SMPS Question: Volt-uS derating for inductors?

[Speakerguy]

Hey guys,

This is a somewhat technical power electronics question. I'm designing the LC output filter for a PWM audio amplifier. I'm trying to pick an appropriate inductor. I'm looking for at least 100W output into any load (4 or 8 ohm nominal). The device I'm using is the TAS5261 output stage with the TAS5518C modulator from TI.

Switching frequency is 384KHz. Voltage is (up to) 50V for an H-bridge into the load (BTL bridge tied load). I am looking at the DR127 series inductors from Digikey since they are easily available.

Digi-Key - 513-1037-1-ND (Coiltronics/Div of Cooper/Bussmann - DR127-100-R)

I am not sure if it is suitable however. Could someone show/explain/give me a link to help me understand how to properly derate an inductor in this application?

Also, does anyone know if this style inductor is as well shielded as a good toroid? Is it significantly better? Any help in the EMI area as far as inductor construction is appreciated too.

Thanks!

 

[dknguyen]

It's probably not as well shielded as a shielded toroid. Vs an unshielded toroid...I would think that's totally up to how it's designed.

I read somewhere that the maximum frequency that should be going through a transformer is 125% of the SRF. Current-wise, for DC-DC converters a I read that a good rule of thumb is that the peak current is 50ish percent of the saturation RMS current- but the datasheet provides some Inductance vs Current graphs so you can just use those and pick a point you like.

As For Volt-Seconds, why don't you just use the core loss derating graph at the very very bottom of the datasheet? I think you just take your maximum pulse on-time and multiply it by the PWM ON voltage to get Volt-Seconds.

Core Loss and Temperature
Allowable losses for an inductor are limited by the maximum permissible temperature. Thus, for most off-the-shelf parts, this limit is a surface temperature of 125°C — although it can be higher. The rms current rating is typically the dc current that leads to a 40°C temperature rise, theoretically allowing operation in an 85°C ambient temperature. However, in most applications, some ripple current resulting from core loss exists. Under such conditions, Irms would need to be de-rated to keep the temperature rise down to 40°C. Also, the specified 40°C rise is normally achieved with no restrictions in airflow due to natural convection, which in most applications is not the case.

The two main issues with this area of inductor selection are calculating the core loss and the required de-rating of Irms to keep the temperature rise down to an acceptable level. Different inductor manufactures have different ways of expressing core loss — some give no details at all, while others provide the information required to calculate dissipation. However, one of the more practical approaches comes from the Coiltronics® catalog, which shows the maximum allowable percentage loss from Irms against applied volt-seconds at various frequencies. Since the volt-seconds product is proportional to core loss, it can be easily determined by using these curves.

 

[OutToLunch]

If your volt-seconds are higher than what is shown on the datasheet, then expect some more core losses - the listed volt-seconds represent a core loss that is 10% of the losses. The main thing you want to do is to make sure that you aren't saturating the inductor at any point - that includes DC and peak ripple current.

The shielding on those inductors is good.

 

[Speakerguy]

think you just take your maximum pulse on-time and multiply it by the PWM ON voltage to get Volt-Seconds.

I think this is what I need to figure out. So it's just voltage across the inductor multiplied by ~1.3uS for a 384khz pwm rate? If that is the case, I think I am well in excess of what it can do with 50V supply. Dangit. No surprise, it's a good bit smaller than the other inductors I am looking at, so I am probably saturating it.

If somebody would stock the inductor that TI used in their eval board (Toko C3B-A0336) I would just buy that.

 

[dknguyen]

Have you checked out Coilcraft? THey seem to have a lot of weird and wonderful inductors.

 

[Speakerguy]

Wow, Coilcraft's site has an awesome loss calculator program for all of their inductors! I just enter the series, the value, the frequency, Irms and Ipk-pk, and it gives me all the power losses (core, DCR, whatnot). Sweetness.