Low-power burst-mode dc converter - Page 1
Forum section: electronics / projects

author: dannyf
author_rank: Super Contributor
timestamp: February 09, 2014, 06:29:25 pm
content: I am building a low - power (output current in a few ma) boost converter, around a mcu (16f684).Input: 5vdcOutput: 12vdc, regulated and user adjustable. To drive a 1kohm loadOperating principle: the mcu acts like a fixed frequency oscillator, with the output on RA0. Q1 acts like a switch, and L1 (R2 represents its dc resistance) + diode + capacitor forms a boost converter. Nothing unique here.RV1 is the voltage divider that samples the output voltage. It goes into a comparator where the output voltage (after being divided) is compared with an internal voltage reference. If the output voltage is sufficiently high, the oscillator is stopped; If the output voltage goes below the reference voltage, the oscillator is turned on. Aka burst mode, similar to Linear's 1071 switching mode regulator. Because of this, it is particularly efficient when the load is light.R3 simulates the load and Q2 is the switch that switches in the load at 50 - 60ms. As you can see, the output voltage is maintained.Uses:1) generate a higher voltage, for example, for 5v lcds in a 3.3v environment.2) generate a negative voltage, for example, to drive the contrast pin of a HD44780 in a 3.3v environment.3) for a hv programmer I am building for AVR chips - My dragon was busted years ago.Downside:1) It requires an inductor. A typical charge pump can be put in here if you don't mind lower current capabilities;2) It requires a comparator: the code can be made to work with just a digital input pin - a CMOS input pin would work great for example. A ST pin would also work but with higher ripples.3) Limited current capability: can be mitigated with beefier inductors and external switches.

author: jwm
author_rank: Regular Contributor
timestamp: February 10, 2014, 10:22:16 am
content: Roman Black also has a page about using a PIC as a SMPS, he just used the push-pull output of a pin directly and used it as a buck converter.http://www.romanblack.com/smps/pic-smps.htmYour design seems much more general and higher power. I actually have a USB project that could use a 14V boost converter in it and free uC pins and clock cycles to spare so may try to build something based on your design.Could you comment on why you used a bipolar transistor in your boost loop instead of a mosfet? It seems to me it would increase efficiency to have a high impedance gate there, but I am not very experienced with SMPS design.

author: dannyf
author_rank: Super Contributor
timestamp: February 10, 2014, 11:49:44 am
content: Blacks design used an internal switch vs an external switch in mine so the current comparison isn't fair for blacks.bjt vs mosfet: mostly two reasons,- coding: for port operations, I'm coding in C with masks. As such, I'm prone to rmw. For that to work with a mosfet, I have to use a resistor to isolate the pin from the gate capacitance. That takes one reason to favor the mosfet vs. But.- I plan to use a bjt later so it makes sense to consolidate my parts.now, you can and probably should use a mosfet for higher power output. But driving a high power mosfet at high speed without a driver can be challenging.

author: dannyf
author_rank: Super Contributor
timestamp: February 10, 2014, 12:40:12 pm
content: Wanted to add that the way I implemented the burst mode also made the use of mosfet less appealing. Rather that stop the oscillator, I inhibited its output by turning the pin into an input.for that to work on a mosfet, you would have to use a pull down resistor to bleed off the gate charge.obviously that's no problem if you don't implement the burst mode that way.

author: dannyf
author_rank: Super Contributor
timestamp: February 10, 2014, 10:28:52 pm
content: Burst mode converters trade ripple and current output for efficiency. For high power applications, you may want to use other approaches - changing duty cycles for example.there are ways to implement a burst mode converter on a mcu entirely in hardware: a clock-out pin, or a pwm generator or a timer can serve as the oscillator. A comparator,s output can inhibit the oscillator. No code after the whole thing is set up.

author: dannyf
author_rank: Super Contributor
timestamp: February 11, 2014, 12:42:48 am
content: Here is a quick simulation to show how it could be done on a mcu without taking up processor mips.V2 is the power supply; V3 is the comparator's reference voltage; V1 is the mcu's clock output - it could be a clock-out pin, a pwm output, or a timer output; U1 is the onboard comparator. The rest of the circuit looks identical to the one I provided earlier.The mcu's clock output drives the switch Q1 and the inductor charges up the capacitor. When the output is less than V3 * (1+R3/R4), U1 outputs a '1' and the clock's signal gets to the switch -> this in turn continues the charging up of the capacitor.Once the output voltage reaches or exceeds V3 * (1+R3/R4), U1 outputs a '0'. This shorts the clock signal to the switch and turns off the switch. Output voltage starts to be discharged via the load R5 (and R3/R4). Once the output voltage falls below V3 * (1+R3/R4), the clock to the switch is turned back on and the capacitor is charged up again.This can be seen on the chart on the bottom. The green trace is the output voltage and the blue track is the signal where D1 meets R1 and R2.So all it takes is a mcu that can output a clock signal, and a mcu that has a comparator + output pin. Most modern mcus meet those requirements.