How Thermal Throttling Breaks USB Charger Testing, and a Custom Rig That Follows It
イチケン / ICHIKEN
Modern compact, high-power USB chargers often cut their own output when they get hot. This is called thermal throttling, and it protects the charger from its own heat. In this video, Ichiken explains why heat has become a central problem for small GaN chargers and why throttling has made the channel's old testing method unreliable. They then introduce a self-built test device that talks to the charger over USB Power Delivery and follows the charger's reduced power limits as they change.
From 5V/1A to GaN Fast Chargers
Ichiken starts with some history, drawing on earlier videos from the channel. USB chargers came into wide use with smartphones. Early models typically supplied about 5V at 1A. As smartphones spread and batteries grew, fast chargers appeared, and every charger lined up on the desk in the video is a fast charger.
Ichiken credits Anker, a frequent subject on the channel, with changing the industry when it introduced its USB fast charger around November 2019. The key change was inside the charger. Instead of conventional silicon power semiconductors, it used gallium nitride (GaN) power semiconductors. According to Ichiken, GaN made relatively low-cost, compact, high-capacity chargers possible without a large price increase, and the market for GaN-based USB chargers has kept growing since.
The charger Ichiken uses most often as a benchmark is the Anker Nano II series, released around May 2021. They describe it as a major bestseller among GaN chargers. After that, competition intensified, and the market gained multi-port chargers and chargers with built-in cables.
Why Small Size and High Power Pull Against Each Other
Among all the performance requirements for a USB charger, Ichiken singles out two: being small and handling high power. People carry chargers when traveling or going out for work, so the smaller the better. Chargers are also increasingly used as AC adapters for laptops and for charging high-capacity mobile batteries, which requires high output.
That combination leads straight to heat. Ichiken says heat remains the fundamental problem for this kind of product no matter how far the technology goes. The switch from silicon to GaN reduced heat generation, and that reduction is what allowed chargers to become smaller and more powerful. Once designers push GaN chargers further toward small size and high output, however, the heat problem returns.
Where the Heat Comes From
A USB charger converts the AC voltage from the wall outlet into DC voltage for charging. During that conversion, the electronic components inside lose some power, and that loss ends up as heat. Ichiken says recent GaN chargers are quite good, with conversion efficiencies often in the low 90% range. The remaining few percent is still lost, and all of it becomes heat.
To make this concrete, Ichiken takes a charger with 90% conversion efficiency supplying 100W. In that case, about 11W is lost inside the charger. Ichiken calls 11W a significant loss for a device that small, and says it can push the surface temperature above 60°C or 70°C.
High temperature causes two problems in their view:
- Reliability: the internal components operate in a hot environment, which shortens their lifespan and raises the risk of failure.
- Safety: a charger is something people touch, so there is a risk of burns.
Reconciling miniaturization with high output therefore depends on two things: reducing power loss, and dissipating whatever heat is still produced.
Reducing Loss Costs Money, So Manufacturers Changed Tactics
Ichiken focuses on the first of those two. Reducing heat means reducing power loss, and there are many ways to do that with higher-performance components. The catch is that the product price then jumps. This is why Ichiken considers GaN such a breakthrough: it improved performance over silicon without a large price increase.
According to Ichiken, balancing heat, price, small size, and high output all at once is very difficult, and manufacturers are struggling with it. They suggest the problem may not be fully solvable without another technological breakthrough.
Their observation is that manufacturers have recently responded differently: they manage overheating with thermal throttling. When the charger's temperature rises, it temporarily lowers the power it can deliver by one step. Less output means less power loss, which means less heat, and that protects the charger. Ichiken notes that Apple, too, now sells a USB charger that throttles this way.
Why the Old Test Method Stopped Working
The channel has tested many USB chargers, mainly measuring how much power they can deliver and how much their temperature rises while doing so. Ichiken says this method no longer works for recent chargers, and thermal throttling is the reason.
The old tests applied a fixed power draw with an electronic load. A throttling charger lowers its output limit as its internal temperature rises, and the fixed-load method could not capture how the charger actually behaves during throttling. To evaluate current chargers accurately, Ichiken needed a test method that communicates with the charger in both directions and follows its throttling.
How USB PD Negotiation Works
Before showing the device, Ichiken explains the relevant part of USB Power Delivery. When a charger connects to a smartphone or computer, the charger (the SOURCE side) and the device being charged (the SINK side) communicate before charging begins.
The source sends a message called Source Capabilities. It lists the maximum voltage and current the charger can supply, made up of entries called PDOs (Power Data Objects). The example on screen lists 5V/3A, 9V/3A, 15V/3A, and 20V/2.25A. These are upper limits, and the charger can output anything within them. The sink reads this list and chooses the voltage and current it wants within that range.
If the sink ignores the Source Capabilities and tries to draw too much current, the charger shuts off its output for safety.
Thermal throttling works through this same mechanism. When the charger needs to throttle, it sends a new, smaller Source Capabilities message and then supplies power only within that reduced range.
What Went Wrong With Electronic Loads
Ichiken says the channel's earlier tests ignored these changes in Source Capabilities. The sink side was an electronic load drawing 100% of the rated power the whole time. When the charger started throttling, the electronic load could not read the new limits, kept drawing the original power, and the charger shut off its output.
Real devices such as laptops behave differently. They read the lowered Source Capabilities and keep drawing power within the new limit, so the output is never cut. That gap between the test setup and real use is why Ichiken concluded that an electronic load alone was no longer a valid test method.
Before building the new device, the channel had tested throttling chargers using real loads such as high-capacity mobile batteries and gaming laptops. That approach had its own problems. Someone had to watch the experiment almost the whole time, efficiency was very low, and reproducibility was limited.
The Self-Built Test Device
The solution was a device Ichiken built that reads the charger's communication while it runs. It keeps drawing power at the upper limit of whatever Source Capabilities the charger is currently advertising. Because load control and communication logging are fully automatic, an experiment can run almost unattended.
The full setup includes power supplies, measuring instruments, an electronic load, and a PC, plus the newly developed board. The board combines a PD trigger device with a microcontroller. It receives the communication from the charger, and the microcontroller processes the data. The board, the electronic load, and the PC work together so that the draw always matches the current upper limit. Ichiken says the detailed operation would take too long to explain here and plans to cover it in a members-only video.
Using it is simple: plug in the charger. The system reads the Source Capabilities, automatically selects the entry that gives the maximum power, and starts drawing that load through the electronic load. In the demonstration it draws a maximum of 3.25A at 20V while measuring temperature, and Ichiken says it behaves as intended as the temperature rises.
Results: With the Board, Load Only, and a Real Laptop
Ichiken compares three measurements of a charger.
With the new board. About 30 minutes after the load started, the charger began thermal throttling. The Source Capabilities initially showed a maximum of 20V at 3.25A, or 65W. As the temperature rose, the charger resent its Source Capabilities with a maximum of 20V at 2.75A, or 55W. Only after that resend did the electronic load lower its draw from 65W to 55W. Ichiken describes this as the charger and the test device working together as intended.
Electronic load only. This is the channel's old method. At about 23 minutes, the charger tried to throttle. The electronic load ignored the change and kept trying to draw 65W. The sink never lowered its power as the charger expected, so the charger cut its output.
A real laptop. When the same charger charged a laptop, it entered thermal throttling around the 27-minute mark and reduced its output. Ichiken says the graph from this test and the graph from the new board are almost identical, and takes that as evidence the board works well.
An Observation From Testing Real Devices
Ichiken also mentions something they learned from testing various chargers with real devices. Sink devices rarely draw power right up to the Source Capabilities limit. They often use about 90% of it.
Next Steps
To sum up, Ichiken has explained thermal throttling, where a charger reduces its output because of heat, and has built an environment that can test chargers correctly even when throttling occurs. They plan to use the board for a range of future charger tests, and they say a 240W version is currently in development.
This video is brought to you courtesy of DigiKey. Hello. Today's video is about thermal throttling in USB chargers and a circuit I made myself to verify it.
As the title of the video states, some modern USB chargers reduce their output power during operation because of the heat they generate. This is an action to protect the USB charger from heating. It's called thermal throttling. Recent small, high-power USB chargers often include this feature.
What is thermal throttling, and why is such a feature needed? I'll explain, referring to past Ichiken Channel videos.
USB chargers have been frequently used since smartphones came out. In the early days, there were many USB chargers that charged at about 5V, 1A. With the proliferation of smartphones and larger battery capacities, fast chargers appeared. All the chargers lined up here are fast chargers.
Anker, which is often featured in my videos, introduced a USB fast charger around November 2019. It changed the direction of this industry in a big way. Here's the product. For the power semiconductors inside the charger, it uses GaN, or gallium nitride, rather than the traditionally used silicon power semiconductors.
By adopting GaN, it became possible to make relatively low-cost, compact, high-capacity USB chargers without significantly increasing their price. These developments led to continued growth in the market for USB chargers using gallium nitride power semiconductors.
But I digress. This charger has appeared frequently in my videos, and I often use it as a benchmark. It's from the Anker Nano II series, released around May 2021. Among chargers using GaN power semiconductors, it has been quite a bestseller. After this, competition in the USB charger market continued, and products such as multi-port chargers and USB chargers with built-in cables appeared.
Amid these trends, one particularly important performance requirement for USB chargers is being small and able to handle high power. Think of the situations where you would use a USB charger. Besides using it at home, you may carry it while traveling or going out for work. In those situations, having the USB charger itself be as small as possible is important.
It's not just about carrying it around. There are also more cases of using a USB charger like an AC adapter. To charge high-capacity power banks and laptop computers, USB chargers capable of high power output are necessary.
As USB chargers get smaller and their output power increases, the next problem is heat: the USB charger itself generates heat. With products like these, it always comes back to heat. Switching from silicon to GaN power semiconductors reduced heat generation and made smaller sizes and higher output possible. But as GaN power semiconductors become more widespread and products are refined to be smaller and more powerful, we face the heat problem again.
First, I'll explain where the heat in a USB charger comes from and how it affects the charger. The role of a USB charger is to convert the AC voltage at the outlet into DC voltage for charging. During this power conversion, power loss occurs in the electronic components inside the USB charger. This is the part here. That power loss ultimately becomes heat.
Recent USB chargers using GaN power semiconductors are pretty good. Their power conversion efficiency is often in the low 90% range. But the power loss from the remaining percentage all becomes heat.
Let's look at some specific numbers. Suppose a USB charger with 90% conversion efficiency is charging something at 100W. That means 11W of power loss occurs in the USB charger. An 11W loss is quite large. In a USB charger of this size, that power loss can cause the surface temperature to exceed 60°C or 70°C.
What is the problem with high temperatures? The internal electronic components are placed in a high-temperature environment, shortening their lifespan and increasing the risk of failure. Furthermore, because this is a product people touch, there is a risk of burns.
To achieve both a smaller USB charger and high output, it's important to reduce power loss and heat generation, and to dissipate the heat that is generated. Here, we'll focus on reducing heat generation. To do that, we need to reduce power loss. There are various ways to achieve this by using higher-performance electronic components in USB chargers, but the product price jumps.
Seen this way, GaN power semiconductors, which improved performance over silicon power semiconductors without a significant price increase, were a major breakthrough in the evolution of USB chargers.
As we've discussed, it's very difficult to balance these three elements: heat, price, and a small size with high power output. Manufacturers are battling this challenge. But it seems the issue itself cannot be solved without the next technological breakthrough. These days, manufacturers are changing their approach. That is overheat protection through thermal throttling.
What is thermal throttling? When the temperature of a USB charger rises, it temporarily lowers the power the charger can output by one step. Apple also offers a USB charger with thermal throttling. This operation reduces power loss, which means less heat generation, to protect the USB charger from heating.
So far, I've explained why heat problems are inevitable as USB chargers become smaller and more powerful, and how thermal throttling is used to counteract them.
On this channel, we've examined many USB chargers, focusing on how much power they can output and how much their temperature rises. With recent USB chargers, the experimental methods we've been using are no longer applicable. The reason is thermal throttling.
USB chargers with this feature automatically limit output power as their internal temperature rises. With our previous method of applying a fixed power load, we couldn't capture the correct behavior of a USB charger during thermal throttling. To accurately evaluate today's USB chargers, we needed a test method that follows thermal throttling while communicating bidirectionally with the charger.
From here, I'll briefly explain PDOs, or Power Data Objects, in USB PD communication, and clarify how output power information is exchanged between the USB charger and the device being charged. Finally, I'll use this circuit I made to see under what conditions thermal throttling is triggered and how much the output power is reduced.
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Let's talk a little about communication between the USB charger and the device being charged. When a USB charger is connected to a smartphone or computer, the charger, or source, communicates with the device being charged, or sink, before charging starts.
At this point, the USB charger on the source side sends information called Source Capabilities. This data indicates the voltage and maximum current the charger can supply. If we look at the Source Capabilities of a USB charger, it looks like this. In this case, the limits are 5V, 3A; 9V, 3A; 15V, 3A; and 20V, 2.25A. It can provide output within those ranges.
The sink receives these Source Capabilities and determines its desired current and voltage within that range. If the sink ignores the Source Capabilities and tries to draw excessive current from the source, the USB charger shuts off its output for safety.
Thermal throttling during charging means changing these Source Capabilities. When the USB charger needs to perform thermal throttling, it sends Source Capabilities again with a lower available output power. It then supplies power only within that range.
Our previous method of testing USB chargers ignored changes in Source Capabilities when thermal throttling occurred. We used an electronic load on the sink side, drawing a 100% load at all times. When the USB charger indicated that it was performing thermal throttling, the sink couldn't read that, so the output shut off.
But in actual use, such as powering and charging a computer, the output doesn't shut off. Power continues to be supplied with reduced Source Capabilities. That's why we needed to update our testing method instead of using only an electronic load.
So I made this testing device myself. It operates while reading communications from the USB charger. The device communicates with the charger and continually consumes power at the upper limit of its Source Capabilities.
To test USB chargers that perform thermal throttling, I had used high-capacity power banks and gaming laptops as loads. But someone had to stay with them and monitor them almost constantly. The experiments were very inefficient, and reproducibility was limited. With the new testing equipment, communication, load control, and recording can all be automated, so experiments can be completed almost unattended.
Now let's try an experiment. Here is an overall view of the setup: power supplies, measuring instruments, an electronic load, a computer, and the board we developed. Here is what the board looks like. It consists of a PD trigger device and a microcontroller. It receives communications from the USB charger, and the microcontroller processes the data. The electronic components for this circuit were purchased from our sponsor, DigiKey.
For the overall operation, this board, the electronic load, and the computer work together to continually draw power from the USB charger at the upper limit of its Source Capabilities. Explaining the details of its operation would take a long time, so I plan to publish a members-only video about that.
In actual use, it looks like this. It's as simple as plugging it in. Around here, it reads the Source Capabilities, automatically selects the option that provides the maximum power, and starts drawing power with the electronic load.
Here it is operating. It draws a maximum current of 3.25A, and the output voltage is 20V. Temperature is also measured at the same time. As the temperature rises, it's doing exactly what it's supposed to do.
Here are the actual measurement results. About 30 minutes after starting to draw the load, thermal throttling occurs, and the upper limit of the output current in the Source Capabilities decreases. The electronic load lowers the current it draws accordingly, so they are working together.
Here are the received Source Capabilities. Initially, the maximum was 20V, 3.25A, or 65W. As the temperature rises and thermal throttling occurs, the maximum output becomes 20V, 2.75A, or 55W. After the Source Capabilities are sent again, the electronic load reduces its power consumption from 65W to 55W.
Here are the next results. In this experiment, we didn't use the board we developed. These are the results of testing directly with an electronic load, which is the way we've always done it. At about 23 minutes, the USB charger tries to apply thermal throttling. The electronic load ignores it and tries to keep drawing 65W. Since the expected behavior on the sink side—reducing power—didn't happen, the USB charger shut off its output.
Finally, here is a graph of charging a laptop with the USB charger. Around the 27-minute mark, the charger enters thermal throttling and reduces its output power. This graph and the results using the board we developed are almost identical. In other words, I think the device we developed is working well.
By the way, testing various USB chargers revealed that the sink side rarely uses power right up to the limit of the Source Capabilities. It often uses about 90%.
Today, we covered thermal throttling, in which a USB charger reduces its output power because of heat, and built a setup that can test USB chargers even when that behavior occurs. I'd like to use this board for various USB charger tests in the future. A 240W version is currently under development. Thank you for watching the video to the end.
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