How to select MOSFETs for PD fast chargers above 30W?
MOSFETs Become Indispensable
Back in the era of 5V/2A charging, MOSFETs were barely used in fast charging power supplies. A 10W output could be easily achieved with just a few diodes and transistors. However, PD 3.0 reshaped the landscape. With a maximum output of 20V/5A (100W), the power rating surged tenfold alongside higher operating frequencies. Diodes could no longer meet the demands due to excessive power loss and severe heat generation. Today, MOSFETs have become an irreplaceable component for PD fast chargers.
30W serves as a clear dividing line. PD chargers below 30W rarely adopt MOSFETs, as transistors, diodes, resistors and capacitors are sufficient. For models above 30W, MOSFET selection turns critical. The higher the power, the more MOSFETs are required, and the stricter the selection criteria become.
MOSFETs in PD Fast Chargers
MOSFETs account for a considerable proportion of the total BOM cost of PD fast chargers. Far from being auxiliary parts, they rank among the costliest components in the entire power solution. MOSFETs are mainly deployed in two key positions in PD fast chargers.
Position 1: Vbus Load Switch
A switching transistor is installed between the Type-C port and the circuit to control the on-off state of Vbus. It turns on the circuit when a device is connected, and cuts off the power path upon disconnection or abnormal conditions to prevent reverse current and overcurrent.
N-channel MOSFETs are the mainstream choice here. Compared with P-channel counterparts, N-channel MOSFETs feature lower on-resistance (Rds(on)), mature driving circuits and better cost performance, dominating the current market.
The core selection criterion is low on-resistance. Since the full load current flows through the Vbus load switch, higher on-resistance leads to greater power loss and more intense heat buildup.
DFN3×3 and DFN5×6 are the prevailing package types, striking a balance between compact size and heat dissipation.
Hekotai Model Recommendations by Power Rating
- 20W (20V/1A): HKTQ65N03, 30V/65A, Rds(on): 3.8mΩ, DFN3×3
- 45W (20V/2.25A): HKTQ40N40, 40V/40A, Rds(on): 7.5mΩ, DFN3×3
Position 2: Synchronous Rectification
Traditional flyback PD chargers use Schottky diodes for rectification on the secondary side. Schottky diodes have a forward voltage drop of 0.3V to 0.7V. For low-voltage high-current outputs (e.g. 5V/3A), this voltage drop accounts for 6% to 14% of the output voltage, resulting in substantial efficiency loss.
Replacing diodes with N-channel MOSFETs for synchronous rectification effectively solves this problem. The voltage drop of a conducting MOSFET equals the product of its on-resistance (Rds(on)) and operating current. When the on-resistance is reduced to the milliohm level, power loss is far lower than that of Schottky diodes.
The voltage rating of synchronous rectification MOSFETs is determined by the output voltage. A 60V rating is adequate for 5V and 9V outputs, while a 100V rating is recommended for 12V, 15V and 20V outputs to ensure sufficient safety margin.
Hekotai Model Recommendations by Power Rating & Output Voltage
- 20W / 30W (5V - 9V output): HKTD50N06, 60V/50A, Rds(on): 13.5mΩ, TO-252
- 30W / 45W (12V - 20V output): HKTG48N10, 100V/79A, Rds(on): 8mΩ, DFN5×6
Summary of Selection Principles
Three key parameters need to be prioritized when selecting MOSFETs for PD fast chargers:
- On-resistance (Rds(on)): The top priority, as it directly determines conduction loss and temperature rise.
- Package: DFN5×6 is a versatile all-round option. Choose DFN3×3 for space-constrained designs, and adopt dual MOSFET parallel connection for high-power applications.
- Gate charge (Qg): It affects switching loss and driving requirements, which deserves special attention for high-power and high-voltage circuits.
Higher power ratings call for stricter requirements on Rds(on), larger packages and higher component prices. That said, total cost is not simply calculated by unit price. A MOSFET with lower on-resistance may eliminate the need for heat sinks, ultimately cutting down the overall cost of the charger.
Customers developing PD fast chargers usually confirm four core requirements first: number of output ports, total power, circuit topology, and specifications for MOSFET package and on-resistance. MOSFET selection is basically finalized once these points are clarified.
Beyond MOSFETs, Hekotai also provides a full range of discrete components for PD fast chargers, including rectifier bridges, fast recovery diodes and ESD protection devices. As MOSFETs are the most widely used components, customers can source transistors, diodes, resistors and capacitors together in one stop for higher procurement efficiency.