schnell
Electrical
- Apr 26, 2010
- 105
hi,
I am making an offline, isolated SMPS which must provide 38W maximum at V(out) = 30V. V(in) = 85 to 265VAC
Sorry in advance for the length of this post!
However, it also has a "standby" mode where it is actually loaded with an average of 170mW.
-This 170mW is not a constant load, but represents a pulsed load, pulsing fully on and off with low duty , at around 4Hz
-When its in "standby", it must draw no more than 500mW from the mains.
I am sure that we all know how badly auxiliary coils are at providing the controller voltage rail when the secondary load varies over such a huge range as in this case.
....the problem of auxiliary rail voltage "wandering" is made worse by the unpredictability of the "wandering"......the voltage range of the auxiliary voltage rail depends on the amount of transformer leakage inductance.........and unless SMPS transformers are wound expensively, the leakage inductance usually has a wide tolerance of values.........so getting the right number of turns on the auxiliary coil in such cases as ours is a truly hit and miss affair.
-the only way to get sufficient voltage output from the auxiliary coil in no-load is to wind too many turns for the auxiliary, and then linearly regulate it so that it does not over-voltage the controller when the SMPS is on maximum load.
and as i said, the amount of turns is by no means a strightforward calculation, and may not work with differing amounts of transformer leakage.
-in "old" times, we simply let the auxiliary coil "drop-out" and the controller would then be supplied from the high voltage BJT regulator connected to the DC bus....but with our new 500mW maximum standby power draw regulations......this kind of thing is no longer possible, as it takes too much power.
With regard to this, i am just thinking of getting rid of the auxiliary coil altogether and using a LNK302 to efficiently provide the SMPS controller power supply.
(-LNK302 is a controller for a very low component count offline buck)
(-This is not a marketing post....since in any case powerint.com do not recommend using LNK302 as i have described......on the contrary , powerint.com seem stuck on the idea of using an auxiliary coil)
So in any case, my question is, since we now have these stringent demands for low standby power, why is it that every SMPS now does not just do away with the auxiliary coil and use a LNK302 to provide the SMPS power supply?
-especially when the secondary load is variable over a wide range.
After all, LNK302 is very very cheap.
I am making an offline, isolated SMPS which must provide 38W maximum at V(out) = 30V. V(in) = 85 to 265VAC
Sorry in advance for the length of this post!
However, it also has a "standby" mode where it is actually loaded with an average of 170mW.
-This 170mW is not a constant load, but represents a pulsed load, pulsing fully on and off with low duty , at around 4Hz
-When its in "standby", it must draw no more than 500mW from the mains.
I am sure that we all know how badly auxiliary coils are at providing the controller voltage rail when the secondary load varies over such a huge range as in this case.
....the problem of auxiliary rail voltage "wandering" is made worse by the unpredictability of the "wandering"......the voltage range of the auxiliary voltage rail depends on the amount of transformer leakage inductance.........and unless SMPS transformers are wound expensively, the leakage inductance usually has a wide tolerance of values.........so getting the right number of turns on the auxiliary coil in such cases as ours is a truly hit and miss affair.
-the only way to get sufficient voltage output from the auxiliary coil in no-load is to wind too many turns for the auxiliary, and then linearly regulate it so that it does not over-voltage the controller when the SMPS is on maximum load.
and as i said, the amount of turns is by no means a strightforward calculation, and may not work with differing amounts of transformer leakage.
-in "old" times, we simply let the auxiliary coil "drop-out" and the controller would then be supplied from the high voltage BJT regulator connected to the DC bus....but with our new 500mW maximum standby power draw regulations......this kind of thing is no longer possible, as it takes too much power.
With regard to this, i am just thinking of getting rid of the auxiliary coil altogether and using a LNK302 to efficiently provide the SMPS controller power supply.
(-LNK302 is a controller for a very low component count offline buck)
(-This is not a marketing post....since in any case powerint.com do not recommend using LNK302 as i have described......on the contrary , powerint.com seem stuck on the idea of using an auxiliary coil)
So in any case, my question is, since we now have these stringent demands for low standby power, why is it that every SMPS now does not just do away with the auxiliary coil and use a LNK302 to provide the SMPS power supply?
-especially when the secondary load is variable over a wide range.
After all, LNK302 is very very cheap.
