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Old 02-24-2020, 09:07 PM   #10
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Default Re: question about Polymodding

Originally Posted by Uranium-235 View Post
How would one know if it used either one?
Originally Posted by stj View Post
number of turns on the inductors, or just get the datasheet for the control chip
There is no precise guide to this really. It's mostly a rough ballpark.

Generally, this "high" frequency vs. "low" frequency VRM thing is mainly meant to rule out much older motherboards. For example, many old ECS motherboard (but also a few other cheapo ones) used a KA7500 or DBL494 or TL494 PWM chip - i.e. the same ancient PWM chip that's been used in ATX power supplies for decades. These PWM chip doesn't really go much above 50 KHz switching frequency, if even that, and the motherboards are 2-phases max. The ECS K7S5A, for example, has 4 coils... but it's only a 2-phase design due to the fact that KA7500/TL494 having only 2 outputs. Either way, motherboards based on these ICs tend to have either bigger inductors or just more larger capacity capacitors... and typically it's the latter. Because of the relatively low switching frequency, ultra-low ESR caps are not needed, but the capacity is important due to the slow frequency at which the output caps are "refilled" from the coils. In other words, it's like having bigger batteries in a device, but charging the device less often.

That's why on many old motherboards with KA7500/TL494/DBL494, you see at least six 2200-3300 uF caps filtering the CPU VRM. The ESR doesn't matter so much, but the capacitance does. And that's why they got away just fine with using barely entry-level low ESR crap caps (like G-Luxon LZ and similar.)

This brings another point: number of VRM phases.
Generally, the more phases a VRM has, the less capacitance is needed on the output. That's because each phase "refills" the output caps when the others aren't. So more phases = lower ripple current per given output capacity. Thus, on some older motherboards with only a single phase VRM (think Pentium 3 era), it's good to keep the capacity similar to the original (or higher, if possible.) As to whether very low ESR caps are needed... that brings us back to switching frequency: inductors/coils with less than 1-2 uH tend to be "higher" frequency IME. Meanwhile, anything with 4-5 turns or more on a big inductor core is typically medium frequency, more or less. These, and the high switching frequency boards will do very well with ultra-low ESR caps (or polymers).
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