Your power supply feeds every other circuit in your product, so when it goes wrong, everything else goes down with it.
Pick the wrong regulator and a battery that should last a year can be dead in about a week.
Ignore peak current and your product can reset every time the Wi-Fi turns on, and a sloppy layout around one tiny chip can fail FCC testing even when the schematic is perfect.
And a couple of these mistakes can start a fire in a customer’s home or give someone a dangerous shock.
Power supplies look like the easiest part of a design, just a regulator and a few capacitors, which is exactly why so many product creators rush through them.
So in this video, I’m counting down 9 power supply design rules you should never break, and what each one will cost you if you do.
Table of Contents
Rule #9 – NEVER Overlook Inrush Current and Startup Sequencing
Inrush current is the big spike of current your product pulls the instant it’s plugged in or switched on.
Most of that spike comes from charging up the capacitors on your power input, and for a split second those capacitors act almost like a short circuit.
With a lot of capacitance there, that spike can trip the current limit on a USB port, make a wall adapter shut down, or cause a spark at the connector every time it’s plugged in.
USB is especially picky, since the spec limits how much capacitance a device can put right on its power input.
The fix is to put an inrush limiter or a load switch with inrush control, which is a small chip that ramps the voltage up gradually, in front of those input capacitors.
A regulator’s soft-start feature helps too, but only with the current charging everything after the regulator, not the capacitors sitting right on its input.
Startup sequencing is the second half of this rule, since some processors, displays, and other complex chips need their power rails turned on in a specific order.
Power them up in the wrong order and the chip might not boot reliably, or in some cases it can even be damaged.
Regulators with an enable pin and a power-good output make this easy, since each one can turn on the next one in line only after its own output is stable.
Check the datasheet of every major chip for startup requirements, because finding this after you’ve built a few hundred units means a board redesign.
Inrush current and startup order are easy to miss because nothing on the schematic looks wrong, and that’s true for a lot of the rules in this video.
Rule #8 – NEVER Charge a Lithium Battery Without a Dedicated Charger IC and Protection
Lithium batteries pack a lot of energy into a small space, and that energy has to be controlled carefully every time you charge them.
A lithium-ion or lithium-polymer cell needs to be charged with a constant current first, then held at a constant voltage, usually 4.2V, until the current tapers off.
Push it past that voltage and the cell can swell, overheat, or catch fire.
Drain it too low and you can permanently damage it, which makes it more likely to fail the next time it’s charged.
So never try to charge a lithium battery with just a resistor, a diode, or a transistor that your firmware turns on and off.
Use a dedicated charger IC with a thermistor input, and connect it to a thermistor, which is a temperature sensor, placed on the battery so charging stops when the cell is too hot or too cold.
Charging below freezing is a big one, since it can permanently damage the cell even if it seems fine afterward.
On the battery side, you also want a protection circuit that cuts the battery off if it’s overcharged, drained too low, or shorted.
Most pre-certified battery packs already include that protection board, which is one more reason to buy an off-the-shelf pack instead of building your own.
Skip either one and you’re not just risking returns, you’re risking a fire in a customer’s home.
Rule #7 – NEVER Ignore Quiescent Current in a Battery Product
Quiescent current is the small amount of current a regulator draws just to keep itself running, even when your product is doing nothing.
In a battery product that spends most of its life asleep, it can be the biggest drain on the battery.
Take the AMS1117, a cheap linear regulator that shows up in a lot of designs because it’s on so many development boards.
It can draw around 5mA just sitting there, which would drain a 1,000mAh battery in about 8 days even if everything else in your product was turned off.
Low-power regulators can draw less than 1uA, which turns the regulator into a tiny part of your battery budget instead of the biggest part.
Regulators aren’t the only culprit, since a voltage divider that measures the battery or an LED left on can draw current around the clock too.
I learned how hard a tiny bit of extra current can be to track down while working on a low quiescent current regulator design at TI, where I spent weeks hunting current the simulations said shouldn’t be there.
It turned out to be a parasitic transistor in the physical layout, something nothing in the schematic could have shown me.
So don’t trust your schematic for sleep current, and measure it on your prototype with a meter that can read microamps.
Rule #6 – NEVER Skip Input Protection
Input protection is what keeps your product alive when a customer does something you didn’t plan for.
And customers will plug in the wrong adapter, put batteries in backwards, and connect cables while the power’s already on.
Barrel jacks are a common problem, since a 12V adapter often has the exact same plug as a 5V one, and most 5V circuits won’t survive it.
For reverse polarity, a Schottky diode in series with the input works, as long as you can afford to lose a few tenths of a volt.
If you can’t afford that drop, a P-channel MOSFET wired for reverse polarity protection does the same job with almost no voltage loss.
For the wrong adapter, an overvoltage protection chip can disconnect the input before the high voltage reaches the rest of your circuit, and a TVS diode handles quick voltage spikes.
A fuse or a resettable fuse, called a PTC, sized to protect your traces and wiring, helps make sure a short circuit turns into a dead product instead of a melted one.
None of these parts cost much, but leaving them out means returns, bad reviews, and units that die the first time someone grabs the wrong adapter.
Rule #5 – NEVER Size Parts for Average Current Instead of Peak Current
Average current tells you how fast your battery drains, but peak current is what decides whether your product actually works.
Many products sit at a few milliamps most of the time, then spike hard for a fraction of a second when a radio transmits, a motor starts, or a display backlight turns on.
The ESP32 is a good example, since its Wi-Fi transmissions create short current spikes, and Espressif recommends a power supply that can deliver at least 500mA.
If your regulator can’t keep up with those spikes, the voltage dips and the microcontroller resets right when it tries to connect to Wi-Fi.
If you’re using a switching regulator, the inductor has the same issue, since going past its saturation current rating makes it lose inductance, and with less inductance the current spikes even higher every time the regulator switches.
Batteries have limits too, and a coin cell is the classic trap because it can’t deliver much peak current at all, so its voltage sags hard during a radio burst.
Add up the peak current of every part on your power rail at the hottest temperature your product will see, since most chips draw more current as they heat up.
Then pick a regulator that can handle that number with some margin, and use the regulator’s datasheet to size the inductor and figure out how much current it will pull from your battery.
Rule #4 – NEVER Ignore the Layout of a Switching Regulator
Switching regulators are efficient because they turn power on and off hundreds of thousands of times per second, but that switching also makes them one of the noisiest circuits on your board.
That’s why the layout matters just as much as the schematic, and a perfect schematic can still fail with a sloppy layout.
For the most common type, a buck regulator that steps voltage down, the most important part is the loop formed by the input capacitor and the regulator, since fast-changing current flows through it every time the regulator switches.
If that capacitor sits too far away, the loop acts like an antenna and broadcasts noise, which can show up as EMI, or electromagnetic interference, during FCC or CE testing.
The switch node, which is the pin that connects to the inductor, is the other trouble spot, so keep its copper area small and keep sensitive traces like the feedback line away from it.
Layout noise can also leak into your radio or your sensor readings and make them unreliable.
The good news is that almost every switching regulator datasheet includes a recommended layout, and the safest move is to copy it as closely as you can.
Rule #3 – NEVER Trust a Capacitor’s Printed Value
Ceramic capacitors are in almost every power supply, and most product creators assume a 10uF capacitor gives them 10uF.
But the common ceramic types, like X5R and X7R, lose capacitance as you put DC voltage across them, and this effect is called DC bias.
A small 10uF ceramic capacitor rated for 6.3V can lose well over half its value at 5V, leaving you with something closer to 3uF.
That’s a big problem for regulators, because most of them need a minimum amount of output capacitance to stay stable.
Fall below it and an LDO can start to oscillate, or a switching regulator can produce much more ripple than you expected.
Capacitor makers like Murata and TDK publish DC bias curves for their parts, so check what you’ll really get at your operating voltage before you pick one.
A larger package or a second capacitor can help, but go by the DC bias curve, not just the voltage rating.
Rule #2 – NEVER Pick an LDO Without Checking Heat and Dropout
An LDO, or low-dropout regulator, is the simplest way to get a lower voltage, but it does it by turning the extra voltage into heat.
The heat is just the voltage drop across the LDO times the current flowing through it.
Drop 12V down to 3.3V at 500mA and the LDO has to get rid of about 4.4W, which is way more than a small package can handle.
Depending on the package and how much copper is around it, a small LDO might only handle around 1W or even less before it gets too hot, and when that happens it either shuts itself down or cooks the parts around it.
For a big voltage drop or a lot of current, use a switching regulator instead, since it can often be more than 90% efficient.
Dropout is the other half of this rule, and it’s the minimum gap the LDO needs between its input and output to keep regulating.
Battery products get caught by this because a lithium battery starts around 4.2V when it’s full and keeps dropping as it drains.
With a 3.3V LDO that needs around 200mV of headroom, the output starts sagging once the battery gets below about 3.5V, and unless your circuits can run on less, you lose a good chunk of the battery’s usable capacity.
So if your product runs from a single lithium cell, look at a buck-boost converter, which can make 3.3V whether the battery is above or below that voltage, or check whether your chips can run at a lower voltage like 3.0V.
Rule #1 – NEVER Design Your Own AC-to-DC Power Supply
Designing your own AC-to-DC power supply means your circuit connects directly to wall power, and that changes everything about your product.
Wall power can kill someone, so the safety rules are strict, and your design will need to pass safety testing to standards like UL or IEC 62368-1.
Those standards control how far apart the high-voltage and low-voltage parts have to be, what insulation you need, and how the product behaves when a part fails.
Get any of it wrong and you’re not looking at a board redesign, you’re looking at a product that can shock a customer or start a fire, and that leads to recalls and lawsuits.
And even if you get everything right, safety certification for a product that plugs straight into the wall can easily cost $10,000 or more and add months to your launch.
The better path is a certified external wall adapter, so the only thing your product ever sees is a safe, low voltage.
That makes your own safety testing much simpler and cheaper, just make sure the adapter you pick also meets the efficiency rules for wall adapters in every country where you’ll sell.
If the power supply has to be inside your product, use a pre-certified AC-to-DC module instead of designing one from scratch, though your wiring, spacing, and enclosure around it will still need a safety review.
Wall power adds a shock risk that can kill on top of the fire risk, and that’s why this rule sits at #1.
If you’d like an expert to look over your power supply before you order boards, whether that’s your regulator layout or your battery charging circuit, you can get help from me and other engineers inside my Hardware Academy.