12 V Push Pull - increase efficiency? - Page 1
Forum section: electronics / projects

author: kaz911
author_rank: Super Contributor
timestamp: February 05, 2012, 12:33:34 pm
content: Hi guys,I have a special "push pull" switchmode power supply designed to deliver constant current from 0-2amp. It is controlled by a PIC.Circuit has one SB560 zener on the DC input - and one SB560 on each of the push/pull channels output. The push pull is driven by a SG2525 to two IRF3205 Mosfets. PIC gets feedback about voltage from a 9.1 zener plus resistor/cap - and current sense via a current resistor between MOSFET SINK and DC ground.Input is "12 volt solar (so 11.8 -> 14.5v) - output voltage is dependent on input but between 10-12 volt (so about minus 20%). Total circuit input Amps is more or less the same as Output Amps minus about 50 ma. (so gives an estimated efficiency of about 80%)But due to always having 2 x SB560 "on tow" I am loosing about 0.6-0.8 volt total from input to output. And since it is solar powered I would rather have my amps put into where it is supposed to go - than wasting them in V forward drop watts.My ideas range from exchanging the SB560's with SB540's to gain a bit but loose some protection - or replace the diodes with MOSFETs.But I am absolutely not a SMPS expert - but a n00b trying to relearn what I was taught some 20+ years ago. :-)Does any of you have any good suggestions? (I do unfortunately not have the schematic - since it is not my design - but I hand traced part of the circuit)/Kasper

author: Simon
author_rank: Unknown
timestamp: February 05, 2012, 12:55:24 pm
content: what exactly do you mean by push pull ? are you talking syncronized rectification ? or is this a strange type of SMPS

author: kaz911
author_rank: Super Contributor
timestamp: February 05, 2012, 12:59:38 pm
content: push pull - 2 mosfets each reacting opposite.http://www.national.com/pf/LM/LM25037.html(typical standard setup)

author: Simon
author_rank: Unknown
timestamp: February 05, 2012, 01:07:37 pm
content: that is a bit different, it involves a transformer, you are creating AC with a pushpull driver and then it is being rectified back to DC on the output. If your SMPS is only dropping a couple of volts I doubt it uses a transformer. Bear in mind that most controller/driver IC's can be made to work in a number of ways for different circuit topologies

author: kaz911
author_rank: Super Contributor
timestamp: February 05, 2012, 01:17:42 pm
content: yes I know - but it does drive a transformer. Mostfets are directly coupled to the transformer. Output of transformer then goes through a half bridge to rectify it back to "semi" DC - or actually PWM.Transformer is just a 1:1 for galvanic isolation.best regardsKasper

author: Simon
author_rank: Unknown
timestamp: February 05, 2012, 01:20:16 pm
content: hm I think you want to just use another SMPS that does not have the transformer, i mean do you need the isolation ?

author: kaz911
author_rank: Super Contributor
timestamp: February 05, 2012, 01:21:28 pm
content: yes - it is for marine use. They have to be galvanic isolated.so I am looking at other ways to improve the circuit.

author: Simon
author_rank: Unknown
timestamp: February 05, 2012, 01:22:49 pm
content: you can't !I'm not sure why marine use makes you need the isolation, it's only 14.5V in and your panels will take a short circuit with no problem

author: kaz911
author_rank: Super Contributor
timestamp: February 05, 2012, 01:33:23 pm
content: Marine Electronics have to be galvanic isolated for multiple reasons. One is to protect the boat from corrosion and wear from stray currents. This goes in the water... The risk is someone else drops a live wire in the water etc. etc.Solar is only for delivering DC input power - and is not really relevant apart from that's why I want better efficiency.