Conventional bridge rectifier loses. - Page 1
Forum section: electronics / beginners

author: davelectronic
author_rank: Frequent Contributor
timestamp: September 05, 2020, 03:17:47 pm
content: I know this has been discussed before, but threads offer mixed views on this.What I'm looking at is the mosfet full wave bridge rectifier, I know mosfets are voltage controlled devices, but I've looked at the IC's that control the 4 X N channel mosfets. My interest is the virtually no voltage drop across the mosfet bridge, the diagram below, is that practical for an AC input of around 15V to 20V can this circuit with the IC be used for this scenario. Thanks for reading, an thoughts appreciated.Oh nearly forgot, does additional components need adding to that schematic.

author: Siwastaja
author_rank: Super Contributor
timestamp: September 05, 2020, 04:22:16 pm
content: Yes, the IC exists just for this purpose. AFAIK they show all components required but read the datasheet carefully.This is easy to do at low frequencies (such as <600Hz specified). Requires a lot of parts and design to do with discrete components, hence the IC which does everything (and you'll pay money for this convenience).

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 05, 2020, 04:47:10 pm
content: Is there a question in this post?I guess I missed it.Have you read (and understood) the datasheet of the LT4320?

author: Siwastaja
author_rank: Super Contributor
timestamp: September 05, 2020, 04:49:52 pm
content: I found the question clearly laid out: can this IC be used in this scenario (15-20V input; I'm assuming 50 or 60Hz AC). The answer is yes, with the caveat of reading and understanding the datasheet, as always.

author: davelectronic
author_rank: Frequent Contributor
timestamp: September 05, 2020, 07:10:40 pm
content: Thank you for your replys. Yes that was my question, I'm going to use an LM2940C for a regulator. I'm not going to have much headroom voltage, even less if I use a regular full wave bridge rectifier. I have never used mosfets to date, this seems like a very good way to start. I had noted 10 Amps as the recommended highest current, is there any way to double that to 20 Amps ? Not continuous, but around a duty cycle of 50% or so.

author: Siwastaja
author_rank: Super Contributor
timestamp: September 06, 2020, 06:55:47 am
content: In principle, just select the MOSFET with low enough Rds(on) to satisfy the current need.For MOSFET selection, note that the datasheet "maximum drain current" specification is 99% bullshit. Start by multiplying the provided room-temp Rds_on by 1.5 to get real-world Rds_on, then calculate power dissipation Pdissip = I^2 * Rds_on. Note the MOSFET package thermal resistance Junction-To-Ambient (or calculate by yourself summing Junction-to-Case + heatsink Rth), and using that, calculate the die temperature Tj = T_ambient + Rth_j-a * Pdissip. Don't go much over 100 degC. This is from the MOSFET survival viewpoint. But seeing you want to minimize losses, you likely want to go lower Rds_on than that. Then just choose the required Rds_on = Vmax_drop_you_can_live_with / I.There is a caveat, though; switching speed. The LT chip can't drive unlimited current to the MOSFET gates. The "recommended" 10A highest current makes you choose smallish MOSFETs, with small gate capacitance. If you design for 20A, you need to use larger MOSFETs, with larger Qg. Pay attention to this parameter, there is a lot of variation; older generation MOSFETs may have massive gate capacitances.