TinySwitch4 constant current mode

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I am working on a power supply for a smart LED lamp. The ouput requirements are: 300mA @ 25V. I used PI Expert to design the circuit based on the TNY285. My prototype works well, but I have a problem getting the device to work in C.C. mode. I made 2 observations:

1. the power supply supplies a lot more current that the designed requirement of 300mA at 25V.

2. When peak power is achieved the device resets.

Am I missing something basic? What is the way to adjust the output power without having the device reset?

Thanks in advance for your help.



For CV/CC LED applications, I recommend checking LYTSwitch-2. Anyway, there are several design examples using TinySwitch for CV/CC. However, there is an additional circuit for CC sensing. Please refer to DER-33 and DER-35 for references. In regard to PI expert, I will ask the PI expert team to check why there appears to be no schematic change between CV and CV/CC modes.


Hi Crumb,

I appreciate you looking into this.

I have gone through the desing with LYTSwitch-2. What are the resons you recommend this device over the TinySwitch-4. Is it the possibly lower power at no load (which is important for us), tighter/simpler current limiting circuit? My worry is that the lower switching frequency of this device is making it necessary to use a larger transformer (I get the CMA values way to low for EE16 and the EF20 might not fit our application).

It looks like for me to use the EE16 transformer I would need to increase the number of primary layers to 9. Would you be able to estimate the effects of this on the operation of the device (losses).

I just want to get the full picture and make the best decision.



Hi Witek,

I recommended LYT-2 because it does not need opto-feedback (less component count overall). In terms of transformer size, you can fit EE16 with LYT-2. I encourage you to use PIexpert online or use PIXLS Designer to design with LYT-2 and check. I did a quick lookup and it's definitely not 9 layers.

Hi Crumb,

Thanks for your quick responce. I have gone through the PIXLS and AN62 again and, yes, you are right, I should need fewer layer than 9. The problem was that I entered 3.1mm into B55. Since I will specify triple insulated wire, I should not need it.

It looks like my design is sorted for now.