SMPS replacement for 7805

This is a collection of switch mode power supply modules designs that are efficient drop-in replacement for the old 78XX series.

Similar projects worth following
The main goal is to promote the use of switch mode power supplies as efficient and/or more flexible substitutions.

Most users simply pick the 7805 because they don't know the more efficient alternatives. Some of these modules have no simple linear alternatives e.g. Boost, isolated or Buck-Boost converters. One would otherwise require the use of multiple transformers windings or operate multiple supplies from a much higher input voltage.

The original goal of this project was to document the modules used in my UPS project. As my collect grow due to the different requirement of the UPS peripheral, so does the type of module.

Most users simply pick the 7805 because they don't know the more efficient alternatives. This is surprising as there are a lot of affordable DC/DC supply modules or PCB. I am hoping that this project would bring a bit more awareness to the matter.

Buck Mode Power Supply

The traditional 78xx linear regulator are simple and convenient to use, but they are highly inefficient especially if you are powering it from a high voltage.

This power supply is pretty much straight from TI/National's LM3485 datasheet with what I have in my parts bin. Just a plug-in regulator module for one of my old projects. Just need to add a bulk electrolytic/tantalum etc. cap for the power input. You should already have that one in your project.

This was developed to be used 24/7 in my DC UPS since 2008 and has proven itself to be reliable.

From the datasheet - Typical Performance Characteristic (i.e. your mileage may vary)

There are all kinds of designs out there. I happen to have samples for this part. This is designed as pluggable modules e.g. 5V or 12V module for my UPS project.

A number of power supply chips have an upper limit on the duty cycles at 90-95%. This basically means that your output voltage is at most 90-95% of the input. They are sort of like the older generation of linear regulator with a higher drop out. For battery operations, such as running a 5V module from 4 AA batteries, don't leave you with a lot of head room before the 5V output is outside of the +/- 10% range needed.

When the input voltage is below the set point for the output, the MOSFET in this design is allowed to run at 100% duty cycle. It connects the input to the output and acts like a straight pass through. This drop out mode makes this design behave like a LDO. My routers runs off a 12V module and when running off the backup battery, the drop out mode allows my router to run all the way down to the low battery cut off voltage of 9.5V.

Buck Module - KIS-3R33S

These are pulled modules that was popular from Chinese site a few years ago. There was some reverse engineering, but most of them require modifications inside the module to use them other than 3.3V modules. I did my homework and show you how to use these without modifications the way the original designer intended. Why else would they put a voltage adjustment pin?

See project log here:

Boost Module

These are modules that has a higher output than the input voltage. This particular module was used in my UPS as the DSL modem uses a 26V supply.

Isolated Module

Both my VoIP (ATA) and cord less phone decided to cut cost and eliminated their isolation circuits as they assume that the other device would have it. The end result is that these devices cannot coexist from power supplies that shared a common ground. This isolated supply module allow these two devices to work together in my UPS.

SEPIC Converter

Unlike the previous modules, SEPIC is a buck boost that can operate when its input voltage is lower or higher than its output voltage. This flexibility comes with a lower efficiency.

Negative rail Analog supply

I used a negative charge pump to generate the negative rail for an analog supply. I reduce the amount of switching noise by using filtering and reducing return path via layout improvement.

Youtube Video


Schematic: Creative Commons 4.0 Attribution-ShareAlike...

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Discrete sync buck 5V

LTSpice files for 12V to 5V/10A Synchronous Buck converter

Zip Archive - 4.02 kB - 11/14/2016 at 18:02


Reddit Discrete

Reddit Buck Regulator Olympics contest LTSpice simulation. (new) with Transient Test

Zip Archive - 4.90 kB - 04/23/2016 at 21:06



Reddit Buck Regulator Olympics contest Excel

ms-excel - 18.00 kB - 04/21/2016 at 19:51


  • 1 × LM3485MM/NOPB (TI Part) Power Management ICs / Switching Regulators and Controllers
  • 1 × do3316p-223 Inductor, Ferrite Core, 22uh, 20±%
  • 1 × MBRS340 Schottky diode, 3A
  • 1 × 220uF/10V tantalum cap (or Niobium alternative)
  • 1 × 22uF ceramic

View all 8 components

  • LED Photoflash

    K.C. Lee11/15/2016 at 18:19 2 comments

    This is a quick and dirty mod of my discrete design. I haven't play with proper MOSFET nor driver yet.

    This is mostly doodles right now for answering question posted by Ted Yapo from

    Caps are from LTSpice models. There are no inductance in the model.


    LED fired at 1ms for 5us duration.

    There are some initial droop. D3 is one of the LED.

    With a 3.3uH, the current becomes constant. This however is dependent on the diodes characteristics and input voltage source, so don't rely on it in real life.

    The flash turns on at the worse time when the MOSFET was just turned off. (Red trace shows the MOSFET gate voltage.) The capacitor supplies the current during this time. It takes about 700ns for the feedback circuit to correct for it.

    The larger inductor value is picked to maintain a constant current.

    Purple trace shows the current drawn from the power source (purple trace).

  • Discrete 5V/10A Synchronous Buck Converter

    K.C. Lee11/14/2016 at 18:05 0 comments

    This is a follow up to the Discrete 3.3V Buck Converter project log. I do not intended to build this as is because there are chips that can do a better job and offer fault protection. Due to the discrete nature of this design using easy to find models, it can be useful for running under LTSpice (or other simulators).

    At one point for the Reddit contest, I thought about adding short circuit protection by sensing current going through the diode D1. The control circuit is a bit complicated and not worth the extra BOM cost to earn a design feature. It'll take a few discretes and would be a lot easier if I can throw in some 74HC parts. The contest rule doesn't allow for that. It is one of those things that a chip could have done it smaller, cheaper and more efficiently.

    The control circuit is a result of the 3.3V design but with a proper 2.5V reference (e.g. TL431) (once again because I am not constrained by contest rules.) R9 and C5 are for slowing down the initial rise time to limit the inrush current. R9 can be increased to slow down the slew rate for real life. ~0.8ms might be too fast for some, but great for speeding up simulation time.

    I am using a synchronous rectifier (M2) to minimize the conduction loss in this design. It is driven off the output of comparator U2. I added (R10, D5, C7) and (R8,D4,C2) on the control signal to introduce a delay to minimize cross conduction between M1 and M2 during cross over.

    I am using a N-MOSFET for the main driver so that I can use N-MOSFET. It comes at a cost of complexity. Q5 inverts the polarity of the signal and R8, D4 and C2 provides the necessary delay. A charge pump (C1, D6, D7 and C6) leeching off the H bridge is used to provide the high voltage for driving M1. The duty cycle to below 100% to keep the charge pump going. An external driver supply can extend the operation to 100%.

    The blue trace shows the gate voltage of M2. The red one shows the gate voltage relative to the source for M1. The delay between the two signals is to prevent cross conduction. D1 picks up the slack during the cross over. Because of the low duty cycle, I get by with a 1A schottky diode. There are more intelligent gate driver parts with control schemes such as Predictive Gate Drive (TM) and Adaptive Gate Driver that can squeeze another few percent of efficiency by tightening the timing and better gate driver circuit to minimize the losses due to slow switching.

    The power components values are picked based on what I have on hand. In general I like to balance the losses between 3 parts: M1, M2 and the discrete to roughly the same amount for more bang per buck. (pun intended)

    M2 is chosen for low R(ON). The power loss is 1.35W at full load of 10A

    In this case I am using the old 20N03 for both parts because I got a pile of them from scrap boards and it is just good enough for this design. The power loss for M1 is 1.82W.

    A copper pour larger than 0.5 square inch should be used to dissipate this amount of heat. You really don't want to use the TJ = 150C (from graph) for a design.

    These are the power components for this design. A fill under these parts is close to 1 sq inch. You are going to want heavy copper pours and traces to carry the high current anyway. Vias to power planes on the solder side can help to dissipate power.

    A better gate drive could improve on this as the losses are during switching. You want to switch this part as fast as you can with lots of gate drive. In contrast, M2 losses are due to I^2*R.

    The green trace is the gate current for M1 and blue for M2. Red trace is the H bridge output. The simple discrete gate driver design is at its upper limit for this MOSFET due to high gate charge. Just using a gate driver chip could easily improve on the efficiency and allow for higher power.

    The losses for diode and inductor (Coiltronic HC1-5R1) is 0.54W. I haven't account for the filter caps and misc. they'll probably be in the 0.2W range depending on the ESR.

    So for a 5V 10A output, the losses is around 1.35W +... Read more »

  • Discrete 3.3V Buck Converter

    K.C. Lee04/20/2016 at 23:46 8 comments

    Congratulations to /u/fpga_computer for his winning entry! This entry features the 2nd place (adjusted for mounting type) cost, 1st place efficiency, and a tremendous write-up!

    saw @oshpark post about Buck Regulator Olympics contest on reddit /r/diyelectronics earlier today. I guess I can't help being interested in this contest. I actually have a lot of discrete boost converter designs, but no buck converters.

    Entry submitted. Elapse time: about 24 hours including design/built/documentation/test.

    This is what I have so far for a high frequency reasonably efficient discrete buck converter with fast transient response. The contest allows comparators, so that's what I am going to be using. See bottom of the page for full schematic.

    Control Circuit

    I am making a voltage mode hysteretic buck converter. The output voltage of the regulator is compared against a 3.3V reference.

    U2 is wired as a comparator with hysteresis with R3 is the series resistor and C4 for positive feedback. C4 injects a large amount of positive feedback that is only active once the falling/rising edge transition started. This ensures that the edges are fast and clean and a minimum on/off time. The feedback soon disappear allowing the comparator to compare voltage accurately.

    V(n006) is the output of U2 and V(n003) is the regulator output. The switching frequency is around 318kHz and output ripple is about 100mV pp.

    Power circuit

    I am going to use a P-MOSFET with a discrete gate driver. Q1 & Q2 forms a pseudo push-pull driver. I am using a 3A schottky diode as the free wheel diode.

    The following shows the MOSFET gate voltage and current.

    Top graph is the MOSFET output voltage and the bottom graph is the power loss.

    Most of the power losses are when the MOSFET switches on/off and not I^2 *R losses. The switching edge is where the voltage * current is the greatest. It is critical to switch the MOSFET as quickly as possible to minimize the time in the transition. So this is the case where a large MOSFET with low Ron would be worse. You want to pick a MOSFET with low gate charge so that it can be switched quickly and use a gate driver circuit correctly sized for it.

    The circuit is delivering 3.3V^2/2R = 5.44W, average MOSFET power loss: ~0.1W (~2%) That's not too bad for an old SOT23 MOSFET. :)

    Power loss for the 3A diode: 0.423W

    Changing to a 5A diode actually makes it worse. Once again, bigger isn't always better.

    Power loss for inductor: CD75-100

    There is a high power loss because of the low duty cycle (high input, low output) so the diode conduct 63% of the time. At 5V input when the duty cycles is higher, the diode loss is reduced to 0.122W while the loss at the MOSFET rises to 0.233W

    So far the overall power loss is: 0.1 + 0.423 + 0.218 = 0.741W The rough estimate of this design to be somewhere below 88% efficiency at 12V (Vin max) which is the worse case for this design.

    Transient Performance:

    In LTSpice, I attached a current source to the output of the supply to provide a load step of 1.5A (0.1A to 1.6A) with 1us rise/fall time to test the output load step response. I don't have a fast transient load tester to test this. May be some day, I'll build one.

    The output undershoot to 3.21V (-0.09V -2.7%) and overshoot to 3.49V (+0.19V +5.7). Not a lot can be done about the overshoot as the load drop except to use a large value capacitor with lower ESR.

    V(n006) is the MOSFET switching node.

    Zoom in view:

    The fast response of this type of converter can be seen here. The duty cycle pretty much changed right away. (0V means on for P-MOSFET). The output slew rate is only limited by the L & C.

    From Linear Tech App note 149 on fast and stable loop.

    I guess I got c).

    The current limiting seems more trouble than it is worth. I'll leave that for the time being.

    Work to be done is a 3.3V reference source out of discrete. The lowest part count approach is to use a 3.3V Zener...

    Read more »

  • SEPIC Converter

    K.C. Lee07/22/2015 at 03:25 0 comments

    aven't forgotten about this project. Pretty much everything I build has its own power supplies, so when the power supply circuits from those project works, they'll get pulled into this project.

    I have a working prototype of a 5V/100mA SEPIC converter. It is designed to work from 3V to 15V. I did some quick test here.

    It is basically a buck-boost converter that can regulate an input that is lower or higher than its output. It sounds too good to be true. There are some draw back as it has a lower efficiency - power going through a few more parts means more losses. When your input voltage can be higher or lower, then this is an interesting and useful building block. This is taken from my Dual charger design which has such a requirement. You are otherwise better off from an efficiency point of view to use either a buck or a boost converter.

    Basically, you can turn a boost converter into a buck-boost by adding a cap (C11) and an inductor (L2). The output is taken from L2 with rectifier diode (D4) and output cap (C14). The cap C11 serves as energy transfer between the two inductors. This particular version is built with 2 separate inductors. There are also version of the design that have L1 and L2 as a pair of coupled inductors. I use a couple of those in my charger project.

    When I get around, I'll clean up the layout and release the tested block.


    This is the current layout I have for it. The switching frequency is 1.5MHz and it deserves a bit of care. It is mostly a single layer design with a full ground plane on the secondary side optimized for home made PCB. I have minimize the high frequency/high current loop area. Use shielded inductors if you want to minimize EMC issues.

    I have ordered the cheapest inductors from China, so it might take a few week to arrive. This is what it would look like.

    I notice that AP3019AKTR-G1 (boost converter for LED) are being sold on Aliexpress at $3.09 for 10 pieces. It is probably a variant of the Diodes Inc AP3012. The feedback voltage is lower, but it is a matter of changing the voltage divider ratios. May be I should use the SOT23-6 package so that it would take either chips.

    Here is the home made PCB.

  • Isolated Module

    K.C. Lee05/14/2015 at 23:18 0 comments

    Both my VoIP ATA box and my cordless phone cheated on the isolation to cut cost. I had to make this isolated flyback module so that they can coexist in my UPS.

    I used a similar circuit to the boost design. L1B provides a feedback for the output voltage. The isolated output at winding L1C is poorly regulated as it is regulated by proxy via transformer ratio. That's all I need for the phone as it has its own regulator.

    Adding a LDO to the output is an easy way to tighten the regulation without adding too much overall complexity. I supposed I could have used the old optoisolator with a TL431 for linear current feedback. The feedback network is a bit too advanced for me to simulate in LTSpice besides it is going to be hard to find a TI part model in LinearTech's tool. This topology will provided a tight regulation as well as being able to use off the shelf SEPIC couple inductor.

    I used a zener diode clamp to limit the voltage across L1A to protect Q1.

    To make the transformer, I counted the number of turns on a 10uH inductor as I unwind it. To reduce the leak inductance, I rewound it with 3 thinner strands of wires at the same time (aka trifilar winding). L1B is for feedback and requires very little current, so you can get by with very thin wires. Keep the same number of turns as before and you'll have the same inductance.

    The output as on the two solder pads - 0V on top and +6V on bottom.
    Sorry have to be the potato picture as it is in use.

  • KIS-3R33S Modules

    K.C. Lee05/14/2015 at 22:28 0 comments

    I bought 20 of used ones from China a long time ago.

    I measured some of the internal component values:

    Note: The current rating is not even close to 2A probably due to the inductor and the cooling problem in the case.

    By connecting an external resistor to the ADJ pin (7) to ground, you can increase the output voltage. By connecting an external resistor between the output and ADJ pin, you can decrease the output voltage. You do not need to modify them if you want to use them as a fixed output supply unless you want to get higher than 10V output. You also need to use a higher input voltage than 12V as the duty cycle is limited.

    JP1 selects 5V/3.3V outputs. JP2 and JP3 are lined up with supply rails on the breadboard. When S1 is closed, the output is turned off.

  • 26V Boost Module

    K.C. Lee05/14/2015 at 21:37 0 comments

    This is designed for my old ST516V6 ADSL modem to be used on my UPS. That modem comes with a huge transformer with a non-standard plug. It uses a high voltage, so I have to make a 26V boost module.

    Design is also pretty much text book from the LM3478 documentation. Due to the high voltage, I use an Al electrolytic cap for the output bulk cap. R5 sets the switching frequency. The output voltage can be tweaked by playing with R6 and R7. R8 controls the inductor current. The chip doesn't have a whole lot of gate drive currents, so I would recommend a MOSFET with small gate capacitance (435pF). You can probably find a smaller MOSFET that does the job.

    This board was designed to be etched at home. All the vias are large and outside of the components footprints and a piece of component lead was used to connect to the other side of the PCB.

    Here is what I actually built:. I have to make do with the parts I have.

    I ended up using a Sanyo 100uF/35V cap because the SMT part was too tall. A 10uH inductor was rewounded for the right values.I didn't have 22nF, so I had two 10nF at C2 and C6. I used two 0.15 ohms in parallel for the 0.075 ohms resistor. That used up the space for the 2 ceramic output cap, but the circuit works fine.

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alan_r_cam wrote 11/28/2015 at 21:54 point

Wow. Isolated, buck, and buck/boost... any plans to add an inverter (5V in -5V out) ?

If only to "complete the set" ?

  Are you sure? yes | no

K.C. Lee wrote 11/28/2015 at 23:09 point

Actually I had built an -5V inverter to fix a HDD (with a blown one).  I don't find much use for it.  I'll keep it in mind and add that to the collection.  

I  got something a bit more interesting.

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Anool Mahidharia wrote 10/08/2015 at 04:40 point

whoa! This is so awesome. I guess we both felt the need for this. Here's what I built recently.

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Parkwoodrehab1 wrote 08/06/2015 at 14:21 point

Pololu S7V7F5    Vin ~ 2.5 - 11 v   buck+boost  T220 sized

CUI V7805-500   Vin ~ 6-35 v   buck   T220 sized

these are  excellent and cheap - I never use lossy linear regs anymore.

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Antti Lukats wrote 07/07/2015 at 18:49 point

easy. and keep a smile :) I we design here boards with tons of DCDC every day, not fun at all no matter what parts you use.

  Are you sure? yes | no

K.C. Lee wrote 07/07/2015 at 16:47 point

Sadly mine is a much older design without synchronous rectifier. Theirs also seem to be one of the newer much higher frequency switchers. 
On the other hand by changing the input cap and the MOSFET, mine can work up to 30V input range with some margins.  Current design is for 20V or so for my UPS project .

With those parts, I would have only use 1/2 of the board space with much a tighter layout to reduce the current loop.  I don't just throw parts on, route a board and call it a day.

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Antti Lukats wrote 07/07/2015 at 16:15 point

is your design somewhat better than that one:

6.95 is of course a rip-off price :(

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leandro wrote 06/13/2015 at 02:32 point

really useful!

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con-f-use wrote 06/12/2015 at 08:18 point

Is my eye-sight that bad, or am I really not seeing the eagle-files?

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K.C. Lee wrote 06/12/2015 at 14:13 point

Project files now available:

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con-f-use wrote 06/12/2015 at 16:14 point

Thank you!

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phaseform wrote 05/14/2015 at 10:02 point

love it, want one! so useful! car usb power etc

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phil6062 wrote 05/14/2015 at 06:56 point


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Nick Sayer wrote 05/14/2015 at 00:41 point

When I was designing Pi Power I gravitated towards the LM3485 as well. One thing I did differently was that I used a sense resistor rather than the MOSFET for ISENSE. That let me be a little bit freer with MOSFET substitutions without impacting the overcurrent protection. But that design is for 2A, so the parts are beefier as well.

One thing I've figured out is that since this is a hysteresis driven device, the operating frequency rises with the delta between Vin and Vout. And as the operating frequency rises, the impact of the MOSFET's capacitance starts to become significant. I had to reduce the maximum input voltage of my design because the MOSFETs were getting too hot. Alas, their failure mode is to fail-short, which is a very, very bad thing for a buck converter...

  Are you sure? yes | no

K.C. Lee wrote 05/14/2015 at 01:09 point

The LM3485 have very limited gate drive, so that limits the input capacitance/size of the MOSFET that it can drive fast enough.  At higher frequencies, there are more on/off transitions so that can get you in trouble fast.  Using as high inductance as possible would lower the switching frequency.

These days I would have used a part with internal MOSFET.

  Are you sure? yes | no

Nick Sayer wrote 05/14/2015 at 01:26 point

The MC34063 is an attractive choice IMHO - and probably would have worked nicely for this application. But for Pi Power, the current requirements were too great, unfortunately.

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K.C. Lee wrote 05/14/2015 at 02:19 point

The same design is used for 12V modules which are used in my UPS.  So when the battery is running low, I want the 100% duty cycle and low drop out with the PMOS not the 2V drop from a bipolar NPN transistor.

Besides, that part's switching frequency is too low (100kHz) and the efficiency isn't that great.  There are lots of synchronous buck converters.

  Are you sure? yes | no

K.C. Lee wrote 07/27/2014 at 17:35 point
That chip is only rated for 35V and it drives an external MOSFET limited to 500pF gate capacitance or so (i.e. good for 2A or so), so I doubt it is the same circuit. There are plenty of ebay stuff out there and they are probably the ones you are talking about.

I use them in my DC UPS which takes a 16V old laptop supply and backup batteries with OR'ing diodes. Some of my network equipment that can't work with the 16V bus are paired with these modules. I made some buck, boost and isolated etc. Pretty solid stuff as they are on 24/7 for the last 3 years.

I have half a box of 100V-240V with active PFC 12V@10A open frame supply Made in Taiwan with real safety certifications. $0.00 ftw. :)

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J Groff wrote 07/27/2014 at 16:42 point
if you are looking for a cheap ready 5v bus I use the USB wall chargers which are basically the circuit you show, works up to 120V DC and no way to hook input up backwards! I use them on 48V DC cells. Take apart the unit and break out the USB or just use USB cables and wall plugs. I've found them up to 4+ amps for around $30. Good luck.

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