MOSFET Selection Guide for 30W+ Medium & Low Power PD Fast Chargers
MOSFETs Become Indispensable
Years ago during the 5V/2A charging era, MOSFETs were hardly used in fast charging power adapters. A mere 10W power output could be realized simply with several diodes and triodes.
USB PD 3.0 has completely changed the landscape. It supports a maximum output of 20V/5A (100W), bringing a tenfold power increase along with higher operating frequencies. Conventional diodes are no longer competent due to excessive power loss and severe heat generation. Hence, MOSFETs have become an essential component that cannot be omitted in PD fast chargers.
30W is a clear dividing line. PD chargers below 30W seldom adopt MOSFETs, where diodes, triodes, resistors and capacitors are fully sufficient. For models above 30W, MOSFET selection turns critical. The higher the power rating, the more MOSFETs are required and the stricter the selection criteria become.
MOSFET Applications in PD Fast Chargers
MOSFETs take up a fairly large proportion of the total BOM cost of PD fast chargers. Far from being auxiliary components, they rank among the highest-cost devices in the entire power solution. In PD fast chargers, MOSFETs are mainly deployed in two core positions.
Position 1: Vbus Load Switch
A switching MOSFET is installed between the Type-C port and internal circuits to control Vbus power on and off. It turns on the power path when devices are connected, and cuts off the circuit upon unplugging or abnormal conditions to prevent current backflow and overcurrent risks.
N-channel MOSFETs are mainstream choices here. Compared with P-channel counterparts, N-channel MOSFETs feature easier achievement of lower on-resistance, mature driving circuits and better cost performance, dominating the current market.
The core selection principle is low on-resistance.Since full load current flows through the Vbus load switch, higher on-resistance leads to greater power loss and more severe heat buildup.
Mainstream packages are DFN3×3 and DFN5×6, balancing compact size and heat dissipation performance.
Hekotai Recommended Models 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 adopt Schottky diodes for secondary-side rectification. However, diodes produce a forward voltage drop of 0.3V to 0.7V. For low-voltage high-current outputs such as 5V/3A, this voltage drop accounts for 6% to 14% of the total output voltage, resulting in obvious efficiency loss.
Replacing diodes with N-channel MOSFETs for synchronous rectification effectively solves this problem. Its conduction voltage drop equals the product of on-resistance and operating current. When the on-resistance reaches milliohm level, the power loss is far lower than that of Schottky diodes.
The voltage rating of synchronous rectification MOSFETs is determined by output voltage:60V voltage rating is adequate for 5V and 9V outputs; 100V voltage rating is recommended for 12V, 15V and 20V outputs for sufficient safety margin.
Hekotai Recommended Models by Power & 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 MOSFET Selection Rules
Three key parameters shall be prioritized in PD fast charger MOSFET selection:
- On-resistance (Rds(on)): The core indicator determining conduction loss and operating temperature;
- Package: DFN5×6 offers excellent universal compatibility; DFN3×3 fits space-limited designs; parallel connection of dual MOSFETs is adopted for high-power products;
- Gate charge (Qg): Closely related to switching loss and driving circuit requirements, worthy of special attention for high-power and high-voltage applications.
Higher power applications demand stricter requirements for lower on-resistance, larger packages and correspondingly higher costs. Nevertheless, overall cost cannot be judged merely by unit price. Slightly lower on-resistance may eliminate the need for heat sinks, thus cutting down the total manufacturing cost of finished chargers.
We usually confirm four core demands first for PD fast charger projects: number of output ports, total rated power, circuit topology, and required MOSFET package & on-resistance. MOSFET selection can be finalized once these points are clarified.
Apart from diversified MOSFET products, Hekotai also supplies a full range of discrete components for PD fast chargers, including rectifier bridges, fast recovery diodes and ESD protection devices. Matching MOSFETs with triodes, diodes, resistors and capacitors in one-stop procurement greatly simplifies component sourcing work.
