC ( active rectification ) that we could obtain an isolated raw DC, somewhere around 28V on the Secondary side.
We would then use low voltage switchers, and or linear regulators to obtain the various voltage rails as described above.
Physical size, cost, and heat are very important..
Does TI have any recommendations for parts?
I don’t mind keeping the current custom transformer design, but I have no room for a larger high voltage bulk capacitor.
The single stage PFC is a thought but of course the secondary side regulation components will make things toasty too.
Is there a fixed frequency current mode valley switching offline controller TI can recommend?
Would it be better to do a 2 stage design using PFC as a replacement to the high voltage capacitor, then use the flyback to provide the secondary voltages?
