DC DC experts - 30Vin 100Vout 300W Sync non-isolated boost - Page 1
Forum section: electronics / projects

author: max_torque
author_rank: Super Contributor
timestamp: July 10, 2023, 01:51:04 pm
content: Hi All,Any dc dc experts have experience with a synchronous boost converter (non isolated topology) for 30v nominal Vin and 100Vout at around 300w continuous power rating?What sort of size would i be looking at, and i want the ability to run complex current control strategies so i'm thinking of using a micro on-board to control the switches directly.I assume that this step up ratio (~3:1) i don't need at voltage conversion via the magentics ie no transformer and i can just use a power inductor?Switching frequency tbc, i guess under 100khz to make things practical with regard to driving the switches and controlling parasitics?Any decent dc dc calc websites i can run the numbers on?

author: Siwastaja
author_rank: Super Contributor
timestamp: July 10, 2023, 02:21:43 pm
content: This is pretty close to what I built - 21Vin, 75Vout, 770W continuous. Although I didn't go synchronous, efficiency gains at such high output voltage are small. With 75Vout, schottky diodes were still an option, I guess at 100V it would be marginal. SiC is great at higher voltage but quite large Vf for 100V, so maybe right around 100V synchronous makes some sense.I ran some numbers in Excel first working out dual-phase but wanting to use low cost COTS inductors (which would become quite large for large currents), ended up with four-phase, controlled by 2x LTC7840 wired for 90deg phase interleave. MOSFET was CSD19533Q5A, diode was PMEG100V060ELPDZ, output capacitors aluminum polymer + MLCC, input capacitors aluminum electrolytic + MLCC. Inductor was WE 74439370330. Design was SMD only with thermal transfer through vias to bottom layer and then with SilPad to aluminum heatsink.This worked flawlessly, never did real efficiency measurements though. It was used to run universal motor 120V 1.2kW vacuum cleaners out of a 6s li-ion battery packs.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 10, 2023, 02:41:48 pm
content: Oh, and the first incarnation was 21Vin -> 150Vout which I did with center-tapped boost topology, and SiC diode. This was single phase, with hand-wound inductor with Kapton tape insulated copper sheet as primary. Like a flyback, it required a RCD snubber connected to Vin and this was dissipating significant amount of power. But why I mention this topology, this was completely microcontroller driven (no DC/DC controller IC), with STM32H743 existent in the system already. Some quickly hacked code around a general-purpose PWM timer, and current sense to ADC, PWM duty cycle adjusted in an ISR. The problem was the cost of that special inductor, which is why I solved the problem by going from a 230VAC shop vacuum to a 120VAC one, which could run at low enough voltage so that simple boost could be used, built from simple COTS components. The final choice of 75VDC was because in EU regulation, this is the upper limit for DC safety extra low voltage.

author: max_torque
author_rank: Super Contributor
timestamp: July 10, 2023, 03:00:38 pm
content: Ive used Wurth's WE-HCF high current surface mount inductors for a high current dc/dc step down before:https://www.we-online.com/en/components/products/WE-HCFThey have a decent range of currents and inductances and simplify the high current inductor part of the design.I'd like to have resonably high efficiency to minimise the thermal disipations required so i'd like to go synchronous, but i'll probably include a footprint for a large freewheeling diode in parallel with the output switch so i could start non-sync for simplicity and getting things up and running quickly (passive output rectification means i don't have to worry about dead time corrected pwms etc)These devices will be deployed groups to drive a common 100V bus, so the micro will also deal with the inter-device coms and control, probably over isolated RS485. I'm going to be over the Low votlage directive min threshold what ever i do in order to keep output current sensible, so i'll deal with the isolation / grounding / leakage issues on each device from the start. I may also require some sort of randomisation / sweeping of the modulation frequencies for each individual device to try to interleave all the outputs togther, although initally i will put in enough output filtering to avoid this requirementWhat i need to do fairly quickly is get to a sensible converter volume so i can issue a concept 3d package envelope asap

author: tszaboo
author_rank: Super Contributor
timestamp: July 10, 2023, 03:31:53 pm
content: You could do it in half brick size, if you have the know how, if this is your first design like such, than I would take a bit more space. Also depends if you need forced air cooling, or you have a target max temperature of the components, and if you can bolt it to a heatsink or not.I'm not sure if I would start with a microcontroller, probably you are much better off with something like the IC mentioned above or LTC3862-2 and adding MCU just for power management. The issue with high power DC-DC microcontroller based systems, they can set themselves on fire if you place a breakpoint the wrong place.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 10, 2023, 03:46:03 pm
content: For reference, the 700W 4-phase thing was as pictured, PCB (4-layer) was approx. 100x100mm. Most of the heatsinking was through the PCB, but there was an aluminum cover (which pressed against the solder-paste-opened edges you can see on the layout), the aluminum cover did also provide extra heatsinking of the inductors, through extra soft, thick silpads (pad-gap type). Cooling of the bottom side aluminum heatsink, approximately 100x150mmx30mm, was passive.

author: max_torque
author_rank: Super Contributor
timestamp: July 10, 2023, 05:02:53 pm
content: Maybe looking at using an IMS pcb for the power stage which also forms the base of the device enclosure. Single layer layout considerations obviously apply in this case!