Home grown switching power supply

[tiger762]

I've been there and done that with respect to linear power supplies with cinder block size transformers. I'd like to explore the area of taking a high potential source, chopped at dynamically changing duty cycles, to achieve a constant output DC voltage.

What I had in mind was to use a LM339 quad voltage comparator, zener diode, power mosfet, and humungous electrolytic capacitor.

I will assume that the input AC mains power is rectified and filtered the same way in a linear supply. I'll refer to this available, high voltage DC source as "mains".

Connect the mains to the negative side of the voltage comparator, through a 1Mohm resistor. At that same terminal of the comparator, connect a zener diode, of the desired output voltage, to ground. This will present a constant voltage to the comparator, as long as the mains is greater.

Connect the output of the comparator to the gate of the power mosfet. Connect the mains power to the Source of the MOSFET and the Drain of the MOSFET will go to the positive terminal of the electrolytic capacitor. The negative of the capacitor will connect to Ground. The positive of the capacitor will feed into the positive side of the comparator. Once powered up, the voltage comparator will allow the MOSFET to conduct when the capacitor voltage is less than the desired reference voltage, and it will turn the MOSFET off when the voltage is greater than the desired output. So if the desired voltage is 14.4 volts, then I want the comparator/MOSFET pair to keep it between 14.39 and 14.41.

The issues that I know of are:

1. Voltage comparator has to be able to fast enough to recognize changes in capacitor voltage as compared to reference voltage.

2. The MOSFET also has to switch fast as well as be able to handle the available mains voltage at its Source terminal.

3. Possible drifting of zener voltage with temperature, although I think there are two temperature dependent factors that supposedly balance each other.

I've had two electrical engineers doubt this is a good design, but no one can explain why.

I just want the ability to change out zener diodes and get different outputs as well as have high efficiency.

Any input is greatly appreciated.

[kreutz]

Originally Posted by tiger762 I've had two electrical engineers doubt this is a good design, but no one can explain why. Any input is greatly appreciated.

Explanation: No current control. RDson and the output capacitor's capacitance value determine the time constant of the charge circuit. Switching delays will make it very non-linear. Introducing an inductor on the charge circuit, in order to control charge current, will make it become a traditional switching power supply type (once you add a couple of components to compensate for the effects of the inductance, and improve on the output voltage control circuit )

[tiger762]

Made by Intersil, I believe, because of the simplicity of it being TTL compatible. In layman's terms, a TTL "1" at the gate will make it be fully "ON". Its specs are 100V max, 12amps max. Yeah the delay incurred by the voltage comparator (LM393, low-power dual) is between 0.3 and 1.3us, depending on how much the delta-V at the inputs is. So I was expecting to see a switching frequency of several hundred kilohertz. During that delay, the MOSFET will stay in whatever state it is in, and be overcharging the capacitor or allowing its charge to be depleted for too long. But that's why I would use a huge (80,000uF) capacitor in the first place. Something that can source a lot of charge with minimal voltage drop.

I guess I'm going to just wire it up and see what happens. I just got in a large order from AllElectronics, including every value of zener diode they carry (3.3V up to 51V), and 20 each of the comparators, MOSFETS, and some other chips.

Thanks for your reply.

[rdpzycho]

considering your capacitor is discharged and your mains is 100V, initial charging current is 100V/.2ohms = 500A.

mains go to drain, by the way. if I am right you want to put the MOSFET above the capacitor, just like in a buck regulator. if that is so, the comparator may not be able to drive the MOSFET even if it can be driven TTL level. you will still need a bootstrap circuit.

hysteretic supplies share the same idea. your switching frequency will be dependent on your load. you must have a pre-load.

[neilw20]

1). Use a UC3842/3/4/5 family of current mode chips. (different max duty cycles).

2). Use the application notes on the chip.

3). To regulate output use a TL431 measuring output, driving an opto coupler.

4).The output of the opto coupler shorts out reference input on the 3842.

5).Overcurrent on output also drives the same opto.

6).These chips have their own oscilator, comparitor, voltage reference, MOSFET/IGBT current limit, and undervoltage protection.

Smoke tests become rare with experience.
Always use a resistor of at least 27 ohms in series with the gate.
Don't turn them ON too quick (high current spkies from diode reverse recovery time being too short)

or OFF too quick (big inductive spikes where you least expect them)
Attention to detail is a MUST.
You must consider the current paths through every high current part of wiring.
Put the driver chip close to the MOSFET/IGBT. Keep Gate/Drain current paths direct to driver chip, with no other common connections along these wires.
ALL WIRES HAVE INDUCTANCE, which becomes very obvious when creating switchmode power. Invisible voltage spikes are usually the cause of mystery failures.
The application notes, followed very closely give really good results.

[NC Cams]

SWitching BIG inductive and/or capacitive loads with semiconductors can not be done with brute force. Things go "bump in the night" and bizarre things happen when electrons go ashtray (sic).

Kreutz's opservations about some things lacking in this variation of a switching power supply are worth considering. Switchers not only "switch" but also recover energy that is induced or goes wanting/lacking. IT is also recovering energy that tries to do nefarious things to the pass elements.

This much I learned from my experience with SMPS's - it aint that simple to simply switch the power off and on so as to regulate voltage AND power regulating IC's designed for SMPS's are worth their weight in gold. They do too much in one package to NOT use them - especiallly with some of the "free protection scheme's" they offer.

[rdpzycho]

switcher's became my first tutor on why I should be too keen to details when wiring and doing PCB layouts. what you think may be working on your schematic may not be working on your prototype. the time when all those hidden schematics come to play.

they are not the easiest projects on electronics land. I love them though. one good challenge especially when you're already thinking resonant switching.

[rdpzycho]

if you have to wire them on protoboard, always twist pairs of current source and current return path. use kelvin connections especially on the gate drives (never use the same wire for high current return path of drain to source for driving the MOSFET's gate to source, use separate wires, the inductance can blow that MOSFET).

[rdpzycho]

when you already gain experience here (especially on the controls, it's not simply when I reach this level MOSFET turns off there are lots of dynamics to stability) consider using average current mode. they're much better based on my experience in SMPS.

[lovesakhi]

Hello smps modifiers
I presume there are several ways to make use of at/x smps. you can use them
as they are and connect in series as mentioned in another thread but they seem to be too bulky and big. I would be interested to modify the existing smps to get only 5v and 12v from a 400w atx smps. any working mods in this aspect would be highly desirable. I am in process of the same and would welcome inputs. I am testing out the smps with variable loads and other stuff and have yet to remove the main transformer and rewinding the windings!. waiting for help/assistance/colloboration?
regards
dr.sanjeevi rao s

[rdpzycho]

if you'll only need the 5V and 12V then the PSU is adequate as much of the power rating is based on the 12V output. If you need a higher supply voltage you need to rewind the transformer and the inductor (although the old inductor might work). if dynamics are not well too important the old control loop compensation may still be used.

if you want to do the transformer winding based on calculation then you will need to measure area of the center leg (if unsure of the transformer size), know switching frequency, etc. what I would normally do is to take off the transformer, count the old windings (they worked anyway), adjust secondary winding turns and wire gauge.

for the inductor, the higher the inductance the smaller the ripple but current slew is slower.

[lovesakhi]

hello rdpzycho,
did u rewind the transformer and what are the winding details. I know they vary from
smps to smps but not much. I and most others would be interested to modify one
400W atx smps to give 5v and 24v with max permissible/possible current.
regards
dr.sanjeevirao s