negative bias generator - Page 1
Forum section: electronics / projects

author: ealex
author_rank: Frequent Contributor
timestamp: January 13, 2015, 11:09:35 am
content: HelloI need to generate ~ -28V at 5mA for a LCD bias voltage, from a 5V supply with some components that i have at hand ( i can't place any order right now + it's a one-off )I've thought of a simple royer circuit using some ring cores i had available:http://www.kaschke.de/fileadmin/user_upload/documents/datenblaetter/Ringe/R30-5.3.pdfhttp://www.kaschke.de/fileadmin/user_upload/documents/datenblaetter/Materialien/MnZn-Ferrit/K2006.pdfthe inductor values are not optimized - i can reduce L1 and L2 to 5 turns without saturating the core.Also L6 - i'll pick up something that i have available ( 470uH for example )Q1 and Q2 form the royer oscillator. C1 sets the oscillator frequency - i'll tune it so it oscillates at ~ 100kHzQ3 is used as an enable signal - there must be a delay between 5V and -28V otherwise the LCD drivers go crazy.If i'm careful i can fit almost all components inside the core ( hot-glue everything in place once it's finished and forget about it )C3 acts as a inrush current limit so the circuit will not overload the 5V rail on start-up and will limit the current if the LCD drivers decide to go crazy.I don't think a charge pump / voltage multiplier will work with this voltage ratio ( almost 6:1 )Are there any other solutions ? Maybe a 1 transistor oscillator ?Or a 1 transistor fly-back - will it work with a simple ring core ? i have some E32 and E42 cores available but they are too big.In the image:green - output voltagered - enable signalyellow - input currentI've attached the LTspice circuit / simulation.

author: Marco
author_rank: Super Contributor
timestamp: January 13, 2015, 11:22:25 am
content: Why wouldn't a invering multiplier work? Just use a higher multiplication and use a (makeshift) regulator to get it to the required voltage.

author: Howardlong
author_rank: Super Contributor
timestamp: January 13, 2015, 11:49:05 am
content: LT1617?http://cds.linear.com/docs/en/datasheet/16171f.pdf

author: dannyf
author_rank: Super Contributor
timestamp: January 13, 2015, 11:55:04 am
content: If you have a pin or two to spare, a mcu can do that.Otherwise, try a 555.

author: paulie
author_rank: Frequent Contributor
timestamp: January 13, 2015, 12:32:04 pm
content: It's true. A PWM pin with transistor, inductor, diodes, and cap. 25 cents tops. I managed to generate 400v that way for a geiger counter tube.(Dannyf is sooooo smart. I wanna be like him when I grow up.)

author: ealex
author_rank: Frequent Contributor
timestamp: January 13, 2015, 01:19:54 pm
content: I've thought of the PWM / inductor, etc but the LCD will be driven by an fpga - the pwm control might not be that easy.the simple boot configuration will not work, i need a negative voltage in respect to system ground.in the spice simulation i've used a positive output voltage just for convenience - the output is isolatedi've seen the polarity inverting buck configuration but i don't have a P-FET available and i don't think i can source one quickly ( i can't order anything right now and i can't find much more than basic stuff around here )i've simulated a voltage multiplier - see the attached image and ltspice files. it needs 14 diodes and capacitors.the input signal is a 100kHz 5V square wave - i can make this from the fpga + a transistor driver.the first image shows the output voltage with no loadthe second with a 2.8mA load - the voltage drops to 22V with 1V ripple => that's why i did not want to use a multiplier.

author: dannyf
author_rank: Super Contributor
timestamp: January 13, 2015, 02:18:16 pm
content: QuoteChange the diodes' polarity, or ground the other end of the bridge.

author: ealex
author_rank: Frequent Contributor
timestamp: January 16, 2015, 08:00:53 am
content: helloupdate: managed to dig up a MC34063 from the scrap boxi've built one of the basic applications from the data sheet - it works better than a simple unregulated royer. It uses ~ 35mA with no load, 5V in and -28V out.i've also managed to find a logic level n-mos ripped from a mother board -> made a simple ON/OFF swithc using it. With 3V on the gate it conducts pretty well.