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Design with LNK3202D and LNK3204D + defining saturation current coils

Posted by: Nomien on

Based on the datasheet of the LNK3202D and LNK3204D, nothing is mentioned about the max. saturation current we shall take into account during the selection of the inductor.
The datasheet states:
"The typical inductance value and RMS current rating can be obtained from the LinkSwitch-TN2 design spreadsheet available within the PI Expert
design suite from Power Integrations. Choose L1 greater than or equal to the typical calculated inductance with RMS current rating greater than or equal to calculated RMS inductor current"

But the peak current that goes through this inductor is much higher than this RMS current (especially at very small loads), so shall it not be better to take this also into account.
Can we assume that the maximum switching current is equal the current limit of the device? So, can we conclude that the inductor shall next to the sufficient RMS current rating shall also
have a saturation current higher than the current limit of the device?

is it possible to explain how we shall next to the RMS current rating of the coil we shall consider the saturation current?

Comments

Submitted by PI-Jango on 07/02/2019

Hi KarelVanHaver,

Your observation is correct that PI expert does not indicate the saturation current for inductor selection.

PI expert calculates the required inductance based (among other things) the device peak current limit. You can base inductor selection on the following variables: ILIMIT_MAX (for saturation), ILRMS (for copper size) and L. You can assume that LNK3202/4 peak current limit is the same as the inductor peak limit, and that this peak current limit is a fixed value (see the datasheet).

For example, please open up the PI expert spreadsheet for LNK3202 with the following default specs:
Input: 85-265 VAC, 50Hz,
Output: 12V, 0.063A
Efficiency: 0.72

You will see that ILIMIT_MAX=0.146 A, ILRMS=0.07 A and L=2200 uH. To minimize copper loss, select an inductor with rms rating > 0.07 A. To ensure that the inductor does not saturate, check the inductor datasheet and see if saturation current is much more than ILIMIT_MAX.

Submitted by Nomien on 07/03/2019

Can you tell me why the simulation uses the value ILIMIT_MAX = 0.146A as mentioned in the datasheet for di/dt = 55mA/us and not ILIMIT_MAX = 0.191A for di/dt = 250mA/us?

For the LNK3204D in RED mode we have some placement constraints, so here we are thinking to use an inductor of 1500uH, Isat = 0.27A. If I check ILIMIT_MAX I have 230mA for 65mA/us which looks ago, but for ILIMIT_MAX for di/dt = 415mA/us the datasheet tells me 289mA

So, this would then not be recommended?

Submitted by PI-Jango on 07/04/2019

Hi KarelVanHaver,

As you have pointed out, the datasheet provides the current limit variations depending on inductor current slew rate (di/dt). The steeper di/dt is, typically ILIM increases.
For a worst case input of 265 VAC and L=2200uH (in prev example), we get di/dt=265*sqrt(2)V / (2200uH) =170 mA/us. You can interpolate for the resulting ILIM by using the ILIM values for 55mA/us and 250mA/us.
For your application, if you cannot use a larger inductor, then you may consider that ILIM_TYP range is well below your ISAT. Also you should evaluate ILIM at your operating slew rate. Can you send your power supply specs?

Submitted by Nomien on 07/04/2019

My conditions are the following:

230Vac + 10% , 50Hz (so 253Vac max)

Design 1
Output: 5V, up to 60mA in active mode, 20mA in stand-by mode
I use the LNK3202D in STD mode with L = 2200uH 10%, Isat = 0.23A

Design 2
Output: 5V, up to 100mA in active mode, 20mA in stand-by mode
I use the LNK3204D in RED mode with L = 1500uH 10%, Isat = 0.27A

Regards
Karel

Submitted by PI-Jango on 07/12/2019

Hi KarelVanHaver,

I would suggest for you to use the PI XLS tool for LNK-TN2, so you can better evaluate the design with your complete specifications (half bridge vs full bridge, input range, bulk capacitance, etc).

But with the information you provided..
For design 1, worst case ILIM_MAX is 191mA < 230mA (Isat) and your worst case di/dt=253*sqrt(2)/2200uH=162mA/us.
For design 2, worst case ILIM_MAX is 289mA > 270mA(Isat), but worst case di/dt=253*sqrt(2)/1500uH=238mA/us is between di/dt=65 and di/dt=415 as presented in page 12 in LNK-TN2 datasheet.
I suggest you evaluate your design margins on the bench for design 2.