• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Bias power requirement of TNY287 Flyback SMPS controller

Status
Not open for further replies.

Flyback

Well-Known Member
Hi,
We are using the TNY287 for a offline isolated Flyback SMPS (10W), and we are powering the TNY287 from a bias winding. (schem attached)

Page 6 of the datasheet says we must supply 1mA to the TNY287, however page 12 says that the TNY287 requires 550uA.

Page 15 of the datasheet states that if TNY287 us supplied with over 4mA , then it will shut down.
Please advise if the TNY287 requires 550uA or 1mA?
We need to know, because when our secondary is light loaded, our bias coil collapses from 24V down to 14V.........we want to set the feed resistor (R18) to as low value as possible (feed resistor is the resistor that feeds the BP/M pin from the bias coil) so as to reduce dropout of the bias coil in light load as much as possible.

If the 550uA figure applies, then we can set the feed resistor to 11k, but if the 1mA figure applies, then we must set the feed resistor to 6k2.
If we set the feed resistor to 6k2, then when the secondary is loaded, and the bias coil thence goes up to 24V, then we are feeding the TNY287 perilously close to the 4mA maximum.
Please advise the actual bias current draw of the TNY287.

TNY287 datasheet:
https://ac-dc.power.com/sites/default/files/product-docs/tinyswitch-4_family_datasheet.pdf
 

Attachments

alec_t

Well-Known Member
Most Helpful Member

Flyback

Well-Known Member
Thanks, i can measure the current into the BP/M pin...but this is consistent of the bias current, plus the current that just get shunted to ground by the internal regulator...and theres no way if splitting these currents out.
 

alec_t

Well-Known Member
Most Helpful Member
Then if the datasheet is giving conflicting values, I'd consult the chip manufacturer for clarification.
 

ronsimpson

Well-Known Member
Most Helpful Member
Page 6 of the datasheet says we must supply 1mA to the TNY287, however page 12 says that the TNY287 requires 550uA.
Please advise if the TNY287 requires 550uA or 1mA?
If the 550uA figure applies, then we can set the feed resistor to 11k, but if the 1mA figure applies, then we must set the feed resistor to 6k2.
Page 6 is a generic 1mA. Not 1mA +/-5%. It covers all 7 parts in the family.
Page 12 is detail. It has typical and max. It has different values for each of the seven parts.
Then if the datasheet is giving conflicting values
I don't think there is any conflict.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
You do not _need_ to supply the BP pin at all; that's purely an option to reduce consumption via the drain pin whilst the PSU is under light load.

Page 10:
Reducing No-load Consumption As TinySwitch-4 is self-powered from the BYPASS/MULTIFUNCTIONAL pin capacitor, there is no need for an auxiliary or bias winding to be provided on the transformer for this purpose. Typical no-load consumption when self-powered is <150 mW at 265 VAC input. The addition of a bias winding can reduce this down to <50 mW by supplying the TinySwitch-4 from the lower bias voltage and inhibiting the internal high-voltage current source. To achieve this, select the value of the resistor (R8 in Figure 16) to provide the data sheet DRAIN supply current. In practice, due to the reduction of the bias voltage at low load, start with a value equal to 40% greater than the data sheet maximum current, and then increase the value of the resistor to give the lowest no-load consumption.

If you really want to use a supply to the bias pin, use two series resistors with a zener from the junction to common, so that point never exceeds a suitable voltage to give something in the approx. 1 to <4 mA range.

It is a trial and error process, according to the data sheet paragraph quoted above.
 
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top