Current mode cycle by cycle limits, and amplification  - Page 1
Forum section: electronics / projects

author: Hiemal
author_rank: Regular Contributor
timestamp: September 02, 2023, 11:47:10 pm
content: Hiya all!So one of the potential issues one may face with a current mode controller is that many, many are made/designed for offline use... Most controllers usually have a cycle by cycle limit of about 0.5v-1.0v. When the primary side currents are relatively tame, they can be measured with a fairly reasonable sense resistor without too much fuss/power loss.However, when you get into step up applications, the primary side currents get a lot higher. Trying to get 0.5v from a sense resistor with 20-30A flowing without burning anything is kinda hard when you want to have a proper current limit.I know CTs are a thing, of course. But, they're typically more bulky, and harder to come by. It's realistically the better option in most cases, HOWEVER.Would it be possible or 'okay' to use a simple op amp with a small gain to help bump up the voltage from said sense resistor? I've done this for a few power supplies I've designed in the past (small gains ~ 5) but I want to know if there's any pitfalls or potential gotchas around this practice that I may have missed. I did find some paper/design document some time ago that detailed how to do this properly (gave some equations relating switching frequency, resistor size, gain, etc) to give what speed and slew rate of op amp you'd need.Many controllers also implement leading edge blanking, or slope compensation using the CS pin; I presume this means the RC filter usually found leading from the sense resistor to the CS pin, ought to be after any amplification to keep these features in place? Obviously without any impedance on the CS pin slope compensation would just be thrown out.I've also thought a little about adding a flat offset to the CS pin, but I don't know if this would impact the loop or stability...If there's a better way of doing this please lemme know!

author: jonpaul
author_rank: Super Contributor
timestamp: September 03, 2023, 03:25:47 pm
content: Bonjour cher monsieur:CM control depends on an accurate current wave feedback.Any noise, spikes or artifacts in the CM signal to the controller will affect the peak current corntol.The current transformer is the industry standard: Low cost ($0.20 ea) any desired ratio, and isloaton from CM noise.Sendse resistors should be four terminal kelvin and are usually used in a DC application.Adding a diff/instrument amp to a sense R is complicated ast the CMRR will depens on the amp CMRR vs FI would refer back to some CM controller IC app notes, and either select an off the shelf CT or wind one with a toroid.Keep sense R and amps on the DC side of a PS.Bon chancej

author: magic
author_rank: Super Contributor
timestamp: September 03, 2023, 07:15:16 pm
content: I thought about externally clamping COMP pin on the jellybean UC3842 - this could prevent it from reaching its built-in limit of 1V.Unfortunately, those input series diodes and their tempco aren't making it easy. Maybe there are more suitable parts.Some chips meant exclusively for off-line don't even have a reference and error amp. An external amp could be clamped easily.A further downside is that offset voltage of the current sense comparator could cause problems if the limit is set too low.This is a poorly specified aspect of those chips.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 03, 2023, 09:20:28 pm
content: Yes, an amp is perfectly fine. Mind it has to have fairly high bandwidth, else the signal is delayed, peak current rises higher than expected, etc. etc. On average, the control adapts to that, it doesn't mean anything, but where it cannot adapt is around minimum duty cycle, which is thus raised by amp frequency response. So, this may affect light-load performance, or EMI or whatever, but not necessarily adversely, just means it's something you'll need to check up on.ISENSE bandwidth of several MHz is acceptable in typical applications, so for a gain of say 5 or 10, GBW of a few 10s MHz is required. This is pretty affordable among low voltage (say 5V) amps, which could be supplied conveniently from VREF -- the pin has a few mA of spare capacity, this is fine.Notice you can bias ISENSE itself, since the range is 0 ... 1V. You can simply restrict it to operate in the, say, 0.7 ... 1V range. This has the downside of restricting the V(COMP) range, adding a deadband below normal operating voltage, which will affect transient recovery (step load) response.Even better [for many applications], you can further do the biasing by blending in some RtCt ramp, to afford slope compensation. When it's a UC3843 for example, you can use the stock circuit in the datasheet (a BJT follower from RtCt, the emitter resistor doing double service as pulldown/bias and dividing the signal into ISENSE).Pulling 3843 ISENSE up above 0.7V or so is unwise, given the poor tolerance on V(ISENSE) max (0.9-1.1V). This limits the sense/shunt resistor to the balance, or say 200-400mV -- it's a definite improvement, but you'll need an amp to do better.For better performance, consider LM3481 or others.Tim