Advice on 0.4V 250A buck converter. - Page 1
Forum section: electronics / projects

author: Boscoe
author_rank: Frequent Contributor
timestamp: January 23, 2018, 09:32:15 am
content: Hi all,I'm looking for a solution for the above. It's a difficult spec to achieve and will require a lot of clever design and so I want to reduce my risks as much as possible with an off the shelf design. I must have adjustable output voltage up to 0.5V too. The TI TPS53681 seems to fit the bill but it's complex, I understand that may just have to be the case but I'd like to ask the community if there are any other options. i will consider modules if they are competitively priced and I can spilt the PSU into 2x 125A for example.Thanks

author: jbb
author_rank: Super Contributor
timestamp: January 23, 2018, 09:57:43 am
content: That’s a beast :-)A couple of questions to help frame the issue:- is this going to be one-off test equipment or do you need several?- do you need good dynamic response? (Eg good voltage control with variable load)- do you need low noise?- will you need it to workbelow[\i] 0.4V output? (This can be an issue with minimum pulse widths.)- what balance do you want between small, efficient and easy to design & build?From a converter perspective I guess you’d look at a 12V DC intermediate bus (PC power supply?) and a multiphase synchronous buck converter.Assuming a minimum of 0.12V out and 12V in, we get 1% duty cycle (0.12/12). For a 100 ns minimum pulse width (shortest on time), we get a minimum period of 10 000 ns, i.e. 100 kHz, for a not-too-fancy design.

author: Boscoe
author_rank: Frequent Contributor
timestamp: January 23, 2018, 01:18:56 pm
content: Quote from: jbb on January 23, 2018, 09:57:43 amThanks for the reply. I should have been more specific in my original question.- This is going in a mass market product.- No, the load will vary but there are no switching loads as in 0-100% less than 1ms.- No, it's for digital circuits, ripple less than 10mV.- I would say 0.38V to be safe.- I need efficient (around 90%) and small is ideal but not vital.- Yes, it can be powered from 5V to 24V as this hasn't been determined and will be done so by this 0.4V PSU.Are you suggesting a bespoke solution could be possible? Small MCU for the control loop? Would this be precise enough? That is something I haven't done before and I'm really trying to reduce risks.

author: nctnico
author_rank: Super Contributor
timestamp: January 23, 2018, 04:50:08 pm
content: I'd go for a design with a transformer with a winding ratio so the duty cycle is around 50% at the nominal input voltage.

author: David Hess
author_rank: Super Contributor
timestamp: January 23, 2018, 05:41:25 pm
content: There are a lot of suitable multiphase synchronous buck converter designs intended for high power CPUs and GPUs and they include detailed design information. Operating at 24 volts might be a problem though.

author: Boscoe
author_rank: Frequent Contributor
timestamp: January 23, 2018, 06:46:56 pm
content: Hi all,Thanks for the feedback.I don't see transformers being very viable because the secondary rectification.I'll have a read of the others.And, for clarification, I don't mean the PSU must operate from a range from 5V to 24V but the input voltage hasn't been decided and can be in that range. So 5V input voltage is fine.

author: David Hess
author_rank: Super Contributor
timestamp: January 23, 2018, 07:21:56 pm
content: Oh, it just occurred to me that there is a solution to the wide input voltage range problem that does not use a transformer. Instead of using a transformer, replace the buck inductor with a tapped buck inductor to make a current boosted buck regulator. The switch drives the whole inductor but the synchronous rectifier drives the tap. The result is a higher switch duty cycle so it can support a higher input voltage without excessively small duty cycles.Figure 12 on page 5 ofLinear Technology application note 35shows an example where this is used to get 10 amps out of a 5 amp switching regulator. Figure 15 on page 24 ofLinear Technology application note 44discusses it in more detail.I suspect nobody does this because once you have a multiphase design, it is easier to add more phases.

author: Zero999
author_rank: Super Contributor
timestamp: January 23, 2018, 10:51:32 pm
content: Quote from: Boscoe on January 23, 2018, 06:46:56 pmWhy is rectification an issue? Synchronous rectification has a very low voltage loss.However, I think David Hess's idea of using a tapped inductor buck converter is a good one.

author: nctnico
author_rank: Super Contributor
timestamp: January 24, 2018, 09:36:47 pm
content: Quote from: Boscoe on January 23, 2018, 06:46:56 pmYou will always need rectification. The thing is though that you can use a transformer (with an air gap to store the energy) in a flyback configuration with a MOSFET to do synchronous rectification. The design is pretty similar to a buck converter.