Forward converter with peak-current mode controller - Page 1
Forum section: electronics / general

author: moonz
author_rank: Contributor
timestamp: August 29, 2023, 07:29:58 am
content: Why stable operation of a forward converter with peak-current mode controller, the magnetization current should be less than the reflected current?I figured out the forward converter, and I'm currently reading app-note(https://www.analog.com/media/en/technical-documentation/app-notes/designing-activeclamp-forward-converters-using-peakcurrentmode-controllers.pdf) where state:QuoteDo I understand correctly that it is needed in order for the feedback to 'know' what is really occurring in the load?If the answer is yes, then how will the forward converter work with a slight load?

author: mtwieg
author_rank: Frequent Contributor
timestamp: August 30, 2023, 05:34:29 am
content: Quote from: moonz on August 29, 2023, 07:29:58 amI've designed peak CMC forward converters before and never observed this, and I can't see any reason why this would be true. In fact, magnetizing current effectively acts as a bit of slope compensation, helping against subharmonic oscillation.

author: Faringdon
author_rank: Super Contributor
timestamp: September 02, 2023, 09:45:42 pm
content: Why stable operation of a forward converter with peak-current mode controller, the magnetization current should be less than the reflected current?It generally is...but yes in light load the magnetizing current peak may be more than the reflected...but who cares, you can still get regulation.Turn the question round.......why do you want magnetizing current to be greater than reflected load current?....or, what is it about magnetizing current that you dont like?But generally, yes, the load current is what you are trying to control, so why do you want much magnetizing current......magnetizing current is what you are forced to put up with.....due to V = Ldi/dt....but as Mtwieg says...magnetizing current does give you a nice bit of slope compensation if you need that.

author: moonz
author_rank: Contributor
timestamp: September 10, 2023, 02:59:56 pm
content: QuoteI don't want. Magnetizing current is a detrimental effect for a forward converter, as it doesn't provide power to the load; instead, it increases power loss. Right?I would like to continue discussing the topic of forward converters. Can someone please review my approach for selecting a core for the converter?So... Input data:Input voltage min: 10VOut Power: 70WOut Voltage: 54VMaximum duty cycle(Dmax): 0.6 (I know, i's not typicaly for forward but for MAX17598 it allow)Frequency switching: 1MHz1)Calculate out current in load:Iout=Pout/Vout = 1.3A2)Calculate turns ratio:n= Vout/(Vin*Dmax) = 93)Calculate secondary current:I skipped the inductor selection step, just say that ripple current are 0.33A. The secondary peak current Isec = Iout + Irip = 1.3+0.33 = 1.63A4)Calculate primary current:Ipri = Isec * n5)Calculate magnetization inductance:Suppose that the magnetization current should be less than 20% of the full load current. Imag = 0.2*IpriThen Lmag = 2uH (from V=L*di/dt)For example choose B66285G0050X187H = Ipri*Turns/le = 451 (for Al = 1520nH enought 1 turns in primary side) = Ipri/le = 586. Ipri iclude magnetization current.B = H * ue * Al = 0.35; It's less then 0.4 (saturation value),and this core is suitable for this design.Is everything correct in my reasoning or did I make a mistake somewhere?

author: jonpaul
author_rank: Super Contributor
timestamp: September 10, 2023, 03:43:58 pm
content: PC200 is the power materialAll others like N87 are not for power applications.Magnetizing current is a natural result of any transformers primary inductance. As it is inductove and not resistive, there is no power loss just VARS.j

author: moonz
author_rank: Contributor
timestamp: September 11, 2023, 10:54:15 am
content: QuoteWhat does the PC200 have that the N87 doesn’t?I wouldn't be so strict because N87 is still used in DC/DC applications. For example TI:https://www.ti.com/lit/ug/tidub83/tidub83.pdfAlso, the manufacturer himself says that the main application is power supplies.But apparently still this material does not suit me because I want to work at a frequency of 1 MHz

author: moonz
author_rank: Contributor
timestamp: September 11, 2023, 11:05:08 am
content: QuoteThis is a very strong simplification. This current still makes losses: I2R (on the transistor and winding) plus dynamic losses in the core.

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 13, 2023, 03:09:07 am
content: Quote from: jonpaul on September 10, 2023, 03:43:58 pmTDK's PC200 is one of several relatively new ferrite materials (like 3F46 from ferroxcube, 80 from fair-rite, etc). For switching frequencies >500kHz they work well, but these new materials didn't make the old materials obsolete. Also last time I checked these high frequency materials are not available in many form factors.Quote from: moonz on September 11, 2023, 11:05:08 amAgreed, though depending on the design magnetizing current isn't necessarily a bad thing. In some converter topologies, magnetizing inductance helps achieve soft-switching in the semiconductors. But in the case of a hard-switched converter like the forward converter this is not the case. Magnetizing current means there will be core loss, and also slightly increased conduction losses in the FETs.

author: MrPWM
author_rank: Newbie
timestamp: September 17, 2023, 02:51:19 pm
content: There is no need to make the magnetizing current "half" of the reflected current. After reading that, I don't know why the author even said that. I've designed quite a few forward converters and the magnetizing current is usually a tiny percentage of the reflected current.