Author:
Matt Hein, Product Line Manager – Current Sensors, Allegro Microsystems
Date
08/21/2024
PDF
Click image to enlarge
Figure 1: Block diagram of a CLLLC Isolated DC-DC Converter
In order to fix the problems with today’s transformers, one solution offers industry-first power conversion capabilities that move at warp speed.
Isolated DC-DC Converter – the common element of power conversion
The isolated DC-DC converter allows for efficient high-voltage power conversion. One common implementation of an isolated DC-DC converter is denoted “CLLLC” due to implementation of two capacitors (C), two inductors (L), and an isolating transformer (another L). This architecture uses a resonant tank with active bridges on either side to achieve efficiencies well over 97%.
There are other methods to implement high-voltage power conversion, but the CLLLC is popular for two reasons. First, the CLLLC architecture has isolation, which is a regulatory requirement for many applications that consumers may interact with. These systems must prevent a hard electrical path to the grid through an isolation barrier, and an isolated transformer meets this need.
Second, the CLLLC architecture provides the highest efficiency compared to most other methods. In applications such as a vehicle on-board charger, power conversion can be rated as high as 22 kW and every percent of power loss results in a slower vehicle charging time and more wasted energy for the consumer to pay for. Just 1% of the total power in a 22kW system equates to 220W – similar to the energy consumption of an active Sony PlayStation 5.
Some systems which convert high voltage to a low voltage such as 12V use a similar architecture called an LLC converter. This structure omits some secondary side components to generate a high-current, low-voltage output.
Common applications for the isolated DC-DC converter
High-voltage power conversion is notably found in electric vehicles, electric vehicle charging infrastructure, solar string inverters, energy storage systems, and datacenter power supplies.
Electric vehicles
In an electrified vehicle powertrain, the high-voltage vehicle battery – typically 400V or 800V – is charged through an on-board charger module. The high voltage is converted down to 12V or 48V using an isolated LLC converter where it is used in vehicle subsystems such as power steering, braking, or infotainment. Key customer pain points for these systems are charging times and vehicle range which are affected by efficiency and system sizes/weight.
Click image to enlarge
Figure 2: High-level block diagram of an electric vehicle power system
EV charging infrastructure
In principle, fast chargers for electric vehicles operate as a very high power off-board charger. They directly charge the vehicle battery and bypass the on-board charger. A DC charger can range in output from 20kW (like the highest power on-board chargers) up to an astonishing 300 kW to fully charge an electric vehicle in as little as 30 minutes. DC chargers are typically constructed from multiple lower power modules of 5 to 20 kW each in parallel to provide such a high current output. The key customer pain point is charging time, which is effected by efficiency.
Solar string inverters
A solar string inverter captures and converts solar energy from a string of solar panels into an energy storage system or to energy for the grid. A typical residential string inverter is rated for 3-10kW and accepts a panel string input of up to 1000V. Commercial string inverters may have power capability up to 100 kW. Even though the power output of a residential string inverter is relatively low compared to other systems, efficiency is a key design focus. Higher efficiency results in more money saved or generated for a household and a quicker pay-back time for the solar installation.
Energy storage systems
Energy storage is a key part of any clean energy strategy. Solar energy is only available during the daytime and needs to be stored to access when the sun is not shining. Clean energy storage systems comprise power conversion to charge a battery bank. While lead acid batteries are still used, lithium-ion batteries have taken over a large part of the market. A typical residential energy storage system needs to store 10 kWh and be able to output 5kW at any given time. Efficiency is key for customers using these systems.
Click image to enlarge
Figure 3: High-level block diagram of a DC fast charger
Datacenter power supplies
The widespread use of cloud computing and artificial intelligence (AI) services have driven increased demand for datacenter processing power worldwide. One server rack in a datacenter installation will contain multiple server power supplies, each using an isolated LLC converter to generate 1-4 kW for consumption by the system on a 12V or 48V supply. While this power output may seem small compared to other applications, the power density needs are very strict. The latest datacenter standards from Open Compute Project M-CRPS require a form factor of only 265 mm by 73.5 mm or smaller.
Trends in high voltage power conversion
The applications listed above benefit significantly from higher efficiency and higher power density.
Many system designers are considering moving from traditional Silicon (Si) MOSFETs to Gallium Nitride (GaN) in the pursuit of a higher efficiency and higher power density system. The two main reasons for this are lower conduction and switching losses and higher switching frequencies. GaN transistors also do not suffer from reverse recovery losses like a traditional MOSFET.
Click image to enlarge
Figure 4: High-level block diagram of a solar string inverter
GaN transistors allow for a higher switching frequency which enables smaller transformers and inductors. Generally, an inductor size reduces by a rate inversely proportional to the operating frequency. For example, the transformer size may reduce by approximately 75% if the frequency increases from 100 kHz to 400 kHz. Since power inductors and transformers are often the largest components on the board, a reduction in their footprint can result in significant size reduction for the whole system.
GaN presents challenges to traditional current sensors
While GaN helps system designers to achieve hgiher efficiency and power density, there are two new challenges related to current sensing. The GaN component can suffer from very rapid thermal runaway requiring a protecting current sensor that can respond in less than 200 ns. Additionally, these higher switching frequencies needed to reduce transformer size presents challenges for traditional current sensors which are too slow.
Sense-resistor-based current sensors suffer in these applications because they are not fast enough. A high-voltage current shunt-based approach uses an isolation barrier, where current information is digitized on the primary side, sent over the barrier, and reconstructed on the secondary side. This process generally limits the bandwidth of sense-resistor-based solutions to 500 kHz and below. The sense-resistor-based solution also requires multiple external components and an isolated regulator to operate - meaning a large solution size.
Click image to enlarge
Figure 5: High-level block diagram of a clean energy storage system
Current transformers (“CTs”) are often used in high-voltage and high-speed current sensing applications because they can be used to measure frequencies exceeding 1 MHz. However, they have some major drawbacks. The CT footprint and height are large, which limits power density. The solution requires multiple external components to rectify and buffer the current transformer output. A CT only measures a narrow range of AC frequencies and cannot measure a DC signal - additional current sensors need to be implemented in the case of DC measurement.
Magnetic current sensors available today are limited in speed to 1 MHz maximum. While these may be used in some applications, they do not have the speed necessary for fast GaN protection.
Allegro Microsystems introduces magnetic current sensor with industry-leading speed
To address the current sensing challenges in GaN adoption, Allegro has released a new product family of current sensors – the ACS37030 and ACS37032 - featuring 5 MHz bandwidth. These products are 5x faster than the previous generation of magnetic current sensors.
The ACS37030/32 5 MHz magnetic current sensor achieves a higher bandwidth than any other
similar current sensor due to a proprietary dual sensing and signal path. Allegro developed an innovative solution combining a chopper-stabilized hall for lower frequencies with a coil-based transducer on silicon for higher frequency sensing. Both signal paths are combined internally to offer a single sensor output spanning DC to 5 MHz. The sensor response time is an astonishing 40 ns – fast enough to help protect GaN from thermal runaway.
Click image to enlarge
Figure 6: High-level block diagram of a datacenter power supply
Complimenting the ACS37030/32 industry-leading bandwidth are features for better system power density. The device integrates reinforced isolation capability, up to 420 VRMS (Basic isolation capable to 840 VRMS), eliminating the need for external isolator components. The solution is packaged in a small, enhanced SOIC-8 package with a low conductor path resistance of 0.68 mΩ to minimize heat generation when high current is flowing.
Despite the high levels of integration and high performance, the ACS37030/32 is a very simple device to implement for current sensing in any system. The product is powered from a 3.3V supply and provides an analog output voltage which is proportional to the current flowing through the conductor. No customer programming or configuration is required. There are two different versions of the device for selection: a reference voltage output (ACS37030) or an overcurrent fault output signal (ACS37032).
Click image to enlarge
Figure 7: Comparing transformer size versus operating frequency
ACS37030/32 integration into power conversion systems
The ACS37030/32 may be used in multiple different locations of the CLLLC circuit to provide current sensing feedback for GaN protection and power conversion control. GaN protection can be accomplished by placing a single current sensor in series with every GaN component, but this may require more components than actually needed to achieve the system’s goals. Typically, a current sensor is placed on the supply and/or ground lines to provide a common current feedback which can protect against GaN hard shorting events – the component presents a low impedance – or a half-bridge short to the power supply or ground. For system control, one or two current sensors ay be placed on either side of the CLLLC circuit in order to monitor the current and calculate the operating power of the power conversion.
