Carbon film vs metal film resistor?

[grizedale]

For the snubber resistor in an RCD clamp of a flyback converter of 50W.

-Is it best as a carbon film resistor or a metal film resistor?

An expert power electronics engineer who was 12 years at GEC in Rugby (plus 12 years at other places) once told me that carbon film resistors are rubbish.

is this true?

 

[edaenrico]

Hi,

> Is it best as a carbon film resistor or a metal film resistor?

I can't answer your question.

I can only tell you that TDK-LAMBDA's KPSA power supplies series have a 0603 THIN FILM resistor as part of the RCD clamp network.

That means the they have an extremely low leakage inductance.

They are open frame power supplies. This is an example: KPSA1515

Enrico Migliore

 

[grizedale]

Hi,

thanks that is amazing.

are you sure thats not an active clamp flyback.?

0603 is phenominally small for an rcd clamp network....specially thin film.

I presume thin film has less power handling capability than thick film though i dont know.

 

[mtwieg]

The resistors in RCD clamp networks don't need to carry high peak current or be low inductance. Other snubber circuits do though. Carbon composition resistors (not film) are good for low ESL and pulse handling, and are cheap.

 

[grizedale]

-though one cannot help but think that the lightness of a carbon film resistor (just pick one up and feel how cheap and light it feels) means that it wont be able to handle power bursts as well as the more substantial metal film resistor

..the very word "metal" tells us that the material composition of a metal film resistor is sturdy and substantial , and it will better handle overloads, than the cheap, light carbon film resistor.?

 

[FvM]

Metal films are usually thinner than carbon films (due to their lower resistivity), but what does this prove? Some highest pulse load resistors are carbon fim types, e.g Vishay CBB and CMB 207. But this doesn't necessary mean much about cheap standard carbon film resistors. You have to refer to the datasheets, some have pulse load rating.

 

[dick_freebird]

Graphite is about the second or third highest melting temp
substance you can get (provided you keep the environment
inert). That's good for pulse loads. I've worked plenty with
thin film metallic fuses where we make use of their poor
pulsed power handling as a positive attribute.

 

[edaenrico]

Hi,

> 0603 is phenominally small for an rcd clamp network....specially thin film.
The capacitor in parallel to the resistor is a 0603 Kemet 1 kV


> are you sure thats not an active clamp flyback.?
I'm sure ;-)

TDK-LAMBDA for that series of AC/DC flyback converters use an external NEC MOSFET which allows 1.5 A

At a fixed power, high primary current means low primary inductance. P = (1/2 * L * i ^ 2) * f

Since the leakage inductance is low (low losses) they can use a 0603 resistor.

The diode, of the RCD network, has a cylindrical ferrite bead on the cathode lead.

The controller is a Fairchild SO8 very common.

Enrico Migliore